Gas fired absorption heat pumps (GAHPs) offer a great ecological potential due to their high efficiency compared to, for instance, modern condensing boilers. A possible scope of application is the so called retrofit application, i.e. the use in older buildings with existing gas or oil boilers, in which the existing infrastructure can be used. Absorption heat pumps that are commercially available today as well as absorption heat pumps in current research projects are condensing systems. The flue gas is cooled below its dew point in order to (partially) use the latent heat of the condensed water vapor in the flue gas. The further the flue gas is cooled down, the higher is the gain achieved by using the latent heat. State-of-the-art flue gas heat exchangers are normally cooled by the return flow of the heating water. In retrofit applications however, the temperature level of the return flow is usually higher than the dew point temperature. As a result no condensation takes place and the latent heat of the water vapor in the flue gas is emitted to the environment instead of being used for heating purposes.
By cooling the flue gas heat exchanger by means of refrigerant from the evaporator outlet instead of heating water, complete condensation of the water vapor in the flue gas can always be ensured, as the temperature level of the refrigerant is around 0 °C in most cases. The latent heat from the water vapor in the flue gas is then transferred to the refrigerant and eventually released to the heating water in the absorber and condenser. This concept is expected to significantly increase the overall efficiency of the GAHP.
In the framework of the present project a refrigerant cooled flue gas heat exchanger shall be designed by means of a pinchpoint analysis and detailed simulations and an appropriate heat exchanger shall be either purchased or built. Due to the aggressive characteristics of both the condensate and the refrigerant (ammonia), choosing suitable materials is of crucial importance in this context. The flue gas heat exchanger shall then be integrated into a near-serial prototype of a GAHP and be characterized experimentally in terms of e.g. capacity and efficiency. Furthermore a control strategy for the modified cycle shall be developed. Finally the economic efficiency of the concept shall be analyzed by means of static and dynamic investment calculations.
One of the expected results of the project is a general proof of concept. Moreover, an experimentally based, stationary characteristic map of the modified cycle shall allow quantitative statements concerning the achievable efficiency enhancement as well as a comparison with a conventional flue gas heat exchanger. Additionally operational experience with such a system shall be gained and it should be possible to judge the suitability of different control concepts. Furthermore a validated simulation model shall be available by the end of the project. In general the project will contribute to a technological improvement and hence to a further market penetration of GAHPs.