PVIR:Solar Power panel using both visible and IR light.

                As I've been working on my house power sources are an ever recurring theme. Right now my house is power via the electrical mains from the local power company. I'm luck where I live because power is very cheap, about $.07 USD per Kw/h. But recently we lost power during one of our later summer storms that hit us with 60mph winds. I was able to hook a battery and get things back up, but what if this happens for a long period of time?? How will I recharge my batteries then? Well that is what brought about this project.

               This is part of a 4 part project that I will working on between a few other things. I'll have some blog post up about the other 3 parts later on this month. The other parts being: The battery Bank, The wind turbine, and the Rectifier system. I hope also to build a control for the whole system but I'm not sure when that will come. The idea behind all of this is to build a system that will have multiple ways of charging my battery backups so that I will also have some kind of power source no matter what happens to the mains. I hope that down the line I'll be able to build a grid tie in system and sell power back to the system, but the cost of those systems are quite high now.

So this part the project will be like a Solar panel PLUS!!!  But before I geek out too much, lets get into some theory first.

Theory of Solar Panel Operation

            So the way that Solar Panels, or Photovoltaic cells, work is wonderfully simple. A photon of 1100mn or shorter in wavelength hit a silicon molecule. This excites one of the electrons in the valence shell of the atom which bumps it up an orbit. This causes a potential difference in the doped silicon which will give you the ability to charge batteries or power a device with enough of it. Now there are a few shortcomings of this system. First any and all wavelengths longer than 1100 nm are just absorbed by the device and converted into heat. As we move to shorter wavelengths than 1100 nm less and less of photons energy is used to create current, so that excess energy also goes into heating the device. So there is a whole lot of heating going on, unless you are only getting 1100 nm wavelength of light. 

So, lets put this into some more practical info, shale we?

Source:http://www.viridiansolar.co.uk/Solar_Energy_Guide_5_2.htm

The above figure gives you a rough breakdown of the spectrum emitted from our star. Peaking around 550 nm or and tapering off from there. Keep in mind that IR is 700 nm and longer, Visible is 700 nm to about 400 nm. UV is about 400 nm and short in wavelength.  Now as you can see from the figure a fair portion of this spectrum is absorbed by our atmosphere. Via that you can see the spectral lines for H20, O2, N, and CO2.  The green section is the section which we want to look at, this is the septicum which with work with silicon PV cells. Everything else that hits our panel with be converted into heat! 

On top of this we will need a transparent material to allow photons of different wavelengths through to our PV cells but keep rain and such out. Luck for us most polycarbonate transparent materials will do the trick.

Source:http://www.plasticgenius.com/2011/05/infrared-and-ultraviolet-transmission.html

We can see here that the bandpass of Makrolon is from about 400nm to about 1700nm. This is plenty to help our panel work. (Note:Lexen and other polycarbonate materials should have a bandpass about like this.)

But we run into a problem, what do we do with all the energy the PV cell can't convert to current??

Converting IR to Current:

             After look at the info above I've guessing it's quite clear that PV cells are not the most efficient power source in the world. In fact the ideal PV cell can only convert 31% of the photons that hit it into current. The remaining 69% will just heat things up. Now there isn't much out there yet in production that is a "solar cell for UV", yet! There are a few things still in testing that may fix this problem later. But until that point we are left just letting the 400nm+ spectrum to hit our devices and heat them up. 

Now we do have some options for using all of that radiation below 1100nm coming off the sun and our devices. Heat has for a long time been used to produce power one way or the other, and this is where will with add the "IR" to our "PVIR" panel.

The most well known way to convert IR into current without any moving parts is with a peltier diode pack. These devices are used to both generate current as well as sense thermal shifts. The way this works is as the temperature shifts the forward voltage for the diode changes. The benefit of this is that if you heat one side up and cool off the other it will create current. 

The other way we can create some current from our IR is to use the heat to move other things, like air, or steam. By making sure the area behind the PV cells will absorb as much IR as it can we will be able to transfer that energy into water. This can be via mount the PV cells onto a thermally conductive pad, like sil-pad, then mounting this on to a copper plate.  You can then solder copper pipes to the back of the plate. The water running through the pipes will heat up and this heat change can be used create steam.(Note: Not test yet!)

Another way would be to heat air up in the are behind the PV cells, this air would then rise through a vent which could have "wind turbine" on the opening. This would then spin to give us some more current.
The water idea can also be used with our Peltier diode packs, attaching one side to the PV Cell copper plate/heatsink and the other side to a copper block soldered to the water in take we will create a thermal difference. This could also work by using the air intake as a cooler as well.

The PV Cells:

              Now this is where things get a bit confusing. Data on PV cells can be a bit flaky, at best. In my search for cheap cells I found a few good deals, with ZERO documentation! So I'm going to have to wing this a bit and hope I can figure out some better specs as I go.

Most solar cells have ratings such as this:

Source:www.spectrolab.com/DataSheets/SJCell/sj.pdf

These are the specs for 7cm x 7cm high output cells. These guys are a bit price and not what we will be working with, but they are the best datasheet I could find. 

Here you can see our current/voltage, Efficiency/wavelength, and the specs of open and short circuit voltage/current. We also have the temp coefficients which will tell use how the out put will change over a range of temps. Based on most the data I have seen the current/voltage is very standard as well as the efficiency/wavelength. Also the open circuit, closed circuit data seems very standard as well. But I couldn't find much info on how the output of the panels changed over a temperature range. I found some sources that state it change up to 33% between 0C and 75C.(Source:jeldev.org/9Tayyan.pdf) Now this is a very extreme shift in temperature, but it gives you some idea of how what your PV cells will be doing over days and year. 

From talking to many people who have built panels before as well as some info on google it seems the standard is 36 cells to charge a 12VDC battery system. Now I questioned this as most sites show 0.5VDC as the Avg voltage for a cell. Well for the above cell that would be right, but now looks at a cheap cell.

Source:http://www.hebesolar.com/solution/Photovoltaic-Cells-2.html

This a data sheet for a cell that is close to the one I'm buying for this project, a 150mmx150mm multicrystalline silicon cell. Here you can see the roll off voltage is right around 0.45 and 0.5VDC. That sure as hell doesn't look average to me!!! This will be very important to keep in mind when we desgin our panel's layout and order the cells we need.

A final note before first design:

                The biggest thing that I took away from my research on this is that PV power sources are by no means very efficient. In fact I would say they are one of the lowest efficiency sources out there, with only a 31% max and an effective 10% - 19%(based on info seen in my produce searches) there is a lot of energy that is been lost to heat in these systems. So I feel it is important to keep an eye on this and see how you can make the best use of that heat to add to our final system output. 

I think the other important thing to keep in mind here is that this will not be the only source, this will only be one part in a larger picture of power sources. So lets make sure not to put all our electrons in one place. 

I hope this was a good primer on photovoltaic cells and some of the practical elements involved. Next time I'll cover some of the first design idea I have and what will be going into to the first build. 

until that time, keep on hacking!