A small steam expander reciprocating engine is proposed for the conversion and utilization of low-grade heat resources of different nature, such as geothermal, solar, and recovery of waste heat. The engine, still to be developed for production, should be able to work with small flow rates and low upper temperature (100-150 degrees C), rejecting heat at a level still interesting for heating or cooling (with an absorption machine), that is, 50-80 degrees C. The device should be compact, simple and capable of easy control in order to match electric production and loads. The thermodynamic model uses real-fluid and real-cycle assumptions. It includes a heat transfer model for non-adiabatic compression and expansion, losses through admission/discharge valves, and the effects of dead space. Quasi-stationary modeling of the system is applied: the results are also compared with those of a dynamic model, working under perfect-gas assumptions. The model allows to calculate and analyze the performance of the system, including its dependence on the main design parameters; it includes a preliminary design, with special reference to valve sizing. The results indicate that the proposed technical solution can be applied and that the performance of the CHP (combined heat and power) system is competitive with respect to other technologies for renewable energies. (C) 2013 Elsevier Ltd. All rights reserved.