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Combined Heat and Power Plant (CHP)
Lecture 6
Combined Heat and Power
Combined heat-and-power, also known as “cogeneration,” refers to the use of recovered exhaust heat of any production unit for another process requirement.
This in turn results in improvement in the energy utilization of the unit. By so doing, the overall thermal efficiency of generation may be raised from 40–50% to 70–90%. The upper limit of 90% holds for large installations with a very well-defined and constant heat demand.
Combined heat-and-power does not have to be a renewable source of energy; in fact, many CHP installations use natural gas as a source.
The use of biomass as a source is the only renewable form of CHP. The direct combustion of organic matter to produce steam or electricity is the most advanced of the different CHP processes and, when carried out under controlled conditions, is probably the most efficient.
Large CHP installations are used for production of steam in industrial installations, for space heating in the agriculture, and for district heating. Agricultural CHP is very common in the Netherlands and in Denmark, where about 25% of electricity comes from CHP. Recently, incentives toward smaller generation units have resulted in a growth in CHP in some countries.
Micro CHP plants used for space heating and electricity receiving a lot of attention. Possible applications are domestic heating, hotels shopping centers and offices.
For example, a somewhat larger unit produces 105kW electricity and 172kW heat. A market for much smaller units may be emerging, intended for heating of domestic premises.
An example is a unit that produces 1 kW electricity together with 7.5–12kW heat.
Categories of CHP applications
Four categories of CHP applications: small-scale CHP schemes: to meet space and water heating requirements in
buildings, based on spark ignition reciprocating engines large-scale CHP schemes: for steam raising in industrial and large buildings,
based on
compression ignition reciprocating engines, steam turbines or gas turbines large scale CHP schemes for district heating: based around a power station or
waste
incinerator with heat recovery supplying a local heating network CHP schemes fuelled by RES: these may be at any scale
Combined Cycle Schematic
EnergyDistribution in a combined cycle system
Two pressure level system
Features Very mature technology Size: 0.5 – 30+ MW Efficiency: electricity (20 – 45%), cogeneration (80 – 90%) Installed cost ($/kW): 400 – 1,200 O&M cost ($/kWh): 0.003 – 0.008 Fuel: natural gas, biogas, propane Emission: approximately 150 – 300 ppm NOx (uncontrolled) below approximately 6 ppm NOx (controlled) Cogeneration: yes (steam) Commercial Status: widely available Three main components: compressor, combustor, turbine Start-up time range: 2 – 5 minutes Natural gas pressure range: 160 – 610 psig Nominal operating temperature: 59 F
Combustion Gas Turbines24
Combustion Gas Turbines
Combustor
PowerConverterCompressor
airfuel
Power Turbine
Generator
HRSG(Heat Recovery
Steam Generator) Feed water
Process steam
Fig. 1 Block diagram of Combustion Gas Turbine System.
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CHP Technologies
Advantages
High efficiency and low cost (particularly in large systems)
Readily available over a wide range of power output
Marketing and customer serving channels are well established
High power-to-weight ratio
Proven reliability and availability
Disadvantages
Reduced efficiencies at part load
Sensitivity to ambient conditions (temperature, altitude)
Small system cost and efficiency not as good as larger systems
Advantages & Disadvantages
Combustion Gas Turbines27
Comparison CHP
Micro-turbines29
Size: 25 – 500 kW
Efficiency: unrecuperated (15%), recuperated (20 – 30%), with heat recovery (up to 85%)
Installed cost ($/kW): 1,200 – 1,700
O&M cost ($/kWh): 0.005 – 0.016
Fuel: natural gas, hydrogen, biogas, propane, diesel
Emission: below approximately 9 - 50 ppm NOx
Cogeneration: yes (50 – 80C water)
Commercial Status: small volume production, commercial prototypes now
Rotating speed: 90,000 – 120,000
Maintenance interval: 5,000 – 8,000 hrs
Micro-turbines
Features
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Advantages Small number of moving parts
Compact size
Light-weight
Good efficiencies in cogeneration
Low emissions
Can utilize waste fuels
Long maintenance intervals
Disadvantages Low fuel to electricity efficiencies
Micro-turbines
Advantages & Disadvantages
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Overview of CHP TechnologiesTechnology Pros Cons
Fuel Cell - Very low emission- Exempt from air and permitting in
some areas- Comes in a complete “ready to
connect” package
- High initial investment- Limited number of
commercially available units
Gas Turbine -Excellent service contracts-Steam generation capabilities-Mature technology
- Requires air permit- The size and shape of
generator package is relatively large
Micro-turbine - Lower initial investment- High redundancy- Low maintenance cost- Relative small size and installation
flexibility
- Relatively new technology- Requires air permit- Synchronization problems
possible for large installations
Recip.Engine
- Low initial investment- Mature technology- Relatively small size
- High maintenance costs- Low redundancy
Benefits of CHP High Efficiency, On-Site Generation Means
Improved Reliability Lower Energy Costs Lower Emissions (including CO2) Conserve Natural Resources Support Grid Infrastructure
Fewer T&D Constraints Defer Costly Grid Upgrades Price Stability
Facilitates Deployment of New Clean Energy Technologies
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Factors for CHP Suitability
High Thermal Loads-(Cooling, Heating)
Cost of buying electric power from the grid versus to cost of natural gas (Spark Spread)
Long operating hours (> 3000 hr/yr)
Need for high power quality and reliability
Large size building/facility
Access to Fuels (Natural Gas or Byproducts)
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GeneratorsTwo Types of Generators
Induction• Requires Grid Power
Source to Operate • When Grid Goes
Down, CHP System Goes Down
• Less Complicated & Less Costly to Interconnect
• Preferred by Utilities
Synchronous• Self Excited (Does
Not Need Grid to Operate)
• CHP System can Continue to Operate thru Grid Outages
• More Complicated & Costly to Interconnect (Safety)
• Preferred by Customers
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Environmental Benefits of CHP (NOx)36
186lb/MMBtu
Power Station Fuel(U.S. Fossil Mix)
117
CHP Fuel (Gas)
Lb/MMBtu
CO2 Emissions Reductions from CHP
39,000 Tons CO2 Saved/Year
Power Plant
6.0MWe
70,000 pphSteamBoiler117
Boiler Fuel (Gas)
Lb/MMBtu CO2 Emissions56k Tons/yr
CO2 Emissions43k Tons/yr
…TOTAL ANNUAL CO2 EMISSIONS…95k Tons 56k Tons
CO2 Emissions
52k Tons/yr
Conventional Generation Combined Heat & Power:Taurus 65 Gas Turbine Efficiency: 31%
Steam
Efficiency: 80%
Efficiency: 82.5%
CHP and Energy Assurance
Combined Heat & Power (CHP) can Keep Critical Facilities Up
& Operating During Outages
For Example, CHP can Restore Power and Avoid:
– Loss of lights & critical air handling
– Failure of water supply
– Closure of healthcare facilities
– Closure of key businesses
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