Behind a chain-link fence at the edge of a substation in the English Midlands, 96 white containers hum at a pitch you feel in your teeth before you hear it. Together they can push 100 megawatts into the grid for two hours. In this episode Priya Venkataraman spends a day with the people who decide, minute by minute, whether the site should be buying power or selling it. Listen in the player below; the chapters and notes follow.
In this episode
- 00:00 The fence line: arriving at the site before the morning peak
- 05:40 What is inside a container, and why it needs air conditioning
- 13:15 The trading desk: selling in five-minute slices
- 22:30 Charging at midday, discharging at teatime
- 30:05 How a battery wears out, and who pays when it does
A fence, a hum and a substation
The site belongs to Greyfield Storage, which runs six battery parks in England and is building a seventh. It was placed here for one reason: the substation next door has spare connection capacity, the scarcest resource in the business. "People think the batteries are the expensive part," said Tom Achterberg, Greyfield's site engineer. "The expensive part is the right to plug them in."
Each container holds lithium iron phosphate cells, racks of inverters and a cooling system that runs harder in August than in January. The hum comes from the transformers and fans, not the cells.
Money by the minute
Nadia Oyelaran, Greyfield's head of trading, works from an office in Birmingham with four screens and a kettle. Her battery earns in three ways: being paid to stand ready to steady the grid's frequency, answering the system operator's calls to add or remove power at short notice, and buying cheap electricity to sell later at a higher price.
A power station sells energy. We mostly sell being quick.
Ms Oyelaran said the mix changes by the season and sometimes by the hour. According to the revenue tracker Break keeps for twelve comparable sites, a two-hour battery earned the equivalent of about £68,000 per megawatt over the past twelve months, with a third of it from frequency services and most of the rest from trading.
Charging at midday
On a sunny day the pattern is plain. Solar output pushes midday prices down, sometimes below zero, and the battery charges. At around five in the afternoon, as the sun drops and kettles go on, prices climb and it discharges. On the day of our visit the spread between the cheapest and dearest half-hour was £112 per megawatt-hour.
Cloudy weeks are harder. When the midday dip never comes, the spread narrows and the software holds back, keeping charge in reserve for the frequency contracts. "Some days the best trade is no trade," Ms Oyelaran said. "The battery sits at half full and gets paid for being available. It looks idle on the screen, and it is one of our better days." In the episode she walks through one of those afternoons, trade by trade.
Cells that wear out
Every cycle costs a little life. Greyfield's cells are warranted for 7,000 full cycles, and the trading software treats wear as a cost on every trade, refusing ones that would not pay for it. The site averages 1.4 cycles a day. At that rate it will need new cells, or a smaller rating, in about thirteen years.
Credits and further listening
Presented by Priya Venkataraman. Produced by Sam Idowu. Sound recorded on site with Greyfield Storage's permission. For more from the energy desk, read about the solar modules stacked in bond and the town that makes heat pumps.
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