WHAT IS CHP?
In Combined Heat and Power (CHP) systems
the fuel source can be natural gas, propane,
fuel oil, coal, wood chips, biogas, other
biomass materials or any combination.
CHP uses this fuel to provide all or a
part of the electric energy and thermal
energy output to a facility at an overall
energy efficiency that is greater than
what would be required if the electricity
and thermal energy were being provided
separately.
Electric power production requires high
temperatures, while lower temperatures
can fulfill space heating or process energy
needs. By capturing unused low temperature
heat energy rejected from the electric
production process, fuel energy is used
more efficiently. Combining heat and power
production reduces the net fuel demands
for energy generation by supplying otherwise
unused heat to residential, commercial
and industrial consumers who have thermal
needs.
A range of commercially available technologies
can be employed in CHP facilities including:
diesel and gasoline engines, fuel cells,
combustion turbines and steam turbine
generators combined with fossil fuel fired
boilers. Although fuel cells are not normally
considered to be cogeneration devices,
they present common issues to CHP.
BENEFITS OF CHP
Combined Heat and Power systems use fuels,
both fossil and renewable, to produce
electricity or mechanical power and useful
thermal(heating and cooling) energy far
more efficiently and with lower emissions
than conventional separate heat and centralized
power systems. Nationally, current CHP
benefits includes:
• Produces over 9% of the electric
power generated in the U.S.
• Saves users over $5 billion each
year in energy costs
• Decreases energy consumption by
almost 1.3 trillion Btus a year
• Reduces NOx emissions by 0.4 million
tons per year
• Reduces SO2 emissions by over 0.9
million tons per year
• Prevents the release of over 35
million metric tons of carbon equivalent
into the atmosphere.
This benefit has come about primarily
from large industrial facilities such
as are found in the paper, refining and
chemical industries. However, new CHP
technologies now entering the market hold
the promise for even larger benefits for
both large and small users by:
• Improving profitability of local
companies,
• Utilizing an environmentally friendly
way to build generation capacity, and
by
• Reducing the load on Electric Transmission
Infrastructure through distributed generation.
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A BRIEF HISTORY
OF CHP ORIGINS
Reciprocating steam engines powered the
first electric generators in the 1880's.
Because these plants were inefficient,
a large amount of waste steam was available
for process use or building heat. Early
electrical developers provided electricity
to customers and sent the waste heat through
steam pipes for space heating. This concept
of what has been referred to as district
heating was first implemented in 1884
to provide energy for the Del Coronado
Hotel in San Diego.
By the turn of the century, larger steam
turbine generators with greater efficiencies
replaced reciprocating engines. Power
engineers, in an effort to satisfy expanding
energy needs, focussed on building larger
and larger steam turbine generating stations.
Generating efficiencies improved from
3.7% in 1902 to 16.5% in 1932.
During the early 20th century abundant
and relatively cheap coal became the fuel
of choice for electric generation. But,
the public nuisance of coal dust and flue
gas particulate emissions forced electric
generation facilities out of the cities.
The remote location of most coal-fired
power plants made capture and transmission
of heat energy uneconomical and brought
an end to use of waste heat in surrounding
buildings. In fact, by the time federal
regulation of the utility industry began
in the 1930's, generation and supply of
electrical energy were separate from generation
and supply of heat energy. This model
would predominate well into the 1980's.
This did not bring an end to the supply
of steam through pipes for space heating.
In many cases, however, because the electrical
loads were increasing in the summer time
when heating was not required, it was
more economical to separate these two
functions.
By 1965, conventional steam turbine technology
had reached its peak efficiency, rising
to roughly 33 percent. Today's advanced
combustion turbine technology can produce
electricity at over 40% efficiency when
operated alone. Because of improvements
spurred by defense and air transport needs,
combustion turbine technology now surpasses
steam technology’s efficiency. This
is due in part to the fact that the hot
exhaust gases from a gas turbine, unlike
fuel fired boilers which feed steam turbine
generators, have a relatively high energy
content which still can be used to make
steam in a Heat Recovery Steam Generator
(HSRG). The greatest efficiency - over
60 percent - occurs when the two cycles
are combined; i.e., when a generator is
driven by a gas turbine, and a second
generator is driven by steam made with
the gas turbine's exhaust heat. Energy
waste drops from two-thirds of the input
fuel to less than half in this process,
known as a Combined Cycle. For non-utility
applications, the high-energy content
of the gas turbine exhaust can alternatively
be used with a waste heat boiler to provide
steam for process or space heating needs
in a CHP process.
In the 1980's the Public Utility Regulatory
Policies Act (PURPA) opened the field
to improved efficiency, which gave industrial
energy users a financial incentive to
adopt CHP. Those that have continued to
generate their own power have realized
fuel efficiencies as high as 90 percent,
depending on how well the electric and
thermal needs are matched and on which
type of system is used.
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STATUS OF CHP
ON THE NATIONAL LEVEL
On a national scale, CHP has the potential
to offset significant quantities of emissions
of CO2 and other so-called greenhouse
gases (see Appendix C). CHP systems can
have an overall energy efficiency that
is more than double that of most electricity-only
fossil fuel power plants by distributing
the waste thermal energy from power generation
that would otherwise be lost as waste
heat. CHP systems increase the energy
efficiency and thus reduce the net amount
of air pollutants per unit of energy derived
from fuels.
Based on the Presidental Climate Change
Initiative, DOE has kicked off a CHP Challenge
in order to “raise awareness of
the energy, environmental and economic
benefits of CHP, and to promote innovative
thinking about ways to accelerate the
use of CHP.” The initiative has
three goals: to facilitate the removal
of barriers to CHP implementation, especially
on the state level; to facilitate the
identification and installation of innovative
CHP projects; and to assist states in
educating end users and the financial
community about the benefits of CHP, especially
new CHP technologies.
APPLICATIONS OF CHP IN PUBLIC
AND PRIVATE POWER PRODUCTION
Facilities choosing to use CHP for their
total electrical power needs may also
incorporate on-site backup power for use
when the CHP is down. Under these conditions
the facility is independent of the electrical
grid. Normally, however, facilities will
maintain a connection to the grid and
continue their service with a distribution
utility. This occurs for a variety of
reasons, such as to supplement their on-site
power generation to meet their peak electrical
requirements, or for backup reliability
purposes during planned or unplanned outages
of their own CHP system.
CHP systems need a balanced relationship
between the thermal energy supplied and
the electric power produced that depends
on the type of CHP system being used (Appendix
B). It is also normally important to have
thermal loads that are coincidental with
the electrical load to make CHP cost effective.
The use of thermal energy storage is not
typically economic unless the electrical
rate structure has a heavy demand charge.
Hence, most CHP facilities use the thermal
energy at the time it is produced and
so thermal energy demand should match
the time that electrical energy is needed.
A facility can be designed to match either
the thermal or electrical load of the
facility. If a facility is designed to
meet the thermal load, the difference
between the electrical power produced
by the CHP system and the power required
by the facility must either be sold to
others or purchased from the grid. Conversely,
if the facility is designed to meet the
electrical load, energy to meet the thermal
load will either need to be supplemented
or disposed of. For example, a gas turbine
can be used to generate electrical power
and then the exhaust gases can be passed
to a heat recover steam generator for
the supplying thermal needs of the facility.
The gas turbine can be used to track the
electrical load requirement and if insufficient
waste heat is available, supplemental
fuel firing can be used to supply the
heat recovery steam generator with additional
energy to produce needed steam requirements.
New technologies are allowing CHP to enter
new markets, including small commercial
buildings and food service operations.
It may also seem that cooling loads are
not consistent with the hot thermal output
from CHP plants. However, by using an
absorption cycle chiller, the hot thermal
output of the CHP plant can be converted
to a chilled water supply for use in the
summer for space cooling.
CHP technology should be considered in
geographical areas where electricity rates
are high, fuel costs are low, and for
applications with a requirement for both
electricity and thermal energy. Typical
candidates for implementation of CHP include:
• any industrial company requiring
coincidental thermal energy,
• schools, hospitals and universities,
• apartment buildings (urban district
heating systems),
• commercial buildings requiring
heating and air conditioning,
• health clubs, laundries, nursing
homes and extended care facilities, and
• facilities considering upgrades
or replacement of existing boilers.
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