Renewables

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Variable renewables
    Wind power
    Solar Photovoltaic
    Tidal and wave generation
Constant renewables
    Hydroelectricity
    Biomass electricity
    Geothermal electricity
    Biomass gas
    Rubbish incineration
Conclusion
Further reading

To date, renewable energy has not been given a high priority. The current implementation of renewable energies is close to zero in most places, with the exception of solar water heating in some countries.

Variable renewables

These are defined as energy that cannot be generated at a constant level. Some examples are tidal, solar and wind power. These are sometimes called intermittent sources, but I prefer the term "variable" over "intermittent", because the latter implies an "all or nothing" energy generation, whereas the output more commonly varies from nothing to full output with every intermediate level possible. It should be noted that it is essential that constant power supplies must be available to cover the maximum demands, so that variable renewables can only serve to allow conventional power stations to be "eased off", thereby reducing fuel consumption. Good weather forecasting is a sine qua non of useful exploitation of variable supplies, so that the constant requirements can be foreseen and the plant brought up to speed accordingly.

It must be noted that there is a limit to the amount of variable energy that a power grid can handle at any one time. Above that limit of 18 - 20 per cent for any one type and about 24 per cent for the aggregate of all types, experience in some countries has shown that the whole grid system may become unstable, leading to black-outs.

Wind power

In some countries in higher latitudes, wind generation, both offshore and onshore, has been well implemented with further expansion planned. Generally speaking, wind generators have a rated output with wind speeds of about 16 - 20 m.s^-1 and the output drops to about half at 11 m.s^-1 and one-quarter at 8 m.s^-1. This means that wind turbines could operate efficiently only in countries with high average wind speeds. This is exacerbated by the fact that these machines shut down when the wind speed exceeds about 23 m.s^-1 such as may happen during gales.

Typical empirical wind turbine curve

Typical distribution of wind speeds in a favourable location

By combining the data of the above graphs, it can be seen that the turbine will give practically zero output for about 23% of the time, will work at its full rated output for 6% of the time and will give about 50% of its output for half the time. However, this is not the full picture because the wind speed is never constant but is always gusty and the turbine/generator combination has considerable inertia. In fact, these two factors reduce the efficiency somewhat, but this can be partially overcome by a wind farm over a considerable area. Another point to be taken into consideration is the diurnal variation of wind speed. If the average speed is higher at night, when the energy demand is least, then it may be less economical to implement than if the velocity is higher during the day. This is a factor to consider particularly in coastal locations, with land and sea breezes.

It is estimated that the average cost of on-shore wind-generated electricity would be over twice that of fossil-fuelled generation. Off-shore installations would be economically even less efficient.

It is feared that the windmills may kill migrating birds, some of which are rare and protected species. It is perhaps not appreciated that a 2 MW wind turbine may have blades of 65 m diameter on a mast of 100 m height. The tips of the blades may rotate at speeds of 100 km/h or more, so that large birds may not have an opportunity to avoid them if the blades interrupt their flight path. The blades themselves can also be damaged if the bird is of the size of a duck or more. This has been a problem in the USA, especially with raptors.

Solar Photovoltaic

This is perhaps the ideal variable renewable energy source for countries with much sunshine, except for its capital outlay. Some countries have well over 2,500 hours of "useful" sunshine per year, so that a 3 - 4 kW system (size which will fit on a typical south-facing villa roof) will generate a theoretical 7.5 - 10 MWh. However, be warned, you will never reach this theoretical limit in sunny climates because the efficiency of the solar panels drops at temperatures above 25°C; it would be wise to budget for 5 - 7.5 MWh respectively. If one were to buy a PV system at its full price and not sell any surplus electricity generated, the payback period would probably exceed the lifetime of the system. With the subsidies and the buy-back price that are offered in some countries, an average household would have a payback period of 8 - 15 years, after which it will become profitable (subject to reserves on the final conditions of subsidies).

The real cost of generating solar PV electricity is very high, typically 35 - 50 c/kWh. However, it can make a real contribution to smoothing out peak demands because it will be reasonably productive at the time when air-conditioning units and chillers would be working hardest. This alone makes it interesting, despite the high cost and the subsidy burden on the ordinary electricity consumer.

Tidal and wave generation

It has been said that one form of tidal electricity generation is like wind generation under water. Where this analogy fails is that tides are largely predictable, wind is not. However, it should be stated that there are four periods per day when tide generation does not and can not work; as the tide turns, hence it being classed as variable, even though it is predictable. Other types include estuarine dams (which may disrupt shipping and create problems for wildfowl), and caisson pressure systems. The latter can be widely distributed in small units of, say, 100 kW capacity with the advantage of smoothing out the slack water periods.

Wave generation is a possibility when the average height of the waves (crest to dip) exceeds 1 metre. The "best" waves for this are oceanic swells, but local wind-driven waves would also work. However, as these are dependent on wind, it is probable that the same conditions as in the paragraph on wind generation would probably apply.

Constant renewables

Hydroelectricity

Hydroelectric generation is the mainstay in countries, like Norway, Switzerland and Austria, where there are large glaciers. The water from the summer melt-off is collected in large dams, at altitude, and penstocks lead the water to pressure-operated turbines in the valleys. Alternatively, large dams across rivers, such as the Three Gorges Dam in China, can turn flow-operated turbines. Both types are environmentally discussional for several reasons and large projects are nowadays very severely criticised. Both types are also potentially dangerous to downstream life if, for any reason, the dam should burst and this does occasionally happen, despite the best efforts of civil engineers. It is estimated that about 250,000 lives have been lost because of dam bursts since 1950.

Small landslide falling into the Yangtze upstream from the 3-Gorges dam
showing the geologic instability of the region
(nearly all the land in this photo will be submerged when the project is completed).

Yangtze tributary, showing where massive rock falls have occurred, upstream
from the 3-Gorges dam.

There is a variant of hydroelectric generation which could possibly have some future relevance to counter the undesirable effects of variable renewables. Imagine two lakes of equal size, say, similar to that of one of the larger dams, but separated in altitude by 300 - 500 m. During the night, when there is a surplus of power generating capacity, water is pumped up from the lower to the higher. At peak demand time or when variable renewables have a low output (e.g., no wind and a cloudy day), the water in the upper reservoir is made to flow down to the lower lake, generating hydroelectricity. This method is the only useful way of "storing electricity" for later use on a reasonable scale with today's technologies. There are several pumped storage systems operational throughout the world but suitable sites are rare.

Biomass electricity

This consists of growing some form of crop, usually wood from quick-growing trees, for gasification or chipping and burning in a thermal power station. The major problem is that harvesting and transporting the wood is very energy-intensive for large installations and become energetically negative.

Geothermal electricity

This method is ideal in places like Iceland but requires volcanic rock strata at a constant temperature of about 200°C. It is, perhaps, the most ideal form of "free" energy, where geological conditions permit it.

Biomass gas

Medium to large-scale poultry, cattle and pig farming, such as is practised in many places, produces large amounts of excrement. If this is placed in a large anaerobic digester, the gas produced by fermentation in the first 48 hours can be collected and large quantities of methane (or natural gas) can be easily separated. This gas is indistinguishable from fossil natural gas and can be used for any similar purpose. If transported to power stations, it could complement other fuels, providing a small percentage of the region's power supply.

Rubbish incineration

In a number of countries, up to ten percent of electricity requirements are being supplied by incinerating household rubbish and other combustible scrap. This is most practical in regions of high population density, such as large cities. This would also reduce the need for the many, unsightly, polluting, insanitary landfills, a few of which would be used only to dispose of the surplus sterile ash. Such power stations are not cheap to construct, as the rubbish has to be pulverised and the exhaust gases have to be scrubbed to eliminate harmful pollutants, but they do make a useful contribution to the environment.

There is, however, a big "but" to this idea: it will require meticulous sorting of household waste (see the essay on Waste). Notwithstanding, this will become a requirement under EU directives, anyway.

Many cities in all parts of the world are already generating electricity successfully from waste incineration, such as Baltimore (USA), Zurich (Europe), Shanghai (Asia) and many others.

Conclusion

In sunny climates, the only potentially large scale variable renewable energy source is the solar photovoltaic panel. Where there are consistent winds of 10 m.s^-1 or more, wind generation may be viable, or nearly so. The only major potential constant renewable energy source, capable of supplying up to 10 per cent of our needs, would be by waste incineration. I strongly urge that this be implemented, not just for electricity supply but also as a logical means of introducing waste discipline. A small contribution to any future natural-gas electricity generation may be made by composting farm animal excrement, the residues being a valuable natural fertiliser to reduce the demand for chemicals which are energy-intensive to manufacture.