Environmental impact of cogeneration in binary geothermal plants

Cogeneration of power and heat from geothermal resources is considered an environmentally alternative to conventional energy, yet the environmental benefits of combined heat and power (CHP) binary plants have so far not been quantified. Here, we apply life cycle assessment to quantify the environmental impacts for three CHP concepts for the hydrothermal plant of Kirchstockach, Germany. A comprehensive, site-specific life cycle inventory is compiled, which encompasses components and processes needed for the plant-specific refrigerant, as well as for construction and operation of a district heating network (DHN). Results show that the CHP options perform equally well in terms of environmental emissions for heat generation (3.9-4.0 gCO2-eq./kWhth) and vastly outperform conventional, fossil heat sources. Although cogeneration reduces the amount of generated electricity, the corresponding increase in the environmental burden is found to be minimal (4.3-6.6 gCO2-eq./ kWhth). Different schemes to share the environmental burden of auxiliary energy between heat and power output showed no significant difference, as long as the auxiliary energy is supplied by the binary plant itself. As 78% of the non-renewable energy demand of the generated heat in Kirchstoackach are associated with DHN construction, sites with an existing network will particularly benefit from cogeneration of geothermal heat and power.