Introduction: Hybrid Solar Panel (photovoltaic and Thermal)

Picture of Hybrid Solar Panel (photovoltaic and Thermal)

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....

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!).


- 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.

Step 2: Building the Heat Exchange System

Picture of Building the Heat Exchange System

Photo 1: Soldering 1/4" (~0.5 cm) flexible copper pipe to the Aluminum backplate. The backplate is about 0.80 m x 0.65 m. I used about 7 m of pipe. Make sure you use the proper tool to bend the pipe to avoid pinching it off! The Al. plate cost about $10. The pipe about $20.

Photo 2: I thought it was impossible to solder to aluminum. Incorrect! You need the right FLUX. I bought this from McMaster-Carr for about $30. Notes: 1) buy a smaller pot, this was way more than I needed. 2) Be careful, this stuff is nasty: it has Fl and breathing its vapors or touching is NOT recommended!

Photo 3: Here my friend Martin is soldering the pipe and plate. Note that we worked outside, there was a good breeze and we used some protection!

Photo 4: End-result: will not win a soldering contest but was not bad for our first time soldering aluminum...

Photo 5:
Close-up of the solder job...  Could be better, eh?

Step 3: Gluing the PV Cells

Picture of Gluing the PV Cells

The aluminum needs to be covered by some electrical insulator (I used Begquist sil-pad 400, cost about $50) because the backside of the PV cells is one of the elctrodes. Sil-pad is a decent thermal conductor too, but I presume that tar-paper would have worked OK too and would be much cheaper! The PV cells are 15 cm x 7.5 cm, and this pane will use 36 cells. I bought the cells on ebay from rebeccayi0904 (nice!) at $150 for 76 cells (used ~1/2 of those for this panel). The cells need to be connected in series (how are your soldering skills?) and then I glued them to the sil-pad using silicone caulk. BE CAREFUL: the PV cells are only 0.20 mm thick and break if you look at them too hard!

Step 4: Electrical Connections

Picture of Electrical Connections

Added electrical interconnection between rows (electrical bus). Notice I cracked some cells, this will reduce the efficiency of the panel, sigh! 

Step 5: Testing Back in Winter... (electrical System)

Picture of Testing Back in Winter... (electrical System)

First test! Just put the panel out and connected a 12 V halogen lamp, the panel was pumping 16.2 V, the lamp was very very bright! Open circuit voltage was just over 20 V. Short circuit current about 2 A. This was in winter, so the sun was not too bright.

Step 6: Testing Water Pump and Pipes

Picture of Testing Water Pump and Pipes

Photo 1: Testing the water system. Note that pump (12 V, bought on ebay for ~ $10) is powered by one half of the solar panel (generating ~ 9 V).  The plastic pipes were recycled from an old medical condition!

Photo 2: Water is circulating! This day was cloudy and 1/2 of the PV panel was only generating 4 V (should be over 9 V on a sunny day!).

Photo 3: cool guy in the "mirror" :-)

Photo 4: water circulates between the panel and a small cooler, will do some calorimetry later to check the thermal output of the system. (not enough sunlight this day...)

Step 7: Insulating the Backside

Picture of Insulating the Backside

Photo 1: Insulated backside. I just glued some Styrofoam to the back using silicone caulk.

Photo 2: detail, I tried to keep it as weather tight as possible.  The electrical cables go through a hole in the aluminum frame and there is plenty of caulk to keep things in place.

Step 8: Two Days of Real Testing (May/5 and May/6)

Picture of 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).

Step 9: Conclusions

Overall I am quite pleased with the results, but I did spend a lot more $$ than I had anticipated!!!  Whether this can be a cost-effective solution for people's homes remains to be seen.  I like the idea of using as much solar power as possible, and it makes for a most complete utilization of the surface area (e.g., a home's roof).  One extra advantage:  the water keeps the temperature of the PV cells lower, increasing their efficiency (or so I hope).

If you find this instructable interesting please comment.  If you have suggestions please comment.  What other types of heat exchangers could be used?  I still have enough PV cells to make a second panel, but I would like a cheaper and simpler way to harness the thermal energy.  Any suggestions are welcome.

Please enjoy and if you decide to make a panel like this let me know. THANK YOU.

Carlos Wexler

Note (added May/13/2010):  a google search reveals that there is apparently some commercial systems that use the same concepts (using both the electricity and the heat in a single package).  Please see, for example:, where they use air convection on the backside of the panels (instead of water).


DJaySingh (author)2016-04-07

Can you tell me what size sil-pad did you use. I am unable to find a sil-pad of this dimension. Please REPLY..!!

carlos1w (author)2015-05-05

A semiconductor will not fry when exposed to reasonably high T's. You should read some more about mechanisms for heat transfer too:

1) The insulation is not perfect. If you could make perfect insulation you'd be really rich.

2) Then there is radiation. The cells are very close to a blackbody radiator. Even with the greenhouse effect of the glass the PV cells will radiate back IR radiation.

3) There are shunt diodes dispersed in the circuit. This makes sure that one shady cell does not fry.

4) Heating because of current is minimal. These are 2-3A maximum, at 0.5-0..7 V per junction this will be 1-2 W of heat. Not too significant for a cell this size.

5) The panel has been working for 5 years without cooling...

carlos1w (author)2015-05-05

Again… be smart and read before you make comments that are not so smart. From 2010 to 2015 PV cells and panels have dropped in price. Surprised? Maybe I have a 286 computer that I can sell you (you know, these used to cost $1,000 or more back in the early 1990's).

carlos1w (author)2015-05-05

1) This was built in 2010. Read the article before making comments that are too smart for their own good. In 2010 this was a good price.

2) It has lasted 5 seasons now and it is still generating electricity, and no… there is no cooling working because in the end this was a prototype and there was no need for the hot water (not worth on a panel this size to do all the piping for the house).

Still charging a battery all these years (for a backyard night illumination using LEDs… I am quite sure you can buy the LEDs cheaper these days too).

carlos1w (author)2015-05-04

No safeguards. It is a prototype. However, I find it highly unlikely that the PV cells would "fry" (unless perhaps you are talking of the Sahara desert). What should happen is that the efficiency of the PV conversion goes down significantly with temperature.

Russell McMahon (author)2012-09-21

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 :-).

robbytesla (author)2012-08-05

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.

servant74 (author)2011-12-13

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

static (author)servant742012-07-24

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.

static (author)2012-07-24

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?

patenaude (author)2010-05-14

 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?

patenaude (author)patenaude2010-05-14

Oops, I just spotted the sil-pad... so that's taken care of, but I'm still concerned about corrosion.

carlos1w (author)patenaude2010-05-14

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:

|        |         |          |          |         |

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.

evad (author)solaralternatives2012-07-24

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 (author)carlos1w2012-07-24

Look into Bosch or 80/20 structural systems.

donde21447 (author)2012-07-07

See size does matter. In size of panel and flow of water or current thru size of cable.

mcarrell (author)2011-08-03

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 (author)mcarrell2011-12-31

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.

Dr.Bill (author)2011-12-01

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.

shastalore (author)2010-09-03

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.

buddhawhisper (author)shastalore2011-08-11

I'm fairly new at all'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.

mikesnyd (author)2011-06-27

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.
I really like your ideas so far. Nice instructable.

bahi (author)2011-04-13

This instructable shows the potential of solar based energy sources.
I wonder why there is no comercial product available using your dual
I tried to find something on the web, and this could interest you


Mechanic2011 (author)2011-03-27

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)Mechanic20112011-03-28

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!!!

Mechanic2011 (author)2011-03-27

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 ?

Foxtrot70 (author)2011-02-04

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.

MacOSJoey (author)2011-01-23

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.

dimovi (author)2010-05-13

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)dimovi2010-05-13

The pump is not solely to cool the cells, it is to extract heat for heating water for household use. 

devonfletch (author)carlos1w2010-12-02

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)devonfletch2010-12-03

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 (author)carlos1w2010-11-27

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.

bethehammer (author)2010-11-27

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..

vontzy (author)2010-11-18

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)vontzy2010-11-19

Thank you for the reply. For anyone interested, the instructable is 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)2010-11-14

Please see this other instructable ( for a likely better way of bonding the Al and Cu without the nasty flux.

Skyriam (author)2010-10-13

Great idea and concept, congratulations!! One question: How exactly do you harness the heat of the water? You could also use peltier plates, although they are quite expensive for large builds like this. Thanks!

carlos1w (author)Skyriam2010-10-14

Hi Kal00! Thank you for your comment. The *idea* would be to use this to heat water for household use. It seems the conditions are ideal (heat water to 40-50 C, perfect for dishwashing, bathing, etc) and the cost would be quite low. The temperature differences involved are not so great to make for an efficient thermal->electric conversion (the PV conversion is more efficient) but the direct use of the heat is very efficient.

BTW: currently I use the panel to recharge some car batteries someone gave me and power a LED light system for my deck. Unfortunately I am not using the hot water part of the system now (too complicated for such a small system, but it would be feasible for a larger installation).

evad (author)2010-05-13

While I know aluminum is cheaper than copper sheet,  you might end up (after a number of years?) with the aluminum being eaten away by being in contact with the copper.  On the galvanic scale, copper is up near the top, one of the most  "noble" metals.  Aluminum is way low on the scale, (ignoble?) down below steel.  With condensation as an electrolite, or even humidity, a potential (voltage) is set up between the two metals, and eventually, the aluminum will begin to disappear via galvanic corrosion- basically, all three make a (weak) battery. 

I'd be curious to know if there is a voltage present:  put your voltmeter on the millivolt DC scale, one probe on the copper and one on the aluminum, and see what voltage you get out.

carlos1w (author)evad2010-05-14

I put my voltmeter with one terminal on the copper tubing and the other on the aluminum plate and the voltage measured is 0.0 mV (that is as low as it goes).  Would this mean that corrosion will be minimal?

trebuchet03 (author)carlos1w2010-06-16

Late to reply, I know.... Galvanic corrosion will occur even in the uV (microvolt) range... BUT, in practice, you needn't be worried. Something else will likely fail first ;) A pertinent example of dissimilar metal contact is the cheaply made mini fridge... If you were to take one apart (which I DO NOT recommend - you must destroy it due to the assembly process used) - you'll find copper tubing being used as the condenser in contact with the sheet metal (low grade steel) enclosure. Now if you were using a steel bolt in an aluminum fixture that needed to be taken apart frequently - then yes, it's something to consider ;)

evad (author)carlos1w2010-05-14

I'd say it's not happening...  As I said, you need three elements- the two dissimilar metals, and an electrolite like rain, or humidity.  I  should  qualify my statement by saying it would be more prone to happening if the unit was constantly outside. But, you seem to have it covered up pretty good.

I live near the ocean, and have a steel boat trailer... About 6 years ago, I repainted it,.  I used some stainless steel hardware here and there, and where the screw heads touch the steel, the steel is rusting around the head- not bad, but, I will replace them with steel this summer.

And I forgot to say- Nice project!

kmpres (author)2010-06-16

Interesting project, and well documented. Your attention to detail is just what I need to help me decide if I want to go ahead with a similar project. I have often thought of combining heat and PV in the same panel but have been discouraged by the fragility of the cells. The last thing I would want to have to do is to take down the panels from my roof (a very difficult task in my case) because of a leak or rain water got in and corrupted or shorted the cells. The panels should be rigid enough to eliminate the effects of thermal expansion and parts degradation and sealed enough to prevent water infestation. Commercial solar panels are usually maintenance free for 20 years or more but they're still too inefficient (12% isn't breaking any records these days) and still too expensive, so, I will wait for the efficiency to approach the cells used on satellites (about 35%) and the prices to drop. Also, .2mm is indeed thin, about the thinness of two sheets of printer paper. A good substrate is imperative for such a panel to last 20 years or more. You've a good start. I look forward to reading about improvements to your design.

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