alt.energy.renewable

Dunc wrote:
> On Nov 29, 8:42 am, "Ron Purvis" wrote:
>> "Dunc" wrote in message
>>
>> news:14f04e4c-c02c-4bb2-9e4b-0b63a43f8103@ ...
>>
>>> Before I retired I work for Bayer . They made a polycarbonate
>>> material
>>> that if I remember correctly was called "LISA". One of it's
>>> properties was
>>> that 80% of the light that fell on it surface came out on the edges. I
>>> saw
>>> plastic sculptures that exhibited this property.
>>> My question is:
>>> Has there been any work done on using this material as a means of
>>> concentrating light to reduce the number of solar cells required to
>>> collect
>>> light from a giving area thereby reducing the cost of solar energy?
>> I haven't heard of this being used, but then I am not as knowledgeable about
>> this as some others.
>>
>> What I don't understand from your post is how this material is supposed to
>> work with solar energy. Can you explain this a little more?
>
> You surround a 4 foot by 8 foot sheet of 1/2 inch thick "LISA"
> poycarbonate with solar cells. 80 percent of the light falling on the
> 32 square feet (4x8) of surface area would be directed to 1 square
> feet ((8+4+8+4)x ( /12) of solar cells along the edge of the sheet.
>
> Assuming the electricity produced is proportional to the amount of
> light received this 1 square foot of solar cells would have the same
> output as 25 square feet of solar cells exposed to direct sunlight.

I'm not aware of anybody doing such a thing. It sounds intriguing, but a
few questions leap immediately to mind:

- Where does the other 20% of the light go? I presume it escapes through
the top and bottom of the material, but is a significant quantity
absorbed? What I'm getting at is that on a cloudless summer day your 32
sq ft sheet of LISA is going to see about 3200 watts of incoming
sunlight -- can it handle whatever amount of that it would absorb with
no ill effects? The polycarbonate page at Wikipedia suggests that about
10% of the light is absorbed and that polycarbonate doesn't shed heat
very well, so that could be problematic (though it's been too long since
I did a heat transfer calculation for me to even attempt one here).

- How much does LISA cost? Even if it can reduce the cost of solar cells
now, I think it would eventually have to cost $4-8/sq ft or less for PV
manufacturers to have sustained long-term interest in it (that's a
back-of-the-envelop estimate on my part). Again based on Wikipedia's
information on generic polycarbonate, I calculate that a 4 x 8 foot
sheet a half-inch thick would weigh 100 lb (about 25% more than a
conventional solar panel) and cost $10-19/ft^2. Does that sound right to
you? At that cost, I estimate it could reduce current PV manufacturing
cost by as much as 50% (or as little as 0%), but to get the factor of
4-5 reduction needed to compete with conventional energy sources the
polycarbonate cost would have to drop substantially.

- Related to the first question, how does LISA perform when exposed to
the elements? Will long-term UV exposure damage it? Wikipedia hints that
it will. Does wear on its surface impair its ability to collect and
direct light? In short, will it hold up with little to no degradation
when sitting outdoors for 30 years? If it has to be protected behind
glass or if it simply won't last 30 years then the required cost I
estimated above will drop, perhaps significantly.

It *is* an intriguing idea, but its long-term commercial potential would
depend on the answers to those questions (at a minimum). Based on what I
found at Wikipedia it seems like it has short-term potential (provided
it weathers well), but that long-term use would probably require a
significant drop in the cost of the polycarbonate. I would be interested
to hear your thoughts, though.