Saturday, October 14, 2006

Solar technology

One of the things I tried this year was alternate ways of powering my radios. I had powered radios from batteries and from the car in the past but this year I added solar and mechanical power to the mix. I'll talk about my experiences with solar power in this post.
There is a great deal of materials on solar cells and solar panels and, of course, I won't attempt to duplicate all of that here. Instead, I would like to talk about some of the things that have impressed me most this summer. The first thing I learned about photovoltaic power was there are many different technologies that are available today with different characteristics and efficiencies. A quick overview of these technologies can be found at the Solar Expert site. The efficiencies discussed all fall somewhere below 30%, meaning much less than 1/3 of the energy falling on the material is converted into something useful. That might not sound like very much, but it is! The Sun puts out about 900 to 1000 watts per square meter so even a system that is only 10% efficient is capable of putting out 100 watts. Some of these numbers might be easier to visualize with an actual panel so let's use the one that I bought as an example.
I purchased a 15 watt 12 volt folding panel from The Alternative Energy Store. First, a quick note about these folks: I like them! I placed my order, they processed it promptly, they sent me a tracking number, and it arrived quickly and in good condition. Recommended. Now, back to the math.
The panel folds out to expose 6 areas with solar cells, each area has two arrays measuring approximately 8 cm by 20 cm. So, the total area with cells is about 8 x 20 x 12 = 1920 square cm of power producing material. A square meter is 100 cm x 100 cm = 10000 cm. Therefore, this panel has about 0.19 square meters of power producing material. If the material was 100% efficient, and if we assume that the Sun puts out 1000 watts per square meter, we would see 190 watts of power coming from this material. That would be handy on a DXpedition!
As mentioned before, these panels are much less than 30% efficient and these flexible panels are probably closer to 10% or even less. If we take a number like 8% efficient and multiply that by the 0.19 square meters, we end up with a number like 15 watts rated output. That's the right answer.
Power ratings for panels are just that: ratings. The two big factors that go into a power rating for a solar panel are (a) the total area of the power generating material, and (b) the efficiency and power producing capability of the materials used to produce the power. The actual power produced in a given situation, though, depends on many more factors. For example, material efficiency decreases as the operating temperature rises. So, as your panel bakes in the Sun charging your battery, the efficiency of the material slowly goes down as the panel gets hotter and hotter. Also, optimal conditions such as the Sun's light hitting the panel squarely to produce maximum power are not likely to happen in your portable operation. In practice, you'll set up the panel as best you can and then hope it produces enough power for your needs.
The panel will only be part of your system. The power we wish to draw should be at 13.8 volts to run our radios or possibly 14.1 volts to charge our sealed batteries. The voltage from a solar panel can be far below or even far above these values, even producing voltages that would be dangerous for our batteries or equipment. What we would really like to have is a device that would take all the energy produced by the panel, no matter what voltage that energy is presented in, and convert it with a voltage-to-voltage converter into a form that is safe and effective for our operation. Further, we'd like this device to protect us from possible spikes in voltage that might happen when a passing cloud produces a knife-edge effect. There are such devices. The are called charge controllers. I purchased a Morningstar SunSaver 6.
The charge controller does a couple of things. First, it does convert the voltage produced by the panel into 13.8 volts (or so) which is useful and safe for our purposes. Secondly, it does protect me from spikes that might be produced which could, if not stopped, destroy my radio. The third thing my particular controller will do is protect my battery from being drawn down too low. Sealed batteries should not be discharged below a certain voltage as they could be permanently damaged. The charge controller will monitor battery voltage and, if the voltage falls below a certain point, disconnect the battery from the load. This low voltage disconnect feature means your battery is safe from abuse even if you aren't paying attention to the voltage level.
On some of my previous trips to Georges Island I would just hook up the panel and toss it on the ground to capture the Sun's rays. On this last trip, taken Monday, the Sun was low enough to the horizon (as it was October here in Boston) that this strategy was no longer viable. Now that I have my new Super Whatt Meter it seemed like a good opportunity to see how much difference "aiming" the solar panel would make. With the panel laying on the ground, it produced about 0.7A (about 9.5 watts). Sandy then suggested we use the small cart we'd brought to prop up the panel and have it face the Sun squarely. The power meter now read about 0.9A (12.5 watts). That's a big difference! So, obviously, the better you can position your panel, the more power you will produce.
I had the Super Whatt Meter watching the power coming out of the battery. With the solar panel and charge controller supplementing the power, the battery was being drawn at a rate of about 0.5A from the radio (with the panel producing the rest). I then removed the panel from the circuit and the current rose to 1.33A from the battery. The panel, therefore, was contributing about 0.83A, or more than half, of the operating current. That effectively doubles the life of your battery on receive!
The effectiveness of a solar panel solution gets even more pronounced when you operate QRP. My K2 draws only 35mA on receive. Even with this very modestly sized panel, I could probably operate during the day and fully charge a battery which could then subsequently be used for nighttime operation. If good weather and bright sunshine continued, I could probably operate indefinitely.
Now that I've had some experience with this technology I'm starting to consider larger panels, possibly a 30 or even 45 watt version of the folding panel I have now. Though expensive, they are certainly lighter and smaller than sealed lead acid batteries, and can produce a significant portion of the power needed for a remote operation.


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