Page 57 - Sustainable On-Site CHP Systems Design, Construction, and Operations

36 CHP B a s i c s

steam from the turbine must be piped to a low-pressure steam heat use, which is needed
at the same time as the steam turbine. Therefore, there must be a balance between a
thermal load which can be supplied by the turbine exhaust steam and the power output
of the steam turbine. Supplemental boilers and thermal storage can help balance loads.
The temperature and pressure of the exhaust steam must match the load requirements.
The lower the temperature and pressure needed by the facility, the more energy avail-
able to the steam turbine.
Boilers and steam-driven turbines offer more flexibility in the fuel which drives the
process. Any fuel which can be burned in a boiler or waste heat derived from a process
can produce the steam necessary to drive a steam turbine. A special application is com-
bined cycle where waste heat from a gas-fired combustion turbine produces steam
which drives a steam turbine or is injected back into the combustion turbine for addi-
tional power (Cheng cycle). A common application today is to install a backpressure
steam turbine in an existing steam boiler and steam distribution system. Boilers are
sometimes operated at higher pressure than is required to deliver the steam, and with
a backpressure steam turbine, steam expands through the turbine to the lower pressure
needed to deliver the steam to the loads served producing power as a by-product.
Where the needed components exist, such a system is highly efficient and low-cost.
As discussed later in this chapter, a fuel cell is a special process that does not depend
on mechanical energy to produce electricity. A chemical reaction occurs within the cells
from the union of hydrogen and oxygen. The products of that chemical reaction are
electrical energy, water vapor, and heat. The source of the hydrogen is often natural gas.
In CHP plants, the waste heat is used to meet the thermal energy needs of buildings or
processes. The temperature of the waste heat depends on the specific fuel cell process.
While the temperature in some fuel cell processes is quite low, in other processes, the
waste heat temperature is high enough to produce steam for a combined process.
The prime mover is the heart of a CHP system. The power produced by the prime
mover is typically used to generate electricity but can also be used for mechanical power
to drive pumps, chillers, and compressors, for example. As noted, heat may be pro-
duced directly in the prime mover and/or heat recovery takes place in the prime mover
exhaust stream. Heat recovery for producing additional power, hot water, steam, chilled
water, and/or desiccant humidification is a critical component of CHP systems and is
discussed further in Chap. 4.
A prime mover CHP process develops rotary power to drive an electrical generator
or other rotary equipments such as fans or pumps. CHP prime movers come in two
varieties: fuel-to-power equipments and thermal-to-power equipments. Fuel-to-
power prime mover equipments are fired with gaseous fuels such as natural gas,
methane from wastewater plants or landfills, or liquid fuels such as light oils, biofuels
(a growing important part of enhanced CHP sustainability), alcohol, or other biomass
in a combustion process to mechanically create power for use by the building, industry,
or facility.
Fuel cells are a fuel-to-energy process, but do not produce rotary power, and are
not, therefore, prime movers.
As discussed in Chap. 2, natural gas (NG) is often the preferred fuel as it is readily
available via a nationwide distribution system. Natural gas is cleaner than fuel oil, coal,
wood, or agricultural waste, since more of NG energy content comes from hydrogen so
NG has less carbon footprint than most other fuels. NG, therefore, may not have the
environmental problems associated with other such fuels.