10.5 Use the airs properties and conditions provided in the case studies on pages 157-160 in the textbook, write the required equations, and re-calculate case 2 only at 1000cfm flow rate: a) the cooling power, cooling load; b) COP from outside air, and COP room load.

4.1.8 Energy balance of sorption-supported air-conditioning 4.1.8.1 Usable cooling power of open sorption Sorption-supported air-conditioning systems are driven with pure fresh air. The cooling capacity 0. is therefore calculated from the enthalpy difference between the outside air status h, and the supply air status hin. The removable cooling load from the room 9 , however, is given by the enthalpy difference between supply air hin and space exhaust air hr, with the space exhaust air temperature usually several Kelvin under the outside temperature. What proportion of the cooling capacity produced is usable depends in particular on the required dehumidifying performance as well as on the necessary fresh air flow rate, which must in every case be cooled from the outside air status, even in conventional air-conditioning systems. 9. = pv (h. hn) = pv ((c. +x, c,)T. + x, h.-((, +x,,C,) 7. + x,, h.)) (4.51) Q = PV (h. - hn) = PV ((+x,(,)T, +x, h.-((c + x,, C, )T + x, h.)) (4.52) For the three most important applications of sorption-supported air-conditioning the cooling power can be determined by Equation (4.51) and the removed cooling load from the room by Equation (4.52). CASES: 1. Pure cooling of the outside air with minimum dehumidifying to 11.5 g/kg. 2. Cooling of the outside air to 16C supply air temperature with dehumidifying to 8.5 g/kg. 3. Pure dehumidifying of the room air to 8.5 g/kg without additional cooling, i.e. the supply air temperature equals 26C. Solar cooling 157 The enthalpy of the outside air remains constant at 62 kJ/kg here, at design criteria of 32C and 40% relative humidity (12 g/kg). The enthalpy of the space exhaust air is 26C, with 55% relative humidity (11.5 g/kg) at 54.9 kJ/kg. Case 1-pure cooling with minimum dehumidifying: With the inlet air humidity specification of 11.5 g/kg, at 95% humidification and good heat recovery efficiencies of 80% a minimum supply air temperature of 17C is possible. The enthalpy difference between the outside air and supply air is h. - h. = 62 45.7 = 16.3 [kJ/kg] From this a cooling capacity for 1000 m/h flow rate results: pV (h. - hm)=1.18 kg/mx1000 m/3600s x16.3x10? J/kg = 5343 W With a flow rate of 1000 m/h, however, only a sensible cooling load of 3 kW can be removed. PV (h. - hn)=1.19 kg/mx1000 m3/3600sx (54.945.7) kJ/kg = 3041 W Thus if only a sensible cooling load is to be removed, without a fresh air requirement existing, the sorption system must produce 1.8 times more cold than is needed as cooling output; an energetically unfavourable application. Case 2-Cooling with dehumidification: If humidity loads of the space must be removed (here for example 3 g/kg from 11.5 g/kg, to 8.5 g/kg), the energy expenditure for air-conditioning clearly becomes higher. At such high air-drying performance of the sorption wheel, the supply air temperature must now be limited to a minimum value, since the usual 95% humidification would produce supply air temperatures far below 16C. The enthalpy difference rises to h. - h., n = 62 37.3 = 24.7 [kJ/kg] and thus the cooling capacity to 8.1 kW per 1000 m/h flow rate. The cooling load of the space now consists of sensible heat and latent heat of dehumidifying, and the enthalpy difference is h, hn = 54.937.3 = 17.6 [kJ/kg] The total cooling power is still 1.4 times higher than the cooling load removal from the room of 5.8 kW per 1000 m3/h, so here too a high fresh air requirement offers a favourable initial position for open sorption cooling. 158 Solar technologies for buildings Case 3-pure dehumidifying: To dehumidify the room air to 8.5 g/kg, i.e. by 3 g/kg, without cooling, the outside air at 32C and 12 g/kg must be dehumidified by 3.5 g/kg. h. - h, = 62 47.3= 14.7 [kJ/kg] The necessary cooling performance per 1000 m/h volumetric air flow is 4.9 kW. If dehumidifying were carried out with recirculating air, i.e. not fresh air but space exhaust air, the enthalpy difference would be reduced to h, - h., = 54.947.3 = 7.6 [kJ/kg] The removable cooling load is around 2.5 kW. The different energy expenditures on the basis of the constant outside air status are summarised in Table 4.3. The outside air status is given with 32C, 40% relative humidity and an enthalpy of 62 kJ/kg. Table 4.3: Total cooling power and cooling load removal of open sorption-supported air-conditioning with different applications. Case Inlet air Enthalpy Enthalpy Cooling power Enthalpy Removable status: inlet air difference per 1000 m3/h difference cooling load temperature (kJ/kg) outside air to (kW) room exhaust and absolute inlet air air to inlet air m3/h (kW) humidity (kJ/kg) (kJ/kg) 1 17C 45.7 16.3 9.2 3.0 11.5 g/kg 2 16C 37.3 24.7 8.1 17.6 5.8 8.5 g/kg 3 26C 47.3 14.7 4.9 7.6 2.5 8.5 g/kg per 1000 1 5.3 An optimal field of deployment for sorption-supported air-conditioning is found in applications with a high fresh air requirement. At high space-cooling loads with dehumidifying needs but little fresh air requirement, a combination of sorption systems with closed cycle coolers is suitable for separating dehumidifying from load removal. From the ratio of cooling power to regeneration heat, the energy performance figures of open sorption can be determined in what follows. 4.1.8.2 Coefficients of performance and primary energy consumption To make available a kilowatt-hour of cold, compression refrigerators with a coefficient of performance (COP) of 3 require a total of 0.33 kWh of electricity. The coefficient of performance is generally defined as the ratio of produced cooling power to the supplied power, either electrical power or heat. Solar cooling 159 COP= cooling (4.53) O supply At an average primary energy conversion efficiency Ncon for electricity production of 35%, 0.95 kWh of primary energy must is used for 0.33 kWh of electricity. The primary energy efficiency npe as a ratio of the produced cooling energy to the supplied primary energy results from the product of COP and the conversion efficiency of the respective energy carrier. O cooling = COPxn.com O primary energy (4.54) In electrical compression coolers, Ype is around 3.0 x 0.35 = 1.05. If compression coolers are operated in a full air-conditioning system, reheating is often necessary after dehumidifying due to the low dew-point temperature, and the mean primary energy efficiency falls to 0.6, i.e. for a kWh of cold, 1.7 kWh of primary energy are used. With sorption-supported air-conditioning systems, both thermal energy for regeneration and electricity for fans and auxiliary aggregates such as humidifier pumps and wheel drives must be supplied. First the purely thermal COP should be considered, i.e. the ratio of cooling power produced to the necessary regeneration heat. cooling h. -h outside / exhaust "supply (4.55) COP = Dregeneration hreg - h. after HX / outside If total cooling power from outside air is considered, the enthalpy difference between outside and supply air (houtside h supply) must be selected. If only room loads are removed, the difference between room exhaust air and supply air (hexhaust hsupply) is to be used. The regeneration power in the denominator is calculated as the enthalpy difference between entry into the (solar) regeneration air heater and exit from the heater with enthalpy hreg. For closed exhaust air systems, the entry air into the heater is the exit air after humidification and the heat exchanger (hux); for open exhaust systems, outside air is used with enthalpy houtside. The enthalpy after heating depends on the regeneration temperature necessary for the respective application. As an example, the respective performance figures for the three applications of open sorption can be calculated, for closed exhaust air systems. As a boundary condition, an outside air status of 32C and 40% relative humidity is selected. The specification is the desired supply air status (cooling with or without drying); the regeneration temperature is calculated as a function of the air to be supplied. 160 Solar technologies for buildings Table 4.4: Performance figures of open sorption-supported air-conditioning with a closed exhaust air system. Case Temperature Temperature Enthalpy Enthalpy and humidity regeneration regeneration exhaust supply air [C] air [C] [kJ/kg] [kJ/kg] after HX Enthalpy COP increase from regeneration outside [kJ/kg] air [-] 17.1 0.93 COP room load [-] 1 53.3 88 70.9 0.53 N 95.1 129.7 79.2 50.5 0.48 0.35 17C 11.5 g/kg 16C 8.5 g/kg 26C 8.5 g/kg 3 3 48.3 83.1 69.3 13.8 1.05 0.55 With a falling regeneration temperature, less dehumidifying occurs and the necessary amount of heat falls. At very low outside air humidities, air-conditioning can take place by energy-neutral evaporative cooling alone, so the thermal performance figure becomes infinite in extreme cases without dehumidifying. However, there is a low limit to the regeneration air temperature dependent on external humidity. For example, at an air status of 32C and 40%, the performance figure (COP thermal) does rise with a falling regeneration temperature. If the regeneration temperature falls below 52C, there is so little dehumidification that in the supply air humidifier humidifying to 95% can no longer take place (due to the maximum admissible room air humidity). This results in an unadmissible rise in the supply air temperatures. 1.6 1.4 22 1.2 1.0 0.8 18 supply temperature [C] COP COP (-) supply air temperature 0.6 0.4 0.2 0.0 100 10 40 50 90 60 70 80 regeneration temperature [C] Figure 4.16: Supply air temperatures and cooling performance figures (COP) as a function of the regeneration air temperature at constant outside air statuses of 32C and 40% relative humidity. Although energetically very interesting, pure evaporative cooling is limited to dry outside air statuses and is only possible for a limited number of hours of operation. By using thermal solar energy, however, the regeneration heat can likewise be produced primary-energy neutrally at full sorption operation. Solar cooling 161 For a total energy balance, the additional pressure losses through the sorption wheel, heat recovery device and humidifier, and the associated electrical power increase, must be considered. At a typical flow velocity of 3 m/s in the sorption system, pressure losses of about 150-200 Pa result in the sorption wheel and heat recovery device respectively, and in the humidifier between 100 and 250 Pa, depending on the design. The total of supply-side and exhaust side pressure losses is between 800 and 1300 Pa. For a 100 m air collector field as a regeneration air heater, pressure losses of about 250 Pa can be expected. From the total pressure losses Ap the electrical power Pel of the fans is calculated as a function of the fan efficiency n. At an efficiency of a large fan of 70%, the result is thus an electrical power demand of 417615W per 1000 m/h of volumetric air flow, with total pressure losses between 1050 and 1550 Pa. In addition there are about 100 W per 1000 m/h for electric drives of the components (circulation pumps, wheel drive etc.). V Ap 1000m3/3600s x1050 Pa n 0.7 417W (4.56) Altogether, therefore, the result is connected electrical loads of some 500700 W per 1000 m/h of flow rate, i.e. about 1.42 kW primary energy requirement. Thus a cooling capacity of between 4.98.1 kW can be produced, depending on the application, i.e. the electrical primary energy efficiency is between 2.45.8. This value contains the pressure losses both for the heat recovery and the humidification function. These must also be considered during conventional cooling by compression refrigerant plants as part of a full air-conditioning system. If the heat is supplied either primary energy-neutrally by solar energy or waste heat is used, the desiccant cooling process is primary-energetically clearly superior to electrical compression refrigerant plants. 4.1.8 Energy balance of sorption-supported air-conditioning 4.1.8.1 Usable cooling power of open sorption Sorption-supported air-conditioning systems are driven with pure fresh air. The cooling capacity 0. is therefore calculated from the enthalpy difference between the outside air status h, and the supply air status hin. The removable cooling load from the room 9 , however, is given by the enthalpy difference between supply air hin and space exhaust air hr, with the space exhaust air temperature usually several Kelvin under the outside temperature. What proportion of the cooling capacity produced is usable depends in particular on the required dehumidifying performance as well as on the necessary fresh air flow rate, which must in every case be cooled from the outside air status, even in conventional air-conditioning systems. 9. = pv (h. hn) = pv ((c. +x, c,)T. + x, h.-((, +x,,C,) 7. + x,, h.)) (4.51) Q = PV (h. - hn) = PV ((+x,(,)T, +x, h.-((c + x,, C, )T + x, h.)) (4.52) For the three most important applications of sorption-supported air-conditioning the cooling power can be determined by Equation (4.51) and the removed cooling load from the room by Equation (4.52). CASES: 1. Pure cooling of the outside air with minimum dehumidifying to 11.5 g/kg. 2. Cooling of the outside air to 16C supply air temperature with dehumidifying to 8.5 g/kg. 3. Pure dehumidifying of the room air to 8.5 g/kg without additional cooling, i.e. the supply air temperature equals 26C. Solar cooling 157 The enthalpy of the outside air remains constant at 62 kJ/kg here, at design criteria of 32C and 40% relative humidity (12 g/kg). The enthalpy of the space exhaust air is 26C, with 55% relative humidity (11.5 g/kg) at 54.9 kJ/kg. Case 1-pure cooling with minimum dehumidifying: With the inlet air humidity specification of 11.5 g/kg, at 95% humidification and good heat recovery efficiencies of 80% a minimum supply air temperature of 17C is possible. The enthalpy difference between the outside air and supply air is h. - h. = 62 45.7 = 16.3 [kJ/kg] From this a cooling capacity for 1000 m/h flow rate results: pV (h. - hm)=1.18 kg/mx1000 m/3600s x16.3x10? J/kg = 5343 W With a flow rate of 1000 m/h, however, only a sensible cooling load of 3 kW can be removed. PV (h. - hn)=1.19 kg/mx1000 m3/3600sx (54.945.7) kJ/kg = 3041 W Thus if only a sensible cooling load is to be removed, without a fresh air requirement existing, the sorption system must produce 1.8 times more cold than is needed as cooling output; an energetically unfavourable application. Case 2-Cooling with dehumidification: If humidity loads of the space must be removed (here for example 3 g/kg from 11.5 g/kg, to 8.5 g/kg), the energy expenditure for air-conditioning clearly becomes higher. At such high air-drying performance of the sorption wheel, the supply air temperature must now be limited to a minimum value, since the usual 95% humidification would produce supply air temperatures far below 16C. The enthalpy difference rises to h. - h., n = 62 37.3 = 24.7 [kJ/kg] and thus the cooling capacity to 8.1 kW per 1000 m/h flow rate. The cooling load of the space now consists of sensible heat and latent heat of dehumidifying, and the enthalpy difference is h, hn = 54.937.3 = 17.6 [kJ/kg] The total cooling power is still 1.4 times higher than the cooling load removal from the room of 5.8 kW per 1000 m3/h, so here too a high fresh air requirement offers a favourable initial position for open sorption cooling. 158 Solar technologies for buildings Case 3-pure dehumidifying: To dehumidify the room air to 8.5 g/kg, i.e. by 3 g/kg, without cooling, the outside air at 32C and 12 g/kg must be dehumidified by 3.5 g/kg. h. - h, = 62 47.3= 14.7 [kJ/kg] The necessary cooling performance per 1000 m/h volumetric air flow is 4.9 kW. If dehumidifying were carried out with recirculating air, i.e. not fresh air but space exhaust air, the enthalpy difference would be reduced to h, - h., = 54.947.3 = 7.6 [kJ/kg] The removable cooling load is around 2.5 kW. The different energy expenditures on the basis of the constant outside air status are summarised in Table 4.3. The outside air status is given with 32C, 40% relative humidity and an enthalpy of 62 kJ/kg. Table 4.3: Total cooling power and cooling load removal of open sorption-supported air-conditioning with different applications. Case Inlet air Enthalpy Enthalpy Cooling power Enthalpy Removable status: inlet air difference per 1000 m3/h difference cooling load temperature (kJ/kg) outside air to (kW) room exhaust and absolute inlet air air to inlet air m3/h (kW) humidity (kJ/kg) (kJ/kg) 1 17C 45.7 16.3 9.2 3.0 11.5 g/kg 2 16C 37.3 24.7 8.1 17.6 5.8 8.5 g/kg 3 26C 47.3 14.7 4.9 7.6 2.5 8.5 g/kg per 1000 1 5.3 An optimal field of deployment for sorption-supported air-conditioning is found in applications with a high fresh air requirement. At high space-cooling loads with dehumidifying needs but little fresh air requirement, a combination of sorption systems with closed cycle coolers is suitable for separating dehumidifying from load removal. From the ratio of cooling power to regeneration heat, the energy performance figures of open sorption can be determined in what follows. 4.1.8.2 Coefficients of performance and primary energy consumption To make available a kilowatt-hour of cold, compression refrigerators with a coefficient of performance (COP) of 3 require a total of 0.33 kWh of electricity. The coefficient of performance is generally defined as the ratio of produced cooling power to the supplied power, either electrical power or heat. Solar cooling 159 COP= cooling (4.53) O supply At an average primary energy conversion efficiency Ncon for electricity production of 35%, 0.95 kWh of primary energy must is used for 0.33 kWh of electricity. The primary energy efficiency npe as a ratio of the produced cooling energy to the supplied primary energy results from the product of COP and the conversion efficiency of the respective energy carrier. O cooling = COPxn.com O primary energy (4.54) In electrical compression coolers, Ype is around 3.0 x 0.35 = 1.05. If compression coolers are operated in a full air-conditioning system, reheating is often necessary after dehumidifying due to the low dew-point temperature, and the mean primary energy efficiency falls to 0.6, i.e. for a kWh of cold, 1.7 kWh of primary energy are used. With sorption-supported air-conditioning systems, both thermal energy for regeneration and electricity for fans and auxiliary aggregates such as humidifier pumps and wheel drives must be supplied. First the purely thermal COP should be considered, i.e. the ratio of cooling power produced to the necessary regeneration heat. cooling h. -h outside / exhaust "supply (4.55) COP = Dregeneration hreg - h. after HX / outside If total cooling power from outside air is considered, the enthalpy difference between outside and supply air (houtside h supply) must be selected. If only room loads are removed, the difference between room exhaust air and supply air (hexhaust hsupply) is to be used. The regeneration power in the denominator is calculated as the enthalpy difference between entry into the (solar) regeneration air heater and exit from the heater with enthalpy hreg. For closed exhaust air systems, the entry air into the heater is the exit air after humidification and the heat exchanger (hux); for open exhaust systems, outside air is used with enthalpy houtside. The enthalpy after heating depends on the regeneration temperature necessary for the respective application. As an example, the respective performance figures for the three applications of open sorption can be calculated, for closed exhaust air systems. As a boundary condition, an outside air status of 32C and 40% relative humidity is selected. The specification is the desired supply air status (cooling with or without drying); the regeneration temperature is calculated as a function of the air to be supplied. 160 Solar technologies for buildings Table 4.4: Performance figures of open sorption-supported air-conditioning with a closed exhaust air system. Case Temperature Temperature Enthalpy Enthalpy and humidity regeneration regeneration exhaust supply air [C] air [C] [kJ/kg] [kJ/kg] after HX Enthalpy COP increase from regeneration outside [kJ/kg] air [-] 17.1 0.93 COP room load [-] 1 53.3 88 70.9 0.53 N 95.1 129.7 79.2 50.5 0.48 0.35 17C 11.5 g/kg 16C 8.5 g/kg 26C 8.5 g/kg 3 3 48.3 83.1 69.3 13.8 1.05 0.55 With a falling regeneration temperature, less dehumidifying occurs and the necessary amount of heat falls. At very low outside air humidities, air-conditioning can take place by energy-neutral evaporative cooling alone, so the thermal performance figure becomes infinite in extreme cases without dehumidifying. However, there is a low limit to the regeneration air temperature dependent on external humidity. For example, at an air status of 32C and 40%, the performance figure (COP thermal) does rise with a falling regeneration temperature. If the regeneration temperature falls below 52C, there is so little dehumidification that in the supply air humidifier humidifying to 95% can no longer take place (due to the maximum admissible room air humidity). This results in an unadmissible rise in the supply air temperatures. 1.6 1.4 22 1.2 1.0 0.8 18 supply temperature [C] COP COP (-) supply air temperature 0.6 0.4 0.2 0.0 100 10 40 50 90 60 70 80 regeneration temperature [C] Figure 4.16: Supply air temperatures and cooling performance figures (COP) as a function of the regeneration air temperature at constant outside air statuses of 32C and 40% relative humidity. Although energetically very interesting, pure evaporative cooling is limited to dry outside air statuses and is only possible for a limited number of hours of operation. By using thermal solar energy, however, the regeneration heat can likewise be produced primary-energy neutrally at full sorption operation. Solar cooling 161 For a total energy balance, the additional pressure losses through the sorption wheel, heat recovery device and humidifier, and the associated electrical power increase, must be considered. At a typical flow velocity of 3 m/s in the sorption system, pressure losses of about 150-200 Pa result in the sorption wheel and heat recovery device respectively, and in the humidifier between 100 and 250 Pa, depending on the design. The total of supply-side and exhaust side pressure losses is between 800 and 1300 Pa. For a 100 m air collector field as a regeneration air heater, pressure losses of about 250 Pa can be expected. From the total pressure losses Ap the electrical power Pel of the fans is calculated as a function of the fan efficiency n. At an efficiency of a large fan of 70%, the result is thus an electrical power demand of 417615W per 1000 m/h of volumetric air flow, with total pressure losses between 1050 and 1550 Pa. In addition there are about 100 W per 1000 m/h for electric drives of the components (circulation pumps, wheel drive etc.). V Ap 1000m3/3600s x1050 Pa n 0.7 417W (4.56) Altogether, therefore, the result is connected electrical loads of some 500700 W per 1000 m/h of flow rate, i.e. about 1.42 kW primary energy requirement. Thus a cooling capacity of between 4.98.1 kW can be produced, depending on the application, i.e. the electrical primary energy efficiency is between 2.45.8. This value contains the pressure losses both for the heat recovery and the humidification function. These must also be considered during conventional cooling by compression refrigerant plants as part of a full air-conditioning system. If the heat is supplied either primary energy-neutrally by solar energy or waste heat is used, the desiccant cooling process is primary-energetically clearly superior to electrical compression refrigerant plants