Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems

Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems

3.12 Solar Hot Water Heating Systems G Faninger, University of Klagenfurt, Klagenfurt, Austria; Vienna University of Technology, Vienna, Austria © 2012 Elsevier Ltd All rights reserved 3.12.1 3.12.1.1 3.12.1.2 3.12.1.3 3.12.1.4 3.12.1.5 3.12.1.5.1 3.12.2 3.12.2.1 3.12.2.1.1 3.12.2.1.2 3.12.2.2 3.12.2.2.1 3.12.2.2.2 3.12.2.2.3 3.12.2.2.4 3.12.2.3 3.12.2.3.1 3.12.2.4 3.12.2.4.1 3.12.2.4.2 3.12.2.5 3.12.2.6 3.12.2.7 3.12.3 3.12.3.1 3.12.3.2 3.12.3.3 3.12.3.4 3.12.3.5 3.12.3.6 3.12.4 References Toward a Sustainable Energy System Solar Heat – Renewable Energy Source with High Potential Solar Water Heating Solar Energy for Developing Countries Market Introduction and Market Deployment of Solar Thermal Systems Solar Heat Worldwide Distribution by application Technologies for Solar Hot Water Systems Components and Concepts Solar DHW systems with natural circulation Solar DHW systems with forced circulation Solar Thermal Collectors High-performance flat-plate collectors Properties of collectors Integration of solar collectors New developments in the collector sector The Collector Circuit Drain-back system Thermal Storage Water storage technology Advanced heat storage technologies Decentralized and Centralized Solar Thermal Systems Auxiliary Heat Sources Hygienic Aspects of Solar Hot Water Heaters Design Principles of Solar Thermal Systems Meteorological Conditions and Simulation Tools The Solar System Collector Orientation and Inclination Solar DHW Systems for Households and Single-Family Houses Solar DHW Systems for Apartment Houses Solar-Combined Heating Systems Summary and Conclusion 419 419 420 422 422 423 423 425 426 426 426 426 429 430 430 431 431 433 434 434 435 435 436 437 438 438 440 441 442 443 444 445 446 3.12.1 Toward a Sustainable Energy System 3.12.1.1 Solar Heat – Renewable Energy Source with High Potential The facts of our present energy supply – limited fossil resources, instability by political influence on the oil and gas markets, and greenhouse gas emission from fossil energy resources – are serious arguments for creating a new energy system The main resources for a future sustainable energy system will be renewable sources And, solar thermal technologies have the potential for a high contribution to the future energy supply The ‘solar source’ for solar thermal systems is immense and inexhaustible The environmental and economic benefits are substantial Today, solar thermal systems are regarded as a well-established, low-tech-technology with an enormous potential for energy production ‘Solar thermal technologies’ for low- to medium-temperature applications can be used all over the world – cold to hot climates A large variety of solar thermal components and systems, mostly for residential applications, are available in the market The products are reliable and have a high technical standard in the low-temperature regime (below 150 °C) There has been a rapid market growth in recent years for small solar hot water systems in countries moving toward partly automatic or semiautomatic fabrication of solar thermal components Solar thermal systems in larger buildings – multifamily houses and apartment blocks – as well as in district heating plants are now emerging in the market The use of solar hot water systems in larger buildings and centralized solar thermal systems has the Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00312-7 419 420 Applications advantage of lower specific investment costs, and thus, the heat production costs can be reduced in comparison with small, decentralized systems The possibilities for a central hot water preparation in multifamily buildings are used increasingly in the market nowadays The component for the conversion of solar energy into heat is the collector – either nonconcentrating or concentrating Collector working temperatures of about 60–80 °C, with conversion efficiency from 40% to 60%, can be achieved with flat-plate collectors, which are typically used for hot water solar systems The properties of this type of collectors are well known today and thus manufactured in many parts of the world In countries with solar radiation ≥1800 kWh m−2 yr−1, it is advantageous to use solar systems for domestic hot water (DHW) preparation as compact system with flat-plate collectors based on the thermosi­ phon principle Synthetic absorbers are preferred to metal absorbers not only for cost reasons but also due to lower corrosion potential Solar heating and cooling (SHC) technologies include solar water heating, solar space heating and cooling, using active technologies and passive system designs, daylighting, and agricultural and industrial process heating The use of solar energy in housing presents remarkable advantages as follows: requires less energy; causes less adverse environmental impacts, for example, CO2; provides open sunlight; improves building esthetics; and provides a new medium for archi­ tectural expression While solar water heating and solar space heating have been in the market for decades, new approaches for solar thermal applications (e.g., for cooling and process heating) are now emerging in the market Solar-assisted cooling is an extremely promising technology as peak cooling requirement coincides with peak solar radiation Small-scale solar cooling systems are now commer­ cially available Figure illustrates the market development from solar thermal technologies 3.12.1.2 Solar Water Heating Today, DHW preparation with solar energy is standard in many countries In the area of building renovation, solar hot water preparation is attractive to increase the efficiency of heating systems Especially, ineffective heating systems for hot water preparation outside the heating season have been replaced by solar hot water preparation Thus, pollutant emissions through heating (wood, coal, and oil boilers) could be reduced, and at the same time, a high comfort in hot water preparation could be reached Solar hot water preparation in high-performance houses is sensible In such houses, the energy needed to heat domestic water can equal or even exceed the energy needed for space heating, since the latter has been so far reduced by insulation and heat recovery In Europe, about 50% of the new detached and row houses and about 15% of apartment houses are designed on this concept Market deployment Swimming pool heating Domestic hot water Hot water in multifamily housing District heating Solar Combisystems Facade collector systems Sea water desalination Process heat Cooling Research and development Market introduction Figure Solar thermal technologies in the market: From research to demonstration and market deployment Market deployment Solar Hot Water Heating Systems 421 Further, demand for heating domestic water is a 12-month energy demand, including the high insulation during the summer months Using a solar system is therefore an effective way to reduce the total primary energy demand Increasingly, the market for solar water heating systems also includes systems that provide, in addition to hot water preparation, space heating in winter, called ‘Solar Combisystems’ For hot water heating in transition countries, such as China and India, and also in countries without space heating systems (e.g., Greece, Cyprus, and Malta), direct electricity is used Large amount of electricity is necessary to meet the hot water requirements in domestic, institutional, and commercial sectors resulting in peak load and load shedding to the shortage of power supply With solar hot water systems, the electricity demand as well as the peak load can be reduced remarkably (Figure 2) Figure Solar hot water systems to replace electricity demand and to reduce peak load 422 Applications Figure Solar heat for developing countries 3.12.1.3 Solar Energy for Developing Countries The utilization of solar energy is considered to be promising in developing countries with suitable meteorological conditions Also, the potential for decentralized (stand-alone) energy systems is huge in developing countries Therefore, the use of solar energy for heat and electricity production is the first step for economic development (Figure 3) It appears essential to promote the development, testing, demonstration, and market introduction of solar technologies in developing countries with the support of industrialized countries Many joint projects were initiated since 1980, with the govern­ mental support of OECD-Member States, the World Bank, UNIDO, and other organizations 3.12.1.4 Market Introduction and Market Deployment of Solar Thermal Systems As a result of the first oil price crisis, the market introduction of solar hot water systems started in most of the industrialized countries in 1976 with the aim of consumers to reduce the dependency from oil imports (First Solar Boom) From 1980 until the mid-1990s, the solar market development was not stable Initially, the collectors and systems were offered by small companies, but due to missing guidance information for design and construction, the consumers were not always satisfied The market deployment decreased, but through new firms and better-educated installers and available experiences on the market, the amount of installed collectors and systems increased again in late 1970s (Second Solar Boom) The situation on the solar thermal market for Austria is illustrated in Figure Favorable applications were the separation of hot water preparation in households from firewood heating systems in small communities, especially outside the heating season With the decrease of oil price at the beginning of 1980, the solar market decreased again In this period, ‘self-built’ solar heating systems were organized, primarily for solar projects for personal use, and were offered in the market Through these private activities, the interest for solar systems was pushed and industry was motivated for more attention and new activities (Figure 5) From early 1990s onward, larger solar firms were found, and the industrial production was based on national standards, guidance for energy-efficient design, construction, and operation With the increase of industrial produced Solar Hot Water Heating Systems 423 Yearly installed flat-plate collector area 1975 - 2009 400,000 350,000 Industrially produced collector Self-built collector Third Solar-Boom Supported by market-proofed technologies and with financial governmental support Collector area (m2 yr–1) 300,000 Second Solar-Boom Driven by "GreenhouseGases" 250,000 200,000 150,000 100,000 50,000 First Solar-Boom Oil-price crises 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1989 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Figure Market deployment of solar thermal collectors in Austria collectors, the production of self-built collectors and systems was focused to ‘social’ projects – to involve unemployed young people as well as handicapped persons with the aim to open perspectives for the job market The products are used in social projects More attention for ‘greenhouse gases’ and their potential for climate change were given – both in policy and by consumers – and this supported the solar market remarkably at the end of 1990s (Third Solar Boom) Today, solar hot water systems are well designed, using materials with an expected lifetime of more than 25 years; the price for installed systems is acceptable; and the results are satisfying the consumers Also, financial support by the governments has influenced the increase of annual growth rates 3.12.1.5 Solar Heat Worldwide Installed solar thermal capacity grew by 9% around the world in 2007 Solar thermal power output reached 88 845 GWh, resulting in the avoidance of 39.3 million tons of CO2 emissions At the end of 2007, the installed solar thermal capacity worldwide equaled 146.8 GWth or 209.7 million square meters The breakdown by collector type is as follows: 120.5 GWth for flat-plate and evacuated-tube collectors, 25.1 GWth for unglazed plastic collectors, and 1.2 GWth for air collectors (Figure 6) [1, 2] 3.12.1.5.1 Distribution by application The use of solar thermal energy varies greatly by country In China and Taiwan (80.8 GWth), Europe (15.9 GWth), and Japan (4.9 GWth), plants with flat-plate and evacuated-tube collectors are mainly used to prepare hot water and to provide space heating, while in North America (the United States and Canada), swimming pool heating is still the dominant application with an installed capacity of 19.8 GWth of unglazed plastic collectors It should be noted that there is a growing unglazed solar air heating market in Canada and the United States aside from pool heating Unglazed collectors are also used for commercial and industrial building ventilation, air heating, and agricultural applications Europe has the most sophisticated market for different solar thermal applications It includes systems for hot water preparation, plants for space heating of single-family and multifamily houses and hotels, large-scale plants for district heating, as well as a growing number of systems for air conditioning, cooling, and industrial applications From the worldwide collectors capacity in operation (2007), 50% are evacuated-tube collectors, 32% flat-plate collectors, 17% unglazed collectors, and 1% air collectors (mainly from the ‘SolarWall’ type) The main markets for evacuated-tube collectors are in China, while most flat-plate collectors are found in Europe In the United States and Australia, unglazed collectors are dominating But in recent years, the worldwide market for new installed glazed collectors has been significantly growing, in Europe with growth rates near and above 100% compared to the capacity installed in 2006 424 Applications Figure Development of collector production and installation The already installed capacity of solar thermal heat is considerably higher than the installed capacity of the other renewable sources The total energy yield of solar thermal heating systems comes in second place behind solid biomass, but it is higher than the energy yield of wind and photovoltaic (PV) power Solar Hot Water Heating Systems Annual installed capacity of flat-plate and evacuated tube collectors from 1999 to 2007 Total capacity in operation of water collectors of the 10 leading countries at the end of 2007 China and Taiwan Europe Others Australia and New Zealand Japan United States 12 000 10 000 8000 6000 4000 20 000 16 000 12 000 8000 4000 2000 1999 2000 2001 2002 2003 2004 2005 2006 C hi U na ni te d St at es Tu rk ey G er m an y Ja pa n Au st lia Is el Br az il Au st ria G re ec e 2007 Total capacity of glazed flat-plate and evacuated-tube collectors in operation by economic region at the end of 2007 70 59.8 50.4 50 38 40 31.5 30 20 9.5 10 5.3 China and Australia and Taiwan New Zealand Japan Europe Others United States and Canada Collector yield per 1000 inhabitants (kWh a–1) Total capacity per 1000 inhabitant (kWth) Europe: EU-27, Albania, Macedonia, Norway, Overseas Departments of France, Switzerland Others: Barbados, Brazil, India, Israel, Jordan, Mexico, Namibia, South Africa, Tunisia, Thailand, Turkey 60 Evacuated tube Glazed Unglazed 72 616 14 000 24 000 280 16 000 Total capacity (MWth) Installed capacity (MW th a–1) 20 000 18 000 425 Annual collector yield of glazed flat-plate and evacuated-tube collectors in operation by economic region at the end of 2007 40 36.89 35 29.176 30 25.916 25 18.67 20 15 8.609 10 4.31 China and Australia and Taiwan New Zealand Air collector 1% Flat-plate collector 32% Distribution of the worldwide capacity in operation 2007 by collector type Total capacity in operation (Gwel,GWth) and produced energy (TWhel,TWhth) Evacuated-tube­ collector 50% Heat 100 Others United States and Canada 190 200 150 Europe Total capacity in operation and annual energy generated 2007 Power Worldwide capacity in operation 2007 by collector type Unglazed collector 17% Japan 147 Total capacity in operation (gwel) Produced energy (twh) 89 94 58 50 10 Solar thermal Wind power heat Geothermal power 9.4 10 0.6 1.5 0.4 0.6 Photovoltaic Solar thermal Ocean tidal power power Figure Worldwide solar thermal market 2007 Source: Solar Heat Worldwide, 2009 Edition To find a more detailed analysis on the market penetration of solar thermal technology in the 49 documented countries representing more than 85% of the solar thermal market, see http://www.iea-shc.org [1] 3.12.2 Technologies for Solar Hot Water Systems The key applications for solar thermal technologies are those that require low-temperature heat, such as for swimming pools, for DHW and space heating, drying processes, and process heating in the low- to medium-temperature range Solar water heating, including pool heating, has been commercially available for over 30 years, and can be considered a mature technology Active solar space heating, while commercially available for almost as long, significantly lags behind 426 Applications solar water heating in the market due to its relatively higher costs as well as special requirements for utilization (only low-energy buildings with low-temperature heat distribution) But in recent years, systems that combine water and space heating, called Solar Combisystems, have emerged in the market and show great promise for further market success [3, 4] Solar heating systems for combined DHW preparation and space heating are similar to solar water heaters in that they use the same collectors and transport the heat produced to a storage device There is, however, one major difference; the installed collector area is generally larger for Solar Combisystems, and in addition, this system has at least two energy sources to supply heat: the solar collectors and the auxiliary energy source The auxiliary energy sources can be biomass, gas, oil, or electricity This dual system makes Solar Combisystems more complex than solar DHW systems with the additional interactions of the extra subsystems These interactions profoundly affect the overall performance of the solar part of the system Figure illustrates examples of solar heating systems 3.12.2.1 Components and Concepts The components of a solar DHW system are collector, storage, collector cycle, heat exchanger, auxiliary heat source, and regulation Solar systems for DHW system are fairly simple and manufactured and marketed today in developed as well as in developing countries Two different principles for solar DHW systems are used: Systems with natural circulation Systems with forced circulation Figure shows the principal schemes of solar water heating systems 3.12.2.1.1 Solar DHW systems with natural circulation Solar DHW systems with natural circulation (thermosiphon systems) are most favorable in areas with a mean annual sum of global radiation on a horizontal surface above 1800 kWh m−2 yr−1 Thermosiphon systems can work satisfactorily only if the storage tank is mounted above the collector and if the collector warms up enough to establish a density difference between the water in the collector and the water in the storage tank The density difference is a function of the temperature difference, and therefore, the flow rate is a function of the useful gain of the collector that produces the temperature difference The systems are self-adjusting with increasing gain leading to increasing collector flow rates (Figure 8) The efficiency of heating systems during summer months could be improved by larger storage volumes or by hot water extraction during the day If a constant water temperature is needed at any time, a backup heating system must be incorporated in the system Due to the meteorological condition in most of developing countries – solar radiation ≥1800 kWh m−2 yr−1 – solar hot water systems according to the thermosiphon principle are suitable for domestic use and can be manufactured at a reasonable price Because of the high lime and salt content of the tap water, special attention has to be paid to possible calcification and corrosion The rubber absorber made of polymeric materials (e.g., ethylene propylene diene monomer (EPDM)) turned out to be useful It is recommended to use glass material for covering purpose, because plastic covers tend to decolorize, which results in a reduction of the solar radiation absorbed Solar hot water systems with collector areas exceeding 10 m2 should be supplied with forced circulation It should be possible to mount the collectors on flat roofs without expensive auxiliary structures, which reduces investment costs and improves economic application considerably 3.12.2.1.2 Solar DHW systems with forced circulation Solar DHW systems with forced circulation are the common concepts in areas with moderate and cold climates The components of a compact solar DHW system with forced circulation – for a household/single-family house – are shown in Figure 3.12.2.2 Solar Thermal Collectors Collectors are the component for the conversion of solar energy into low- and high-temperature heat ‘Nonconcentrating’ collectors fully utilize the global radiation but ‘concentrating collectors’ use only the direct beam component of the radiation by concentrating irradiation on the absorber, thus increasing the intensity of radiation on the absorber Concentrating collector systems are the preferred technology in regions with more than 2500 annual sunshine hours (Figure 10) The simplest design of a nonconcentrating collector is the ‘flat-plate collector’ The properties of this collector are well known As absorbers, black painted metal (copper, aluminum, or steel) or plastic plates are used and in order to reduce the useful heat losses – which increase with rising temperatures – transparent covers are placed on the collectors and appropriate insulation is provided at the back side of the absorber (Figure 11) With this type of collector, temperatures up to 80 °C with conversion efficiency of about 40–60% can be achieved Applications of this type of collector are swimming pool heating, water heaters, agricultural drying, desalination, and space heating Solar Hot Water Heating Systems Solar hot water Solar Combisystem Solar Combisystem Solar small district heating Solar district heating Solar district heating Solar cooling Solar process heat Figure Examples for solar thermal systems for low- to medium-heat production 427 428 Applications Hot water Schematic of natural circulation solar water heater Hot water Storage tank Collector pipe Thermosyphon system Collector Auxiliary Tank Collector Cold water Cold Water Figure Solar domestic hot water (DHW) systems with natural circulation Solar compact system for hot water preparation Collector 12 13 Hot water 11 (5) Auxiliary heat (1),(2) Collector-pipes (6)–(13) Control and regulation 10 (3) Tank (4) Heat exchanger Cold water Figure Components of a compact solar domestic hot water (DHW) system for a household/single-family house For temperatures above 100 °C, advanced designs, like some ‘evacuated-tube collectors’, have been developed To obtain fluid temperatures above 150 °C, ‘concentrating solar collector’ systems must be used The concentrator (a mirror or lens) is normally equipped with a tracking device that follows the sun The absorber in this system is located close to the geometric focus of the concentrator to intercept most of the incident direct radiation In general, there are two types of concentrators: (1) the linear focusing concentrator and (2) the point focusing concentrator In summary, the type of collector to be used Solar Hot Water Heating Systems (a) 433 Evaporation in the collector Condensate To the storage Poor location of the check valve Check valve Pump From the storage Good location of the check valve Expansion vessel Heat store (b) Heat store Drain-back tank Drain-back tank Figure 14 The collector circuit and drain-back concepts (a) Arrangement of the components of the primary solar circuit; (b) implementation of the drain-back concept when the collector and the heat store are at the same level that the heat from the collector goes to the right level in the store Low flow should not, in general, be used with internal heat exchangers, as these cannot fully use the high-temperature built up in the collector, and the resulting temperature in the store is much lower as the water in the store gets mixed rapidly Moderate flows can be used, but in this case, the internal heat exchanger should have a greater vertical extent than when using high flows 3.12.2.3.1 Drain-back system In ‘drain-back systems’, the collector is drained of fluid when it is not in operation (see Figure 14) The heat transfer fluid is removed from the collector each time the collector pump stops This method is used for protection from both frost and overheating Another method of overheating protection involves keeping the collector circuit pump in operation and dumping heat in the ground or some other heat sink Some systems even cool the store at night so that the risk for overheating the next day is reduced A system design that can withstand high pressures (up to bar) in the collector circuit enables the fluid to remain in the collector at all times However, this approach can lead to rapid deterioration in the glycol and is not to be recommended for systems with stagnation temperatures over 140 °C Drain-back technology provides an interesting alternative for overheating protection of fluid in the solar collector loop and also prevents the heat transfer fluid from freezing When the collector circuit is not running, the circulation can operate using plain water without (antifreeze) additives due to drain-back of the collector fluid This system concept is based on draining the water from the tilted collector and outdoor collector pipes using gravitational force and replacing the liquid with air from the top By replacing water in the collector with air, ice cannot be formed and damage is, therefore, avoided The water also drains back if the heat store is fully 434 Applications charged, thereby avoiding boiling of water and high pressures inside the system When using polymer materials in the collector circuit, both stopping the pump in time and a permanent opening in the collector loop to the atmosphere are needed to avoid overpressure In comparison with the use of heat transfer fluids, drain-back technology using water features both advantages and disadvantages Following are the advantages of drain-back technology using water: • Water does not face the aging drawbacks exhibited by collector fluids with additives, such as a change in material properties and possible corrosion of the collector loop • Heat transfer properties of water, that is, both heat capacity and viscosity, are better than those of other heat transfer fluids • Water is much cheaper than all other collector fluids and easily available • The collector circuit generally does not face high overpressures, possibly leading to additional guarantee for safety • The level of maintenance for drain-back systems is lower The disadvantages of drain-back systems are as follows: • less flexibility in the choice of the solar collector and • special attention for drain-back collector loop design and installation The implementation of the drain-back concept in solar heating systems is simple and inexpensive; draining a solar collector requires special qualities in hydraulic design The major feature is that all of the water must run down to the level of the drain-back storage part of the system when the pump stops This requirement means that every pipe from the top of the solar collector loop to the drain-back volume must slope downward When the collector loop is in operation, the drain-back volume is filled with air This volume can be part of the heat stored or integrated into the collector side heat exchanger within the store or it can be designed as external drain-back tank When the pump in the collector circuit stops, water drains from the collector to the drain-back volume due to gravity This process stops when water levels in both pipes are equal or when the collector loop is empty When complete, the collector and all outdoor pipes must be fully filled with air Drain-back collector circuits can be implemented as closed or open loops to the environment Closed loops are commonly used in collector circuits that can withstand pressures up to at least bar, which usually requires metal absorbers and pipes After some time, the metal absorbs the oxygen in the circuit and no further corrosion occurs Open loops are applied in systems with plastic materials Pressures higher than the hydrostatic level should be avoided as combination of high temperature and pressure may cause weeping and may damage the plastic materials The properties of the collector have to withstand stagnation temperatures with no fluid (empty) without deterioration, thermal shock when the collector is hot and suddenly fed with cold water, and repeated thermal cycling The conditions for good emptying behavior of collectors in the event of stagnation can be achieved by the simple repositioning of the check valve in relation to the expansion vessel as shown in Figure 14 Different implementations of the drain-back systems were analyzed in the framework of IEA-SHC Programme, Task 26 Solar Combisystems [4] 3.12.2.4 Thermal Storage A heating system needs thermal storage when there is a mismatch between thermal energy supply and energy demand, for example, when intermittent energy sources are utilized The need for thermal storage in solar hot water systems is often short term In such instances, water is a very efficient storage medium Water storages are sensible heat energy storages with the advantage of being relatively inexpensive, but the energy density is low and decreases during the storage time [5–7] 3.12.2.4.1 Water storage technology The ‘hot water tank’ is one of the best known thermal energy storage technologies The hot water tank serves to bridge sunless periods in the case of solar hot water and combined heating system, to increase the system efficiency in combination with cogeneration systems, and to shave the peak in electricity demand and improve the efficiency of electricity supply in the case of an electrically heated hot water tank Water storage tank technology is mature and reliable Sensible heat storage in water is still unbeaten regarding simplicity and cost In refined systems, the inlet–outlet heights in the tank can vary according to the supply and storage temperatures Three types of water storage concepts are in the market: (1) bivalent storage, (2) tank-in-tank storage, and (3) stratified storage (Figure 15) Thermally stratified water tanks can improve the annual system efficiency by 20% and more ‘Short-term storage’ for solar hot water systems typically has a storage volume between 1.5 and 2.0 times of the daily hot water demand Even with short-term storage, generous insulation of the tank is essential For short-term and mid-term storages, one- and two-storage concepts are used (see Section 3.12.3.2) Solar Hot Water Heating Systems 435 Solar storages for hot water and space heating Bivalent heat storage Tank in tank Stratified storage Water storage with thermal stratification Figure 15 Concepts for water storage technology ‘Mid-term storage’ for solar-combined heating systems and solar-supported district heating should cover the heat demand for 3–5 days For detached and row single-family low-energy houses, a storage volume of about 800–1500 l will be suitable ‘Seasonal storage’ is one means to achieve a high annual share of solar heat in northern latitudes A realistic target to provide a heat capacity of months in existing housing or months in low-energy housing is provided Mid-term and seasonal storages are used in solar heating plants (district heating) 3.12.2.4.2 Advanced heat storage technologies For a widespread market deployment of solar thermal systems, it is necessary to store heat efficiently for longer periods of time in order to reach high solar fractions, and therefore efficient and cost-effective compact storage technologies with high heat capacity are needed Advanced storage technologies, such as concepts with a phase-change material or with thermochemical materials, are still in the research and development stage (Figure 16) Latent heat storage uses the principle of the change of phase of a material named the storage medium The physical principle of latent storage is a reaction of phase change The storage capacity of the storage medium is equal to the phase-change latent heat at the phase-change temperature + sensible heat stored over the whole temperature range of the process Storage systems based on chemical reactions can achieve much higher energy density than storage systems based on sensible heat or even latent heat, but are not yet commercially viable The storage systems based on chemical reactions have negligible losses whereas sensible heat storage system dissipates the stored heat to the environment and needs to be insulated strongly if the storage period will be long 3.12.2.5 Decentralized and Centralized Solar Thermal Systems Solar heating systems may distinguish between a ‘decentralized’ and a ‘centralized’ approach In a decentralized approach, the storage and collectors are placed within the individual houses like in an ordinary active solar heating system but of a larger size In the centralized concepts, these components are centrally situated, that is, all solar heat is collected in one storage unit, from which the heat is distributed to the houses Figure 17 illustrates the schematics of solar-supported heating plants For central solar thermal systems – for example, for apartment housing – the concept of the heat distribution network is of high importance For solar-supported heating systems, four- and two-pipe networks are used Based on experimental data, two-pipe 436 Applications Seasonal storage for solar heat Development of new storage materials • Sensible heat ≈ 100 MJ m–3 • Latent heat ≈ 300 – 500 MJ m–3 • Thermochemical heat ≈ 1000 MJ m–3 Latent heat 335 kJ kg Ice °C Sensible heat kg Water °C 335 kJ kg Water 80 °C Temperature T T2 Latent Tmelt T1 Sensible Solid Sensible Melting liquid Heat Q Figure 16 Advanced heat storage technologies in development networks have obvious advantages over four-pipe networks when it comes to the plant efficiency and utilization of the solar system Two-pipe networks can be operated in combination with decentralized heat exchangers or decentralized boilers in the row houses With individual storages, it is possible to operate the network at different temperatures: lower temperature for space heating (about 40 °C) and higher temperature for hot water preparation (about 65–70 °C) Therefore, the heat losses in the network can be reduced compared to a network with heat exchangers, which is operated on the highest temperature all the time On the other hand, the investment costs for decentralized storages are higher than those for heat exchangers The major advantage of having a centralized system is the reduced unit costs and heat losses from the storage In general, a centralized system may make better use of the economy of scale (unit prices decrease with the size) than a decentralized one The solar unit costs decrease sharply up to approximately 100 m2, as can be seen in Figure 18 This translates to lower kilowatt-hour costs for the solar heat, as illustrated in the example of Austria On the other hand, the heat losses in the pipes of the heat distribution net have to be considered The relatively high losses in small district heating systems mainly through the pipes in summer are caused by the lower heat consumption during the summer months Smaller systems could be found in countries with moderate climate for multifamily housing and heating systems in commu­ nities The aim of such systems is to cover the hot water demand outside the heating season Larger district heating plants will be found in Denmark and Sweden 3.12.2.6 Auxiliary Heat Sources In DHW compact systems for households, mainly electricity is used as the auxiliary heat source Otherwise solar hot water systems are combined with fossil or biomass boilers or heat pumps for space heating, mainly during the heating season The hot water preparation outside the heating season should be covered up to 100% by solar This goal is also for Solar Combisystems, with combined hot water preparation and space heating For example, with a combined solar–biomass heating system the contribution to the heat demand of a building (space heat and hot water) is covered 100% by renewable energy Natural gas is mainly used as backup system in gas district heating Solar Hot Water Heating Systems 437 Solar supported district heating Midterm storage Building Collectors Heat station Auxiliary heating Collector area Boiler Heat distribution net work Heat storage Collector pipes Solar thermal system with central storage in combination with decentralized hot water storage tanks a Energy storage tank re a or ct lle Co Solar thermal system with central storage in combination with decentralized heat exchangers Heating circuit Storage tank Cool water Hot water Storage tank Cool water Hot water boiler Two-pipe network Storage tank Cool water Hot water Two-pipe network Figure 17 Schematic diagrams for district heating plants and examples for small district heating in Austria 3.12.2.7 Hygienic Aspects of Solar Hot Water Heaters In some active solar system configurations, the storage tank contains drinking water There is then the risk of the so-called Legionnaires’ disease, Legionella pneumonia It is caused by Legionella, or rod-shaped, mobile, aerobic bacteria that occur naturally in surface water and groundwater They begin to propagate at temperatures between 20 and 50 °C, with optimum growth occurring between 30 and 40 °C Above 60 °C, they die off quickly A long residence time in water at favorable temperatures may result in high concentrations of Legionella Stagnant water in pipes or in parts of an installation that have not been flushed is a breeding ground for these bacteria 438 Applications Collector area and collector costs Costs for installed collector area 800 Maximal costs Costs (Euro m–2) 700 Average costs 600 Minimal costs 500 400 300 200 100 Maximal costs: installation on flat roofs Minimal costs: building integrated 50 100 150 200 250 Collector area (m ) 300 350 400 Solar hot water system Heat production costs Costs (Euro kWh–1) 0,30 Lifetime of solar system 0,25 Annual solar share: 45%–50% 0,20 15 years 20 years 25 years 0,15 30 years 0,10 0,05 0,00 10 20 30 40 50 60 70 Collector area (m2) 80 90 100 Figure 18 Economic aspects of solar water heating systems To prevent the growth of Legionella, the water temperature should be either below 25 °C or above 50 °C Disinfecting a contaminated system can be done by flushing it, then heating the water to 60 °C for 20 In general, solar thermal systems for hot water preparation are backed up by an auxiliary heating system to achieve temperatures above 50 °C In this manner, the risk of Legionella-contaminated water can be minimized A distinction is made between small and large systems Small systems are considered to have a very low risk and need no special attention Small systems are installations in one- or two-family houses, or installations with volume 230 0 nonstudy area METEONORM 4.0 Global irradation: year (kWh m–2) 70 60 50 40 30 20 10 –1 0 –2 0 ? ?30 –4 0 –5 0... For short-term and mid-term storages, one- and two-storage concepts are used (see Section 3. 12 .3. 2) Solar Hot Water Heating Systems 435 Solar storages for hot water and space heating Bivalent