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!

Project #1:Portable USB style solar charger for portable devices.

I find the effects of electronic devices on society very fascinating. How things like the person computer, the calculator, and the cell phone has completely changed the way we act, our expectations, and what we consider productive in our world. Specificity the effect of the "smart phone", the current apex of portable computing. Its is the do all device, it has access to many different mediums of communications, it has any kind of function you want limited only by hardware, and it can work just as well as a PC but fits(mostly) in your pocket. Its what we hackers where dreaming of 5 years ago. But, just as with every device it has a few limitations. Its only effective when in range of the cell system, its only as capable as its OS and its operator, and its only good as long as it has battery power. This leads me to our topic, the smart phone owners constant bane,...... a dead battery!!!!
With the addition of internet access on your phone you can now stay connected 24/7, as long as your device is powered on and in range of the cell network or a WiFi AP. Every time your device talks to the network to sync email, check facebook or load a web page its transmitting. For a short period of time(the duration of the packet being transmitted) its using as much power as it would when you where on a phone call(in ratio to time duration). So of course being that the transmit duty cycle is much higher the battery lasts a shorter time. The way most people fix this is by having chargers every where they go so it only runs on battery for a short time. But what about those of us who walk, cycle or are other wise are away from a ready charger or easy power source? Well here are one of my ideas to over come this.

Charging on the go.
Most phones now charge off of a USB 5VDC power sources. So you now are finding every where 120AC to USB or car socket to USB adapters. These are great if your near a wall outlet or a car that is running. But what about on a bike, or hiking or when the car is off? My answer is solar.
Now there are several products out on the market now that are solar powered phone chargers. But my goal in this project is to build a charger that is simple, rugged and will last as long as it can with little maintenance.

Batteries vs. Supper Caps
Most products that will charge your phone from solar power use a photovoltaic panel hooked to a voltage regulator or charging circuit. This then charges a small battery which in turn charges your phone. This works as long as the battery is in good health, but over time it will fail or retain a shorter and shorter charge life and will die at some point.
The other way to do this is use a capacitor in a RC circuit. The challenge with this is to get a large enough capacitance to do this would render this device none portable. Standard capacitors with values grater than 1 farad and voltages higher than 5VDC are 3cm in diameter and at least 5cm higher, if not taller. Even though they are much lighter than a battery you would still have to carry a large pack of these things around with you. But we are in luck with the advent of double sided capacitors, or what they call supper caps. These have a value of 10 farads and a voltage of 2.5VDC and are only 3cm high and .5cm in diameter. Now with several of this our pack is looking much smaller, but how effective would they really be?
Power density
When looking at the effectiveness of a power source for a portable device one must look at power density. This has two factors to, the power(in Watt Hours) per volume and the power per weight. This will help us to determine the best balance between longevity, size, weight.
Now, most people would do research on line to out this info. But, that sounds boring! So I have put together information based on tested done in my shop to fit this required function of charging a smart phone.

What's next?
The next step in our project is to determine the power requirements of the device we will be charging. This will give us parameters to work with in when designing our power source.
Second is to test several different power sources to see which will best meet our requirements and have the highest power density.
Third, we put it all together!

In my next blog installment I'll cover the first step, determining the phones power requirements.

So stay tuned for the next installment of electrical goodness!