Ground truthing the environmental benefits of a polygeneration system: When to combine heat and power?

Energy systems are major contributors to critical environmental problems including climate change and air pollution. Over half of global energy end use is in the form of heat, three-quarters of which is from fossil fuels. We assess different ways to provide heat, cooling, and electricity to a university campus, comparing lifecycle primary energy requirements, greenhouse gas (GHG) emissions, and particulate matter impacts. For this case study, replacing cogeneration of cooling, heat, and power with grid electricity and building level heating and cooling can reduce GHG emissions by 30%, while cogeneration of heating and cooling in an electrically powered heat recovery chiller system with thermal energy storage can reduce GHG emissions by 45%. GHG reductions from grid electricity remain even if it is assumed to come from more carbon intensive marginal electricity generation. Prominent factors affecting the environmental benefits of polygeneration are identified, most notably the fuel mix and energy efficiency of regional grid electricity, and combined heat and power system efficiency. Thermal energy storage adds resilience to the system while reducing environmental impacts. A heuristic charts the viability of combined versus separate heat and power production for a general case, based on cogeneration efficiency and regional electricity carbon intensity. Low carbon grid electricity strengthens the case for a shift in polygeneration systems from combined heat and power to combined heating and cooling. (C) 2018 Elsevier B.V. All rights reserved.