1 SOLAR COOLING An Overview of European Applications & Design Guidelines Costas Balaras, Ph.D. Member ASHRAE, Mechanical Engineer, Research Director GRoup Energy Conservation (GR.E.C.) INSTITUTE FOR ENVIRONMENTAL RESEARCH & SUSTAINABLE DEVELOPMENT NATIONAL OBSERVATORY OF ATHENS (NOA) SACE project, 5 th Framework programme (NNE5/2001/00025) European Commission, D.G. XII Research Disclaimer: The content of this presentation reflects the views of the authors. It does not represent the opinion of the Community. The authors and the European Commission are not responsible for any use that may be made of the information contained therein. BACKGROUND Fundamentals of absorption refrigeration were patented in France by Ferdinand Carré (1859). First machine was introduced in the market by Edmond Carré in 1886. Peak cooling demand in summer is associated with high solar radiation availability => excellent opportunity to exploit solar energy with heat-driven cooling machines. Obstacles: High first cost, limited practical experience with the design, control, operation, installation and maintenance of these systems. Limited commercially available low power cooling systems.
2 HEAT DRIVEN COOLING TECHNOLOGIES CLOSED CYCLE SYSTEMS e.g. Absorption and Adsorption cycles They produce chilled water that can be used in combination with any AC equipment such as an air handling unit, fan-coil systems, chilled ceilings, etc. OPEN CYCLE SYSTEMS e.g. Desiccant systems The refrigerant is discarded from the system after providing the cooling effect and new refrigerant is supplied in its place in an open-ended loop. OPERATING PRINCIPLES Closed Cycle Systems - ABSORPTION Most common thermally-activated cooling devices in solar AC Single-effect configuration Cycle: A refrigerant expands from a condenser to an evaporator through a throttle, like in the conventional vapour compression system. Cooling is produced in the evaporator through the evaporation of the refrigerant at low temperature. A second working fluid the absorbent - is employed, which absorbs refrigerant vapour from the evaporator at low pressure in the absorber, and desorbs into the condenser at high pressure, when heat is supplied to the desorber. Absorbent-refrigerant pairs: LiBr/H 2 O & H 2 O/NH 3 From cooling tower H.X. Generator Strong solution Condenser Vacum Absorber Evaporator Water Weak 4 o C 39 o F solution To cooling tower Steam or hot water 80-170 o C 176-338 o F Multiple stages: Utilize the heat rejected from the condenser to power additional desorbers, to double or triple the amount of refrigerant extracted out of solution. Chilled water
3 OPERATING PRINCIPLES Closed Cycle Systems - ADSORPTION Instead of refrigerant absorption in an absorbing solution, adsorb the refrigerant on the internal surfaces of a highly porous solid. Cycle: The refrigerant previously adsorbed in one adsorber is driven off through the use of hot water (right compartment). The refrigerant condenses in the condenser and the heat of condensation is removed by cooling water. The condensate is sprayed in the evaporator and evaporates under low partial pressure, producing chilled water. The refrigerant vapour is adsorbed into the other adsorber (left compartment). Heat is removed by cooling water. Working pairs: water/silica gel water/zeolite, ammonia/activated carbon or methanol/activated carbon The two chambers may be directly coupled for some time between the changes in their function for heat recovery (phases 2 & 4), since the hot chamber has to be cooled in the next step and vice versa. Higher efficiency than absorption at low driving temperatures; No moving parts; No crystallization Intermittent operation (periodic cycle), require more effort in system design and operation control; Larger physical dimensions and heavier; More expensive per kw cooling capacity; Few manufacturers OPERATING PRINCIPLES Open Cycle Systems - DESICCANTS Sorption air dehumidification using Solid or Liquid desiccants. SOLID DESICCANTS (e.g. silica gel) Usually employ a rotary bed carrying the sorbent material (desiccant wheel), to allow continuous operation. Makes sense, if the air change and/or the dehumidification of the indoor air are necessary or strictly prescribed (e.g. supermarkets, museums, and assembly halls with high occupancy). LIQUID DESICCANTS Essentially an open cycle absorption system, where water serves as the refrigerant. Includes an air dehumidifier (absorber) and a solution regenerator (desorber) in the form of packed towers. Fewer components (no condenser since refrigerant condensation uses the environment); system operates at atmospheric pressure; more efficient utilization of low heat driving temperatures (down to 60-70 C or 140-158 F). Yet no market availability; need further system optimizations.
4 SOLAR ASSISTED COOLING SYSTEMS Solar Collectors Heat Storage Heat Distribution Heat-driven Cooling Unit Cold Storage (optional) Air Conditioning System Cold Distribution Auxiliary (backup) integrated at different places in the overall system: as an auxiliary heater parallel to the collector or the collector/storage or as an auxiliary cooling device or both The SACE Project Data base with Projects / Applications Automated processing & assessment Evaluation Tools Easy Solar Cooling Guidelines - Building professionals - Decision makers
5 The DATA BASE The user is able to: - Screen/Identify different solar cooling technologies - Review project reports & evaluation results - Possible to add new projects in the data base using the automated survey form - Automated evaluation for new applications A total of 54 applications/projects were identified, documented and evaluated: - 12 in Germany - 2 in Austria - 3 in Malta - 1 in Croatia - 5 in Hellas - 1 in Spain - 1 in Kosovo - 4 in Israel - 15 from Cordis & 10 IEA projects CASE STUDIES Presentation
6 Thermal COP ratio of cooling capacity to the heating power delivered to the system by solar, directly or indirectly through storage Single-effect Absorption: 0.50-0.73 LiBr/H 2 O average 0.66 H 2 O/NH 3 average 0.60 Double effect 1.3 Adsorption average 0.59, but operate at a lower temperature 0.59 0.60 0.66 0.85 0.74 0.51 0.49 Driving 52-82 Temp. o C 126-180 o F 60-110 o C 140-230 o F 117 165 66 120 o C Solar Collectors Flat plate (63%) 60-90 C (140-194 F) Evacuated tube (21%) 80-120 C (176-248 F) Parabolic (16%) 97-165 C (207-329 F) COP increases with the driving temperature
7 Solar Collectors Adsorption & Absorption: 2 m²/kw (76 ft 2 /ton) to 5 m²/kw (189 ft 2 /ton) Avg specific solar collector area = 3.6 m 2 /kw Initial Cost Depends on: - Cooling capacity - Solar collector type - Stage of development - Working principle Avg initial cost 4000 Euro/kW
8 Actual Performance COP Best Performance LiBr/H 2 O systems The adsorption systems are generally less efficient Avg annual performance thermal COP = 0.58 Actual Performance Auxiliary Energy Consumption Average annual value is 0.255 kwh/kwh Absorption systems have the lowest consumption LiBr/H 2 O systems = 0.018 kwh/kwh Desiccant cooling systems have the highest consumption Avg auxiliary fans & pumps = 225 W/kW Solid desiccant systems = 0.631 kwh/kwh
9 Actual Performance Water Consumption Highest consumption Adsorption: 7.1 kg.h - 1 /kw (54.8 lb/ton-h) For the majority of projects: 4-6 kg.h -1 /kw (30.9 to 46.4 lb/ton-h) Avg water consumption = 5.3 kg.h -1 /kw (41 lb/ton-h) Actual Performance Exploitation Cost Most expensive Solid desiccant systems: 1.05 Euro/kWh Least expensive LiBr/H 2 O absorption systems: 0.16 Euro/kWh Avg annual exploitation cost = 0.65 Euro/kWh
10 Guidelines - GENERAL ISSUES Each technology has specific characteristics that match the building s HVAC design, loads and local climatic conditions A good design must first exploit all available solar radiation and then cover the remaining loads from conventional sources Collector and storage size depend on the employed technology Hot water storage may be integrated between the solar collectors and the heat driven chiller to dampen the fluctuations in the return temperature Guidelines - ABSORPTION Single-effect machines provide best results with a heat supply temperature of 80-100 o C (176-212 o F) In hot and sunny climates need 3-4 m 2 collectors / kw cooling (114-152 ft 2 /ton) Double and triple-effect machines require higher supply temperature using higher-cost evacuated tube or concentrating collectors, and may need a high temperature storage Most large scale applications (300 kw or 85 ton and up) employ LiBr/H 2 O, produce chilled water at 6-7 o C (43-45 o F) LiBr systems must be water cooled (need cooling tower), while NH 3 systems can have an air-cooled condenser LiBr chillers usually have large physical dimensions
11 Guidelines - ABSORPTION In LiBr systems the refrigerant freezes at 0 o C (caution during winter, while the machine is idle) Potential crystallization of LiBr solution at high concentrations (high generator temperatures or inadequate temperature control at other parts of the machine) Heat supply temperature from the solar collectors or heat storage must be adequately controlled A fuel-fired boiler usually covers the need for a backup system to heat the desorber of the heat-driven chiller Caution: during low solar radiation availability, the collectors connected in series with a backup boiler can turn into a heat sink Guidelines - ADSORPTION Periodic nature of the cycle results to fluctuations at all temperature levels For stable operation, use heat storage on the high temperature level and on the chilled water side Need variable speed pumps to adjust the volumetric flow of the heat transfer medium through the solar collectors and to provide the desired outlet temperature that matches the operating conditions Commercially available adsorption chillers use a constant period between cycles (6-7 minutes per half cycle). Under part load conditions, extending the cycle time can increase the COP (exploit the adsorption potential of the sorptive materials) Use a chilled water storage to avoid freezing in the evaporator (operate chilled water pump for 10 min after completing the heating cycle of the adsorption chiller, to avoid that the regenerated sorption material causes continued evaporation of water in the evaporator below freezing point)
12 SOFTWARE Easy Solar Cooling CONCLUSIONS Solar assisted air conditioning is still in a status of early development Need to overcome shortcomings, mainly in the hydraulic design and system control Various installations exist, but data is often incomplete Reliable monitoring data and operation experience are available only from a few systems More experience from actual installations is necessary to prepare standardized configurations and solutions for the design of the hydraulic scheme and the control Further funding is necessary in order to support market introduction of solar assisted air conditioning, BUT should require that systems achieve a certain primary energy saving (e.g. > 30 %) compared to conventional technology and a cost of primary energy savings below a maximum value (e.g. 1 0 Eurocent per kwh of primary energy).
13 For more information Web site http://www.ocp.tudelft.nl/ev/res/sace.htm ASHRAE JOURNAL Vol. 48 (6), p. 14-22, (2006) SOLAR COOLING An Overview of European Applications & Design Guidelines Thank you for your kind attention... Q & A DISCUSSION?