TRIGENERATION

A PROJECT REPORT
ON
TRIGENERATION


Birla Institute of Technology and Science - Pilani,
Pilani Campus

PREPARED UNDER THE SUPERVISION OF
MR. DILEEP KUMAR GUPTA
(DEPARTMENT OF MECHANICAL ENGINEERING)


SUBMITTED BY

SHREYAS SAUMITRA 2011ABPS572P
RACHIT GOYAL 2011ABPS619P
INDEX

1. Introduction
2. History
3. Cogeneration
4. Tri-generation
5. Generic Tri-generation System
6. Conclusion
7. References


1. Introduction
Thermal power plants (including those that use fissile elements or burn coal, petroleum, or
natural gas), and heat engines in general, do not convert all of their thermal energy into
electricity. In most heat engines, a bit more than half is lost as excess heat (see: Second law
of thermodynamics and Carnot's theorem). By capturing the excess heat, CHP uses heat that
would be wasted in a conventional power plant, potentially reaching an efficiency of up to
80%, for the best conventional plants. This means that less fuel needs to be consumed to
produce the same amount of useful energy.
Steam turbines for cogeneration are designed for extraction of steam at lower pressures
after it has passed through a number of turbine stages, or they may be designed for final
exhaust at back pressure (non-condensing), or both. A typical power generation turbine in a
paper mill may have extraction pressures of 160 psig (1.103 MPa) and 60 psig (0.41 MPa). A
typical back pressure may be 60 psig (0.41 MPa). In practice these pressures are custom
designed for each facility. The extracted or exhaust steam is used for process heating, such
as drying paper, evaporation, heat for chemical reactions or distillation. Steam at ordinary
process heating conditions still has a considerable amount of enthalpy that could be used
for power generation, so cogeneration has lost opportunity cost. Conversely, simply
generating steam at process pressure instead of high enough pressure to generate power at
the top end also has lost opportunity cost. The capital and operating cost of high pressure
boilers, turbines and generators are substantial, and this equipment is normally operated
continuously, which usually limits self-generated power to large scale operations.


A cogeneration plant in Metz, France. The 45MW boiler uses waste wood biomass as energy
source, and provides electricity and heat for 30,000 dwellings.
Some tri-cycle plants have used a combined cycle in which several thermodynamic cycles
produced electricity, then a heating system was used as a condenser of the power plant's
bottoming cycle. For example, the RU-25 MHD generator in Moscow heated a boiler for a
conventional steam power plant, whose condensate was then used for space heat. A more
modern system might use a gas turbine powered by natural gas, whose exhaust powers a
steam plant, whose condensate provides heat. Tri-cycle plants can have thermal efficiencies
above 80%.
The viability of CHP (sometimes termed utilization factor), especially in smaller CHP
installations, depends on a good base load of operation, both in terms of an on-site (or near
site) electrical demand and heat demand. In practice, an exact match between the heat and
electricity needs rarely exists. A CHP plant can either meet the need for heat (heat driven
operation) or be run as a power plant with some use of its waste heat, the latter being less
advantageous in terms of its utilizations factor and thus its overall efficiency. The viability
can be greatly increased where opportunities for Tri-generation exist. In such cases, the
heat from the CHP plant is also used as a primary energy source to deliver cooling by means
of an absorption chiller.
CHP is most efficient when heat can be used on-site or very close to it. Overall efficiency is
reduced when the heat must be transported over longer distances. This requires heavily
insulated pipes, which are expensive and inefficient; whereas electricity can be transmitted
along a comparatively simple wire, and over much longer distances for the same energy
loss.
A car engine becomes a CHP plant in winter when the reject heat is useful for warming the
interior of the vehicle. The example illustrates the point that deployment of CHP depends on
heat uses in the vicinity of the heat engine.
Thermally enhanced oil recovery (TEOR) plants often produce a substantial amount of
excess electricity. After generating electricity, these plants pump leftover steam into heavy
oil wells so that the oil will flow more easily, increasing production. TEOR cogeneration
plants in Kern County, California produce so much electricity that it cannot all be used
locally and is transmitted to Los Angeles[citation needed.
CHP is one of the most cost-efficient methods of reducing carbon emissions from heating
systems in cold climates and is recognized to be the most energy efficient method of
transforming energy from fossil fuels or biomass into electric power. Cogeneration plants
are commonly found in district heating systems of cities, central heating systems from
buildings, hospitals, prisons and are commonly used in the industry in thermal production
processes for process water, cooling, steam production or CO2 fertilization.
Tri-generation is the production of coldness alongside heat and electricity from the sys- tem
with the purpose of using cogeneration technology more efficiently. An absorption
refrigeration system can be operated utilizing the waste energy of a cogeneration system,
or the systems can be combined by the method of obtaining the shaft power directly from
the motor or turbine that is necessary for a refrigeration cycle with vapor compression.
Another method is the production of coldness with an electrical conventional machine
that works totally independent of turning components. Among these methods, the most
common and efficient one is the absorption refrigeration system

2. History
Tri-Generation technology was first conceived at the NFCRC in 2002 and then developed
further through research and collaboration with Air Products & Chemicals, Inc. and Fuel Cell
Energy, Inc., eventually leading to the current demonstration at the Orange County
Sanitation District. The partners involved in the program include Air Products and Chemicals,
Fuel Cell Energy, the U.S. Department of Energy, the California Air Resources Board, South
Coast Air Quality Management District, and the Southern California Gas Company. During he
past decade, natural gas combined cycles has become the technology of choice for new and
replacement power plant in the EU. The technology is also widely applied in the USA.
Cogeneration in Europe
The EU has actively incorporated cogeneration into its energy policy via the CHP Directive. In
September 2008 at a hearing of the European Parliament’s Urban Lodgment Intergroup,
Energy Commissioner Andris Piebalgs is quoted as saying, “security of supply really starts
with energy efficiency.”[35] Energy efficiency and cogeneration are recognized in the
opening paragraphs of the European Union’s Cogeneration Directive 2004/08/EC. This
directive intends to support cogeneration and establish a method for calculating
cogeneration abilities per country. The development of cogeneration has been very uneven
over the years and has been dominated throughout the last decades by national
circumstances.As a whole, the European Union generates 11% of its electricity using
cogeneration, saving Europe an estimated 35 Mtoe per annum a day.[36] However, there is
large difference between Member States with variations of the energy savings between 2%
and 60%. Europe has the three countries with the world’s most intensive cogeneration
economies: Denmark, the Netherlands and Finland.[37] Of the 28.46 TWh of electrical
power generated by conventional thermal power plants in Finland in 2012, 81.80% was
cogeneration.Other European countries are also making great efforts to increase efficiency.
Germany reported that at present, over 50% of the country’s total electricity demand could
be provided through cogeneration. So far, Germany has set the target to double its
electricity cogeneration from 12.5% of the country’s electricity to 25% of the country’s
electricity by 2020 and has passed supporting legislation accordingly.[39] The UK is also
actively supporting combined heat and power. In light of UK’s goal to achieve a 60%
reduction in carbon dioxide emissions by 2050, the government has set the target to source
at least 15% of its government electricity use from CHP by 2010.[40] Other UK measures to
encourage CHP growth are financial incentives, grant support, a greater regulatory
framework, and government leadership and partnership.
According to the IEA 2008 modeling of cogeneration expansion for the G8 countries, the
expansion of cogeneration in France, Germany, Italy and the UK alone would effectively
double the existing primary fuel savings by 2030. This would increase Europe’s savings from
today’s 155.69 Twh to 465 Twh in 2030. It would also result in a 16% to 29% increase in each
country’s total cogenerated electricity by 2030.Governments are being assisted in their CHP
endeavors by organizations like COGEN Europe who serve as an information hub for the
most recent updates within Europe’s energy policy. COGEN is Europe’s umbrella
organization representing the interests of the cogeneration industry.The European public–
private partnership Fuel Cells and Hydrogen Joint Undertaking Seventh Framework
Programme project ene.field deploys in 2017 up 1,000 residential fuel cell Combined Heat
and Power (micro-CHP) installations in 12 states. Per 2012 the first 2 installations have taken
place.
Cogeneration in the United States
Perhaps the first modern use of energy recycling was done by Thomas Edison. His 1882
Pearl Street Station, the world’s first commercial power plant, was a combined heat and
power plant, producing both electricity and thermal energy while using waste heat to warm
neighboring buildings.[45] Recycling allowed Edison’s plant to achieve approximately 50
percent efficiency.
.
By the early 1900s, regulations emerged to promote rural electrification through the
construction of centralized plants managed by regional utilities. These regulations not only
promoted electrification throughout the countryside, but they also discouraged
decentralized power generation, such as cogeneration. As Recycled Energy Development
CEO Sean Casten testified to Congress, they even went so far as to make it illegal for non-
utilities to sell power.
By 1978, Congress recognized that efficiency at central power plants had stagnated and
sought to encourage improved efficiency with the Public Utility Regulatory Policies Act
(PURPA), which encouraged utilities to buy power from other energy producers.

2.1 Current status, manufacturers and installations

1. The combined cycle plants have a smaller polluting effect due to a better use of the
final energy cycle.
2. The plants are designed to support the load variations.
3. The installation can be completed in two phases: the gas turbine subsystem is
installed first, which can be ready for operation in 12-18 months. While this is in
operation, the steam subsystem is installed. The installation period is of 2 – 3 years,
and the life cycle is about 15 – 25 years.


2.2 Manufacturers

There are no manufacturers specialised only on combined cycles. Manufacturers of
gas turbines and steam turbines (that in some cases are the same) provide and install also
combined cycles. So the list of references of manufacturers for steam and gas turbines are
also valid for combined cycles. Some of these manufacturers provide turn key solutions for
combined cycles, such as, Alstom, Siemens or Turbomach.
It is not usual to find combined cycles based on reciprocating engines instead of gas
turbines, but some manufacturers of large reciprocating engines such as Wärtsilä have some
references of these systems.

2.3 Expectations and Timeline

To overcome the barriers, attention must be paid to:
1. Raising the efficiencies of gas turbines. This may be achieved through:
2. Developments that allow them to operate at higher temperatures, ie improving new
materials and advanced blade-cooling techniques
3. Improving design aspects, e.g. to minimize the performance of existing materials and
developing thermal barrier coatings aerodynamic losses.
4. Continued work aimed at reducing both capital and plant operating costs.
5. Reducing emissions, particularly those of NOx.
6. Improving the efficiency of part-load operation.
7. Turbine burner development for the use of unconventional fuels, e.g. coal gas and
gasified biomass, and in combination with natural gas.
8. Combustion developments to reduce NOx emissions, e.g. by staged and catalytic
combustion.

3. Cogeneration

Cogeneration is the simultaneous generation of usable heat and power (usually electricity)
in a single process. Useful outputs can be more varied: increasingly, heat is being used to
drive absorption chilling, and in some cases power can be mechanical power, for example to
drive a compressor. The term cogeneration is synonymous with combined heat and power
(CHP). Cogeneration uses a variety of fuels and technologies across a wide range of sites,
and schemes sizes. The basic elements of a cogeneration plant comprise one or more prime
movers (a reciprocating engine, gas turbine, or steam turbine) driving electrical generators,
or other machinery, where the steam or hot water generated in the process is utilised via
suitable heat recovery equipment for use either in industrial processes, or in community
heating and space heating. Whereas an electricity-only plant is typically large, and
connected at very high voltage to the grid transmission system, a cogeneration plant is
typically much smaller, attached to a site which consumes the heat and power produced (or
a large proportion of it), is sized to make use of the available heat, and connected to the
lower voltage distribution system.


Cogeneration is more efficient through utilisation of heat, but it also avoids significant
transmission and distribution losses, and can provide important network services such as
the ability to continue to supply the site if the grid goes down. A cogeneration system can
provide power for remote locations and generate cost efficient onsite power. Cogeneration
typically achieves 25 to 35 % reduction in primary energy usage compared with electricity-
only generation and heat-only boilers. This can allow the host organisation to make
substantial savings in costs and emissions where there is a suitable heat load.



















4. Tri-generation


Tri-generation, the simultaneous production of electricity, heat and refrigeration from a
primary source of energy, such as natural gas or bio-fuel, is a natural extension of co-
generation. From a strictly thermodynamic viewpoint, a tri-generation system is simply a
traditional combined heat and power (CHP) system plus absorption and/or a vapour or
compression chiller (CCHP combined cooling heating and power). However, the advantages
of tri-generation, such as primary energy savings and greater overall efficiency have, in
recent years, attracted authors, researchers and the construction community [1–11]. Tri-
generation is clearly of importance in connection with pollution control. According to
Meunier who studied the impact of co- and tri-generation on the environment, CO2
emissions could be reduced by at least40% if sorption heat pump technology were to be
developed.

Moreover, tri-generation plants are economically viable in situations where electric energy
is scarce and/or costly. Despite the attractiveness of tri-generation as an energy-integrated
scheme, new energy demands arise, implying further operational constraints not present in
co-generation. In most tri-generation systems heat, refrigeration and electricity are
produced by a combination of an absorption chiller and a Diesel or gas turbine generator.
While the choice of a Diesel engine or gas turbine as a prime mover is dictated by a
thermodynamic cost–benefit relation, the possibility of transforming low
grade heat from CHP units into cold has made absorption chillers an almost indispensable
component of most commercial applications. Absorption chillers offer good partial load
performance, low maintenance and high availability and this may account for the fact that
most research in the area has concentrated on tri-generation with absorption chillers. Some
investigations have focused on the food industry and supermarkets where items must be
kept chilled or frozen in refrigerated cabinets throughout the year, subject to appreciable
seasonal variations. These investigations highlighted the possible gains to be had from this
new energy technology. However, less attention seems to be paid to the more important
problem of the assessment of thermodynamic performance. After studying tri-generation
plants consisting of gas turbines and internal combustion engines driving ammonia-water
absorption chillers an index of electric equivalent efficiency without, however, taking into
account the various modes of energy encountered in tri-generation. Nevertheless, they
confirmed the expected superiority of internal combustion engines over gas turbines by
comparing the cold produced by tri-generation with that generated by a conventional
compression chiller plant.

5. Generic tri-generation system
5.1 System description

Figure below shows that a heat engine, which may be a gas turbine, a reciprocating internal
combustion engine or even a Stirling engine, drives an electric generator. Electricity from
the generator is used to power the vapour compression chiller which, apart from producing
cold, rejects heat from the condenser. The electricity demand is met by the surplus of
electricity production from the generator whereas the heat demand is met by recovering
heat from the heat pump and from the heat engine exhaust gases and, in the case of
reciprocating engines, from the coolant system as well.



A heat recovery boiler extracts rejected heat from the engine exhaust gases, supplying
steam to a steam turbine which, in turn, drives a vapour compressor chiller. Heat is
recovered from the condensers of the three heat pumps. Such a generic system will
hopefully be of use in the assessment of real systems.
In this generic case, it is assumed that the temperature at which heat is recovered from the
engine coolant is high enough to drive the absorption chiller. In the case of a gas turbine,
heat is extracted from the exhaust only, as there is no coolant fraction, and the two values
of take a different representation, corresponding to the fuel energy fractions that go to the
heat recovery boiler and to the absorption chiller. The efficiency of the heat recovery boiler
is.
The corresponding Rankine cycle, required for the conversion of steam from the heat
recovery boiler to the steam turbine shaft work, is not represented in Figure below. Each
heat pump, the electrically driven vapour compression, the steam turbine driven vapour
compression and the absorption chillers, are each represented by a pair of COPs, for heating
and cooling.

Again, in contrast to the case of an ideal heat pump, the difference between the two COPs,
heating and cooling, may not be equal to unity, due to the heat losses and gains. The
diagram below shows that the tri-generation scheme runs entirely on a single energy source
(denoted by “fuel”) and has three so-called energy products, namely heat, refrigeration and
electricity.


5.2 Thermodynamic model

The following assumptions are made:
(a) The vapour compression and absorption chillers are sized to supply the entire
cooling load;
(b) The heat engine/electric generator compound is designed to provide sufficient
power to meet the electricity demand and to drive the heat pump compressor;
(c) The temperature at which rejected heat is recovered is sufficiently high to meet the
heat load demand and to drive the absorption chiller. These assumptions deserve
some comments. A peak boiler is
Indispensable in meeting the heat demand as all (energy) demands are hardly ever met at
the same. Furthermore, only a high temperature heat rejection is

The energy conversion ratio, ECR, is defined as the total energy delivery (heat, electricity
and refrigeration) divided by energy input, i.e., the fuel burned in the engine and in the peak
boiler:


Two other dimensionless ratios, RHE and RCE, compare the magnitudes of the heating,
cooling and electricity loads:


Using the fact that total fuel consumption is distributed between the peak boiler and the
engine, both burning the same type of fuel:

One has:

And, from Equations. (2) and (3):

The heating load is met by the total heat recovered and by the peak boiler:

Since heat is recovered from the condensers of the absorption and vapour compression
heat pumps. The total heat recovery rate is given by:

For the peak boiler,

Rejected heat is recovered from the engine coolant (to drive the absorption chiller) and
exhaust gases (to steam turbine) at rates that depend on the energy balance of the heat
engine and on the heat recovery efficiencies of both engine exhaust boiler and coolant heat
exchange so that:


Electric power supplied to one of the vapour compression chillers, and both
cooling and heating power are obtained from the evaporator and condenser,
respectively. The cooling and heating coefficients of performance describe the
energy balance in the electrically driven vapour compression heat pump
cycle:

Taking into account the fraction of fuel energy that goes to power shaft in the heat engine,
and the efficiency of the electric generator, the compressor electrical power consumption is
given by:

From Equations . (13)–(15), the cooling capacity and condenser power output of the
electrically driven chiller are, respectively:

For the steam turbine driven vapour compression chiller, similar equations can be written:

For the heat-driven absorption chiller, the equations become:

The cooling load is provided by the three chillers:

Summing the three equations for 16), (18) and (20), one has:


where an overall cooling efficiency factor, can be defined as:



where an overall heat pump heating efficiency factor,˙h, is equally defined:



Taking Equations. (24)–(28) into Eq. (6), a final expression for ECR is obtained, in terms of
the load ratios, RHE and RCE, and the characteristic parameters of the system components:


The amount of fuel burnt to meet all three loads (electricity, heating and refrigeration) can
be represented by the energy rate
Equivalent of fuel consumption to electricity load ratio, RFE, defined as:


Heat demand less than heat recovered
If the heat load is less than or equal to the total heat recoverable,
the peak boiler can be dispensed with (this corresponds to ˙Qpb = 0and, consequently, ˙Hpb
= 0). Then:

From Eq. (27), the range of RHE where the heating demand, ˙Qhe is less than the heat
recovered is given by:




Minimum cooling load
For a given electric load, in a power-matched system, a certain amount of cooling effect will
be present from the steam driven
and absorption chillers, for they are driven by the heat engine waste heat (exhaust and
coolant, respectively). If the demand for
cooling power is greater than the cooling power produced by the waste heat-driven chillers
systems (steam turbine and absorption),then the electrically driven vapour compression
chiller comes into play. This lower bound for the cooling load is defined by

In this situation, all electricity produced goes to the electrical power load, so that, from
Equations. (18) and (20),making the compressor power equal to zero, Eq. (15):

Thermal efficiency
Every heat engine is subject to the theoretical efficiency limits of the Carnot cycle. When the
fuel is natural gas, a gas turbine following the Rankine cycle is typically used.
[13]
Mechanical
energy from the turbine drives an electric generator. The low-grade (i.e. low
temperature) waste heat rejected by the turbine is then applied to space heating or cooling
or to industrial processes. Cooling is achieved by passing the waste heat to an absorption
chiller.
Thermal efficiency in a tri-generation system is defined as:

Where:
= Thermal efficiency
= Total work output by all systems
= Total heat input into the system
Typical tri-generation models have losses as in any system. The energy distribution below is
represented as a percent of total input energy:
[14]

Electricity = 30%
Heat + Cooling = 55%
Heat Losses = 13%
Line Losses = 2%


Conventional central coal- or nuclear-powered power stations convert only about 33% of
their input heat to electricity. The remaining 67% emerges from the turbines as low-grade
waste heat with no significant local uses so it is usually rejected to the environment. These
low conversion efficiencies strongly suggest that productive uses be found for this waste
heat, and in some countries these plants do produce by product steam that can be sold to
customers.
But if no practical uses can be found for the waste heat from a central power station, e.g.,
due to distance from potential customers, then moving generation to where the waste heat
can find uses may be of great benefit. Even though the efficiency of a small distributed
electrical generator may be lower than a large central power plant, the use of its waste heat
for local heating and cooling can result in an overall use of the primary fuel supply as great
as 80%. This provides substantial financial and environmental benefits.


5.3 Results

Eqs. (29) and (34) were applied for a set of typical data



Fig. 2 depicts the variation of the energy conversion ratio as a function of RHE and RCE. It
can be seen that, for a given cooling-to-electricity load ratio, there is a certain value for the
heating-to-electricity load ratio for which an optimum value for ECR is obtained.


It corresponds to the situation where the heat load is exactly the recovered heat from the
chillers. As explained before, for greater values of RHE, a less efficient heating solution (the
peak boiler) is necessary. For lower values, excess waste heat is produced from the fuel,
with no use for it. For greater values of RCE, the heat engine is set to operate at greater
generator loads (to cope with the chiller compressor power) and a large amount of
recovered waste heat is available, thus eclipsing the less efficient effect of the peak boiler
operation.


Fig. 3 displays the variation of RFE, related to fuel consumption, with cooling and heating
loads, RCE and RHE. For the same reasons described in the preceding paragraph, RFE
remains constant for values of RHE. Above it, RFE increases linearly, with Eq. (35).




6. Conclusion

Tri-generation may play a significant role in the international effort (on the part of the
signatories of the Kyoto Protocol) to reduce CO2 and other greenhouse gases emissions,
since the highly integrated character of tri-generation implies higher energy conversion
ratios and hence lower pollutant emissions.


Furthermore, tri generation is sufficiently flexible to permit installation in locations where
electricity from the national grid is, for some reason, not available. Given the various
conflicting design criteria such as cost, efficiency and environmental impact, plant designers
need some sort of “thermodynamic yardstick” to select the best or the most appropriate
plant configuration. The first-law analysis presented here for a generic tri-generation system
accurately reflects the energy conversion efficiency of the case studied but can, in fact, be
applied to any tri-generation system. The introduction of non-dimensional parameters
relating cooling and heating demands and fuel energy-equivalent consumption to the
electrical load demand, RCE, RHE and RFE, has proven to be an adequate approach that can
be extended to second-law or thermo economic analyses. The present study is in no way
comprehensive, but thermal engineers may find the method of value when sizing and
choosing thermal equipment for buildings and other systems.




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