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Select Committee on Science and Technology Fourth Report


APPENDIX 8: ENERGY PAYBACK TIMES

How much energy is required to build and operate power stations, and how does this compare with their lifetime energy outputs? Some estimates for coal and nuclear stations and wind farms are shown in the table:

ProcessCoal FissionFusion (design concept) Wind (no storage or backup)
  Terajoules per GW-year of electrical output
Mining and fuel preparation 1,2581,288 48-
Fuel transport1,059 8- -
Materials (other than fuel) 5558 302581
Plant construction 6199 335242
Operation283 384435 517
Waste disposal and transport -172 16-
Decommissioning10 1955 72
Land reclamation3 0.1Negl. Negl.
Total 2,737 2,0281,191 1,387
Energy Payback Period (yrs) 3.32.5 1.51.1
  
Assumptions:
Plant Size: MW1,000 1,0001,500 25
Plant lifetime: years 4040 4025
Capacity factor: % 7575 7525


Source: S.W. White and G.L. Kulcinski (1998). Energy Payback Ratios and CO2 Emissions Associated with the UWMAK-I and ARIES-RS DT-Fusion Power Plants. Fusion Technology Institute: University of Wisconsin. Paper UWFDM-1085.

Estimates of the payback period vary with the scale of the power plant, since there are appreciable scale economies in the use of materials per unit of capacity, and with the plant's capacity factor. The high transportation costs for coal presumably reflect the long distances over which coal is transported in the United States. The above estimates for wind were made 7 years ago, when turbines were one quarter of their size today; capacity factors can also be higher. The Danish Wind Industry Association[96] claims that modern wind turbines recover the energy use in their manufacture and installation within three months, a statistic shared with the Committee by Risø on a visit to Denmark.

For offshore wind, the payback times are expected to be shorter. The higher capacity factors—they are expected to be 1.3 to 1.5 times higher than those for onshore wind—would more than offset the added energy costs of installation. A 30 percent improvement in capacity factor for instance equates to 6-8 years of extra operating life.

For solar photovoltaic systems, estimates vary greatly with the material and with the scale and design of the manufacturing process. The payback period is also declining with technical progress in manufacturing. The following estimates of payback periods are provided by the World Energy Assessment Report by the World Energy Council and the UNDP (2000):[97]

  • For crystalline silicon. In year 2000, 4-9 years. Prospectively 2-3 years or less;
  • For thin film. In year 2000, 3-4 years. Prospectively 1-2 years or less.

For biomass, estimates of the energy input to produce useful energy from biomass for the consumer vary with the source (forest residues, straw, miscanthus, short-rotation coppice); the yield per hectare (higher yields lower the collection effort); and the type and efficiency of the plant (e.g. whether it is for heat, electricity or CHP). For producing the biomass at the 'farm gate' the energy output/input ratio varies from 10-20, and are often outside this range.[98] Taking a typical ratio of 15, and a CHP plant of 70 percent efficiency, the energy output/input ratio is reduced to 10 (in round numbers). Thus for a plant with a 25 year lifetime, it would take about 2.5 years for the energy of the biomass to be paid back in energy terms. The construction and operation of the power plant may require another a year (about twice that for the coal plant in the above table, on account of the lower energy density of biomass). Overall, an energy payback time of around 3.5 years would not be untypical—similar to that for coal, though rather more than for other renewable energy technologies and nuclear power.

The energy consumed in transporting biomass is equivalent to about 0.8 kg of oil fuel per tonne of biomass (which has the energy of about 0.4 tonnes or 400 kg of oil) for each 10 km transported by road, and each 100 km transported by ship,[99] equivalent to an increase in the energy payback times of around one-fifth of a percent.

Professor Dennis Anderson, Specialist Adviser


96   www.windpower.org/en/tour/env/enpaybk.htm Back

97   New York: UNDP. Back

98   Hall, Rosillo-Calle, Williams and Woods (p625) in Johansson et al (1993) Renewable Energy. Washington DC: Island Press. Back

99   Estimates inferred from the data in Table 4.4 of the 2004 report of the Royal Commission on Environmental Pollution, which reports that the CO2 equivalent emissions from road transport per oven dried tonne of biomass are 0.18-0.27kg/km, and by ship 0.012-0.024kg/km, such that the weight of the oil consumed would be approximately one third of these quantities (the ratio of the molecular weight of oil to that of CO2). The above estimates take the upper range of these figures. Back


 
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