Prospects for solar cooling – An economic and environmental assessment

Available online at www.sciencedirect.com

Solar Energy 86 (2012) 1287–1299 www.elsevier.com/locate/solener

Prospects for solar cooling – An economic and environmental assessment Todd Otanicar a,⇑, Robert A. Taylor b, Patrick E. Phelan c b

a Department of Mechanical Engineering, Loyola Marymount University, Los Angeles, CA 90045, United States School for Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia c School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, United States

Received 25 August 2011; received in revised form 16 November 2011; accepted 21 January 2012 Available online 17 February 2012 Communicated by: Associate Editor Yogi Goswami

Abstract Producing refrigeration and/or air conditioning from solar energy remains an inviting prospect, given that a typical building’s cooling load peaks within 2 or 3 h of the time of maximum solar irradiation. The attractiveness of “free” cooling obtained from the sun has spawned a wealth of research over the last several decades, as summarized in a number of review articles. Obstacles—especially high initial costs—remain to the widespread commercialization of solar cooling technologies. It is not clear at the present time if thermally driven systems will prove to be more competitive than electrically driven systems. We therefore describe a technical and economic comparison of existing solar cooling approaches, including both thermally and electrically driven. We compare the initial costs of each technology, including projections about future costs of solar electric and solar thermal systems. Additionally we include estimates of the environmental impacts of the key components in each solar cooling system presented. One measure of particular importance for social acceptance of solar cooling technologies is the required “footprint,” or collector area, necessary for a given cooling capacity. We conclude with recommendations for future research and development to stimulate broader acceptance of solar cooling. The projections made show that solar electric cooling will require the lowest capital investment in 2030 due to the high COPs of vapor compression refrigeration and strong cost reduction targets for PV technology. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Solar cooling; Photovoltaic; Solar thermal; Absorption chiller

1. Introduction Using sunlight to produce cooling is a long-sought goal. Intuitively, the need for cooling is proportional to the solar intensity, thus nearly matching the time of peak cooling demand with the time of maximum sunlight. Given this close coincidence between resource and need, it is no wonder then that considerable effort has been devoted to producing economical solar cooling technologies. These can be divided into roughly two approaches—heat-activated systems which rely on solar thermal energy, such as an ⇑ Corresponding author. Tel.: +1 310 338 3872.

E-mail address: [email protected] (T. Otanicar). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2012.01.020

absorption refrigeration cycle that is driven primarily by heat input, and work-activated systems like the conventional vapor-compression cycle which requires compressor work input that is normally electrically powered. The question remains as to which approach is more practical, i.e., more economical? The answer to this question depends in part on the scale of the system. Here, we restrict our analysis to a size suitable for a typical single-family residence. In short, our work attempts to provide an answer to this question: for a given climatic zone (here we will assume the southwestern USA), for a typical residence (5 ton, or 17.5 kW of cooling), is it better to use a solar thermal cooling system, or one driven by solar photovoltaic (PV) panels? We base our analysis entirely on the initial cost of

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the system. We believe this is appropriate since solar systems essentially require the user to come up with lifetime system costs up front while fuel/electricity costs are usually negligible. That is, operation and (ideally) maintenance costs of a solar cooling system are low when compared to the initial capital cost. As such, this analysis considers performance and initial costs for a number of different types of solar cooling technologies. Comparisons for both current costs as well as projected future costs are presented. It is first instructive to provide a brief review of solar cooling technologies. We do not attempt here an extensive technical review, as that has been admirably done in a number of publications (Balaras, 2007; Best and Ortega, 1999; Desideri et al., 2009; Florides et al., 2002; Fong et al., 2010; Gordon and Ng, 2000; Grossman, 2002; Hang et al., 2011; Henning, 2007; Hwang et al., 2008; Kim and Ferreira, 2008; Klein and Reindl, 2005; Papadopoulos et al., 2003; Pesaran and Neymark, 1995; Zhai and Wang, 2009) – especially in comparison of the various solar thermal systems (Anyanwu, 2004a,b; Critoph, 1988; Halliday et al., 2002; Srikhirin, 2001; Wang et al., 2009). Rather, we focus on a strictly economic comparison (first cost) between solar–thermal-driven and solar–PV-driven air conditioning technologies, again at the residential scale. The first type of solar cooling technology considered is PV-driven air conditioning. This type of system uses a conventional vapor compression air conditioning cycle in which the electrical input is provided by solar PV panels. This is compared to the following selected solar–thermal cooling technologies: the first type that we consider is based on a solid desiccant, in which solar heat is used to regenerate the desiccant after it has absorbed water from an incoming air stream. Water is sprayed into the dehumidified air stream, thus lowering its temperature and providing a cooling effect. The second type of thermally driven system is absorption cooling, in which the refrigerant vapor is absorbed into a liquid, thus allowing its pressure to be economically increased by a pump, rather than by a vapor compressor that requires much more mechanical input. As described below, we consider both NH3/H2O and H2O/aqueous LiBr types of absorption cycles. The third, and final, type of thermally driven system is the adsorption cycle, where the refrigerant vapor is adsorbed onto the surface of a solid adsorbent, which when heated desorbs the vapor and thus pressurizes the vessel in which the vapor is contained. This, in effect, creates a “thermal compressor” that replaces a conventional electrically driven compressor. In all cases, means for storing energy—thermal storage for the thermally driven systems, and electrical storage for the PV-driven system—are included in the analysis. More details on the refrigeration cycles and storage systems are provided below. Most previous studies of solar air conditioning tended to focus on just thermally driven technologies, and did not provide a rigorous comparison between thermal and PV systems. There are, however, some notable exceptions. The study that is the closest in intent to the present work is

also the oldest report that we have discovered, as it was published in 1983 (Ayyash and Sartawi, 1983). It was found, upon consideration of both the initial and operating costs that a PV-assisted vapor-compression system could be cost-competitive with an absorption system driven by solar thermal energy. Much later, Klein and Reindl (2005) concluded that only PV-driven cooling would be viable for providing sub-zero (freezing) solar refrigeration, compared with an NH3/H2O absorption system, and a second thermal system in which solar heat powers a Rankine cycle that in turn provides mechanical input to a vaporcompression cycle. In a somewhat similar manner, Casals (2006) compared local (decentralized) solar absorption cooling with cooling provided by centralized solar thermal power plants, which generate electricity that is distributed to conventional vapor compression units at the point of use. No clear conclusions were reached after a fairly rigorous evaluation of cost and other variables. Kim and Ferreira (2008) reported a comprehensive study of several solar thermal and solar PV cooling systems, based on both technical and economic considerations. Their conclusion was that solar thermal cooling, in particular a single-effect H2O/aqueous LiBr absorption system, followed next by H2O/silica gel adsorption and double-effect H2O/aqueous LiBr absorption systems, are more competitive than the other solar cooling technologies, including PV-driven systems. Finally, an extensive evaluation of solar cooling technologies coupled with building cooling demand for Hong Kong (Fong et al., 2010) reported that solar PV-driven systems had the greatest potential to deliver the highest annual energy savings, compared with a number of solar thermal technologies. Cost, however, did not seem to be considered in this analysis. In summary, relatively few studies have undertaken a technical and, perhaps more importantly, an economic comparison between solar thermal and solar PV cooling systems. The results to date are mixed, motivating our interest to conduct further analysis.

2. Economic analysis The economic analysis focuses on current and projected costs for the equipment associated with the proposed solar powered cooling schemes outlined in Figs. 1 and 2. It is not expected that the maintenance costs over the life of the systems will be significantly different enough to alter the results and are not considered in the resulting analysis. As mentioned above, the amount of cooling to be provided by these systems is 5 tons (17.58 kW). Thus, all systems are normalized by the same amount of cooling, but will have different solar collector area and storage capacity requirements to deliver the necessary energy. The solar irradiance is assumed to be a peak value of 1000 W/m2. This ideal condition is chosen because solar cooling will be most likely sited in high flux locations, this is the ASTM standard flux value, and using a constant flux value such as

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Fig. 1. Schematic of potential solar photovoltaic cooling system.

Fig. 2. Schematic of potential solar thermal cooling system.

this will make comparison between various technologies straight forward. In order to achieve a stable cooling solution we also include a storage system capable of providing up to 8 h of energy storage. Since the heart of the analysis is to determine the initial (capital) cost variation, the output throughout the year for each system is not considered, but readers are referred to the detailed performance analysis in Fong et al. (2010). 2.1. Solar photovoltaic systems As shown in Fig. 1 the solar electric system is comprised of four major components: PV modules, inverter, battery and vapor compression cooling system. For each of these four components it is necessary to determine the current efficiency or coefficient of performance (COP) and to project these values into 2030. Since research and development into photovoltaic technology has been widely supported by a variety of government research agencies, many efficiency forecasts and projections are available. Based on the International Energy Agency projections for single crystal silicon PV modules, efficiency values of 17%, 19% and 21%

are used for years 2010, 2020 and 2030 respectively (Solar Photovoltaic Roadmap, 2010). The inverters used in most PV systems have already achieved high levels of efficiency (90% by 2010) with projections forecasting efficiencies at levels of 95–98% by 2020–2030 (Navigant, 2006). The projections for battery efficiency reveal relatively stable levels of efficiency for PV systems at 80%. Projections for new technologies being developed for large-scale energy storage suggest 80% efficiency is a reasonable expectation (U.S.C.T. Program, 2005). These three technologies comprise the power input side of the solar cooling system, while the vapor compression refrigeration unit is the actual cooling system. Regulations on new installations of air conditioners require COP values of at least 3 ( Energy Savers, 2010) while systems with COPs nearing 6 are readily commercially available, albeit at greater expense (Infinity Series Central Air Conditioner, 2010). These efficiencies form the basis for calculating the necessary system sizes given the desired cooling demand and input solar irradiance. To determine the size of the PV collector, inverter, and battery bank, the efficiencies referenced are used in combination with the peak solar flux and cooling load. The required electrical power output from the PV system is determined

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only from the cooling demand, COP, storage efficiency and inverter efficiency: qcool ð1Þ QPV ¼ COP gst ginv where qcool is the ideal cooling energy, ginv is the inverter efficiency, and gst the storage efficiency. Again the above equation is only used to determine the output power from the PV system; the sizing of the PV system itself is completely tied to the PV system efficiency. The footprint of the system (i.e., the collector area APV), another important aspect of system adoption, can be found with the following: qcool ð2Þ APV ¼ COP gst ginv gpv gsun where gPV is the PV efficiency and qsun is the input solar irradiance. In a similar method the amount of power into the inverter required to meet the cooling load can be found as follows: qcool qinv ¼ ð3Þ COP ginv The amount of energy stored can be found based on the assumption that 8 h of cooling capacity will be stored: Qst ¼

tst qcool COP gst ginv

ð4Þ

where tst is the storage time. These three parameters form the basis for calculating the cost associated for each component of the proposed solar cooling system. The associated costs for each component are based on estimates from a variety of references that compile average prices as well as generate forecasts of projected prices. Because of this, an approach was taken that looks at the ceiling and floor estimates of the projections to determine ranges of potential overall costs. A challenging aspect of compiling the economic projections are the wide variety of data sources as well as determining what components (for example PV projections often are for the full system including inverter) are included. This is an additional reason for using a ceiling-floor approach. For the PV module, not including the inverter, the following values are taken as the prices for the noted year (the ceiling prices are in parentheses): 2010 – $4.8/W ($5.35/W), 2020 – $1.81/W ($2.45/ W), and 2030 – $1.25/W ($1.9/W) (Solar Photovoltaic Roadmap, 2010; Projection of PV System Prices, 2004; Itron, 2007). Inverter prices are drawn from a variety of projections based on current prices as well as from the target price goals set forth by the United States Department of Energy, and are assumed as the following: 2010 – $0.65/W ($0.85/W), 2020 – $0.25/W ($0.58/W), and 2030 – $0.2/W ($0.55/W) (Navigant, 2006; Itron, 2007). Due to the relatively stable and widespread use of electrical energy storage costs for battery storage are projected to remain constant at $150/kW h (Ton et al., 2008). The resulting component costs for current and future years of solar PV cooling are displayed in Fig. 3. As displayed,

the largest decrease in cost comes from the PV module itself while the decrease in the cost of the inverter has less impact. The cost of the 5-ton vapor compression refrigeration system with a COP of 3 is considered to have a present and future unit cost of $3501 (EnergyStar, Savings Calculator, 2010) while it is assumed that a system with a COP of 6 will be three times the cost (EnergyStar, Savings Calculator, 2010). 2.2. Solar thermal systems The main promise of using thermal systems is that they can utilize more of the incoming sunlight than photovoltaic systems. Fig. 4 shows, roughly, what happens to the solar spectrum when it strikes a conventional PV collector. One can see that much of the incoming solar power is converted to heat and cannot be used to generate electricity in a PV system. Since the purpose of a thermal collector is to convert light into heat (which is rather easy to do) thermal collectors have no such limitation. Depending on the absorbing medium, a thermal system can absorb over 95% of the incoming radiation (Duffie and Beckman, 2006). Of course, not all of this is converted to useful energy due to inefficiencies/losses along the way. Nonetheless, collection efficiencies for commercial solar thermal collectors are generally more than double that of crystalline photovoltaic solar collectors (Choudhury et al., 2010; Joshi et al., 2009). In general, a solar thermal cooling system consists of four basic components as shown in Fig. 2: a solar collector array, a thermal storage tank, a thermal air conditioning unit, and a heat exchange system to transfer energy between components and the conditioned space. In each component category there are several options. At present, no particular combination of these components has proven dominant, but most have been built and tested over the years (Anyanwu, 2004a,b; Critoph, 1988; Halliday et al., 2002; Srikhirin, 2001; Wang et al., 2009). Options for the first component, the solar array, can vary significantly in complexity. These options can be roughly categorized as the following: flat plate, evacuated tube, and concentrating collectors. In order to choose between these, one must define the temperatures needed to run the thermal A/C system. In most cases this is between 60 and 100 °C – i.e., falling into the medium temperature class of collectors. Thus, selected flat plate collectors, evacuated tube collectors, and concentrating collectors of low concentration are all technically viable options. Due to the added complexity of tracking, most concentrating collectors are expected to be too expensive as an input for residential solar cooling systems. It should be noted that the efficiency for any thermal collectors goes down as the temperature difference between the working fluid and the ambient is increased. Looking at this fact the other way, the efficiency of thermal collectors improves as the ambient temperature is increased. This is opposite to how PV modules respond to ambient temperature changes.

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Fig. 3. Current and projected component and storage costs of solar electric technologies (blue-inverter component cost, red-PV collector component cost, black-battery storage cost). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Solar spectrum used in a PV system. Energy distribution from (Duffie and Beckman, 2006).

To find common ground between different designs, we will assume solar thermal collection efficiency can be approximated by the following general equation (Duffie and Beckman, 2006): gtc ¼ ao  a1

Tm  Ta ðT m  T a Þ2  a2 qsun qsun

ð5Þ

That is, we assume thermal collector efficiency, gtc, is a function of the difference between the mean temperature, Tm, in the solar collector and the ambient temperature, Ta. It is also a function of the solar irradiance, qsun, and constants a0, a1, a2. These constants account for different geometries and collector types. If constants a1 and a2 are large, collector efficiency will drop off quickly at high operating temperature. For our analysis we chose the following

constants to represent selected commercial evacuated tube and flat plate collectors, respectively: a0 = 0.39 and 0.69, a1 = 0.83 and 3.39, a2 = 4.7  103 and 1.9  103. The efficiency of the thermal collector is expected to increase over time. To estimate future efficiencies we have extended the trends of historic efficiency improvement (assuming a logarithmic shaped curve). This projection is shown in Fig. 5 – data were collected from AET (2011), Apricus (2011), and EIA (2010). Thus, solar thermal collection efficiencies are expected to stay in the range of 20–40% between now and 2030. Note: this range is valid for the outlet temperatures that are needed to run a thermal A/C system. Options for the second component, the thermal storage tank, mainly involve the type of storage medium and the temperatures desired. Because of its low environmental impact and high specific heat, we will limit our analysis to using water. Since most thermal A/C systems have COPs less than unity, we will assume that we need a storage system which can store cold. (Note: If COP is greater than unity, however, hot storage would be a more efficient system design choice – akin to storing electricity as discussed above.) That is, all things being equal, cold storage will require a smaller volume tank than hot storage for low COP systems. For example, if 300 kW h of thermal energy is put into an A/C system with a COP of 0.7, it will pull 210 kW h energy out of a cold storage tank. This means a significant reduction in tank size, assuming the storage medium has the same approximate storage capacity per unit volume at those temperatures. In this analysis we will consider sensible chilled water storage and ice (water) storage for the cold storage. Of course, if ice storage is used, the COP of the thermal A/ C system is decreased because the system is forced to oper-

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Fig. 5. Current and projected efficiencies for the thermal A/C systems and medium temperature solar thermal collectors.

ate at a lower temperature. This is accounted for in our analysis by de-rating the system efficiency by a fraction of the Carnot COP at the given temperature. The following equation is used to do this:   COP ice

¼ COP normal 

T ice T a T ice

T normal T a T normal



ð6Þ

where the subscripts ice, normal, and a represent ice conditions (0 °C), normal cold-side operating conditions (5– 25 °C), and the absorber temperature input conditions (60–85 °C), respectively. This equation was derived from the Carnot efficiency of absorption refrigeration systems (Carmargo et al., 2003). One assumption made in using this equation is that we are in a linear region where (all things being equal) changing the cold side temperature proportionally changes the COP. Another important parameter is the efficiency of storage. For this analysis, we will conservatively assume that the round trip efficiency of the storage system is 90%. This means that over the course of a day, one can expect to get back 90% of the energy that was put into storage. In order to meet demand overnight, we will need to assume that the storage tank is big enough to store 8 h worth of cooling. Options for the thermal A/C component are the real focus of this study. As such, our analysis will include the following potential cooling options: Desiccant, absorption using lithium bromide (LiBr), absorption using ammonia (NH3), and adsorption cooling. If we take a simple, toplevel view of these systems, the main differences between them are found in their overall COP and their necessary fluid input temperature. In general, the COP is defined as the following: q COP ¼ cool ð7Þ qth

where qcool is the heat removed from the conditioned space and qth is the heat input to the thermal A/C system. In this analysis all the systems will provide the same amount of cooling (5 tons) to be directly comparable to each other and to the PV system discussed above. Briefly, each solar-powered A/C unit operates as follows. 2.2.1. Desiccant A desiccant system is usually an open cycle where two wheels turn in tandem – a desiccant wheel containing a material which can effectively absorb water, and a thermal wheel which heats and cools inward and outward flows. Warm, humid, outside air enters the desiccant wheel where it is dried by the desiccant material. Next, it goes to the thermal wheel which pre-cools this dry, warm air. Next, the air is cooled further by being re-humidified. When leaving, cool, conditioned air is humidified to saturation and is used to cool off the thermal wheel. After the thermal wheel, the now warm humid air is heated further by solar heat in the regenerator. Lastly, this hot air passes through the desiccant wheel so that it can dry the desiccant material on its way out of the cycle. 2.2.2. Absorption Both absorption cycles that we are using in this study work in a similar manner. The main difference between them is which substances are used as the refrigerant and absorbent. In an LiBr system, LiBr is the absorbent and water is the refrigerant. In an NH3 absorption system, water is now the absorbent and NH3 is the refrigerant. In both cases, the job of the compressor (in a conventional vapor compression system) is replaced by an absorber and a generator. Concentrated absorbent enters the absorber, which is connected to the evaporator. When

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refrigerant is boiled off in the evaporator (removing heat from the conditioned space), vapor (of relatively high pressure) then moves to the LiBr/water absorber where it is absorbed. Next, the mixture moves to the generator where solar heat is supplied to boil off the refrigerant. High-pressure refrigerant vapor then travels to the condenser where heat is rejected to the surroundings to condense the refrigerant back to liquid. Liquid refrigerant goes back into the evaporator, where it can be used again to take in heat from the conditioned space, which completes the loop. 2.2.3. Adsorption In this cycle, solar heat is directed to a sealed container containing solid adsorbent saturated with refrigerant. Once this reaches the proper temperature/pressure the refrigerant desorbs and leaves this container as pressurized vapor. That is, the vapor has been compressed with thermal energy. This vapor then travels to a condenser where it turns to liquid by rejecting heat to the surroundings. Expanded, low-pressure liquid refrigerant then flows over the evaporator which pulls heat from the conditioned space to boil off the refrigerant. The refrigerant vapor can then be adsorbed again by the cool adsorbent material easily at night. Thus, the diurnal adsorption cooling cycle is complete. Note that diurnal cycles are convenient, but not necessary. As mentioned above, the most important parameter in this analysis is the COP. Based on historic improvements, the COP is likely to increase over time. Our assumptions for thermal A/C unit COP improvements to 2030 (assumed to follow logarithmic curves) are shown in Fig. 5. Data for historic trends come from Balaras (2007), Fong et al. (2010), Carmargo et al. (2003), Florides et al. (2002), Harrison and Sasaki (1978), Henning et al. (2001), Pita (1991), Robur (2011), Sozen (2001), Tchernev (1979), Wang et al. (2010), and Yazaki (2011). Overall, desiccant cooling systems currently have the highest COP and are projected to keep that advantage, while adsorption system COP is improving the fastest. Absorption system COP is found between the adsorption and desiccant systems with relatively minor differences projected between LiBr and NH3 into 2030, according to our assumptions. Options for the final component, the heat exchange systems, are numerous as well. Many researchers and companies have developed heat exchangers which can be optimized for almost any application. Since we intend to exchange heat between two liquids we will simply pick a good parallel-plate, counter-flow heat exchanger with an effectiveness of 0.9 (Geankoplis, 2003). In our analysis we considered the thermal collectors, the A/C unit, and the cost of thermal storage to be the main contributors to the thermal cooling capital cost. Thus, the heat exchange system cost is assumed to be accounted for in the other components. It is important, however, to include a heat exchanger effectiveness since it increases the size and cost of the thermal collector array and the thermal storage system to make up for heat exchange losses.

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A similar ceiling-floor approach is used to determine current and future costs of each system component. The results of this approach are shown in Fig. 6. Note that these prices are deceptively low since they are normalized on a per thermal W (Wth) or kW h (kW hth). That is, it will take many more Watts or kWatt–hrs to achieve the same cooling effect, since thermal A/C units run at a much lower COP. For the thermal collectors, the following values are taken as floor (and ceiling) prices for the given year: 2010 – $0.83/Wth ($0.84/Wth), 2020 – $0.68/Wth ($0.89/Wth), and 2030 – $0.50/Wth ($0.95/Wth) (EIA, 2010). Thermal storage prices are estimated to currently be $1585/m3 ($6/gallon) (Hot Water Tank Price, 2010; Garday and Housley, 2007; Zalba, 2003) and we estimate this price will fall at 1% per year to 2030. The major difference between our estimated ceiling and floor price is in sensible versus latent thermal storage. That is, the ceiling price reflects storage using chilled water as sensible heat (with a temperature change of 10 °C) and the floor price is for latent heat in ice (water) storage. This makes a very large difference since more energy can be stored per cubic meter with latent storage – 11.7 kW hth/m3 as compared to 85.1 kW hth/m3. Thus, the price floor (and ceiling) prices for storage are the following: 2010 – $25.15/kW hth ($129.23/kW hth), 2020- $22.75/kW hth ($116.94/kW hth), and 2030 – $20.59/kW hth ($105.81/kW hth) . Thermal A/ C system (5-ton) costs are difficult to estimate, since with the exception of some absorption systems, not many commercial systems are on the market. Since cost trends in that area are hard to predict with little historic data to draw from, we will conservatively assume the unit will have a constant price over time. The following costs were assumed for each 5-ton unit: a LiBr absorption unit – $1.14/Wth of cooling (Yazaki, 2011), a NH3 absorption unit – $0.28/Wth of cooling (Robur, 2011), an adsorption unit – $1.14/Wth of cooling (Wang et al., 2010), and a desiccant unit – $1.42/Wth of cooling (Carmargo et al., 2003). A summary of the range of component efficiencies and costs for both the solar thermal and solar electric cooling systems can be found in Table 1. 3. Environmental impact Although the economic cost represents one of the major obstacles preventing widespread adoption of solar cooling systems more and more emphasis will be placed on the environmental impact of future refrigeration systems. The environmental impact of utilizing solar energy as a means to offset fossil fuel usage has seen widespread investigation (see e.g., Alsema et al., 2006; Ardente et al., 2005; Fthenakis and Alsema, 2005; Kalogirou, 2004) while only limited studies have assessed the impact of the cooling technologies themselves (Florides et al., 2002; Heikkila, 2004). In order to provide a more broad approach to the environmental impact of solar cooling, four categories are investigated for their respective impact on carbon dioxide (CO2) emissions. These categories address the following impacts: life-

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Fig. 6. Current and projected component and storage costs of solar thermal technologies (blue-thermal A/C component cost, red-solar thermal collector component cost, black lines-storage costs). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

time (20 years) impact of solar collection, the lifetime (20 years) impact of the storage system, the direct impact of the refrigerants used, and lastly, the indirect effects from any backup power supplied to the chiller. For the PV systems the indirect effect is comprised of the CO2 emissions related to the creation of electricity, set at 784 g CO2/ kW h of electricity used, while the thermal systems all assume natural gas backup at 170 g CO2/kW h of natural gas energy consumed (EPA, 2010). The environmental cost associated with utilizing PV cells to produce electricity has been projected to have anywhere from 25 to 35 g CO2/ kW h of electricity produced over the lifetime of the cell (Alsema et al., 2006; Fthenakis and Alsema, 2005), while solar thermal collectors are projected to have embodied energy requirements resulting in 12 g CO2/kW h of thermal energy provided over the lifetime (this assumes a 20year life, 70% efficiency, and 6 kW h/m2/day of irradiance) (Ardente et al., 2005; Kalogirou, 2004). The associated costs of storage can be found in a similar fashion with thermal storage having an embodied energy resulting in 66– 77 g CO2/kW hth of stored thermal energy (Ardente et al., 2005) while lead-acid batteries result in 24 g CO2/kW h of stored electrical energy (Rydh, 1999). As can be seen all of the numbers are normalized based on the energy output of the component while it is useful to normalize the results in terms of the cooling provided based on the component outputs (input energy to refrigeration system). This normalization, which accounts for the cooling system COP, is shown below: g  CO2 g  CO2 1 ¼  kW hcooling kW hinput COP

ð8Þ

In addition to the component environmental impacts the impact of any used refrigerant on potential global warming can be assessed based on the global warming potential (GWP) of the refrigerant. Based on the results of Florides et al. (2002) the impact of using R-22 in a 5-ton refrigeration system can be estimated at 18 g CO2/kW h of cooling provided. As R-22 is phased out of use due to the large ozone impact other refrigerants may scale this number based on the GWP (Bovca et al., 2007) of the refrigerant. Likely replacements such as R-410A are noted to have lower ozone impact but similar GWP and will not have a drastic result on the projected equivalent carbon dioxide release over the life of the system (Bovca et al., 2007). The thermal systems that use refrigerants, absorption and adsorption, have the advantage of not having any associated GWP (Bovca et al., 2007), while the use of a desiccant system would not be expected to lead to any GWP either. It should be noted that the embodied energy of the mechanical equipment associated with each of the different refrigeration systems is assumed to be equal. Thus, we have not factored it into any of the forthcoming discussions. 4. Results and discussion The results of the cost projections for solar electric cooling are shown in Fig. 7. Two major observations can be made from the projections. First, as the projected price of the solar module goes down, as outlined in Fig. 3, the cooling system cost decreases in a similar fashion. Second, the COP of the vapor compression system has a drastic effect on the overall system cost for current PV prices. However, this is less important as PV system prices

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Table 1 Component efficiency and cost ranges. System/component

Efficiency/COP range

Reference

Cost range

References

Photovoltaic cell

11–21%

Pesaran and Neymark (1995)

$1.25–4.80/W

Solar thermal collector

26–38%

$217–317/m2

Inverter Battery Thermal storage Vapor compression cooling Absorption cooling (LiBr)

90–98% 80% 90% 3–6

U.S.C.T. Program (2005), Energy Savers (2010) and Infinity Series Central Air Conditioner (2010) Zhai and Wang (2009) Anyanwu (2004a) – Anyanwu (2004b) and Critoph (1988)

Pesaran and Neymark (1995); Halliday et al. (2002); Srikhirin (2001) Anyanwu (2004a)

$0.20–0.75/W $150/kW h $21–135/kW h $3501–10,503

Zhai and Wang (2009); Srikhirin (2001) Wang et al. (2009) EIA (2010); Carmargo et al. (2003) Ayyash and Sartawi (1983)

$20,000

Ton et al. (2008)

Absorption cooling (NH3)

0.57–0.62

$5000

EnergyStar, Savings Calculator (2010)

Adsorption cooling

0.40–0.61

$20,000

Joshi et al. (2009)

Desiccant/evaporative cooling

1.06–1.22

$25,000

AET (2011)

0.76–0.83

Gordon and Ng (2000), Kim and Ferreira (2008), Itron (2007), Ton et al. (2008) and Duffie and Beckman (2006) Gordon and Ng (2000), Kim and Ferreira (2008), Itron (2007), EnergyStar, Savings Calculator (2010), Duffie and Beckman (2006) and Choudhury et al. (2010) Gordon and Ng (2000) and Joshi et al. (2009) Gordon and Ng (2000) and AET (2011)

decrease-shown by the merging of the two COP-based cost projections. That is, as the cost of the PV system decreases the difference between a high performance and an average vapor compression system is minimized. As a percentage of the overall system cost the PV component represents greater than 69% and 52% of the total cost for 2010 for a system with a COP of 3 and 6 respectively. In 2030 these percentages are reduced to 40% and 23% for a system with a COP of 3 and 6 respectively. This demonstrates that as the PV cost is reduced the remaining components begin to have larger impacts, especially for systems with lower COP values. Fig. 8A and B shows the costs for the solar thermal cooling technologies. The cost reductions over time are in the

Fig. 7. Current and projected cooling system costs for solar electric cooling.

neighborhood of those expected by the International Energy Agency. The IEA projects a drop of 35–45% reduction in total system cost for solar thermal cooling by 2030 (IEA, 2007). These projected reductions are much lower than those for PV cooling. This is likely due to the fact that the market is much smaller for the components in a thermal cooling system. Almost 1 GW of solar PV has been installed in the United States, whereas, at most 20 MWth of thermal collectors are installed (depending on efficiency numbers) (EIA, 2010). Further, conventional vapor compression units compete in a large, mature residential market, where thermal A/C units are rarely found. Fig. 8A shows the cost projections for absorption-based technologies and reveals very minor differences in the overall costs of cooling. It should be noted that NH3 absorption systems are of lower cost simply because the A/C unit price is much lower $5000 compared to $20,000 for a LiBr system. Fig. 8B compares the cooling costs of adsorption and desiccant-based systems. The adsorption system has the highest projected and current costs mainly due to the low COP, while desiccant-based systems are the most affordable, due to the highest value of COP for the thermal systems. Comparing the costs of cooling for solar electric and thermal systems reveals some important considerations regarding solar cooling. For solar electric cooling the COP of the system has a large impact on the system cost due to the large impact on the PV system cost. Additionally decreasing cost projections for PV systems lead to large cost reduction potential whereas little cost reduction is forecast in the solar thermal technologies leading to relatively flat cost projections. In terms of overall cost it appears that solar–thermal-based cooling systems, particularly ammonia absorption and desiccant-based systems are

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Fig. 8. Current and projected cooling system costs for solar thermal cooling: (A) absorption systems (LiBr darker shade) and (B) desiccant and adsorption systems.

currently competitive with solar electric systems using high performance vapor-compression systems. By 2030 the costs of solar electric cooling will decrease to levels at or below that of solar thermal cooling for both COP level vapor compression systems. That is, given our assumptions, we predict that solar PV-powered cooling will become more cost effective than thermal-based systems going forward. While the difference in cost of the proposed systems represents an important factor in the decision of installing a solar cooling system it is also important to consider the collector footprint or area necessary. Fig. 9 shows the projected areas for the discussed systems with the PV systems typically requiring half of the area as a thermal based system. Again this is almost entirely due to the COP of the system. The electric systems all have high COPs resulting in lower collector energy requirements than a thermal system of the same load. Some of the disadvantage in COP of the thermal systems is made up in the higher collection efficiency of solar thermal collectors in comparison to a PV system. This can especially be noted in the desiccant based system with a COP near 1 requiring nearly triple the energy input of COP = 3 solar electric system but needing only about 1.6 times the footprint. In addition to the importance of the economic cost the environmental impact of the technologies needs to also be considered. One metric that can be used is the amount of carbon dioxide associated with each system. Table 2 presents the results for the total CO2 impact for each proposed solar cooling system analyzed. The results of Table 2 show the associated amount of CO2 needed for the production and operation of the solar collection, storage, and refrigeration system (refrigerant impact only) based on the amount of cooling energy provided. This shows total CO2 release

Fig. 9. Footprint of solar collector to meet 5-ton cooling load.

T. Otanicar et al. / Solar Energy 86 (2012) 1287–1299

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Table 2 Environmental impacts of proposed solar cooling components. Storage (g CO2/ kW h cooling)

Direct effect (g CO2 eq/ kW h cooling)

Indirect Effect (g CO2/ kW h cooling)

8

18

261

6

4

18

131

0.76

16

87

0

263

0.57

21

116

0

351

0.62

19

106

0

323

0.45

27

147

0

444

System/component

COP

Collector (PV includes inverter) (g CO2 eq/kW h cooling)

PV-electric (vapor compression with battery) PV-electric (vapor compression with battery) Flat plate thermal (Li Br absorption) Evacuated tube thermal (NH3 absorption) Flat plate thermal (adsorption) Flat plate thermal (desiccant/evaporative)

3

12

6

per kW hth of cooling for solar electric cooling is less than the other systems analyzed, even when the global warming impact of the refrigerant is included. This is mainly due to the high values of COP associated with vapor-compression refrigeration resulting in smaller collectors and energy storage mechanisms. The impact of COP is also reflected in the comparison of the thermal systems since the highest COP systems (absorption) result in the lowest values of CO2 impact. Additionally the indirect CO2 impact, resulting from emissions if backup energy is needed, was found by looking at the amount of CO2 released from electricity or natural gas consumption. It should be noted that the proposed system would have enough storage capacity to not require backup during normal operation but if periods of low daytime irradiance occurred the resulting backup energy to create cooling is captured by the indirect effect. Although natural gas results in lower values of CO2 for a given amount of energy in comparison to electricity, the lower COP of the thermal-based cooling systems results in larger indirect effects. The impact of CO2 is only one metric for evaluating the environmental impact and it should be noted that each technology has additional issues resulting in a variety of environmental concerns. 5. Conclusions The results of the economic and environmental analysis of a variety of solar cooling schemes reveal some key details regarding system choice. For solar electric cooling the system cost is highly dependent on the system COP when PV prices remain at the current levels, but when prices are lowered the impact of COP becomes diminished. For solar thermal cooling the cost of solar collection is much lower as a percentage of the overall cost, but the cost of the refrigeration system often represents a larger percentage of the cost. Additionally the costs for solar thermal cooling are not projected to decrease as much as PV cooling over the next 20 years due to the relatively stable cost of collection and storage. If the costs of refrigeration were to

come down as well as thermal refrigeration COP increases, especially to values greater than 1, it could be expected that solar thermal cooling costs would be competitive with solar electric cooling costs. From an environmental standpoint solar electric cooling, even with the associated impact of refrigerants with global warming impact, have a lower projected emission value of carbon dioxide per kW hth of cooling than any of the thermal technologies. The major reason for this is due to the much larger COP values associated with solar electric cooling. One additional favorable aspect to solar electric cooling systems is the collector area footprint. For solar PV systems, expected sizes in 2010 are between 24 and 48 m2 – depending on system COP. On the other hand, solar thermal system footprints are expected to be between 78 and 106 m2. Thus, if solar thermal systems are to be cost-competitive by 2030, COP improvements and/or thermal collector costs will need to see some considerably favorable shift(s) beyond current trends. Acknowledgment PEP gratefully acknowledges partial support provided by Arizona Public Service, Inc. References AET, 2011. Flat Plate Solar Thermal Collector Technical Information. Alternative Energy Technology. . Alsema, E., de Wild-Scholten, M.J., Fthenakis, V.M., 2006. Environmental impacts of PV electricity generation – a critical comparison of energy supply options. In: 21st European Photovoltaic Solar Energy Conference, Dresden, Germany. Anyanwu, E., 2004a. Review of solid adsorption solar refrigerator I: an overview of the refrigeration cycle. Energy Conversion and Management 44, 301–312. Anyanwu, E., 2004b. Review of solid adsorption solar refrigerator II: an overview of the principles and theory. Energy Conversion and Management 45, 1279–1295. Apricus, 2011. Solar Hot Water Evacuated Tube Collector Technical Information. .

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