18 May 2017
The efficiency of a gas-fired absorption heat pump (GAHP) with 40 kW nominal capacity has been examined under transient operation conditions at a test facility at TU Berlin. The GAHP was operated in continuous modulation mode (50% part load), continuous on-off mode (equivalent heat output of 50% of nominal capacity), and discontinuous operation mode (with nominal capacity heating demand for 20 to 70 minutes followed by a cooldown of the entire system). Furthermore, two field applications with hot water storage have been analyzed: One single GAHP and one system of six parallel connected GAHP. For comparison a normalized operational gas utilization efficiency (gue) is applied, which relates the measured GUE of the heat pump to a GUE at maximum heat output conditions for given temperatures.
In modulation mode the efficiency shows a nonlinear increase with increasing gas input. The results show that in this mode, a higher GUE compared to on-off-mode at the same mean heat output is achieved. A strong decrease down to 50 % of the steady state GUE in discontinuous mode for operation periods below 40 minutes has been measured. Compared to continuous on-off mode and modulation mode, the heat pumps perform better in discontinuous operation mode for operation periods exceeding 60 minutes.
As part load operation efficiency affects the overall system performance, its influence on storage sizing and hydraulic design has to be considered in an energy efficient heating system design. The startup and shutdown time is longer than in common systems and has a significant effect on the dynamic behavior of the system. An efficient control strategy has to differentiate between modulation and discontinuous mode.