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Overall view of the source HiL test rig. The eight available hydraulic circuits provide three different performance classes.
© E.ON Energy Research Center
Heat pump technology
Projektinfo 04/2018

Structure of the demonstrator developed for the combined use of outside and exhaust air.
© Fraunhofer ISE

Principle of the sink HiL: Depicted heating network, simulated environment (grey background).
© E.ON Energy Research Center
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Double heats better

Air-to-water heat pumps are providing increasingly more heat for new or refurbished single-family homes with low energy requirements. Compared with ground source heat pumps, they have significantly lower acquisition costs and can be easily integrated into low-temperature heating circuits. Scientists from research and industry want to increase the efficiency of the systems by, on the one hand, using a second heat source for the heat pump and, on the other hand, by reducing the heat demand by means of centrally controlled individual room and supply control systems.

Heat pumps work more effectively the lower the temperature difference between the heat source and the heat sink. In residential buildings, underfloor heating or other large heating surfaces therefore usually ensure that the heat distribution works at low temperatures. However, the temperature level of the energy sources is usually predetermined. Whereas geothermal heat pumps are supplied with almost constant temperatures in the 10 °C range, with air source heat pumps the heat supply and demand are divergent. In particular they work less efficiently when a high heating capacity is needed, i.e. with cold outside air.

Scientists at Fraunhofer ISE have therefore investigated how additional heat sources at comparatively high temperatures can supplement the primary energy supply. Possibilities include heat from solar thermal systems or the central ventilation systems of highly insulated residential buildings.

The research was done in close collaboration with colleagues from the E.ON Research Centre (ERC) at RWTH Aachen University, who closely investigated the heat distribution. They showed that there is potential for optimisation resulting from improved control strategies for supplying needs-based heat to individual rooms.

In simulation calculations and laboratory experiments, the project partners combined their respective research focus areas in regards to heat sources and sinks. As an industrial partner, the Vaillant heating engineering company led, coordinated and supported the research work.

The investigations focussed on newly built or refurbished single-family homes with an energy requirement less than 120 kWh/p.a. To establish comparability with other studies, the researchers used a 140 m2, two-storey residential building, as modelled by IEA SHC Task 44, as their reference in simulation calculations. The boundary conditions are a heating demand of 60 kWh/m2 p.a.) and a domestic hot water consumption of 17.7 kWh/(m2 p.a).


Heat from two sources

Preliminary studies have shown that air-to-water compact units are simple in construction and inexpensive to use when they use exhaust air from the building as a secondary source of heat. This concept has therefore formed a focus of the research, which has ranged from initial simulations and the optimisation of the evaporator to the construction of a demonstrator.

Another focus was on conventional air source heat pumps with outdoor units where a solar thermal system acts as a supplementary heat source. However, the calculations showed that the investment costs compared unfavourably with the additional yield. In addition, the regulation is particularly complex here. It has to decide, in accordance with the operating conditions, whether the solar thermal system should act as a heat source for the heat pump or feed directly into the supply line for the heating distribution system. The concept was not pursued for economic reasons, since a major advantage of air-water heat pumps are the low system costs. The researchers did not want to relinquish this plus factor. The extensive simulations of various system variants also showed that comparably large photovoltaic systems are advantageous relative to solar thermal systems in terms of the primary energy.


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Source management
Fraunhofer ISE

Sink management


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