Transcript ECE 7800: Renewable Energy Systems

ECE 7800: Renewable Energy
Systems
Topic 5: Distributed Generation with
Fossil Fuels
Spring 2010
© Pritpal Singh, 2010
Distributed Generation
With the restructuring of the electric
utility industry, the new model of
power generation is that of distributed
generation (DG).
DG allows for higher efficiency
because locally generated electricity
can be combined with district heating
and cooling.
The scales of DG technologies are
shown on the next slide.
Distributed Generation (cont’d)
kW
0.02
0.1
1
10
100
1,000
10,000
100,000
Distributed Generation Technologies
• Reciprocating engines
• Combined Heat and Power (CHP)
• Micro-Combined Heat and Power
(Micro- CHP)
• Microturbines
• Fuel Cells
Use fossil
fuels
• Stirling engines
Renewable
• Wind Generators
energy
technologies
• Solar Electric systems
• Micro-hydroelectric systems
Distributed Generation with Fossil Fuels
The most widespread distributed
generation sources are gasoline and
diesel generators (using IC engines)
used for backup power in various
applications, e.g. hospitals, first
responders, data centers, etc.
High and Low Heating Values of Fuels
An important parameter associated
with fuels is the heating value. When
a fuel is burned, some of the heat
goes into latent heat for vaporization
of water into steam. There are two
types of heating value:
1) High heating value – takes into
account the heat given to latent heat
2) Low heating value – does not take
into account the heat given to latent
heat.
High and Low Heating Values of Fuels (cont’d)
The high and low heating values for
some common fuels are given in the
below table:
Energy Density of Batteries and H2
Energy density of lead acid batteries
= 40Wh/kg = 144,000 J/kg = 144 kJ/kg
Energy density of lithium ion batteries
= 460-720 kJ/kg
Energy density of H2 = 143,000 kJ/kg
(however, volumetric energy density is
much less than gasoline)
- 5.6 MJ/L (compressed H2) vs. 34.2MJ/L
(gasoline)
Reciprocating Internal Combustion Engines
Distributed generation is dominated
by reciprocating, i.e. piston-driven
internal combustion engines (ICE’s)
connected to constant-speed ac
generators. They range in power
from 0.5 kW to 6.5 MW with LHV
efficiencies of 37-40%. They can run
on diesel, gasoline, kerosene,
natural gas, fuel oil, alcohol, wastetreatment plant digester gas, and
hydrogen. They are the least
expensive DG technology and with
natural gas, one of the cleanest.
Reciprocating I C Engines (cont’d)
The engines may be four-stroke or twostroke and may be spark-ignited or
compression-ignited. Four-stroke engines
are more efficient than two-stroke and are
cleaner burning; therefore two-stroke are
generally not used. Spark-ignition
engines use easily ignitable fuels, e.g.
natural gas, gasoline and propane.
Compression-ignition engines require
heavier petroleum distillates such as
diesel or fuel oil. A supercharger, which
compresses the air-fuel mixture prior to it
entering the cylinder, can be used to
improve the efficiency of engines.
Reciprocating I C Engines (cont’d)
The basic cycle of a four-stroke engine
is shown below:
Reciprocating I C Engines (cont’d)
Since emissions are a major constraint,
most of the interest in reciprocating
engines is in natural gas engines. The
below diagram shows the heat balance for
present and target reciprocating engines:
Reciprocating I C Engines (cont’d)
For combined heat and power
applications, waste heat from IC
engines can be extracted using water
jackets around the engine. An example
of such a system is shown below:
Commercial Natural Gas Generator
•60kW, 80 kW, 100 kW, 150 kW and 200 kW sizes,
synchronous or induction.
•Units may be operated in parallel for larger installations
up to 3,000 kW
•High-efficiency dedicated natural gas industrial engines.
•Solenoid-operated automatic inlet gas cut-off valve.
•Unit-mounted natural gas pressure regulation system.
•Solid-state automatic air/fuel ratio controller system.
•Extended-life replaceable air filtration system.
•Electronic engine speed governor system accurate to ±
0.25% steady-state.
•Pressurized closed-loop cooling system with oversized
heavy-duty radiator.
•12 volt D.C. engine electrical system.
•Solid-state 110V battery charging system.
•Heavy steel base with built-in connection points and
lifting sockets.
•All components mounted on composite vibration
isolators.
•All-weather 14-gauge steel enclosure with locking doors.
•Sound Attenuation packages available for commercial,
residential or critical applications.
•All-weather exhaust silencer system options available for
various sound attenuation level requirements.
•Microprocessor based, digital readout auto-start control
panel with programmable set points available.
•Double-sealed, permanently lubricated single-bearing
reconnectable brushless generator with SAE housing and
positive alignment flexible drive plate connection.
Induction or Synchronous generators available.
•Guaranteed to meet or exceed all United States air
quality requirements.
•Customized Cogeneration options available for hot
water, heated air, process heat, chilled water and air
conditioning, specifically designed for the facility served.
•Remote PC operation options for pre-alarm, alarm and/or
control functions.
•Paralleling switchgear, intertie protection packages,
automatic transfer switches and numerous other types of
control/operation packages available.
•Extended Warranty, Maintenance Plans and
Refurbishment programs available.
http://www.genergypower.com/System%20Features.htm
Microturbines
Rankine cycle and combined cycle gas
turbines (described earlier) have been
used as peaking power generators at
scales of few MW to hundreds of MW.
More recently, a new generation of
small gas turbines ranging from about
500W to several hundred kW have
become available. These are referred
to as microturbines. A schematic
drawing of a microturbine is shown on
the next slide.
Microturbines (cont’d)
in
Ref: Vanek and Albright,
Energy System Engineering
Microturbines (cont’d)
Incoming air is compressed to 3-4 atm.
pressure. This is sent to a heat
exchanger (recuperator) where it is
heated by hot exhaust gases. The hot
compressed air is mixed with fuel in the
combustion chamber and burned. The
expanding hot gases spin the turbine
and the generator and compressor
which are co-located on the same shaft.
The exhaust gases are cooled in the
recuperator transferring heat to the
incoming air.
Microturbines (cont’d)
Some commercial microturbine specs.
are given below:
Capstone C60 Microturbine
http://capstoneturbine.com/prodsol/techtour/index.asp
Microturbines (cont’d)
See Table 4.3 Capstone C60 info.
Microturbines (cont’d)
Examples 4.1 and 4.2
Biomass for Electricity
Biomass uses plant material
for electricity production
which is derived from waste
from agriculture, forestry and
municipal waste.
World wide there are about
14GW of biomass plants with
about half in the US. About 2/3
are co-generation plants. Almost
all operate on a conventional
steam-Rankine cycle.
The McNeil power
station built in 1998 in
Vermont is capable of
generating 50 MW of
power from local wood
waste products.
Ref:
http://www.ucsusa.org/clean_energy/technology_and_impacts/en
ergy_technologies/how-biomass-energy-works.html
Biomass for Electricity (cont’d)
Biomass plants tend to be lower
efficiency (~20%) than conventional
power plants due to high water
vapor content of waste and lower
operating temperatures and
pressures. This results in relatively
expensive electricity (~9¢/kWh). Coal
and biomass can be co-fired and this
increases overall efficiency and
reduced emissions compared to only
coal-fired power plants.
Biomass for Electricity (Cont’d)
A two-step process for gasifying
biomass fuels is shown below:
Biomass for Electricity (Cont’d)
Step 1:
Raw biomass fuel is heated, causing
it to undergo pyrolysis in which
volatile components are vaporized.
Moisture is driven off first then
syngas comprising H2, CO, CH4, CO2
and N2 is produced. Char and ash
are byproducts of pyrolysis.
Step 2:
The char is heated to 700ºC reacts
with O2,H2 and steam to produce
more syngas.
Carbon Emissions of Biomass
Riding the Carbon Cycle: The carbon cycle is nature's way of moving
carbon around to support life on Earth. Carbon dioxide is the most
common vehicle for carbon, where one carbon atom is bound to two
oxygen atoms. Plant photosynthesis breaks the carbon dioxide in two,
keeping the carbon to form the carbohydrates that make up the plant,
and putting the oxygen into the air. When the plant dies or is burned, it
gives its carbon back to the air, which is then reabsorbed by other
plants.
Ref: http://www.ucsusa.org/clean_energy/technology_and_impacts/energy_technologies/howbiomass-energy-works.html
Stirling Engines
An alternative approach to IC engines
is external combustion engines, i.e.
the heat source is located outside the
engine (rather inside the engine as in
an ICE). A Stirling engine is an
example of a reciprocating engine that
uses an external combustion source.
It can run on any almost any fuel or
other source of high temperature heat,
e.g. concentrated sunlight on a black
absorber plate.
Stirling Engines (cont’d)
The basic operation of a Stirling
engine is shown in the below figure:
Stirling Engines (cont’d)
The engine comprises two pistons in
the same chamber. One is on the hot
side and the other on the cold side.
They are separated by a short-term
thermal storage device called a
regenerator. This is usually a wire or
ceramic mesh that allows gas to flow
in either direction. As the gas passes
through the regenerator it either heats
up or cools down depending on the
direction it is traveling.
Stirling Engines (cont’d)
There are four states of operation:
1)->2) The hot piston is stationary
while the cold one moves to the left,
compressing the gas while
transferring heat to the cold sink.
2)->3) Both pistons move
simultaneously to the left. The gas
heats up as it passes through the
regenerator, increasing temperature
and pressure while its volume
remains constant.
Stirling Engines (cont’d)
3)->4) The gas in the hot space absorbs
energy from the hot source and
expands, pushing the hot piston to the
left. This is the power stroke.
4)->1) Both pistons move simultaneously
to the right. The gas passing through
the regenerator into the cold space
drops in temperature and pressure.
The piston movement can be controlled
by a rotating crankshaft. When this is
tied to a generator, electricity can be
generated.
Stirling Engines (cont’d)
The pressure-volume diagram for a
Stirling engine is shown below:
Stirling Engines (cont’d)
Animation of Stirling engine:
http://en.wikipedia.org/wiki/Stirling_engine
Stirling Engines (cont’d)
Stirling engines range in size from
< 1kW to about 25 kW. Their
efficiencies are relatively low (< 30%).
However, progress is being made in
improving efficiency and the external
combustion nature makes them
relatively quiet engines when
compared to IC engines.
Stirling Engines (cont’d)
•The AC WhisperGenTM is set to revolutionise the
home energy market. It is an innovative system
developed to provide central heating, water
heating and electricity in your home.
•The WhisperGenTM is a co-generation (heat and
power) system based on a small four cylinder
Stirling Engine. The AC WhisperGen is a unique
micro combined heat and power (microCHP)
system providing efficient, low maintenance
generation of heat and electricity. It operates as a
floor mounted, fully automatic boiler, providing
up to 8.0kW of thermal energy for hot water and
space heating while generating a max. 1.2kW
230V AC power output.
•Designed and styled as a whiteware appliance
with minimal noise and vibration, the
WhisperGenTM replaces the traditional domestic
boiler, and supplements the commercial
electricity supply to your home.
•Running on natural gas or LPG, the AC
WhisperGenTM efficiently uses over 90% of the
fuel energy resulting in a cleaner and more cost
effective alternative to traditional electricity
generation. Electricity generated can be fed back
into the electricity grid or used in the home,
reducing electricity costs even further.
•The WhisperGenTM is currently being evaluated
internationally with systems operating in several
countries, including the UK, Netherlands,
Germany and France.
http://www.whispergen.com/main/residential/
(Based in Christchurch, New Zealand)
Combined Heat and Power
Some DG technologies produce waste heat
(e.g. combustion turbines, fuel cells) and
others do not (e.g. solar panels, wind
turbines). By utilizing this waste heat, both
the energy efficiency and the economic
value of the energy generator can be
greatly enhanced.
High temperature waste heat can be used
for process steam, absorption cooling, and
space heating while low temperature heat is
normally used for water heating.
Energy Efficiency Measures of CHP
Energy Efficiency Measures of CHP (cont’d)
One approach to comparing systems
with and without CHP is to compare
their thermal efficiencies:
Overall thermal =  30  48  x100% = 78%
 100 
Efficiency (w/CHP)
30  48 

= 
 x100% = 52%
 90  60 
Overall thermal
Efficiency (w/o CHP)
Energy Efficiency Measures of CHP (cont’d)
For an industrial facility, it needs heat
anyway for process heat. An alternative
measure that considers how much extra
heat is needed to generate electricity is
the energy-chargeable-to-power (ECP)
given by:
ECP =
total thermal - displaced thermal
input
input
Electrical output
Units: Btu/kWh or kJ/kWh
Energy Efficiency Measures of CHP (cont’d)
Example 5.13
CHP Economics
The ECP depends on the amount of
usable heat recoverable from the
boiler or furnace and the efficiency
of the process. When ECP is
modified to account for the cost of
the fuel, a measure of the added cost
for electricity generation is the
operating cost chargeable to power
(CCP) given by:
CCP = ECP x unit cost of energy
Units: $/kWh
CHP Economics (cont’d)
Example 5.14
Design of CHP System
The design of CHP systems is
complex since it involves balancing
the amount of heat and electrical
power that the CHP system delivers.
The power-to-heat (P/H) ratio may be
constant for an industrial plant but
vary significantly in a residential
building (see figure on next slide).
Residential Thermal and Electrical Loads
Smoothing the P/H Ratio
As seen in fig. 5.11, the P/H ratio can
vary widely throughout the year due
to high demand for electricity for air
conditioning in the summer and high
demand for space heat in the winter.
There are several approaches that
can be considered to smooth out this
P/H ratio throughout the year.
Smoothing the P/H Ratio (cont’d)
Some examples are:
• Heat pumps can displace heat with
electricity in the winter
• Absorption cooling systems can
displace electricity with heat in the
summer.
Another strategy (that does not affect the
P/H ratio) is:
• Ice making for cooling water for
chillers in the summer displaces the
time of day that the electricity is used.
(see section 5.6 textbook)
Distributed Generation-Related Standards
An important issue related to distributed
generation is interconnection standards.
The utilities have put up several
roadblocks related to interconnecting
distributed generation sources to the grid.
These revolve around maintaining power
quality, e.g. harmonics injected onto the
grid, constant frequency, safety for
maintenance crew, etc.
Efforts are underway at the federal level
and within individual states to develop
fair and uniform interconnection
standards to help facilitate the
deployment of distributed generation.
Distributed Generation-Related Standards
The National Electric Code has
procedures and practices related to
installation of commercial and
residential Photovoltaic Systems.
The IEEE has a working group
(P1547) developing standards for
interconnection of DG equipment to
the electrical grid.
Distributed Generation-Related Standards
The IEEE Standards are shown below:
http://grouper.ieee.org/groups/scc21/dr_shared/
Distributed Generation-Related Standards
Interconnection standards for the
state of Ohio are given at this
weblink:
http://www.puco.ohio.gov/emplibrary
/files/smed/Technical_Requirements.
pdf
Distributed Generation-Related Standards
Example: IEEE 929-2000 for Photovoltaic Systems
IEEE 929-2000: Safety
Utilities and the PV community now have an approved interconnection
standard that ensures the safe operation of a PV system connected to a
utility grid. Safety for the utility lineman, for the utility equipment, and for
the customer was the primary concern throughout the development of
the interconnection standard. The IEEE 929-2000 standard includes
tightly-defined specifications that require the PV system inverter to cease
to energize the utility line for specific out-of-tolerance conditions such as
voltage and frequency trip settings when values are outside of acceptable
limits. These inverters also include sophisticated and reliable antiislanding protocols that include active detection functions to ensure that
the inverter does not deliver power to the utility system when utility
power is cut off or disconnected from the inverter. Additionally, detection
functions ensure that the inverter will cease to energize the utility line
when an excess of dc current is present at the ac interface.
IEEE 929-2000: Power Quality
The quality of power provided by the PV system must meet specifications
for voltage, flicker, frequency, and distortion. Out-of-bounds conditions
for any of these variables require the inverter to cease to energize the
utility line. Voltage and frequency set points for systems larger than 10
kW may be altered by the utility to accommodate system-specific needs.
http://www.solarelectricpower.org/interconnection/position_statement.cfm