Combined Heat and Power (CHP) - Beyond Cogeneration

by Neal Elliott

Conventional generation is inherently inefficient, only converting on average about a third of fuel's potential energy into usable energy. Engineers have long appreciated the tremendous efficiency opportunity of combining electricity generation with thermal loads in building and factories, capturing much of the energy that would be otherwise wasted. President Carter, in the 1970s coined the word "cogeneration" to describe this practice. At that time, the dominant configuration of systems was a boiler that generated steam, some of which was used to turn a steam turbine that generated steam. Due to cost and complexity of these systems they were largely confined to large systems of over 50 MW, thus precluding their installation at most manufacturing facilities.

Recent advances in electricity-efficient, cost-effective generation technologies, in particular advanced combustion turbines and reciprocating engines, have allowed for new configurations of systems that combine heat and power production, expanding opportunities for these systems and increasing the amount of electricity they can produce. These combined heat and power (CHP) systems now come in many more configurations than co-generation systems of the 1970s. Many of these CHP systems place the electricity generation equipment, the turbine or engine, first in the system, using a waste heat recovery boiler to capture the heat. The captured heat can then be used to satisfy heating requirements, provide cooling using advanced absorption cooling technology, and even generate more electricity with a steam turbine. These systems can even satisfy compressed air requirements by bleeding high pressure air off the compressor stage of a combustion turbine.

New turbines are now cost effective for systems down to 500 kW and reciprocating engines for systems down to 50 kW. These smaller sized equipment dramatically expand the number of sites where CHP can be installed. In fact, many existing industrial boilers can be repowered with advanced generation equipment, replacing existing fuel burners, adding electricity generation capability while reducing emissions of pollutants.