Arup Thoughts: Achieving resilient power and clean air

The increased use of standby diesel generators to plug gaps in the national electricity supply poses a threat to local air quality.

By Christine McHugh, Arup, London January 13, 2017

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

During a recent project visit, a reminder was received that a nearby building would be conducting tests of their standby generator. It was advised to avoid any fumes or smells entering the office and 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 to prevent the polluted air from entering the building.

Previous tests during office hours had already resulted in a visible plume of exhaust from the generators. While occasional nuisances 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 U.K., 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 U.K. governments have not succeeded in doing that.

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

As it stands, standby generator installations are essentially unregulated. Conversely, emissions from nonstandby plants with more than 50-kW thermal input must either comply with limit values from the European Union’s Directives for large- and medium-combustion plants or the Greater London Authority’s Sustainability Design and Construction Supplementary Planning Guidance. Other chimneys serving smaller boilers and furnaces may require stack-height approval from the local authority under the Clean Air Act of 1993.

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

The City of London Corp. 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 and fine particulate matter. Their current (and clear) advice is summarized in a short guidance note aimed at minimizing 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."

The 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, innovative solutions to standby power provision that doesn’t threaten air quality in cities are needed. 

Christine McHugh leads Arup’s London-based air quality team. This article originally appeared on Arup Thoughts Blog. Arup is a CFE Media content partner.