Re: Solar concentrator design.

From: Duane C. Johnson (redrok_at_redrok.com)
Date: 12/03/04 

Date: Fri, 03 Dec 2004 07:58:09 -0600
To: Pete Lynn <pete@peterlynnkites.com>

Hi Pete;

Pete Lynn <pete@peterlynnkites.com> wrote:

> I am trying to define this a bit more.

> Solar power scales with area such that many small concentrators are in
> theory much lighter than one big one. Individual solar concentrators
> scale with volume, so to speak.

Yes, I think of it this way:
1. Individual concentrating collectors increase
    output by the square of the linear dimension.
2. But the costs increase by the cube of the linear dimension.

Multiple devices increase output linearly with
the number of devices.
Solar devices just don't benefit from the normal
scaling factors that conventional power benefit from.

> Currently off setting this scale effect are the cost of computer control
> and the inability to scale down the more efficient heat engines,
> assuming they are used. Obviously complexity is independent of scale.

> However it is possible to slave many concentrators together and computer
> control will continue to reduce in cost at any rate. Micro turbines
> also offer promise on the heat engine side of things, though solar cells
> are currently easier and scale down. Assuming a lower cost concentrator
> is possible efficiency matters less, cost becomes the dominant factor.

> Dishes should probably be less than one metre in diameter so as to keep
> costs down. Assuming the costs of other aspects of the design can also
> be scaled down. This should still work with the likes of water cooled
> CHP approaches.

> The crux of the problem is the concentrator itself, it needs to be
> cheaper.

I agree.

> Glass is too heavy, fragile and expensive to be used in a brute force
> fashion. There are two possible ways around this, use a reflective
> plastic film, or use a very thin tiled glass approach as per a mirror
> ball. The likes of foam or even custom wood could be used to support
> such reflective surfaces, assuming they were sealed appropriately.
> Within the above constraints, thin plastic fresnel lenses might also
> similarly work, though I need to study this further.
>
> With such approaches order of magnitude cost reductions might eventually
> be possible.

Don't forget another possibility, Heliostats.

Multiple dish designs have logistic problems:
1. Dish receivers need to be connected with flexible couplings.
2. The hot zones are spread out among the multiple dishes.
3. The hot zones are up in the air and moving.
4. Multiple dishes are relatively expensive.

I propose a combination of several heliostats and 1 dish.
1. The dish is stationary and near the ground.
2. The single hot zone doesn't move.
3. The hot zone is in a convenient location.
4. The basic collection area is with the heliostats.
6. The multiple flat heliostat mirrors are easier and cheaper
    to make than multiple dishes.
7. Small "Receiver Axis" heliostats don't require computer
    controlled tracking, although computer control may be
    desired.
8. Light conversion to heat is not done in the array of
    heliostats but is done at the single hot zone receiver.

Look at it this way:
1. The cheap parts, the heliostats, do the basic collection.
2. The medium cost part is the single dish.
3. The expensive cost parts are at the single receiver.

> Pete.

Duane 

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