Saturday, June 26, 2010

Concentrating Solar Power Generation.

Concentrating Solar Power (CSP) systems employ lenses or mirrors coupled with tracking systems to concentrate a large area of sunlight into a small beam, rather in analogy with the simple and familiar burning-lense. The concentrated energy may be used to heat a central “boiler” to run a power plant fitted with a conventional steam-turbine from which electricity is generated in the usual manner. A quite broad range of methods may be used to accomplish this, e.g. the parabolic trough, the solar (parabolic) dish and the solar power tower.

All such systems contain a working fluid which is heated by the concentrated sunlight, and then used to generate power or to store energy. In a parabolic trough there is a linear parabolic reflector which concentrates sunlight onto a receiver oriented along its focal line. By means of a tracking system, the reflector follows the Sun during the daylight hours along a single axis. Trough systems are the most efficient of any solar technology in regard to the land area occupied by the plant. The SEGS plants in California and the Acciona Nevada Solar One near Boulder City, Nevada are based on trough systems.

A parabolic (solar) dish system consists of a single parabolic reflector which concentrates light at the focal point of the reflector, which tracks the Sun along two axes. Of all the CSP technologies, parabolic dish systems are the most efficient. The 50 kW Big Dish in Canberra, Australia is an example of this technology. The Stirling solar dish combines a parabolic concentrating dish with a Stirling heat engine which drives an electric generator. The term “Stirling” refers to the fact that the device operates on a simple heat-engine principle. Stirling solar energy production is more efficient than photovoltaic cells and the technology has a longer lifetime.

A solar power tower consists of an array of dual-axis tracking reflectors (heliostats) that concentrate light on a central receiver at the top of a tower. The receiver contains a working fluid to absorb the heat, and can be seawater. The working fluid in the receiver is heated to 500-1000 °C and then used as a heat source to generate power or to store energy. Concentrating thermal power is the main technology proposed for a cooperation to produce electricity and desalinated water in the arid regions of North Africa and Southern Europe by the Trans-Mediterranean Renewable Energy Cooperation Desertec.

The potential and future of concentrated solar power was investigated and reported from a study by Greenpeace International, the European Solar Thermal Electricity Association, and the International Energy Agency's SolarPACES group. Remarkably, it was concluded that concentrated solar power could provide 25% of the world's energy needs by 2050. To achieve this, however, would require an increase in world investment would from 2 billion euros to 92.5 billion euros over that same time interval, although it further predicted that the price of electricity would drop from the present 0.15 - 0.23 euros currently per kilowatt, to 0.10 - 0.14 euros a kilowatt.

We always hear this, however, in the inauguration of all new technologies that the power production by their means will be cheaper, most notably (or notoriously) atomic power that was supposed to provide “electricity too cheap too meter”. Spain is the world leader in concentrated solar power technology, with more than 50 projects underway. The Desertec scheme has been described as being part of an overall intention to create "a new carbon-free network linking Europe, the Middle East and North Africa".

Related Reading.
C.J.Rhodes, "Solar Energy: Principles and Possibilities", Science Progress, 2010, Vol. 93, 37 - 112.


Anonymous said...

Hi Chris,
Some time ago there was talk of solar towers, consist of simple tall black columns in which a vertical axis turbine was fitted. The updraft caused by the heating of the tower spun the turbine and so on. Have you heard of these things, and if so, do you know if any progress was made?
The idea caught my imagination since I was aware that the greater part of maritime trade in the formative period of the industrial revolution was enabled by steamships using natural draught boilers. There is no doubt at all that this remarkably simple technology
very well indeed. There were some big boilers around, pushing high horse-powered steam engines. Thousands of them, (a basic requirement in our curriculum was calculations of funnel draught effects).
The systems would only work during daylight hours of course, but would be wind independent. As I have mentioned before, efficiency experts demand 24/24 operation for their economic calculations, yet there is no intrinsic reason why the periodic nature of such things as wind towers cannot be accepted. In comparison with wind turbines which must have a useful wind factor of about 30% factored in, it would seem that a tower with a sunlight availability factor of 50% compares favourably.
Aesthetically speaking, it seems to me that simple black, or even dark matt columns would be less intrusive in our landscapes than the mighty windmills which now adorn them.
Safety speaking, the funnel turbine, being protected at the base of the chimney, would be more secure than a wind turbine.
Constuctionally speaking, tall chimney constructors and maintenance specialists was a long established and respected trade, whether they still exist is a moot point. Certainly the gigantic cranes required for wind turbines would not be needed.
Best regards.
Peter Melia

Professor Chris Rhodes said...

Hi Peter,

yes, I have heard of such things. There is a wikipedia site which gives some information:

My understanding is that to get really useful power from such a device needs a fairly large additional collector area so to drive the air "up" through the tower to spin the turbine.

As I sit here thinking, however, I can imagine that such devices of the lower complexity you allude to might prove useful for generating power for isolated/small communities or other stand-alone applications.

I agree that the "periodic nature" of energy production need not rule it out as being useful and e.g. there is (was? It's gone a bit quiet now) the plan to build a European power grid into which would be fed electricity from a combination of power plants including solar and wave, which would have a practically constant output.

I note too from the wikipedia article that the Australian tower is couched in commercial secrecy so it's not clear exactly what it's design is to be. That leaves me thinking that indeed the technology is "on-to-something".

Best regards,


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