Solar Energy Perspectives - IEA

Chapter 4: Buildings
radiators or heating floor directly transfer heat to the colder transfer fluid in the ground. This
“free cooling” option saves electricity in not running the compressor of the heat pump since
no temperature lift is required. The third option is to send some solar heat from the collectors –
again, a relatively small area may suffice – into the boreholes during summer. This heat will
be efficiently recaptured by the heat pump in winter, while the solar collectors can also be
used to pre-heat the fluid that enters the heat pump, further increasing its efficiency (CoP).
This can be done with glazed or unglazed collectors, as shown on Figure 4.8. Even for singlefamily
houses, heat losses in this combination are limited by the relatively low working
temperatures of this sort of inter-seasonal ground storage.
Figure 4.8 Combination of GSHP with solar collectors
Unglazed solar
collector
Heating system
Storage
Hot
water
Heat pump
Borehole
Cold
water
Source: Henning and Miara/Fraunhofer Institute for Solar Energy Systems.
Key point
Unglazed solar collectors can increase the efficiency of ground-source heat pumps.
Indeed, there are many ways to combine solar heat and heat pumps. These combinations
increase the solar fraction of water and space heating, up to 50% or more, with limited solar
collector areas and without the need for very large heat storage systems. They increase the
SPF of heat pumps and can provide long-term ground temperature stabilisation to GSHP.
Another combination of great interest in urban renovations and many other cases where
access to the ground is limited links solar collectors with the less efficient ASHP (Figure 4.9).
Glazed collectors are likely to be preferred in this case for better performances, to compensate
for the likely lower temperature of the ambient heat. This raises the SPF by about 20%.
A more sophisticated combination that added an intermediate latent heat storage system
could lift the SPF by 40%.
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© OECD/IEA, 2011