|
Library
- Frequently Asked Questions
What
is combined heat and power, CHP?
Combined
heat and power refers to recovering waste heat when electricity
is generated and using it to create high temperature hot water or
steam. Steam or hot water can then be used for space heating, producing
domestic hot water, or powering dehumidifiers and water chillers
for air conditioning.
Back
To Top
Why
is there so much interest in CHP?
There
are two different driving forces behind CHP. First, recent problems
in electrical supply and distribution have heightened concerns about
availability and cost of electricity. These have led in turn to
interest in distributed generation and subsequently use of waste
heat from power generation. The Department of Energy is interested
in CHP because of “resource efficiency.” If coal or
natural gas is burned at a power plant to produce electricity, less
than a third of the energy content of the fuel is delivered to customers
as useful power. The “resource efficiency” is less
than 33%. If a CHP plant captures 68% of the energy in the exhaust
gas and for space heating or hot water, the resource efficiency
becomes 78% (0.33 + 0.68*0.67). Much more of the fuel energy content
is used, fossil fuels consumption and CO2 emissions are reduced.
Back
To Top
Is
CHP the same as cogeneration?
CHP and
cogeneration are basically the same thing, although cogeneration
has been identified with district heating and large utility owned
power plants or industrial power production and plant operation.
CHP is generally a smaller scale, privately owned operation. It
frequently refers to generation of heat and power for university
campuses, military bases, hospitals, and hotels. New technologies
for small scale power production are opening opportunities for CHP
in medium and small sized buildings.
Back
To Top
What
is the difference between CHP, CCHP, BCHP, DER, IES?
Many new
terms and acronyms are entering common usage that mean basically
the same thing, generation of electricity near the customer’s
facility so that waste heat can be recovered and used. The terms
differ in where the emphasis is placed. CCHP stresses that combined
cooling, heating, and power production occur, whereas combined heating
and power in CHP may or may not use the recovered heat for cooling
purposes. BCHP is just CHP applied to a building as opposed to a
district heating system or industrial process. DER is distributed
energy resources, the use of small generating facilities distributed
close to the consumers either with or without heat recovery.
IES is
an integrated energy system that recovers waste heat from on-site
or near-site power generation to provide hot water, steam, heating,
cooling, or dehumidifying air for buildings.
Back
To Top
Why
can't I use my backup generator for on-site power production?
The primary
problem with using backup generators for on site power generation
concerns their emissions, NOx and SOx, although noise and durability
can also be problems. Most urban areas limit the maximum number
of hours that IC engine driven backup generators can be operated
each year because of their NOx and SOx emission levels. Generators
for CHP systems can operate upwards of 8000 hours per year which
greatly exceed backup generator usage, typically limited to less
than 200 hours per year. Some models may be able to handle such
high usage, others may not.
Back
To Top
Backup
generators have been around for decades, what is new about on-site
power generation?
Recent
developments have pushed to make on site power generation cleaner,
cheaper, and quieter. Backup generators typically use internal combustion
engines with a multitude of moving parts and relatively high emissions
of pollutants NOx and SOx. Microturbines have been developed which
have very low emissions of pollutants and extremely few moving parts
making them attractive from an environmental and maintenance point
of view. Gas turbines are also being marketed in smaller capacities
so that they have appeal beyond large utilities and factories. Fuel
cells continue to be developed with a promise of higher efficiencies
and lower emissions than any other source of electricity and heat.
Finally, strides are being made to reduce emissions from IC engine
driven generators to reduce their environmental impact.
Back
To Top
What
types of power generators can I buy?
The most
common type of on site power generation is using an IC engine-driven
generator. They are available in a broad range of capacities and
can have very high efficiencies. A couple of manufacturers are producing
microturbine generators and there are products under development
by additional companies and in additional sizes from the current
manufacturers. Gas turbine generators are sold for applications
requiring greater capacities and one brand of fuel cell is available.
Many different companies are in the process of developing fuel cells
for on site power generation and more products will become available.
Back
To Top
How
are generators classified, what is a kW?
Generators
are classified by the “combustion” system and their
rated electrical output. Combustion refers to whether an IC engine,
microturbine, gas turbine, or fuel cell is used to convert the fuel
to mechanical energy. It is in quotes because while most of these
technologies use a combustion process, fuel cells use a chemical
process without combustion. The electrical output or capacity is
the number of kilowatts (kW) or megawatts (MW) of power generated.
A kilowatt or megawatt is a measure of the rate of energy use or
production. How much energy is consumed or produced is measured
in kilowatt- or megawatt-hours. One kilowatt is equal to 1000 watts.
A 100 watt light bulb has an electrical load of 0.100 kilowatts;
if the bulb is left on for 10 hours it consumes 1000 watt-hours
or 1.0 kilowatt-hours (kWh).
Back
To Top
What
are gas turbines?
A gas
turbine burns a gas or liquid fuel to produce rotary motion, the
turbine blades spin about a central axis. The turbine and air compressor
are mounted on a central shaft; the electric generator can be mounted
on the same shaft or on a second shaft and driven by a gear drive.
The rotary motion requires fewer moving parts than the reciprocating
action of an IC engine and consequently produces fewer vibrations
and needs less maintenance. Gas turbines were developed for marine
engines in boats and jet engines in airplanes as well as in large
industrial turbines for utility power generation. The smaller gas
turbine generators are aeroderivatives, descendants of jet aircraft
engines.
Back
To Top
What
are microturbines?
Microturbines
are a fairly recent innovation bringing the advantages of gas turbines
to markets for smaller applications. They employ an air compressor
and turbine blades on a single shaft. Some employ a recuperator
to boost their efficiency and air bearings to reduce maintenance
costs. Products are available ranging from 30 kW to 75 kW of capacity;
this range will eventually expand to include 200 to 300 kW generators.
Back
To Top
What
is a recuperator and why is it important?
A recuperator
is an internal heat exchanger that is used to recover energy from
the turbine exhaust and use it to pre-heat inlet air. Using some
of the exhaust energy to heat the air before mixing it with the
fuel for combustion allows the same combustion temperatures and
generating capacity to be reached using less fuel. Recuperators
can double the efficiency of microturbine generators.
Back
To Top
What
is an HRSG?
A heat
recovery steam generator, or HRSG, is used to recover energy from
the hot exhaust gases in power generation. It is a bank of tubes
that is mounted in the exhaust stack. Exhaust gases at as much as
1000°F heat the tubes. Water pumped through the tubes can be
held under high pressure to temperatures of 370°F or higher
or it can be boiled to produce steam. The HRSG separates the caustic
compounds in the flue gases from the occupants and equipment that
use the waste heat.
What
are fuel cells?
Fuel cells
are devices that use a chemical reaction to produce an electric
current at very high efficiencies. They are frequently compared
to batteries where the chemicals needed for the reactions are stored
within the battery itself. Fuel cells differ in that they are connected
to a source of fuel, almost always molecular hydrogen. Hydrogen
is combined with oxygen from the air to produce water and electric
current; electrons flow between the cathode and anode of the fuel
cell through an external circuit and while positive chemical ions
flow in the opposite direction within the fuel cell itself. Fuel
cells are categorized by the substance used for ionic flow in the
fuel cell; phosphoric acid (PAFC) proton exchange membranes (PEMFC),
solid oxide (SOFC), molten carbonate (MCFC), etc.
Back
To Top
Can
I buy a fuel cell?
There
is only one fuel cell suitable for CHP applications is commercially
available in the spring of 2001. It is a 200 kW phosphoric acid
fuel cell. Many other products are under development worldwide but
are not yet on the market.
Back
To Top
What
is a reformer?
Generally
speaking, fuel cells use molecular hydrogen as their fuel and oxygen
from the air to produce electricity. A reformer is a device that
allows a fuel cell to use a hydrocarbon fuel like natural gas or
propane as the fuel. It uses a catalyst, water, and heat to break
down the hydrocarbon releasing hydrogen as fuel to the fuel cell
and carbon dioxide to the atmosphere.
Back
To Top
What
is a desiccant dehumidifier?
Dehumidifiers,
naturally, remove humidity from the air. Normally this is done by
cooling finned tubes in a heat exchanger below the dew point temperature
so water condenses and drips into a condensate pan or drain. This
process is energy intensive because it requires cooling the tubes
and air below temperatures that are comfortable for air conditioning.
Desiccants are chemical compounds that have an affinity for water
vapor, in a sense they absorb it like a sponge. A desiccant dehumidifier
employs desiccants deposited on honeycombed surfaces to provide
lots of area for water vapor to be absorbed. Blowing air through
these surfaces remove moisture from it before it enters the building
and reduces humidity levels. Liquid desiccants are also available
in spray systems.
Back
To Top
How
do desiccant dehumidifiers use waste heat in a CHP system?
Desiccant
materials can be heated to remove water vapor from them. This is
done in a practical application by building the desiccant into a
wheel that rotates through the building supply and exhaust air.
For example, supply air being brought into a building is passed
through the left side of the wheel where it absorbs water vapor.
Exhaust air is heated and blown through the right side of the wheel
and then vented outdoors removing water vapor from the desiccant.
The wheel is rotated slowly so the desiccant passes through both
supply and exhaust air flows. Steam or hot water from a HRSG can
be used to provide the heat needed to heat the exhaust air and regenerate
the desiccant.
Back
To Top
What
is a chiller?
Most small
buildings like homes use a forced air distribution system to provide
hot or cold air for comfort conditioning. Large buildings frequently
use a hydronic distribution system and pump chilled water to air
handling units to provide cool air for air conditioning. A chiller
is the machine that cools water to around 44°F for distribution
to the air handling units.
Back
To Top
What
is an absorption chiller?
Absorption
chillers use heat and a chemical solution to produce chilled water.
A gas burner is usually used to produce the heat with a mixture
of lithium bromide and water as the chemical solution. Recovered
waste heat in the form of hot water or steam can be used to power
an indirect-fired absorption chiller (they use electricity for solution
pumps, but only a small fraction of the electricity that electric
motor driven chillers require).
Back
To Top
What
are single- and double-effect absorption chillers?
Without
getting technical, the number of “effects” in a chiller
reflects the number of times energy is used. A single-effect machine
uses heat just once to produce chilled water. A double-effect machine
contains heat exchangers to recover heat left over from the first
stage of cooling to produce additional refrigerant vapor and more
cooling. Double-effect is more efficient than single-effect. Triple-effect
chillers are under development.
What
is a cooling tower?
Every
type of air conditioning or refrigeration process is a means of
moving heat from where it is not wanted to medium where it can be
rejected. The radiator of a car is a dry, finned-tube heat exchanger
that is used to reject engine heat to the outdoor air efficiently.
A cooling tower is essentially a wet heat exchanger used to reject
heat from a chiller or excess heat from a HRSG. The water spray
over tube banks in a cooling tower is more efficient at rejecting
heat than a dry heat exchanger. It allows lower operating pressures
in the chiller and greater efficiencies.
Back
To Top
What
is power conditioning?
Utilities
in the U.S. distribute electricity at standard conditions with specifications
for voltage, frequency, and type. Consequently most of our electrical
appliances are designed for 60 Hz, alternating current. Power conditioning
is the process of taking whatever electricity is produced by a generator
and converting it to meet the industry standards so it can be used
without damaging whatever is plugged in, be it a hair dryer, television,
or computer. Power conditioning is an essential part of on site
power generation.
Back
To Top
What
is NOx and why is it called a pollutant?
NOx is
an abbreviation or acronym used to refer to nitric oxide (NO) and
nitrogen dioxide (NO2). Both of these chemical compounds contribute
to urban smog and can contribute to acid rain so their emissions
are carefully controlled by government agencies. They can be formed
during high temperature combustion from nitrogen in the air. Careful
control of the combustion process or treatment of exhaust gases
is needed to keep emissions low.
Back
To Top
What
is SOx and why is it a pollutant?
SOx encompasses
a group of chemical compounds of sulfur and oxygen, but it predominantly
it refers to sulfur dioxide, SO2. Sulfur dioxide is formed during
combustion from sulfur compounds in the fuel and oxygen in the air.
Liquid and solid fuels like gasoline and coal contain sulfur compounds
and cause SOx in the flue emissions; SOx is not an issue with gaseous
fuels like natural gas and propane. Sulfur dioxide dissolves in
water forming sulfuric acid, the principal constituent of acid rain.
SOx emissions are strictly regulated.
Back
To Top
What
is SCR?
SCR stands
for selective catalytic reduction and is a process for removing
NOx from exhaust gases in order to meet pollution control requirements.
Back
To Top
|
