A Simple Look at the Efficiency Penalty Cost of Renewables

This post is a response to Wind Integration vs. Air Emission Reductions: A Primer for Policymakers, which claims the efficiency penalty of turning down fossil fuel power stations offsets the benefit of renewable power generation. I did some simple modelling to see if this is true.

The carbon benefit of renewables arises from avoiding burning fossil fuels. Most generation (machines in general) is designed for optimum efficiency at maximum (or near maximum) load. Operation of the machine above or below the optimum load will incurr an efficiency penatly. This applies to fossil fuel generation such as coal boilers and gas turbines.

Below I look at what the breakeven efficiency penalty would be to offset the benefit of renewable generation

• total electricity demand of 1 GW
• renewable output is varied from 0 GW to 0.5 GW
• coal supplies the balance
• coal operates at a 50 % HHV efficiency at full load

I then looked at various efficiency penalty factors in the form of reduced efficiency per reduction in load (% HHV / % load). The efficiency penalty was modeled as linear. You can download a copy of the model here.

Figure 1 – The effect of various assumed efficiency penalties on fossil fuel consumption

For this simple model 5 % HHV / % load is the break even. If the efficiency really reduces at this rate then generating electricity from renewables is giving us no carbon benefit.

The real question is what is the actual relationship between fossil fuel power station output and efficiency. It’s likely to be non-linear. I also expect it would not be as harsh as 5 % HHV/% load – so likely renewables are providing a carbon benefit.

Is it is useful to know that this is a carbon penalty we could be paying somewhere on the system as renewables penetration increases. This penalty will net off some of the maximum benefit that renewable generation could supply.

This comparision is between renewables and coal. Because natural gas is cleaner than coal, a comparison with gas would be more in favour of renewables - i.e. the negative effects of any efficiency penalty will be less. Permanently shutting down fossil fuel power stations means this effect doesn’t occur - so lets turn them off completely.

Thanks for reading!