Page 82 - Sustainable On-Site CHP Systems Design, Construction, and Operations

60 CHP B a s i c s

The electric efficiency (total electrical output divided by the total energy input
energy) of the different technologies ranges from as low as 20 percent with the micro-
turbine to over 50 percent with the best fuel cells. The natural gas reciprocating engine
has electrical efficiencies ranging from 25 to 45 percent and power output ranging from
50 kW to 5 MW. The CTG has efficiencies ranging from 25 to 40 percent with a simple
cycle and 40 to 60 percent with a combined cycle based on the HHV of the fuel. CTG
power output typically ranges from 3 to 200 MW, though some manufacturers are pro-
ducing units with power output as low as 1 MW or as high as 1000 MW.

Heat Recovery Potentials
Different CHP technologies have different heat recovery potentials. Some CHP tech-
nologies may produce low temperature hot water (LTHW) generation (less than 250°F),
low-pressure steam production (15 psig or less), or medium-pressure steam. Some heat
recovery systems are a part of the equipment served. For example, an exhaust gas-fired
absorption chiller-boiler, which directly intakes the exhaust from the prime mover and
uses the hot exhaust gases directly to drive the absorption process and also to produce
hot water for other uses. Another occasional thermal use is direct heating or drying,
which can be highly efficient as exhaust gas transfers its energy as it cools to ambient
temperature. In some industrial applications, the exhaust from a gas turbine is directed
to a process such as drying agricultural products or wood. This application beneficially
uses the waste heat without an intermediate recovery process. Such applications are
also extremely cost-effective.
LTHW is typically recovered from IC reciprocating engines, although low-pressure
steam (less than 30 psig) can be obtained from high temperature engine exhaust.
Medium-pressure steam (up to about 250 psig) is typically recovered from a HRSG that
uses the CTG exhaust as a heat source. Fuel cells generate LTHW (about 180°F depending
upon the fuel cell technology), which can be used for hydronic heating or domestic hot
water production.
CTG generally have higher thermal-electric ratios and generate substantially more
heat than do IC engines.
The useable temperature of the recovered heat varies. Some applications can use
cooling tower water from a steam turbine plant to heat agricultural processes, fish
farms, or air drying using heat at 80 to 90°F. On the high end, a duct-fired combustion
turbine may recover heat from 1100°F exhaust.
Fuels and Fuel Pressures
As discussed, CHP systems can be designed to operate on myriad fuels including, but
not limited to, natural gas, diesel fuel, landfill gas, digester gas, propane, wood or agri-
cultural waste, and so on. As an example, a large power generation plant in California
burns walnut husks. In Oregon, CHP plants burn wood waste in sawmills to drive
steam turbines for power and use the waste heat to dry lumber. However, the most
commonly utilized fuel in CHP systems is natural gas. Natural gas is widely available
through local utilities and is rarely subject to interruption in service. When considering
the use of natural gas, or any other fuel, the engineer must ensure that the selected
prime mover equipment is able to operate with that fuel source. CTGs and microtubines
can be designed to operate on fuels including natural gas, biogas, propane, and distil-
late oil. Natural gas and diesel IC engines, although similar in mechanical function,
are designed to operate on different fuels. Natural gas IC engines can be designed to