Integrated Control of Heat Pumps

Licentiatavhandling, 2003

The purpose of the work presented in this thesis is to investigate the potential and prerequisites for improved energy efficiency of heat pump systems using efficient components and proper methods for optimisation of operation. The background is that several investigations have reported on possibilities for increased energy efficiency of heat pumps if variable-speed capacity control is used instead of conventional on/off control. Further improvements are possible if variable speed pumps or fans and electronically controlled expansion valves are used as well. To take full advantage of these new controls they should be properly co-ordinated and optimised. In this work, a measuring system consisting of three temperature sensors and two pressure sensors was used for on-line measurement of the coefficient of performance in an experimental investigation. Knowledge of the performance provides a basis for self-optimising closed-loop control. The measurement system can also be used for Fault Detection and Diagnosis (FDD) and Performance Indication and thus contribute to an integrated control system. The Nelder-Mead simplex method was applied for on-line optimisation. The method was evaluated by computer simulations and shows promising results. It was possible to find the optimal set point, maximising COP, and the method could also handle soft constraints, such as a desired set point for the heating capacity. The potential of variable-speed capacity control was investigated both by laboratory tests and annual energy savings calculations. The laboratory tests were made on a brine-to-water heat pump with a variable-speed piston compressor and an electronic expansion valve. The results from the tests showed a decrease in efficiency when applying variable-speed capacity control to the compressor when compared to conventional on/off control. Analyses indicate that this is due to losses in the frequency converter and increased losses within the compressor, most likely in the electric motor. The electronic valve gave only a marginal increase in efficiency compared to a conventional thermostatic expansion valve. Using these test results as a basis, the possible annual energy savings were calculated. This analysis indicates that there exists an energy saving potential also for variable-speed controlled brine-to-water heat pumps but it requires product development mainly of the compressors. A conservative estimate indicates savings of 10 % compared to on/off control.