Saturday, February 04, 2006

Energy yield factors for the generation of electrical energy

By Joachim Grawe

In partnership with Energie-Fakten

The yield factor of a power plant is – in simplified terms – how many times energy generated during plant operation covers the energy used for constructing the plant. The exact definition is: 'The yield factor is the ratio of net energy production during plant life and the cumulative energy used for construction, operation and operating supply items'. The concept is only meaningful in the context of using regenerative energy sources.

The yield factor has been investigated mainly for electricity generation plants in several studies. Sporadic data is also available for solar collectors, heat pumps, district heating systems and insulation measures. The most solid dataset on this subject is available from the 'Institut fuer Energiewirtschaft und Rationelle Energieanwendung' of the University of Stuttgart, that has developed a series of detailed studies on the subject. It is also worthwhile to mention the 'Forschungsstelle fuer Energiewirtschaft Muenchen' and the Chair of Prof Franz-Jozef Wagner, Gesamthochschule Essen.

The literature on this subject is not fully consistent in its treatment of energy uses for operating power plants (for example maintenance works, or fuel consumption) as well as for demolition and disposal of the plant, or in its treatment of energy conversions (for example from final energy to primary energy). Except for strictly scientific papers, many publications unfortunately make only very superficial statements on these issues. In addition, various assumptions relating to lifetime, loading, technology etc have a significant impact on the outcome. As a result, declarations about the yield factor for selected energy generation technologies will fluctuate a lot. There is however some consistency in the declarations of leading institutes.

As a benchmark for current technology, the values in the following table can be used, which give the net energy yield factor, not including energy required for operating the plant, or demolition. Electricity is converted to its primary energy equivalent. For lifetime, typical values of the respective facilities are used. These are based on the constraints defined in the most solid publications on the subject.

This gives following yield factors:

Nuclear power station: 100 – 200
Coal-fired power station: 100 – 150
Large hydro station: 100 – 200
Small hydro station: 40 – 100
Wind power plant: 10 – 50
Solar photovoltaics: 2 – 8

When including the energy required for operating the plant, these values become:

Nuclear power station: significantly below 100
Coal-fired power station: significantly below 80

Another characteristic is the energy amortisation factor (energy payback), i.e the period needed by the power station to generate the energy consumed for its construction. In case of photovoltaics, this period can be as long as 8 years – depending on cell-material used. For small hydro power stations, it is in the range of 2-3 years. For other power plants, the factor is similar between 1-2 years.


This question has prompted me to revisit the subject 'energy yield for fossil, nuclear and regenerative energy generation systems'. This has highlighted an error in my thinking on this subject. The latest comparative reviews at the University of Stuttgart and the Technical University of Munich include for nuclear and coal-fired power plants already the energy required for fuel (coal) respectively fissile material (Uranium), or more accurately: their extraction, transport and disposal is included. Hence, for these 2 electricity generation technologies, the yield factor is more than 100.

We can easily crosscheck with the energy amortisation time. The yield factor corresponds to the lifetime divided by the energy amortisation time. This is the time period until the corresponding system has generated the energy consumed for all stages of its lifetime from 'craddle' (material extraction for construction) to 'grave' (demolition and disposal). Expressed in months, it is on average:
  • hard coal just below 4 months
  • brown coal just below 3 months
  • natural gas (combined cycle) just below 1 month
  • nuclear energy *) just below 3 months
  • hydro power below 14 months
  • wind power **) 7-16 months
  • solar energy ***) 70 – 100 months
*) Pressurised Water Reactor, 1300 MW, direct disposal used fuelrods
**) 1 MW, average wind velocity 4.5-5.5 m/sec
***) 5 kW (roof-mounted installation), polycrystalline and amorphous silicon
We can then derive following yield factors:
  • coal-fired power stations (lifetime 30 years = 360 months): 90
  • nuclear power station (lifetime for old facilities 40 years = 480 months, for new facilities 60 years = 720 months): 160 – 240
If we normalise the expected useful life (as for photovoltaic and wind power plants) to 20 years (240 months), we obtain a yield factor of 60 for coal and 80 for nuclear power plants.

This contribution was originally published on October 6, 2005 by Energie-Fakten. The addendum was added November 24, 2005

No comments: