The number of buildings heated by heat pumps continues to rise. In recent years, air-water heat pumps have been the dominant force in this regard. More energy efficient alternatives exist, however, to outside air as a heat source. Examples include geothermal energy, ground water, exhaust air and solar thermal energy. The variety of potential heat sources and of heat exchangers tapping these presents an incredible challenge for planners. Validated concepts, objective decisionmaking guidance and analyses are consequently called for. This endeavour is supported by two research groups.

Geothermal heat collectors require a relatively large surface area, which is why they are something of a rarity. A research group of the Institute for Solar Energy Research in Emmerthal consequently hopes to slash the necessary geothermal heat collector surface area through the regeneration of ground soil with solar thermal energy. In an effort to analyse the concept with greater precision, the researchers, together with the German heat pump association (Bundesverband Wärmepumpe) and tewag GmbH, have developed a simulation model for geothermalheat collectors and validated it experimentally using data from a prototype system. Parameter studies reveal how best to design such hybrid systems without causing thermal depletion of the ground soil during operation.

In a further project, researchers at the Institute for Building Services and Energy Design in Braunschweig hope to support planning efforts for heat pump systems by providing clear and objective decision-making guidance for the early planning phase. This involves the development of a simple, Excel-based tool. The tool facilitates the selection of suitable heat sources and heat exchangers and the rapid assessment of these based on current market data and empirical system parameters. Comparisons in energy and economic terms are possible for various building and usage types. The calculations are validated using measurement data of genuine systems and simulation.

Ground soil exhibits considerably less temperature fluctuation than ambient air from as little as one meter depth, according to season. The higher average temperature in the warmer months makes it appealing as a heat source for heat pump systems. The slight downward trend observed in the proportion of newly installed plants accounted for by ground source heat pump systems is therefore quite remarkable. Compared to boreholeheat exchangers drilled into the earth, horizontally laid geothermal heat collectors often entail reduced outlay in terms of structural works and licensing regulations. They are also largely unaffected by the geologicalsituation of the underground.

The required surface area for laying collectors at one to one-and-a-half meter depth often presents a problem. Thermal energy produced using simple, unglazed solar collectors can, in the approach followed by researchers at the Institute for Solar Energy Research (ISFH), help facilitate effective operation using far fewer geothermal heat collectors, without producing critical frost conditions in the ground soil.

This is because in such cases, further electrical heating must be directly implemented, with considerably reduced efficiency.

To facilitate the accurate assessment and configuration of geothermal heat collectors, a numerical simulation model was developed. It allows for the precise discretisation of a two-dimensional cut through the ground soil, factors in edge influences, freezing processes in the ground soil and the thermal capacity in the ground collector fluid. Embedded in the TRNSYS simulation program and optimised for rapid, dynamic system simulations, it can be used to investigate the influence of individual parameters on overall system properties.