Approaching Free Electricity: How the Real World Differs from Thermodynamic Models

ABSTRACT

As distributed generation moves from concept to reality, industrial and institutional steam customers are realizing that they can often use steam turbine/generator technology in their existing steam networks to make “opportunistic” electric power and significantly reduce their annual energy costs. Historically, turbines operating in this “backpressure” mode have been assumed to have a net fuel-to-electric efficiency that is approximately equivalent to the on-site steam boiler, because every unit of energy removed as electricity must be replaced with a unit of heat. Under this assumption, backpressure turbine/generators easily become the most efficient power generation technology ever invented. While this logic is thermodynamically true, it often breaks down in practice, where the nuances of steam plant operation make it possible to generate electricity at an even higher efficiency with backpressure turbine/generators. In some cases, the power produced by backpressure turbine/generators is actually free.