Costing methods for combined heat-and-power plants fueled by zero-marginal cost energy sources

•Levelized cost metrics and costing methods for combined heat-and-power plants.•(Near) zero-marginal cost renewable energy sources are considered.•Concepts levelized cost of exergy & net present value based on exergy are proposed.•Novel concepts account for two products and temperature levels (usefulness of heat).•Application to a low-temperature geothermal combined heat-and-power plant.