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Which Solar Concentrator?

First published May-2010
last update 5-April-2016

There are of course many solar technologies, but in order to replace the majority of (non-food) energy use in a relatively short period of time — under a wide range of social conditions, both present and likely to occur, such as the depletion of fossil fuels and collapse of related infrastructure — we are searching for a technology that can be built with the minimum of resources and skills but is powerful enough to satisfy a large majority of real and perceived energy needs.

We can posit the following working design principles for such a technique:

1: Simplicity

Since the advantage of direct solar energy is that it can be used on location, our technology must be capable of being built, operated and maintained locally, using only basic tools and materials, a minimum of specialized knowledge and no electronics.

2: High temperature

Processes that require high temperatures can never be run with low temperatures, so a viable technology must be able to reach the highest temperatures a community needs.

However, since high temperatures can be easily diluted into lower temperatures, which means a high power concentrator can run lower temperature processes, we do not need to multiply solar devices for every temperature range. Better yet, the waste heat of a high temperature process, like baking ceramics, can be used for a low temperature task, like heating water.

What’s more, efficiency is directly dependent on temperature, the higher the temperature the higher the potential efficiency. So a high temperature concentrator can not only accomplish a wide variety of tasks, but can do those tasks more efficiently, meaning less surface area and material is required.

Not only does a high power concentrator avoid a proliferation of single use low power devices, it would occupy less surface.

3: Local but large

Though the temperature difference will determine what and how efficient our applications can be, the surface area of our concentrator will determine how much energy we have to work with. Or more plainly, the temperature of our concentrator answers “what can we do?” and the surface area answers “how much can we do it?”.

So we require a technique that can be built easily at large enough scales to provide the quantity of energy a community needs.

4: Fixed focal point

A solar concentrator (simple enough to be built locally) cannot avoid needing to move to track the sun. The focal point however can remain fixed relative to the ground.

Though there can be theoretical benefits for a moving focal point, the practical disadvantage is that any application must be attached to the concentrator and move with it. This movement makes the application difficult to access, any pipes to and from the focal point (for steam for instance) requiring a much more complicated design, and finally attaching the application to the concentrator creates a significant limitation in the weight of the application (as leverage will significantly increase the force acting on the concentrator). [1]

In brief, a fixed focal point solves so many problems that in most cases the convenience far outweighs a loss of theoretical efficiency. At the very least, a fixed point solar concentrator is a good starting point and switching designs to a moving focal point concentrator should be done only when it is clear the benefits outweigh the inconveniences.

5: Manual tracking

Manual tracking is essential since automatic tracking can easily increase the complexity of a solar concentrator, both to build and maintain, many fold. To truly accomplish our goal of a technique simple enough to be built anywhere, our basic designs must track the sun by hand.

And in most cases the inconvenience is minor. Many applications require being close by to observe and work with, and so in these cases adjusting the concentrator every 10-20 minutes is not a problem, and soon becomes second nature.

Tracking should require little physical and mental effort (which is usually the case since the focal point is very visible and a solar concentrator can be easily balanced so minimum energy is required to move it).

A tracking system can easily be more expensive than the concentrator itself and can easily break down and be difficult to repair, but even when automatic tracking would be a significant help, such as for steam production, it makes sense for most people starting a solar concentration activity to first test the idea with the minimum at first, hence with manual tracking, and only once the concept is proven and running smoothly think of installing automatic tracking.

So the best designs would be though for manual tracking, but have the possibility of installing automatic tracking later. And of course, if the automatic tracking ever breaks down, the manual option remains a backup. As there are more and more solar concentration artisans in the world, such smooth development of a solar activity will become easier, and the mastery of gravity based tracking will also arise; but before then we should not imagine that manual tracking is a limiting factor, as it remains less work than collecting, cutting and storing wood in many regions.

6: Open Source

For a rural locally built design to proliferate as widely and as fast as possible it must be Open Source. This is all the more necessary with solar concentration since the available materials and end uses can vary widely from region to region, and person to person, so an effective design must be open source so that anyone can adapt the technique to their conditions and needs. Many places may only be effectively reachable by open source as no closed-source organization could hope to find the right people needed to build and adapt the technology for make millions of projects each with unique circumstances, but millions of projects can be organized spontaneously by the right people if there is an open source design available.

For larger machines for small industries open source is not critical as it’s possible to reach these end users by conventional engineering means. Though open source would be idea, In my experience developing solar fire, it’s difficult to raise the funds required to develop larger machines in a open source framework; so a balance must be struck between doing things in open source and doing something at all.

Moreover, as we will see in the chapters to come, what is critical in solar concentration is the design of applications. For applications to be designed requires an easily accessible solar concentrator design.

7: Hybrid power or co-generation

When it’s not practical to reach the desired temperature with only solar concentration, as too much or a too constant energy is required, then in this case direct solar should be used as much as possible and the energy difference completed by combustion or electricity.


Now, with these design principles we have eliminated most solar technologies that currently exist.

However, this does not mean such technologies, as well as other renewables, would not play a part in a sustainable solar based society; what is meant is that complicated, low temperature, moving focal point, and/or limited energy sources would be used in limited situations and for limited tasks, and could not in themselves form a feasible basis for the majority of society’s real and perceived energy needs in the near future.


[1Rotating the reflector around the focal point can resolve some of these problems, but not all.

Written by Eerik Wissenz.

Table of Contents

*Chapters in grey are in progress.

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