Achieving resilient power and clean air

+ Power suppliers are increasingly relying on businesses to run their standby diesel generators but the government also faces a challenge to meet air quality limits. Is it possible to have both?

In designing a building’s energy system, trade-offs must often be made. Business needs resilient energy, so backup generators have become a fact of life. But the increased use of standby diesel generators to plug gaps in national electricity supply poses a threat to local air quality. The challenge is for designers to find an innovative solution that means we don’t have to choose between resilient power supply and air quality. 

During a recent project visit, I and other colleagues received a reminder that a nearby building would be conducting tests of their standby generator. We were warned that to avoid any fumes or smells entering the office, we should ensure that all windows were closed each evening before the tests were carried out. The office’s air handling units were also to be shut off during this time so the polluted air would not enter our building.

Previous tests during office hours had already resulted in a visible plume of exhaust from the generators. While occasional nuisance may be a price worth paying for continuous operation in an emergency of critical facilities, it points to a new threat to local air quality from increased use of standby generators. 

Standby generator use is becoming a regular occurrence. Across the UK, backup generators are being called upon to supply electricity to the grid at times of peak demand, such as in the early evening. This could be avoided if the grid’s capacity was better managed, but successive UK governments have not succeeded in doing that.

The use of diesel in emergency generators is understandable: it’s tried and tested technology, reliable and allows almost instantaneous start-up. But diesel generators are more polluting than gas-fired engines, and if operated at a time that coincides with considerable road congestion or during unfavourable winter weather, they can substantially worsen local air quality. 

As it stands, standby generator installations are essentially unregulated. Conversely, emissions from non-standby plant over 50kW thermal input must comply with limit values from the EU’s Directives for large combustion plant and medium combustion plant, or, in London, the Greater London Authority’s Sustainability Design and Construction Supplementary Planning Guidance (SPG). Other chimneys serving smaller boilers and furnaces may require stack height approval from the local authority under the Clean Air Act 1993. 

Standby plant are excluded from all of these regulations and the prospect of such plant operating for an increased number of hours, during times of year unfavourable for air quality, is a loophole that should be closed.

The City of London Corporation was one of the first local authorities to identify use of standby generators to feed electricity into the national grid as a growing concern in terms of the adverse impact on air quality, for both nitrogen dioxide (NO2) and fine particulate matter (PM10). Their current (and clear) advice is summarised in a short guidance note aimed at minimising emissions from standby generators and states: 

‘Standby generators in the City should not be used to feed electricity into the utility grid. They should be used in emergencies only.’ 

Its Air Quality Strategy also recommends working with major businesses to help phase out standby generators that run solely on diesel.

In an era when electricity generation is being continually tested for its green credentials, we need innovative solutions to standby power provision that doesn’t threaten air quality in cities. 

If you’ve encountered any solutions that overcome this problem, please share them in the comments below.