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
3.14 Solar Cooling and Refrigeration Systems GG Maidment and A Paurine, London South Bank University, London, UK © 2012 Elsevier Ltd All rights reserved 3.14.1 3.14.2 3.14.3 3.14.4 3.14.4.1 3.14.4.2 3.14.4.3 3.14.4.4 3.14.4.5 3.14.4.6 3.14.4.7 3.14.5 3.14.5.1 5.14.5.2 5.14.5.3 3.14.6 3.14.7 5.14.8 References Introduction Solar-Powered Cooling Need for Solar-Powered Cooling Solar-Powered Cooling Technologies Desiccant Cooling System Solid Desiccant Liquid Desiccant Absorption Systems Adsorption Systems Ejector Systems Photovoltaic–Compression Systems Relative Comparison of Solar Cooling Technologies Solar Coefficient of Performance Capital Cost Comparison Life-Cycle Cost Comparison Application of Solar Cooling System Integration with Solar Hot Water and Solar Tthermal Systems for Cost-Effectiveness Conclusions 481 481 481 482 482 483 483 484 486 489 489 490 490 491 491 492 492 493 493 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 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 components These include the solar collector for harnessing solar energy by converting it into heat or mechanical work, a 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 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 doi:10.1016/B978-0-08-087872-0.00314-0 481 482 Applications G [W] P [W] Congruence Global radiation Cooling load Heating load Surplus of solar in summertime Jan Dec Figure Relationship between incidence of solar radiation and cooling [1] Solar collector Heat or work Refrigeration or cooling plant Cooling Building or process Heat sink Figure 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] Qu COP GT ẵ1 SCOP ẳ coll COP ẵ2 SCOP ẳ or where coll 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 Solar Cooling and Refrigeration Systems 3.14.4.2 483 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 It should be noted that in Figure 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: � � _ process air Cp T8 T7 ị ỵ ðg8 − g7 Þhf g m ε T ; DW ẳ ẵ3 _ regeneration air h4 h3 ị m 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: εR ; DW ¼ _ process air ðg8 −g7 Þhf g m _ regeneration air ðh4 h3 ị m ẵ4 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 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: εC ; LD ¼ _ Process Air ðgB −gC Þhfg m _ Regeneration Air hE hD ị m ẵ5 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: C P ð1 À Þ ðT2 −T1 Þ εC ; LD ¼ CP ð3 À Þ ðT3 −T2 Þ ½6 where C P ð1 À Þ ≈ CP ð3 À 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 Evap cooler Exhaust air Outside air Desiccant wheel + + Regen coil Solar coil Return air 10 Supply air Heating coil Thermal wheel + Solar coil + − Cooling coil Figure 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] 484 Applications Supplementary cooling battery − Air passing through a soak media Incoming outside PROCESS air A B C Treated supply air (CaCl2 or LiCl) dilute salt solution Supplementary heater battery Supplementary cooling exchanger Liquid desiccant heat exchanger Hot & humid exhaust air E + Solar collector (CaCl2 or LiCl) concentrated salt solution Incoming outside REGENERATION air D Liquid desiccant regenerating heat exchanger Figure Liquid desiccant solar cooling system Therefore, eqn [5] simplifies to the following expression below: εC ; LD ¼ ðT2 −T1 ÞðT3 −T2 Þ− ½7 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: SCOP ẳ coll C ; LD ẵ8 where coll 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 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 (HTX 2) via pip in Figure 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 (HTX1) and incoming outside regeneration air (IORA) onto the liquid desiccant regenerating heat exchanger using the following expression: SCOP ¼ ηcoll _ IPOPA ðgA − gB Þhfg εHTX2 m _ IORA hH hG ị m ẵ9 Alternatively, this can be defined in terms of the incoming secondary outside process air (ISOPA) onto HTX 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: SCOP ¼ ηcoll _ ISOPA ðhE − hF Þ m _ IORA ðhH − hG Þ m ½10 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 Solar Cooling and Refrigeration Systems Supplementary cooling battery − Incoming primary outside PROCESS air 485 Primary warm exhaust air Incoming secondary outside PROCESS air A E D Cooling water (CaCl2 or LiCl) concentrated salt solution HTX B F C HTX 2 Supplementary heater battery Treated supply cool air H Hot & humid exhaust air + G Incoming outside REGENERATION air Liquid desiccant regenerating heat exchanger Figure Solar air cooling system with indirect evaporative liquid desiccant Weak solution absorbent High pressure refrigerant Condenser Strong solution absorbent Generator QG QC High-pressure refrigerant Ex.valve Low-pressure refrigerant Low-pressure refrigerant Evaporator Absorber QA QE Figure 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: 486 Applications Carnot efficiency : W Th Ts ẳ Th QG ẵ11 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 Carnot efficiency : ẳ ẵ12 Tl Qs 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: COP ¼ Qs Tl ðTh Ts ị ẳ T h Tl Ts ị QG ẵ13 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 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 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 Cooler unit Solar collector tubes Insulated ducts Refrigerator Figure Solar Polar’s commercial product Solar Cooling and Refrigeration Systems 487 Figure Rotartica commercial product four-temperature discontinuous cycle that consists of one or several adsorbers connected to heating sources, condenser, and evaporator Figure below shows a single adsorbent bed intermittent adsorption cycle The adsorption cycle presented in Figure 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 QC Condenser Adsorbent bed + Intermediate refrigerant receiver Supplementary heater battery Solar collector Flow & return cooling media Evaporator − Air handling unit cooling battery Figure Schematic diagram of an intermittent adsorption cycle 488 Applications Decreasing isosters Ln(P) Liquid–vapor equilibrium Pc Condenser B Sensible heating desorption C Valve (c) Isosteric sensible heating Throttling valve Pe Evaporator A Valve (e) Isosteric sensible cooling D Sensible cooling adsorption Tevap Tcond Tmax −1/T 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 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: SCOP ¼ ηColl Tevap ðTs −TCond Þ � � Ts Tevap −TCond ½14 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 QC Condenser + Supplementary heater battery Absorbent bed Absorbent bed Evaporator − Air handling unit cooling battery Figure 11 Schematic diagram of a continuous adsorption cycle Solar collector Flow & return cooling media Solar Cooling and Refrigeration Systems 3.14.4.6 489 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 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 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 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 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 SCOP ¼ ηcoll ½15 h1 −h6 _ g (entrainment ratio), ηcoll is collector efficiency, and h is vaporization latent heat of the refrigerant _ e =m where ¼ m The SCOP of 0.25 was reported at evaporating, condensing, and generating temperatures of °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 °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 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 Condenser Generator QG Figure 12 Basic ejector cycle Ejector 490 Applications QC Condenser Photovoltaic solar panel AC Inverter DC Battery DC Evaporator − Air handling unit cooling battery 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 Solar coefficient of performance (−) 1.2 0.8 0.6 0.4 0.2 −50 −40 −30 −20 −10 Cooling temperatures (°C) 10 20 Standard Single-effect LiBr/water absorption Double-effect LiBr/water absorption PV - Vapor compression Improved single-effect LiBr/water absorption Figure 14 High-level SCOP map for a range of cooling temperatures 30 491 150 125 100 750 500 Total cost Solar collectors Chillers Ancillaries FPC+SE Absorption Conversional(nonsolar) Heat rejection FPC+SE Absorption PV+Centrifugal PV+Water cooled ETC+DE Absorption 250 ETC+SE Absorption Capital cost of cooling (£/kW) Solar Cooling and Refrigeration Systems Figure 15 A comparison of the capital cost of solar cooling systems 5.14.5.2 Capital Cost Comparison Figure 15 subdivides the three generic families and compares the capital cost composition of solar cooling equipment normalized per kilowatt of cooling The following observations from Figure 15 are noteworthy: • The collector cost ranges from to 26 times the chiller cost depending on their types and operating temperatures • About a sixth of the investment is required for procuring flat-plate collectors for supplying hot water at a temperature of 75 °C compared with concentrating collectors for higher temperatures The lower capital cost system consists of flat-plate collectors and single-effect absorption chillers sized for low hot water temperatures • The lowest cost option is currently times the cost of a conventional (nonsolar) vapor compression system 5.14.5.3 Life-Cycle Cost Comparison LCC depends on the SCOP as well as capital and running cost of systems Only those systems that show lower SCOP could have better LCC if they have lower capital costs These two have the highest SCOP; however, the options that have a marginally lower SCOP were also considered in an LCC evaluation In practice, the SCOP will depend on the availability of solar energy and whether additional thermal energy is required The solar fraction SOLF is sometimes used to describe the utilization of solar energy for cooling Therefore, SOLF is described as a ratio of thermal solar energy or electrical solar energy input to total energy input (including ancillary energy) It should be noted that a solar fraction of unity is achievable if a system is energized entirely with solar energy This is given by either of the following equations: Qsol Qsol ỵ Qgas ẵ16 Wpv Wpv ỵ Wpoh ẵ17 SOLFthe ẳ SOLFw ẳ where Qsol is the solar thermal energy, Qgas is the gas auxiliary energy, and Wpv is the solar electrical energy Therefore, the overall LCC is given by the following equation: � � � � � � � � i EFLH W ỵ POH W C ỵ Q ỵ M LCC ẳ Z ỵ C wg g ng poh eflh Qe 11 ỵ i ị n ẵ18 where Z is the capital cost, Qg is the generator load, Cng is the natural gas tariff, Cwg is the grid electricity tariff, EFLH is the number of equivalent full load hours (EFLH), POH is the plant on hours which is typically 1.5 EFLH, Weflh is the electrical Applications LCC difference relative to conventional (nonsolar) cooling (%) 492 700 FPC+SE absorption (75 °C/70 °C) PV+air-cooled screw FPC+SE absorption (85 °C/80 °C) ETC+SE absorption (115 °C/110 °C) ETC+DE absorption PTC+ejector PV+centrifugal PV+water-cooled screw 600 500 400 300 200 100 500 1500 2500 3500 4500 5500 Equivalent full load hours (hours) Figure 16 Annual difference in life-cycle cost between solar and nonsolar cooling systems energy input as shaft power to mechanical compressor, Qe is the evaporator load, Wpoh is the ancillary plant power, and M is the maintenance cost An LCC comparison was carried out to consider the combined effect of capital and running cost on system performance It was found that due to the high capital cost of currently available solar collectors, annual LCC savings with solar cooling systems compared with the conventional system cannot yet be realized This is indicated in Figure 16, which provides the difference in LCC of solar cooling systems and a conventional centrifugal vapor compression system with a base of 100% (indicating LCC equivalence) against EFLH As the systems are run for longer EFLH, the LCC difference diminishes due to the impact of cost of saved energy Interestingly, this results show that the lowest capital cost option reflects the lowest LCC difference for a number of EFLH of cooling The result of an LCC sensitivity study has shown that solar cooling using flat-plate collectors and single-effect absorption chillers at the lowest driving temperatures could become economical if the collector capital cost is reduced [11] 3.14.6 Application of Solar Cooling System A recent study funded by the European Union investigated the current and future potential use of the solar cooling systems in Europe It created a database of installed systems and was able to draw some key conclusions There is a significant potential stated for solar cooling technology and particularly in the