Investigation of field methods for evaluation of air-to-air heat pump performance
Examensarbete för masterexamen
Air-to-air heat pumps have gained an increasing popularity to supply domestic heating and cooling in the building service sector since the 1990s. Due to the challenge from depletion of fossil energy sources as well as global warming, it is essential for the manufacturers to improve the heat pump efficiency in order to satisfy the requirement of living standard without consuming excessive electricity. This diploma work focuses on studying different air-to-air heat pump COP (Coeffi-cient of Performance) field testing methods and standards. Practical measurements with three test methods were carried out in parallel in the laboratory, namely SP Method, Climacheck Method and Calorimeter Method. Results were collected from both long and short term testing with the corresponding outdoor weather conditions. Hence, heat pump performance characteristics can be studied. Furthermore, uncer-tainty evaluation and limitation analysis of each testing method were made so that further improvement could be suggested for field measurement. While defrosting is a key factor to ensure high efficiency, the dynamic performance within that period is extremely hard to monitor, and it is critical for the long term HSPF (Heating Seasonal Performance Factor) measurement of air-to-air heat pumps. In this study it has been discovered that neither the SP Method nor the Climacheck Method is optimal for the field test, due to the inaccuracy during the defrosting proc-ess. The Calorimeter Method is feasible to serve as the reference method but it is not suitable for field testing. Results achieved by the SP Method seem to be in accord with what is achieved by the Calorimeter Method. However, the influence of the air volume flow measurement devices upon the testing unit itself, and the bulky equip-ment setup, make the SP Method infeasible for field testing during long time periods. The Climacheck Method is designed as a convenient way to carry out continuous measurement, with a user friendly interface and less bulky instrument. But it does not work properly if, for instance, the superheat is too low to avoid liquid droplets in the suction line to the compressor. Droplets result in an overestimated isentropic effi-ciency and consequently the heating capacity is also overestimated. On the other hand, the Climacheck unit warns the user and indicates the cause of the problem. Based on this study, further improvements could be suggested for each testing method. For the Climacheck method, an additional testing point is available and should be mounted close to the inlet of the condenser. For the SP Method, pre-testing in the lab could be made concerning air volume flow rate under different indoor fan speeds, so that the bulky air collecting system would be avoided for field testing af-terwards. Nevertheless, how to measure air volume flow rate accurately, especially under dynamic conditions, remains a challenge for the external COP testing method.
Building Futures , Byggnadsteknik , Building Futures , Building engineering