VoxSolaris: The Voice of the Sun
Concentrated Solar Voltaics

Focusing the sun

It used to be the case that solar cells based on silicon crystal, the most established technology, were inherently expensive because silicon is hard to purify and the crystals hard to fabricate. Prices had dropped but were stuck at $5 per peak watt. The $1 per watt was a distant dream and it was not at all clear we would ever get there. It has since come to pass and indeed, bulk prices of some panels are as low as $0.50 per peak watt. This has largely undermined the case for using solar concentrators to produce electricity. A good design could meet the $1 per watt and in the right climate beat it but solar concentrators have not seen much change in the factors affecting their price. But they have not been left behind completely and are still part of the mix for electricity.

There are four important forms of solar concentrator as follows:-

a) The parabolic dish mirror which produces very high temperatures (up to 3000°C) at a focal point.

b) The rectangular parabolic mirror which produces moderately high temperatures (around 450°C) along a focal line.

c) The solar tower which produces large amounts of heat (megawatts) by serving as the focal point of many ground based mirrors reflecting sunlight upon it. In effect the solar tower is a parabolic dish mirror but comprised of thousands of small mirrors spread over a large area (several hectares).

d) The Fresnel lens. This replaces a conventional lens by a set of concentric annular sections thereby reducing the volume and mass of material required to construct it. This comes at the expense of reduced image quality which is not an issue here. It can achieve concentration ratios of 500:1

Operation of solar concentrators

There are several ways in which solar concentrators can produce electricity with perhaps the most obvious being to use the heat produced in a heat engine to drive a generator. The heat engine might be a turbine driven by steam or other working fluid although small turbines are generally less efficient. For this reason smaller installations commonly use a stirling engine, pressurized for compactness. Some home built concentrators even use adapted model steam engines. Electricity producing solar concentrators don't have to this cumbersome though. One novel approach and a promising line of research, employs a vacuum valve to produce electricity. The high intensity sunlight is focused on the valve's cathode giving rise to both thermionic emission and the photoelectric effect. The results in terms of efficiency are not spectacular but very promising economically. Valves allow for smaller concentrators which require less material per unit cross section area (sunlight gathered) and increases opportunities for deployment. Valves have no moving parts and are a lot cheaper than stirling engines or steam turbines.

Efficiency Factors

Of the sun's energy that falls onto a solar reflector, about 20% is in the form of useless scattered light. Of the remaining 80%, some 3% is absorbed by the mirror, a small amount by the air between the mirror and the absorber and some is radiated back by the absorber. The extent to which re-radiation occurs is mostly due to the operating temperature of the absorber. The surface of the absorber will be just as good at radiating energy as it is at absorbing and the extent to which selective coatings can mitigate against this is extremely limited. The maximum temperature the absorber can reach depends on how concentrated the sunlight is. A very high quality dish reflector could concentrate sunlight up to 10,000 times. If ordinary sunlight can heat things up to say 60°C (330°K) and the radiated energy rises as a power of 4, such as reflector could produce temperatures 10 times this, 3300°K or 3000°C. But at the maximum temperature the absorber is radiating as much energy as it receives leaving no energy available to be converted into anything useful like electricity! To actually get anything out of the concentrator we need the temperature of the absorber to be lower. For steam we only want 450°C or 825°K, one quarter of what a high quality dish can produce. At that temperature the re-radiation would be a mere 1/4096th of that absorbed leading to the question as why even bother to mention it!

The reason why it matters is because high quality dishes are very expensive. They need an awful lot of polishing. To be viable we want something of reasonable quality we can mass produce. Such dishes are not going to get anywhere near a concentration ratio of 10,000 and a more typical value would be around 500. This would still produce a maximum temperature of some 1650K but as this is only double our hoped for operating temperature, one 16th of the energy is re-radiated and that is a significant loss. And it is much worse with the rectangular design as these have a focal line rather than a focal point. With the quality of reflectors that are viable to mass produce, concentration ratios are going to be more like 60. This corresponds to a maximum temperature of about 1300K and if we still want the same operating temperature we would lose about one 6th of the energy. And it is more of an issue with the chemical water splitting concentrators that will need operating temperatures of 830°C and the thermionic concentrators which need around 1200°C.

Other Design constraints

The larger the dish or rectangle the more sunlight it can capture but this relationship is linear. But structural factors are not linear. The larger the mirror the thicker the material must be to withstand wind sheer etc. So purely in terms of the structure, it cost less to build 10 small concentrators than 1 concentrator 10 times the area. But other factors go the other way. The cost of the stepping motors needed to track the sun does not rise in line with size. Moreover, if the area of the mirror is too small it will not generate enough steam to efficiently run even the smallest of turbines and using a piston type engine is less efficient. On the other hand pistons can operate at lower temperatures which mitigate re-radiation losses so lower costs by allowing lower quality reflectors.

A typical dish might be between 3 and 6 meters in diameter garnering between 5KW and 20KW of thermal energy at temperatures of around 450°C and converting this by a stirling engine to produce a peak electrical output of between 1.5KW and 6KW.

The tracking systems on some of the dishes we have evaluated have complex tracking systems that in our view are overkill. When surveying possible sites for solar dishes you do need to take account of how much direct sunlight the site will provide during the course of the year and what if anything might obscure the dishes' view of the sun. But during operation the dish only has to point at the sun. Nothing less. Nothing more. You don't really need a database of coordinates describing the exact trajectory of the sun at a given longitude and latitude. We recommend a simple arrangement comprising a short pole surrounded at the base by a ring of photocells. The dish is pointing at the sun if the pole casts no shadow (photocells are seeing equal levels of light). If the cells on one side are seeing more light than the other the dish need to swing towards the brighter side. Likewise the top and bottom.

High Temperature Processes

Many common industrial processes require copious amounts of heat which is often provided by burning fuel. Much of this heat could be provided by solar concentrators, particularly of the solar tower class. Numerous opportunities exist including processes that split water and produce syngas. This area is important because industrial chemical processes are a massive emitter of carbon dioxide - more significant than even the much derided car. The savings are very considerable because solar concentration produces high temperature much more efficiently than any solar device can produce electricity.