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Why there is no rooftop parabolic trough

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Posted by azuredu on January 4, 2009, 1:56 pm
 

I expose first the difficulties facing a home parabolic trough
attempt. Did I overlook something?

Parabolic trough is a very attractive solar technology. It can output
thermal energy with temperature ranging from 60C to 400C at high
efficiency, or make direct heat and electricity cogeneration. A great
majority of the non-mobile energy need of the society can be met by
this technology: electricity generation, heating, cooling, industrial
process, cooking, etc.

In fact, big parabolic trough power plants have been generating
electricity in the deserts since several decades. With a cost that,
although having difficulties to compete with other sources such as
coal, natural gas or even wind, is at a very interesting level. If it
can be miniaturized and put onto family rooftops, there are great
advantages with respect to other solar technologies such as flat panel
water heaters, evacuated tubes, or photovoltaic panels. If further the
cost can be brought down, it becomes the ideal product.

However, we have seen none of them operating in ordinary homes. Of
ourse, this is due to technical difficulties. In fact, it is not one
difficulty, but a series of them. You have to overcome all of them at
once, in order to make a viable commercial product.

1. Mirror formation and optical precision.

The parabolic reflective surface is a spacial curved surface, which
should be formed with precision. However, no material is perfect, no
process is perfect. We want to make mirrors with very high precision,
but the more precision we want, the higher will be the cost. In actual
practice, a compromise is taken between the precision and the cost,
which is usually neighter very precise nor very economic.

Moreover, the big parabolic troughs require field alignment by
xperienced experts using sophisticated instruments. It is hard to
imagine applying the same method on a consumer product.

To address this problem, surfaces based on elastic deformation are
roposed. However, elastic deformation does not yield exactly the
surface we want. And people usually correct the problem by adding
precise ribs behind the trough. The problem is that between adjacent
ribs, the surface is never very correct, resulting in defocussing. And
the durability of the glue between the surface and the rib may become
a big trouble, as the natural elasticity of the plate tends to tear
itself off from the rib.

2. Deformations.

Deformations of the reflective mirror surface have many causes:
gravity, wind, thermal expansion, aging of materials.

For example, the gravity makes the surface deform once the trough
changes its position due to tracking. In the big parabolic troughs,
sophisticated support structures are designed in order to rigidify the
tructure without adding much to the weight. Such a method does not
apply to smaller collectors.

3. Thermal isolation.

Pipe and receiver insulation of smaller parabolic solar collectors are
more difficult than bigger ones, because the conductive thermal loss
basically depends only on the length of the pipe, but not on its
diameter. As smaller collectors will inevitably lead to longer and
smaller pipes, the conductive thermal loss may grow inverse
proportionally with the width of the collectors, and the thermal
isolation becomes much more difficult.

4. Tracking mechanism.

This one is locking up most of the do-it-yourself attempts due to the
need of microcontroller programming, but in fact it is the easiest
part of the equation.

The difficulty is again the precision requirement. In the early days,
electronics is more costly and less powerful than today, so the
designs were often quite complicated. For example, the photodetector
alone in the big trough fields costs 100$ a piece. Such a cost is much
too high for a small product.

There have been many design attemps for alternative tracking
mechanisms, using passive methods, clocks or preprogrammed positions
etc. But none of them seems to be able to meet the sub-milimeter
precision requirement of a high performance parabolic trough.

5. Resistance to bad elements of the nature.

A parabolic trough is exposed to all kinds of bad elements: strong
wind, hailstorm, rain and snow, humidity, ultraviolet, temperature
variations, sand and dust, cleaning, etc.

6. Resistance to aging.

Here the single most important point is the reflective mirror. The big
parabolic troughs use back surface mirrors, that is, the reflected
light travels through the substrate of the mirror. It is expensive,
and does not suit well for small collectors.

On the other hand, if a front surface mirror (one under which the
reflected light does not travel through the substrate) is exposed to
rain and humidity, it usually does not last more than 3 years. There
are solutions, but they tend to drive the cost up.

7. Patent lockups.

There are a huge number of patents on solar technology out there, a
great majority of them being worthless. A few of them have good
points, but the problem for any engineer is that nowadays it is
impossible to design anything without running into a patent of
somebody else. Moreover, as these inventors were unable to solve all
the above problems at once, even the "good" patents usually finish
their days without bringing up anything concrete.

Posted by Steve O'Hara-Smith on January 6, 2009, 11:41 am
 
On Sun, 4 Jan 2009 05:56:34 -0800 (PST)


    Just two things I think

    Many homes are in areas not suitable for concentrated solar
applications (ie. do not have have direct sun most of every day). Flat
plates, evacuated tubes and even PV panels will pick up useful amounts of
energy on cloudy days and so are a better fit for temperate to cold
climates.

    The real benefit of a trough over a flat panel is that it can make
steam and so power a turbine - this is not really useful at home because
steam turbines are not cheap easy things to produce or particularly safe to
have where skilled maintenance is not available.

--
C:>WIN                                      |   Directable Mirror Arrays
The computer obeys and wins.                | A better way to focus the sun
You lose and Bill collects.                 |    licences available see
                                            |    http://www.sohara.org/

Posted by azuredu on January 6, 2009, 12:34 pm
 
Energy in diffuse light is a myth. Often exagerated by flat panel
vendors.

I've made many tests and statistics. The average diffuse light
intensity is less than 100W/m^2. With the thermal loss and optic
efficiency of a flat panel, the loss exceeds the contribution of
diffuse light once the water temperature is more than 15C above the
ambient temperature. This temperature level is strictly useless in a
cold weather.

Evacuated tubes do better, striking even at 20-30C. But the output is
ridiculously low if there is no direct sunlight.

I agree that small turbine for home is a problem. But it should be
possible to make something.

Posted by azuredu on January 6, 2009, 4:12 pm
 
Here is the page for how to overcome these difficulties.

http://wims.unice.fr/xiao/solar/difficulties.html

Posted by RF on January 6, 2009, 6:12 pm
 azuredu wrote:

And then there is the problem of wind effects.
Everything would need to be
held down to the roof with brackets and strong
bolts. Lying flat on the roof would present minor
problems.

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