Page 74 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 53

In CHP applications, the waste heat from the microturbine is used to produce hot
water to heat building(s), to drive absorption cooling, desiccant dehumidification equip-
ment, and to supply other thermal energy needs in a building or industrial process.

Sizes
Microturbines are presently available with electrical outputs varying from about 25 to
250 kW. While this range of electrical outputs is relatively low compared to other prime
mover technologies, the smaller footprint of the microturbine makes it ideal for installing
them in parallel, creating large banks of microturbine generator sets to create larger
power production arrays. This concept offers some benefits over a single, larger com-
bustion turbine generator. One benefit to a microturbine array is that if one machine is
out of operation, the entire electrical generation capacity is not lost. A microturbine
array also can maintain good efficiency throughout a variable electrical demand as sin-
gle machines can be shutdown during periods of decreased load, leaving the remaining
machines in the array to operate at full load efficiency. One negative aspect with the
microturbine array is that a microturbine array will likely be more expensive to con-
struct than a single large prime mover of the same total capacity.
Efficiencies and Heat Rate
Microturbines exhibit shaft efficiencies between 20 and 30 percent, based upon the HHV
of fuel burned, which corresponds to a heat rate between 11,300 and 17,000 Btu/kWh.
Because microturbines reduce power output by reducing mass flow and combus-
tion temperature, efficiency at part load can be below that of full-power efficiency. Thermal
output ranges from 400 to 650°F which is suitable for supplying heat for a variety of
building thermal needs.
Emissions
Low inlet temperatures to microturbines and high fuel-air ratios result in NO emissions
x
of less than 10 parts per million (ppm) when operating with natural gas.
Equipment Life, Operation, and Maintenance
Microturbines are relatively new, and, therefore, do not have a long operating history to
analyze; however, microturbines can be expected to operate for approximately 10 years
when well maintained. As microturbines are packaged into self-contained units, main-
tenance is usually more limited than with larger custom built CHP systems.
Fuel Cells
Engine or gas turbine–based CHP systems rely on the combustion of fuel to produce
high-pressure, high temperature gas that can expand to provide useful work as previ-
ously described. The expanded gases are harnessed by the specific equipment, and pro-
vide mechanical and thermal energy. In fuel cells, the oxidation process occurs across
membranes which cause electron transfer. Fuel cells directly create electric power with-
out a prime mover or generator. The process is usually thought of as a chemical reaction
rather than as a combustion process although most typical systems have a fuel and an
oxidizer and so the process is technically a combustion process. Combustion is of course
a chemical reaction.
Fuel cells are similar in some ways to batteries. With batteries, the chemical reaction
that produces the electric power consumes the materials out of which the battery is con-
structed. As a result, modern batteries, even the rechargeable type, eventually wear out.