By Paul Homewood
https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html
There has been some discussion around BP’s switch from Mtoe to Exajoules, as their measurement of energy consumption. I have only done a few quick checks, but the change does not seem to have made any real difference to the trends.
However I want to focus now on how they measure energy from wind, solar and nuclear power, as this can make big differences.
Measuring fossil fuel use is relatively simple – a tonne of oil is a tonne of oil, although assumptions may need to be made about its grade. Similarly, gas and coal can be converted to Mtoe/Exajoules, given assumptions about calorific values. (The BP spreadsheet gives conversion factors for anybody interested.
However, wind, solar, nuclear and hydro power are another kettle of fish. Let’s start with a simple example (ignore the actual numbers please, they are just for show):
Suppose it takes 3 Mtoe of oil to produce 1 TWh of electricity in a oil-fired power station. It could therefore be said that producing 1 TWh from a wind farm is equivalent to 3 Mtoe.
However, the energy content of that 1 TWh clearly is not 3 Mtoe, because power stations do not work at 100% efficiency. Typically, it is more likely to be around 33%, meaning that two thirds of the energy has been lost.
In our example, therefore, the energy content of the electricity is only 1 Mtoe.
BP use the first method, called the “input equivalent basis”, so would value 1 TWh as 3 Mtoe. (Conversion to Exajoules this year does not affect the result in any meaningful way, merely serving to fine tune the calculation).
This is how they describe it:
Traditionally, in BP’s Statistical Review of World Energy, the primary energy of non-fossil based electricity (nuclear, hydro, wind, solar, geothermal, biomass in power and other renewables sources)
has been calculated on an ‘input-equivalent’ basis – i.e. based on the equivalent amount of fossil fuel input required to generate that amount of electricity in a standard thermal power plant.
For example, if nuclear power output for a country was 100 TWh, and the efficiency of a standard thermal power plant was 38%, the input-equivalent primary energy would be 100/0.38 = 263 TWh or about 0.95 EJ.
For many years, the efficiency of this standard power plant has been assumed to be 38%. However, in reality, the world average efficiency of fossil fuel-based power changes over time and has risen from around 36% in 2000 to over 40% today.
Moreover, given the much higher efficiency of the most modern power plant (e.g. the thermal efficiency of a modern gas turbine plant is above 55%), the global average is expected to increase in the future.
However, the UK government uses the latter, output-based methodology. (My understanding is that this follows EU and UN rules):
The energy value for hydro-electricity is taken to be the energy content of the electricity produced from the hydro power plant and not the energy available in the water driving the turbines. A similar approach is adopted for electricity from wind generators where the electricity is regarded as the primary energy form because there are currently no other uses of the energy resource “upstream” of the generation.
https://www.gov.uk/government/publications/energy-balance-methodology-note
I suppose both methods could be said to be right. If wind power has replaced, say, oil-fired power, each TWh has displaced 3 Mtoe.
On the other hand, though, if wind power is replacing the direct use of oil for, say, heating or industrial purposes, then it is the energy content that matters.
Given BP’s assumed 38% efficiency of a thermal power plant, they effectively triple the energy value of wind, solar, nuclear and hydro power.
If we look at the UK data for last year, government figures show that wind/solar/hydro only accounted for 3.8% of primary energy consumption:
https://www.gov.uk/government/statistics/total-energy-section-1-energy-trends
On the other hand, BP say that wind/solar/hydro provided 0.73 EJ, out of a total of 7.84 EJ, ie 9.3%.
You pays your money, and you takes your choice!
