Solar Cooling Technology Review Curt Robbins Desert Research Institute This paper provides a brief summary of current solar cooling technologies based on a literature review of 16 journal articles. The REDD facility at DRI currently uses two methods of cooling: an absorption chiller driven by solar thermal, and an evaporative cooler which can be run off of solar photovoltaic. This literature search is to identify what are the best technologies to convert solar energy into space cooling. Air conditioning is an attractive area for solar energy use as it is estimated that 45% of household energy consumption is used for cooling. Furthermore, 10-20% of all electricity produced is consumed for refrigeration and air conditioning. 1 Solar energy is a prime target as an energy source as peak radiation levels typically coincide with peak refrigeration and air conditioning demands. Solar cooling can be broken down into two main categories, electrical and solar thermal. Electrical solar cooling systems compose of electron producing solar technologies that drive conventional air condition methods, the most common being the vapor compression cycle. The evaporative cooler in the REDD workshop falls under the electrical category. Solar thermal based solar cooling systems fall under two categories, open cycle systems, and closed cycle systems. Closed-cycle systems consist of several technologies, the most common being absorption cooling systems. It is estimated that 59% of the solar cooling systems in Europe use absorption cooling.2 Other systems include adsorption cooling, ejector, and solar assisted heat pumps. Open-cycle systems provide direct treatment of air, such as desiccant cooling. Also falling under this category are passive systems such as stack-effect ventilation (SEV), or solar chimney’s.3 Absorption cooling is by far the most widely used and widely publicized, with many variations with increased performance. Second to absorption cooling, is adsorption cooling. Absorption Cooling- The thermodynamic process of a single-effect absorption chiller can be broken down into three basic stages: evaporation, absorption, and regeneration. The main components of an absorption cooling system are the absorber, pump, expansion valve, regenerator, and generator. Systems can be made more efficient by adding a stage to increase heat transfer efficiency; known as double-effect. The common refrigerant-absorbent used are ammonia/water, and LiBr/water. A singleeffect chiller has a COP of 0.6 for NH3/H2O and 0.7 for LiBr/H2O; while a double-effect chiller can have COP’s ranging from 0.8 to 1.2. 4 Absorption cooling has the advantages of a relatively high COP for solar thermal applications, as well as mature technology with several commercially available products. A disadvantage of these systems is that they are complicated, with moving parts. The process requires several different working fluids, including a cooling tower to dissipate heat. Adsorption Cooling- This process consists of pressurization heating, desorption at constant condenser pressure, depressurization cooling, and adsorption at constant evaporator pressure. Solar adsorption can be divided into two categories, physisorption and Chemisorption. 1 Both categories utilize a surface phenomenon where gas molecules are attracted to the adsorbent surface by van der Waals force (physisorption) or by valence forces (chemisorption) and are then released through the application of

heat. These systems have many advantages to absorption cooling, as shown below; however, their disadvantages have proven difficult to overcome. Advantages    

Low heat application: can be driven by temperatures as low as 50°C 1 High storage capacity and energy densification No need for moving parts, or a cooling system Can withstand harsh environments

Disadvantages    

Requires high vacuum pressure Large volume and weight of system: requires cold storage Low COP: poor heat and mass transfer Two or more units are required to maintain continuous cooling

Other Technologies- Another technology for solar cooling is known as desiccant cooling in which heat is absorbed during an evaporation process. A diagram is shown in Figure 1.4 This process utilizes the same difference between wet bulb and dry bulb temperatures as evaporative cooling. The effectiveness of these systems limited by a low COP of 0.3 and is dependent on weather. Systems in dry climates have demonstrated energy savings of 80%. Another technology available is known as ejector refrigeration which operates with a heat source above 80°C, by replacing the compressor in a vaporrefrigeration cycle with a boiler, pump, and ejector. 4 Lastly, Omojaro et al. reported on the use of direct expansion solar assisted heat pumps for cooling applications. They showed a COP of 2.2-6.2 with a system that adds an expansion valve, and evaporator before the working fluid is pumped back into the solar collector. In their literature search, they only found 1% of literature reporting on the use of such a system for cooling. 5 Figure 1: Desiccant cooling system4

Technology Comparison A critical operating point for a solar cooling system is the working fluid temperature required to drive the process. A graph showing the typical COP for different technologies based on the heat source temperature is shown in Figure 2. Although several technologies can produce a cooling effect in the same temperature range, the performance can be affected by small temperature changes. For example, Choudhury et. al. noted that for a Yazaki WFC-600 absorption chiller (similar unit to the REDD’s), the cooling capacity was cut in half with hot water temperature change from 88°C to 80°C. 6 Furthermore, the produced working fluid

temperature is highly dependent on the type of solar collector used in the system; both flatplate collectors and evacuated tube collectors are reported in literature. Comparisons can be made between solar cooling technologies based on performance, life-cycle, and economics. Beccali, et al. performed a life-cycle analysis on a LiBr absorption chiller that uses evacuated tube collectors as a heat source. They ran TRNSYS Figure 2: COP Comparison6 simulations for Zurich, Switzerland and Palermo, Italy to compare energy and environmental impacts. This system was compared to a system with a conventional vapor compression unit. In all situations tested, the Global Energy Requirement (GER) and Global Warming Potential (GWP) were reduced by 25% to 50%. 7 Similarly, Otanicar, et al. performed an economic and environmental assessment comparing a solar photovoltaic vapor compression system to solar thermal systems. The focus of this assessment was to determine which technology will be economically feasible in the future. In terms of performance, anticipated increases in system efficiency is shown in Figure 3. This graph infers that the greatest gains will be achieved in adsorption cooling systems, followed by desiccant systems. Furthermore, they anticipate that system costs will reduce the most in the future with photovoltaic system. Total system costs for comparable sized systems can be seen in Figure 4a, 4b, Figure 3: Anticipated Efficiency Increases8 and 4c.

Figure 4a: Projected Absorption Cooling System Costs8

Figure 4b: Projected Adsorption and Desiccant Cooling System Costs8

According to their study, the IEA projects a drop is system cost of 35% to 45% for solar thermal cooling systems, however, a greater reduction in a photovoltaic system will lead to a lower total system cost for a photovoltaic vapor compression system compared to a solar thermal cooling system. 8 The reported use of different types of solar thermal collectors complicates economic calculations. Figure 4c: Projected PV/Vapor compression Cooling System Costs8

Conclusion

The following reported research areas will be essential in order to enhance the development of solar cooling systems in order to compete with conventional cooling methods:      

Adsorbent materials for adsorption cooling technologies Hybrid systems: adsorption with desiccant, adsorption with ejector triple-effect absorption chillers Efficient PV cells to drive vapor compression Environmentally safe refrigerants Efficient solar thermal collectors

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Cabrera, F. J, Fernandez-Garcia, A, Silva, R. M. P, and Perez-Garcia, M. Use of parabolic trough solar collectors for solar refrigeration and air-conditioning applications. Renewable and Sustainable Energy Reviews 20(2013), 103-118. 11-19-2012. Gomri, R. Simulation study on the performance of solar/natural gas absorption cooling chillers. Energy Conversion and Management 65(2013), 675-681. 4-17-2012.