Fig. 35 Heat pumps and solar energy can be combined.
© obs/Schüco International KG

Fig. 36 Solar storage tank for heat pumps.
© Bundesverband Wärmepumpe e. V., Berlin

Fig. 37 System diagram for a heat pump system with a flat plate collector connected to the domestic hot water storage tank.

Fig. 38 System diagram for a heat pump system with an unglazed solar collector that feeds solar heat to the ground or directly to the heat pump as a second heat source.

Fig. 39 System diagram for a heat pump system with flat plate collectors for heating the domestic hot water storage tank and as a general heat source for the heat pump.

Fig. 40 System diagram for a system with an air source heat pump and solar collector for directly heating domestic hot water and as a second heat source.
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Solar-assisted heat pumps

In many cases, heat pump systems can be successfully combined with solar thermal systems so that solar thermal energy can be used to meet a large proportion of the hot water requirements in summer and part of the heating load during transitional periods. Alternatively, the efficiency of heat pumps increases significantly when the temperature of the heat source is increased with solar thermal energy.

Solar thermal systems are frequently designed in residential buildings so that the collector surface area and storage tank meet around 60% of the annual hot water requirement. Larger sized systems can also be used for providing space heating support. The remaining energy required for domestic water heating and space heating is provided by a further heat generator, for example a gas boiler or a heat pump. The solar collector is connected to a storage tank via a solar circuit.

“Conventional” incorporation of solar thermal systems

Fig. 37 shows a system configuration that combines a solar thermal system with a heat pump. The solar thermal system charges a domestic hot water storage tank that – whenever required – is also heated by the heat pump. In this example the space heating is directly and exclusively provided by the heat pump, i.e. without a heat storage tank.

It is important that the heat pump control system prioritises the solar heat generation. The high seasonal performance factor of the solar thermal system means that the electrical energy requirement lowers and the system efficiency increases. The improvement in efficiency and the operational cost savings depend on many parameters such as the solar irradiance, the type of main components and their sizing. With systems that use ground source heat pumps, the solar thermal system reduces the heat absorbed from the ground in summer. The extent to which this reduces the drop in ground temperature during the heating operation as a result of this, or the extent to which the borehole heat exchanger or horizontal ground heat exchanger can be made smaller depends among other things on the relative reduction in the heat removal and the existence of groundwater flows.

Solar thermal energy as a heat source for heat pumps

A further approach is to integrate the solar thermal system on the source side of the heat pump so that the solar thermal energy is either the sole heat source for the heat pump or provides supplementary heat. A growing number of systems have been available on the market for several years now that sometimes differ only to a slight extent but are also sometimes fundamentally different from one another. The main differences are in terms of the following aspects:

  • Heat source: Type and sizing of the heat source(s)
  • Heat storage system: Is a storage tank connected on the heat source side? Which type of storage tank is used?
  • Collector type: At which temperature level is the solar thermal system available?
  • Incorporation: In addition to being incorporated on the source side, is the solar thermal energy also utilised on the sink side, i.e. for directly heating domestic hot water and water for space heating?

Three examples from the diverse range of existing system concepts are described below.

Fig. 38 shows a system with a ground source heat pump that incorporates unglazed collectors to provide heat cost-effective at a low temperature level. The solar heat is exclusively injected on the source side. The solar thermal energy stored in summer can be used for regenerating the ground. This stabilises the heat source against any possible, unforeseeable increases in the heat removed and (slightly) increases the heat source temperature of the heat pump.

Fig. 39 shows a system that exclusively uses the solar thermal system as the heat source for the heat pump. The flat plate collector gives precedence to directly heating the domestic hot water storage tank. If the solar thermal energy generated surpasses the requirements or the generated temperatures are too low, the solar heat is fed into a buffer storage tank that is used as the heat source for the heat pump. The buffer storage tank ensures that the solar thermal energy can be used at a later stage following its generation.

Fig. 40 illustrates a system concept with an air source heat pump and a flat plate collector. The solar thermal energy principally heats a domestic hot water storage tank. If the generated temperature is not sufficient, the solar thermal energy is directly used as an additional heat source for the heat pump during the heat pump operation. This increases the source temperature of the heat pump, which for air source heat pumps is only very low on cold days during the heating season.

System assessment

Field measurements and simulation studies show improvements in the efficiency when using solar thermal systems on the heat sink side (“standard system”). Their incorporation on the heat source side requires a sophisticated, robust control concept; the added value needs to be considered in a more differentiated way. Until now there have been no comprehensive analyses of the different concepts with different application areas and boundary conditions. Whereas relevant guidelines exist for heat pumps and solar collectors, no corresponding standards are available for combined systems. But established simulation-based test methods of solar thermal heat generation systems were already extended to cover heat pumps as auxiliary heating equipment.

The “Solar and Heat Pump Systems” project is being organised by the International Energy Agency (IEA) and lasts from January 2010 until December 2013. It is concerned with comparing solar thermal heat pump systems, whereby among other things it is developing evaluation parameters and methods for investigating the systems using simulation methods, laboratory measurements and field tests. This joint project is also incorporating the results from a project conducted by the University of Stuttgart and funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, which is concerned with developing performance tests and ecologically evaluating combined solar heat pump systems (HPSol).


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