When Origin Energy announced recently that it planned to close its 2922-megawatt (MW) Eraring Power Station, the largest coal-fired power plant in the country accounting for 20% of NSW's generation capacity, the NSW Government said it would underwrite the construction of a 700 MW battery by the time Eraring ceases to operate in August 2025.
However, in the words of Chief Brody of Jaws fame who said, "You’re gonna need a bigger boat", surely NSW is "Gonna need a bigger battery."
Measuring energy and capacity
To begin, we need to clarify two important power industry measures. First, there is 'capacity'. The typical rating measure for power stations, it is the maximum output of electric power able to be produced at a point in time. For example, Eraring has the capacity to generate a maximum 2922 MW of power at a given instant.
Second, there is the measure of 'energy', which is the amount of power output over a period of time, measured in watt hours (Wh). The annual output rating of Eraring therefore being 25,596 gigawatt hours (GWh), (2922 x 8760 hours in a year/1000), 1 GW = 1000 MW. Energy is the usual power rating for batteries (or energy that can be stored), therefore talk of "construction of a 700 MW battery" is obscure, as it does not indicate the period of time the battery can continuously supply 700MW of power.
In reality, power plants will never deliver full capacity over an entire year, for reasons such as energy demand, and planned outages for maintenance. This is where 'capacity factors' come in. An annual 'capacity factor' is the actual output measured over a year, as a percentage of the theoretical maximum output.
Required battery capacity
In the five years to the end of 2021, the average annual output out of the Eraring plant was 16,012 GWh. That is, an average capacity factor of 62.5% (= 16,012 / 25,596), which is the equivalent of 1828 MW continuous power generated (1828 = 0.625 x 2922).
Suppose then that to replace Eraring, continuous power rounded up to 2000MW will be needed in future. And assume that Eraring will be replaced with solar power plus battery storage, so that when the sun is shining, the solar plant produces 2000MW continuously at full capacity. What size battery will be required?
Assume also that on average, there are 10 hours of full sun per day, therefore battery storage will be needed for the remaining 14 hours. Meaning 14h x 2000 MW = 28,000 MWh of output storage required. The battery will also need to be charged daily when the sun is shining, so an additional 28,000 / 10 = 2800 MW of continuous power generation will be necessary. To deliver 2000MW all day every day will therefore require a power plant capacity of 4800 MW, and 28,000 MWh of battery storage.
Making up for cloudy days
But of course, these numbers assume continuous sunlight in the daytime hours, day in day out. The power plant’s generating capacity and necessary battery storage blows out considerably in the event that the sun doesn’t shine.
To demonstrate, suppose we have one full cloudy day every three days. One day without the sun shining requires additional storage of 24 hours times 2000 MW, which equals 48,000 MWh. With two full days of sunshine before the cloudy day, that means 20 hours are available to generate the 48,000 MWh needed for one full cloudy day. That is, an additional 2400 MW of generating capacity required. So we are now up to a generating capacity need of 7200 MW, plus storage of 76,000 MWh across probably multiple batteries, to ensure continuous supply of 2000 MW of power.
Now sometimes there will be more than one cloudy day in a row, including partial cloudy days, and at other times, more than two days of sunshine between cloudy periods. So there will be occasions when the additional generating capacity will not be enough, and times when it will be too much. Such is the unpredictability of sunshine. Add in the fact that batteries don’t actually fully charge or discharge, and the numbers here are understated.
Similar calculations could be undertaken for wind generated energy, but being even more volatile than solar, would imply an even greater amount of generating capacity and storage required.
Clearly these calculations are simplistic, but they serve to give an order of magnitude, and highlight the inadequacies of the battery proposal, and the uncertainty that surrounds the intermittent nature of weather-dependent energy sources.
Tony Dillon is a freelance writer and former actuary. This article is general information and does not consider the circumstances of any investor.