P V Panels vs. Passive Solar Heating

Currently the panels on the market do very little on cloudy days (MIT is doing research on all weather panels, as are others). You'd be better off keeping the heat you have with this stuff called insulation. If you do it right it does a great job at slowing the migration of heat.

I was taken in a couple of years ago, when solar panel christmas lights came on the market. The sad truth is, these lights don't work at christmas! There's not enough hours of daylight this time of the year (at my location), for the PV to power the string of LEDs.

My guess is that if you are far enough north to need home heating in winter, solar PV panels will not be productive in the circumstances where home heating is required. Using PV's power for air conditioning in summer might be a more realistic goal - but I don't know how many panels you'd need to generate enough energy for that purpose.

PV is not economical for heating.

Look at the power ratings of your electrical heating appliances and compare that with the output of a PV array.

Solar thermal harvests about 1kW(heat)/sqm, PV harvests about 100W(electrical)/sqm

You could warm your hands with an incandescent lamp with PV not the interior of a building.

If the PV array was big enough you could do it but the cost.......

Hi wal, You are a bit over optimistic for solar thermal and a bit pessimistic for PV. 1kW/m2 would be 100% conversion efficiency on a sunny summer lunch. You can't use that ideal value for design calculations, especially if we are talking winter heating. Have a look at my long answer to the initial question further down.

Works fine here for rough dimensioning.

Go for ground source heating coupled to a heat pump & fully insulate the building, you should do away with heating costs altogether, There's a chap in England who has converted a concrete ex pumping station to a house, he put insulation on the outside of the building then rendered the whole of the external, he then made the building as air tight as possible fitted heat exchangers to the extracts and intake fans, The roof is flat so he fitted solar pv panels to the whole area, A third of these were enough to make him completely independent of the power company's, the other two thirds he sold to the grid. Be cause this was a water pumping station there was a 90meter deep artesian well which he used for ground source heating and to get his water from, so he has got no utility bills at all.

There are now panels that do both, ie produce electricity and heat, some do electricity and hot water, others include heating air, - that seems to me to be the answer you are looking for? One company I am aware of is called Medynique, http://www.meldynique-solar.com/site/Solar_Modules.html I don't know much about them but there are also others out there, so one can say that the technology is available right now. Cheers, Geoff Thomas.

Oops, that should have been Meldynique, and certainly insulation and thermal storage are good to include, Geoff.

What a great idea.

If area is limited then this is an answer.

Not sure about cost though....

These hybrid panels are a compromise. Because the PV elements are the ones exposed to the sun, the heat absorption is not as good as with dedicated thermal collectors. Also, it means the PV cells tend to get hotter than in a normal PV modules and that lowers efficiency (about 0.4% relative per centigrade for cristalline Si). So you might end up with a compromise that suffers esoecially in winter for not being able to produce hot enough water but still the cells will be hotter than on a normally ventilated PV module (very roughly Ambient+20centigrades). If you consider, a freezing point winter day the PV module will not reach more than 20centigrades. The hybrid panel would have to reach at least 35 degrees C even for floor heating, probably 50 degrees C or more for traditional heating systems. So your PV module efficiency would go down by 6-12% relative of what the dedicated PV module would have brought.

I also just had a look at the data sheet of a hybrid collector and from that they don't reach 70% of thermal efficiency at deltaT of 10 deg C (who needs ambient T+10degC for their hot water needs? Looks like the marketing department told the engineer he had to make the numbers look good). Expect efficiency to get worse as delta T increases and from the above you see that it will more likely be 30-50deg C. Those hybrid collectors that I have seen were classical PV-modules with a thermal collector mounted instead of the back sheet. Now, dedicated thermal collectors instead use an air gap between the front glass and the collector surface to insulate much in the same way as double glazing does in a window. Also, their absorber has a low emissivity to avoid radiation loss. Just by a rule of thumb estimate I would guess that a hybrid collector that does not have these features will see its efficiency go down about 1% more per degree difference than a dedicated system (if someone has real numbers I am all ears). At a delta T of 40degC that is a loss of about 40% on the thermal part and 10% on the PV part.

it might not seem like much for the PV part but considering it is in winter where the system will struggle anyway and especially considering the thermal loss, I would prefere dedicated systems in most cases.

That said, there are certainly situations where this makes sence. For example, if you have a free source of cooling water (i.e. you live by a lake or stream) you could keep your cells cooler than in a standard PV module during summer. This would actually give you a more efficient PV system (if the pump consumes less than the efficiency gain)? Also, if the PV part is much more valuable to you, you have limited space and you still need quite some warm water (like for pool heating at the public pool).

in those cases what will decide is likely the cost balance.

Wal, where do you life?

Tropics.

That is very nice, it means you get sufficient sun all year round. I would guesstimate that you get somewhere between 4-5kWh/m2/day with a solar thermal collector.

At 6 good insolation hours per day that's about right for thermal.

Look at evacuated tubes for solar thermal. Or build your own sun-tracking solar concentrator (Google "CSP") if you live in a tundra.

Both systems you name have as their main advantage to be able to make hotter water than flat plate collectors. This comes in very handy in winter. Each one has an inconvenience though, especially if we are talking about winter weather as I know it from Europe or north America:

- solar concentrators depend on direct sunlight. They collect nearly nothing on the winter days we have so often when the light is very diffuse due to light clouds. I would never opt for these for a zone with lots of diffuse light, especially in winter where you need the energy most.

- In general you will have to liberate your solar collectors from snow in winter. This can be difficult with vacuum tubes without risking to break them. At the last Intersolar I saw a vacuum tube collector that was covered by a flat glass pane. This seems like a very sensible step to me and technically would be my solution of choice.

(there used to be vacuum tubes that work as concentrator devices. I haven't seen those in a while but obviously the same thing as for any optical concentration as above counts for them as well)

The vacuum tubes don't melt the snow that may accumulate on them due to their intrinsic insulating properties but the gaps in between the tubes let the snow fall through. An uber cover of glass will allow the snow to pileup even worse.

Those glass tubes are pretty tough. They are hail resistant.

What do I know? It doesn't snow here.

Valid point if the installation is in a place with little snow accumulation. If the installation is in a region where significant snowfall is more common the amount that comes down in a single day can easily be 30cm. If you reach such snow heights the snow covers everything and you have to go up with a shovel and broom to keep your collector working. That's when I prefere the flat plate over it.