CHP systems help to squeeze out the last few joules of energy from their fuel. If all of the heat and energy produced by a CHP installation is used they can have efficiencies of up to 80%, compared to the 35% of an average fossil-fuel plant.

Back in 2012 President Obama signed an executive order to encourage more industrial CHP. These days CHP is popular in the US, accounting for around 7% of generation capacity. Forecasts indicate that CHP is likely to become more prevalent in commercial and institutional settings in the US, with buildings with high hot water needs such as schools, hospitals, prisons, swimming pools, nursing homes and laundries, likely to benefit most. So what impact might this trend have on energy costs, energy transmission and greenhouse-gas emissions?

To investigate this question, Jeremy Keen and Jay Apt from Carnegie Mellon University, US, modelled increasing CHP penetration in New York, based on the assumption that commercial CHP will increase by 2 to 9% annually over the next decade.

According to the model, as the number of CHP systems increases, energy system costs would decrease by around 7% compared to a scenario with no CHP, and network congestion would also decrease. But at the same time, the researchers found that greater penetration of CHP was also likely to lead to an increase in carbon dioxide emissions, with a 5% penetration of small commercial CHP producing a 2% increase in carbon dioxide emissions.

The increase in emissions occurred because of a tendency to run CHP continuously, even when the extra heat wasn't needed, leading to production of excess energy. It was the buildings that had the greatest variation in heat demand, such as schools, offices and retail outlets, which produced the greatest increase in carbon dioxide emissions. Meanwhile, buildings with a more constant heat demand, such as hospitals, large hotels and full service restaurants, were less likely to let their CHP energy go to waste. The findings are published in Environmental Research Letters (ERL).

In general, CHP is not designed to be turned off more than once a day, but carefully timed use could help to minimize the extra carbon dioxide emissions. "Any policy that encourages users to turn off their CHP when heat demand is low, typically during the summer and night-times, will reduce inefficiencies," said Keen. In their model the researchers show that seasonal rates like this could reduce emissions at a typical warehouse from 3% more than the bulk power grid to 1% less.

New York gets much of its power from hydropower and natural gas, but in regions like the Midwestern US, where emissions-heavy power stations are more dominant, any CHP installations are likely to reduce emissions, regardless of how they are run.

Nonetheless, the results show that CHP systems vary in their ability to tackle emissions, depending on the type of building they are situated in. "If policymakers chose to incentivize commercial CHP for the purpose of reducing emissions, it would be a mistake to incentivize them equally because different buildings in all regions of the world will have different abilities to reduce emissions," concluded Keen and Apt.

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