Master's Thesis Hannah Krützfeld


Optimal design of heat pump systems in consideration of the operational strategy

Framework optimization problem Copyright: EBC Framework of the applied optimization problem. Three sub-steps are shown: I. Preparation of the boundary conditions; II. Linearization of the model; III. Application with two alternatives for the integration of the control.

Heat pumps offer the possibility to reduce carbon dioxide emissions in the building sector by electrifying the heat supply and thereby avoiding fossil energy sources. In order to increase the number of heat pump systems in refurbishments, systems need to be designed more economically. Current studies show that in order to improve the design, the interactions between the components and the operating behaviour should be taken into account during the design process.

At the Institute for Energy Efficient Buildings and Indoor Climate at RWTH Aachen, Germany, the potential design improvement is being investigated through simulation studies. In addition, mathematical optimization calculations are carried out to set the design of energy systems. Currently, there are no approaches for the simultaneous optimization of component dimensioning and operation while taking the control strategy into consideration.

Within the scope of this work, the applicability of anMILPmodel for the optimization of the design of a heat pump system is investigated. The operation and the influence of the control strategy are considered during the dimensioning of the components. Representative days are used as basis for the optimization to reduce the calculation time. Annual simulations of the systems are carried out to compare the optimized design with the design according to standards.

The exact data basis and thus also the choice of the representative days have a large influence on the design of the heat pump system. In order to increase the overall annual efficiency, the dependence of the efficiency on the ambient temperature should be taken into account when selecting the representative days.

In comparison to a normative design, the size of the heat pump is reduced by up to 50% whereas the storages are enlargened up to six times by the design optimization. In annual simulations of the optimized design, the total costs are slightly lower than with a standard design, whereas the operating costs are higher due to an increased use of the heating rod.

The MILP modeling of the system requires a simplification of the system description which leads to numerical difficulties and in some cases results which cannot be interpreted physically. Particularly for the implementation of more complicated control strategies, the formulation as MILP does not appear to be suitable. It is therefore recommended to investigate simulation-based optimization methods, to model nonlinearities as well as differential system states.