1. “Energy Efficiency Guide for Industry in Asia”
Training Session on Energy Equipment
Cogeneration
Presentation from the
“Energy Efficiency Guide for Industry in Asia”
Thermal Equipment/
Cogeneration
TO THE TRAINER
This PowerPoint presentation can be used to train people about the basics of cogeneration. The information on the slides is the minimum information that should be explained. The trainer notes for each slide provide more detailed information, but it is up to the trainer to decide if and how much of this information is presented also.
Additional materials that can be used for the training session are available on under “Energy Equipment” and include:
Textbook chapter on this energy equipment that forms the basis of this PowerPoint presentation but has more detailed information
Quiz – ten multiple choice questions that trainees can answer after the training session
Workshop exercise – a practical calculation related to this equipment
Option checklist – a list of the most important options to improve energy efficiency of this equipment
Company case studies – participants of past courses have given the feedback that they would like to hear about options implemented at companies for each energy equipment. More than 200 examples are available from 44 companies in the cement, steel, chemicals, ceramics and pulp & paper sectors
© UNEP 2006

2. Training Agenda: Cogeneration
Introduction
Types of cogeneration systems
Assessment of cogeneration systems
Energy efficiency opportunities
Thermal Equipment/
Cogeneration
© UNEP 2006

3. What’s a Cogeneration/CHP System?
Introduction
What’s a Cogeneration/CHP System?
Generation of multiple forms of energy in one system: heat and power
Defined by its “prime movers”
Reciprocating engines
Combustion or gas turbines,
Steam turbines
Microturbines
Fuel cells
Thermal Equipement/
Cogeneration
A Cogeneration system or a Combined Heat & Power System (CHP) is the sequential or simultaneous generation of multiple forms of useful energy. It is usually mechanical (power) and thermal (heat) in a single, integrated system.
It is the type of equipment, or prime mover that drives the overall system, that typically identifies the CHP system.
Prime movers for CHP systems include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical energy.
© UNEP 2006

4. Efficiency Advantage of CHP
Introduction
Efficiency Advantage of CHP
100 68
24 Units
34 Units
6 Units (Losses) 60 40
36 Units (Losses)
 = 85%
 = 40%
10 Units (Losses)
Conventional Generation (58% Overall Efficiency)
Combined Heat & Power (85% Overall Efficiency)
Thermal Equipment/
Cogeneration
The picture illustrates the efficiency advantage of CHP compared to the conventional central station power generation and on-site boilers.
When both thermal and electrical processes, a CHP system typically requires only three-fourth of the primary energy compared to separate heat and power systems.
This reduced primary fuel consumption is key to the environmental benefits of CHP as burning the same amount of fuel more efficiently means fewer emissions for the same level of output.
(UNESCAP, 2004)
© UNEP 2006

5. Benefits of Cogeneration / CHP)
Introduction
Benefits of Cogeneration / CHP)
Increased efficiency of energy conversion and use
Lower emissions, especially CO2
Ability to use waste materials
Large cost savings
Opportunity to decentralize the electricity generation
Promoting liberalization in energy markets
Thermal Equipment/
Cogeneration
Provided the cogeneration is optimized in terms of sized according to the heat demand, it brings the following benefits
Increased efficiency of energy conversion and use
Lower emissions to the environment, in particular of CO2, the main greenhouse gas
In some cases, biomass fuels and some waste materials such as refinery gases, process or agricultural waste (either anaerobically digested or gasified), are used. These substances which serve as fuels for cogeneration schemes, increases the cost-effectiveness and reduces the need for waste disposal
Large cost savings, providing additional competitiveness for industrial and commercial users while offering affordable heat for domestic users also
An opportunity to move towards more decentralized forms of electricity generation, where plants are designed to meet the needs of local consumers, providing high efficiency, avoiding transmission losses and increasing flexibility in system use. This will particularly be the case if natural gas is the energy carrier
An opportunity to increase the diversity of generation plant, and provide competition in generation. Cogeneration provides one of the most important vehicles for promoting liberalization in energy markets.
© UNEP 2006

6. Training Agenda: Cogeneration
Introduction
Types of cogeneration systems
Assessment of cogeneration systems
Energy efficiency opportunities
Thermal Equipment/
Cogeneration
© UNEP 2006

7. Type of Cogeneration Systems
Steam turbine
Gas turbine
Reciprocating engine
Other classifications:
- Topping cycle
- Bottoming cycle
Thermal Equipment/
Cogeneration
© UNEP 2006

8. Type of Cogeneration Systems
Steam Turbine Cogeneration System
Widely used in CHP applications
Oldest prime mover technology
Capacities: 50 kW to hundreds of MWs
Thermodynamic cycle is the “Rankin cycle” that uses a boiler
Most common types
Back pressure steam turbine
Extraction condensing steam turbine
Thermal Equipment/
Cogeneration
Steam turbines are one of the most versatile and oldest prime mover technologies that are still in general production. Steam turbines are widely used for combined heat and power (CHP) applications.
The capacity of steam turbines can range from 50 kW to several hundred MWs for large utility power plants.
The thermodynamic cycle for the steam turbine is the Rankine cycle. This cycle is the basis for conventional power generating stations and consists of a heat source that converts water to high-pressure steam. The developed condensate from the process returns to the feedwater pump for continuation of the cycle.
The two types of steam turbines most widely used are the back pressure and the extraction-condensing types. The choice between backpressure turbine and extraction-condensing turbine depends mainly on the quantities of power and heat, quality of heat, and economic factors.
© UNEP 2006

9. Type of Cogeneration Systems
Back Pressure Steam Turbine
Steam exits the turbine at a higher pressure that the atmospheric
Boiler
Turbine
Process
HP Steam
Condensate
LP Steam
Advantages:
Simple configuration
Low capital cost
Low need of cooling water
High total efficiency
Disadvantages:
Larger steam turbine
Electrical load and output can not be matched
Thermal Equipment/
Cogeneration
Back pressure steam turbine is the most simple configuration. Steam exits the turbine at a pressure higher or at least equal to the atmospheric pressure. This is why the term back- pressure is used. After the steam exits the turbine, it is fed to the load where it releases heat and is condensed. The condensate then returns to the system.
(click once) The back pressure system has the following advantages: Simple configuration with few components; low capital costs; reduced or no need of cooling water; high total efficiency as there is no heat rejection to the environment through a condenser.
(click once) The back pressure system has the following disadvantages: The steam turbine is larger for the same power output because it operates under lower enthalpy difference of steam. The generated electricity is controlled by the thermal load. This results in little or no flexibility to match electrical output to electrical load.
Fuel
Figure: Back pressure steam turbine
© UNEP 2006

10. Type of Cogeneration Systems
Extraction Condensing Steam Turbine
Boiler
Turbine
Process
HP Steam
LP Steam
Condensate
Condenser
Fuel
Steam obtained by extraction from an intermediate stage
Remaining steam is exhausted
Relatively high capital cost, lower total efficiency
Control of electrical power independent of thermal load
Thermal Equipment/
Cogeneration
In an extraction condensing steam turbine system, the steam for the thermal load is obtained through extraction from one or more intermediate stages at appropriate pressure and temperature. The remaining steam is exhausted to the pressure of the condenser.
In comparison to the back - pressure system, the condensing type turbine has a higher capital cost and generally a lower total efficiency. To its advantage is that the extraction condensing steam turbine can control the electrical power independent of the thermal load by proper regulation of the steam flow rate through the turbine to a certain extent.
Figure: Extraction condensing steam turbine
© UNEP 2006

11. Type of Cogeneration Systems
Gas Turbine Cogeneration System
Operate on thermodynamic “Brayton cycle”
atmospheric air compressed, heated, expanded
excess power used to produce power
Natural gas is most common fuel
1MW to 100 MW range
Rapid developments in recent years
Two types: open and closed cycle
Thermal Equipment/
Cogeneration
Gas turbine systems operate on the thermodynamic cycle that is known as the Brayton cycle. In a Brayton cycle, atmospheric air is compressed, heated, and then expanded. It is the excess of power produced by the turbine or expander, over that consumed by the compressor, that is used for power generation.
Gas turbine cogeneration systems can produce all or a part of the energy requirement of the site. Though natural gas is most commonly used, other fuels such as light fuel oil or diesel can also be employed. The typical range of gas turbines varies from a fraction of a MW to around 100 MW.
Gas turbine cogeneration has probably experienced the most rapid development in the recent years due to the greater availability of natural gas, rapid progress in the technology, significant reduction in installation costs, and better environmental performance.
There are two types of gas turbine cogeneration systems: open cycle and close cycle. These are explained on the next slides
© UNEP 2006

12. Condensate from Process
Type of Cogeneration Systems
Open Cycle Gas Turbine
Air G
Compressor
Turbine
HRSG
Combustor
Fuel
Generator
Exhaust Gases
Condensate from Process
Steam to Process
Open Brayton cycle: atmospheric air at increased pressure to combustor
Thermal Equipment/
Cogeneration
Old/small units: 15:1 New/large units: 30:1
Exhaust gas at oC
High pressure steam produced: can drive steam turbine
Most of the currently available gas turbine systems operate on the open Brayton cycle where a compressor takes in air from the atmosphere and derives it at increased pressure to the combustor. This is also called Joule cycle when irreversibilities are ignored. The air temperature is also increased due to compression.
Older and smaller units operate at a pressure ratio in the range of 15:1, while the newer and larger units operate at pressure ratios approaching 30:1.
Looking at this figure, the air is delivered through a diffuser to a constant-pressure combustion chamber, where fuel is injected and burned. Combustion takes place with high excess air and the exhaust gases exit the combustor at high temperature and with oxygen concentrations of up to 15-16%. The highest temperature of the cycle appears at this point; with current technology this is about 1300°C.
The high pressure and high temperature exhaust gases enter the gas turbine and produce mechanical work to drive the compressor and the load. The exhaust gases leave the turbine at a considerable temperature ( °C), which makes high-temperature heat recovery ideal. The produced steam can have high pressure and temperature. This makes it appropriate not only for thermal processes but also for driving a steam turbine thus producing additional power.
Figure: Open cycle gas turbine cogeneration
© UNEP 2006

13. Condensate from Process
Type of Cogeneration Systems
Closed Cycle Gas Turbine
Heat Source G
Compressor
Turbine
Generator
Condensate from Process
Steam to Process
Heat Exchanger
Working fluid circulates in a closed circuit and does not cause corrosion or erosion
Any fuel, nuclear or solar energy can be used
Thermal Equipment/
Cogeneration
In the closed-cycle system, the working fluid is usually helium or air and it circulates in a closed circuit. It is heated in a heat exchanger before entering the turbine, and it is cooled down after the exit of the turbine releasing useful heat. This way the working fluid remains clean and it does not cause corrosion or erosion.
Source of heat can be the external combustion of any fuel. Nuclear energy or solar energy can also be used.
Figure: Closed Cycle Gas Turbine Cogeneration System
© UNEP 2006

14. Type of Cogeneration Systems
Reciprocating Engine Cogeneration Systems
Used as direct mechanical drives
Many advantages: operation, efficiency, fuel costs
Used as direct mechanical drives
Four sources of usable waste heat
Thermal Equipment/
Cogeneration
Reciprocating engines are well suited to a variety of distributed generation applications, as well as industrial, commercial, and institutional facilities for power generation and CHP.
Reciprocating engines have many advantages:
Operation: reciprocating engines start quickly, follow load well, have good part-load efficiencies, and generally have high reliabilities. In many cases, multiple reciprocating engine units further increase overall plant capacity and availability.
Efficiency & costs: Reciprocating engines have higher electrical efficiencies than gas turbines of comparable size, and therefore lower fuel-related operating costs.
Reciprocating engines are also used extensively as direct mechanical drives in applications such as water pumping, air and gas compression and chilling/refrigeration.
While the use of reciprocating engines is expected to grow in various distributed generation applications, the most prevalent on-site generation application for natural gas SI engines has traditionally been CHP, and this trend is likely to continue.
There are four sources of usable waste heat from a reciprocating engine: exhaust gas, engine jacket cooling water, lube oil cooling water, and turbocharger cooling.
Figure: Reciprocating engine cogeneration system (UNESCAP, 2000)
© UNEP 2006

15. Type of Cogeneration Systems
Topping Cycle
Supplied fuel first produces power followed by thermal energy
Thermal energy is a by product used for process heat or other
Most popular method of cogeneration
Thermal Equipment/
Cogeneration
Cogeneration systems are normally classified according to the sequence of energy use and the adopted operating schemes. A cogeneration system can be classified as either a topping or a bottoming cycle on the basis of the sequence of energy use.
(click once) In a topping cycle, the supplied fuel is used to first produce power and then thermal energy, which is the by-product of the cycle. This is used to satisfy process heat or other thermal requirements. Topping cycle cogeneration is widely used and is the most popular method of cogeneration.
Examples include a combined-cycle topping system; steam turbine topping system; and gas turbine topping system
© UNEP 2006

16. Type of Cogeneration Systems
Bottoming Cycle
Primary fuel produces high temperature thermal energy
Rejected heat is used to generate power
Suitable for manufacturing processes
Thermal Equipment/
Cogeneration
In a bottoming cycle, the primary fuel produces high temperature thermal energy and the rejected heat from the process is used to generate power through a recovery boiler and a turbine generator.
Bottoming cycles are suitable for manufacturing processes that require heat at high temperature in furnaces and kilns, and that reject heat at significantly high temperatures. Typical areas of application include cement, steel, ceramic, gas and petrochemical industries.
© UNEP 2006

17. Training Agenda: Cogeneration
Introduction
Types of cogeneration systems
Assessment of cogeneration systems
Energy efficiency opportunities
Thermal Equipment/
Cogeneration
© UNEP 2006

18. Assessment of Cogeneration Systems
Performance Terms & Definitions
Overall Plant Heat Rate (kCal/kWh):
Ms = Mass Flow Rate of Steam (kg/hr)
hs = Enthalpy of Steam (kCal/kg)
hw = Enthalpy of Feed Water (kCal/kg)
Overall Plant Fuel Rate (kg/kWh)
Thermal Equipment/
Cogeneration
Overall Plant Heat Rate is Ms x (hs – hw) / Power output (kW), where Ms = Mass Flow Rate of Steam; hs = Enthalpy of Steam; hw = Enthalpy of Feed Water.
(Click once) Overall Plant Fuel Rate is Fuel consumption *(kg/hr) / Power output (kW)
© UNEP 2006

19. Assessment of Cogeneration Systems
Steam Turbine Performance
Steam turbine efficiency (%):
Thermal Equipment/
Cogeneration
Gas Turbine Performance
For steam turbine performance the steam turbine efficiency is calculated according to the formula (let audience reflect over the formula before continuing).
(click once) For gas turbine performance, the overall turbine efficiency is calculated according to the formula.
An example calculation of the performance of a steam turbine cogeneration system is included in the cogeneration chapter.
Overall gas turbine efficiency (%) (turbine compressor):
© UNEP 2006

20. Assessment of Cogeneration Systems
Heat Recovery Steam Generator (HRSG) Performance
Heat recovery steam generator efficiency (%):
Ms = Steam Generated (kg/hr)
hs = Enthalpy of Steam (kCal/kg)
hw = Enthalpy of Feed Water (kCal/kg)
Mf = Mass flow of Flue Gas (kg/hr)
t-in = Inlet Temperature of Flue Gas (0C)
t-out = Outlet Temperature of Flue Gas (0C)
Maux = Auxiliary Fuel Consumption (kg/hr)
Thermal Equipment/
Cogeneration
In addition to the turbine, the cogeneration system also has a heat recovery steam generator, which also impacts the performance of the cogeneration system. The formula used to calculate its efficiency is shown on this slide. The chapter “Waste Recovery” has more details.
© UNEP 2006

21. Training Agenda: Cogeneration
Introduction
Types of cogeneration systems
Assessment of cogeneration systems
Energy efficiency opportunities
Thermal Equipment/
Cogeneration
© UNEP 2006

22. Energy Efficiency Opportunities
Steam Turbine Cogeneration System
Steam turbine:
Keep condenser vacuum at optimum value
Keep steam temperature and pressure at optimum value
Avoid part load operation and starting & stopping
Boiler & steam – see other chapters
Thermal Equipment/
Cogeneration
First we will look at the energy efficiency opportunities in steam turbine cogeneration systems.
Condenser vacuum or back-pressure is the most important factor as a small deviation from optimum can produce a significant change in efficiency. There are a number of reasons why condenser vacuum may vary from the optimum value such as when the cooling water inlet temperature is different from the design value. This is the most common reason for variations in condenser vacuum because the temperature of the cooling water is significantly influenced by weather conditions such as temperature and humidity. Condenser vacuum may also change because of cooling water flow rate, or that the condenser tubes are fouled, and due to air leaks into the condenser.
If the steam temperature and pressure conditions at the inlet to the steam turbine vary from the design optimum conditions, the turbine may not be able to operate at maximum efficiency. Variations in steam conditions can be due to errors in plant design, incorrect plant operation or fouling within the boiler.
Market decisions to operate the generating unit at certain loads for certain periods can have the major influence on its average thermal efficiency. Similarly, market decision on when the plant is to come on and off line also has a bearing on average thermal efficiency because of energy losses while starting or stopping the system.
© UNEP 2006

23. Energy Efficiency Opportunities
Gas Turbine Cogeneration System
Gas turbine – manage the following parameters:
Gas temperature and pressure
Part load operation and starting & stopping
Temperature of hot gas and exhaust gas
Mass flow through gas turbine
Air pressure
Air compressors – see compressors chapter
Heat recovery system generator – see waste heat recovery chapter
Thermal Equipment/
Cogeneration
Finally, we will look at the energy efficiency opportunities in gas turbine cogeneration systems.
If the gas temperature and pressure conditions at the gas turbine inlet vary from the design optimum conditions, the turbine may not be able to operate at maximum efficiency. Variations in gas conditions can be due to errors in plant design or incorrect plant operation.
Generating unit efficiencies at part loads can be maintained close to the design values by giving attention to all the above items. However, once again market decisions to operate the generating unit at certain loads for certain periods will have the major influence on its average thermal efficiency.
The temperature of the exhaust gas is also a factor. Increased temperature generally results in increased power output. Reduced temperature generally results in increased power output. Also, higher mass flows result in higher power output.
© UNEP 2006

24. Cogeneration Training Session on Energy Equipment Thermal Equipment/
THANK YOU
FOR YOUR ATTENTION
Thermal Equipment/
Cogeneration
Hello.
© UNEP GERIAP

25. Disclaimer and References
This PowerPoint training session was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP). While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. © UNEP, 2006.
The GERIAP project was funded by the Swedish International Development Cooperation Agency (Sida)
Full references are included in the textbook chapter that is available on
Thermal Equipment/
Cogeneration
© UNEP 2006