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.

25

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

30

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

31

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

33

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)

34

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

35

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

38