Firm pushes for diesel as back-up power

Hundreds of diesel generators could provide a large slice of the electricity grid’s back-up power needs in spite of their high carbon emissions.

Edinburgh-based Flexitricity is using businesses’ standby diesel generators – among the most carbon-intensive, air-polluting forms of electricity generation – to provide back-up power to National Grid. It claims this will bring significant greenhouse gas benefits, although this is difficult to substantiate.

Flexitricity has been providing back-up services to National Grid since 2008. It sets up ‘smart’ connections with its clients’ sites to control them in response to the grid’s needs by temporarily lowering power use or turning on generation.

The company “flexes” a range of electricity-generating equipment in this way, including combined heat and power and hydro plants. But around half its response is provided by standby diesel generators, according to Alistair Martin, the company’s chief strategy officer.

Flexitricity is paid by National Grid for providing back up power. Its clients get a share of this revenue, which can be over £100,000 per site per year.

The firm has 17 clients so far. This includes the London exhibition centre ExCeL, which has two standby diesel generators that can provide a combined six megawatts (MW) of peak capacity reserve.

The list of clients is expected to grow rapidly. RWE Npower has started promoting the company’s services to its 17,000 industrial and commercial customers under the title SmartSTOR. And Flexitricity has started targeting the cash-strapped public sector, particularly hospitals.

The company estimates there is 14 gigawatts of standby diesel generation in the UK, but it sees only 1GW of this playing a role in providing back-up power in future – equivalent to 330 units.

That would be significant; National Grid’s current back-up needs are 3.5GW, and it says that will rise to 8GW in 2020.

The company’s marketing focuses heavily on the potential environmental benefits of using existing generators for reserve. Each MW saves 300-750 tonnes of carbon dioxide a year, it says, because it prevents the need for existing oil or coal generation, and for such plants to run on “hot standby” just in case they are needed.

Flexitricity does not run diesel generators more than a normal testing regime, Dr Martin adds, so its service is simply making use of generation that should occur anyway. Manufacturers recommended 50 hours testing a year, he says.

Although these arguments can appear compelling, the firm cannot prove that using diesel would save carbon in all occasions. A diesel generator has emissions of around 750 grams of CO2 per kilowatt hour of electricity produced, Flexitricity says. This is lower than the average emissions of UK coal plant of 915gCO2/kWh. But it is almost double gas’ 405gCO2/kWh.

Gas supplies about a half of National Grid’s reserve needs.

Flexitricity says emissions associated with running gas on part load, and other factors, must also be taken into account. But the figures it gave ENDS only increased gas’ emissions to 485gCO2/kWh.

Dr Martin recognises this problem. “The carbon cost of reserve vary continually,” he said and gas is regularly the marginal back-up plant. But he claims that for the majority of the time, diesel would be replacing higher-carbon plant.

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