Instructables

Hybrid solar panel (photovoltaic and thermal)

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Some time ago I bought some PV cells (6"x3", from rebeccayi0904 on ebay, very nice seller!) with intention of building a PV panel (after reading a plaethora of instructables on solar panels here!).  While testing the individual cells out there in the sun I noticed that they got quite hot.  I then realized that PV panels convert to electricity only about 12% of the solar power that gets to them.  What about the rest?  It becomes heat (about 88%).  I figured that perhaps in the same surface one can harness both the electricity and the thermal energy of the panel....

 
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Step 1: Overall panel characteristics, items, etc.

I am not going to repeat all the details on how to build a solar panel, there are plenty of other instructables for this (search tool is your friend!).  I will give some basics, though... and then focus more on the "hybrid" nature of my panel (PV + thermal).

General characteristics:

- about 0.5 m^2 area, at a maximum of 1 kW/m^2 of irradiation and 12% efficiency this should produce UP TO 60 W of electrical power.  (at the same time this means that about 440 W of thermal power could potentially be harnessed!).

Materials:

- 36 cells, 3"x6".   Cost:  about $150 from rebeccayi0904 (ebay, nice seller!) for 80 cells (used 36 for this panel).
- aluminum backplate (26"x32", can't remember thickness):  about $10 in a sheetmetal store.
- small roll of Begquist sil-pad 400, cost about $50 from ebay (can't remember seller)
- glass front cover, about $15 at the hardware store
- aluminum rails for borders, about $12 at the hardware store
- about 25 feet of 1/4" copper pipe, about $20 at the hardware store
- some 2-3 tubs of silicone caulk
- aluminum flux paste from McMaster-Carr (about $30, but you can buy a smaller quantity, I only used about 1/20 of the tub)
- solder
- a 12 V water pump, search on ebay "12 V pump laser & cpu cooling", cost about $10.

Caution: Aluminum Flux Paste is a very nasty material.  It contains fluoride and if handled improperly it can cause serious harm to you.  Read all instructions and the material safety data sheet (MSDS), and if you are not 100% confident that you can work safely, do not.
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shortw2 months ago

Would be a lot easier and cheaper to use black pvc tubing.

Very nice.
(1) Note that you REALLY don't want to use broken cells if only a few are nbroken as it affects the WHOLE panel output in proportion to the % area missing in the ONE cell. eg if a single cell has 10% area missing then the whole panel output will be 10% lower if all cells are otherwise the same. This is because current out is proportional to cell area while voltage is essentially nnaffected by area. So if you wire an -800- 800- 800- 600- 800- 800- mA cell string in series the 60- mA of the lowest cell "throttles the while string. In many cases if you have only one low output cell you may be better off shorting it out than leaving it in. eg if you have a 36 cell 18V nominal panel, if you short out one cell you get a 17.5V nominal panel. In practice it wioll usually be somewhat higher and in most cases when driving a 12V system with Vmax battery lead acid = 13.8V the 17.5V at full current is better than 18V at reduced current.
(2) The cooling water will add 5% to 10% to panel output on a hot day. I have tried running a very thin film of water over a panel surface with excellent results. Needs a continuous water supply or a pump :-).
robbytesla1 year ago
A very good instructable. Is it possible to double the copper tubing to increase the efficiency of your heat exchanger?, Like two staggered "U" configurations both interconnected? Also what about adding (radiator) coolant to increase the heat transfer and use that in conjuction with a secondary heat exchanger to heat water or preheat water for the house?I will follow your work , you are doing research in a field and opening doors to many of us neophites. Thank you again for your article.
servant742 years ago
Did you figure how much additional electricity you get from the PV with the 'cooled' vs 'not cooled' use?

Cooling the PV is supposed to allow them to be more efficient.

Thanks ... Jack
That thought came to mind as well but in a different manner. Solar hot water and PV may not be compatable. The water temperature that would be good for PV may not be satisfactory for domestic hot water and vice versa.
Respectfully aesthetics is a relative topic,relative to the individual. Personally I go for function first aesthetics last. All I can say is to look for collectors with the least amount of bulk & can be painted to match the surfaces the are mounted on. The frame and mounting hardware can be painted, not sure how to mask the actual collector while retaining the function. Was a time a TV antenna was a bit of a status symbol, but a lack of one indicated we can afford cable TV also a bit of status symbol :) Perhaps a good decorator could give advise how to mount a collector so it would blend in with other structural elements?
Yeah you can cover them in a thick coat of paint the same color as your roof.

Can you tell me what exactly is "dis"-pleasing about them? I really wish you could describe in detail what it is you object to about them because a local h.o.a. forbid its members to install them for the same reason (yet water panels, not PV were OK.. why?)...If we could understand this phobia of the looks of PV panels, maybe we could do something to make it less objectionable to those who find it revolting....

If it's the color, there isn't much that can be done about that without infringing on the panel's efficiency... They must admit all the blue light they can which means red to match roof tiles or green to match trees is out of the question. But purple or cyan might be possible..... :)

What's interesting is that the removal of heat from the thermal section should actually help act as an active cooler for the PV section, BOOSTing its efficiency!

:)
snotty mcarrell2 years ago
Environmental apocalypse is pretty ugly too.

Sometimes I wonder if we hesitate to be sustainable because changing our ways would mean we've doing something wrong. If we kill for oil in Iraq, perhaps subconsciously we must believe this was the correct thing to do; otherwise we would be accessory to murder. And that's an uncomfortable conclusion where cognitive dissonance kicks in. Thus we choose coal and nuclear over wind and solar. Which is ugly? Which is pretty?

Just a theory.
However, if you paint the copper black you shouldn't have that problem, plus you'll have the added benefit of even more efficient heat transfer.
patenaude4 years ago
 It seems like mixing copper and aluminium is going to cause corrosion problems... probably on the aluminum. I would also assume that the aluminum backplane is electrically connected to one leg or the other of the cell, meaning you might be shorting all the cells together, wouldn't you?
Oops, I just spotted the sil-pad... so that's taken care of, but I'm still concerned about corrosion.


carlos1w (author)  patenaude4 years ago
Probably would be an issue in the long run (years?), but for a prototype it is probably OK (months...).  I guess it will be important to keep the whole thing dry (at least where Cu and Al are in contact).  Hopefully my liberal use of silicone caulk will take care of it! 

Does anyone know whether there is some kind of hollow aluminum panel of the right dimensions that could carry water inside?  Something with a profile like this:

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Why not use stainless steel scrap sheets? Better heat transmission capability. Even though greater weight, can be thinner as its a much stronger material.
aluminium or even better copper are much better heat conductors than SS..
In even slightly damp conditions, copper is very prone to corrosion, so aluminum or stainless is preferable.
Stainless is a horrible conductor of heat:

Thermal conductivity is measured by the formula BTU*ft/hr*ftE2*F

Copper: generally over 220
Aluminum 73 to135, depending on the alloy
SS 304 or 316 is 9.4

Data source: Materials Selector, 1993

Even the "worst" aluminum has 7 times the thermal conductivity of stainless.
However, if you paint the copper black you shouldn't have that problem, plus you'll have the added benefit of even more efficient heat transfer.
evad carlos1w2 years ago
Look into Bosch or 80/20 structural systems.
donde214472 years ago
See size does matter. In size of panel and flow of water or current thru size of cable.
Dr.Bill2 years ago
When the water heats up are the tubes on the back of the voltaic section hot as well?

If they are hot then where is the cooling advantage?

I just don't see it.
shastalore3 years ago
Great idea!

I've always noticed that those solar panels really heated up in the sun, but never considered harnessing that extra heat to produce hot water. Wow! It'll be fun to see how this bright idea develops and spreads.

One tip: With my trusty little HotPot solar cooker, I could sometimes force through 3 gallons of ambient temperature water, to boiling, on a given day. But the real challenge was keeping that hard-earned water hot.

And those commonly used stainless steel vacuum bottles just can't hold their heat for more than a couple of hours. You see, the neck of a stainless steel vacuum bottle is a serious design/materials flaw that allows heat to escape.

But glass-lined vacuum bottles, although fragile, can really hold their heat. For example, a typical 3 liter pump action airport pot, full of boiling hot water, will still be:
   - Scalding hot 24 hours later.
   - Hot 48 hours later.
   - Warm 72 hours later.

And several 3-liter glass-lined vacuum easy-to-use airport pots can be conveniently distributed to the kitchen, dining room, bathroom, and easily loaned out to a neighbor.

As boiling hot water has to usually be mixed with equal amounts of cold tap water, in most tasks, a 3-liter vacuum bottle goes a long way:  My wife washed and rinsed a large batch of dishes using only one 3-liter vacuum bottle of hot water -and the windows of the kitchen completely fogged up. That is testimony to the tremendous amount of energy stored in those bottles. And even unused warm water can be dumped back into the solar cooker, taking only half the time to bring back to boiling.
I'm fairly new at all this...no...I'm really new. Just one question. what is a 3 liter pump action airport pot and where do I find one? thanks.
Rather than trying to store the water, what my High School electronics teacher did with his solar heating panel was that he heated a reservoir, which was also sourced by his house's water heater. The solar heat reduced his water heater usage by doing a sizable part of the heating and was supplemented by the other heater when there was high use to keep the reservoir hot.

As an adaptation to your HotPot, you could try "recharging" you're already hot water since it would get up to temp faster than tap water. If you already have warm water from before, you shouldn't need as much newly heated water. Mixing leftover and tap would also speed up the heating process.

One draw back is that you then have to keep track of when the reheated water reaches the desired temperature to switch to the next batch.
mikesnyd3 years ago
You are right with the right solder you cna acomplish this but i think you would have been more successful with brazing the copper pipe to aluminum. Faster less fumes and no flux needed. Check this instructable out. http://www.instructables.com/id/Quick-and-easy-brazing-aluminum-copper-and-nonfer/
I really like your ideas so far. Nice instructable.
bahi3 years ago
This instructable shows the potential of solar based energy sources.
I wonder why there is no comercial product available using your dual
concept.
I tried to find something on the web, and this could interest you

http://www.websesame.co.uk/wp8.pdf

It's the DESIGN OF A DUAL FLOW PHOTOVOLTAIC/THERMAL COMBI PANEL.
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)  Mechanic20113 years ago
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!!!

Wow this is a great instructable !!
Thank you for posting it .
I have been looking for for one like this .
I want to build from scratch a whole array of these types of panels.
If you have a good thermal conversion/extraction from this panel , you might be able to get around 800 BTU per day/square foot of collector.

I have one question the though , wouldn't it be better to use 3/8" copper tubing ?
From 1/4" to 3/8" tubing would increase your flow of coolant 2.25 times.

I realize the coolant will have to flow through it slowly to pick up the heat but if I have a whole bunch of these on an array together then the flow becomes an issue.
What do you think ?
Foxtrot703 years ago
Love the Instructable, a friend of mine is overhauling the tracking system on his photovoltaics I suggested this Instructable to him. With the back panels as you show the tubing, better heat transfer can be accomplished by brazing the tubing to the aluminum. Another method of heat transfer would be to use a series of aluminum angles brased to the back side, this would give more surface area for air to pass and improve cooling.
MacOSJoey3 years ago
Great instructable! One idea: Run the pipes inside of the panel instead of behind it. I know you don't really have a lot of room on this panel, but if you made more room and had the pipes run alongside the cells (assuming you don't have any leaks), you should be fine and have hotter water.
dimovi4 years ago
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)  dimovi4 years ago
The pump is not solely to cool the cells, it is to extract heat for heating water for household use. 
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)  devonfletch3 years ago
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.
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.
bethehammer3 years ago
nice... solar power and heat exchanger... the water is not only heated but carries heat away from the solar cells, theoretically making them more efficient. this is a great start however you may want to put more "loops" of copper... perhaps using fittings rather than bending, or utilizing the pre-moulded panels used for solar water systems..
vontzy3 years ago
Carlos1w, there is another Instructable entitled "Solar-Thermal-Water-Heater-For-Less-Than-Five-Dollars" that might offer a less expensive way to collect the Heat from your PV array. Instead of getting the heat from the Sun the $5 Collector could absorb the heat from the PV array. The Copper Pipe alone, in your parts list, is $20. Love your instructable and hope merging the two ideas coulds save some bucks.
carlos1w (author)  vontzy3 years ago
Thank you for the reply. For anyone interested, the instructable is http://www.instructables.com/id/Solar-Thermal-Water-Heater-For-Less-Than-Five-Doll. Note that the above mentioned instructable makes it chep because they get the heat exchanger from the dumpster. This is obviously excellent (I love recycling!). I have thought about using a refrigerator coil instead of the copper pipe too but could not find one. I am also not sure how easy it will be to bond the coil to the back of the solar panel. Perhaps some glue instead of solder? (for the skeptics: you do not NEED a very high heat conducting material because the power per unit area is actually not that big, though obviously it is better to use something that is highly conductive)
carlos1w (author) 3 years ago
Please see this other instructable (http://www.instructables.com/id/Quick-and-easy-brazing-aluminum-copper-and-nonfer/) for a likely better way of bonding the Al and Cu without the nasty flux.
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