Technical and economical design of a combined heat pump/low temperature storage system. Results from system simulations.

A computer program simulating a whole heating system for space heating in large multifamily houses or small district heating systems is presented. The system consists of an ambient-air natural convective heat collector, a heat store with vertical pipes in the ground, and an electrically driven heat pump.The most important design parameters from both a technical and an economical point of view are the sizes of the storage, the ambient-air heat collector, and the heat pump. The influence of these parameters on the technical result is discussed. With the aid of computer simulations it is shown how this influence can be presented in a generalized way. From this presentation a method to optimise this kind of system is outlined. Using typical costs for Sweden the total annual cost for some designs is calculated and a cost comparison is made with an oil fired burner.In the optimisation calculations optimum values for the main parts were determined. For the heat collector and the storage size flat optimums were found. The heat pump size shall in most cases be as large as is practically possible, when such aspects as freezing of the soil have been taken into account. Compared to an already existing oil fired burner and to a heat pump without storage this system was found to be more expensive. Decreasing investment cost and increasing energy costs in the future can however make the system economically favourable