Page 33 - Sustainable On-Site CHP Systems Design, Construction, and Operations

12 CHP B a s i c s

Microturbines
Microturbines are essentially miniaturized combustion turbine generators and are pres-
ently available in sizes up to approximately 250 kW. Microturbines can be ganged
together to provide greater capacity and some systems have been designed with more
than 1 MW of capacity.
Fuel Cells
Fuel cells are becoming more popular due to their high efficiency and low emissions;
however, price hurdles, as compared to other CHP technologies, remain. Engine or com-
bustion turbine–based CHP systems rely on the combustion of fuel to provide the
mechanical and thermal energy. In fuel cells, the process takes place as a chemical reaction
rather than as combustion. A fuel cell is an electrochemical device that converts hydrogen
to DC electricity, with heat and water as by-products. There are different types of fuel cells
such as phosphoric acid (PA), proton-exchange membrane (PEM), and molten carbonate
(MC). The type of fuel cell determines the electrolyte used to separate the hydrogen ions.
Fuel cells are similar to batteries, except that in batteries the chemical reaction that pro-
duces the electric power consumes the battery internals. As a result, batteries, even the
rechargeable type, eventually wear out. Fuel cells on the other hand use a continuous
supply of fuel for the chemical reaction, and provided the fuel supply continues, can
operate for extended periods. Although many variations exist, the most common type of
fuel cell uses hydrogen as the fuel source and the oxygen in air to complete the chemical
reaction. The source of the hydrogen is typically natural gas (which is cracked to release
the hydrogen) and the by-product of the chemical reaction is hot water.
The advantages of fuel cells are that they are practically emission free, they operate
at very low noise levels, and they are able to respond rapidly to changes in electrical
loads. Heat recovery allows the fuel cells to reach an energy conversion efficiency of
80 percent or more. Fuel cells are potential candidates for CHP because the water by-
product is produced at temperatures in the 160 to 180°F range (PEM), which is suitable
for space heating and other low-temperature uses (e.g., domestic hot water generation
and swimming pool heating).

Heat Rate
The heat rate is the ratio of fuel input in British thermal units (Btu) to electric power
output in kilowatts (kW), and is a measure of the CTG’s (or engine’s) fuel-to-electric-
power conversion efficiency. The lower the heat rate, the more efficient the CTG or
engine. That is, prime movers with lower heat rates deliver the same amount of power
than those with higher heat rates with less fuel combustion.
Published heat rates and power outputs are nominal values only. For example, the
entering air temperature dramatically affects both the heat rate, and the power output
of a given CTG. Output power decreases and the heat rate increases (i.e., efficiency
decreases) with increasing combustion inlet air temperature. The CTG nominal values
are typically based on an inlet air temperature of 59°F. The inlet air can be cooled on hot
days with evaporative cooling or chilled water in a water-to-air heat exchanger, for
example, to maintain at least the nominal heat rate and power output values.
In addition to the heat rate, it is important to look at the overall system efficiency.
As shown in Chap. 17, the total system efficiency is equal to the sum of the power output
plus the thermal energy output divided by the total fuel input in consistent units. It is
possible to have a low heat rate (i.e., high electric power generation efficiency) but have