Monday, January 11, 2010

Playing Around With Lightly Concentrated PV

The application I've been thinking about lately is concentrated light on solar panels. Usually, when you are talking about concentrated solar for PV (photovoltaic), you mean high concentrations (100-500 suns) focused on a few square centimeters of high efficiency (and expensive) triple-junction cells. I'm looking at much lower levels of concentration, say, 10 suns, concentrated on something very close to a normal solar panel.

According to backyard experimenters, typical solar panels seem to put out 1.5-2 times their rated output under concentrated light but the higher temps generated by multiple suns can ruin the panels. Though reflecting light onto heliostats is done frequently, I haven't seen the basic info of how hot surfaces get under 2x sun, 4x sun, 10x sun and so on -- preferably under different weather conditions. I'd like some data on how hot things get under multiple suns. Given this, one could pair mirror configurations to solar panels operating parameters.

We know too that solar panels waste most of the of solar energy that hits them since they're only sensitive to specific light frequencies (wikipedia et al). Ideally, one could concentrate, say 10 suns, on a PV panel. Prior to the panel, there would be filtration films to eliminate IR or UV rays and so reduce the heat load. The ideal configuration would be 10 suns filtered precisely by a film down to the small band of frequency that the panel uses. If 80% of the heat were filtered out (overly optimistic? yes.), then you're down to 2x sun. Tweak the panel to handle high heat and higher currents and go for it.

Admittedly, this is an experiment undertaken with plenty of ignorance There are multiple effects to worry about if you actually focused 10 suns on an off-the-shelf solar panel for a chunk of time. Overheating the panel say, or size of the wiring at the back of the panel. Would the panel need to be excessively doped to produce the desired current? Sure, who knows.

But for all it's flaws, I still want to focus 10 suns on a brick wall and measure the temperature. Then, add a filter and see how much it helps. I'll let you know the results. Then, back to heliostats.


  1. Heliostats and PV Panel concept:

    Conventional PV panels are very Expensive, we may as well
    maximize the daily collected energy, make them work harder.

    Of course, one method is to use direct solar tracking
    methods. Don't get me wrong, this works well. But can we
    get even more.

    Heliostats are, generally, flat mirrors that track the sum
    and reflect sunlight onto a stationary target. This target
    could be a PV panel.

    If you talk to a PV salesman they say you can't concentrate
    at all as it would void the warrantee. But if you talk to
    the PV engineers they say you can concentrate just don't
    over heat the panel. If overheated the encapsulant turns
    brown, there goes the warrantee.

    Lets use several heliostats, say 4, and aim them on the
    PV panel. This is a concentration of about 4X. While the
    panel won't deliver 4 times the power it'll be considerably
    more than 1X. Within reason, increasing the concentration
    always increases power output.

    But wait, won't the panel overheat? Yes, it may. So a
    temperature sensor is installed on the back of the panel.
    If the temperature gets to high them move a heliostat away.
    If it gets hotter then back off more heliostats.

    Ok, so if the panel is cold you can concentrate more, if
    its hot less concentration can be employed. Hey, when it's
    winter it's cold the because sun is out less. And more
    concentration is acceptable.

    Perfect. When it's hot we have more sun hours and less
    concentration and when the weather is cold we can have more
    concentration with less sun hours.


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  2. Agreed Duane.

    A nice cheap heliostat is a very generic tool. If we have an open development platform so building and controlling heliostats is easy, then people will soon be able to solve physical problems (like PV overheating, wind, snow weight,...) through software. For example, your point -- add a sensor to the system, if PV material is too hot, re-point the heliostat or turn on fans. I think it's very achievable.

    By the way, I'm a fan of your site. Great public service