Why there was no rooftop parabolic trough

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Parabolic trough is a very attractive solar technology. It can output thermal energy with temperature ranging from 60°C to 400°C 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 course, 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 experienced 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 proposed. However, elastic deformation does not yield exactly the surface we want. And people usually correct the problem by adding precise ribs behind the trough, for example see Soponova 4. 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.

    Instead of this, we use elastic redressing to correct the elastically formed surface. The result is much more precise, and the cost is negligible. This method has the extra advantages of being able to adapt an imperfect material, and to readjust the surface should any deformation occur after a few years of service.

  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 structure without adding much to the weight. Such a method does not apply to smaller collectors.

    We use a closed parabolic trough whose design is auto-rigidifying. As a result, most deformations are non-existent. The worse deformation that occurs is due to unequal thermal expansion in a plastic box, when it is partly hot and partly cold, for example due to a shaded exposure to the Sun. Using silver-coated mirror generally solves this problem.

    Aging of a plastic plate is a new problem brought up by the new design. But test shows that with a correct choice of the plastics, the effect of aging-brought deformation is rather undetectable.

  3. Receiver and thermal insulation.

    The big parabolic troughs use evacuated tube-in-tube receiver. Such a design relies on intensive metal-to-glass sealings that work under high temperature. It is known that the quality and the durability of the sealing is very hard to ensure, resulting in a very high initial cost and a shortened life. Moreover, a phenomenon common to all evacuated insulation is that helium in the atmosphere, as well as hydrogen resulting from thermal oil decomposition in this case, permeate slowly into the insulation layer and destroy the insulating property.

    The situation will much worsen if the same design is used in small collectors.

    For this reason, we designed a new method, making partial evacuation and filling the insulation layer with a low-conductivity inert gas. See my article.

    See also this page.

  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.

    However, today's microcontrollers are very powerful and very economic. So it is now relatively easy to design a microcontroller-based tracker at a minimal cost, whose precision can go down to micrometer levels, and who can support software adjustments.

  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. Careful design should be made to achieve a product that has sufficient resistance to all these elements, without driving up the cost too much. See this page.

  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.

    Our closed parabolic trough has no such problem: the reflective surface is well protected by the closed box and by a thin plastic sheet in front of it, so it practically does not age.

  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.

    Fortunately, a big rush to solar patenting has taken place during late 1970s and early 1980s, and all these patents are now expired. So it has been possible for me to make my design, "infringing" nothing more than expired patents, adding only things that are really my own inventions.

There are other known problems with the big parabolic troughs, such as the cosine effect, important seasonal variations of the output (at a latitude of 35 degrees, the winter daily output is already only 20% of that of the summer). See the book Renewable Energy Cannot Sustain a Consumer Society, Chapter 3 for more details. But these problems are non-existent for small collectors, so we did not list them in the above.

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