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!).

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.

Step 2: 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

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

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)

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

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

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)

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
([e-mail])

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:  http://solarwall.com/en/products/solarwall-pvt.php, where they use air convection on the backside of the panels (instead of water).