The main problem with regard to the performance of the absorption chiller is that it does not reach the projected COP. The following reasons can be found:

• The required temperature for driving the chiller is not provided constantly (see Error! Reference source not found.).

• The re-cooling temperature is mostly at about 28° C and not at 27° C as planed (see Error! Reference source not found.). According to the manufacturer’s characteristics for the machine this causes a performance loss of 10 to 20 %.

• In simultaneous operation of the compression chiller the required flow rate in the cold-water circuit is not achieved.

• The cold-water temperature is lower than planned.

The integrated frequency curves for the temperatures in the driving and re-cooling circuits are shown in figure 6. The minimum required temperature of 75° C in the supply line of the driving circuit is reached for about 3,600 hours of almost 5,000 hours of total operation. That’s nearly 75 % of the time. Only for 2,000 hours a temperature of over 80° C is achieved, less than half of the operation time. The temperature for the supply line of the re-cooling circuit is always higher than 27° C, the temperature is quasi-constant at 28° C.

The parasitic energy consumption of the solvent pump and the refrigerant pump is independent from the refrigerating capacity. Therefore, the total COP declines and the primary energy coefficient and the carbon dioxide emission of the chiller rise disproportional.

The demand of electric energy of the installed absorption chiller is very high compared to other products. Comparable products from other manufactures only need a tenth of the electric connected load. Solely the reduction of the electric energy consumption of the absorption chiller itself to this level would reduce the primary energy coefficient to the level of the compression chiller. The emission of carbon dioxide would fall far below the level of the compression chiller.

4. Optimisation

To improve the conditions for the operation of the absorption chiller several changes where developed and partly realized. For a more constant heat supply additional storage capacity was installed. This storage is reserved for the wood pellet combustion unit. So the existing storage capacity can be used only for the solar collectors. At the same time three bigger units replaced the four wood pellet boilers. The total output of the boilers rises from 128 to 168 kW.

The cold-water outlet temperature will be adjusted according to the demand. Because of the strong power loss of the absorption chiller when the cold-water outlet temperature falls below 9° C, the temperature will be raised to 10° C in times where no de-humidification is needed. This temperature is sufficient for sensible cooling of the air. The cold-water outlet temperature is lowered to 6 to 8° C only in times with high external humidity loads.

To supply the required re-cooling temperature minor improvements to the hydraulics where implemented in 2006 already. However, these improvements did not show any effects in 2007. Further investigations where undertaken to find the reasons. The result was that the control strategy for the re-cooling pumps had to be optimized.

Furthermore, there is the consideration to run the absorption chiller solely by solar heat. The very high consumption of electric energy of the absorption chiller could be only reduced by a complete replacement with another machine.