# Hybrid solar panel (photovoltaic and thermal)

## Step 8: Two days of real testing (May/5 and May/6)

Finally we have some sunny days!  I can do some testing.  Pity can't do it until 4:00 pm and later...  Sun is bright but not at its brightest.

Photo 1: First day of real testing (May/5). Half of the panel is powering some 12 V halogen lamps. Connecting 1 lamp this half panel was providing 9.3 V at 1.47 A (power = 14 W), connecting 2 lamps the voltage dropped to 7.9 V at 2.69 A (power = 21 W). Extrapolating to the whole panel this would be at least 42 W of electrical power (not too bad for doing this experiment at 4:30 pm, considering that the theoretical maximum for this panel is 60 W, assuming 1 kW/m^2 solar power and 12% efficiency of the PV system). The other half of the PV panel powers the water pum, which makes water circulate between the panel and the cooler. There was 3 kg of water in the system.

Photo 2 and Photo 3: Results from May/5.  Given water's heat capacity [4.2 kJ/(C kg)], one can conclude that the thermal power transferred was about 200 W maximum (again not bad considering that the panel received at most 500 W of solar light!). Note that I am neglecting possible other possible heat transfers (cooler, etc). See the second day of experiments below. The max. temperature reached was 52 C (126 F). Quite hot to the touch!

Photo 4 and Photo 5: Results from May/6.  Second day of experimentation. Again I used 3 kg of water in the cooler. Note that at the 94 minute I turned off the water pump and the water started to cool off (at an energy loss rate of ~ 56 W). See the next picture. I estimated that in the "more stable" region of temperature rise the thermal power was about 210 W. This day the solar flux was similar, notice that the electrical power for 2 lamps was almost the same as the previous day. Also notice that connecting 3 lamps in parallel reduces the net electrical power: this is an important factor when designing a complete PV system, you need to optimize the IV operational point!  In the plot, the very high rate of temperature rise in the first few minutes is probably an artifact of having used cold water rather (BTW: the air temperature was about 24 C). At the 94 minute I turned off the water pump and the water started to cool off (at an energy loss rate of ~ 56 W). Notice that as happened before the maximum temperature was about 52 C (126 F).

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Mechanic2011 says: Mar 27, 2011. 7:53 AM
You won't know for sure how a particular panel will perform until you build it and test it like this instructable shows .
By the way great instructable !!

IF a person had the regular type of flat plate solar collection system already then it is possible that the lower level heat coming from this type of panel could be used to preheat the coolant going to the flat plate collectors providing you built enough of these to get the coolant flow rates that you need for 2 arrays plumbed in series.
Run it through this type pf panel first then to the flat plate collectors.
You could even use the same pumps and not have to add any.

I just wonder though if a person would need more pump volume (i.e. a multiple speed pump) on a really good solar collection day to increase the flow a bit so as not to overheat any of the panels?

I guess I'll find out when I build it.

As long as the home built panels are to be used for things off-grid then there won't be any legal issues with certifications on the photovoltaic panels hooked to the power grid.
carlos1w (author) in reply to Mechanic2011Mar 28, 2011. 6:34 AM
Thank you for the comments (this and the earlier one, I am responding to both at once).

1) I used the thinner pipes because they were easier to bend and cheaper (I had already spent much more than I had planned, note: it is possible to use a cheaper insulator than the sil-pad if you are careful enough even tar paper will do). For this size panel I think that the 1/4" tubing is sufficient (I think it is hard to believe one will be able to collect more than 50% of the sun light). If anything, I would have liked to use a thicker aluminum backplate (wasn't patient enough to wait for one, and this is the thickest I could source locally). Another thing would be to place the pipes closer together.

2) For larger panels you may want to have larger water flow to avoid the chance of it heating too much (at which point your heat capture would drop drastically). If you have several panels, I would probably connect the pipes in parallel rather than series, again because once the water becomes too hot your heat capture will be reduced.

3) Please check my comment on "May 14, 2010. 7:31 PM" about aluminum extruded panels. I think these are used for the side walls of 18-wheeler trucks and maybe are not too expensive if you can source them (I couldn't). This will be the best option (no soldering, best water flow, best thermal connection, probably quite good rigidity).

4) Also, please check my comment on "Nov 14, 2010. 3:01 PM" about aluminum and copper brazing without flux. I bought some of these products on ebay and they work great (and no nasty fumes, or at least not that I could tell). Note that you do need to get the materials quite hot for this technique to work (hotter than tin soldering).

5) Please give me an update (even better, post an instructable and put the link here) when you build your system!!!

dimovi says: May 13, 2010. 8:27 AM
I don't know how useful that heat is, however the solar panel is going to be more efficient the colder it runs. So, you should see if the heat you remove from the panel improves your efficiency enough to justify a water pump.
carlos1w (author) in reply to dimoviMay 13, 2010. 8:57 AM
The pump is not solely to cool the cells, it is to extract heat for heating water for household use.
devonfletch in reply to carlos1wDec 2, 2010. 4:50 PM
Of course, if you put the storage tank HIGHER than the top of the absorption system it will self-circulate, ( simple convection) and automatically stop circulation when the sun goes down, and the collector is the coldest part...this leaves the opportunity to harvest the hot water (take a shower). No need to process the power/energy any further. Hot water is a pretty expensive commodity, energy-intensive (carbon-polluting!) and it's by far the best way to harvest solar energy. Twin-walled polycarbonate roofing is a much larger and more durable example of the soft real-estate sign material. It should only be sealed using a neutral-curing silicone. I used Polyurethane (Sika-Flex) on a project, and it caused bad cracking of the Polycarbonate. Also, I'd check possible leaching of nasties from the polycarbonate, and any other plastics in the system.
Using a convection circulation, you would need to use considerably larger pipes.
carlos1w (author) in reply to devonfletchDec 3, 2010. 7:51 AM
Thank you for your comment! Convection would work, though with such thin pipes I am not sure it would be fast enough, and sometimes forcing the circulation with a pump (in this case solar powered too!) would be more convenient. Regarding polycarbonate: I have read that it may leech some chemicals into the water, so perhaps it would then need to operate with a separate circuit, and not circulate the water to be used. Then again, this is perhaps advisable for other reasons: (i) use of anti-freeze for safety, (ii) elimination of build-up problems in pipes (calcium, etc) sicne we would use a closed-system.
bethehammer in reply to carlos1wNov 27, 2010. 11:58 AM
With sizeable enough system you could also recirculate it through an under floor network of tubing to provide radiant heat as well as just warm water for consuming.
danbenedix says: May 22, 2010. 11:19 PM
Why not take the heat you are moving and run it across some peltier (thermoelectric) coolers.  These are the modules used in those iceless coolers.  This is my recollection of how they work.

When current is applied one side gets and one side gets cold to change the hot and cold side just swap the polarity.

Here is the neat part.  If heat or cold is applied they will generate current.
So since you are already capturing the heat and have a system setup to flow electricity why not tie the peltiers back to the inverter?
manleyjames62 says: May 13, 2010. 2:14 PM
fasinating thankyou for sharing this.
excellent craftsmanship.
i would very much like to see if there is a difference with and without the pump on say 30 gal of water
carlos1w (author) in reply to manleyjames62May 13, 2010. 7:17 PM

Well, it would take some time (with this 0.5 m^2 panel) to get 30 gal up to 52 C (several hours).  If you think about it, it is not so bad.  In 1 day you get enough hot water for a shower.  Alternatively, one should get a bigger panel!
achituv says: May 13, 2010. 7:17 AM
Why pump the water? simply place the 'cooler' in a place that is higher than the panel and use thermodynamics.
unclejoe in reply to achituvMay 13, 2010. 1:47 PM
yes. Thermosiphon. no pumps needed just checkvalves. lowers cost of system.
carlos1w (author) in reply to unclejoeMay 13, 2010. 7:16 PM
The pump was only \$10.  Considering the rest of the cost of the system, I did not mind having a more controlled convection.
urbanprimate says: May 9, 2010. 7:07 AM
I like the concept, but was mislead by the title into thinking you were converting heat into electricity as well.  I suppose the idea is to provide off-grid power as well as hot water?  Maybe by using a black barrel to hold the water (and the heat) you could replace your water heater?  Those things suck resources (mine uses electricity, gas, and city water..I want to use the sun and water filtered from the creek that runs through my property to replace it.  Thanks for the inspiration...I'd like to see what you do with it with regards to practical application!

Shiftlock in reply to urbanprimateMay 13, 2010. 9:34 AM
Instead a barrel, why not feed the heated water from something like this into your existing hot water heater?  It already exists and is tied into your plumbing, it's made to hold water, and it's insulated.  That way, the hot water heater would use less power to reach the temperature you set it at.
aasbu02 in reply to ShiftlockMay 13, 2010. 5:04 PM
Could you run your water pipe past the heat source (PV panel) to heat it? Should the water sit there for a time? Trying to think of the most efficient/easy way this could work.
Shiftlock in reply to aasbu02May 13, 2010. 5:34 PM
I don't think letting the water sit in the PV panel would do much good, since the volume of water that the pipe holds is rather small, and you want it to continuously draw the heat away from the heat source the way a liquid-based CPU cooler or a car radiator works.  The water should keep recirculating through the panel, using your hot water heater as the holding tank.  I think the pump should return the water to the top of the tank, and draw it from the bottom of the tank.  That way it will re-heat the water as it cools and falls to the bottom.

Someone correct me if I'm wrong.