E. Bertram1*, J. Glembin1, J. Scheuren1, G. Rockendorf1, G. Zienterra2

1 Institut fur Solarenergieforschung Hameln (ISFH),

Am Ohrberg 1, 31860 Emmerthal; Germany

2 RHEINZINK GmbH & Co. KG, Bahnhofstrafie 90, 45711 Datteln; Germany
* Corresponding Author, s. bertram@isfh. de

Abstract

Two heat pump-systems with borehole and unglazed solar thermal collector are measured and simulated in TRNSYS as part of a research project. Compared to systems without collector the collector yield increases the average temperature level of the heat pump system on the evaporator side. A collector model is developed and evaluated considering the long­wave radiation exchange and the condensation heat gains. The annual collector yield is measured as 545 kWh/m2a, of which 4% are determined as heat gains through condensation. Further simulations in TRNSYS show the interdependency of collector area, borehole length and heat pump system performance. The additional heat source component collector reduces the required borehole length and simultaneously improves the heat pump system perform­ance in comparison to a solely borehole supported heat pump. In addition the system sensi­tivity for the heat source parameters is reduced significantly, thus resulting in a more certain system planning and operation.

Keywords: heat pump system, unglazed solar thermal collector, condensation heat gains

1. Introduction

Unglazed solar collectors (SC) provide a high collector yield at a low temperature level. They may therefore be applied to the best advantage as heat source in heat pump systems (HPS) [1]. During winter, in the period of maximum heating demand, unglazed SC can gain heat on a very low tem­perature level only. Thus a second heat source is needed, which offers ambient temperature inde­pendent heat to the HPS. As such heat sources vertical borehole heat exchangers (BHE) are applied.

The role of an unglazed SC is to increase the source temperature level of a HPS, showing an enormous potential for reducing the electrical consumption of the heat pump. If the average tem­perature level of the heat source is increased by 5 K the annual HPS performance factor (HPF) im­proves from 3.4 to 4.0. The HPF is defined as the heat supplied by the heat pump divided by its electricity consumption for one year of operation.

In such a two source HPS application the solar heat is either supplied directly to the heat pump condenser or to the BHE. The heat is transferred to the BHE for thermal regeneration of the cooled soil surrounding the BHE. A direct use of the solar heat for space heating or domestic hot water preheating is not regarded in this paper. These solar assisted ground coupled systems offer a high dynamic, a complex interdependency and particularly unknown behaviour, which can not be de­scribed properly with common steady state methods used for BHE dimensioning. Hence detailed numerical simulations are required. The realization and evaluation of two HPS pilot plants were in

the focus of a research project, where a TRNSYS-simulation configuration could be validated and used for further extrapolating studies.