Page 78 - Sustainable On-Site CHP Systems Design, Construction, and Operations

56 CHP B a s i c s

$1500 to $2000 per kilowatt sometime in the future. This pricing is closer to what might
be considered feasible for installation in a cost-effective CHP plant.
Efficiencies and Heat Rate
Fuel cell electric generation efficiencies range from 40 to more than 50 percent with
hydrogen supplied fuel cells (versus hydrocarbon supplied), which corresponds to a
heat rate of less than 7000 to about 8500 Btu/kWh.

Equipment Life, Operation, and Maintenance
Although the fuel cells have existed for over 150 years, fuel cells, as noted, are relatively
expensive compared to other CHP technologies and largely in a state of research to
improve performance and lower costs. As a consequence, there is limited operating
history and fuel cells should be thought of as newer technology. Because there is not a
long operating history to analyze, it is difficult to comment on the life expectancy of fuel
cells. As fuel cells are packaged into self-contained units, maintenance is usually more
limited than with larger custom-built CHP systems.

Thermal-to-Power Equipment
Thermal-to-power generating equipment in CHP systems utilizes heat produced by
some other process to generate electricity or rotary power. The most common thermal-
to-power generating equipment is a steam turbine generator which is driven by either
steam produced in a boiler or steam recovered from the waste heat of the fuel-fired
prime mover(s) discussed above. When waste heat from a prime mover produces steam
for use in a steam turbine, the waste heat/thermal energy produces additional power.
Thermal energy can also be used to generate hot water, steam, or chilled water that
would have otherwise (in non-CHP systems) required fuel or power. Those systems are
discussed in greater detail in Chap. 4.

Steam Turbines
A steam turbine is a mechanical device that converts steam energy (enthalpy) into rota-
tional mechanical power. The rotational power can drive pumps, centrifugal chiller
compressors, and other mechanical devices. Steam turbines are often used to drive an
electrical power generator.
A steam turbine generator (STG) can make use of the thermal energy produced in a heat
recovery steam generator (HRSG) to generate additional power. With a conventional boiler
system, to qualify as combined heat and power, boiler produced steam must be used for
both heating (and/or thermally driven cooling) and power. Sometimes, this means steam is
produced at temperatures and pressures greater than needed for the facility’s heating or
cooling needs/applications, and the steam is expanded through a STG to the pressure
needed for use by the facility. This type of CHP system produces power in direct relation to
the thermal load. As a retrofit project, where there is an existing steam boiler and steam
distribution system such a system is often very cost-effective to install. The backpressure
turbine used is very efficient because all the steam exiting the turbine is beneficially used.
In fact, most of the power produced in the United States is generated by conven-
tional steam turbine generator power plants (fuel is burned in a boiler to produce steam
to drive a STG). As noted, when HRSG produced steam is used to produce additional
power in a STG, the CHP thermodynamic cycle is known as combined cycle.