1. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Part I D. Yogi Goswami, Frank Kreith, Jan F. Kreider Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

2. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering  In some ways solar energy is better suited to space cooling and refrigeration than to space heating, but this application of solar energy has not found much commercial success.  The seasonal variation of solar energy is extremely well suited to the space-cooling requirements of buildings.  The principal factors affecting the temperature in a building are the average quantity of radiation received and the environmental air temperature.  Since the warmest seasons of the year correspond to periods of high insolation, solar energy is most available when comfort cooling is most needed.  The two principal methods of lowering air temperature for comfort cooling are refrigeration with actual removal of energy from the air or evaporation cooling of the air with adiabatic vaporization of moisture into it. Chapter 6: Solar Cooling and Dehumidification Solar Space Cooling and Refrigeration 6.1 Solar Space Cooling and Refrigeration

3. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering  Refrigeration systems can be used under any humidity condition of entering air, whereas evaporative cooling can be used only when the entering air has a comparatively low relative humidity. Chapter 6: Solar Cooling and Dehumidification Solar Space Cooling and Refrigeration  The most widely used air conditioning method employs a vapor-compression refrigeration cycle.  The vapor compression refrigeration cycle requires energy input into the compressor which may be provided as electricity from a photovoltaic system or as mechanical energy from a solar driven engine. Schematic diagram illustrating the basic refrigeration vapor-compression cycle.

4. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Another method uses an absorption refrigeration cycle similar to that of the gas refrigerator. Chapter 6: Solar Cooling and Dehumidification Solar Space Cooling and Refrigeration  In humid climates, removal of moisture from the air represents a major portion of the air conditioning load.  In such climates, desiccant systems can be used for dehumification, in which solar energy can provide most of the energy requirements. Schematic diagram of a solar-powered absorption refrigeration system.

5. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering  The cooling load of a building is the rate at which heat must be removed to maintain the air in a building at a given temperature and humidity.  It is usually calculated on the basis of the peak load expected during the cooling season.  For a given building the cooling load depends primarily on  Inside and outside dry-bulb temperatures and relative humidities,  Solar-radiation heat load and wind speed,  Infiltration and ventilation, and  Internal heat sources.  The steps in calculating the cooling load of a building are as follows.  Specify the building characteristics: wall area, type of construction, and surface characteristics etc.  Specify the outside and inside wet- and dry-bulb temperatures.  Specify the solar heat load and wind speed.  Calculate building cooling load resulting from the following: heat transfer through windows; heat transfer through walls etc. Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings

6. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering For unshaded or partially shaded windows, the load is: Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings For shaded windows, the load (neglecting sky diffuse and reflected radiation) is For unshaded walls, the load is For shaded walls, the load (neglecting sky diffuse and reflected radiation) is

7. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering For the roof, the load is Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings Sensible-cooling load due to infiltration and ventilation is Latent load due to infiltration and ventilation is

8. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings Example. Determine the cooling load for a building in Phoenix, AZ, with the specifications given in the table. Solution. To determine the cooling load for the building, calculate the following factors in the order listed. 1) Incidence angle for the south wall i at solar noon can be written as,

9. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings 3) South-facing window load, 4) Shaded-window load, 5) South-facing wall load,

10. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Cooling Requirements for Buildings 6) Shaded-wall load, 7) Roof load, 8) Latent-heat load (30 percent of sensible wall load), 9) Infiltration load, 10) Total cooling load for the building described in the example

11. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle The principle of operation of a vapor-compression refrigeration cycle can be illustrated conveniently with the aid of a pressure-enthalpy diagram as shown in the figure. Simple refrigeration cycle on pressure-enthalpy diagram Simple refrigeration cycle on pressure-enthalpy diagram Process I is a throttling process in which hot liquid refrigerant at the condensing pressure p c passes through the expansion valve, where its pressure is reduced to the evaporator pressure. p e‘. This is an isenthalpic (constant enthalpy) process, in which the temperature of the refrigerant decreases. Since the expansion process is isenthalpic, the following relation holds:

12. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle Process II represents the vaporization of the remaining liquid. This is the process during which heat is removed from the chiller. Thus, the specific refrigeration effect per kilogram of refrigerant q r is, Process III represents the compression of refrigerant from pressure p e to p c'. The process requires work input from an external source, which may be obtained from a solar-driven expander-turbine or a solar electrical system. The work of compression W c is,

13. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle The highest coefficient of performance for any given evaporator and condenser temperatures would be obtained if the system were operating on a reversible Carnot cycle. Under these conditions,

14. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle Example. Calculate the amount of shaft work to be supplied to a 1-ton (3.52-kW) refrigeration plant operation at evaporator and condenser temperatures of 273 K and 309 K, respectively, using Refrigerant 134a (R-134a) as the working fluid. The properties of Refrigerant 134a are tabulated in the table. Also calculate the COP and the mass flow rate of the refrigerant. Solution. From the property table the enthalpies for process I are, Properties of refrigerant 134a saturated vapor at 273K h ve = 247.2 kJ/kg, saturated liquid at 309K h Ic = 100.3 kJ/kg saturated liquid at 273K h Ie = 50.0 kJ/kg. Therefore,

15. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle The specific shaft-work input required is The entropy s e of the saturated vapor entering the compressor at 273 K and 292.8 kPa is 0.919 kJ/kg. K. From the property table, superheated vapor at a pressure of 911.7 kPa has an entropy of 0.919 kJ/kg.K at a temperature of 313 K with an enthalpy of 270.8 kJ/kg. Thus, the energy input to the working fluid by the compressor is,

16. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Vapor Compression Cycle Finally, the heat-transfer rate from the refrigerant to the sink, or cooling water in the condenser, The COP of the thermodynamic cycle is whereas the Carnot COP is 273/36 or 7.6.

17. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Compressor Power input

18. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

19. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

20. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Diagram of heat and fluid flow of absorption air conditioner, with economizer. Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Absorption air-conditioning is compatible with solar energy since a large fraction of the energy required is thermal energy at temperatures that currently available flat-plate collectors can provide. Absorption refrigeration differs from vapor-compression air- conditioning only in the method of compressing the refrigerant. In absorption air-conditioning systems, the pressurization is accomplished by first dissolving the refrigerant in a liquid (the absorbent) in the absorber section, then pumping the solution to a high pressure with an ordinary liquid pump.

21. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning  The low-boiling refrigerant is then driven from solution by the addition of heat in the generator.  The effective performance of an absorption cycle depends on the two materials that comprise the refrigerant-absorbent pair, The absence of a solid-phase absorbent. A refrigerant more volatile than the absorbent so that separation from the absorbent occurs easily in the generator. An absorbent that has a strong affinity for the refrigerant under conditions in which absorption takes place. A high degree of stability for long-term operations. Nontoxic and nonflammable fluids for residential applications. This requirement is less critical in industrial refrigeration. A refrigerant that has a large latent heat so that the circulation rate can be kept low. A low fluid viscosity that improves heat and mass transfer and reduces pumping power. Fluids that must not cause long term environmental effects.

22. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning  Lithium Bromide-Water (LiBr-H 2 O) and Ammonia-Water (NH 3 -H 2 O) are the two pairs that meet most of the requirements.  In the LiBr-H 2 0 system, water is the refrigerant and LiBr is the absorber, while in the Ammonia-Water system, ammonia is the refrigerant and water is the absorber.  LiBr has a tendency to crystallize when air cooled, and the system cannot be operated at or below the freezing point of water.  Therefore, the LiBr-H 2 0 system is operated at evaporator temperatures of 5°C or higher.  The ammonia-water system has the advantage that it can be operated down to very low temperatures.  However, for temperatures much below 0°C, water vapor must be removed from ammonia as much as possible to prevent ice crystals from forming.

23. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning If the pump work is neglected, the COP of an absorption air-conditioner can be calculated as, The COP values for absorption air-conditioning range from 0.5 for a small, single-stage unit to 0.85 for a double-stage, steam-fired unit. These values are about 15% of the COP values that can be achieved by a vapor- compression air-conditioner. It is difficult to compare the COP of an absorption air-conditioner with that of a vapor- compression air conditioner directly because the efficiency of electric power generation or transmission is not included in the COP of the vapor-compression air conditioning.

24. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Example: A water-lithium bromide, absorption refrigeration system is to be analyzed for the following requirements 1.The machine is to provide 352 kW of refrigeration with an evaporator temperature of 5°C, an absorber outlet temperature of 32°C, and a condenser temperature of 43°C. 2.The approach at the low-temperature end of the liquid heat exchanger is to be 6°C. 3.The generator is heated by a flat-plate solar collector capable of providing a temperature level of 90°C. Determine the COP, absorbent and refrigerant flow rates and heat input.

25. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Lithium bromide-water, absorption refrigeration cycle Solution: For analytical evaluation of the LiBr-H 2 O cycle, the following simplifying assumptions are made. 1.At those points in the cycle for which temperatures are specified, the refrigerant and absorbent phases are in equilibrium. 2.With the exception of pressure reductions across the expansion device between points 2 and 3, and 8 and 9, pressure reductions in the lines and heat exchangers are neglected. 3.Pressures at the evaporator and condenser are equal to the vapor pressure of the refrigerant, i.e., water, as found in steam tables. 4.Enthalpies for LiBr-Hp mixtures are given in the next figure

26. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Thermodynamic properties of refrigerant and absorbent Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Enthalpy-concentration diagram for Lithium-Water Bromide solutions. Enthalpy-concentration diagram for Lithium-Water Bromide solutions.

27. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning As a first step in solving the problem, set up a table (Table 6.3) of properties; for example, given Generator Temperature = 90°C = T 1 = T 7 Evaporator Temperature = 5°C = T 9 = T 10 Condenser Temperature = 43°C = T 8 Absorber Temperature = 32°C = T 4 Since the fluid at conditions 7, 8, 9 and 10 is pure water, the properties can be found from the steam tables. Therefore, P 7 = P 8 = Saturation pressure of H 2 0 at 43°C = 8.65 kPa, and P 9 = P 10 = Saturation pressure of H 2 0 at 5°C = 0.872 kPa.

28. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Therefore, P 1 = P 2 = P 5 = P 6 = P 7 = 8.65 kPa, and P 3 = P 4 = P 10 = 0.872 kPa, Enthalpy, h 9 = h 8 = 180 kJ/kg (Saturated liquid at 43°C), h 10 = 2510 kJ/kg (Saturated vapor enthalpy at 6°C), and h 7 = 2760 kJ/kg (Superheated vapor @ 8.65 kPa, 90°C), For the LiBr-H 2 O mixture, conditions 1 and 4 may be considered equilibrium saturation conditions T 4 = 32°C and P 4 = 0.872 kPa, Xr = 0.53, h 4 = 70 kJ/kg-Sol.

29. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Equilibrium chart for Lithium Bromide-Water solutions

30. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Therefore, h 5 = 70 kJ/kg-Sol. And for T 1 = 90°C and P 1 = 8.65 kPa, X ab = 0.605, h 1 = 215 kJ/kg - Sol., for T 3 = 38°C, X 3 = 0.605, h 3 = 110 kJ/kg - Sol., h 2 = h 3 = 110 kJ/kg Relative flow rates for the absorbent (LiBr) and the refrigerant (H 2 0) are obtained from material balances. A total material balance on the generator gives,

31. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning while a LiBr balance gives, Concentration of LiBr in absorbent of solution Concentration of LiBr in refrigerant-absorbent of solution

32. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning Substituting for X s and X ab from the table gives the ratio of absorbent-to-refrigerant flow rate: T 6 and h 6 which may be found from an energy balance at the heat exchanger. Hence,

33. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning The temperature corresponding to this value of enthalpy and a LiBr mass fraction 0.53 is found from Lithium Bromide-Water solutions equilibrium chart to be 74°C. The flow rate of refrigerant required to produce the desired 352 kW of refrigeration is The flow rate of the absorbent is while the flow rate of the solution is

34. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning This requirement, which determines the size of the solar collector, probably represents the maximum heat load that the collector unit must supply during the hottest part of the day. The coefficient of performance COP is The rate of heat transfer in the other three heat-exchanger units-the liquid heat exchanger, the water condenser and the absorber-is obtained from heat balances. For the liquid heat exchanger this gives,

35. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Chapter 6: Solar Cooling and Dehumidification Absorption Air-Conditioning The rate of heat removal from the absorber can be calculated from an overall heat balance on this system:  Explicit procedures for the mechanical and thermal design as well as the sizing of the heat exchangers are presented in standard heat-transfer texts.  In large commercial units, it may be possible to use higher concentrations of LiBr, operate at a higher absorber temperature, and thus save on heat-exchanger cost.  In a solar-driven unit, this approach would require concentrator-type or high efficiency flat-plate solar collectors.

36. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering Desiccant Air Conditioning

37. Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering

38. D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar Engineering