Using radiated heat from PV panel

[betwixt]

I'm not sure whether this is a PV or solar thermal question, it crosses both topics.

I currently have a huge solar thermal panel (3m x 25m) made of about 2km of black neoprene pipes. I pump chlorinated swimming pool water through them to heat it and it works really well.
I now want to increase my current 300W or so of PV to be much bigger, I'm thinking 1.5KW or thereabouts.  The trouble is lack of space and I'm wondering if it would be feasible to lay framed panels on top of the pipes and utilise the radiated heat from the rear of the PV panels to warm them. I realise it wouldn't be as efficient as direct solar insolation but has anybody tried this and can comment on their experiences. Sadly, I can't let the water directly touch the PV, it would always be spaced away by the thickness of the frame.

Brian.

[supremetwo]

if it would be feasible to lay framed panels on top of the pipes and utilise the radiated heat from the rear of the PV panels to warm them

As I read it, you are proposing to block the infra-red on your pipes by placing PV panels on top.

So no thermal into the pipes.

PV panels have little electrical output when irradiated by infra-red and that part of the solar spectrum cannot be used by a PV cell.

A photon with wavelength around 1000 nanometres (corresponding to short wave infra-red light) has just enough energy to promote an electron in a silicon atom. All photons with a longer wavelength than this have insufficient energy to promote the electron and either pass straight through the PV cell or are absorbed as heat. It's shorter wavelengths in the visible spectrum that are needed for PV.

In fact, the cooler the PV panels are, the greater the amount of electricity generated, so the last thing you want is to heat them.

There is research to use more of the spectrum for PV, e.g. http://www.fullspectrum-eu.org/1_1.html

[asorton]

I think the only way to do what your suggesting is with PVT:

http://www.newformenergy.com/photovoltaic-thermal

Volther PowerTherm
 ◦The PowerTherm collector has been developed to maximise the thermal return of the panel, essentially making it an enhanced thermal collector capable of electrical production. The peak outputs of this panel are155/610 watts electrical/thermal respectively.
◦The PowerTherm collector will produce roughly 80% of the output of the equivalent area of solar thermal but it also produces electricity.
◦The PowerTherm collector is ideal for customers with restricted roof space trying to maximise the energy return from an available area.
◦The collector is ideal for use with swimming pools, operating at lower temperatures supplying year round heat to the pool and electrical energy to offset the running of pool equipment.
◦PowerTherm can be used in conjunction with ground source heat pumps, feeding excess summer heat into the ground which is then recovered during colder periods to heat a building. PV-T is capable of producing a zero-carbon building as part of such an integrated solution in a low energy building.

[betwixt]

Supremetwo, sorry if my message was ambiguous. I'm wondering if exactly the opposite is feasible. The PV panel goes on top of the pipes (water filled black neoprene, like a hose pipe not glass) and the heat radiated from the rear of the PV warms the water. It would mean the space under the collector surface is actually cooled by the water pipes although by convection and not by direct contact.. I've seen how very hot my existing PV installation gets after a few hours of direct sunshine and the pipes, at least the inlet end of them, would generally be at about 27 - 28C.  In essence it would be utilising the hot air trapped within the PV frame.

Asorton, that's nearer what I tried to describe but sadly the pipes contain chlorinated water which looks to be unsuitable in their product. This whole system was installed a few years ago to replace a crazy installation that was present when I bought the house. The previous owner had made a tilted frame, insulated it's top and then placed 10 large central heating radiators on it to heat the pool water. They painted the bottoms of the radiators silver and the tops matt black, they even built a glass cover to preven loss by convection. What they didn't do was use a heat exchanger and what went in at one end as clear pool water came out the other like chocolate. It was abandoned after only a few days and within a few weeks had corroded beyond recovery.

Brian.

[rogeriko]

The pipes need direct sunlight to heat up if you cover them you will have cold water. There is no measurable heat radiated behind a solar panel, go put your hand 3 or 4 inches behind a panel and see.

[ovonrein]

I think the only way to do what your suggesting is with PVT

In *theory* this is a very good idea.  In summer, when our PV panels get so hot they become less efficient, the PVT panels receive cooling from the water flowing through them.  In winter, when snow gathers on those panels, warmer water will help de-ice them.  The issue is price.  But it's a long time since I did the maths.  You may be able to make the economics stack up today, using the FIT and RHI together, perhaps.  And may be the PVT panels have come down in price too?