Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps

Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps

3.15 Solar-Assisted Heat Pumps DA Chwieduk, Warsaw University of Technology, Warsaw, Poland © 2012 Elsevier Ltd All rights reserved 3.15.1 3.15.2 3.15.2.1 3.15.2.2 3.15.2.3 3.15.2.4 3.15.3 3.15.3.1 3.15.3.2 3.15.3.3 3.15.3.4 3.15.3.5 3.15.3.6 3.15.4 3.15.4.1 3.15.4.2 3.15.4.3 3.15.4.4 References Introduction to the Concept of Solar-Assisted Heat Pumps Heat Pump Fundamentals Principles of Heat Pump Operation Thermodynamic Cycles Classification of Heat Pumps Renewable Heat Sources Solar-Assisted Heat Pump System Classification, Configurations, and Functions Direct Solar-Assisted Heat Pump Systems Series Solar-Assisted Heat Pump Systems Parallel Solar-Assisted Heat Pump Dual-Source Solar-Assisted Heat Pump Other Configurations Solar-Assisted Heat Pump System with Seasonal Storage Fundamental Options of Seasonal Energy Storage Classification and Evaluation of Seasonal Ground Storage Heat and Mass Transfer in the Ground Store, General Consideration Applications 495 495 495 497 501 502 506 506 507 508 511 516 517 519 519 521 524 525 527 3.15.1 Introduction to the Concept of Solar-Assisted Heat Pumps There are some limitations on the use of solar radiation for heating purposes, mainly because of its stochastic and intermittent character There are changes in solar radiation availability in the long term, that is, over a year, and also in the short term, that is, over a day When solar energy is used for space heating, the time and peak values of available solar radiation are quite opposite to the time and peak values of space heating demand This is especially true in high-latitude countries, where in winter the solar radiation level is low and the duration of solar irradiation is short, and consequently there is a cold climate and long heating season with high heating demands In many high-latitude regions, heating of buildings is a major component of the total energy used in the building sector Usually ‘traditional’ solar active heating systems alone cannot provide all the heating needs There are different options to solve this problem One of them is to couple a solar heating system with a heat pump in one combined heating system, as can take place in a small- or medium-scale application, that is, in single-family houses and multifamily or public buildings, respectively This type of a heating system is called a solar-assisted heat pump (SAHP) system Another option is to use the seasonal solar energy storage in the form of sensible heat of a large storage volume Usually the temperature of the heat stored is too low to be used directly for heating The low-temperature heat stored can be converted into higher temperature heat by applying a heat pump In this way also a heat pump is incorporated into the heating system Such heating systems can be used for medium- or large-scale applications, that is, in multifamily or public buildings, or blocks of buildings (i.e., in communes and small city districts), respectively In the case of medium-scale applications, the heating system is termed a solar-assisted heat pump system with seasonal storage (SAHPSS) system, and a common example is a solar-assisted heat pump system with ground storage (SAHPGS) In the case of large-scale applications, such a system is called a central solar heating plant with seasonal storage (CSHPSS) There are a variety of underground thermal energy storage (UTES) system configurations and modes of operation Solar collectors and a heat pump are the major system components This chapter describes the concept, classification, and operation of SAHP systems, including systems with seasonal storage For better understanding of the idea of SAHP system operation and application, the fundamentals of heat pumps are initially presented Heat pumps applied in SAHP systems are vapor compression heat pumps, and this type of heat pumps are described and analyzed 3.15.2 Heat Pump Fundamentals 3.15.2.1 Principles of Heat Pump Operation Heat pumps, refrigerators, and heat engines are heat machines Their operation is based on thermodynamic processes that are governed by the first and second laws of thermodynamics [1, 2] Heat pumps and refrigerators operate in reversed cycles compared with heat engines The processes and energy flows are in the opposite direction to those in the power cycles of heat engines, as presented schematically in Figure Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00321-8 495 496 Applications Power cycle Reverse cycle Heat source at T2 Heat sink at T2 Q2 Engine Q2 Refrigerator or Heat pump Wout Win Q1 Q1 Heat sink at T1 Heat source at T1 Figure Concept of power cycle for a heat engine and reverse cycle for a refrigerator or a heat pump A heat engine’s function is to generate work using a heat source In a heat engine, the high-temperature T2 heat source is used giving the heat input Q2 to get the energy output in the form of work W However, there is an amount of heat Q1 at lower temperature T1 that must be removed to the heat sink to fulfill the first law of thermodynamics The first law of thermodynamics is the conservation of energy law, and can be written (see Figure 1) as follows: Q2 ẳ jWj ỵ jQ1 j ẵ1 Unlike a heat engine, a heat pump’s function is to lift a certain quantity of heat Q1 from a heat source at a lower temperature level T1 to a heat sink at higher temperature T2 However, to fulfill the first law of thermodynamics, the upgrading of heat must be done by the work W that is supplied to the machine Following this and Figure 1, it can be seen that the amount of heat Q2 to the heat sink is the sum of heat Q1 extracted from the heat source and the amount of work W required in the process, and the first law of thermodynamics, described by eqn [1], is accomplished As it has been already mentioned, a heat pump is used to supply heat Q2 at high temperature T2 Conversely, a refrigerator is used to extract heat Q1 at low temperature T1 It means that a refrigerator’s function is to cool down a given heat source extracting a certain quantity of heat Q1 from this source (at lower temperature T1) In a refrigerator, as in a heat pump, to fulfill the first law of thermodynamics, to extract the heat from the heat source the work W must be supplied to drive the cycle and heat must be removed at a higher temperature In practice, the functions of both heat pump and refrigerator can be combined in one machine, when both heating and cooling are required simultaneously For example, such situations can be found at a sport center where some chillers are used to cool an ice skating area and at the same time they also provide heat for hot water for swimming pools, operating like a heat pump The efficiency of a heat engine, a refrigerator, and a heat pump is defined using the first law of thermodynamics In the case of a heat pump and refrigerator, efficiency is measured by the coefficient of performance (COP), which is the ratio of the energy that is used for heating (at the heat sink) or for cooling (at the heat source) to the work that has to be supplied to drive the cycle The efficiency of a heat engine, a refrigerator, and a heat pump can be expressed by eqns [2a], [2b], and [2c], respectively, in the following way: η¼ COPr ¼ W Q2 − jQ1 j jQ1 j ¼ ¼1− Q2 Q2 Q2 ½2aŠ   Q2 −jWj  Q2  jQ1 j jQ1 j ¼ ¼ ¼ −1 jWj Q2 − jQ1 j jWj W ẵ2b Q2 Q2 ẳ jWj Q2 jQ1 j ẵ2c COPhp ẳ It can be seen from eqn [2a] that the efficiency of the heat engine is always lower than 1, and the COP of a refrigerator (eqn [2b]) is one less than that of a heat pump (eqn [2c]) Summarizing, a heat engine is a machine generating work from the heat that is provided to the process and rejecting some amount of the heat at a lower temperature Conversely, a heat pump is a machine that lifts a certain quantity of heat from a lower temperature level to a higher temperature level using the work provided to the machine A refrigerator is a machine that extracts a certain quantity of heat from a lower temperature level and transfers it to a higher temperature level also using work provided to the machine Solar-Assisted Heat Pumps 497 According to the second law of thermodynamics, heat cannot flow from a lower to a higher temperature without the expenditure of energy This law for the reversible cycle can be expressed by the following equation: X Qi ẳ0 Ti ẵ3a Taking into account eqn [3a] and referring to the nomenclature used before, the following can be written: Q1 Q2 − ¼0 T1 T2 ½3bŠ Referring to eqn [1], eqn [3b] can be rewritten in the following way: Q1 Q ỵ W ẳ0 T2 T1 ẵ4 Q1 Q W ẳ0 T1 T2 T2 ẵ5 After rearranging the terms, it is as follows: Because T2 > T1, the difference between the first and the second term of eqn [5] is larger than zero, and the work input must be sufficiently big to make the sum to be equal to Thus, there is a minimum work that is required for the machine, that is, the heat pump, to operate reversibly For the vapor compression heat pumps considered, the work supplied is in the form of mechanical energy provided by electrical energy to drive the heat pump compressor There are other types of heat pumps, where the energy required to drive the system is supplied in the form of heat, and these are sorption heat pumps (not analyzed in this chapter) 3.15.2.2 Thermodynamic Cycles In principle, to achieve the reversible cycle in a heat pump, a condensable fluid must realize a reversed Carnot cycle This allows heat to be input and output at a constant temperature by means of boiling and condensation This meets the requirement that all heat transfer to and from the system must be reversible The Carnot cycle [1–4] for a heat pump in a temperature–entropy diagram is shown in Figure on the left and the main components of a heat pump based on the Carnot cycle are shown on the right The following processes presented and numbered in Figure take place: 1–2 Isentropic compression Two-phase liquid–gas of the working fluid from the evaporator flows into the compressor and is compressed to the required level of pressure and temperature 2–3 Constant pressure and temperature heat rejection – condensation Vapor of the working fluid at high pressure and temperature flows from the compressor to the condenser At constant pressure and temperature, the working fluid condenses giving up the heat (latent heat – condensation heat) to the sink, for example, space heating medium that provides the heat required by the space heating demand 3–4 Isentropic expansion Liquid of the working fluid from the condenser flows into the ideal expansion device (at high pressure and temperature) and is expanded to the required level of pressure and temperature to close the reversible cycle 4–1 Constant pressure heat absorption (evaporation) The two-phase liquid–gas working fluid at low pressure and temperature flows from the expansion valve to the evaporator At constant pressure and temperature, the working fluid evaporates, because of the existing temperature gradient between working fluid and the low-temperature heat source Then the process repeats T Q2 T2 Q2 2 Win T1 Condenser Expansion valve Compressor Q1 Δs Evaporator s Q1 Figure The Carnot cycle for a heat pump and schematic concept of a vapor compression heat pump Win 498 Applications The ideal reversed Carnot cycle is realized between heat source and heat sink that have constant temperature, that is, T1 = const and T2 = const The transition process between these two states (lines) is adiabatic, and can be written in the following way: dQ ¼ dS ¼ T ½6Š Equation [6] is true for only reversible adiabatic processes and then S = const Analyzing Figure it can be seen that two processes are adiabatic: 1–2 compression and 3–4 expansion Because of the equity of entropy differences, S4 – S1 = S3 – S2, the efficiency of the Carnot cycle for a heat engine (eqn [2]) is as follows: ẳ T1 S4 S1 ị T1 jW j jQ1 j ¼1− ¼1− ¼1− Q2 Q2 T2 ðS3 −S2 Þ T2 ½7aŠ The same can be written for the efficiency (i.e., COP) of a refrigerator and a heat pump operating in reversible adiabatic processes 1–2 and 3–4 (Figure 2), and according to eqns [2b] and [2c] respectively, it is as follows: COPr ¼ T1 ΔS T1 jQ1 j ¼ ¼ Q2 −jQ1 j ðT2 −T1 ÞΔS ðT2 −T1 Þ ½7bŠ T2 ΔS T2 jQ2 j ¼ ¼ ðT2 −T1 ịS T2 T1 ị jQ2 j Q1 ẵ7c COPhp ẳ The work W that is necessary to drive the machine is equal to the area limited by lines 1–2, 2–3, 3–4, and 4–1, which represent all ongoing processes, as shown in Figure The Carnot cycle has the maximum possible COP between any two temperature levels T2 and T1 and is used as an ideal cycle for comparison with practical ones From eqns [7b] and [7c], it is clear that the COP is the biggest when the temperature difference between T2 and T1 is the smallest (the work W – the matched area – is the smallest) It means that the operation of a heat pump and refrigerator is most efficient when the temperature of a heat source is as close as possible to the temperature of a heat sink This is very important for the practical selection of heat sources and heat sinks The main components of a heat pump specified below are responsible for processes presented and numbered in Figure 2: 1–2 Compressor 2–3 Condenser 3–4 Theoretically it could be a turbine; however, a Carnot cycle is impractical for power generation, therefore it is expansion valve (and in practice a throttle) 4–1 Evaporator To determine the COP of an ideal Carnot cycle heat pump, eqn [7c] can be used For example, if outside air at °C (273 K) is a heat source and the air inside the house at 20 °C (293 K) is a heat sink, then the COP of the heat pump considered is equal to 14.65 In practice, it is not possible and a real machine would have a COP much smaller, of about 3–4, as is described in the following paragraphs In reality, a Carnot cycle cannot easily be used for a heat pump (or refrigeration) cycle and the reversed Rankine, Perkins/Elmer, or Linde cycles are used, all being vapor compression cycles [3, 4] Figure presents the basic vapor compression cycle for heat pumping in a temperature–entropy diagram The following processes presented and numbered in Figure take place: 1′–2′ Isentropic compression Saturated dry vapor (of the working fluid) at low pressure flows from the evaporator into the compressor and is compressed to the required level of pressure and temperature in the superheating vapor region T 2� x=0 T2 T1 x=1 Q2 4� 1� Q1 s Figure The vapor compression cycle for a heat pump presented in a temperature–entropy diagram Solar-Assisted Heat Pumps 499 2′–2 Isobaric heat rejection Superheated vapor of the working fluid at high pressure and temperature flows from the compressor to the condenser The heat rejection takes place due to temperature gradient and represents desuperheating of the vapor 2–3 Isobaric and isothermal heat rejection – condensation Desuperheated vapor (of the working fluid) at high pressure flows through the condenser At constant pressure and temperature, vapor condenses giving up the heat to the sink (e.g., space heating medium) Saturated liquid leaves the condenser There is no pressure drop in the condenser and connecting piping, 3–4′ Isenthalpic expansion Saturated liquid (of the working fluid) from the condenser flows into the throttle (at high pressure and temperature) and is expanded to the required low pressure and temperature Expansion losses mean that the cycle is not reversible (not isentropic expansion process) 4′–1′ Isobaric and isothermal evaporation Two-phase mixture of the working fluid at low pressure and temperature from the throttle flows through the evaporator At constant pressure and temperature, the working fluid evaporates, taking heat from the low-temperature heat source There is no pressure drop in the evaporator and connecting piping Comparing the evaporation process of this cycle and the Carnot cycle, it is evident that now the end of the evaporation process is exactly on the saturation line (point 1′) It must be underlined that in reality stopping evaporation at just the right dryness fraction, as it is in the Carnot cycle, is very difficult In addition, real compressors might be damaged because of compressing two-phase mixtures (saturated liquid and vapor) to a saturated vapor state In practice, to ensure that the evaporation is really completed and there is only one-phase fluid (saturated vapor) at the compressor inlet, a small amount of superheat (typically a few degrees) is transferred to the vapor just after leaving the evaporator This is shown in Figure in the T–S diagram where state 1″ (beginning of ‘suction’) is slightly superheated Using a throttle in the place of an ideal expansion device means that the expansion process is not reversible There is saturated liquid leaving the condenser which flows into the throttle However, the reduction in pressure in the throttle causes some of the liquid to boil and a two-phase mixture is formed As a result, the temperature of the working fluid drops The whole process is isenthalpic, because the enthalpy of the stream of fluid is the same before and after the throttle However, two phase mixture leaving the throttle contains liquid with lower enthalpy than the fluid before throttle and vapor with higher enthalpy than the fluid before throttle This means that the working fluid enters the evaporator at point 4′ not and the entropy of expansion process in the throttle increases and the process is irreversible Entropy difference at the heat source is not equal to that of the sink, the former being bigger than the latter At such cycle, the work input W to the device must be bigger than that for the Carnot cycle (the rectangular area matched in Figure 2) As a consequence, the vapor compression COP is reduced in comparison with a Carnot cycle working between the same temperature limits, and can be written as follows: COPhp ¼ T2 ΔS2 − T2 ΔS2 − jQ2 j ¼ ¼ ¼ COPhpCar ηhpCar < COPhpCar ðT2 −T1 ÞΔS4 ′ − ðT2 −T1 Þ ΔS4 ′ − jQ2 j −Q1 ½8Š As mentioned above, to ensure complete evaporation, the quantity of heat absorbed from the heat source is increased (evaporation ends at point 1″ not at for the Carnot cycle, or 1′ for the Linde cycle), as shown in Figure This effect of slightly overheating the dry vapor before it enters the compressor results in an increase in the refrigeration effect, which is positive for a refrigerator, but not for a heat pump The vapor compression cycle is very often presented as the ln(p)–h diagram An example of such diagram for the vapor compression cycle is shown in Figure In Figure 5, most of the same state points are presented in the ln(p)–h diagram as before (see Figure 4) However, there is an extra point 2‴, which represents the state of the desuperheated vapor after the compression, which in reality is not an isentropic process The points in Figure represent the following processes: T 2� T2 Q2 1� Carnot T1 4� 1� Q1 s Figure T–S diagram of the ideal vapor compression with the beginning of ‘suction’ slightly superheated 500 Applications h2 �3� np Q2 4� 1� 2� 2� 1� Q1 h1�4� h1� � h Figure The vapor compression cycle presented in a ln(p)–h diagram 1″–2‴ Nonisentropic compression in the superheating vapor region, 1″–2″ isentropic compression, dotted line 2‴–2 Isobaric heat rejection representing the desuperheating of the vapor 2–3 Isobaric and isothermal heat rejection – condensation 3–4′ Isenthalpic expansion 4′–1′ Isobaric and isothermal evaporation Analyzing the ln(p)–h diagram, it can be seen that q1 = h1″ – h4′ = h1″4′ – isobaric evaporation (w = 0); q2 = h3 – h2‴ = –h2‴3 – isobaric condensation (w = 0); win = h1″ – h2‴ = –h1″2‴ – adiabatic compression process (q = 0) It is very convenient to use the ln(p)–h diagram to determine the COP of a heat pump, because the COPs are simple ratios of length (enthalpies) in the ln(p)–h diagram, as presented in Figure A COP of a refrigerator and a heat pump (referring to eqns [2b] and [2c], respectively) can be expressed as a function of enthalpy The enthalpies of different refrigerants as functions of pressure and temperature can be found in the literature (e.g., in tables and charts [5]) Thus, the COP of a refrigerator and a heat pump, using the diagram in Figure 5, can be expressed as follows: h1 ″ ″ h1 ″ ‴ ½9aŠ h2 h1 ẵ9b COPr ẳ COPhp ¼ The COP of a heat pump described by eqn [9b] is higher than the real one and there are many reasons for this First, in practice, liquid leaving a condenser is often subcooled to ensure that only the liquid phase of the working fluid enters the throttle Therefore, apart from the so-called ‘superheat horn region’ (see Figures and 4), there is a ‘subcooled region’ outside the saturated line (point is moved into the liquid region, to the left) This makes the input work W to drive the machine bigger than for a Carnot cycle, as indicated by the increased area enclosed by the cycle in a T–S diagram, and as a consequence the COP is smaller It must be also underlined that in real heat exchangers there is a temperature difference between evaporating working fluid and a heat source (e.g., about 10 °C) and between condensing working fluid and a sink (e.g., °C) It makes the temperature difference in a vapor compression heat pump between a heat source and a heat sink bigger than that in an ideal Carnot cycle heat pump, and makes the COP smaller Another reason for the reduction in COP is the nonideal compression process Compressors operate with a certain isentropic efficiency and in addition electric motors driving a compressor operate with an efficiency less than one Therefore, in practice, COP drops to 3–4 It should be mentioned that nowadays some compressor heat pumps can offer apart from the heating an additional function by being able to cool buildings There are two main different methods for cooling with a heat pump In the first method, a heat pump can operate in a reversible cycle, so in summer it operates like a refrigerator (the fundamentals have been described in the beginning of this chapter) In the second method, the so-called direct ‘natural cooling’ takes place It means that a heat pump is switched off, except for the control unit and the circulation pumps The ‘natural cooling’ is applied in ground and underground water heat pumps when the brine of underground heat exchangers or the groundwater system absorbs the heat from the heating circuit (e.g., floor heating system) in a building and transfers it to the heat source medium in the ground This method can also be considered as thermal regeneration of the ground heat source (cooled during heating season and heated during summer) Solar-Assisted Heat Pumps 501 One of the most important issues of heat pumps is requirements for refrigerants Refrigerants should have low pressures (all subcritical) at heat exchange temperatures They must be nontoxic, not flammable, and nonpolluting, that is, environmentally safe Considering their influence on the environment, the following indexes are usually used: • ODP – ozone depletion potential; • GWP – global warming potential; • TEWI – total equivalent warming impact (for the whole system) It is also required that the refrigerant’s density is high for low-volume flow rate and thus a smaller compressor, piping diameter, and heat exchangers can be used In the past, the most popular refrigerant for domestic and light commercial heat pumps was R22 Nowadays, it has been replaced mainly by R 134a (a hydrofluorocarbon (HFC)) However, there are a number of new (and old) refrigerants such as other HFCs, propane and butane mixtures, ammonia, or carbon dioxide Another important issue is the selection of a heat source suitable for heating demands and the type of heat pump as described below 3.15.2.3 Classification of Heat Pumps As has been described in previous paragraphs, the general principle of a heat pump operation is to extract heat from a low-temperature heat source and to transfer it to a heat sink at a higher temperature The useful energy output must be significantly greater than additional energy required to drive a heat pump to achieve a real reduction in primary energy use Heat pumps can use renewable energy or waste heat as a heat source Energy extracted from these sources is converted into useful heat in the low-temperature range This low-temperature heat can be applied with high efficiency, for example, for space heating and domestic hot water (DHW) [6] A number of different classifications of heat pumps can be made; the main one is according to the form of energy that is used to drive them In this case, the following types are considered: • mechanically driven, that is, compressor heat pumps; • thermally driven, that is, sorption heat pumps In this chapter, the mechanically driven compressor heat pumps are considered Among compressor heat pumps, we can list • electrical heat pumps in which the compressor is driven by electricity (e.g., by an electric motor); • heat pumps in which the compressor is driven by an internal combustion engine; these heat pumps operate with natural gas, diesel, or biofuel (rapeseed oil) The heat pumps considered can be classified according to the type of the end user as follows: • domestic heat pumps for small- and large-scale application used for space heating and DHW; • domestic heat pumps for small- and large-scale application used for space heating, cooling, and DHW; these heat pumps can work depending on the season of the year (winter and summer) in heating or cooling mode, which means that a heat pump function can be reversed; • light commercial heat pumps for different heating purposes (applied in offices, schools, hotels, hospitals, public buildings); • heat pumps with dehumidification function (for swimming pools; for drying of vegetables, fruits, plants, etc.); • large commercial and industrial heat pumps (for town districts and towns, industrial applications) These mainly use waste heat sources The sources of waste heat can be sewage, exhaust gases, or technological waste heat in air, water, and vapor Classification can also be made according to the heat pump’s construction There are two fundamental options: • compact or unitary heat pump – all the components are in one compact unit, there is a heat exchanger between a heat pump evaporator and a heat source, and between a heat pump condenser and a heat sink Usually the heat source is outside the building heated; however, when the waste heat is used it could be also inside a building; • split heat pumps – the components are split (divided) usually into two units: one of them is located in a separate room or outside a building Usually an evaporator or a condenser can be located directly at the heat source or at the heat sink, respectively It means that evaporation and/or condensation take part directly at a heat source or at a heat sink When the evaporator is located directly at a heat source, such systems are also called direct expansion The other way of classification can be done in accordance with the role of a heat pump in a heating system, and it is as follows: • monovalent heat pump – it operates throughout the year in monovalent mode providing all heating requirements by itself; • bivalent or hybrid heat pump – to provide all heating requirements it has to operate in conjunction with another heating device or system Heat pumps can also be classified according to the type of a heat source medium, which can be generally air, water, and brine (antifreeze mixture) They can use waste or renewable energy heat sources Renewable energy sources are 502 Applications mainly used for domestic and light commercial (including swimming pools) applications The renewable energy sources are as follows: • • • • • ambient air; ground (soil); geothermal water; surface water; solar radiation Another way of heat pump classification is according to the type of heat sink medium, Water and air are the two main heating mediums depending on the type of heating system Sometimes, heat pumps are named (classified) according to the type of heat source and heat sink medium in the following way: • • • • • • • • air–air; air–water; water–air; water–water; ground–water, or brine–water; ground–air, or brine–air; solar–water (SAHP-w); solar–air (SAHP-a) However, it can be mentioned that the last four types are also called in general ground source heat pumps and solar (solar-assisted) heat pumps (the last two) respectively Selection of a heat source suitable for a heat sink and for a given heating demand is very important and influences considerably the heat pump operation and in turn its performance (COP) 3.15.2.4 Renewable Heat Sources Generally, when heat pumps are considered for effective use, the following characteristic features of a heat source are taken into account [6]: • • • • • • • • good availability; coherency between the source and the user; high thermal capacity; constant in time and of relatively high temperature; natural energy equilibrium of the source (environment) and its physical characteristics are not affected by heat extraction; high purity (to avoid corrosion); no pollution, damage to environment, and other ecological issues; low cost of heat extraction Renewable energy heat sources are described below and their characteristic features that have been just mentioned are analyzed briefly The simplified idea of utilizing different renewable energy sources for a heat pump at a single-family house is presented in Figure Considering availability of heat sources ambient air is the best Unfortunately, it is not coherent with space heating demand When space heating demand is the highest, the air temperature is the lowest The temperature of ambient air is not constant in time and it can fluctuate very rapidly Ambient air (heat source) heat pumps operate usually with a COP of about When the temperature of ambient air is just above °C, then the problem with ice formation on the evaporator surface can occur This surface ice effect causes worse heat transfer conditions and together with the low ambient air temperature they result in low thermal performance of a heat pump, that is, low COP Nowadays, these possible problems are overcome by applying regular automatic defrosting (the high-temperature working fluid from the compressor, instead of flowing into the condenser, is recirculated into the evaporator) Modern air–water heat pumps can operate down to an outside temperature of − 15 °C However, then their COP is much lower than and they no longer can meet the heating demand completely Air heat pumps operate in bivalent (dual) mode using an auxiliary heater in times of low outside temperature Usually, this bivalent mode is monoenergetic mode, that is, electric heater is used The heating water is preheated by the heat pump, to the selected flow temperature, and then an electric heating cartridge is used to provide auxiliary peak heat It should be mentioned that an air–water heat exchanger must circulate a large volume of air (e.g., 3000–4000 m3 h−1) As a consequence, they can generate a lot of noise Air heat pumps can be constructed and installed as compact heat pumps and in this case air supply duct is used to supply outside air to the heat pump evaporator inside the building and exhaust air duct is used to take off the air used In this case, a problem with noise generation is very likely Therefore, the split construction is used very often, and the intake and outtake of air and evaporator are located outside the building, and other heat pump elements inside the building There are also heat pumps utilizing exhausted air as a heat source They are applied mainly in buildings with very low energy demand, for example, in passive buildings, where they are coupled with domestic ventilation system with heat recuperation The Solar-Assisted Heat Pumps 503 Solar radiation Ambient air Heat pump Ground Well Underground water Figure Idea of utilizing different renewable energy sources for a heat pump at a single-family house part of heat of air extracted from a building from the ventilation system that cannot be recovered in a direct way (in a recuperative heat exchanger) is used as a heat source for an integral exhaust air–water heat pump This type of a heat pump can also be used in other so-called ‘low-energy buildings’, but it cannot operate only in monovalent mode and an electrical supplementary heater is usually used to provide the additional auxiliary heating energy required to meet the total heat demand The main advantages of air heat pumps, apart from very good availability of a heat source, are the following: • heat extraction from outside air does not disturb the natural energy equilibrium of a heat source (environment) and its physical characteristics; • high purity; • there is no pollution because of extraction and exhaustion of air cooled, and no damage to the environment; • there are very low costs of heat extraction, the lowest among all types of renewable energy source heat pumps, and the method of heat extraction is the simplest one However, the main disadvantage, that is, no coherency between a heat source and a heat sink (space heating demand), causes the operation of a heat pump with low COP, and as a consequence a higher amount of electricity is used to drive the air heat pump than that required, for example, for the ground source one The ground constitutes a suitable heat source for a heat pump considering small-scale low-temperature heating systems [7] The seasonal temperature fluctuations are much smaller than those of the ambient air even at small depths Ground at small depth is under the influence of solar radiation, rain, melt snow (water), and other environmental factors At a depth of m, underground temperatures range from to 13 °C during the heating season (in most European countries) With the increase of ground depth down to 10 m the temperature becomes nearly constant throughout the whole year and is approximately equal to the annual mean outside (ambient) air temperature [8] With the further increase of depth, the ground temperature increases but relatively very slowly The influence of geothermal energy is weak even at a depth of 50 or 100 m [9] The energy flowing from deeper layers upward represents only 0.063–0.1 W m−2 Energy stored in a natural way in the ground medium is extracted by means of large-area horizontal plastic pipework buried underground or longer length plastic tubes set into drilled vertical ducts or bore holes [7, 10] Heat exchangers are installed in an area next to the building In horizontal heat exchanger systems, the plastic pipes (e.g., polyurethane (PE)) are buried underground at a depth of between 1.2 and 1.5 m Individual pipe runs (loops) are usually limited to a length of 100 m If the length of pipe runs is too big, then there is pressure drop in piping and as a consequence the required pump capacity would be too great All loops have to be of the same length, because the pressure drop must be the same to achieve identical flow conditions in every pipe run As a consequence all heat exchanger loops can extract heat evenly from the ground medium Usually, heat extracted from the ground is transferred via water and antifreeze mixture (brine) to an evaporator of a heat pump Because of the liquid used, these heat pumps 504 Applications are called brine/water heat pump (brine in the primary and water in the secondary (heating) circuit) In the ground outside the building or just directly inside the building, there is a header duct with a brine distribution that consists of two brine distributors, flow and return, where the pipe ends come together Return and flow headers are installed slightly higher than piping (venting) It is important that each loop is able to be shut off separately A circulation pump circulates the brine through the pipes that extract the heat stored underground The heat extraction from the ground varies from 10 W m−2 (in the case of underground areas of dry sandy soil) to 35 W m−2 (in the case of ground with groundwater ways) In central European countries, the freezing zone in the soil is m and in some regions 1.5 m deep More to the north (high-latitude countries) the freezing depth is bigger This makes it preferable to use vertical ground heat exchangers rather than horizontal ones [6] Heat is extracted from the ground by vertical ground heat exchangers that are mainly in the form of U tubes (so-called ground ducts or probes), double U tubes (so-called duplex probes), or concentric tubes (popular in the past, not at present) These tubes are coupled with a heat pump evaporator similar to the horizontal heat exchanger system Water and antifreeze mixture (brine) circulate in pipes and the pipe ends come together in a header duct with a brine distribution (with flow and return distributors) located outside or inside the building If a header duct is located outside the building, it is recommended to insulate the underground collector pipes (flow and return) The location of heat exchanger tubes can vary, and they can be set into the ground in rectangular, hexagonal, or cylindrical configuration The distance between vertical tubes depends on the thermal and hydrological characteristics of the ground Usually, this distance can be at least about m for single U tubes and m for double U tubes Possible heat flux extraction for vertical ground probes depends on the type of the ground [7] and it can be at a level of 20 W m−1 of a tube length for dry sediment with relatively low thermal conductivity (lower than 1.5 W m−1 K−1) or even 70 W m−1 (of a tube length) for solid rock with high thermal conductivity (higher than W m−1 K−1) In the case of ground formed by gravel and sand with waterways, the specific heat extraction for double U tube can be about 55–65 W m−1, and for moist clay and loam about 40 W m−1 Under standard hydrological conditions, an average possible heat flux extraction (so-called probe capacity) of 50 W m−1 probe length can be expected (according to Reference [11]) The most important advantage of a ground heat pump, especially with vertical heat exchangers, is the fact that heating of a building can be accomplished in a monovalent mode of operation Of course, at the end of the heating season, due to heat extraction the ground medium is cooled down However, because of the natural heat and mass processes, that is, influence of ambient environment from the top, undisturbed ground surrounding the sides, and geothermal energy from the bottom, the ground can come back to its initial thermal balance This way, in an annual cycle, the natural thermal state of the ground source cannot be disturbed When bigger heat demand is expected, then it is very good to apply artificial charging of the soil, for example, by solar energy [12] (as described in Section 3.15.4), to ensure return to initial undisturbed thermal conditions Of course, when heating requirements of a building are rather small, then it is quite sufficient to apply ground heat pumps, using the ground as a natural heat source without artificial charging of the ground medium Ground source heat pumps can use the earth in direct or indirect mode In direct mode, heat exchangers buried horizontally or set vertically in the ground constitute the evaporator coils (evaporation process takes place just in the ground medium) When ground heat exchangers constitute separated closed loops or pipe runs and are coupled with a heat pump evaporator located inside a building, then a heat pump uses the ground in indirect mode It means that heat exchange between the ground body and working fluid in the heat pump evaporator (refrigerant) is accomplished through an additional medium In this case, a heat carrier fluid (brine), as an additional medium, circulates in the ground heat exchangers The possibility of utilizing ground as a heat source varies from place to place and depends mainly on local geology and size of system To design a system it is good to make a general review of a place proposed for location and positioning of the ground system and to estimate heat demand and its distribution in time Having made this preliminary study on ground source potential and heating needs, the selection of the type and configuration of the ground heat pump can be made In most ground heat pump systems for small-scale applications, only very rough analysis is needed This analysis becomes more complicated when the size of the system, that is, heat demand, is bigger It is very useful to know the geological and hydrogeological characteristics of the ground As a result, the thermal behavior of a ground system can be predicted more easily Geological and hydrological conditions are not so important in the case of small-scale ground-coupled heat pump systems When a high level of underground water (waterways) exists, then it is a great advantage for a ground system Heat from the surrounding undisturbed ground body at a higher temperature is transferred more quickly to the ducts and probes of a ground system In the case of small systems, very often there is not much area available to install the system; therefore an analysis of disposition and arrangements of the system components, especially tubes of ground heat exchangers, is needed As mentioned before, vertical ground heat exchanger systems are more recommended than horizontal coils, especially in high-latitude countries The main reasons are as follows: • Heat extraction conditions are better deeper in the ground than just approximately m below the earth surface, especially when freezing phenomena develop Deeper in the ground the temperature is higher (than just below the earth surface), and a heat pump uses the heat source of the higher temperature and operates in better thermal conditions As a consequence, the COP is higher (it can be at a level of 4–4.5), and hence a smaller amount of electricity is used to drive the heat pump • Problem with ice formation on the evaporator surface (of a heat pump) does not occur; there is no need for defrosting The heating mode can be more predictable and stable • The operation of a heat pump can be accomplished in a monovalent mode, as the heat pump is the only heating device providing all heating requirements 514 Applications (not via storage) [13] Usually the heat exchanger that links the heat pump condenser and store is located in the upper part of the storage tank, above the solar collector heat exchanger Sometimes, a heat pump condenser can be put directly into a storage tank If the storage tank is also for DHW, the inlet of cold water is located at the bottom At the top of the tank the outlet of hot water for DHW is installed, to extract the heat QhDHW for DHW There is another heat exchanger in the tank, below the DHW outlet, that connects the store with the heating circuit for space heating, usually low temperature, for example, floor heating circuit The heat Qheat needed for heating a building is extracted through this heat exchanger Very often an auxiliary heater, usually an electric one, as the peak source is also integrated into the storage tank at the top If necessary, when the temperature of the storage tank, even at the top, is too low to meet the heating requirements, the electric heater is turned on and it supplies auxiliary heat Qaux to the storage tank Most of the modern parallel SAHP systems contain a storage tank, which is a main core component of the system that integrates all the other components In such a configuration of the system, even if the solar thermal system and the heat pump not have direct contact, through the common storage tank they interact with each other There are positive effects of this interaction, because the solar thermal part and heat pump are complementary to each other This makes the operation of the whole heating system very reliable A parallel SAHP system can provide all heating loads and there is no need to install and use any other heating device, extra burner, or boiler This is very convenient for the user However, due to the fact that a heat pump and solar collectors deliver heat to the same storage medium, sometimes operation of one part of the system, usually the heat pump, limits the operation of the other one, that is, the solar collectors For example, in winter in high-latitude countries, very rarely is the temperature of the working fluid of the solar collectors higher than the temperature of the heat carrier fluid extracting heat from the heat pump condenser As a consequence, the heat pump operates most of the time and limits the utilization of solar energy In addition, sometimes installers (through the automatic control system) set too high a limit for the temperature of the working fluid of the solar collectors to circulate If this value is too high (e.g., above 40 °C) the working liquid does not circulate and supply heat to the storage tank in winter and on cloudy days, which limits significantly the operation of the solar thermal part of the system Figure 10 presents a scheme of modern parallel SAHP system and Figure 11 presents the main components of the system in the indoor ‘boiler room’ (heat pump in the middle, combined buffer storage at the right side) This system has been operating recently In Figure 10 the symbols T with numbers in indexes represent main temperature sensors linked to the control This system contains the following main components: solar collectors – flat plate with antifreeze mixture as a working fluid; a ground source heat pump with U-shaped vertical heat exchangers and antifreeze mixture as a working fluid; combined buffer storage with water as a storage medium; storage tank for DHW with peak electric heater There is a low-temperature floor heating circuit in the building The combined buffer storage consists of a big tank and a small one inside the big one The solar collector loop is closed and heat is transferred through a heat exchanger to the big storage tank The big tank is also supplied by a ground source heat pump The small tank inside the big one is used as a buffer for the DHW There is an inlet of cold water at the bottom and an outlet of warm water at the top The outlet is connected to the DHW storage tank, which can be also supplied directly from the heat pump and if necessary T1 Tg Heat pump Cold water Combined buffer storage T3 T2 Cold water Mixing valve T6 T4 DHW storage tank Figure 10 An example of the parallel SAHP system operating since 2010 Electric heater T7 Floor heating Tin T5 Solar-Assisted Heat Pumps 515 Figure 11 Components of the parallel SAHP system shown in Figure 10 the electric heater can be on Heating of the building is accomplished by the heat stored in the big tank of combined buffer storage Referring to eqn [12] written for the averaged storage temperature Ts, the energy balance of the combined buffer storage in unsteady state of the system considered can be written in the following way: Vcị dTs ẳ Qu tị ỵ QhpBS tị Qloss tị QhDHWBS tị Qh tị dt ẵ19 In eqn [19], there is heat QhpBS supplied by the heat pump to the combined buffer storage It can be the total heat provided by the heat pump, QhpBS = Qhp, or only part QhpBS = xQhp of that heat, if there is some quantity of heat QhpDHW =(1 – x)Qhp provided by the heat pump to the DHW storage tank In a given time, there can also be extraction of some water heated from the small tank to feed the DHW storage tank, QhDHWBS = mC(TDHWBS – Tin) Thus the energy balance of the DHW storage tank can be written in the following way: Vcị dTDHW ẳ QhDHWBS tị þ Qhp ðtÞ− QhpBS ðtÞ þ QauxE ðtÞ− Qloss ðtÞ− QhDHW tị dt ẵ20 There is no inlet of cold water to the DHW storage but only outlet for direct use Some cold water is provided to the three-way valve out of the storage tank to protect the user against too high water temperature from the DHW system The parallel SAHP system presented in Figure 10 supplies heat for building heating and for a DHW system The operation of the system is based on solar collectors and a ground source heat pump that supply heat to one or both the storage tanks The main modes of operation of the system considered can be described in a general way as follows: • Solar heating only: storage tanks: combined buffer and DHW storage are supplied by solar collectors; the heat pump is off and no auxiliary energy is used • Solar heating and peak auxiliary heating for DHW: Storage tanks are supplied by only solar collectors; the heat pump is off, for a peak load (or to protect against Legionella bacteria) the auxiliary electric heater is on; depending on thermal and environmental conditions, the useful heat Qu from solar collectors can be transferred to storage tanks • Solar heating and heat pump heating in parallel: If the temperature of collected or stored heat is too low to meet total heat load, for DHW and space heating, the heat pump is switched on and supplies heat to one or two storage tanks; the useful solar energy can be collected and stored in the combined storage tank, if possible • Heating via the heat pump only: When the temperature difference between the outlet of solar collectors loop and the storage (at a given point) is below the limit value, the solar collectors not operate, and the heat pump provides all heating requirements and supplies one or two tanks • Heating via the heat pump and auxiliary heating: When there is no available solar energy and the heat pump cannot provide all the heat for DHW, auxiliary electric heater is on during peak time The COP of the system under consideration that is applied for space heating and DHW can be expressed in a general way as follows: COP ẳ Qhd ỵ Qhpcon ỵ QauxDHW Qheattotal ẳ W ỵ WpumpSd ỵ Wheat ỵ Whp þ WDHW þ WauxDHW Wtotal ½21Š The equation above is written with the assumption that total heat requirements are provided by the system considered The use of electric heater (WauxDHW) is included in the total work required to accomplish all heating requirements; however, electric heater is 516 Applications used for only DHW In the total work WDHW there is also work required to drive circulation loop and pumps in the DHW system Of course it is the automatic control system that is responsible for the effective operation of the system [34] In some parallel SAHP systems, it is possible that a heat pump can supply heat directly to the heating system (usually to the storage tank as in the system presented in Figure 10), depending on the heating demand and temperature level of working fluid The automatic control of the system considered can be organized in a different way and priorities could be given to different heat sources The solar thermal collectors and the heat pump are not connected together They can operate in alternative ways, that is, each of them at a different time, but they can also operate together, both supplying heat at the same time The main idea of the parallel operation is to use two heat sources: solar for solar collectors and the other one (not solar) for the heat pump in a parallel way However, as it was presented, there is interaction between operations of the main components of the system even if they are not coupled together Perhaps such systems could be called flexible parallel SAHP systems The modern control systems based on microprocessor techniques make it possible to apply different operation strategies for different applications and heat demand 3.15.3.5 Dual-Source Solar-Assisted Heat Pump In a dual-source SAHP system, there are two heat sources for a heat pump [19, 21, 35] A heat pump is equipped with two evaporators or one evaporator but supplied by two heat sources (through two heat exchangers) One heat source is solar energy and the other source is usually air or ground or other heat source When air solar collectors are used, then solar energy collected is transferred directly to the heat pump evaporator When liquid solar collectors are used, then solar energy is absorbed by solar collectors and can be sent directly to the heat pump evaporator or can be stored in the form of sensible heat in the storage tank Then heat stored can be used as heat source for the heat pump evaporator if the temperature of the storage medium (water) is high enough If not, the heat pump can use the other heat source (usually ground or air) Solar or the other renewable heat source is used depending on which source results in a higher COP of the heat pump It can be said that the dual-source SAHP system is in some way a combination of two systems: parallel and series A standard dual-source SAHP system with liquid (flat plate) solar collectors is presented in Figure 12 In a traditional dual-source SAHP system, presented in Figure 12 (numbers represent the main system components and are given in the legend of Figure 8), the following main modes of system operation can be used: • Solar DHW heating: Heat stored in the main storage tank is transferred to the DHW tank (description of this mode of operation is the same as for series SAHP system presented in Figure 8) • Solar direct heating: When solar energy is used directly for heating, if temperature Ts of stored heat is high enough to supply heat to the heating system directly, that is, if Ts > Tsmin, then Qhd = Qheat and a heat pump does not operate, Qhp = 0; depending on the 15 11 14 7 17 23 23 12 13 10 14 11 16 9 21 21 20 18 19 Figure 12 Standard dual-source SAHP system See Figure caption for legends Solar-Assisted Heat Pumps 517 solar radiation level and the difference between the temperature of working fluid in solar collectors and storage medium, the solar collector loop can operate gaining useful energy (Qu > 0) or not (Qu = 0) • Solar indirect heating via the heat pump: If the temperature of collected or stored heat is too low for direct heating, Ts ≤ Tsmin, but if it is higher than the temperature of the other heat source of the heat pump evaporator Ts > Tnotsol, then the heat pump can use the heat stored to meet heating demand, so Qhpcon = Qheat and Qhd = 0, and Qu > or Qu = 0, depending on whether the solar collectors operate or not; the heat Qhp extracted out of the storage tank as a heat source for the heat pump can be calculated using eqn [13] • Heating via the heat pump using the renewable energy other than solar energy: When the storage temperature Ts is too low for effective use of the heat stored as a heat source of the heat pump evaporator, and/or the temperature Tnotsol of the other heat source is higher than the solar one, that is, Ts ≤ Tnotsol, then Qhpnotsolcon = Qheat, and Qhd = 0, Qhp = 0, and Qu > or Qu = 0, depending on whether the solar collectors operate or not • Heat pump and auxiliary heating: The rule of this mode of operation is the same as for the series heat pump shown in Figure 8, with the only difference being that there are two heat sources for the heat pump: solar or the other renewable heat source is used depending on which source results in a higher COP of the heat pump; heating load is provided by the heat pump and an auxiliary heater, so at the heat sink it is always true that Qhpcon + Qaux = Qheat If needed, additional modes of operation can be applied The automatic control system is responsible for the effective operation of the system The energy balance of the storage tank of the dual-source SAHP system presented in Figure 12 can be described in the unsteady state in the same way as in the case of the series SAHP system, that is, by eqn [12] The heat pump is used only for space heating; therefore, the COP of the system considered is presented without DHW function and can be written (referring to eqns [15] and [17]) in the following way: COP ¼ Qhd þ Qhpcon þ Qaux Qheattotal ¼ W þ WpumpSd þ WpumpShp ỵ Wheat ỵ Waux ỵ Whp Wtotal ẵ22 When the heat pump of the dual-source SAHP system also supplies heating energy for DHW, then eqn [22] includes more terms, similar to eqn [18] or [21] 3.15.3.6 Other Configurations Studies and experiments in the past showed that the parallel systems were much better than the series systems and slightly better than dual source [36] However, at present, it is difficult to state definitely which system is the most effective and has the highest thermal performance; more results of the operation of different systems are needed and comparative and optimization studies are necessary Nowadays, different strategies of operation are possible in one system due to well-developed automatic control systems and any system can be a mixture of different configurations and operate as a multifunctional system in a flexible way Nowadays, most heat pump systems are used for heating of buildings and DHW supply These systems must include a hot water heater (an electric heater (booster)) The electric heater provides the auxiliary energy when there is peak demand, usually in cold winter days Heat pumps at the small scale, for example, for single-family houses, should be designed to cover about 60–70% of the maximum designed heat load of a building, but not more, because of high investment costs It means that a heat pump can cover nearly most, up to 90–95%, of the annual heat demand of the building The rest is covered by the auxiliary peak electric heater included in a storage tank or in the heat pump This construction of the heat pump makes the SAHP system configuration different from standard series or parallel systems used in the past However, these modern heat pumps can also be coupled to a solar system in a series or parallel way Therefore, it is difficult to classify them as ‘other’ configuration of the SAHP systems, but the past definitions of series, parallel, or dual-source SAHP system also not suit them fully Some of the SAHP systems using the ground as a heat source for the heat pump have a function of recharging (regeneration) the ground (duct, borehole, etc.) An example of such a system is presented in Figure 13 As shown in Figure 13 (the legend is the same as for Figure 8), solar collectors can be connected to the return pipe from the evaporator going back to the ground The antifreeze mixture (based on glycol) circulating into the solar collector loop can be sent to the ground heat exchanger loops Some results show that the recharging process results in an increase of the ground temperature by only a few degrees [13] However, these few degrees can improve much the operation of the heat pump When solar collectors and ground heat exchangers are linked together and the same antifreeze mixture can circulate in both loops, then apart from the recharging effect it is also possible to use another mode of operation When the temperature of the working fluid in solar collectors is higher than the temperature of the working fluid in ground heat exchangers, the antifreeze mixture can circulate first in ground heat exchangers, which behave as a preliminary heat source Next the antifreeze mixture circulates through solar collectors gaining more heat and then it flows into the heat pump evaporator In this way, the heat extracted from the ground is upgraded by solar energy and the heat pump operates longer in better thermal conditions (higher temperature of the heat source – ground upgraded by solar), in consequence with higher COP This type of SAHP system is a very specific configuration and it represents a hybrid of parallel and series and dual-source SAHP system A solar-assisted ground storage heat pump system with latent heat energy storage [23] is another example of a rather complicated system in configuration Its effective operation depends on automatic control systems giving possibilities of different strategies for 518 Applications 15 14 22 DHW 23 12 11 16 9 21 21 Cold water 18 Figure 13 A combined SAHP system: hybrid of parallel–series–dual-source SAHP system See Figure caption for legends operation and it is very difficult to classify this system according to traditional categories The main components of the system are the following: solar collectors, latent heat storage with PCM material and a serpentine heat exchanger, U tube ground heat exchangers, heat pump, and a heating system in the form of fan coils These components can operate on their own, or in cooperation with some others or with all of them A system of valves controlled automatically play a very important role in managing the energy flow in the system It is possible that the same fluid can flow through different loops linked together The main modes of the system operation are as follows: • Direct solar heating: The building is heated directly by heat gained by solar collectors and the heat pump is switched off; depending on solar and ambient conditions, the useful heat Qu from solar collectors can be transferred to a latent heat storage with PCM material • Direct heating from the latent heat storage: The building is heated directly from the latent heat storage; the heat pump is switched off, and solar energy cannot be stored and used • Solar heating via the heat pump and storage of solar energy: Solar energy is collected by solar collectors and then stored in the form of latent heat in storage; loading and unloading of the latent heat storage tank take place all the time; the heat stored in the tank becomes the heat source of the heat pump evaporator, the process of supplying heat from the store to the heat pump evaporator can be considered as the low-temperature cycle; working fluid flows out of the evaporator and comes back to solar collectors; heat from the heat pump condenser is transferred to the fan coil system in a building, the process of supplying heat from the condenser of the heat pump to the fan coil system can be considered as the high-temperature cycle • Solar heating via the heat pump: Solar heat stored in a latent heat storage tank is used as a heat source of the heat pump; heat from the store is sent to the heat pump evaporator for the low-temperature loop; heat from the heat pump condenser is transferred to the fan coil system in a building in a high-temperature loop; solar energy is not collected or stored • Heating via the ground source heat pumps: Heat extracted by ground heat exchangers is used as a heat source for the heat pump; this constitutes the low-temperature circuit; heat from the heat pump condenser is transferred to the fan coil system in a high-temperature loop • Solar heating and ground heating via the heat pump in series: Working fluid circulates first in the ground heat exchangers, which preheats the working fluid, then the working fluid circulates through the solar collectors gaining more heat, and finally it flows through the heat pump evaporator and comes back to ground heat exchangers; this is the low-temperature circuit; the heat from the heat pump condenser is transferred to the fan coil system in the high-temperature circuit • No heating; only storing heat from solar collectors in the latent heat storage tank • No heating; only storing heat from solar collectors in the ground, regenerating the ground source Solar-Assisted Heat Pumps 519 This is a very brief description of SAHP system operation, which shows how complicated the configuration of the system can be and how complex are the strategies of operation of such a heating system, especially when there are multiple heat sources The complication of the system structure makes the investment costs high but at the same time its operation is much more effective Apart from traditional functions such as heating or cooling, SAHP systems can be used for dehumidification including drying purposes Some of SAHP drying systems have already been manufactured and tested [27, 37, 38] One of the interesting examples of such systems [38] proposes the use of air solar collectors and an air heat pump operating in a parallel way Even if these two heating systems operate in parallel, it is difficult to classify this system strictly as a parallel one The main modes of operation of the system can explain this statement and these modes are as follows: • Solar heating and ventilation: When solar irradiance is high enough, the ambient air is heated by solar collectors and sent to the granary to dry grains • Solar heating and heat pump heating and ventilation: When solar irradiance is not high enough to meet drying needs, the ambient air is heated by solar collectors and by an air heat pump, the airflow from both sources is mixed and sent to the granary to dry grains, the return air (from granary) flows to the heat pump evaporator for heat recovery • Heat pump heating and ventilation: When solar irradiance is very low or it is nighttime, the fans of solar collectors are off and the ambient air is heated by an air heat pump and sent to the granary to dry grains, the return air flows back to the heat pump evaporator for heat recovery • Heat pump dehumidification and ventilation: It is used on rainy and cloudy days Ambient air is sent first to the heat pump evaporator to be cooled and dehumidified and then to the heat pump condenser to be heated again It is possible to find an example of SAHP systems that are combined with systems that produce electricity One such system has been presented in Section 3.15.2.2 and it represents DX SAHP system with PV/T collector–evaporator [28] The other example can be the solar-assisted geothermal heat pump coupled with small wind turbine systems for heating agricultural and residential buildings [39] By analyzing the examples presented above it is evident how many different technologies and modification of standard categorization of SAHP system are available nowadays and how flexible are the modes of operation of these systems 3.15.4 Solar-Assisted Heat Pump System with Seasonal Storage 3.15.4.1 Fundamental Options of Seasonal Energy Storage Energy storage of different forms of thermal energy is a very efficient way of energy conservation Energy storage can very much improve the efficiency of the whole thermal process and the rate of useful energy conversion Heat supply and demand are often at quite different times Energy storage is widespread with many typical everyday applications In addition to the different types of energy, the basic difference in energy storage systems is the duration of the storage period The most common heat stores are used as short-term storage systems (e.g., hot water boilers in domestic use) The application of long-term storage is currently much less common, mostly due to economic reasons However, there is a large amount of surplus heat in summer and surplus cold in winter, which could be stored for a longer period of time, for example, for a season, to be used when it is really needed In the literature [20] we can find some general and important information about requirements regarding energy storage In combina­ tion with seasonal energy storage, solar energy can make a major contribution to heating of buildings The incoherency of the solar radiation peak season and space heating demand creates interest in applying the ground as a seasonal storage medium of solar energy A seasonal storage facility can be designed in many different ways Heat can be stored in the ground (clay, sand), in unfractured rocks, and in water [7, 40–42] Four fundamental options for long-term solar thermal energy storage are presented in a schematic way in Figure 14 and they are mentioned below: • • • • water tanks (including water pits, water–gravel, and solar ponds); rocks (boreholes in rocks, rock caverns, pits); soil storage (ducts in earth, earth coils); aquifers A water tank contains a mass of water heated by solar collectors and stored in a tank The water tank can be located on the ground surface (see Figure 14, top left) or partly embedded in the ground, or fully embedded in the ground (see Figure 14, top right) Water tanks should have appropriate storage volume, usually a few thousand cubic meters The water tank construction is usually of reinforced concrete It should be insulated partly or fully depending on its position, but at least over the roof area To ensure water tightness, the tank can be built with extra steel liners or special high-density concrete material with very low vapor permeability can be used Water pits are usually expensive due to the cost of excavation They need lit construction; however, it is possible to keep excavation cost low if they are built on sites with soft ground A water–gravel pit is a pit with a watertight liner and is filled with gravel–water mixture, which forms the storage medium This store should have thermal insulation on its top and side walls The specific heat capacity of the mixture is lower than water; therefore, for the same amount of heat stored, the volume of the gravel–water store should be bigger (50%) than that of the water storage tank 520 Applications Figure 14 Basic options of long-term solar energy storage A solar pond is a mass of relatively shallow salty water (about one and half meter depth) used as a solar collector Solar radiation is absorbed at the bottom of the pond Due to the density gradient of salt water in the pond, the concentration increases with depth and convection is suppressed The density gradient causes the temperature gradient to increase with depth The heat stored in the lower layer can be utilized for heating purposes Due to heat extraction, a lower convection layer exists At the surface of the water, due to wind an upper convection layer exists In this manner, the nonconvective layer (about m thickness) between the two convection layers acts as thermal insulation Rocks can be used as a storage medium by excavating in rock caverns and pits, which in most cases are filled with water But more popular is drilling in rocks making a great number of boreholes, which are filled with plastic tubes in which water flows Rock is a very good storage medium due to high thermal conductivity and thermal capacity The typical depth of the boreholes is from 40 to 150 m and spacing is about m When the ground surface area available for the system is limited, then the boreholes require other arrangements (e.g., increasing duct spacing as depth increases) The heat exchanger may consist of a single plastic tube in a borehole or a configuration of multiple boreholes with plastic tubes inside In the simplest arrangements, the fluid flows down in the plastic tube and flows up in the channel, which constitutes the plastic tube and borehole wall The fluid is extracted from the top of this channel to the heat distribution system Such a borehole with a concentric inner tube is defined as an open system The main advantage of this system is that heat carrier fluid is in direct contact with the surrounding rock; this provides the best heat transfer conditions Unfortunately, the hydrogeological and geochemical conditions are not very often good for open systems The most common type of heat exchanger is therefore a closed system The close system composes a U-shaped loop of plastic tube in the boreholes [43] The volume of the borehole outside the pipe contains groundwater or filling material as sand to improve the thermal contact between the plastic pipe and the borehole wall The heat transfer conditions for closed systems are not as good as for open ones In summer, the heat from the solar system can be transported to the rock store via a fluid circulating in the channels of the heat exchanger This is the time the store is loaded (charged) During the heating season, when heat is needed for heating purposes, the store is unloaded (discharged) The fluid circulating in the channels of heat exchangers coupled with a heat pump is heated by the surrounding rocks When the temperature of the circulating fluid is expected to be below °C, due to heavy energy demand in winter, then an antifreeze mixture in a closed loop must be used All systems mentioned above are systems operating in an indirect mode with a heat pump Indirect mode of the system operation means that heat stored in rocks is transferred to the heat pump evaporator through a fluid circulating in heat exchangers, that is, a matrix of vertical pipes in boreholes Some investigations have been made using a direct mode of the system operation, that is, direct evaporation of the working fluid of the heat pump in copper pipes in the boreholes Nowadays, a duct system is the most common type of ground heat storage device The main advantage of the duct systems is the low construction cost, because of no ground excavation However, it is very important that duct system must be designed in accordance with the geologic conditions of the site When the geologic conditions are not checked properly, unexpected disadvan­ tages of the system may appear There are two basic components of a duct ground heat store: • the geological medium, which provides the storage capacity; • the ground heat exchanger Solar-Assisted Heat Pumps 521 In the ground storage system, the ground (soil) is used as a storage medium of sensible heat Heat is injected or extracted from the ground by means of ground heat exchangers The heat exchanger consists of vertical or horizontal tubes, which are inserted into the soil What is most important, considering ground as a storage medium, is the low construction cost and the fact that heat losses from the storage are relatively small These heat losses depend on the size and shape of the store, the average storage temperature during a cycle, and the physical and thermal properties of the ground storage medium Heat losses from the store increase with higher temperature and higher water flow through the store The heat loss density decreases with larger volume of the store In the ground (clay, sandy soil, peat) deposits, a duct system is obtained by drilling holes and inserting vertical U-shaped loops of thin plastic tubes Heat exchangers are set into a depth of 30–100 m below the surface The typical spacing between tubes of a ground heat exchanger is about m This distance is shorter than in rocks due to the lower thermal conductivity of the clay The volume of the ground store must be a few times more than the volume of a water tank for the same quantity of heat to be stored The storage volume is defined as the volume of the ground perforated by the ducts (see Figure 14, bottom left) The heat transport in the ground is mainly by heat conduction The heat transfer mechanism in the ground depends on the physical and thermal properties of the ground Typically, the ground region has a parallelepiped shape or it is a cylinder with a vertical axis When the store is of shallow depth, a high temperature gradient between the ground surface and the upper parts of the storage exists To avoid large heat losses from these upper parts, thermal insulation is used between heat exchangers and ground surfaces Even when deep duct systems are applied, very often the upper part of the heat exchanger tubes is insulated Sometimes, the land area above the store is covered with a shallow layer of soil with low thermal conductivity When there are waterways in the vicinity of the store, with water flow more than m per year, then special protection (thin foil) around the store can be set (see Figure 14, bottom right corner) The important advantage of the system is its modularity; therefore, it is easy to extend the store by inserting more heat exchangers, when more heat is needed because of the increasing number of houses being supplied An aquifer is a special hydrogeological formation; therefore, its practical application is limited to few locations An aquifer is very often defined as a saturated permeable geologic unit, which transmits significant quantities of water under an ordinary hydraulic gradient This geologic unit can be sediment or permeable rock capable of conducting groundwater There are two main types of aquifers: confined and unconfined When one layer of the aquifer is enclosed by two layers of low permeability (e.g., clay), then this layer in the middle is called confined, and the whole aquifer is called confined Heat can be stored in aquifers by heating the ambient aquifer water During summer or times when there is no heat demand (or heat demand is low), heat from solar collectors is injected to aquifer Besides solar energy, waste heat from industry or power stations can be used When the aquifer is loaded, water is extracted from the aquifer through a supply well, then is heated by solar energy or waste heat (through a heat exchanger), and as a hot medium is injected into an injection well When the aquifer is unloaded, hot water from aquifer is taken and is used as a heat source for the heat pump evaporator Then cooled water is rejected into the supply (cold) well 3.15.4.2 Classification and Evaluation of Seasonal Ground Storage Ground storage systems can be classified in many different ways One of them is based on the type of storage medium: liquid – water; solid – rocks, ground; mixture of liquid and solid: water–gravel; aquifer; or saturated soils The other important classification, mainly for ground systems, is made according to the temperature range of the store, as is done in the following way: • • • • CT – cold-temperature store – < 10 °C; LT – low-temperature store – 10–30 °C; MT – medium-temperature store – 30–50 °C; HT – high-temperature store – > 50 °C The cold store (CT) is applied to small heating systems when heat demand is not high, for example, in single-family houses and in small farm buildings The cold store system can consist of vertical or horizontal heat exchangers using undisturbed soil as a storage medium of sensible heat In a CT system, no artificial charging (loading) of the soil is applied The ground is used as a natural source of energy, where natural thermal regeneration process, because of the influence of the environment, takes an important role To meet the house heating demand, the heat exchangers are coupled with a heat pump The CT store is usually called a ground source heat pump system and has already been described in a previous section The temperature in a ground body and the temperature of the fluid that circulates between ground heat exchanger and the heat pump evaporator may drop below zero Therefore, an antifreeze mixture (ethylene–glycol–water) is used The CT systems are usually designed to operate in a monovalent mode, that is, only one source of energy (energy extracted from the ground) for the heat pump is used and heat demand is provided only by ground heat exchangers coupled with the heat pump In monovalent CT systems, no auxiliary conventional source of energy is used In LT system, artificial charging of the ground is required The low temperature range allows applying simple flat plate solar collectors, which can be roof-integrated collectors For LT application, solar collectors not need to have any covers, therefore unglazed collectors can be used In most LT stores, the storage volume is created by constructing vertical channels for vertical ground heat exchangers The spacing between the tubes depends mainly on the type of the ground The depth of the heat exchanger depends on the size of the heating load together with technical restrictions The LT system has to be coupled with the heat pump Heat from the heat pump is transmitted to the heating system in a house Both LT stores and CT systems require a low-temperature space heating system in the house In order to obtain sufficient area for the heat exchange between heating fluid and air in the house, floor 522 Applications or wall heating systems are mainly used When typical radiators are applied in low-temperature space heating systems, then the surface of these radiators must be sufficiently large, approximately two times more than in conventional high-temperature heating systems The LT storage systems can be used for heating small houses, bigger residential and attached houses, or apartment buildings, according to the size of solar collector matrix, storage volume, and capacity of the heat pump In the MT storage system, during the loading period solar energy or waste heat is transferred to the ground storage body During unloading of the ground store, heat can be transferred directly to the heating system in the building It takes place when the temperature of the fluid circulating in the ground heat exchangers is high enough for direct space heating In the other case, when the temperature of the store and consequently the temperature of the heat carrier fluid are too low, the heating mode is accomplished via the heat pump For MT storage systems, solar collectors with a higher working fluid temperature can be useful Therefore, evacuated tube solar collectors are very often applied In some projects with MT stores (e.g., Kulavik Project, Sweden), the heat storage is divided into two regions The first storage region is in the center of a store and is designed for storing heat at higher temperature This region is surrounded by a region in which heat at lower temperature is stored Heat from the central region can be used directly for the space heating system But heat from the outer region has to be used via a heat pump The main advantage of this system is that radial heat losses from the inner region have been withdrawn in the outer region The distance between the heat exchanger tubes in the center region is much less (e.g., 0.5 m) than this distance in the outer region (e.g., 1.5–2.0 m) The center region and outer region ought to be situated at some distance (e.g., about m) It can also be mentioned that the heat exchanger tubes in the outer region should be buried deeper than tubes in the center region (to reduce heat losses from the inner region) The MT storage system can be applied to a group of single-family houses, a group of apartment buildings and commercial buildings, that is, office buildings, school and administration buildings, and garages The HT storage systems are still under development The storage of high-temperature heat would be very attractive since it allows utilizing waste heat and it is very important to transfer this high-temperature heat directly to the district heating system Therefore, there is no need to apply a heat pump However, an auxiliary burner ought to be used to cover, if it is necessary, the peak load The most common types of solar collectors, which can be applied in HT systems, are high-temperature evacuated tube collectors Considering HT stores it ought to be mentioned that these systems need a special duct technology which depends on geologic conditions It is necessary to ensure a good thermal contact between the heat carrier fluid and the ground, a good thermal conductivity of the ground, especially in the near vicinity of the duct, also small heat losses, especially in the near vicinity of the duct, and to minimize heat losses, especially through the top In HT stores, two main problems occur, groundwater flow and as a result heat losses and moisture migration, the latter producing a drying effect in the ground HT stores cannot be used in small systems; heat losses in such systems would be extremely large Therefore, HT store projects are made only for large loads, a group of apartment or commercial buildings The general idea of underground storage through application of vertical heat exchangers is presented in a schematic way in Figure 15 However, it should be underlined that the possibilities of utilizing ground as a heat source vary from place to place and depend mainly on the local geology and on the size of the system As mentioned, usually systems of middle scale, that is, for a small group of heating systems, or large scale for district heating are considered Planning of ground seasonal storage systems requires some preliminary study and careful design There are places not suitable for seasonal heat storage application from a hydrogeological point of view To evaluate a seasonal ground store in a large-scale application, a detailed study has to be made Thus to evaluate a project of central heating plant with seasonal storage (CHPSS), the following study steps have to be performed: – site selection, a review of proposed sites taking into account • heat load and its nature, • land availability, • possibility of integration of the CHPSS to existing system, • site geology; Solar collectors Heat pump Heated house Ground storage Figure 15 The idea of underground storage through application of vertical heat exchangers Solar-Assisted Heat Pumps 523 – investigation of the storage properties: • geological conditions, • hydrological conditions, • hydrochemical conditions (for HT systems), • thermal and physical properties of the store, • review of construction techniques suitable for the given geology; – preliminary analysis of the system performance: • preliminary evaluation of mathematical model, • simulation study including economic analysis, using special design tools, that is, computer simulation programs; – recommendations for the proposed system based on the simulation results The design of the seasonal storage system depends on whether CHPSS is planned for a new building and new heating system, or the CHPSS is expected to be integrated to existing building and heating system If CHPSS is integrated to existing space heating system, it is good if this system is the low-temperature type Buildings using steam for heating are not favored because they are likely to require extensive retrofitting Sufficient land area for seasonal storage system is a very important factor A special review study about land availability for storage, solar collectors in CSHPSS system, and other coupled equipment should be made A suitable land area for solar collectors with good insolation conditions is required These sites are favorable when they have got well-documented geology, for example, geological characteristics are known from geological survey maps and sometimes experimental borings are made during the construction of other buildings and plants at the chosen site The lack of information pertinent to the geology of the sites means that the site will not be favored for the CHPSS applications When the heating building and the land area are selected and they are found to be the potential site for the application of the CHPSS or CSHPSS project, further investigations can be made The storage medium should be investigated from the point of view of its geology and its physical and thermal properties The local geology of the store is characterized by geological, geothermal, hydrogeological, and hydrochemical conditions Hydrogeological studies are very important to estimate the water movement and the water table in the store and its surroundings As it is known, water flow through the store leads to heat losses Physical and thermal properties influence the thermal storage capability Geotechnical investigation is necessary for the proper design of a system particularly when clay is used as a storage medium Inaccurate soil properties can result in different than assumed actual performance of the system Lessons from Groningen project [44] show how important is a detailed storage properties study Higher heat losses and lower thermal capacity and consequently lower efficiencies in real projects are due to convective flow in saturated, especially highly permeable ground Therefore, it is very important to take into account the geological conditions to choose suitable sites for ground store location In many countries, soil conditions are characterized by a high groundwater table that leads to high water movement in the store When the effects of natural and induced convection to the surrounding can be reduced, then a higher store efficiency can be obtained Groundwater can flow through the pores or voids in soils The water flow depends on the permeability and the hydraulic gradient in the soil The flow of water through the soil for the proposed storage system has to be analyzed before planning the type and size of such a system Thermal properties of the soil, that is, thermal conductivity and volumetric heat capacity, describe the thermal storage ability [45, 46] Thermal conductivity of any kind of rock within the water-saturated zone is higher than for the nonsaturated zone The thermal parameters of the store depend on the property–temperature variations of the soil medium The dependence of heat conduction on temperature is noticeable; however, it is not as strong as dependence on density and especially on water content As it is known, thermal parameters are influenced by the physical properties of the store such as water content, bulk density, dry density, and porosity and by the soil mechanics The water content and porosity are very important for thermal properties of the soil, so they are important in designing the heat store The water content is defined as the ratio of mass of water to mass of dry material It was found by Adolfson et al [47] that water pore pressure increases with increasing temperature in the soil This is due to the fact that water has greater coefficient of volumetric expansion than the soil solids With the increase of temperature, water in the vicinity of the heat exchanger will move away from its tube It can be explained by the fact that water in the soil will tend to move from a high-pressure to a low-pressure zone The significant amount of water migrating away from the tubes may reduce the effectiveness of the heat exchange into storage Drying conditions at the contact surface between the heat exchanger tube and the soil may occur Air voids may take the place of the expelled water and add resistance to heat flow and consequently cause the drying conditions at the contact surface, for example, at the contact surface of heat exchanger tube and clay Dry conditions are not expected in the store where operating temperatures are not so high, for example, for LT storage systems The study shows that even for MT systems, the risk of significantly drying the saturated clay is very small indeed If the ground is saturated and has a low permeability, then these sites are preferable for high-temperature storage For soils with higher permeability, a large store volume is needed for the same store efficiency Considering the HT seasonal storage systems, a more detailed study ought to be performed due to more complicated heat and moisture phenomena and more evident chemical processes in the storage medium In HT systems, apart from moisture reduction and dissolution, additional phenomena such as effects of desorption of water from clay minerals, dehydration of gypsum, and structural decomposition of specific clay minerals ought to be taken into account To apply HT systems in the ground, very intensive and careful geological studies are needed Geochemical, geotechnical, and hydrogeological problems in HT systems are described in the literature [48] 524 Applications It can be said that sites suitable for seasonal thermal energy storage especially for HT systems should be chosen in a soil with high water content, due to the fact that porous sediment in nonsaturated conditions has poor thermal properties If the ground is unsaturated, then it should have a high thermal conductivity To increase the thermal efficiency of the storage, boreholes around the heat exchanger tubes are filled with a special material, for example, quartz sand, Portland cement, bentonite, and water It helps to keep soil (sand) around the tube moist and helps to reduce the overall thermal resistance from heat exchanger tube to the clay (sand moist has higher thermal conductivity than clay) Sand also has larger pores than clay and water can move easily in the filling material in the borehole, which results in adding a convective heat transfer contribution to the process Application of filling material in boreholes gives better performance of the heat store and can reduce some geothermal problems in a clay heat store However, the additional cost of applying and installing filling materials in boreholes needs to be justified by the increased performance of storage and reduction of borehole numbers It has to be mentioned that in the case of large-scale seasonal ground storage systems, a very careful analysis is necessary to achieve efficient performance of the operating system It turns out that in some cases when no or very poor preliminary design studies were used, the actual overall performance of the storage systems was far away from predictions The most common way to predict thermal performance of CHPSS or CSHPSS systems is to simulate the operation of the system and thermal behavior of each of the system components using computer models 3.15.4.3 Heat and Mass Transfer in the Ground Store, General Consideration Thermal processes in ground storage are characterized by rapid heat flow in the near vicinity of the duct and by relatively slow heat flow in the surrounding ground It is proposed to analyze thermal processes in the ground in two groups [46]: • local thermal processes around each ground heat exchanger, so-called microscale, • global thermal processes in the storage volume and the surrounding ground, so-called macroscale To analyze the heat transfer phenomena and to solve the problem, it is necessary to determine the temperature field in the ground body, that is, in the near vicinity of the ground heat exchangers and in the whole storage medium and surroundings The complete temperature field may be considered as a superposition of a global (slowly varying in time) temperature field and local temperature field with steep gradients near the duct The theory of thermal processes and analysis of duct storage systems can be found in the literature [35, 40–51] However, the approach given by scientists sometimes differs The basic concepts of heat transfer, between the heat-carrying fluid of ground heat exchanger and the surrounding ground, and the heat flow process in the storage region are described briefly in this section At the beginning it is essential to describe the initial ground store state Usually initial conditions are determined by the undisturbed ambient ground temperature, which increases with depth due to the geothermal heat flow The governing equation of heat transport in a solid body is based on the principle of energy conservation The density of heat flow is a function of spatial coordinates and time Taking into account Cartesian coordinates, the three components of heat flux density are qx(x,y,z,t), qy(x,y,z,t), and qz(x,y,z,t) The energy balance equation is as follows: −∇q ¼ ∂ρcp T ∂t ½23Š where ∇ is (∂/∂x, ∂/∂y, ∂/∂z) The density and specific heat of ground medium can be also functions of spatial coordinates and time The heat flow takes place by conduction and convection According to Fourier’s law, the conductive heat flow is proportional to the temperature gradient: ! q cond ẳ T ẵ24 The ground is composed of grains of different minerals The ground structure varies very often with spatial directions, which gives rise to anisotropic physical parameters Anisotropic thermal conductivity in a large-scale store may have some consequences if the ground grains have a preferential orientation within the structure Thermal anisotropy in the case of small ground samples does not affect the thermal properties of the whole ground store so much Therefore, the averaged thermal parameters as well as averaged thermal conductivity can also be considered In addition, by analyzing the dynamic processes of heat flow in low-temperature storage range, the thermal conduction can be assumed to have a constant value The convective heat transport can be represented by the following relationship: À Á! ! q conv ẳ cp q w T Tref ị ẵ25 where Tref is an arbitrary reference temperature, which can be set to zero The convective heat transfer is a result of groundwater flow This heat transport is caused by: • regional groundwater flow – forced convection; • buoyancy effects caused by the temperature difference between the storage region and the surrounding ground – natural convection Solar-Assisted Heat Pumps 525 The magnitude of the groundwater flow in the ground body is determined by: • permeability of ground – intrinsic property of a porous material; • hydraulic conductivity of ground – property of a ground material and fluid; • local water gradient The groundwater flow qw through a porous material is described by Darcy’s law Now taking into account conductive and convective heat transfer (eqns [24] and [25]), the combined heat transfer in the ground body can be written as h À! À Á ! i ! Á ½26Š −∇ q cond ỵ q conv ẳ T ỵ ρcp w q w T When it is assumed that groundwater can be treated as an incompressible fluid, then ! ∇ qw ¼ and the divergence of convective term can be written as À ! Á ! ! ! T qw ẳ qw T ỵ T qw ¼ qw ∇T ½27Š ½28Š Additionally, when we assume an isotropic ground medium with constant thermal properties, the divergence of the heat conduction term can be written as ∇ðλ ∇ Tị ẳ T ẵ29 When we assume that inner heat sources exist in the ground medium (in some models loading of store is regarded as an additional inner heat source), then the energy balance in the ground store can be written as À ρcp Á ∂T À Á ! ¼ λ ∇2 T − ρcp w q w T ỵ qv g t ẵ30 To solve this equation it is necessary to determine boundary conditions Modeling of heat transfer in the ground body is very complicated Different numerical methods are used to solve the problem To describe undisturbed ground temperature field and temperature distribution during loading and unloading of the ground store, different assumptions are made The most typical models assume that heat convective transfer is neglected This assumption can be made for low-permeability ground In this case, the governing partial differential equation for heat transport in a solid body is an unsteady heat conduction equation, known as Fourier–Kirchhoff’s equation, which takes the following form: À Á T cp g ẳ T ỵ qv ∂t ½31Š In most models, axis-symmetrical thermal processes in a cylindrical storage region and a duct are considered Then it is convenient to write eqn [31] in two-dimensional cylindrical coordinates, using radial and axial coordinates The boundary and initial conditions are also written for the cylindrical heat transfer model The temperature distribution of the ground medium, during the heat extraction, depends on the total length of the tubes of ground heat exchangers (number of tubes and length of one tube) and the distance between them When the tube number and their length are small, then the ground is cooled down significantly [37] In a CT store system for very small number of tubes, the freezing phenomena of the ground body in the near vicinity of the heat exchanger can appear at the end of the heating season When the tube number and their length are large enough, then the temperature of the natural ground store is relatively high all the time That leads to more efficient operation of the heat pump 3.15.4.4 Applications A lot of information on energy storage problems, including seasonal storage of solar energy, and information on different applications, projects, and demonstration plants of SAHPSS including large-scale CSHPSS can be found at the website of the IEA Energy Storage Program, http://www.iea-eces.org, or at the websites of different tasks and annexes of the Energy Conservation through Energy Storage Program A distinguishing feature of ground systems with seasonal storage applying to large heating demand, that is, CSHPSS, is a large store with soil, water, or rock as a storage medium and a large solar collector array to charge the store So the main components of CSHPSS are the following: • solar collectors, • storage system, • distribution network for the load The CSHPSS systems are really very large To justify the use of such systems, a forecast heating load of a few hundred MWh should be expected per year Therefore, it is obvious that these systems are not practical for single-family houses and other 526 Applications small loads The CSHPSS systems can be applied in large residential districts, schools, hospitals, and public or commercial facilities The CSHPSS concept is also suitable for retrofitting existing building stock and for the integration of solar energy with other energy sources The Northern and Central European countries are leaders in large-scale application for seasonal storage systems The first operational system was built in 1978 at Studsvik Laboratory, in Sweden This system was designed to provide heat demand for an office building In 1979, the IEA Solar Heating and Cooling Projects created a new Task VII on Central Solar Heating Plant with Seasonal Storage This task was established to investigate the feasibility and cost-effectiveness of CSHPSS systems and to promote and assist in the establishment of this technology in the participating countries, which in alphabetic order were as follows: Austria, Canada, CEC (JRC Ispra), Denmark, Federal Republic of Germany, Finland, Italy, The Netherlands, Sweden, Switzerland, the United Kingdom, and the United States In 1990 Task VII was finished However, promoting the very successful results of this task’s investigations led to the creation of the new Task XV on Advanced Central Solar Heating Plants in Built Environments Later other tasks that were a kind of successors of Task XV were developed Since the beginning of 1990, when Task VII was established, many CSHPSS systems were built and tested in many countries, mainly in Europe Some of those projects, in some way historic projects with seasonal storage, are presented briefly below: Kerava, Finland The Kerava Solar Village (KSV) Project was put into operation in 1983 It was to provide heat for 44 apartments (3756 m2 total area) The solar energy was stored in a stratified water tank (1500 m3 volume) excavated in bedrock Fifty-four boreholes in rocks (11 000 m3 volume) were additionally used The system was coupled with a heat pump The storage capacity was found to be too small to achieve the design solar fraction Scarborough, Canada The purpose of this project was to test the performance and economic feasibility of seasonal storage in aquifers This project, completed in 1985, provided cool and heat for 30 470 m2 Scarborough Canada Center Building, the major federal government building in the eastern part of Toronto The Scarborough building had a cooling load greater than heating load; therefore, the aquifer was used to store cold water to be used for summer cooling A vacuum tube solar collector array (700 m2 area) was used for DHW demand Groningen, The Netherlands Dutch CSHPSS at Groningen was put in operation in 1984 The system was designed for heating purposes (1200 MWh) of 96 houses divided into nine blocks and grouped around the seasonal duct heat store Vacuum tube solar collectors (2400 m2 area) were used The storage system consisted of short-term (daily) and long-term storage The short-term storage was a water tank (100 m3 capacity) embedded in the center of clay seasonal storage (23 000 m3 capacity) In the clay storage, 360 polybutylene U-shaped vertical tubes (20 m deep) were used The system performance evaluation showed that the solar contribution of the system (628 MWh or 52% fraction) was about 15% lower than expected This lower solar fraction was due to lower solar collector efficiency, higher storage losses through the top insulation, slightly higher minimum useful temperature in the system, and greater regional groundwater flow than expected Treviglio, Italy The Treviglio Project was put in operation in 1982 The system was designed to provide heat for five apartment buildings (35 062 m3 total heating volume) Flat plate solar collectors were used (2727 m2 total area) Heat was transferred into the ground by 55 horizontal U-shaped pipes and 414 vertical boreholes The total storage volume was about 43 400 m3 The system was assisted by a heat pump The evaluation of the system showed a very close agreement between the expected and measured solar fraction (expected 76%, real value 72%) Sunclay, Sweden The Sunclay Project was designed to supply low-temperature heat to a school It was put in operation in 1980 Unglazed roof-integrated solar collectors (15 000 m2 area) and duct store in clay (87 000 m3 storage capacity) were coupled with a heat pump (diesel type) The annual heating requirement of the school was 1650 MWh Lambohov, Sweden The CSHPSS at Lambohov was put in operation in 1980 This system used flat plate solar collectors (2900 m2 area) and an insulated water storage volume (10 000 m3) in bedrock to provide heat for 50 small houses (7000 m2 total area) The annual heating demand was equal to 940 MWh Lyckebo, Sweden Lyckebo was the largest and most widely known plant in Sweden Lyckebo was put in operation in 1983 The system consisted of water storage in a rock cavern (105 000 m3 capacity) and high-temperature flat plate solar collectors (final area 28 800 m2) The rock cavern was not insulated The heat losses from the cavern differed from the design study due to convection losses through the old access tunnel that was used during construction The storage was designed to supply 100% of heating requirements for 550 houses at a temperature of 70 °C The heat supplied to the system was approximately 8500 MWh per year Stuttgart University, Germany An SAHP system with an artificial aquifer was built in 1985 This was the first project with seasonal heat storage in Germany on a large scale The artificial aquifer gravel- and water-filled pit (1050 m3 capacity) and unglazed solar collectors (211 m2 area) and an electric heat pump (66 kW) were used The system was designed to supply heat for an office building (floor area 1375 m2) of the University Institute The annual heat load was 150 MWh and consumption of energy for hot water was about 25 MWh The heat distribution system was a low-temperature system (50/40 °C) Two different storage concepts combined in one store were used (artificial aquifer, man-made, that constituted primary and a secondary system: coils in gravel) Solar-Assisted Heat Pumps 527 Since the 1980s, some new CSHPSS systems have been installed in other places Some of them are not coupled with heat pumps There is a new type of large-scale system, the so-called central solar heating plants with diurnal storage (CSHPDS) They have ‘small’ diurnal storage and are connected with the main heating plant, usually a conventional one, and the central district heating system Some examples of both types of systems are listed below [52–55] • Falkenberg, Sweden, CSHPDS in operation since 1989, collector area 5500 m2, 1100 m3 water tank, annual load size 30 GWh; • Ry, Denmark, CSHP in operation since 1990, collector area 3025 m2, directly connected to district heating, annual load size 32 GWh; • Hamburg, Germany, CSHPSS in operation since 1996, collector area 3000 m2, 4500 m3 water-filled concrete tank, annual load size 1.6 GWh; • Friedrichshafen, Germany, CSHPSS in operation since 1996, collector area 2700 m2, 12 000 m3 water-filled concrete tank, annual load size 2.4 GWh; in 2003 the collector area was extended to 3500 m2, in 2004 to 4050 m2, annual heat demand increased to 3–3.4 GWh, due to enlargement of residential area • Marstal, Denmark, CSHPDS in operation since 1996, collector area 18 300 m2, 2100 m3 water tank + 4000 m3 sand water store + 10 000 m3 water pit (2003), annual load size 28 GWh; • Aeroskobing, Denmark, CSHPDS in operation since 1998, collector area 4900 m2, 1200 m3 water tank, annual load size 13 GWh; • Neckarsulm, Germany, CSHPSS in operation since 1999, in 1999 collector area 2636 m2, in 2002 extension to 5044 m2, in 2007 next extension to 5670 m2, in 1999 20 000 m3 duct heat store, in 2001 store enlargement to 63 400 m3, in 1999 annual load size 1.25 GWh, in 2007 increase to 2.8 GWh; • Kungalv, Sweden, CSHPDS in operation since 2000, collector area 10 000 m2, 1000 m3 water tank, annual load size 90 GWh; • Rise, Denmark, CSHPSS in operation since 2001, collector area 3575 m2, 4500 m3 water tank, annual load size 3.7 GWh; • Steinfurt, Germany, CSHPSS (second generation) in operation since 2000, collector area 510 m2, 1500 m3 gravel–water, annual load size 0.325 GWh; • Rostock, Germany, CSHPSS (second generation) in operation since 2000, collector area 1000 m2, 20 000 m3 aquifer, annual load size 0.497 GWh; • Hannover, Germany, CSHPSS (second generation) in operation since 2000, collector area 1350 m2, 2750 m3 hot water, annual load size 0.694 GWh; • Attenkirchen, Germany, CSHPSS (second generation) in operation since 2000, collector area 800 m2, 500 m3 hot + 9350 m3 duct, annual load size 0.487 GWh During the last five years the next generation CSHPSS systems were realized in Germany thanks to R&D program Solartermie2000plus [56] For example, in 2007 in Munich the new CSHPSS with 2900 m2 of solar collectors and water tank (pit storage) of 5700 m3 was put into operation The annual heat demand is 2.3 GWh In the same year the other CSHPSS in Crailsheim was realized with 7300 m2 of solar collectors and duct ground store of 37 500 m3 ground volume (there are also two buffer water tanks of 100 and 480 m3) The annual heat load is 4.1 GWh The solar fraction of the new plants is about 50%, which is really high Apart from the CSHPSS and CSHPDS, there are systems for combined heat and cold storage Aquifer thermal energy storage (ATES) systems are especially good for cold storage Such systems can be found in Sweden in Solna/Frosundavik at the SAS head office, and in Berlin in the German Parliament building and in other places However, this technology is still not so popular at present, even though some of the systems represent high 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