HOW CLEAN SOLAR POWER MAY REPLACE DIRTY KINDS OF POWER
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If the potential of "clean power from deserts" can be realised, it should be possible to replace some 'dirty' forms of energy with clean electricity and thus reduce emissions of CO2 and other pollutants. This page explores some of the possibilities.
Electrification of transport by rail and road
There is great scope for the use of electricity in powering trains and road vehicles. To replace fossil fuels in overland transport with renewable electricity would require less than 50% more electricity than we currently use (see below).
Trains have been powered by electricity for many years and the technology is mature, but electric road vehicles currently have a ‘milk float’ image in the UK. However, this is really out of date since there are now several attractive and practical designs for electric cars and other kinds of road vehicles. A very useful source of information in this connection is the website of Electric Vehicles UK.
Every day, the great majority of road vehicles travel less than the 100 mile range that is well within the scope of EVs now. But it will always be true that any vehicle may need to cover a longer distance in any one day, or will be needed for some important journey when the battery is flat. Hence, there is a good case for hybrid vehicles that can draw power from an ordinary engine when required. Plug-in Hybrid Electric Vehicles (PHEVs)—which are like ordinary hybrids but with larger batteries—give ample scope for the use of green electricity. There is scope for incorporating photovoltaic panels in the roofs of vehicles so that they can draw some of their power directly from the sun. And PHEVs open up interesting possibilities for load-balancing on the grid (see also vehicle-to-grid technology).
How much extra electricity would be needed?
If the electrification of transport by road and rail were taken to its logical extreme, an obvious question is “How much extra electricity would be needed?” The calculations shown here are based on a snapshot of energy uses in the recent past without any attempt to predict changes in the future.
In 2005, the quantities of energy consumed in transport by road and rail in the UK were as shown in the following table (From “Table 2.1. Transport energy consumption by type of transport and fuel, 1970 to 2005”, Department of Trade and Industry statistics):
Type of transport 1000s TOE* TWh
Cars 26834 312
Freight 15501 180
Electric 740 9
Fossil fuels 869 10
Total 43944 511
* 'TOE' = tonnes of oil equivalent
To simplify things in the following calculations, I shall focus on the 312 + 180 = 492 TWh of energy used for road transport and ignore the relatively small amounts of energy used for rail.
If the whole of road transport was electrified, then at first sight, the UK would need an additional 492 TWh of electricity, over and above the 407 TWh that was used in 2005 (“Energy consumption in the United Kingdom”, Department of Trade and Industry, 2006, p. 114). But batteries and electric motors are relatively efficient:
- “The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out.” (http://www.powerstream.com/NiMH.htm)
- “Electric motors often achieve 90% conversion efficiency over the full range of speeds and power output and can be precisely controlled.” (Wikipedia, “Fuel efficiency in transportation”, 2007-06-24)
If we take those two figures as representative, the overall efficiency of an electric vehicle would be about 90% of 66% or 59% overall, ignoring losses of energy elsewhere in the vehicle. By contrast, the efficiency of an internal combustion engine is normally about 20% (Wikipedia, “Internal combustion engine”, 2007-06-24).
With our current road transport system, based almost exclusively on the internal combustion engine, the useful energy obtained from an input of 492 TWh will be about 492 x 0.2 = 98.4 TWh. To obtain this amount of useful energy from electric vehicles with an efficiency of 59% would require 98.4 x 100/59 = 166.8 TWh of electricity. This is less than 50% of the 407 TWh that we currently use.
A possible worry about a large-scale switch to EVs or PHEVs is that some of the raw materials needed for batteries—such as Lithium in Lithium-ion or Lithium-Iron-Phosphorus batteries—might not be plentiful enough to meet demand. This is a complex subject that is well beyond the scope of this short piece.
Another option for reducing the use of fossil fuels is to create synthetic substitutes—hydrogen, methanol or even hydrocarbons—using the power of the sun, as outlined on our page about solar-powered industrial processes.
Of course, the burning of such fuels might create the same kinds of local pollution—particulates, oxides of nitrogen, or ozone—as are produced by the burning of fossil fuels. But their use should mean a net reduction in releases of CO2 into the atmosphere because any carbon required for their synthesis would be extracted from the air.
Possible uses for synthetic fuels include aviation, providing power for the internal combustion engine in a hybrid vehicle or a PHEV, and space heating (discussed next).
The key to space heating in a low-carbon economy appears to be the use of high levels of insulation, air tightness with the use of heat exchangers for ventilation, and passive solar power, in the manner of the German Passivhaus (see also zero-carbon eco-renovation).
Any residual need for heating could be met by the use of synthetic fuels (as mentioned above) but a better option is probably the use of a ground-source heat pump powered by clean solar electricity. Naturally, all other electrical power for the house would be clean and green.
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Last updated: 2009-08-20