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Experimental system for operating a heat pump with low-temperature heat from near-surface geothermal heat and solar collectors. Solar thermal energy helps significantly reduce the surface area requirements of geothermal heat collectors, increasing the appeal of these systems.
© ISFH, Institut für Solarenergieforschung, Emmerthal
Heat pump systems
15.05.2018

The chart illustrates the extent to which geothermal collector surface area can be reduced as solar collector service area is increased. The thermal conductivity of the ground is also an important factor in this regard.
© tewag Technologie - Erdwärmeanlagen - Umweltschutz GmbH, Starzach-Felldorf

Solar thermal energy reduces surface area of geothermal heat collectors

For heat pumps with geothermal heat collectors, unglazed solar collectors offer several advantages as a secondary heat source. The surface area requirement for laying geothermal heat collectors is reduced by up to 50 percent without resulting in depletion of the ground or critical freezing conditions. Researchers at the Institute for Solar Energy Research (ISFH) in Emmerthal closely investigated this together with partners in a research project. The simulation model developed is available for simulation software TRNSYS, information materials for simplified designing rules for planners will be published shortly.

Ground-coupled heat pumps benefit from low seasonal temperature fluctuations in the ground. Their performance capability and efficiency remain stable even at low outside temperatures and with high heating demand. For some buildings, borehole heat exchangers are simply not feasible nor optimal in terms of cost. Horizontal geothermal heat collectors are a viable alternative, though so far require an unsealed surface area in considerable measure for integration into the ground. By combining geothermal heat collectors with solar collectors, which is the idea of the research project, the surface area requirement can be considerably reduced while maintaining at least comparable system efficiency.

Dynamic examination of ground

For detailed simulation-based investigation, the researchers developed a numerical model for geothermal heat collectors. The model accounts for dynamic thermal processes in considerable detail in a two-dimensional sectional view through the ground, including the thermal capacity of the piping-routed heat transfer medium and ice formation in the ground. This is due to freezing that occurs and the significant changes to thermal conductivity and heat capacity when large volumes of heat are drawn from the ground.

Dynamic examination is necessary to be able to accurately gauge the behaviour of the geothermal heat collector in clocked operation. The model has been implemented as a computational time optimised component for simulation software TRNSYS and validated experimentally based on trials on a purpose-built experimental system at the Institute for Solar Energy Research. The simulation software is provided free of charge by the Institute for Solar Energy Research on request (TRNSYS Type 710).

Greater efficiency over half the space

Various system configurations were investigated and assessed in simulation studies. For a typical modern single-family dwelling, ground type, piping length and layout spacing of the geothermal heat collector and the solar collector type and surface area were varied as key parameters. This revealed that two effects limit the surface area reduction of geothermal heat collectors: Thermal depletion and critical freezing conditions in the ground. Thermal depletion means conditions in which the geothermal heat collector is no longer able to supply the minimum necessary source temperature to operate the heat pump. In such circumstances, an electric resistance heater usually takes over the job of supplying heat in heat pumps available on the market, which considerably reduces system efficiency. Critical freezing conditions result from ice formation around collector piping in the ground.

Research has been able to demonstrate how effectively unglazed solar collectors in particular help reduce the surface area requirements of geothermal heat collectors. Even with geothermal heat collectors reduced by more than 50 percent, intermittent regeneration with solar heat helps prevent critical freezing and depletion conditions in the ground. This means collectors can be configured for ground-coupled heat pumps over around half the surface area. That drastically boosts application options.

Simplified designing rules available shortly

To facilitate the planning of heat pump systems with combined geothermal and solar thermal heat sources with maximum possible reliability, simplified designing rules have been developed. Simulation results reveal that surface area reduction is achieved with reduced layout spacing of geothermal heat collector piping. Solar thermal energy from solar collectors in this regard prevents depletion and freezing conditions in the ground critical to system efficiency. The designing rules describe the connection between installed solar collector surface area and the reduction of layout spacing depending on the thermal conductivity of the ground.

An information sheet with simplified designing rules will be released in the third quarter of 2018. For details, see product presentation “Geothermal and solar collectors complementary as heat sources” (in German) of the ENERGIEWENDEBAUEN research initiative.

(jl)

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