Hybrid solar panel (photovoltaic and thermal)

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).

Comments

[MacOSJoey]

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.

[patenaude]

I really like this project, but I see some problems and possible improvements.

The first problem is the corrosion/electrolysis caused by using differing metals that I mentioned a few pages back. And the second is how you use the hot water.

While you might be able to fix the electrolysis by using a copper backplane rather than aluminum, that would either be very expensive to be structurally rigid enough for the PV cells.  A way to handle that is to solder the copper pipe to a very think copper backplane, and make it rigid by attaching the pipes to sheetrock or plywood. Alternatively just build a form and pour a thin layer of mortar around the pipes.  You'd need to experiment to see what will be would be rigid enough to keep the PV cells from cracking, and also if it expanded or contracted too much with the temperature.

Even better, you might be able to get rid of conductive pipes altogether.  While not as good as copper, you could use the same plastic tubing used in floor heating systems attached to some plywood, covering the tubing with a layer of mortar as in those systems.  You could attach the PV cells to the mortar, which acts as a small thermal sink while the water carries the excess heat away. This would allow you to get rid of the silicone insulation barrier altogether, and reduce potential problems with corrosion or shorts.  While thermal transmission isn't quite a good, it would probably reach equilibrium with the PV cells getting only a few degrees hotter.

Lastly, if this were being used in my house, I think I might use this in a closed loop filled with mineral oil rather than water (to protect the system from freezing.)  This loop would heat the tank of a standard hot-water heater (most are 40-50 gallons~150-180 liters/Kg.)  I would either use or replace my existing hot water heater, and have that feed a tankless, on-demand style water heater. Since the tankless system is getting preheated water, it will use less gas/electricity.  (If you can get the large tank to 126F, then that's close to the temp. that domestic hot water should be anyway, and further heating mostly unnecessary.)

I'd also probably build a simple circuit that checks to see if the system temp was higher than the tank temp before turning on the oil circulating pump. That's maybe $20 in electronics parts.. a simple comparator or microcontroller feeding a relay to control the pump.  The microcontroller would also allow you to track and publish the data back to your home computer, and you can make plots of hours of operation along with system and tank temperature.

[carlos1w]

Thank you for your comments.  All of your comments are quite valid and would certainly make a lot of sense for a permanent installation (please bear in mind that this was merely a proof-of-concept device, and I really wanted portability and to reduce issues with possible spills-hence the use of pure water).  There are quite a few alternatives that can be used instead of the Cu/Al mix and all have positive and negative aspects: a) all Cu (good but too soft and expensive), b) all Al: I would think that it could be possible to have an extruded form like mentioned below, I think this would work best; c) polypropylene "corrugated plastic", and others.

[carlos1w]

Here is what I think should work well: an extruded aluminum pane like shown in one of these figures:

[ChrisVincent]

You dont need such a complex Aluminum form, even a single celled structure would work.

If you want to get more efficient and loose the pump look into heat pipes, in particular pulsating (or oscillating)  heat pipes. You can use the heat to transfer it's self to the store water and get to use all of the solar power.

[linesofflight]

Great Idea! 
Looking at your graphs, I noticed that if you hooked your heat exchanger up to a closed loop preheat domestic water or for radiant heating, the water would spend more time in its peak linear phase due to having more mass to heat.  It would also keep your panels more cool - obviously, that depends on rate of flow through the preheat-tank/heating system and total number of panels, but it does offer a nice opening for optimization.

[carlos1w]

Indeed, every time the reservoir temperature drops it automatically increases the heat transfer.  In fact, most solar water heaters have two thermometers and the water pump is only activated if the temperature difference is bigger than some threshold.  One thing I have still to test is how the efficiency of the PV panels would decrease if the water pump is off (since the back is insulated and there is a layer of air in the front, I presume they will get really hot!).

[dddddd]

It sounds liike you already know that efficiency plummets as cell temperature increases.
A cell at 105 degrees produces only 90 percent of the power of a cell at 77 degrees.
At 122 degrees it produces only 85 percent of the power of a cell of a cell at 77 degrees.
On the other hand, if you could limit your cell temperature to 105 degrees during the day, it would probably be cooler than an equivalent air-cooled cell would be at the same time under the same sunlight.

I didn't know you could solder aluminum to copper, using the right flux. Thanks!

[Woodenbikes]

Nice Instructable!  I'm glad you put in the graphs of test results.  It seems you are able to get 3-6 times as much energy (electricity and thermal) from the combined panel as from a solo electric panel.  Good idea for taking advantage of small sunny portions of roofs. 

Perhaps an easy low cost method would be to start with plastic or metal solar swimming pool panels and then apply a flexible adhesive backed thin film solar PV panel.  Then you have an unglazed low temperature solar panel.  You could put it in a glazed box if you wanted to boost the thermal production at the expense of some of the electric production (losses from glass reflectance and absorbency as well as PV efficiency loss as cell temperature rises.) 

Maybe even better would be the unglazed pool panel type of system (with PV film layered on it) with a heat pump extracting heat from warm fluid coming off the panels to make sub cooled fluid returning to the panel at or below ambient temperature for zero heat loss.