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Energy Saving Technologies

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Title: Energy Saving Technologies


1
Energy Saving Technologies
  • Cogeneration

2
  • The course is developed within the frames of
    project Development of Training Network for
    Improving Education in Energy Efficiency
    acronym ENERGY, grant Nr. 530379-TEMPUS-1-2012-1-
    LVTEMPUS-JPCR.
  • Project was approved by the European Commission
    in frame of program Tempus IV Fifth call for
    proposals (Programme guide EACEA/25/2011).
  • Sub-programme Joint Projects
  • Action Curricular Reform
  • Deliverable 2.1 Development and translation of
    study courses within the frame of direction
    enhancement of energy efficiency (EEE).
  • This project has been funded with support from
    the European Commission Project. This publication
    reflects the views only of the author, and the
    Commission cannot be held responsible for any
    use, which may be made of the information
    contained therein.

3
RTU Course "Energy Saving Technologies"
Code
Course title Energy Saving Technologies
Course status in the program Courses of Free Choice
Course level Undergraduate Studies
Course type Academic
Field of study Power and Electrical Engineering
Responsible instructor Anastasia Zhiravetska
Academic staff Anastasia Zhiravecka Nadezhda Kunicina Anatolijs Zabašta Ansis Avotins
Volume of the course parts and credits points 1 part, 2.0 Credit Points, 3.0 ECTS credits
4
Course outline
Theme Hours
Introductive class 2
Co-generation 2
Smart metering 4
Distributed generation 2
DC transmission lines 4
Approaches to reduction of electric energy losses 4
Electrical motors and drives 6
Effective lighting 2
Supercapacitors 2
Standartization and legal bases 4
5
Study subject structure
Part Semester CP ECTS Exam Lectures Practical Lab.
1. Autumn 2.0 3.0 2.0 0.0 0.0
6
Cogeneration or combined cooling and
heating
  • According to the Directive 2004/8/EC of the
    European Parliament and of the Council of
    February 11, 2004 on the promotion of
    cogeneration based on a useful heat demand in the
    internal energy market and amending Directive
    92/42/EEC cogeneration means the simultaneous
    generation in one process of thermal energy and
    electrical and/or mechanical energy . In
    literature the following definitions are often
    used
  • Cogeneration is the combined production of
    electrical (or mechanical) and useful thermal
    energy from the same primary energy source
  • Cogeneration is the sequential production of
    thermal and electric energy from a single fuel
    source
  • Cogeneration is on-site generation and
    utilisation of energy in different forms
    simultaneously by utilising fuel energy at
    optimum efficiency in a cost-effective and
    environmentally responsible way.

7
Cogeneration concept
  • The mechanical energy produced by cogeneration
    can be used to drive auxiliary equipment as well.
    The thermal energy can be used either for heating
    or for cooling. Cooling can be obtained by
    thermally driven chillers (usually adsorption or
    absorption chillers).
  • In a conventional thermal power plant, large
    amount of heat (50-70) is wasted with exhaust
    gases and cooling agent. A large portion of the
    waste heat can be recovered and used by combining
    the electrical generation and heat production
    processes, increasing in this way the overall
    efficiency to 80-90. This combination of the
    electrical generation and heat production
    processes represents the combined heat and power
    (CHP) generation or cogeneration concept.

8
The advantages of cogeneration in electricity and
heat production
The main advantages of cogeneration systems
are the following improve energy efficiency at
national level leading to conservation of fossil
energy resources enable locally generation of
electricity and reduce the heat losses enable
the use of different fuels can be used in
remote areas reduce the environmental impact
due to higher efficiency of fuel conversion.
9
Comparison between individual generation of
electricity and heat v/s cogeneration
a) b)
generation of electricity
cogeneration
generation of heat
10
The main disadvantages of cogeneration systems

have high investment and operation
costs require utilisation of the generated
heat in the case the generated electricity is
fully utilised require back-up system in
order to ensure supply security of electricity
and heat, increasing the investment cost.
11
The main components of a cogeneration system

a prime mover an electrical generator a
heat recovery exchanger operating control
systems
12
Cogeneration technology in electricity and heat
production (1)
The prime mover is a thermal engine (Rankine,
Brayton, Diesel, Otto, Stirling) or a combination
of thermal engines which converts chemical energy
of fuel into mechanical energy transmitted to
electrical generator. A special system, which
converts fuel chemical energy directly into
electricity, is the system that uses fuel cell as
prime mover. The heat recover see maybe a heat
exchanger or a network of heat exchangers which
transfers the heat from exhaust gases or engine
cooling agent to the heating agent or to water
(domestic hot water). The most important indices
used to compare different cogeneration systems
are the following mechanical efficiency of
prime mover (heat engine) where is
mechanical power of prime mover
is heat flow produced by fuel combustion
is fuel mass flow rate, kg/s or
Nm3/s
13
Cogeneration technology in electricity and heat
production (2)
LHV is lower heating value (net calorific value)
of fuel, kJ/kg or kJ/Nm3. electrical efficiency
where is electrical power
generated by system (electrical power
output) thermal efficiency where
is the heat flow rate generated by the system.

14
Cogeneration technology in electricity and heat
production (3)
overall efficiency or total energy
efficiency This equation is a subject of
discussions because it is not appropriate to add
heat with electricity since the heat quality is
lower than that of electricity. It is very
difficult to obtain 1 kW electric from 1 kW
thermal due to the losses of heat conversion into
electricity. We can use another index that uses
the exergy, as a measure of energy quality, of
system input and output, instead of the overall
efficiency.

15
Cogeneration technology in electricity and heat
production (4)
Exergetic efficiency Where is exergy
flow rate of generated heat is
exergy flow rate associated to fuel
ef is specific exergy of fuel, kJ/kg or
kJ/Nm3.

16
Cogeneration technology in electricity and heat
production (5)
power to heat ratio fuel energy savings
ratio where is fuel power input in
individual generation system of heat ( )
or electricity ( )
is fuel power input in the system that
cogenerates the same amount of heat ( )
and electricity ( ).

17
Types of cogeneration systems
The size of cogeneration systems can vary from
small micro systems, which can serve the needs
of a single building to large systems that can
serve a town. The cogeneration systems can be
divided by the sequence of energy use and the
operating schemes adopted into topping cycle
systems and bottoming cycle systems. In a topping
cycle system, electricity or mechanical power is
produced by a heat engine using fuel and the heat
is recovered to meet heating demand. These
systems are used in applications that do not
require high process temperature. In bottoming
cycle systems, the heat is produced directly from
fuel combustion to meet high temperature heat
demand of industrial process. The wasted heat is
recovered and used as energy source to generate
electricity or mechanical power. In other way
the cogeneration systems can be classified
according to the type of prime mover or
thermodynamic cycle used.

18
Rankine cycle cogeneration systems
A steam turbine cogeneration system includes a
heat source, a heat sink and a steam turbine
driving an electrical generator. The system runs
on improved Rankine cycle (steam reheat and
regenerative preheating of feeding water). The
heat source can be a boiler, a nuclear reactor or
a waste incinerator. The boiler can use any type
of fuel or combinations of fuels and solar
radiation to produce superheated steam with high
temperature and high pressure. The power of
steam turbine cogeneration system varies from 0.5
to over 100 MW. The generated steam can have up
to 450C and 100 bar in the commercial or
industrial sector and about 540C and over 100
bar in the utility sector.

19
Rankine cycle / steam turbines
The most widely used types of steam turbines
are back pressure steam turbines condensing
steam turbines bottoming cycle steam turbine
systems bottoming Rankine cycle systems with
organic fluids.

20
Types of cogeneration systems
Topping cycle system
Bottoming cycle system

21
Rankine cycle with steam reheat

The choice between types of steam turbines
depends mainly on the quantities of power and
heat, quality of heat, and economic factors.
22
cogeneration system with back pressure steam
turbine and steam reheat
A backpressure steam turbine means a steam
turbine from which the steam exits or is
extracted at a pressure and temperature depending
on the temperature level of heat required by the
processes. The cogeneration system with
backpressure steam turbine. The steam from
turbine transfers the heat to technological/indust
rial process reaching the liquid state
(condensation). The condensate is then returned
to boiler.

23
Advantages of the cogeneration system with back
pressure steam turbine and steam reheat
the construction is simple the high cost
of low pressure turbine stages are avoided
low capital cost due to the lack of cooling
plant high overall efficiency as almost all
entire generated heat in boiler is
utilised (there is no heat transferred to the
heat sink).
The disadvantages of the systems is that the
steam mass flow rate through the turbine depends
on the thermal demand. This means that the
electricity generated by turbine is controlled by
thermal demand.

24
Condensing steam turbine systems
In these systems, the steam is extracted from one
or more intermediate stages at the pressure and
temperature suitable for the heating process. The
main part of the steam exits the turbine at the
condenser pressure. A lower condenser pressure
(implicit temperature) means a higher thermal
efficiency of cycle. The condenser temperature is
limited by the temperature of a cooling agent
(water, air). The cogeneration systems with
condensing steam turbine have a higher capital
cost and lower overall efficiency than
backpressure systems. The electricity load can be
adjusted independently on the thermal load by
controlling steam flow rate through the turbine.

25
Cogeneration system with condensing steam turbine


26
Bottoming cycle steam turbine systems
Evacuated gases from many industrial thermal
processes have high temperature (500-600C). The
sensible heat contained in gasses can be
recovered to generate steam in a Rankine based
cogeneration system. The steam turbine can be a
condensing turbine or a backpressure one.

27
bottoming cycle cogeneration system with
condensing steam turbine

28
Bottoming Rankine cycle systems with organic
fluids
For effective use of low temperature level heat
sources (80-300C) have been developed plants
that operate on Rankine cycle with organic
working fluid. Organic Rankine cycle is similar
to the conventional steam turbine cycle, but uses
as working fluid a high molecular weight
substance and low boiling point, which is
suitable for recovery of heat available at low
temperature.

29
An Organic Rankine Cycle (ORC) has the following
advantages
the heat source cooled to a lower temperature
by using organic fluids so more electric power
can be generated from heat the evaporation
process takes place at lower pressure and
temperature the expansion process ends in the
vapour region and hence the risk of blades
erosion is avoided turbines with organic
fluids can provide higher efficiency at part
loads systems have great flexibility, high
safety and low maintenance the smaller
temperature difference between evaporation and
condensation means that the pressure drop/ratio
is much smaller and thus simple single stage
turbines can be used. The turbine cost is
therefore lower.

30
Temperature profile of water and an organic fluid
at heat addition

31
An Organic Rankine Cycle (ORC) substances
Substances that can be used in ORC are
hydrocarbons (HC) hydrofluorocarbons (HFC)
hydrochlorofluorocarbons (HCFC)
chlorofluorocarbons (CFC) perfluorocarbons
(PFC) siloxanes alcohols aldehydes ethers
hydrofluoroethers (HFE) amines fluids mixtures
(azeotropic and non-azeotropic). The heat losses
through irreversible heat transfer from the heat
source and to sink influence the overall
efficiency of a thermodynamic conversion cycle.
These losses depend mainly on matching heat
capacity of working fluid, heat source and sink.
In order to achieve the matching of working fluid
and source heat capacities, the multi-component,
non-azeotropic working fluids featuring
non-isothermal heat addition are used.

32
An Organic Rankine Cycle (ORC) plant energy
production depends of
good thermodynamic properties high
thermodynamic performance (high
energetic/exergetic efficiency) good thermal
and chemical stability during plant
operation low environmental impact (ozone
depletion potential, global warming potential,
atmospheric lifetime) good safety
characteristics (non toxic, non corrosive low
flammability and auto-ignition properties). good
heat transfer properties (low viscosity, high
thermal conductivity) low cost and good
availability.

The electric power generated by cogeneration
systems with organic Rankine cycle is in the
range from 2 kW to 2.5 MW. The electric
efficiency is about (10-30).
33
Temperature profile of a multi-component,
non-azeotropic working fluid at heat
addition

34
Gas turbine cogeneration systems (1)
The gas turbine can operate in a simple Brayton
(called Joule cycle when irreversibilities are
ignored) thermodynamic cycle or in a combined
Brayton-Rankine cycle. The gas turbine systems
have been developed initially for industrial and
utility applications and as aircraft engines.
Later, for stationary applications was modified
are called aeroderivative turbines. The main
advantages of gas turbine cogeneration systems
are the following low capital cost low-cost
maintenance low installation cost fast
start-ups rapid response to changing load
fuel-switching capabilities, high efficiency of
larger plants high temperature level of heat
(450-600C) which can be recovered good
environmental performance. The main disadvantage
is in low heat to power conversion efficiency.
The electric power output ranges from few
kilowatts (micro turbine systems) to 250 MW.

35
Gas turbine cogeneration systems (2)
The most gas turbine cogeneration systems operate
on the open Brayton cycle. The atmospheric air is
compressed in compressor at a pressure ratio up
to 301 and then introduced in combustion chamber
where fuel combustion takes place. The generated
flue gas exits the combustion chamber at high
temperature and pressure and with high oxygen
concentration (15-16).

36
Cogeneration system with open-cycle gas
turbine

37
Cogeneration system with open-cycle gas
turbine
The higher flue gas temperature is at the turbine
inlet, the higher cycle efficiency is. The upper
temperature limit at which turbine can operate is
determined by the resistance of turbine materials
and by the cooling efficiency of blades. The
current higher cycle temperature is 1400C. The
heat content of exhausted gases from turbine can
be increased by using a burner inside the heat
recovery boiler. The necessary air for additional
fuel combustion is taken from the exhaust gases
passing through the boiler. The gas turbine
cogeneration systems can use a variety of fuels
natural gas, gas oil, Diesel oil, biogas,
landfill gas and syngas. The life cycle of
systems depends on fuel quality and varies
between 15 and 20 years.

38
Cogeneration system with closed-cycle gas
turbine

38
39
The electric efficiency of gas turbine systems
A reduces number of gas turbine cogeneration
systems are running on closed Brayton cycle. The
source heat (nuclear energy, solar energy, heat
from a solid waste combustor) is used to heat the
working fluid (helium, air) which circulates in a
closed circuit. The heat contained in working
fluid after expansion in turbine is released to a
process fluid. The electric efficiency of gas
turbine systems is in the range of (25-35) for
small and medium systems and in the range of
(40-42) for larger systems. This lower electric
efficiency is due to the power consumption of air
compressor, which is about 50 of generated power
by turbine. To reduce the power consumption of
air compressor, the following techniques are
used precooling of the compressor inlet air
cooling of air at intermediate stage of
compression and regenerative air preheating
before the combustion chamber inlet. The overall
efficiency of gas turbine cogeneration systems is
about (60-80) and the power to heat ratio is in
the range 0.5-0.8.

40
Reciprocating engine cogeneration systems
For cogeneration applications in institutional,
commercial and residential sectors are preferred
the reciprocating engine based cogeneration
systems. The engines drive an electrical
generator and heat contained in the exhaust gases
and cooling systems, which represent (6070) of
the inlet fuel energy, is recovered to generate
hot water or steam. Most of the waste heat is
available in the exhaust gases and jacket
coolant, while smaller amounts can be recovered
from the lubricating oil cooler and the
turbocharger's intercooler. The exhaust gases
can also be used for drying or other direct heat
processes.

40
41
The main advantages of reciprocating engine
cogeneration systems
  • low capital cost
  • fast start-ups
  • good operating reliability
  • high efficiency at partial load operation.
  • The main disadvantages are
  • high vibrations and high acoustic noise
  • high maintenance costs
  • high pollutant emissions (especially NOx)
  • the waste heat is available at different
    temperature levels making difficult full
    utilisation of heat.


41
42
Otto cycle and Diesel cycle
The reciprocating engine cogeneration systems may
be classified by the engine cycle in Otto cycle
and Diesel cycle. The Diesel engines are
classified as high speed, medium speed and low
speed engines. The Otto engines can use beside
the gasoline many gaseous fuels such as propane,
biogas, and landfill gas. When they use gaseous
fuel they are called gas engines. The Diesel
engines can operate on Diesel oil, petrol,
bio-oils, natural gas, biogas, landfill gas,
syngas, alcohols and residual fuel oil.

42
43
Classification on the size of the engine
  • Another classification may be made on the size of
    the engine as follows
  • small units with gas engine (15-1000kW) or Diesel
    engine (75-1000kW)
  • medium power systems (1-6MW) with gas engine or
    Diesel engine
  • high power systems (gt 6MW) with Diesel engine.


43
44
Cooling system and temperature of exhaust gases
The water passing through the cooling system is
heated up to (75-80)C before entering the heat
recovery boiler. When there is no heat demand the
cooling water is guided to the auxiliary cooler.
Medium power systems generate saturated steam
with (180-200)C while high power systems
generate superheated steam at pressure of (15-20)
bar and temperature of (250-350)C by using an
auxiliary boiler. The minimum temperature of
exhaust gases at boiler outlet depends on the
fuel type. For fuels containing sulphur the
minimum temperature is (90-100)C and for fuels
with no sulphur content is (160-170)C.

44
45
Closed-loop cooling
Closed-loop cooling is the most common method of
heat recovery from reciprocating engines. In
these systems, the engine is cooled by forced
circulation of a cooling agent through the
coolers and when there is no heat demand through
an auxiliary cooler (cooling tower or radiator).
Another cooling method of reciprocating engines
is ebullient cooling. This method consists of a
natural circulation of a boiling coolant through
the engine. The cooling water at its boiling
point is placed to the bottom of the engine where
it is heated and starts to boil generating
bubbles. The bubbles generation leads to a lower
coolant density, causing natural circulation to
the top of engine. As the heat transfer from the
engine to the coolant occurs at constant
temperature, the engine thermal stress is lower.
The mixture of liquid and vapour after exiting
the engine is introduced in a steam separator for
steam separation. The saturated steam is used in
thermal process.

45
46
Cogeneration system with reciprocating
engine
Hot water or steam
Hot water or steam,
Electricity,

46
47
Overall efficiency of reciprocating cogeneration
systems
The overall efficiency of reciprocating
cogeneration systems is about (70-85) and the
power to heat ratio is in the range of 0.8-2.4.
The electric efficiency is in the range of
(35-45) for small and medium size engines and
about 50 for large engines. The system
performances are not so dependent on ambient
conditions or load.

47
48
Stirling engine cogeneration systems
The Stirling engines are not so well developed,
but they present high interest due to their
advantages high efficiency, good performance at
partial load, low temperature operation, low
pollutant emission level and low vibration and
noise level due to the continuous combustion
process. The Stirling engines can use a large
variety of fuels and different energy sources
(combustion gases, solar and nuclear energy). The
fuels can be changed during operation without
stopping the engine.

48
49
Main disadvantages of the Stirling engines
  • high capital cost
  • long start-ups
  • short lifetime of certain parts (shaft seal,
    piston rings, bearings leakage).
  • Unlike the reciprocating engines, at the Stirling
    engines the fuel combustion occurs outside the
    engine and the heat is transferred to a working
    fluid (usually helium or air) through a heater.
  • There are three configurations of Stirling
    engines Alpha, Beta and Gamma (figure next
    slide). Alpha configuration consists of two
    pistons in separate cylinders, which are
    connected in series by a heater, regenerator and
    cooler. Beta configuration uses the
    displacer-piston arrangement inside the same
    cylinder connected to a heater, regenerator and
    cooler, and Gamma configuration uses the
    displacer-piston arrangement placed in separate
    cylinders connected to a heater, regenerator and
    cooler.


49
50
Stirling engine configurations
50
51
The ideal Stirling cycle consists of the
following processes
isothermal compression (1-2), during which the
heat is removed from the engine at the cold sink
temperature constant volume heat addition
(2-3), in which both pistons move simultaneously
(compression piston towards regenerator and
expansion piston away from regenerator), so that
the volume between pistons remains constant and
working fluid is transferred from compression
volume to expansion volume through porous media
regenerator isothermal expansion (3-4) during
which heat is added to the engine at the hot
source temperature (heater) constant volume
heat rejection (4-1), in which both pistons move
simultaneously to transfer working fluid from
expansion space to compression space through
regenerator at constant volume

51
52
Stirling cycle
52
53
Stirling cycle
53
54
Stirling cycle
54
55
The ideal Stirling cycle consists of the
following processes
The cylinder volume connected to heater is
expansion volume and connected to cooler is
compression volume. The regenerator between the
heater and cooler absorbs and releases heat
alternatively. The temperature difference between
cylinder volumes (Tmax-Tmin) is maintained.

55
56
Conceptual diagram of a cogeneration system with
Stirling engine
The Stirling engine drives an electrical
generator and the waste heat from the heat source
is used to heat up the water. To obtain a high
electrical efficiency of cogeneration system, the
temperature of heating source should be as high
as possible. This is achieved by combustion air
preheating using the heat contained combustion
gases leaving the heater. The combustion air
temperature usually increases up to (500-600)C.

56
57
Conceptual diagram of a Stirling system
Flow Temperature C
1 20
2 600
3 1300
4 800
5 270
6 130
a 60
b 75
c 90
t water to process,
Electricity,
57
58
System thermal efficiency
To increase the system thermal efficiency, the
engine cooling water (heat sink) is used to
recover the waste heat in an economizer (heat
exchanger). Stirling engines are usually
installed horizontally downstream the combustion
chamber. The air preheater and economizer are
placed above the combustion chamber to obtain a
more compact structure. The Stirling engines
have initially developed as car engines with
power in the range of 3kW to 100 kW. Now the
engines are built for cogeneration systems with
the power up to 1.5MW especially for
micro-cogeneration systems. The electric
efficiency is up to 50, the overall efficiency
lies within the range of (65-85) and the power
to heat ratio is 1.2-1.7.

58
59
Fuel cell cogeneration systems
Fuel cells may generate power in the range of
0.1kW to 50MW. A fuel cell is an electrochemical
cell which can continuously convert the chemical
energy of a fuel and an oxidant to electrical
energy by a process involving an essentially
invariant electrode-electrolyte system. The
structure of a simplified fuel cell consists of
an electrolyte layer in contact with two gas
permeable electrodes coated with a catalyst is
shown on next slide. The electrodes are connected
to a device that completes an electric circuit.
The hydrogen fuel is fed continuously to one
electrode and the oxidant (oxygen from air) is
fed continuously to the opposite electrode. The
hydrogen fuel is oxidised into hydrogen protons
loosing its electrons to the electrode, which
becomes in this way anode. The electrolyte
membrane permits only the positive ions to flow
from anode to cathode. Therefore, the electrons
move to the cathode through the external
electrical circuit and electrical current begins
to flow. The electrons react with oxidant and
hydrogen protons at the cathode forming water and
producing heat.

59
60
Fuel cell working diagram
t water to process,
Electricity,
60
61
The chemical reactions involved at electrodes

61
62
Efficiency of a fuel cell
The efficiency of a fuel cell as the ratio of
electrical energy produced and the chemical
energy of the fuel is about (35-55). The working
temperature is in the range of ambient
temperature to 1000C. By using the generated
heat, the overall efficiency of a fuel cell
reaches up to (75-90), the highest efficiency
amongst all conversion systems. The main
advantages of fuel cells are high efficiency
size flexibility (single cells can be stacked to
provide the appropriate voltage for any
application) high reliability due to the lack of
moving parts low pollutant missions. The main
disadvantages of fuel cells are high cost large
size and weight relative to generated power
large start-up time fuel availability some fuel
cells require expensive catalysts some fuel
cells are susceptible to poisoning some fuel
cells suffer from corrosion and breakdown.

62
63
Fuel cell cogeneration system
Electricity,
Hot water to process,
t water to process,
Electricity,
63
64
Other subsystems and component
  • As the fuel cell voltage is too low (about 1.2V)
    it is necessary to stack many cells to increase
    the power output.
  • Besides the stack, a fuel cell system includes
    other subsystems and component such as
  • fuel preparation unit. When other fuels than pure
    fuels (hydrogen) are used, fuel preparation
    (impurities removal, thermal conditioning) or
    fuel reforming is required
  • air compressor or blower for air supply
  • temperature control system
  • water management system
  • electric power conditioning system.


64
65
Classification of fuel cells
The fuel cells are classified according to the
type of electrolyte and fuel as follows - proton
exchange membrane fuel cell (PEMFC) o direct
formic acid fuel cell (DFAFC) o direct Ethanol
Fuel Cell (DEFC). - alkaline fuel cell
(AFC) o proton ceramic fuel cell
(PCFC) o direct borohydride fuel cell
(DBFC). - phosphoric acid fuel cell
(PAFC) - molten carbonate fuel cell
(MCFC) - solid oxide fuel cell (SOFC) - direct
methanol fuel cell (DMFC).

65
66
Combined cycle cogeneration systems
Some thermodynamic cycles work between high
temperature levels and others work between
moderate temperature levels. In order to obtain a
higher efficiency, a high-temperature topping
cycle is combined with a medium- or
low-temperature bottoming cycle. The rejected
heat from the topping cycle is recovered in the
bottoming cycle to produce mechanical/electrical
energy. The most common combined cycle systems
are the combined Brayton Rankine cycle based
systems. The gas turbine plant operates between
300K and 1700K and rejects heat at 800K. The
steam turbine plant in its turn operates between
300K and 750K and rejects heat at 300K. By
combining the cycles, a large part of rejected
heat in topping cycle is used in the bottoming
cycle. The disadvantage of gas topping cycle
consisting in high exhaust temperature becomes
advantage for the steam bottoming cycle. We can
notice in the T-s diagram that the combined cycle
covers a larger area, resulting in a higher
efficiency.

66
67
Brayton - Rankine combined cycle cogeneration
system
Exhaust gases
Heat recovery steam generator
Electricity,
Electricity,
V
Pump
Hot water to process,
Condensate from process
Electricity
1
2
Supplementary burner
Electricity,
Fuel
Fuel
3
Electricity
IV
II
Combustor
III
Generator
Backpressure steam turbine
4
Gas turbine
t water to process,
Steam to process
Electricity,
Generator
Air compressor
I
Air
67
68
The overall efficiency of the Brayton - Rankine
combined cycle cogeneration system

where
Electricity,
Hot water to process,
are the electrical power generate by gas turbine
and steam turbine, respectively
is the heat flow generated by steam system

are the heat flow produced by fuel combustion in
gas turbine and heat recovery steam generator
respectively.
t water to process,
Electricity,
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T-s diagram of combined cycle (Brayton cycle
with Rankine cycle)
Electricity,
Hot water to process,
t water to process,
Electricity,
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Combined cycle cogeneration systems
In order to find what the cycles are suitable for
combination they should be ranked according to
their operating temperature range. The Rankine
and Stirling cycles are suitable both for topping
and bottoming cycle. The Brayton, Otto and Diesel
cycles and also the high temperature fuel cells
can be better used as topping cycles. The Kalina
cycle (a modified Rankine cycle operating with
ammonia-water mixture), organic Rankine cycles
and low temperature fuel cells can be used only
as bottoming cycles.

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Thermodynamic cycles arranged according to their
temperature range of operation
T (C)
2000
Electricity,
Hot water to process,
1500
1000
t water to process,
Electricity,
500
Low temper fuel cells
Organic Rankine cycle
0
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Distributed energy resources
In the last decades a novel technical concept in
energy supply emerged, the distributed/decentraliz
ed energy resources (DER). DER is defined as an
electricity-generation system located in or near
user facilities, which provides electrical and
thermal energy simultaneously to meet local users
in top-priority. They can be divided into two
major sections. The first section includes
high-efficiency cogeneration or combined cooling
and heating (CCHP) systems in industry and
buildings, using prime mover technologies as
reciprocating engines, gas turbines,
micro-turbines, steam turbines, Stirling engines
and fuel cells. The second major area of DER is
on-site renewable energy systems with energy
recycling technologies, including photovoltaic
and biomass systems, on-site wind and water
turbine generators, plus systems powered by gas
pressure reduction, exhaust heat from industrial
processes, and other low energy content
combustibles from various processes. The combined
cooling, heating and power (CCHP) systems are
derived from the cogeneration (combined heating
and power-CHP) systems.

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High-efficiency cogeneration or combined cooling
and heating
In a CCHP system the thermal or
electrical/mechanical energy is further utilized
to provide space or process cooling. The CCHP
systems are known also as trigeneration systems
and as building cooling heating and power (BCHP)
systems. One can say that a cogeneration system
is a CCHP system without any thermally activated
equipment for generating cooling power. Thermally
activated equipment is the equipment that uses
waste heat instead of electricity to provide air
conditioning and/or dehumidification loads such
as absorption chiller, adsorption chiller and
desiccant dehumidifiers. The CCHP systems are
classified into two categories - traditional
large-scale CCHP systems - relatively small
capacity distributed CCHP units with advanced
prime mover and thermally activated equipment to
meet multiple energy demands in commercial,
institutional, residential and small industrial
sectors.

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74
High-efficiency cogeneration or combined cooling
and heating systems
In a CCHP system the thermal or
electrical/mechanical energy is further utilized
to provide space or process cooling. The CCHP
systems are known also as trigeneration systems
and as building cooling heating and power (BCHP)
systems. One can say that a cogeneration system
is a CCHP system without any thermally activated
equipment for generating cooling power. Thermally
activated equipment is the equipment that uses
waste heat instead of electricity to provide air
conditioning and/or dehumidification loads such
as absorption chiller, adsorption chiller and
desiccant dehumidifiers. The CCHP systems are
classified into two categories - traditional
large-scale CCHP systems - relatively small
capacity distributed CCHP units with advanced
prime mover and thermally activated equipment to
meet multiple energy demands in commercial,
institutional, residential and small industrial
sectors.

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75
Distributed cogeneration or combined cooling and
heating systems
The distributed CCHP systems are classified in
accordance with their capacity as
follows - micro systems (capacity under 20
kW) - mini systems (capacity under
500kW) - small scale systems (capacity under
1MW) - medium scale systems (capacity from 1 to
10MW) - large-scale systems (capacity above
10MW). A typical CCHP system comprises the prime
mover, electricity generator, heat recovery
system and thermally activated equipment. The
waste heat from the engine is used to heat up the
domestic water, to generate heating power during
the winter and to drive the adsorption chiller
(for cooling power) during the summer.

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The main advantages of distributed cogeneration
or combined cooling and heating systems
The main advantages of distributed CCHP systems
are high fuel energy utilization low emission
increased reliability of the energy supply
network. The prime mover selected to meet diverse
demands and limitations can be steam turbines,
reciprocating internal combustion engines,
combustion turbines, microturbines, Stirling
engines and fuel cells. The thermally activated
systems include absorption chillers, adsorption
chillers and desiccant dehumidifiers. The diagram
below is useful in choosing the cogeneration
system for a given application (electrical power
demand).

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Schematic diagram of a micro combined cooling,
heating and power system
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Distribution of main cogeneration systems
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References
  1. Home page of Directorate-General for Energy
    http//ec.europa.eu/energy
  2. EDUCOGEN, The European Educational Tool on
    Cogeneration Second Edition, December 2001.
  3. A. Thumann, Plant engineers and managers guide to
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  5. C. Beggs, Energy Management, Supply and
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  8. United States Environmental Protection Agency.
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    http//www.epa.gov/chp/
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  11. EG G Services Parsons, Inc., Science
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Questions?

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