The demand for electricity varies daily and seasonally and therefore some centralized power stations may only be required for short periods or to operate at limited capacity. In general, a fully interconnected electricity network will use low-cost, very large power stations for the base load and more expensive units for peak loads.
The fossil fuels used for either base load or peak units are easily stored and trans- ported. Most renewable energy sources with the exception of biomass cannot be stored and transported easily, unless they are converted into electricity or other energy carriers. In addition, some renewable power systems only supply electricity intermittently, for example wind, wave and photovoltaics. In order to incorporate these intermittent electricity sources and to deal with peaks and troughs in electricity
Table 3.13. Typical peak electrical power output from renewable energy systems in the UK.
(From Dti, 2006b.)
Peak power MW
output MW electricity
System of units produced Comments
Solar photovoltaic 0.01–0.09 6 Panels are available but costly Wind single grid Up to 1 – Some turbines are 3 MW, others
connected 300 kW
Wind farm 10 2016 Many exist in UK but planning difficult
Geothermal 30 0 None in UK
Hydro large Up to 130 1058 Largest in UK Loch Sloy at 130 MW
average 30 MW
Tidal barrage 240 – Severn barrage would have yielded 8640 MW, France producing 240 MW Wave shoreline 0.18 7 Only prototypes
Wave offshore 5.25 – Only prototypes Burning biomass/waste 10–50 540 Size can vary
Landfill gas 1–5 781 A number of sites exist
demand some form of storing either electricity or energy which can be converted back into electricity is required. The advantages of storage would be the following:
Bulk storage of energy would allow the decoupling of production from supply.
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Allows the incorporation of smaller power stations into the network.
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Improves power quality and reliability.
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Reduces transmission losses as transmission distances reduced.
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Cost reduction, as smaller, more efficient power stations can be constructed.
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Allows the use of intermittent renewable power sources.
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Decreased environmental impact associated with renewable sources.
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Strategic advantages of generating energy from indigenous energy sources, avoid-
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ing imports.
At present two large-scale energy storage systems are in operation: pumped hydro storage and compressed air energy storage (Dell and Rand, 2001).
In the case of pumped hydro storage, excess electricity at times of low demand is used to pump water into a lake or reservoir some distance above a hydroelectric power station. When a peak in electricity demand occurs conventional power stations are too slow to respond but the stored water can be released and the hydroelectric plant comes online rapidly. In the UK, there is such a system in Wales. The second large-scale energy storage system is to compress air in large reservoirs when electricity is in excess and release this to drive electricity-producing turbines. Such systems have been operat- ing for some time in Germany and the USA (van der Linden, 2006).
On the small scale a number of systems are under development including the following:
Flywheels.
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Hydrogen production.
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Batteries.
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Thermal storage.
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Superconducting magnetic coils.
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Flywheels have also been used to store energy and using new technology small high- density systems have been constructed and megawatt modules can be installed.
On the island of Utsira, Norway, electricity is provided by wind turbines as there is no link to a mainland power station. Wind power is intermittent so that any excess electricity generated when the wind blows is used to electrolyse water, producing hydrogen. The hydrogen is stored and burnt to produce electricity when the wind is insufficient to run the turbines. The feasibility of a wind-photovoltaic system using compressed hydrogen has also been tested in Australia, where the costs of the hydro- gen storage was the most critical factor (Shakya et al., 2005).
Five types of batteries can be used to store electricity. The lead-acid battery was developed a long time ago and is used widely in the automotive industry. These batter- ies have also been used for small wind and solar installations but they require periodic maintenance and are poor at low and high temperatures. Alkaline batteries, nickel–iron and nickel–cadmium, were also developed a long time ago, around 1900. The best is the nickel–cadmium which performs better than the lead-acid at high and low temperatures. It is however more expensive but the nickel–metal–hydride has been developed. This battery, although more expensive, holds more charge and has seen widespread use in mobile phones and laptop computers. It has also been used in
electric and hybrid vehicles. The third type of battery is the flow batteries, sometimes known as ‘regenerative fuel cells’ (rated to 12 MW). The cells are charged, converting electricity into chemical energy. The two compartments of the cell are separated by an ion-exchange membrane and the electrolyte in the compartments is circulated in a closed-loop system. The last two types of battery are the high temperature battery and the rechargeable lithium battery. The high temperature battery uses molten sodium at 300–400°C, and both these types have problems for large-scale use, although lithium ion batteries are widely used in portable electronic devices.
The thermal storage of energy from electricity using hot water or solid material is used to heat buildings in the form of night storage radiators where off-peak electri- city is used. The heat cannot efficiently be converted back to electricity so this is not suitable for energy storage. However, phase-change materials have been used to store solar energy (Kenisarin and Mahkamov, 2007).
In systems where there is fluctuating power, superconductive magnetic energy storage can be used, and though the system is expensive, it can respond in millisec- onds. Energy is stored in a magnetic field formed by a DC current in super-cooled superconductive coils.
Conclusions
The reports by Stern (2006) and the IPCC (2007) outline the consequences of global warming, and it is clear that efforts should be made to reduce the emissions of green- house gases from fossil fuels globally.
The IPCC has come up with four scenarios predicting the global atmospheric carbon dioxide levels depending on what measures are taken towards their reduction.
If the amount of carbon dioxide released per year was retained at present values, the carbon dioxide would reach 550 ppm by 2050. Carbon dioxide emissions are still increasing so that 550 ppm may be reached before 2050. A level of 550 ppm is pre- dicted to give a 2°C increase in global average temperature. At present there is hope to reduce carbon dioxide release, so that a value of 450 ppm is reached by 2100.
Small increases in temperature seem insignificant, but these can have far-reaching effects such as the melting of sea ice. Some consider that even if we stopped carbon dioxide emissions now, the tipping point may have already been reached and a rapid and long-lasting increase in temperature is inevitable.
The chapter outlines the methods that are currently available or under develop- ment for the reduction of atmospheric carbon dioxide which include: burn less fuel, sequester the carbon dioxide and use non-carbon-dioxide-producing energy. Within these broad categories there are many options and no one option will provide a com- plete solution, but in concert they may well affect the outcome. The solution is not the science but rather the politics where countries have to reduce carbon dioxide emissions, while at the same time producing growth in their economies and increasing prosperity. It seems that if energy supply is to be sustainable and carbon-neutral it cannot be obtained at the same time as continued growth of the economy. However, developing countries will not stop their development in order to reduce carbon diox- ide emissions despite the warning that global warming will affect developing coun- tries the most. Considerable political effort and legislation will be needed if global warming is to be halted.