Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems

Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems Volume 3 solar thermal systems components and applications 3 14 – solar cooling and refrigeration systems

GG Maidment and A Paurine, London South Bank University, London, UK

© 2012 Elsevier Ltd All rights reserved

References

3.14.1 Introduction

Thermally driven cooling offers a more sustainable and low-energy solution for refrigeration and air-conditioning applications For most cooling applications, there is a coincidence between peak solar gain and peak cooling demand By using solar thermal energy

to drive a cooling cycle, it is possible to produce cooling virtually coincident with the demand for cold, and thus solar-powered cooling is a potential technology for domestic, commercial, and industrial buildings The coincidence of cooling with demand is shown in Figure 1

This chapter presents an overview of the state-of-the-art of solar cooling It describes the general theory, the technologies, and their relative performance and applications Several competing technologies for solar energy collection for sorption and vapor compression refrigeration are compared in terms of efficiency, life-cycle cost (LCC), and primary energy basis

3.14.2 Solar-Powered Cooling

Solar cooling is a technology for converting heat collected from the sun into useful cooling into refrigeration and air-conditioning applications Solar thermal energy is collected and used by a thermally driven cooling process, which in turn is normally used to generate chilled water or conditioned air for use in the building A typical solar cooling scheme essentially includes three refrigeration or air-conditioning plant for producing cooling, and a heat sink for heat rejection A diagram of the main components

of a solar cooling scheme is shown in Figure 2

components These include the solar collector for harnessing solar energy by converting it into heat or mechanical work, a

3.14.3 Need for Solar-Powered Cooling

Many buildings require cooling to offset heat gains In most temperate countries, solar heat gains represent a large proportion of the overall load to the building For the United Kingdom, according to Jones [2], for the typical hypothetical office block, the solar gain contributes between 25% and 40% of the total cooling load Low-energy and more sustainable cooling systems have been proposed

as an alternative to traditional energy-intensive methods Interest in solar cooling systems was first shown during the energy crisis of 1970s These systems used solar thermal energy to energize absorption cycles or light to provide electrical power from photovoltaic (PV) panels for vapor compression refrigeration cycles As these systems utilize solar energy, they require minimal grid-derived electricity, unlike conventional vapor compression equipment The main advantage of such systems is that they provide virtually

‘free cooling’ that is coincident with the occurrence of solar gains Also, solar energy is freely available in moderate to hot climates where more than 50% of the world’s population reside [3]

Comprehensive Renewable Energy, Volume 3 doi:10.1016/B978-0-08-087872-0.00314-0 481

G

[W]

P

[W]

1 3

Congruence Global radiation

1

Jan

2

Dec

Surplus of solar in summertime

Cooling load Heating load

2

3

Figure 1 Relationship between incidence of solar radiation and cooling [1]

Solar collector

Refrigeration or cooling plant Heat or

work

Cooling

Building or process

Heat sink Figure 2 Scheme of a typical solar cooling

3.14.4 Solar-Powered Cooling Technologies

There have been numerous projects worldwide relating to the systems used for converting solar thermal energy into useful cooling These systems have included the use of flat-plates, evacuated-tube, PV, and concentrating solar collectors in combination with desiccant cooling, adsorption chillers, absorption chillers, vapor compression systems, and ejector refrigeration system

The relative efficiency for each of these solar cooling systems is determined based on the efficiency of the collector device (ηcoll) and the coefficient of performance (COP) of the cooling cycle The overall system efficiency, also referred to as solar coefficient of performance (SCOP), is indicated in eqn [2]

Q SCOP ¼ u

T  COP

or

where ηcoll is collector efficiency, Qu is the useful heat gained by the collector, and GT is the solar insolation

The SCOP is used because it gives a simple but combined index of system efficiency as well as capital cost Therefore, it should be noted that the collector and heat rejection component size and cost for technologies (described below) are significantly affected by SCOP

3.14.4.1 Desiccant Cooling System

In a desiccant cooling system, air can be passed over common solid desiccants such as zeolite or silica gel for dehumidification and

to sensibly cool the air well below ambient temperature conditions in some form of evaporative cooling process Also, liquid dessicants such as lithium or calcium chloride have been used for air dehumidification processes In either case, the desiccant requires regeneration and this can be achieved using solar thermal energy to dry it out, in a cost-effective, low energy, and continuously repeating cycle

A number of desiccant-based solar cooling demonstration projects have been cited in the literature reviews conducted by Lu et al [16], Pesaran and Wipke [15], Ahmed et al [4], and Gommed and Grossman [18] The main advantage of desiccant-based systems when combined with solar cooling is that regeneration takes place at relatively low temperatures, a factor suited for use with solar energy

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3.14.4.2 Solid Desiccant

A range of solar air-conditioning systems utilizing solid desiccants in open-cycle configuration have been reported The original concept was applied to the Pennington cycle A system that produces dehumidification and cooling as modeled by Halliday et al [12] is shown in Figure 3

It should be noted that in Figure 3 a rotating desiccant wheel containing silica gel particles is deployed to dehumidify and provide supplementary sensible cooling of incoming outside air The desiccant wheel operates such that as a heat exchanger sensibly increases the temperature of the process air while decreasing its latent heat The thermal effectiveness (εT,DW) of the desiccant wheel

is given by the following expression, where notation relates to the numbering in Figure 3:

m_process air CpðT8 ′ − T7Þ þðg8− g7Þhfg

regeneration air ðh4− h3Þ where T8′ is the temperature of process air at vapor pressure similar to that of the outside air entering the desiccant wheel and equivalent to dry bulb temperature of air leaving the desiccant wheel and g and hfg are moisture content and vaporization latent heat

of water, respectively

The effectiveness of the thermal wheel can also be expressed in the terms of the vaporization latent heat rate of the adsorbed water and the regeneration input heat rate into the system The regeneration effectiveness (εR,DW) of the desiccant wheel is given by the following expression:

m_process air ðg8 −g7Þhfg

regeneration air ðh4− h3Þ

By using a perfectly designed and optimized desiccant wheel with high regeneration effectiveness, it is possible to achieve high COP with desiccant-based cooling systems

3.14.4.3 Liquid Desiccant

Liquid-based desiccant systems are described in the literature [5] and these are reported to also clean the air and improve indoor air quality There are some liquid desiccant cooling systems that use water–lithium chloride (LiCl–H2O) and water–calcium chloride (CaCl2–H2O) solutions for sorption purposes In comparison with solid desiccant cooling systems, the liquid desiccant cooling systems have higher rate of air dehumidification at the same range of driving temperatures and in that they have high energy storage capacity when used in the concentrated solutions Figure 4 shows a distinctive liquid desiccant solar cooling system

The effectiveness of the liquid desiccant solar cooling can be defined based on the process and regeneration air inlets and outlets Assuming there is minimal or no heat loss from the system described in Figure 4, the cooling effectiveness (εC,LD) of the liquid desiccant system is given by the following expression:

m_Process Air ðgB −gCÞhfg

Regeneration Air ðhE −hDÞ where g and hfg are moisture content and vaporization latent heat of water, respectively

Alternatively, the εC,LD can be defined in terms of the temperature of the salt solution in the system:

ðT2 −T1Þ CP 1 ð  2 Þ

3 −T2Þ

CP 3 ð  2 Þ where CP 1ð  2 Þ≈ CP 3 ð  2 Þ is the average specific heat capacities of the salt solutions in the two heat exchangers and T is the temperature of the salt solution

Exhaust air

Outside

Solar coil

Regen

coil

7

10

Heating coil

Evap

cooler

Supply air

Return air

Solar Cooling

coil coil

Figure 3 Solar desiccant cooling system Developed from Halliday SP, Beggs CB, and Sleigh PA (2007) The use of solar desiccant cooling in the UK: a feasibility study Applied Thermal Engineering 22: 1327–1338 [12]

Supplementary cooling battery

Incoming

outside

−

1

Air passing through a soak media PROCESS

(CaCl2 or LiCl)

Liquid desiccant dilute salt solution

heat exchanger Supplementary 2

Supplementary heater battery Hot & humid

exchanger

Solar collector

Liquid desiccant

concentrated salt

heat exchanger

REGENERATION air

Figure 4 Liquid desiccant solar cooling system

_

Therefore, eqn [5] simplifies to the following expression below:

Þ− 1

The cooling effectiveness in this case is similar to relative efficiency, and therefore, the SCOP for liquid solar cooling system can be established as follows:

where ηcoll is solar collector efficiency

Liquid desiccants are not popular in the supply airstream due to possible health risks such as Legionnaires’ disease and the risk of desiccant droplet carryover causing corrosion in metal ducts A number of liquid desiccant cooling systems have been developed to eliminate the use of the humidifier and desiccant in the supply airstream One arrangement utilizes two heat exchangers, where the cooled airstream of the first indirect evaporative liquid-desiccant air-cooling heat exchanger is used in a second heat exchanger to cool a clean supply airstream without direct contact with either the cooling water or the liquid desiccants This is shown in Figure 5 Although the use of the second heat exchanger eliminates the possibility health risks and ductwork metal corrosion due to droplet carryover, it does impinge on the overall effectiveness of the cooling system Since the liquid desiccant flowing from heat exchanger 2 (HTX 2) via pip 5 in Figure 5 is minimal, it does not significantly influence the performance of the regenerating heat exchanger Therefore, the overall SCOP can be defined in terms of incoming primary outside process air (IPOPA) onto the heat exchanger 1 (HTX1) and incoming outside regeneration air (IORA) onto the liquid desiccant regenerating heat exchanger using the following expression:

εHTX2m IPOPAðgA − gBÞhfg

_

mIORA Alternatively, this can be defined in terms of the incoming secondary outside process air (ISOPA) onto HTX 2 Since the air will not be in direct contact with the liquid desiccant, it will be sensibly cooled only, and therefore, this can be established using the following expression:

mISOPA ðhE − hFÞ

ðhH −hGÞ

IORA where εHTX2 is effectiveness of the heat exchanger

While there are relatively few suppliers/installations of solar-based desiccant systems, they have been used extensively in certain niche applications where the ability to independently control air humidity at low levels provides additional benefits

3.14.4.4 Absorption Systems

The absorption cycle consists of four basic components operating at two pressure conditions and uses an absorbent–refrigerant solution such as water lithium bromide (LiBr-H2O) as the working fluid These components include the evaporator, absorber, generator, and condenser as shown in Figure 6

ðhH − hGÞ

Supplementary cooling battery

(CaCl2 or LiCl)

concentrated salt

solution

Supplementary heater battery Hot & humid

exhaust air

Liquid desiccant regenerating heat exchanger

Incoming primary

outside PROCESS

air

Incoming secondary outside PROCESS air

Treated supply cool air

Primary warm exhaust air

Cooling water

HTX 2

1

2

3

4

C

H

G

D

E

F

A

B

−

+

HTX 1

Incoming outside REGENERATION air

Figure 5 Solar air cooling system with indirect evaporative liquid desiccant

Weak solution High pressure refrigerant absorbent

Absorber

Generator

Ex.valve

QC

QG

Condenser

Evaporator

High-pressure refrigerant Low-pressure refrigerant Low-pressure refrigerant

Figure 6 The basic absorption cycle

The system works such that high-pressure liquid refrigerant flows from the condenser through an expansion device, which reduces the pressure, to the evaporator condition The refrigerant evaporates in the evaporator, cooling the secondary air or water (cooled medium) and the resulting low-pressure vapor passes to the absorber, where it is absorbed into a strong solution absorbent

It is necessary to continually reconcentrate the solution to maintain the low evaporation temperature required The ‘weak’ solution with high percentage of refrigerant is recirculated to the generator where most of the refrigerant is boiled off and the resulting ‘strong’ solution is passed back to the absorber via an expansion device The heat input into absorption chillers can be supplied via an array of solar thermal collectors

There are many different variants of absorption chillers including half-, single-, double-, and triple-effect systems There are also open- and closed-cycle absorption units that utilize liquid/vapor, solid/vapor, and sorbent/refrigerant combinations The variants of solar absorption system are detailed in the paper by Syed et al [6] The main practical difference between these systems is the driving temperature required for regeneration of the solution and their relative COPs as well as capital cost and complexity The most common cycle is the single-effect LiBr–H2O system that requires heat at around 80 °C will typically achieve a COP of around 0.7 for

a chilled water application

The COP of a typical absorption chiller can be expressed by assuming an ideal heat engine operating in a Carnot cycle Therefore, relation between work and heat for an ideal heat engine is given by the second law of thermodynamics:

W Th −Ts

G Th where W (kW) is the work, QG (kW) is the heat input rate, Th (K) is the temperature of heat source, and Ts (K) is the temperature of heat sink

The relation between work required and refrigeration load for an ideal mechanical refrigeration machine operating as a reverse Carnot cycle is as follows:

W Tl −Ts

s Tl where Qs (kW) is the refrigeration load, Tl (K) is the temperature of refrigeration load, and Ts (K) is the temperature of heat sink (assumed to be the same as for heat engine)

The COP of the two ideal cycles is given by the following expression:

Qs TlðTh −TsÞ

G ThðTl −TsÞ Absorption-type cooling is the most common type of solar cooling technology used in practice According to a survey of pilot plants

by International Energy Agency [7], small capacity machines in the range of 1–10 kW were mainly solar-energized as they can be operated with low-grade heat of temperatures below 100 °C Most of the machines in the survey were air-cooled, and during good climatic conditions, a theoretical COP in the range of 0.7–0.75 was attained In a more recent survey [8], it was reported that absorption technology dominated the other technologies applied in solar cooling industry, that is, it is incorporated in 67% of the installations Moreover, it was reported that there has been growth in small-scale systems (< 20 kW), which were not really on the market a few years ago

A number of commercial solar-powered absorption products are on the market now, such as that shown in Figure 7 marketed by Solar Polar This is reported to provide better integration of the components, and also offers scope to provide combined cooling and heating The main issues reported with the application of LiBr absorption chillers were that of mismatch between cooling load and chiller capacity for commonly used residential applications in Europe [8] There are also spatial constraints reported in accommodating a water-cooled chiller in a typical three- to four-bed house This problem has now been reportedly overcome with a smaller commercially available air-cooled (rotary) LiBr-H2O chiller of 4.5 kW nominal cooling capacity to replace a split DX unit Some pictures of commercially available packaged units are shown in Figure 8 These are designed to be installed outside the building and piped to high-wall fan coil unit in an adjacent room There is no external condenser circuit and the hot water cylinder tee-offs are piped directly to the chiller, hence saving cost and space

3.14.4.5 Adsorption Systems

The adsorption cycle is similar to absorption cycle, but it uses solid sorbent rather than a liquid The system uses an adsorption medium such as zeolite and activated carbon together with a refrigerant to achieve a cooling effect The natural mineral zeolite has the property to attract water vapor and to incorporate it in its internal crystal lattice while releasing heat at the same time The operation of adsorption heat pumps and refrigerators is therefore based on the ability of porous solids (the adsorbent) to adsorb vapor (the adsorbate or refrigerant) when at low temperature and to desorb it when heated The adsorption system is a

Solar collector tubes

Insulated ducts

Refrigerator

Figure 7 Solar Polar’s commercial product

QC

Condenser

Solar Adsorbent

bed + Supplementary heater battery

collector

Intermediate

Evaporator

Air handling unit

− cooling battery

Figure 8 Rotartica commercial product

four-temperature discontinuous cycle that consists of one or several adsorbers connected to heating sources, condenser, and evaporator Figure 9 below shows a single adsorbent bed intermittent adsorption cycle

The adsorption cycle presented in Figure 9 consists of four stage processes that are detailed in Figure 10

The processes demonstrated in Figure 10 are as follows:

• Process A to B Heating and pressurization, where the adsorbent is heated while the adsorber is closed and therefore raising the pressure from the evaporating to the condensing pressure

• Process B to C Desorption (generation) and condensation, where the adsorbent temperature continues to increase and hence inducing desorption of vapor which is then passed into condenser for condensation by rejecting heat to the environment The heat necessary to regenerate the adsorbent is a low-grade heat source such as solar energy or waste heat

• Process C to D Cooling and depressurization, where the adsorbent releases heat while the adsorber is closed and therefore decreasing the pressure from the condensing to evaporating the pressure

• Process D to A Adsorption and evaporation, where the adsorbent temperature continues to decrease and hence inducing adsorption of vapor being vaporized in the evaporator The heat of evaporation is drawn from the space by means cooling medium, which is usually air or water in the case of air conditioning

Figure 9 Schematic diagram of an intermittent adsorption cycle

Tevap Tcond Tmax −1/T

Throttling valve

Ln(P)

Condenser

Pc Valve (c)

Evaporator Pe

Valve (e)

Decreasing isosters

2

1

A

D

Isosteric sensible

sensible cooling

Sensible heating desorption

Sensible cooling adsorption

Liquid–vapor equilibrium

Condenser

Evaporator

Solar collector Supplementary

Absorbent bed 1

QC

Absorbent bed 2

+ heater battery

Flow & return cooling media

Air handling unit

− cooling battery

Figure 10 Clapeyron diagram for an adsorption system [9]

Although the heating and cooling provided by a single generator is discontinuous, it can be made continuous by operating two or more generators out of phase Figure 11 shows a schematic diagram of a typical conventional adsorption system with two adsorbers (generators)

The heating and cooling water media interchange between the two adsorbent beds and therefore ensuring the adsorption of vapor is continuous in order to maintain the cooling condition requirements

Assuming, heat supplied to the systems in Figures 9 and 11 is derived from solar energy, the SCOP can be established based on the two ideal Carnot cycles and this is given by the following expression:

TevapðTs −TCondÞ

s Tevap −TCond where Ts> Tmax (K) is the temperature of heat source, Tevap (K) is the temperature of refrigeration load, and TCond (K) is the temperature of heat sink

There are a few solar-powered adsorption systems operating According to Solair [8], 11% of the systems installed utilize adsorption cycles

Figure 11 Schematic diagram of a continuous adsorption cycle

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3.14.4.6 Ejector Systems

An ejector cooling system (ECS) is a mechanical system utilizing the Rankine cycle and the gas dynamic effect of an ejector (a thermal compression process) The basic ejector refrigeration cycle is illustrated in Figure 12 The system consists of two loops, the power loop and the refrigeration loop In the power loop, low-grade heat, QG, is used in a generator to evaporate high-pressure liquid refrigerant (process 1–2) The high-pressure vapor generated, known as the primary fluid, flows through the ejector where it accelerates through the nozzle The reduction in pressure that occurs induces vapor from the evaporator, known as the secondary fluid, at point 3 The two fluids mix in the mixing chamber before entering the diffuser section where the flow decelerates and pressure recovery occurs The mixed fluid then flows to the condenser where it is condensed rejecting heat, QC to the environment

A portion of the liquid exiting the condenser at point 5 is then pumped to the boiler for the completion of the power cycle The remainder of the liquid is expanded through an expansion device and enters the evaporator of the refrigeration loop at point 6 as a mixture of liquid and vapor The refrigerant evaporates in the evaporator producing a refrigeration effect, QE, and the resulting vapor

is then drawn into the ejector at point 3

The ECS is reported to compete with absorption on the grounds of simplicity, reliability, and low installation cost; however, its COP is much lower (typically 0.3 for a single-effect system)

The efficiency of the integrated cycle or SCOP for an ECS is established using the following equation:

h5 −h4

1 −h6 where  ¼ m_e =m_g (entrainment ratio), ηcoll is collector efficiency, and h is vaporization latent heat of the refrigerant

The SCOP of 0.25 was reported at evaporating, condensing, and generating temperatures of 8 °C, 32 °C, and 95 °C, respectively Furthermore, Mostofizadeh and Bohne [13] reported a COP of 0.3–0.35 at evaporator, condenser, and generator operating temperatures of 4 °C, 40 °C, and 90 °C

Huang et al [10] have reported on the development of a solar-driven ECS of 10.5 kW cooling capacity with a 65 m2 double-glazed flat-plate solar collector Wolpert and Riffat [17] theoretically obtained a COP of 0.62 for a PTC-driven steam ejector system of 13 kW cooling capacity The experimentally derived COP of their system operating in Loughborough, UK, was only 0.3

3.14.4.7 Photovoltaic–Compression Systems

These systems combine PV cells with electrically driven vapor compression refrigeration systems PV cells convert insolation to DC electricity, which is then inverted into AC to produce shaft power for an electromechanical compressor According to Best and Pilatowski [14], these systems have strong market pull mainly due to the lower cost and higher COP of the refrigeration machine A typical system is shown in Figure 13

However, research is needed to improve the efficiency and lower the cost of PV panels Various materials for PV cells such as cadmium sulfide (CdS), amorphous silicon (a-Si), copper indium diselenide (CuInSe2), cadmium telluride (CdTe), and poly­ crystalline silicon have been tested It has been established that the maximum power delivered is limited by the relatively low efficiency of the panel (< 20%) Results of the Solair project have shown promise in air-conditioning and cold storage A 1 kW (COP = 2.5–4) prototype air conditioner was built and connected to a 1.2 kW output array of PV cells activated by threshold insolation of 450 W m−2 The cold storage prototype was built and tested in Spain Results show that the operation has been satisfactory over a range of climatic conditions and a variety of foodstuffs

QE

Evaporator

QC

Ejector

Condenser

Generator

QG

Figure 12 Basic ejector cycle

0

0.2

0.4

0.6

0.8

1

1.2

Cooling temperatures (°C) Standard Single-effect LiBr/water absorption Double-effect LiBr/water absorption

PV - Vapor compression Improved single-effect LiBr/water absorption

30

Air handling unit cooling battery

Inverter

Evaporator

AC

QC

Condenser

Battery

Photovoltaic solar panel

− Figure 13 Typical photovoltaic vapor compression cycle

3.14.5 Relative Comparison of Solar Cooling Technologies

The relative use of solar cooling systems will depend on their relative performance in terms of efficiency, capital cost, and LCC Work carried out by Syed et al [6] investigated these and the results of this investigation are detailed below

3.14.5.1 Solar Coefficient of Performance

In assessing the relative efficiency of a solar cooling cycle, we are concerned with the efficiency of the cooling cycle and efficiency of the collector device itself Figure 14 shows the range of SCOP data reported for four competing solar cooling technologies at different application temperatures

Figure 14 High-level SCOP map for a range of cooling temperatures