Has anyone here installed PV-T panels? They would seem a perfect option for us as we are not allowed to put solar on our roof, but may be able to put them onto a smaller outbuilding that has an east-west facing roof.

We've pondered whether to use solar thermal (we have an unused solar coil in our heatbank) as we'd get more energy from this smaller surface area, but then solar PV has more uses and will work in lower light conditions.

I stumbled across an article about PV-T panels and thought that this would be the perfect solution for us. The same panel produces both hot water and electricity, maximising the amount of energy we could get from the roof. They are MCS approved and so we could receive the FITs payments, but they would also qualify for RHI (if that ever happens).

Here is the article and a snippet of the content:

http://www.mcsapprovedinstallers.co.uk/blog/what-is-solar-pvt/

"Solar PVT is a solar system that combines both solar PV (for generating electricity) and solar thermal (for generating hot water). Typically these two types of energy are collected through separate panels, but with solar PVT only one panel is used.

As Solar PV modules are semiconductors, they have one draw back, which is degradation in high temperatures. In hot summer days the heat between the solar array and the roof can reach up to 100ºc, which makes them much less efficient. Solar PV panels therefore work best in low temperatures. With solar PVT the thermal part of the system takes away some of the excess heat, meaning the solar PV part performs more efficiently and the excess heat is utilised."

The article goes on to discuss the pros and cons. There are more technical articles available on the subject, but this ones gives a decent overview for those (like me) who haven't heard of these panels before.

Note: PV-T aren't those Solar Thermal panels with a small square of PV to run the water pump on solar energy, but a full sized PV panel combined with a full sized solar thermal panel.

Sorry, I did a quick search on 'PV-T' to see if it had been discussed but didn't find anything.

As far as the temperature of the panels, I guess it really depends on what you are doing with the hot water. If you are feeding into into a standard (small) hot water cylinder then it could quickly heat up the store and then and then the return to the panel could be high temperature. If you have a large heatbank/thermal store then the return to the panels could be quite cool for a much longer period. If you are using it to recharge a GSHP loop, either the slinky type ot borehole then the return will constantly be cool (about 14C?)

It would be interesting to know what sorts of temperatures PV panels reach in direct sunlight, full sunlight (noon) and overcast conditions. I read that in full sunlight PV can get well over 100C and I'd imagine that in overcast conditions it could still get up around 20C/40C? If that's the case, then the solar thermal part will eliminate the very high temperatures in full sunlight, reduce the high temperatures in direct sunlight and have little effect in overcast conditions because it won't get hot enough for the pump to come on.

I can't really imagine a scenario where the PV panels get hotter because of the solar thermal element, especially if the system is designed correctly with a method dumping excess heat once the thermal store/HW cylinder is full?

If it were a new build you could no doubt use the heat in a seasonal thermal energy store under the house. But, would the heat loss be sufficient?

Thanks ST. I'll get a brew, pop into that link and have a read.

I've just been looking at research on the internet and found:

"The Effect of High Temperatures
The energy production efficiency of solar panels drops when the panel reaches hot temperatures. A field experiment in the United Kingdom revealed a drop of 1.1% of peak output for every increase in degrees Celsius of a home photovoltaic solar panel once the panel reached 42 degrees Celsius, or about 107 degrees Fahrenheit See Reference 1, conclusion). Laboratory experiments at the Rivers State University of Science and Technology at Port Harcourt, Nigeria in 2008 found similar results; solar panel energy production dropped off steadily once the panel temperature reached 44 degrees Celsius, or 111 degrees Fahrenheit (See Reference 2, Table 1). "

http://greenliving.nationalgeographic.com/effects-temperature-solar-panel-power-production-20500.html

If the efficiency only starts to drop off at 42 to 44C, then this is a much more reasonable temperature to be able to reduce the return feed from a heatbank, heating system or dump radiators. Even if the best you could do is to get a return temperature of 60C, it'd make a PV panel more efficient than having it heat up to +80C in direct sunlight?

Everything I have read points to this technology returning far more solar energy from a given roof space than PV or solar heating alone. In our circumstances with a small roof space and a eatbank with solar coil, it would seem to be ideal.

Thanks for the links ST. I've continued the discussion on the original thread so this thread can now be closed.

Posted By: Ed Davies

Posted By: Pile-o-StoneIf the efficiency only starts to drop off at 42 to 44C,

Nope, I think it's across the range.

The national geographic report that I cited mentioned the 42C to 44C drop off point.

"A field experiment in the United Kingdom revealed a drop of 1.1% of peak output for every increase in degrees Celsius of a home photovoltaic solar panel *once* the panel reached 42 degrees Celsius"

Basic laws of physics with semiconductors Im afraid - increase in temperature = increase in resistance = drop in performance. Havent we all had a hot laptop which just goes slower and slower? As the processor heats up, the PC management sheds operations thus making things slower to prevent frying.