Supply temperature control concepts in heat pump heating systems
Aachen / E.ON Energy Research Center (2015) [Book, Dissertation / PhD Thesis]
Page(s): XXVIII, 169 S. : graph. Darst.
In recent years, electrically driven compression heat pumps have come to be widely used for the heating of buildings. Their efficiency strongly depends on the temperature lift which is influenced by the supply temperature of the heat sink. When used with radiator heating systems it is challenging to operate heat pumps efficiently because high supply temperatures are required. Therefore, in order to efficiently operate heat pumps, this work analyses advanced control concepts for heatpump heating systems that adapt the supply temperature according to the demand. The heat output of radiators is influenced by the mass flow rate and the supply temperature. Today, usually only the outdoor air temperature is used for controlling the supply temperature. However, control strategies exist that consider additional disturbances. In this work, heat pumps in existing buildings are analyzed on the basis of field test data. The temperature of heat source and heat sink influence the efficiency of the heat pump. But additional influences such as the usage of additional appliances can be found which were studied and documented comprehensively. However, without data on user influences, room temperature and control settings, the potential of the field test cannot be fully realized. Field test data are used to evaluate models of the heat pump and the buffer storage of the heating system. The heat pump is implemented as a table-based model, the buffer storage is modeled as a stratified fluid volume. The validated models and existing model components make up a model of the overall system. It includes a model of a one-family home with nine heated zones. User influences implemented as different user schedules, the building physics, dimensioning of components and control are analyzed in simulations of heating periods. The usage and insulation standard of the building have a strong influence on the energy demand and thermal comfort. The system model is also used to analyze different supply temperature control strategies. Three control concepts are modeled: A rule-based one which adapts the set temperature step-wise, a model predictive one which calculates the optimal supply temperature for each heated zone, and a continuously operating controller which uses an integral element controlling the supply temperature according to the room air temperature. The advantages and disadvantages of each strategy are evaluated on the basis of the simulation results. A decrease of the mean supply temperature leads to considerable savings in energy demand. However, an increased thermal discomfort cannot be fully avoided. Attained energy savings crucially depend on the type of building physics and the user influences. Energy savings in real buildings can exceed the ones simulated using ideally parametrized systems.