Ranking the Devices

To establish which energy source is best, we use six key metrics. Specifically, these are energy price, life span, energy generated per square kilometre, payback period, carbon offset period and global potential. In this article, we explain what these metrics are and how we use them.

Energy Price

This is the total cost of producing energy using a renewable device and is measured in cents per kilowatt-hour. It includes the build cost, installation cost, the running costs and any other costs related to the device. The price also considers performance fluctuations due to bad weather and downtime due to maintenance. An example of an energy price is ¢8 per kWh, the rough cost of producing energy using photovoltaics.1

Life Span

This is the length of time a renewable device can be used before it needs to be replaced. For example, a typical offshore wind turbine will last around 25 years.2 In contrast, a hydroelectric dam can last for 200 years or more.3

Energy per Square Kilometre

This is the amount of energy a renewable device can generate each year per square kilometre of land or water. For example, a typical photovoltaic array can generate some 300 GWh per square kilometre of land.4 That’s enough energy to power over 20,000 homes.5 In contrast, offshore wind turbines can generate just 26 GWh per square kilometre of water.6 That’s less than 10% of the amount that photovoltaics can produce within the same area.

Payback Period

This is the time period required to offset the total cost of a device using the revenue generated from energy sales. As an example, it would take around 13 years to offset the cost of a typical onshore turbine and around 21 years to offset the cost of a tidal stream.7 The economic offset period is based on a standard electricity price of ¢10.5 per kWh and an energy inflation rate of 5.4%. The electricity price reflects the current average electricity price in the USA and the inflation rate reflects the average energy inflation rate over the last ten years within the USA.8

Carbon Offest Period

This is the time required for a device to generate enough clean energy to offset the carbon dioxide equivalent emitted from its manufacture and use. For example, a tidal lagoon would need to operate for around four years to offset the carbon emissions that occur due to its manufacture and use.9 In contrast, a hydroelectric dam would most likely never offset the carbon emissions that result due to its manufacture and use.10

Global Potential

This is the amount of energy the renewable device could produce in a single year through the utilisation of all prime locations. For example, if photovoltaics were installed in all viable prime locations on the planet, an estimated 470 million GWh of energy would be generated.11 That’s more than four times the world’s current energy demand. In contrast, if offshore wind turbines were installed in all viable locations, just 6 million GWh would be generated.12 That’s less than one-tenth of the world’s energy demand. Because of the huge numbers involved, the global potential will be measured using petawatt-hours (PWh), with each petawatt-hour equal to one million gigawatt-hours.

The Renewable Solution

Now we know the key metrics used to assess which renewable device is best, let’s take a look at our first device – onshore turbines.

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General Notes

All figures presented in this section are estimates based on best available data, assume optimum locations, and, wherever possible, are based on comparable studies. That said, many of the studies assume different economic conditions, climatic conditions, time frames and locations. Furthermore, the technologies discussed in this section are in a constant state of development. As a result, the figures presented within this section provide a rough guide only and should not be viewed as a definitive performance level.

For the total world energy demand, a figure of 104.4 million GWh has been used. The figure is based on 2012 data and sourced from International Energy Agency – 'World Balances for 2012' – www.iea.org.

All UK to USA currency conversions have been set at $1.656 USD for each £1 GBP. The figure is based on the conversion rate as of the 1st January 2014 and sourced from XE – 'XECurrency Table: USD - U.S. Dollar' – www.xe.com.

Article Endnotes

  1. Lazard – ‘Levelized Cost of Energy Analysis – Version 8.0’ – Page 16.
  2. European Wind Energy Association – ‘The Economics of Wind Energy’ – Page 35.
  3. Figure based on a standard hydroelectric dam having a typical lifespan of between 50 to 100 years and a large hydroelectric dam having a lifespan of between 200 to 300 years. Sourced from Chiras, Daniel D. – ‘Environmental Science’ – Page 333.
  4. Based on photovoltaic cells having an efficiency of 20%, 2,000 kWh of sunlight striking each square metre per year and the photovoltaic cells covering 75% of each square metre exposed.
  5. Data based on typical UK home demanding 18,740 kWh of energy per year. Sourced from UK Department of Energy and Climate Change – ‘Energy Consumption in the UK’ – Page 7.
  6. Data based on three watts of energy being generated per square metre. Data sourced from MacKay, David J.C. – ‘Sustainable Energy – Without the Hot Air’ – Page 60.
  7. Offset period for onshore turbine calculated within the ‘Renewable Solution’ section of the ‘ZERO EMISSION WORLD Energy Database’ and based on a lifespan of 25 years. Lifespan sourced from International Energy Agency – ‘Projected Costs of Generating Electricity’ (2010) – Page 43. Offset period for tidal stream calculated within the ‘Renewable Solution’ section of the ‘ZERO EMISSION WORLD Energy Database’ and based on a lifespan of 20 years. Lifespan sourced from The Carbon Trust – ‘Accelerating Marine Energy’ – Page 14.
  8. Electricity rate based on 2014 data and sourced from United States Energy Information Administration – ‘Electric Power Monthly with Data for October 2016’ – Table 5.3. Inflation rate based on 2005 to 2015 data and sourced from OECD – ‘Consumer Prices: Consumer prices – Annual inflation, energy’ – stats.oecd.org.
  9. Tidal Lagoon Swansea Bay PLC – ‘Environmental Statement’ – Page 16.
  10. Based on detailed studies undertaken by Ivan Lima and his colleagues from Brazil’s National Institute for Space Research (INPE) that demonstrates registered dams greater than 15 metres tall emit a combined 104 million tonnes of methane annually via reservoir surfaces, turbines, spillways and rivers downstream. Data sourced from International Rivers – ‘Greenhouse Gas Emissions from Dams FAQ’ – www.internationalrivers.org.
  11. Hoogwijk, Monique and Graus, Wina – ‘Global Potential of Renewable Energy Sources: A Literature Assessment’ – Page 39.
  12. Hoogwijk, Monique and Graus, Wina – ‘Global Potential of Renewable Energy Sources: A Literature Assessment’ – Page 39.

For further information about any of the above sources, please visit the ZERO EMISSION WORLD Works Cited page.

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