NORWEGIAN UNIVERSITY OF SCIENCE AND TECHONOLOGY DEPARTMENT OF PETROLEUM ENGINEERING AND APPLIED GEOPHYSICS

Natural Gas in Trigeneration

Generation of Electricity, Heat and Cooling

Oluwatosin Ajayi

Lorenzo Angelo Veronelli

Davide Genini

Hui-Gyeong Jang

Ho Jung Jung

TPG4140 Natural gas

Trondheim, November 2011

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Abstract

This work presents a technical illustration of the operation of Trigeneration systems using

Natural Gas as fuel and outlines the possibilities for future Trigeneration systems powered

by fuel cells. It demonstrates these possibilities by comparing the feasibility of Trigeneration

systems based on fuel cell with Trigeneration systems using conventional fossil fuel. Primary

Energy Saving index and first law efficiency were solved to demostrate that Trigeneration

Systems are visible solutions to the spiralling demand for energy across the globe. Although

not well explored at present but with many developed plans and new researches for

trigeneration underway, Trigenration systems signal themselves as a viable energy solution

in the very near future.

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Content

Abstract ..................................................................................................................................i

1. Introduction ....................................................................................................................1

2. Reasons for trigeneration ...............................................................................................2

3. How it works ...................................................................................................................3

4. Gas turbines ....................................................................................................................3

4.1 Operating principle ..............................................................................................................3

4.2 Technical components .........................................................................................................4

4.3 Emissions.............................................................................................................................5

4.4 State-of-art ..........................................................................................................................6

5. Internal combustion engine ............................................................................................6

5.1 Operating principle ..............................................................................................................6

5.2 Technical Components ........................................................................................................7

6. Absorption chiller ...........................................................................................................7

6.1 Compressor chiller and absorption chiller ............................................................................8

7. Fuel cell application ........................................................................................................9

7.1 Operating principle ..............................................................................................................9

7.2 High temperature fuel cells................................................................................................ 10

7.3 Hybrid systems: gas turbine and fuel cells .......................................................................... 10

7.4 Solid Oxide Fuel Cell .......................................................................................................... 11

7.5 Molten Carbonate Fuel Cell ............................................................................................... 11

7.6 Internal reforming ............................................................................................................. 11

7.7 Fuel cells versus traditional combustion engines and small gas turbines in cogeneration ... 12

8. Exergy analysis ..............................................................................................................12

9. Indices for trigeneration and cogeneration ..................................................................13

10. Trigeneration and cogeneration now and in the future................................................15

10.1 Pfizer Singapore API manufacturing facility ........................................................................ 17

11. Conclusions ...................................................................................................................18

12. References ....................................................................................................................19

13. Tables ...........................................................................................................................21

14. Figures ..........................................................................................................................23

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1. Introduction

Trigeneration is the combined generation of electricity, heat and cooling, all simultaneously

produced from a fuel source often referred to as Combined Heat Power and Cooling CHCP.

Trigeneration takes cogeneration of heat and electricity further with the utilization of waste

heat for cooling purposes through an absorption chiller. A Trigeneration system is an

integration of two major technologies: The combined heating and power CHP or

cogeneration technology and cooling technology through compression or absorption

systems. CHP technologies based on gas reciprocating engines and combustion turbines are

the most mature technologies. Fuel cells are entering into the market of Trigeneration.

Natural Gas is the most appealing fuel for driving Trigeneration Systems because of its

reliability, efficiency, low environmental effects and low maintenance costs. It burns

efficiently in the combustor ensuring lower emissions of local pollutants than heavier fuels.

Natural Gas contains mainly methane, a gas with high hydrogen to carbon ratio which leads

to lower CO2 emissions per unit of energy produced. According to the U.S. Department of

Energy in the year 2009, 2.5 billion tons of CO2 were emitted by power plants in the U.S.,

which correspond to 576g of CO2 per kWh. A wide use of Trigeneration would reduce the

amount of green- house gases emitted per unit of electricity.

The most intriguing development in the quest for efficient and cost saving Trigeneration

systems to match Energy Demand is the possibility of using Fuel Cells as alternative engine

for Trigeneration systems. A technical analysis shows that fuel cells provide the next

possibility for making Trigeneration System at a very low operating cost, maintained high

efficiency, with no waste nuisance to the environment (Casalegno 2010). It presents an

environmentally clean technology for the future Trigeneration Applications. Fuel cells can be

fed via syngas produced with steam reforming CH4 + H2O → CO + 3 H2 and Water gas shift CO +

H2O → CO2 + H2.

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2. Reasons for trigeneration

Energy cost is growing, and trigeneration technology in the long term offers a cheaper and

affordable technology for producing energy when compared to other conventional energy

generating technologies. The reduction in cost in trigeneration is achieved with higher

overall cycle efficiency which decreases the amount of fuel used to produce one unit of

usable energy. Governments also offer subsidies to energy made in cogeneration

comparable to the ones given to renewable energy, making the investment more profitable.

Microtrigeneration is becoming common in warm countries that are developing the idea of

“distributed power generation” (small machines placed close to the consumers). The

application of cogeneration and trigeneration as well, in residential places has always been

obstructed by the high variability of loadings and humongous costs of long thermal energy

networks. Distributed power generation and microtrigeneration go together because the

trigenerative application compensates the inevitable lower efficiency of the small machines

and the higher costs. Distribute generation can be applied in crowded places where the

structures are shared by many people, and the cost of the insulated pipes to transport heat

is acceptable because they don’t have to be too long.

Distributed power generation diminishes the transport losses, since energy does not have to

travel long distances to reach the customers. With the increased need of energy in

populated areas new power lines have to be built to transfer the power from the generation

sites in a business as usual scenario. Dispersed power generation can avoid the invasion of

pristine areas by new power lines, preserving the environment and saving money.

The technology challenge in developed countries is the reduction of air pollution and

greenhouse gases. The application of Trigeneration in cities is an effective way to solve this

challenge because of the use of clean fuel such as natural gas and the high efficiency of the

system. Pollution in populated areas of developing countries is a huge problem which at the

moment is not taken into account, but it will be in the near future.

Trigeneration systems have usually very short start-up times because of their small

dimensions and low thermal inertia. Therefore, they can also be used for peak shaving to

help the grid to handle the rising amount of renewable energy connected to the net. Such

dispersed systems can be remotely controlled, operated by the network management

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company as an integration of the grid. The ability of the system to store thermal energy

allows a flexible management of electricity production, giving the opportunity to make

electrical power when it’s required by the grid.

3. How it works

Trigeneration is a new form of power generation that is becoming common in numerous

countries placed in the warm region of the world. In these countries the heating required

during the year is mainly concentrated in the winter season, while in summer the demand of

refrigeration is no more negligible (for air conditioning – household or industry), as shown in

figure 3. A constant demand of electrical power, heating and cooling comes from different

structures such as hospitals, public buildings, universities, shopping malls and gyms.

A Trigeneration plant is similar to a cogeneration power plant plus an absorption chiller to

produce a cold flow with the heat recovered from the hot flue gases. Regarding the electric

power generation, it can be provided by different kind of engines: internal combustion

engines, gas turbine cycles, and Stirling engines (to name a few). They can be evaluated

according to cost, efficiency and environmental effects.

4. Gas turbines

4.1 Operating principle

In micro turbines, electricity comes from a common Joule-Brayton cycle fitted with a

regenerator. Air is sucked up by a compressor that can work with a lower pressure ratio than

usual, just between 2 and 12 (G. Lozza, 2006). A combustor burns the fuel and presents flue

gas at 1000 °C to the first rotator of the expander. After the expander, the exhaust gas

enters to a regenerator to recover some heat by warming up the air coming out of the

compressor. This is a practice that is required to elevate the efficiency, which is deeply

affected by the temperature of exhausted gas that is again affected by the pressure ratio. If

the pressure ratio goes down, the temperature of the flue gas goes up and the efficiency

lessens. A Pressure-Volume and a Temperature-Entropy graph are attached (Figures 4.1a-

4.1b).

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The heating is obtained by cooling down the exhausted gas coming from the regenerator in a

gas-fluid heat exchanger. Here water can be warmed up to 90 °C-115 °C (M. Sileo, 2006), so

the ability of producing steam is not very high in micro turbines with regenerators. It is

evident that this heat is useful where the thermal demand is at low temperature like in

residential buildings, hotels and sport structures. The thermal efficiency is around 50 %,

while the electrical efficiency is approximately 30% considering the energy coming from the

Low Heating Value of the fuel used. Therefore, approximately 80% of the energy in the fuel

is used.

The cooling is provided by an absorption chiller. This device is based on the phase change of

water together with a specific salt. The low temperature that the water can reach when is

warmed up by the exhausted gas is enough in order to make this system working properly.

4.2 Technical components

Micro turbines could be still out of the market if the design of the machine had not been

completely altered. They are characterized by radial machines that work at an impressively

high RPM to ensure good performances keeping the dimensionless parameters in an optimal

range.

The turbo machines (expander and compressor) have been dramatically modified to face

different needs in respect of common large gas turbines (G. Lozza, 2006). They have to

rotate at 70000-120000 RPM, sustained by magnetic bearings because the low power

produced requires treating low flow rates of air and exhausted gas (0.2-0.5 kg/s). These

radial velocities come out considering that the peripheral speed (u = ω*r) is limited by

material resistance: if the radius(r) is small, then the angular speed (ω) has to be high.

Furthermore, from performance optimization analysis, it’s understandable that a high RPM

is necessary. The small radius forces to choose a centrifugal compressor and a centripetal

expander that are able to cope with high pressure ratios (4-6) with a single stage, providing

good performances even with a small rotor. Considering the relatively low temperatures

(950 °C), the rotators can be made up of nickel alloys and they don’t need any cooling

systems.

Small pressure ratio causes a high temperature of the exhausted gas released in the

atmosphere while the inlet temperature of the combustor is low: the efficiency is negatively

affected. The adoption of radial single stage machines implies smaller pressure ratio than

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usual. For example, with a pressure ratio of 4, the outlet temperature of the flue gas is 710

°C and the inlet temperature of the combustor only 184 °C with an efficiency of 16, 45 %. To

overcome this problem, a common solution is the regenerator, which is a decisive device in

the development of a micro turbine. Regenerator can be of two types: a surface regenerator

or a rotary matrix regenerator. The former is a common exchanger with a physical

separation between pressurized air and exhausted gas; it has a particular geometry

optimized to improve forced convection. The latter is based on a package of metal or

ceramic material rotating slowly, that acquires heat when is on the hot side and releases

heat to the air in the cold side. This system provides a high thermal exchange efficiency (85-

90 %), reduced costs (because of compact surfaces and long life) and space, but it must be

taken into account that the two fluids could contact each other.

The combustor is not very different from the combustor of a common gas turbine cycle. The

only difference from usual combustors comes from the opportunity of reaching low NOx

emissions, since lower combustion temperatures reduce the NOx formation. This helps save

money because no emissions treating system is required. Usually the combustor of a micro

turbine emits 10-15 of NOx which is ten times lower than a common gas turbine

(G. Lozza, 2006).

The generator is designed in order to avoid the use of gear-reducers to improve the

efficiency. For these reasons, it is usually equipped with permanent magnets incorporated in

carbon fiber matrices, and it rotates together with the shaft of the expander producing

electrical energy at high frequency (for example 3000 Hz AC with 90000 RPM 4 poles). Then

this energy is converted in a static rectifier and carried to 50 Hz (or 60 Hz) tri-phase 400V by

a static inverter (G. Lozza, 2006). Usually the generator can work at variable speeds: this

peculiarity prevents the remarkable decline of performance typical of gas turbines at partial

loads. This is a noticeable property of a trigeneration system, because it makes easier to

follow the loads imposed by consumers. The efficiency of the generator is usually close to

92-94 %.

4.3 Emissions

Combustion in a gas turbine cycle is designed for reducing NOx emissions. The combustor

works with a great excess of air that quenches down the flame temperature. A low

temperature inhibits the formation of nitrogen oxides, while the excess of air prevents the

formation of CO and unburned gas (G. Lozza, 2006). According to this condition the gas

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turbine doesn’t need any other device to reduce emissions, but catalysts to improve the

environment effect are being studied. (Table 4.3)

4.4 State-of-art

Some international-famed societies have been developing micro turbines for some years

with good results. General Electric, Honeywell and Siemens have commercialized machines

for 30-250 kW, with an electrical efficiency of 24-30 % and Turbine Inlet Temperature of

1000 °C (Table 4.4). At the moment these turbines offer a good availability and reliability

even after long working time. According to Capstone- one of the most important companies

in the field, a micro turbine costs about 1300$ per kW installed. And the forecasts are for a

sharp decrease in prices.

5. Internal combustion engine

5.1 Operating principle

For trigenerative applications only a four-stroke engine can be used, that can be based both

on an Otto cycle or Diesel cycle. The Otto cycle is made up of four transformations: two

isochoric and two isentropic processes. The piston goes from the bottom dead center to the

top dead center causing a high increase of pressure then combustion takes place. Then the

piston does the opposite movement of before producing work and finally the exhausted gas

goes out from the cylinder. In reality, two other operations take place: the exhausted gases

are expelled through a drain valve and the fuel-air mixture is sucked up by an inlet valve.

The Diesel cycle ideally differs from the Otto cycle only because the combustion should

occur at constant pressure instead of constant volume.

The Otto cycle engines fuelled with natural gas have pressure ratios oscillating between 9:1

and 12:1 similar to gasoline engines, even if natural gas has a higher antiknock (M. Sileo,

2006) . The gas is injected into the carburetor forming required stoichiometric mixture which

is compressed in the cylinder.

The Diesel cycle engines are “dual fuel”, they are mainly fed with methane with a small

addition of gas oil to avoid detonation. The gas oil is usually injected at high pressure. The

gas can follow two ways: direct injection at high pressure, or injection in the collector and

then compression as in an Otto cycle. The choice among the two solutions depends on the

gas pressure in the distribution network: if it’s at low pressure the direct injection is better

to avoid expenses and maintenance related to a compressor to pressurize gas.

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5.2 Technical Components

The internal combustion engines can use a wide variety of fuels both liquid (gas oil, gasoline,

heavy oil) and gas (natural gas, propane, and biogas). It’s not easy modeling the emissions

coming from an internal combustion engine. This is because of numerous parameters that

affect emissions in different ways: piston movement, passage of the combustion from

laminar to turbulent, low wall temperature. Generally, the main pollutants emitted are NOx,

HC, soot and CO.

An internal combustion engine for a trigeneration system using natural gas ensures low

emissions, but the environmental norms are actually very strict and will get stricter in the

future. Therefore, this engine requires proper devices for reduction of emissions. These

devices are different according to the type of cycle used.

Inside the exhaust pipes, it’s common practice using systems that react with catalysts to

reduce the emissions. When air and fuel are mixed in a stoichiometric ratio, like in Otto’s

cycles, trivalent catalysts are used. They are called trivalent, because they can reduce

contemporaneously emissions of three pollutants: NOx, CO, HC. To guarantee that this

catalyst works, a strong control of stoichiometry is necessary. To do that a lambda sensor

measures the percentage of O2 in the exhaust gases, and a feedback control system

regulates the percentage of O2.

6. Absorption chiller

Absorption chillers are a practical alternative to compression chillers. Their main advantage

is that they don’t require any electrical power consumption except for the pump moving the

solutes.

An absorption chiller works with a mixture of two fluids. The fluid with the lowest vapor

pressure is the solvent; the fluid with the highest vapor pressure is the solute. Usually the

couple of fluids used can be water (solvent) and ammonia (solute) or lithium bromide

(solvent) and water (solute).

There are different kind of absorption chillers that can be chosen according to the

constraints of the project and the type of heat source available. They can use directly the

exhausted gases that pass through a heat exchanger integrated with the chiller. An

alternative is to use the fluid flowing in the engine jacket, or a combination of the options.

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An absorption chiller is made of four main parts: evaporator, absorber, generator, and

condenser. (Figure 6)

The evaporator is the heat exchanger in which the refrigerant absorbs the heat from the

source at low temperature and becomes vapor. Considering that the refrigerant it’s at low

pressure, its boiling point is low and evaporates absorbing heat from the stream which

needs to be cooled.

The absorber is the device in which the vapor, produced in the evaporator, turns back into a

liquid solution at constant pressure. The solute is absorbed by the liquid mixture coming

from the generator. The absorption process takes place here because of the affinity between

solute and solvent, producing heat. The pump raises the pressure of the rich solution coming

from the absorber. The generator receives this mixture and separates solute from solvent in

a process similar to distillation using the heat source available. The condenser is the heat

exchanger in which the vapor, produced from the generator, condenses releasing heat to the

environment. The two lamination valves cause an isenthalpic expansion of the fluid: water

from the condenser passes through one of them, the solution coming out the generator

passes through the other. A regenerator is used to improve the performance of the system,

exchanging heat from the flows between the absorber and the generator.

The cooling effect is usually provided between 7 °C and 12 °C when water is used as

refrigerant. When temperatures under 0 °C are required, mixtures of glycol-water or other

mixtures are used.

An absorption chiller uses refrigerants which are known not to have a high Green House Gas

potential or to cause harm to the ozone layer. It doesn’t require to run a compressor, so

there are no emissions coming from power generation.

6.1 Compressor chiller and absorption chiller

The comparison between an absorption chiller and a compression chiller is not easy. Looking

at the investment, an absorption chiller is 30 % to 100 % more expensive than a compression

chiller. This comes from cooling towers for an absorption chiller that must be 2 or 2.5 larger

than the ones for a compression chiller. But, according to M. Sileo (2006), an absorption

chiller offers evident management advantages: no problems during blackout, silent

operation, 20 years of lifetime (it has no moving parts), recovers heat that otherwise would

have been wasted. A good way of comparing should be the cost of energy, but considering

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that the prices of natural gas and electricity are variable, it’s hard to say which device is

better. A careful analysis should be made for each case to find the best solution that suits

the situation.

7. Fuel cell application

7.1 Operating principle

A fuel cell is a device capable of converting directly the chemical energy in the fuel into

electrical energy. Normal energy systems, which involve combustion, have first to pass

through thermal, mechanical and finally electrical energy conversion. The combustion of the

fuel is the biggest source of inefficiency of the energy converting system (Pedrocchi, 2011).

This is due to the fact that the combustion is never done at adiabatic temperature and the

heat exchange between the flue gas and the fluid is done with high temperature differences.

Heat transfer under high temperature gradient destroys huge amounts of exergy (the ability

of a system to make work). Fuel cells provide solutions to these challenges.

ADVANTAGES

High efficiency, not limited by Carnot’s theorem.

Efficiency independent from dimension and just slightly dependent from the load,

modular system and flexible working condition.

High availability and reliability, no moving mechanical parts, gradual performance

decline (predictable).

Low environmental impact, no emissions of secondary pollutants.

DISADVANTAGES

Cost, depending on the type, on average not less than 5000$/kW.

Short life cycle.

Most of the fuel cells work with expensive high quality and purity fuel such as

hydrogen.

LOSSES

A fuel cell has an intrinsic efficiency depending on the type and the boundary conditions of

operations, above all the current output (Groppi, 2010). Fuel cells skip all the passages of

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energy conversions but they pay different energy “tolls” from normal energy converting

systems (Figure 7.1), such as:

Activation losses: due to kinetic reasons at the electrodes.

Ohmic losses: they increase with the increase of current flowing and they are due to

resistivity.

Mass transport losses: they occur when high current is flowing through the cell

during high loads.

Crossover: the un-reacted fuel passes through the membrane and it is oxidized on

the cathode without any real benefit for electrical production.

7.2 High temperature fuel cells

High temperature fuel cells are perfect to be combined with a cogeneration plant for

industrial purposes, giving high temperature heat as a byproduct. They can work with

different fuels such as Natural Gas converted by an internal reformer into hydrogen, carbon

monoxide and dioxide. This kind of fuel cell can run with “dirty” fuels containing a lot of

impurities, for example there has been studies at the Georgia Institute of Technology

regarding the possibility to run SOFC (Solid Oxide Fuel Cells) with gasified coal. Other

advantages of this type of fuel cells are the higher efficiencies, lower costs and longer term

stability when compared with low temperature fuel cells.

Working at high temperatures brings some serious problems concerning long startup times

and hard thermal stresses on the components. For these reasons high temperature fuel cells

are more suited for power generation combined with high temperature (high exergy)

cogeneration(Galliani et al, 2006). Instead low temperature fuel cells find better applications

in portable electronic devices, automotive and small stand-alone micro-power generation

systems.

7.3 Hybrid systems: gas turbine and fuel cells

Another interesting application of high temperature fuel cells is their combination in gas

turbines, replacing the normal combustor. Fuel cells need a source of cooling in all cases and

the use of the excess heat to generate power in a Joule Brayton cycle is a natural

consequence. This system is designed to reduce the losses due to normal combustion with

high air excess, taking complete advantage of the chemical exergy in the fuel. To increase

even more the efficiency of the plant the heat can be recovered from the flue gases coming

from the turbine with a Heat Recovery Steam Generator. The steam produced can be used

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both to run a steam turbine and make more power, as a source of heat or to cool down a

fluid with an absorption cycle. This kind of complex plant working at full electric generation

reaches efficiencies close to 70%, 10% more than the best available technology for normal

combined cycle (Campanari S. et al, 2002).

7.4 Solid Oxide Fuel Cell

These types of fuel cells are characterized by the use of a solid oxide as electrolyte usually

made of ceramic material such as YSZ (Yttria Stabilized Zirconia). Instead the anode is made

with nickel –zirconia ceramic-metal, with the first component promoting the internal

reforming and the second one inhibiting the nickel sinterization at high temperature. They

can work flexibly with a wide range of sulphur-free fuels ranging from hydrogen to light

hydrocarbons. With a pre-reformer they can even work with normal “heavy fuels” such as

gasoline, diesel or biofuels. Normal “sandwich geometry” SOFCs stacks require a few hours

of start-up time, but new tubular geometries SOFCs promise to lower this time down to a

few minutes. The main advantage of these classes of fuel cells is that they don’t require

expensive platinum based catalyzer since they work at temperatures approaching 1000°C

and consequently they don’t have problems with carbon monoxide poisoning, though

materials working at such high temperature are expensive (Groppi, 2011).

7.5 Molten Carbonate Fuel Cell

The electrolyte used in this fuel cell is a molten mixture of alkali metal carbonate held in a

ceramic matrix. At low temperature the electrolyte is not conductive, when the temperature

rises above 600-700°C the material becomes highly ionic conductive. The unique feature,

and disadvantage, of this fuel cell is the necessity of having CO2 at the cathode side to make

carbonate ions. Carbon dioxide is recycled from the anode side and the flue gas from the

anode is mixed with air to preheat it and oxidize the unreacted carbon monoxide and

hydrogen.

7.6 Internal reforming

The fuel fed to the fuel cells is usually natural gas which has to be converted to hydrogen

and carbon monoxide via a steam reforming reaction. For example considering a natural gas

made with just methane:

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We can have direct reforming where the catalyzer, usually nickel, is spread directly on the

anode, and indirect reforming where the reaction takes place separately close to the anode.

Since this reaction is endothermic we need to provide thermal heat, this is usually done with

the excess heat from the fuel cell itself. The steam needed comes from the combustion of

hydrogen. Fuel cells with internal reforming are a technology which is still in a developing

phase, while external reformers are a mature technology.

7.7 Fuel cells versus traditional combustion engines and small gas turbines in cogeneration

Presently, the open window for fuel cell as a commercial application in cogeneration is for

systems under 2 MW, in this case fuel cells can be a great alternative to conventional power

generation units. Combustion engines have great characteristics: they are cheap, reliable

and efficient even at partial loads, but they are noisy and they make great amounts of

pollutants. Small gas turbines with centrifugal compressor are not efficient and noise-free. A

great advantage of fuel cells used in residential areas is the ability to produce electric power

and heat with virtually zero-emissions of local pollutants such as carbon monoxide, NOx and

soot. Fuel cells are stationary machines which require only pumps and fans as moving

components, this unique characteristic enables them to work practically noiseless. Having

high efficiencies Fuel Cells produce less carbon dioxide per energy unit produced. At the

moment costs and lifetime limit the application of fuel cells but it is definitely a promising

technology for the future.

According to Casalegno (2010), fuel cells around 1-100 MW to be competitive in power

generation, have to cut down their costs from 12 M$/MW to 1.5 M$/MW, which is a very

hard to achieve. More research has to be done on fuel cells to get these systems on the

market at a reasonable price. In contrary gas turbine, intrinsically more complicated from a

thermo-fluid-dynamic point of view, took the advantage of extensive research for military

aviation (heavily financed by governments in the past).

8. Exergy analysis

Exergy combines the concept of energy and entropy. Exergy expresses the idea of the

amount of work theoretically extractable from a fuel in a given environment at a certain

temperature, pressure and composition. A system in equilibrium with the environment has

an exergy equals to zero. Theoretically it would be possible from a fuel to produce an

amount of work nearly equal to its calorific value with a reversible process where entropy is

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not generated. The real amount of work which can be produced has to take into account

exergy losses:

A very basic example regarding the use of 1kg of natural gas (chemical exergy approximately

50MJ/kg) to heat a house at 20°C with an ambient temperature of 0°C can be made (Table

8).Concluding:

Burning natural gas in a gas-fired power plant making electricity to heat a house with

an electric resistance is a thermodynamic disaster. A smarter way to use electricity to

produce heat consists of a heat pump.

Burning the same amount of gas in a cogeneration unit close to the customer to

produce heat as a by-product and electricity to run a heat pump is the most efficient

way to accomplish the same task.

Burning the natural gas in a boiler on site to make heat is the cheapest way in terms

of overall investment costs but it is very inefficient.

The amount of exergy delivered in the cogenerative case is 3.6 times higher than the case

with no cogeneration and no heat pump. This can be easily explained with an exergy

analysis, taking into consideration that the heat is produced at a temperature close to the

environment one. The greatest inefficiency is the one generated in the resistance,

downgrading valuable electricity into low temperature heat. With a reversible process one

unit of electricity can be transformed in nearly 15 units of heat, though heat pumps do not

reach such great conversions.

Exergy losses are much easier to understand than entropy loss since it’s something that can

affect directly the ability of the system to produce work. Exergy is closely related to the

money that will be spent for fuel. It is a useful tool to understand where improvement can

be made, measuring the efficiency of each power conversion step.

9. Indices for trigeneration and cogeneration

A trigeneration system can save lots of energy (and money) to produce the same amount of

electricity, heat and cold from the same source as compared to a separated generation of

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these forms of energy. To evaluate this saving, it is common to use two specific indices: PES

and first law efficiency (according to the European Ministry of Economic Development). PES

stands for Primary Energy Saving index, and it quantifies the energy saving obtained by a

cogeneration system providing the same amount of electrical and thermal energy of two

separated plants:

In which:

Efuel = energy coming from the fuel consumed

Eel = electrical energy produced

Qrec = thermal energy produced

ηel = reference electrical efficiency

ηgrid,ref = reference grid efficiency

ηth,ref = reference thermal efficiency

If the PES index of a cogeneration (or trigeneration) plant is greater than a certain value ( 10

% in Italy), that plant gets the right of being considered cogenerative, then gets subsidies. A

high efficiency utilization allows to produce some surplus energy (which is comparable as a

renewable source) that otherwise would be wasted in a normal plant. This index can change

for the same plant according to the reference efficiencies decided by the government. In

fact, the reference electrical efficiency can be the efficiency of the Best Reference

Technology (almost 60 % for Combined Cycles) or a national average among the power

generation plants available(usually lower than 40%). Using high reference electrical

efficiency lowers the value of PES. The reference grid efficiency is taken into account

because a trigeneration system saves the energy related to the transport of electricity, since

it’s really close to the users. The reference thermal efficiency instead is usually fixed over 90

%.

The first law efficiency comes from thermodynamic concept. It’s computed as follows:

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In which:

PME=total useful electric (or mechanical) power

QU= total thermal power, net of losses due to heat transmission

PF= total power coming from the primary energy source

This is mainly for cogeneration systems, but can be applied to trigeneration systems as well.

Of course it underestimates the overall efficiency of the trigenerative system, because it

doesn’t take into account the cooling part of the cycle.

In Italy (AEEG, 2002), another index has to be computed before a plant can be considered

cogenerative: the Thermal Limit. It considers the proportion between electrical and thermal

energy:

In which:

= thermal energy sent to the users

= electrical energy produced

= thermal energy sent to the civil use

= thermal energy sent to the industrial use

In order to be considered as an effective cogeneration plant, the LT of the system has to be

greater than 15 %.

10. Trigeneration and cogeneration now and in the future

Trigeneration is not so spread in the world at the moment because of costs and climate

conditions. The investment necessary for such a technology is higher compared to other

power plants (G. Lozza, 2006), because all the machines are more complicated than the

16

usual sized ones. In cold countries a trigenerative system can’t appear as an interesting

solution, because the requirement of cold is negligible. But even in these countries

application of Trigeneration could take place: in systems requiring comparable amounts of

heat, electricity and cold, such as supermarkets.

Trigeneration benefits the same subsidies given to cogeneration, because their development

is strongly linked together. The support given by the government can be classified into four

categories: special taxation, low interest loan, investment subsidy, and subsidy for new

technology development. This reduction in taxation and subsidy policies will help

trigeneration to become more cost competitive. (Environment and Development Division ,

2000)

Each country has a different policy to promote the application of cogeneration and

trigeneration. For example, E. ON which is one of the world's largest investor-owned power

and gas companies in Germany supports cogeneration and trigeneration. According to their

own press, under a nationwide support program, E. ON gives buyers of micro-CHP units a

subsidy of 1000EUR, if they sign a gas supply contract with E.ON. UK also has subsidy policies

for supporting cogeneration which can be applied to Trigeneration market as well. It is

estimated that about 1000 micro-CHP systems were in operation in the UK in 2002. Since

2005, the UK government has cut the taxation from 17.5% to 5% for micro-CHP systems (E.

ON, 2011).

The National Energy Plan by the American Council for an Energy Efficiency Economy (ACEEE)

has a specific chapter to promote CHP. This association estimates that an additional 95GW of

cogeneration capacity could be added before 2020. It is expected that cogeneration and

trigeneration will cover 29% of total power generation capacity according to ACEEE (Monty

Goodell, 2006).

In ASIA, many countries have also policies and plans to support cogeneration and

Trigeneration in the electrical network. For example, Japan supports 30% of the installation

costs, and provides loans with low interest (2.3% per year). In large scale Trigeneration and

Cogeneration plant, the government pays 15% of the investment, up to a maximum of

US$5million (Environment and Development Division, 2000).

Australia, a country in which coal has a large share in electricity production, has energy plans

to reduce its emissions of greenhouse gases. For example, The City’s Sustainable Sydney

17

2030 plan commits the city to produce by 2030 70% of the electricity from trigeneration

using natural and waste gas. Many projects have been undertaken to achieve the goal, and

even the city of New York is interested in this experiment (Sydney 2030 Plan, 2011).

Governments are interested in promoting trigeneration to achieve Renewable penetration in

power generation. Europe, particularly Germany and England, are experiencing a huge

increase in electricity production from wind energy which requires strong back up power in

periods of low wind (Eurostat, 2006). Remotely controlled Distributed Trigeneration

applications fuelled with natural gas will help this country to achieve high penetration of

renewable sources in the grid.

10.1 Pfizer Singapore API manufacturing facility

In the past years Singapore has had problems caused by energy shortage - increases in sea

level and sudden changes in climate. Therefore, many companies in Singapore changed their

traditional energy plants to cogeneration or trigeneration system. Pfizer is also one of those

companies which, due to plant expansion, adopted a trigeneration system in 2008 (Figure

10.1). This facility used a conventional system whose average power consumption was: 6.5

MW Electricity bought from the grid, 5MW heat to produce steam with a gas fired boiler, and

8.8MW of refrigeration power subtracted from chilled water via a normal refrigeration cycle.

This system’s first law efficiency was 65%, given by: the boiler’s thermal efficiency 85% and

an average efficiency of 45% in a common gas fired generation plant (Pfizer Asia Pacific Pte

Ltd, 2008).

Now, this facility adopts a single integrated Trigeneration system (Figure 10.2) which

comprises: Gas Fired Turbine (5MW), Waste Heat Recovery Boiler (8MW) and Absorption

Chiller (9.1MW). Trigeneration improved energy efficiency compared to the old system, and

its energy efficiency is almost 83% now. Figure 10.3 shows the comparison of thermal

efficiency between these two systems.

Using trigeneration system brought two more good impacts: reduction of electricity

consumption adopting an absorption chiller and lower greenhouse gas emissions. 17 %

reduction in CO2 emissions, which corresponds to 5976 ton/year when compared to the old

system. The higher efficiency is responsible for saving 587000 $/year in fuel cost, as shown in

Table 10.1.

18

11. Conclusions

Trigeneration can be considered as one of the most environmentally friendly fossil fuel

utilization. The overall efficiency is so high that emissions/kWh is much lower than other

fossil fuel plants. Distribute power generation reduce the environmental impact caused by

power lines. The application of Trigeneration can easily help renewable energy to get into

the market, supplying energy when the sun, wind or other are not available. Furthermore,

with the improvement in fuel cell technology trigeneration will be even more appealing.

The economic good point of a Trigeneration system is represented by its operational costs.

Its efficiency reduces the fuel required to produce the same amount of heat, electricity and

cold as other power generation plants. In this way it offers a sort of surplus of energy from

the same source with a resultant cost saving. This also brings to a low impact of a CO2 tax on

the economic balance of the plant. Even if natural gas is not the cheapest fuel, Trigeneration

system will be economically competitive.

19

12. References

1. AEEG, “Delibera n. 42, AEEG”, Italy, 19.03.2002

2. Baldini M., Simeoni P, Mattiussi A., “Trigenerazione, fare freddo con il calore di

scarto”, Supplemento dell’informatore agrario, 40/2008

3. B.C. Chung - Century Corporation Korea, “Trend and application of absorption

chiller”

http://www.eurocooling.com/index1.htm, read 20.10.2011

4. Campanari S., Macchi E. “Future potentials of MTGs: hybrid cycles and tri-

generation”, in “Micro Turbine Generators”, ISBN 1-86058-391-1, pp. 43-66,

Institution of Mechanical Engineers, England, 2002

5. Campanari S. “microcogenerazione e trigenerazione:come trasformare le opportunità

in un mercato reale.”, Energy Department, Politecnico di Milano, 2007

6. Casalegno Andrea, “Fuel cell technology”, Energy department, Politecnico di Milano,

2010

7. Chwieduk D, Pomierny W, Restuccia G, Freni A, De Boer R, Smeding S.F., Malvicino C.,

“Trigeneration in the tertiary sector”, paper presented at the world renewable energy

congress VIII, USA, 2004

8. Environment and Development Division (EDD), “Guidebook on Cogeneration as a

Means of Pollution Control and Energy Efficiency in Asia”, pp. 49-63, 2000

9. E.ON, “E.ON supports micro cogeneration”,

http://www.eon.com/en/media/news-detail.jsp?id=10452, 22.07.2011

10. European Environment Agency, “EN20 Combined Heat and Power”,

http://www.eea.europa.eu/data-and-maps/indicators/en20-combined-heat-and-

power-chp, 01.04.2007 read 25th September

11. European Ministry of Economic Development, “Regulatory framework for high

efficiency cogeneration”, Department of Energy, 2004

12. Galliani A., Pedrocchi E., “Exergy analasys”, Polipress, ISBN 8873980252, 2006.

13. Groppi Gianpiero “Fuel cells”, hand-outs from the course “Fundamentals of Chemical

Processes”, Energy Department, Politecnico di Milano, 2010

14. Malico I., Carvalhinho A.P., Tenreiro J., “Design of a trigenereration system using

high-temperature fuel cell”, International Journal of Energy Research, pp. 144-151,

2009

20

15. Monty Goodell, M.B.A., “Trigeneration”, 2006

http://cogeneration.net/Trigeneration.htm, , read 17.11.2011

16. Pfizer Asia Pacific Pte Ltd, “Trigeneration facility at Pfizer”,

http://www.e2singapore.gov.sg/docs/Case_Study_of_Trigeneration_Project_in_Pfize

r.pdf, 2008, read 1.11.2011

17. PolyGeneration in Europe (Front Page Image), “Trigeneration”, 2011

http://www.polygeneration.org/cms/front_content.php?idcat=77, read 11.11.2011

18. Rayment C., Sherwin S.,“Introduction to Fuel Cell Technology”, Department of

Aerospace and Mechanical Engineering, University of Notre Dame, IN 46556, 2003

19. Sileo Michele, “The micro-combined heat and power production: A new way for

energy saving”, Ambiente e diritto, 2006

20. Sydney 2030 Plan website,

http://www.sydney2030.com.au/, 2011

21. U.S. Department of Energy, “Emissions from Energy Consumption at Conventional

Power Plants and Combined-Heat-and-Power Plants”,

http://www.eia.gov/cneaf/electricity/epa/epat3p9.html, read 25.09.2011

21

13. Tables

Table 4.3 Emissions from Gas Turbines (M. Sileo, 2006)

Table 4.4 Gas Turbines Efficiencies (M. Sileo, 2006)

Table 8 Exergy Use of 1kg of Natural Gas (Lozza G., 2006)

Builder and model Power (kW)

Flue gas flow rate

(kg/s)

NOx ppm

(15% O2)

CO ppm

(15% O2)

Noise dB (A) (10 m)

Capstone C30 30 0.31 <9 n.d. 58

Capstone C60 60 0.49 <9 n.d. 65

Turbec T100 100 0.81 <15 <15 70 (1m)

IngersollRand MT70 70 0.73 <9 <9 58

IngersollRand MT250

250 2.0 <9 <9 n.d.

Elliott TA 100 100 0.79 14 <24 <65

Manufacturer Electrical

Power (kW)

Thermal Power (kW)

ηe %

ηt %

ηg %

Capstone C30 30 55 24 50 74

Capstone C60 60 115 28 54 82

Turbec T100 100 167 30 48 78

IngersollRand MT70 70 112 29 46 75

IngersollRand MT250 250 383 30 46 76

Elliott TA80 80 135 28 47 75

Elliott TA 100 100 165 29 46 75

no-cog

no-cog hp

Ηcycle 0,55

ηcycle 0,55

Ηline 0,9

ηline 0,9

Ηres 1

COP hp 3,5

Ex(kJ/kg) 1689,42

Ex(kJ/kg) 5912,969283

Cog

boiler

Ηel 0,4

ηboiler 0,9

Ηth 0,45

Ex(kJ/kg) 3071,672355

COP hp 3,5

Ex(kJ/kg) 6313,993

22

Table 10.1 Pfizer CO2 emissions (Pfizer Asia Pacific Pte Ltd, 2008)

Conventional System

Package Boiler

Steam Pressure 8 Bar

Steam Output 8 T/h

NG Used(boiler@85% efficiency) 15120 mmBtu/mt

h

CO2 Emission 0.19 kg/kWh

CO2 Emission per month(Boiler) 826 T/mth

Power from power station

Power from Power Station 6.5 MW

CO2 Emission (Average Power Station) 0.45 kg/kWh

CO2 Emission per month (Power Station) 2106 T/mth

Total CO2 Emission per month

(Conventional)

2932 T/mth

Trigeneration System

Power generated 4.6 MW

Steam generated from Trigen 11 T/h

TriGen Output (Electricity + Steam) 12.4 MW

CO2 Emission 0.22

9

Kg/kWh

CO2 Emission per month(TriGeneration) 2045 T/mth

Power from Power Station

Power from Power Station (less 0.7MW

from Absorption Chiller)

1.2 MW

CO2 Emission (Average Power Station) 0.45 kg/kWh

CO2 Emission per month (Power Station) 389 T/mth

Total CO2 Emission per month

(Trigeneration)

2434 T/mth

23

14. Figures

Figure 3 Typical annual energy demand for a warm country (S. Campanari, 2007)

Figure 4.1a: Pressure - Volume chart Joule Brayton cycle (M. Sileo, 2006)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Load

(kW

)

Month

Energy demand

Heat Load Electricity Coldness

24

Figure 4.1b Temperature – Entropy chart Joule Brayton cycle(M. Sileo, 2006)

Figure 6 Absorption Chiller Cycle (B.C. Chung)

25

Figure 7.1 Fuel Cell Losses (Casalegno, 2010)

Figure 10.1 Pfizer Facility (Pfizer Asia Pacific Pte Ltd, 2008)

26

Figure 10.2 Pfizer Trigeneration System (Pfizer Asia Pacific Pte Ltd, 2008)

Figure 10.3 Efficiency comparison between conventional and trigeneration system in Pfizer

(Pfizer Asia Pacific Pte Ltd, 2008)