SOLAR HEATING AND COOLING SYSTE WITH LOW TEPERATURE LATENT HEAT STORAGE C. Keil, S. Hiebler, H. Köbel,. Helm, H. ehling, C. Schweigler Bavarian Center for Applied Energy Research, Walter-eißner-Str. 6, D-85748 Garching, Germany Fax. 89/329442-12; schweigler@muc.zae-bayern.de ABSTRACT Heat activated chillers are integrated into solar thermal systems in order to provide solar heating and cooling. During the cold season solar heat serves for space heating. During the warm season solar heat is converted into useful cooling by utilizing the solar heat as driving heat for the sorption cooling device, simultaneously avoiding overheating of the solar thermal system. A favourable situation is given when radiative heating and cooling facilities. e.g. floor or wall heating systems or activated ceilings, are applied for transferring heating and cooling to the room at moderate heating and cooling temperatures, respectively. A low-temperature latent heat storage based on phase-change-material can be applied to cope with the given system requirements: During heating operation the latent heat storage balances heat generation by the solar system and other heat sources and the supply to the consumer. Thus a low operating temperature of the solar thermal system is accomplished yielding efficient operation with optimum solar gain. In cooling mode the latent heat storage serves as a reject heat sink for the absorption chiller in addition to a dry cooling system. By that means heat rejection of the chiller is shifted to periods with lower ambient temperatures, i.e. night time or off-peak hours, and thus the necessity of a wet cooling tower is eliminated allowing for a substantial reduction in operational effort, water consumption, and cost. A discussion of the system concept and of the thermal design of the latent heat storage is given. Experimental results of a functional model of the heat storage and operational experience of a first full-scale demonstration of the solar heating and cooling system incorporating a recently developed compact water/lithiumbromide absorption chiller are presented. 1. INTRODUCTION Absorption cooling systems based on water/lithium bromide (LiBr) solution typically require an open wet cooling tower to transfer the heat rejected by absorber and condenser to the ambient. The necessity of a wet cooling tower and the dimension and cost of the solar thermal system in standard installations are major issues concerning the application of this energy-saving technique. The application of a latent heat storage supporting the heat rejection of the absorption chiller in addition to a dry cooling system allows eliminating the wet cooling tower and thus yields a substantial reduction in operational effort, water consumption, and cost (Schweigler et al. 25). 2. SYSTE CONCEPT In conventional absorption cooling installations, wet cooling towers designed for coolant supply/return temperature 27/35 C are applied. When a dry air-cooler is to be used, cooling water temperatures have to be increased to 4/45 C. As a consequence of the increase of the cooling water temperature, the temperature level of the driving heat supplied to the regenerator of the absorption chiller has to be increased accordingly. This principal dependency of the required driving temperatures on the coolant temperatures of an absorption
chiller can easily be estimated from the linear characteristic-equation model (Furukawa. 1983, Hellmann et al. 1998, Storkenmaier et al. 1999). For chilled water 18/15 C, starting from a chiller design with 8/75 C the required driving hot water temperature rises with increasing cooling water temperature to 9/85 C and further to 15/1 C when a dry air cooler together with a latent heat storage or solely a dry air cooler is applied, respectively. Analogously, for chilled water temperature 12/6 C the required driving hot water temperature increases from 9/85 C to 1/95 C for the system with dry air cooler assisted by a latent heat storage. For this chilled water layout, i.e. standard temperature 12/6 C, complete dry air-cooling is not feasible. The external heat carrier parameters for the discussed system configurations governed by linear correlation according to the characteristic-equation model - are given in Figure 1. Driving Heat Temp. 15/1 C 221/212 F 1/95 C 212/23 F infeasible Chilled Water 12/6 C 53.6/42.8 F Chilled Water 18/15 C 64.4/59 F 8/75 C 176/167 F 27/35 C 8.6/95 F Wet Cooling Tower 32/4 C 89.6/14 F PC + Dry Air-Cooler 4/45 C 14/113 F Dry Air-Cooler Reject Heat Temp. Figure 1: Impact of chilled water temperature and reject heat temperature on driving heat temperature By integration of a heat storage into the heat rejection system of the absorption chiller, a part of the reject heat of the chiller can be buffered during the operation of the solar cooling system, allowing for lower coolant temperatures during peak load operation of the chiller. The stored reject heat then can be discharged during off-peak operation or night time when more favourable ambient conditions, i.e. lower ambient temperatures, are available. As discussed above, as a consequence of the limited coolant temperature arising form the integration of the latent heat storage, lower temperatures of the driving solar heat are required to operate the absorption chiller. Thus a higher solar gain is obtained for a given size of the solar collector system. A detailed analysis of the overall system design has been presented earlier (Schweigler et al. 27). During heating operation, the latent heat storage balances the heat generation by the solar system and other heat sources and the heat supply to the consumer. Thus, a low operating temperature of the solar thermal system is accomplished yielding efficient operation with optimum solar gain. A simplified system structure for cooling (top) and heating operation (bottom) is given in Figure 2.
CHILLER Absorptionswärmepumpe bzw. Kältemaschine 15 C 59 F E G A/C 4 C 32 C 14 F 89.6 F 9 C 194 F 85 C 185 F Wärme- Speicher AUX. BOILER Heizkessel 18 C 64.4 F Solarkollektoranlage Luftwärme- 32 C tauscher 89.6 F DRY AIR COOLER 4 C 14 F SOLAR SYSTE 36 C PC- 96.8 F Speicher HEATING / COOLING SYSTE NT- Kühl-/Heizsystem LATENT HEAT STORAGE 32 C 89.6 F Absorptionswärmepumpe bzw. Kältemaschine Wärme- Speicher AUX. BOILER Heizkessel 4 C 14 F 4 C 14 F PC- Speicher 4 C 14 F NT- Kühl-/Heizsystem HEATING / COOLING SYSTE 3 C 86 F Solarkollektor- SOLAR anlage SYSTE LATENT HEAT STORAGE Luftwärmetauscher 35 C 95 F 32 C 89.6 F Figure 2: System structure of a solar heating and cooling system with absorption chiller (with main components evaporator E, absorber/condenser A/C and generator G) and latent heat storage in cooling mode (top) and heating mode (bottom). 3. PILOT INSTALLATION Within the framework of the German Solarthermie 2plus Programme a pilot installation of the innovative solar heating and cooling system has been erected. The installation will simultaneously serve as field test project for a recently developed compact water/libr absorption chiller with 1 kw nominal capacity (Schweigler et al. 22, Storkenmaier et al. 23, Kühn et al. 25). The chiller has been designed for chilled water supply/return temperature 15/18 C and allows for utilization of rather moderate driving hot water temperature 85/75 C when operated with open wet cooling tower. These conditions have been chosen as nominal data for the design of the chiller with a nominal chilled water capacity of 1 kw. When operated with higher driving hot water temperature of maximum 95 C at the generator inlet the capacity increases to 16% of its nominal value. The chiller exhibits a rather efficient part load behavior with COP above.7 throughout a wide capacity range. Operational data during full and part load of the chiller are plotted in Figure 3. At the demonstration site in unich during the summer cooling period the daily peak temperature ranges from 2 to 3 C. During nighttime, the ambient temperature falls below 15 C. Typically, the relative humidity reaches 8 to 9% during nighttime and decreases below 6% during the day. According to
ASHRAE design rules excluding.4% hottest hours of the year, for the unich climate air-conditioning equipment is to be sized for ambient temperature 29 C and 18.7 C wet bulb temperature. Figure 3: Hot Water Temp. 12 C 248 F 11 C 23 F 1 C 212 F 9 C 194 F 8 C 176 F 7 C 158 F 6 C 14 F 5 C 122 F 4 C 14 F COP Hot Water Supply Temp. Return Temp. 5 kw 1 kw 17*1 3 Btu/h 34*1 3 Btu/h Chilled Water Capacity 15 kw 51*1 3 Btu/h Performance data of the compact 1 kw absorption chiller for solar cooling: Required driving hot water temperature and coefficient of performance (COP) as a function of the chilled water capacity (cooling water supply/return temperature 27/35 C). 1..9.8.7.6.5.4.3.2.1. Coefficient of Performance (COP) With respect to the solar cooling capacity of about 1 kw, a thermal storage capacity of about 12 kwh is required in order to cover 5% of the daily reject heat output of the chiller. The rest of the reject heat is directly transferred to ambient by means of a dry air cooler, which operates with 36/4 C cooling water temperature under peak load conditions. Therefore, the design of the latent heat storage will be based on 36/32 C cooling water supply/return temperature, as given in Figure 2 (top). Radiative Heating Heiz-/Kühldecken / Cooling Absorption Chiller Absorptionskältemaschine Solarkollektorfeld Collector G A / K V Heat Distribution (Low Temp.) Verteiler: Kälte, Heizung (NT) Heat Distrib. (High Temp.) Verteiler: Heizung (HT) + + Dry Air Cooler Rückkühler Latent Heat Storage (PC) Latentwärmespeicher Ambient Außenluft Air Figure 4: Hydraulic scheme of the solar heating and cooling system, operated in cooling mode. Apart from the solar cooling mode, during the heating season the system will serve for solar-assisted heating.
Assuming a solar insolation of 5 W/m² for a duration of 6 hours, again a heat storage capacity of 12 kwh is required to absorb the solar gain of the 4 m 2 solar collector field. The solar heat has to be stored above the return temperature of the building floor heating system, which is controlled in dependence of the ambient temperature. Under moderate winter heating conditions, the heating system return temperature typically ranges from 22 C to 26 C. Of course, phase transition of the PC has to take place above this temperature level in order to accomplish heat transfer from the heat storage to the heating system. Supply of useful heat and cooling for conditioning of the office building is provided by activated ceilings which receive chilled water from the evaporator of the absorption chiller during cooling mode and solar thermal heat during the solar heating season, respectively. A detailed hydraulic scheme of the system operating in solar cooling mode is given in Figure 4. 4. DEVELOPENT OF THE LATENT HEAT STORAGE For the given application in a solar heating and cooling system, heat has to be stored in a very narrow temperature range in order to fulfill both tasks: support of the heat rejection of the chiller at about 35 C and contribution to the heating of the building at temperatures above 22 C. Heat latent Q latent sensible Q sensible T Temperature Figure 5: Schematic comparison between latent and sensible heat storage. Due to this limitation of the available temperature swing, the heat storage has been designed as a latent heat storage. According to its melting temperature in the range of 27-29 C the aqueous salt solution calcium chloride hexahydrate (CaCl 2 6H 2 O) has been chosen as phase change material (PC), providing a heat capacity of about 15 J/g or 24 J/L between 25 C and 3 C. This value of the specific storage capacity comprises both sensible and latent heat. Of course the dominating portion is to be attributed to the latent contribution. For the comparison of the storage density of the latent heat storage to a conventional water heat storage, the specific heat of the liquid, i.e. water, and the temperature swing have to be taken into account. Given a temperature swing of 5 K only, a conventional water storage exhibits a storage density of 4,2 kj/(kg K) x 5 K = 21 kj/kg. Thus for the given application in the solar heating and cooling system, the latent heat storage allows for a reduction of the storage mass by a factor of 7, requiring less than 1% of the volume of a conventional water heat storage. Figure 5 gives a schematic description of the working principle of both latent heat storage and sensible heat storage, respectively. The design of the latent heat storage can either be based on a so-called direct-contact principle, where storage material and heat carrier are immiscible, or a heat exchanger structure has to be applied in order to separate the heat carrier from the active storage volume. Within the current project a commercial capillary tube system commonly used for wall heating installations has been applied as heat exchanger. Because of the high storage density of the PC and its low thermal conductivity, a tube pitch between the capillaries of only a few centimetres has been chosen.
Aiming at 12 kwh heat storage capacity, two storage modules with a volume of 8 L each have been designed and constructed. The capillary tube package has been properly configured in order to fully activate the entire storage volume, as shown in Figure 6. The heat exchanger has finally been immersed in a Polyethylene tank and hermetically sealed in order to avoid uptake of humidity from the ambient air. Figure 6: Capillary tube heat exchanger for activation of the latent heat storage volume (left). 12 kwh latent heat storage, formed by two 8 L storage tanks (right). For the sake of economics and in order to achieve a marketable design the heat storage prototype has been based on standard components. The cost of the PC is about.37 Euro/kg, leading to total cost of the latent heat storage of about 3, Euro, assuming 1,6 Euro for the capillary tube register and 95 Euro for the storage containers. Based on this investment cost for the PC heat storage, the system promises to be economically feasible (Schweigler et al 27). Apart from pure economic aspects, operational issues are of major relevance for the feasibility of small scale solar heating and cooling installations. Here the system with dry air cooler and latent heat storage offers substantial reduction of the maintenance effort and improved performance during the solar heating season. 5. OPERATIONAL RESULTS During preparation of the full-scale pilot installation a 1:1 functional model with a volume of about 16 L has been built and tested (Schweigler et al. 27). A few hundred loading/undloading cylces have been conducted without any degradation of the thermal performance of the storage. Thus reliable operation of the pilot installation may be expected. Operation of the pilot installation comprising the two 8 L latent heat storage modules started in late winter 27. Figure 7 shows the temperature-time curves, the capacity (power) and the thermal content (stored energy) of the two modules during loading (left) and unloding (right). The internal storage temperatures (, ) clearly show the effect of the latent heat in the temperature range of 27-29 C where the PC melts and solidifies, respectively. The achieved capacity (power) and the thermal storage content of the two modules matches the initial thermal design for 1 kw and 12 kwh.
temperature [ C] 36 32 28 24 2 outlet outlet inlet g temperature [ C] 36 32 28 24 outlet outlet inlet 2 1 power [kw] -2-4 -6-8 power [kw] 8 6 4 2 stored energy [kwh] -1-1 -2-3 -4-5 -6-7 time gy, [h:min] g stored energy [kwh] 7 6 5 4 3 2 1 Figure 7: Operational results of the two 8 L latent heat storage modules. Temperatures (top), capacity (center) and stored energy (bottom) during loading (left) and unloading (right) of the latent heat storage. Following the heating season 26/27, during summer 27 the system will be tested in solar cooling mode with heat rejection of the absorption chiller via latent heat storage and dry air-cooler. SUARY AND CONCLUSION By applying a low temperature latent heat storage together with a dry air cooler substantial improvement for the design of a solar-driven absorption cooling system is accomplished: The reject heat of the sorption chiller is buffered by the heat storage and transferred to the ambient during periods of low ambient temperatures, e.g. night time or off-peak situations. Especially in low capacity applications, the absence of a wet cooling
tower substantially facilitates the introduction of absorption technology. An analysis of the thermal design of the different system components showed that a latent heat storage allows for moderate temperatures of the driving heat and thus substantially reduces the over-sizing of the solar collector system arising from the application of dry air cooling as compared to a standard system design with wet cooling tower. During the heating season, the PC storage facilitates low operating temperature of the solar collectors with a positive effect on the solar gain. The described system concept has been implemented in the frame of a pilot installation for solar heating and cooling comprising a recently developed compact absorption chiller. For the latent heat storage the phase change material calcium chloride hexahydrate (CaCl 2 6H 2 O) with phase transition, i.e. melting and solidification, in the temperature range of 27 to 29 C is applied. Due to the limited temperature swing available for the given application, the latent heat storage provides a 1 times higher volumetric storage density in comparison to a conventional water heat storage. Operational results of a 1:1 functional model as well as of the full scale latent heat storage providing a capacity of 1 kw and 12 kwh thermal storage content are in good agreement with the thermal design and thus prove the feasibility of the technical concept. ACKNOWLEDGEENT The project is supported by funds of the German Federal inistry of Environment (BU) under contract number 32965D. REFERENCES 1. Furukawa, T. (1983): Study on characteristic Temperatures of Absorption Heat Pumps. 2th Japan Heat Transfer Conf. Proceedings, p. 58-51. 2. Hellmann, H.-., Schweigler, C., Ziegler, F. (1998): A simple method for modelling the operating characteristics of absorption chillers. Seminar Eurotherm, Nancy, France. 6.-7. 7. 1998. 3. Kühn, A., Harm,., Kohlenbach, P., Petersen, S., Schweigler, C., Ziegler, F.: Betriebsverhalten einer 1 kw Absorptionskälteanlage für die solare Kühlung. Ki Luft- und Kältetechnik, (41. Jahrgang) 7/25, S. 263-266. 4. Schweigler, C., Costa, A., Högenauer-Lego,., Harm,., Ziegler, F.: Absorptionskaltwassersatz zur solaren Kühlung mit 1 kw Kälteleistung. Ki Luft- und Kältetechnik, S. 21-25, 4/22. 5. Schweigler, C., Dantele, T., Kren, C., Ziegler, F., Clark, D., Fulachier,.-H., Sahun, J.: Development of a direct-fired partly air-cooled double-effect absorption chiller. Proc. of the 29 th International Congress of Refrigeration, Sydney, 19.-24. September 1999. 6. Schweigler, C., Hiebler, S., Kren, C., ehling, H.: Low temperature Heat Storage for Solar Heating and Cooling Applications. 1 st International Conference Solar Air-Conditioning, Oct. 25, Bad Staffelstein, Germany. 7. Schweigler, C., Hiebler, S., Keil, C., Köbel, H., Kren, C., ehling, H.: Low temperature Heat Storage for Solar Heating and Cooling Applications. ASHRAE Transactions, ASHRAE Winter-eeting, Jan. 27, Dallas, USA. 8. Storkenmaier, F., Schweigler, C., Ziegler, F. (1999): Die charakteristische Gleichung von Sorptionskälteanlagen, Tagungsbericht der Deutschen Klima-Kälte-Tagung 1999, Berlin. Deutscher Kälte- und - Klimatechnischer Verein, Stuttgart. 9. Storkenmaier, F., Harm,., Schweigler, C., Ziegler, F., Kohlenbach, P., Sengewald, T.: Small-capacity LiBr Absorption Chiller for Solar Cooling or Waste-Heat driven Cooling. Proc. of the 3 th International Congress of Refrigeration, Sept. 23, Washington, USA.