A test rig of an advanced absorption heat pump (AHP) for domestic heating has been built and tested in steady-state operation. The AHP is intended to replace conventional boilers used in hot water heating systems, which are very common in Europe and gaining more interest in the United States. To be competitive against conventional boilers, the AHP must be adaptable to different heat sources and heating systems. An attractive heat source is ambient air, which is easily accessible at relatively low cost.The main disadvantage of ambient air is the greater variation in daily and seasonal temperature and the lower temperature compared to other heat sources such as ground heat. A power controlled AHP with adjustable composition is proposed to match these challenging conditions. A single-stage absorption heat pump was chosen as the base for the development. To enable a monovalent system (i.e., the AHP is the only device to provide heat) and to permit operation at heat source temperatures below 0°C (32°F), ammonia-water has been chosen as the working pair. The absorption heat pump consists of a generator, which possesses a storage volume for strong solution, a solution heat exchanger combined with a solution-cooled partial condenser, a condenser, a precooler, an evaporator, an absorber, a solution pump, and refrigerant storage.

The AHP is designed for a maximum heating capacity of 20 kW (68000 Btu/h) at -15°C (5°F). The maximum hot water temperature is 55°C (131°F). The burner, the solution valve, and the expansion valve are continuously adjustable. The stor-age allows shifting the mass fraction of the strong solution from 17 mass% to 50 mass%.

The combined partial-condenser/solution heat exchanger works well at mass fractions from 25 mass% to 50 mass%. This corresponds to heat source temperatures from -10°C (14°F) to 10°C (50°F), whereby a maximum hot water temperature of 55°C (13°F) is possible. Within this range, the process heating coefficient of performance (COP_H) varies from 1.4 to 1.7. Below -10°C (14°F), the COP_H decreases drastically. It is possible, however, to operate the AHP in heat pump mode at temperatures as low as -20°C (-4°F).

Units: Dual