A solar cooling system consisting of solar thermal collectors, a heat storage (optional), an absorption chiller, a cold storage (optional) and the application system (e. g. room cooling) is a very complex arrangement. The performance of some components (e. g. collector and cooling tower) depends on ambient conditions (e. g. air temperature, air humidity). Because of the high number of interrelations between the different components and between some components and the ambient conditions it is not possible to calculate the performance of the whole system and to compare different types of certain components (e. g. collectors) manually. Simulation programs have to be used.

Using the capabilities of the TRNSYS /1/ package the performance of the small capacity absorption chiller in combination with different kinds of collectors and re-cooling options was evaluated /2/. For flat plate and vacuum tube collectors a comparison of different models (manufacturers) was done using published test data. For each type a reference collector model with a good performance and an acceptable cost-performance ratio was chosen. The properties of these models were used for further calculations. For parabolic trough collectors the STEC-Library was used (DLR model of the IST trough) /3/.

At first a system with a fixed cold water inlet temperature (12 °C; 18 °C) to the absorption chiller was simulated with climate data of Huelva/Spain.

For all combinations of the three collector types and two re-cooling options (wet cooling tower and dry cooler) the delivered cooling energy from April to August was calculated for five collector areas. The results are shown in Figure 8 and Figure 9.

The results show the great influence of the re-cooling version. If a dry cooler is used in­stead of a wet cooling tower the collector area has to be increased to obtain the same quantity of cooling during the summer.

Comparing the figures 8 and 9 the influence of the cold water temperature can be evalu­ated. If the room cooling system is capable to meet the cooling load with a higher cold wa­ter temperature (greater heat exchanger area needed) the cooling energy delivered by the absorption chiller increases (or a smaller collector area is sufficient to receive the same amount of cooling).

The simulations also provided other information such as the maximum cooling perform­ance of the absorption chiller during the operation period, distribution of the cooling energy over the month (and days), water demand for the wet cooling tower, delivered cooling en­ergy per collector area. By comparing the results for different collector areas an optimal size can be chosen.

At a second step a more realistic system with changing cold water temperatures and in­cluding the thermal behaviour of the building (cooling load) was simulated. To compare the different arrangements

— the excess temperature: ATe = TRoom — 26 °C

— the duration (tet) of the periods with inside room temperature above 26 °C and

— the integral: |ДТЕ dTET [Kh]

were calculated. Figure 10 shows the results for one configuration with different collector areas.

Simulations using sea water for re-cooling the absorption chiller were also done. The re­sults for Huelva showed that there is no higher performance of the absorption chiller than using a wet cooling tower. Sea water cooling has some other restrictions too (only near the sea, high cost, need to use suitable materials).

All the results are valid for the specified equipment (collectors, coolers) only. For evalua­tion of a certain project simulations have to be done again with the characteristics of the equipment to be used and suitable climate data.

The experiences gained during the simulations and supervising solar cooling installations showed that great care should be taken regarding system control, storage integration and design of the room cooling system. As mentioned before the cold water temperature is important for the achievable cooling output.

Conclusions

A H2O/LiBr absorption chiller with a nominal cooling capacity of 15 kW was developed. A special heat exchanger design permits the use of low temperature heat such as solar or waste heat to drive the absorption chiller.

After prototype testing a field test at three sites was carried out in the summer of 2003. The field test showed good results. The absorption chiller is working reliable and flexible over a wide range of external conditions.

Because of the high number of cooling applications in the capacity range below 50 kW the new chiller has the potential to increase the usage of solar thermal energy for cooling pur­poses.

The waste heat of cogeneration systems can also be used to drive the absorption chiller creating new possibilities for trigeneration systems (power, heat and cold).

The performance of solar thermal cooling systems can be predicted using the capabilities of system simulation programs.

Nomenclature

Literature

/1/ Solar Energy Laboratory, University of Wisconsin (2000). TRNSYS 15, A Transient System Simulation Program, Madison, WI, USA.

/2/ Heinrich, C. (2004). Modelling and Simulation of solar thermal driven single effect absorption chillers of small capacity for climatisation, Diploma Thesis (German), Dresden, Germany.

/3/ Schwarzbozl, P.; Eiden, U. e.; Pitz-Paal, R.; Jones, S. (2002). A TRNSYS Model Library for Solar Ther­mal Electric Components (STEC) — Reference Manual — Release 2.2, Cologne, Germany.