Waste heat recovery from spark ignition generators is a means of increasing its overall efficiency and viability for distributed generation. The Rankine Cycle is a heat engine capable of being used as a bottoming cycle to convert low grade heat into useful work and ultimately electricity. In this, way the greenhouse gas emissions per unit of electricity produced can be reduced.

Although there is considerable interest in the Rankine Cycle for waste heat, solar thermal and geothermal applications, the heat considered is normally from a single source in the system. For a small generator, both the coolant and exhaust are possible heat sources and the Rankine Cycle can be configured in a number of ways, although the challenge is to maximize the total heat utilization.

The working fluid selection normally matches the boiling point of the fluid with the proposed Rankine Cycle system pressure and temperature. In this work, the overall objective was to maximize the shaft power output from the Rankine Cycle. A spreadsheet model was developed for each of the four different cycle configurations proposed. Each spreadsheet model was linked to a database, allowing calculations based on real thermodynamic fluid properties and an ability to easily select fluids of interest.

Thirteen fluids were considered for each cycle configurations, including seven alkanes (C3 to C7), methanol, ethanol, acetone, R245FA, ammonia and water. The results were analysed in terms of system shaft power output, pressure and temperature, and are discussed with an emphasis on constructing a system from off-the-shelf components.