How mirrors can light up the world

Monday, November 27th, 2006

How mirrors can light up the world:

Two German scientists, Dr Gerhard Knies and Dr Franz Trieb, calculate that covering just 0.5% of the world’s hot deserts with a technology called concentrated solar power (CSP) would provide the world’s entire electricity needs, with the technology also providing desalinated water to desert regions as a valuable byproduct, as well as air conditioning for nearby cities.

Just “0.5% of the world’s hot deserts” sounds pretty small, but, of course, it’s not. Desert land is cheap though.

CSP is not new technology; it’s the form of solar power that’s been used in the Mojave desert for over a decade:

There are different forms of CSP but all share in common the use of mirrors to concentrate the sun’s rays on a pipe or vessel containing some sort of gas or liquid that heats up to around 400C (752F) and is used to power conventional steam turbines.

The mirrors are very large and create shaded areas underneath which can be used for horticulture irrigated by desalinated water generated by the plants. The cold water that can also be produced for air conditioning means there are three benefits. “It is this triple use of the energy which really boost the overall energy efficiency of these kinds of plants up to 80% to 90%,” says Dr Knies.

Note that these impressive efficiencies only come about if you manage to use the shade and the desalinated water for horticulture and the cold water for air conditioning — with no additional inefficiencies.

Is it competitive?

The German reports put an approximate cost on power derived from CSP. This is now around $50 per barrel of oil equivalent for the cost of building a plant. That cost is likely to fall sharply, to about $20, as the production of the mirrors reaches industrial levels. It is about half the equivalent cost of using the photovoltaic cells that people have on their roofs. So CSP is competitive with oil, currently priced around $60 a barrel.

Dr Knies says CSP is not yet competitive with natural gas for producing electricity alone. But if desalination and air conditioning are added CSP undercuts gas and that is without taking into account the cost of the carbon emissions from fossil fuels.

I must admit, this part threw me at first:

[T]he reports recommend a collaboration between countries of Europe, the Middle East and Africa to construct a high-voltage direct current (HVDC) grid for the sharing of carbon-free energy. Alternating current cables, which now form the main electricity grids in Europe, are not suitable for long distance transport of electricity because too much is lost on the way. Dr Trieb, of the German Air and Space Agency, says the advantage of DC cables is that the loss in transport is only about 3% per 1,000 kilometres, meaning losses between North Africa and Britain of about 10%.

Thomas Edison’s original power transmission system used direct current (DC), but it was quickly supplanted by Tesla’s alternating current (AC) system. I didn’t realize exactly how AC transmission was superior, and thus how an HVDC system might have its place:

Low voltage is convenient for customer loads such as lamps and motors. Early electric power distribution schemes used direct-current electrical generators located near the customer’s loads, which distributed power at the same voltage as the lamps and motors needed. As electric power became more widespread, the distances between loads and generating plant increased. Since the flow of current through the long wires resulted in a voltage drop, it became difficult to regulate the voltage at the distribution circuit extremities. Customers near the generator would have a higher voltage than those at a distance. This was undesirable because lamp life was reduced by excess voltage, and performance of motors was reduced by low voltage.

For a given quantity of power transmitted, higher voltages reduce the transmission power loss. Power in a circuit is proportional to the product of voltage and current, and the power lost as heat in the wires is proportional the square of the current. So, higher transmission voltages increase the efficiency of transmission, for a given size of conductors. Another way to reduce transmission loss is to increase the size of conductors, but since the cost of wires is approximately proportional to their weight per unit length, this strategy becomes un-economic.

The principal advantage of AC is that it allows the use of transformers to change the voltage at which power is used. With the development of efficient AC machines, such as the induction motor, AC transmission and utilization became the norm (see War of Currents). Manipulation of DC voltages is considerably more complex, and has only become economically feasible with the development of high power semiconductor devices: Thyristors, IGBTs, MOSFETS, GTOs, etc.

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