Page 75 - Sustainable On-Site CHP Systems Design, Construction, and Operations

54 CHP B a s i c s

The concept of fuel cells has been around for far more than 100 years; however, the
first practical use of the technology was made by NASA in the 1960s for use in manned
spacecraft providing clean electrical generation and water as a waste product. Over the
years, fuel cells have become more practical due to research and development of
consumer-focused products, demonstration projects, transportation use, and military
hardware use. As shown in Table 3-3, there are several types of fuel cells with somewhat
different processes, with different fuel-to-electric efficiencies, and differing waste heat
temperatures. Most fuel cells are in the research stage. The fuel cell processes are similar
in that fuel (typically hydrogen) and an oxidizer (typically oxygen) are introduced into
cells across a membrane separation and the union of oxygen and hydrogen causes ions
to flow between the sides of the cells. Many individual cells are combined to form
“stacks” which provide power and have water and waste heat as by-products.
Advantages of fuel cells include the fact that they are practically emission free of
undesired exhaust gases, are, in some cases, highly efficient, operate at very low noise
levels, and that fuel cells are able to respond rapidly to changes in electrical loads. The
most common fuel cells (phosphoric acid process) reject heat (the chemical reaction by-
product) in the 150 to 200°F range and are about 40 to 55 percent efficient in generating
electricity. Other processes have different efficiencies, different temperature heat dis-
charge and different cost per watt. Molten carbon as an example is about 55 percent
efficient and discharges heat at high enough temperatures (600 to 650°F) to produce
high-pressure steam. In demonstration projects, this steam has been discharged to a
steam turbine to add additional electrical power.

Types
Fuel cells differ from simple batteries in that they use a continuous supply of fuel for the
chemical reaction, and, provided the fuel supply continues, can operate for extended
periods of time. 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 (however, pure hydrogen, propane,
and diesel fuel can also be used). The by-product of the chemical reaction is hot water.
As hydrogen (the fuel) enters the fuel cell and is mixed with air (containing oxygen), the
fuel is oxidized, broken down into protons and electrons. In the proton exchange mem-
brane fuel cell (PEMFC) and phosphoric acid fuel cell (PAFC), positively charged ions
move through the electrolyte across a voltage to produce electric power after which the
protons and electrons are recombined with oxygen in the air to make hot water. As this
water is removed from the fuel cell, more protons are pulled through the electrolyte,
resulting in further power production.

Sizes and Availability
Although fuel cells are excellent candidates for CHP, they have one drawback; at this
time, the capital cost per installed kilowatt remains high relative to other available CHP
prime mover technologies. The availability of other low-cost energy sources, combined
with concerns about the exotic materials and developing technologies used in fuel cells,
have resulted in limited specific commercial applications. A commercial producer of
fuel cells in the United States produces a 200-kW unit that sells for approximately U.S.
$1,000,000. This price is equivalent to U.S. $5,000 per kilowatt, which is approximately
3 to 4 times the cost of an equivalent IC engine or combustion turbine generator system.
Larger fuel cells (1000 kW) are also in development and are expected to sell for U.S.