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Solar PVT: hybrid panels with a dual function

Newform PVT panelsSolar PVT panels generate electricity and supply hot water using the same roof space.

Solar PVT stands for 'solar photovoltaic-thermal', meaning that the same 'hybrid' panel generates electricity and heats water using the sun, instead of needing different panels for each function. Hence, PVT panels can make full use of the renewable energy from the sun where space (e.g. roof area) is restricted, and help toward creating a low-carbon building as part of an integrated system. But at a cost - they are more expensive than either type of panel alone.

The captured heat can potentially be used for hot water or space heating, and a heat store, such as a water tank or vertical borehole, is required. This can store excess heat produced during sunny days or seasons, which can then be withdrawn to supplement the heating or hot water system. Such a system requires a heat pump and controller to balance inputs and outputs appropriately.This means that PVT panels are most suitable for new-build properties off the gas grid.

Space-saving and efficient

Apart from the space-saving aspect, another advantage of PVT panels is enhanced efficiency of the solar PV cells because of the cooling effect of the fluid circulating through the panels. The efficiency of conversion of light into electricity by solar PV panels declines as the ambient temperature rises, and in summer on certain roofs this can exceed 100°C. The active cooling effect is claimed to give roughly 20% more output over a 12-month period compared to conventional solar PV panels. However, the thermal efficiency of the hot water collection is reduced compared to a standard solar thermal system.

Costs and payments

PVT panels are generally integrated with a heat store and heat pump. A typical 16-panel PVT array will cost in the order of £10,000, and deliver 4 kWp of solar electricity and 10 kWp of heat. Added to this would be the cost of the ground source heat pump, control systems, and groundworks, depending on the nature of the setup. Hence a straight comparison with stand-alone solar PV and solar thermal is misleading. However, solar PV is typically now around £1500 to £2000 per kilowatt peak, while solar thermal by itself would cost around £4500 to £5000.

Like other solar PV, PVT panels qualify for the Feed-in Tariff (FIT) payments, but currently not the domestic Renewable Heat Incentive (RHI).solar house

The zero-carbon Solar House, Great Glen, Leicestershire, is equipped
with a 5.6kW array of solar PVT panels and incorporates
an 'energy bank' to store excess heat.

Summary

Benefits

  • maximizes yield from sun's energy where roof space is restricted
  • cooling circuit improves efficiency of solar PV cells
  • panels can absorb heat at night
  • cost of manufacturing and installation reduced
  • low maintenance
  • solar PV attracts income from FIT.

Drawbacks

  • heat output does not qualify for domestic RHI payments (whereas a separate solar thermal system would be eligible)
  • heat pump requires electricity
  • most suitable for larger new-build projects
  • price relatively high
  • panels are slightly heavier and bulkier than conventional solar PV
  • installer must be MCS approved for both solar PV and solar thermal
  • potential hazard if heating fluid leaks onto electrical components