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SEEP2010 Conference Proceedings, June 29th – July 2nd, Bari, ITALY
Trigeneration – A Way to Improve Food Industry Sustainability
S.A. Tassou, IN. Suamir
Brunel University
Uxbridge, Middlesex UB8 3PH
*E-mail:savvas.tassou@brunel.ac.uk
Abstract: In industrialised countries the food industry constitutes one of the largest industrial manufacturing groups with
significant environmental impacts. For Western Europe as a whole it is estimated that food is responsible for between 20%
and 30% of Greenhouse Gas Emissions. Sources of greenhouse gas emissions of the food industry include CO2 emissions
from energy used in the manufacturing processes and for the environmental control of buildings, emissions of refrigerants
from food refrigeration equipment and organic waste. A technology that offers the potential to make significant reductions in
GHG emissions and sustainability is trigeneration. This paper reviews and discusses the main technologies employed in
trigeneration systems, and outlines research, development and challenges in their application to the food industry. Particular
attention is given to applications in the food retail industry and the integration of trigeneration with CO2 refrigeration
systems to minimize greenhouse gas emissions from both the reduction of fossil fuel use and refrigerant leakage.
Keywords: GHG emissions, food industry, trigeneration, supermarkets, energy savings.
1. Introduction
The food industry, food manufacturing, storage and
retail, has a need for heating and electrical power as
well as refrigeration. Invariably, plant is installed
which consists of heating systems employing low
pressure hot water, high pressure hot water or steam,
vapour compression refrigeration systems and an
electrical power supply derived from the National
Grid. The overall utilisation efficiency of these
processes is low, around 50%-55%, because of
seasonal variations in demand and the relatively low
electricity generation efficiency in power stations and
distribution losses in the grid. One way of increasing
the energy utilisation efficiency of food processing
and retail facilities is through local combined heat
and power generation (CHP) or as otherwise known
co-generation. The fuel energy utilization efficiency
of CHP can be as high as 80%, almost 30% higher
than the separate production of electricity and heat,
but to achieve high efficiencies CHP systems have to
operate at maximum load for the for the vast majority
of time and make maximum utilization of the
generated electrical power and heat. If sufficient
demand for the generated electricity and thermal
energy is not available on site, consideration can be
made to export electricity back to the grid and
thermal energy to neighbouring facilities. This
approach, however, introduces complexities and
reduces the economic attractiveness of CHP as the
purchase price of electricity generated locally by the
electricity supply company in the majority of cases is
significantly less than the sale price of electricity.
Where it is not feasible to export electricity or heat a
variety of strategies can be used to optimise the
sizing and control of the CHP system to maximise
efficiency. These strategies which can be heat
demand or electrical demand lead will depend on
many factors, one of the most important being the
purchase and sell (spread) of electricity price.
Another way of ensuring that high energy conversion
efficiencies of CHP systems are maintained
throughout the year is to use some of the excess heat
available in periods of low heat demand to drive
sorption refrigeration systems and provide cooling or
refrigeration. The integration of CHP and sorption
refrigeration or other technologies to provide
simultaneously electrical power, heating and cooling
or refrigeration is known as CCHP, CHRP or
trigeration. The term polygeneration is also
sometimes used when local plant is used to produce
different forms of energy.
Trigeneration systems have been in operation for
many years but only in a small number of food
manufacturing industries. Recent increases in fuel
prices, concerns about the environmental impacts of
the food industry and developments in technology
have increased interest in the application of
trigeneration to the food industry.
This paper reviews the main technologies employed
in trigeneration systems. The paper also outlines
research, development and application challenges and
explores the influence of the performance
characteristics of trigeneration systems on energy
performance and environmental impacts.
2. Environmental Impacts and Carbon Footprint
The environmental impact and cost of each food
product is a function of the type of the raw materials
used, the manufacturing processes employed for its
production, the design of the packaging used, its
distribution and retail and final consumption (Figure
1).
Disposal/
recycling
Manufacture
Raw
materials
Distribution
/ retail
Consum
er use
Figure 1. Life Cycle Stages of food products
In order to improve the sustainability of food and
other products, it is necessary to use techniques and
tools to assess the environmental impacts associated
with the different stages of the product’s life cycle,
identify the ‘hot spots’ and develop processes and
systems that make the greatest contribution to the
reduction of environmental impact at an acceptable
cost.
Over the last twenty years a number of approaches
and tools have been developed for the assessment,
management and monitoring of environmental
impacts and sustainable development. Many of these
tools are complimentary and a number use a form of
Life Cycle Assessment (LCA) to provide a
quantitative estimate of the environmental impacts of
a product or process [1]. Life Cycle Assessment
which is also sometimes known as Life Cycle
Analysis or Ecobalance is the quantification of the
environmental impacts that arise from the complex
interactions between a product and the environment
over its whole life cycle-from cradle to grave.
Application of LCA can be quite complex due to its
multidisciplinary nature which involves assessment
of the impacts of the technical systems involved in
the production and transport processes, the impacts
of resulting emissions, the potential impact of
alternative choices of materials and processes and
wider social and environmental issues. The use of
LCA can be simplified by the selection of specific
methods and impact categories to be considered in
the analysis. PAS2050, a publicly available
specification for the assessment of the GHG
emissions of goods and services provides a
simplified approach to life cycle assessment by
focusing solely on GHG emissions rather than wider
environmental, social and economic issues [2]. The
Guide to PAS2050 provides guidance on the use of
the methodology and examples on its application to
specific goods and services [3].
The PAS2050 uses the carbon footprint as a measure
of the greenhouse gases (GHGs) associated with a
process or a product. The carbon footprint converts
emissions of individual GHGs into a single carbon
dioxide equivalent (CO2e) value using the global
warming potential (GWP) of the individual gases
over a 100 year period [4]. In a farming and food
context, carbon foot print normally represents the
total emissions of carbon dioxide (CO2), methane
(CH4) and nitrous oxide (N2O). It is expressed in kg
or tonnes of carbon dioxide equivalent (CO2e) per kg
or tonne of output and can be calculated for any
system or product. For industrial processes as well as
food processing and retail, carbon footprinting tends
to focus on energy use since most emissions are CO2
from energy production. This is normally an auditing
process where results are fairly precise with minimal
variation.
Figure 2 shows the percentage impact of the life
cycle stages of a cottage pie ready meal determined
using PAS2050 [5].
Figure 2. Impact of the life cycle stages on the carbon
footprint of a cottage pie ready meal.
It can be seen that for the case of the pie ready meal
most of the impacts arise from the raw material of
which mince meat is a significant contributing factor.
Manufacturing and distribution and retail of the pie,
however (Figure 3), also make a significant
contribution of the order of 24%, 12% each, with the
use phase and disposal accounting for the remainder.
Energy
supply
Raw
Materials
Waste
Organic Waste
Waste water
Air
Pollutants
FOOD FACTORY SERVICES
Distribution centre
Retail Outlets
products
Emissions of
refrigerants from food
refrigeration
equipment (HFCs)
Emissions of gases such as
methane from organic waste
(landfill sites)
CO
2
emissions from energy used
in the manufacturing processes,
transport and for the
environmental control of
factories/distribution centres/retail
food stores
Steam
Air
Cooling
Heating
Refrigeration
Figure 3. Emissions from the manufacture and retail
phase of the product life cycle.
The energy used in food manufacturing as a whole
in the UK which should not be much different from
other developed economies, is shown in Figure 4
[6]. It can be seen that approximately 60 to 70% of
the energy is used by fuel fired boilers and direct
heating systems for process and space heating and
the remainder is electrical energy used by electric
motors, electric heating, refrigeration equipment and
air compressors.
Figure 4. Energy used in food manufacturing in the
UK.
In food retail more than 70% of the energy is in the
form of electricity for refrigeration equipment,
heating and ventilation Equipment and lighting and
the remainder thermal energy for space heating and
in certain cases for in-store baking. Invariably,
electricity is provided by central power stations and
electricity supply grids whereas thermal energy is
produced locally by gas boilers and distributed to the
building by low pressure hot water.
The percentage contribution to GHG emissions of
the distribution and retail phase of a cottage beef pie
ready meal is shown in Figure 5 [7].
Figure 5. Percentage contributions to total GHG
emissions of beef cottage pie
The analysis assumed that the refrigerant employed
in the refrigeration systems of retail food stores is
R404A and the annual leakage rate is 15% of
refrigerant charge. It can be seen that most of the
emissions (45%) arise from the refrigerant leakage
due to the high global warming potential of R404A
and from the energy consumption of the refrigerated
display cabinets (41%).
The GHG emissions from the distribution and retail
phase of chilled and frozen food products can be
reduced significantly from the use of natural
refrigerants such as CO2 or hydrocarbons and local
power generation using low emission fuels such as
natural gas or biofuels.
A number of studies and projects have been
performed on the application of trigeneration in the
food industry. Bassols et.al [8] presented several
examples of this. In these applications, a range of
prime movers were employed in conjunction with
ammonia-water refrigeration systems.
Maidment et.al. [9] considered the feasibility of
application of CHP in a supermarket. The
investigators found the system to be viable, with a
projected payback period of approximately 4 years.
Because of the low heat demand for space heating in
the summer months, however, it was found that
considerable amounts of heat would be rejected to the
atmosphere. To improve the utilisation the authors
also considered the feasibility of using the waste heat
to drive an absorption chiller to provide refrigeration
at -10 oC for secondary refrigeration chilled food
display cabinets. It was concluded that such a system
is feasible and would lead to a payback period of 5
years.
A project funded by the European Union,
‘OPTIPOLYGEN’ considered the application of
polygeneration to the food industry. The aim of the
project was to investigate the application of
polygeneration and to develop tools, data and
guidelines to promote its application [10]. The results
of the project indicated that with suitable policy
measures, in the EUR-15 countries that were
considered in the study, the potential for electricity
generation from CHP in the food industry was 40
TWhe, from tri-generation applications 15 TWhe,
and 16 TWhe from the use of biomass or biogas
electricity generation. According to the study, 70%-
80% of the energy needs of the food industry could
be satisfied by polygeneration but at present only
25% of this potential is exploited. The study also
identified a number of technology gaps that need to
be addressed in order to accelerate the application of
polygeneration technologies in the food industry.
These include:
• Improvement of the electricity generation
efficiency of CHP systems,
• Improvement of efficiency of absorption
refrigeration systems.
• Increase of electricity to heat ratio of
commercially available CHP systems.
• Development of off the shelf (packaged) systems
to simplify integration and reduce capital cost.
• Reduction in capacity and footprint of
commercially available biogas plants.
• Development and application of fuel independent
technologies such as Stirling engines.
Work at Brunel University in recent years has
investigated the practical application of tri-generation
systems in the retail food industry to provide
electrical power for the supermarket and refrigeration
to the refrigerated display cabinets. The work which
is funded by Defra and is supported by food retail
and refrigeration companies has lead to:
• the development of tools for the evaluation of the
economic and environmental performance of
Combined Heat, Power and Refrigeration
schemes [11,12]
• The establishment of test facilities in the
University for the testing of systems and
evaluation of components with electricity
generation capacities up to 100 KWe,
• The establishment of the performance
characteristics of microturbine based tri-
generation systems for medium temperature
retail food refrigeration applications and
optimum integration of components.
A current project funded by Defra is investigating the
integration of tri-generation and CO2 refrigeration
systems for retail and other food engineering
applications.
3. Trigeneration Technologies
A trigeneration system is normally an integration of
two major technologies: the CHP system and a
thermally driven refrigeration technology. CHP
systems, many of which are available in packaged
form, consist of a prime mover which drives a
generator to produce electrical power and a heat
recovery system that recovers heat from the exhaust
gases and the engine cooling water in the case of
internal combustion engine based prime movers. A
schematic diagram of a trigeneration system is
shown in Figure 6.
The following CHP technologies are currently in
widespread use.
• Steam turbines
• Gas turbines
• Combined Cycle systems (gas and steam turbines)
• Internal combustion engines (Diesel and Otto).
These technologies are readily available, and fairly
mature, and reliable.
Three other technologies have recently appeared on
the market.
• Microturbines
• Fuel cells
• Stirling engines.
The following CHP technologies are currently in
widespread use.
• Steam turbines
• Gas turbines
• Combined Cycle systems (gas and steam turbines)
• Internal combustion engines (Diesel and Otto).
Figure 6. Schematic of a trigeneration system
Table 1 details the main characteristics of these
technologies, all of which can use either gaseous or
liquid fuels.
The following sections provide more information on
the three newer technologies.
3.1 Microturbines
Microturbines are a new type of combustion turbine
suitable for use in distributed energy generation
applications. A schematic of a microturbine power
generation system is shown in Figure 7 and the major
components of a commercial unit are illustrated in
Figure 8 .
Microturbine power generation units comprise a gas
compressor, a combustion chamber, an air
compressor, a turbine and an alternator. Compressed
air is mixed with fuel and burned in the combustion
chamber. The hot gases produced are expanded in a
turbine which drives an alternator. Recuperated units
recover heat from the turbine exhaust which is used
to preheat the air entering the compressor. This
improves the electrical generation of the unit but
reduces the temperature of the exhaust gases and the
amount of recoverable heat.
Microturbines offer advantages of compactness,
higher exhaust gas temperatures and lower
maintenance requirements than internal combustion
engines. Their electrical generation efficiency,
however, is lower than those of internal combustion
engines.
Fuel
input
Electricity
Refrigeration
Waste heat
Prime mover
Heat
recovery
Sorption
refrigeration
system
Table 1. Characteristics of CHP systems
Technology Size
MWe
Electrical
Efficiency
(%)
Overall
Efficiency
(%)
Average Capital
Cost
(£/kWe)
Average
Maintenance Cost
(£/kWh)
Steam Turbine >50 7-20 60-80 450-900 0.0013
Gas Turbine 0.5 - 25 25-42 65-87 200-450 0.002-0.005
Combined Cycle
>10
35
-
55
73
-
90
200
-
450
0.002
-
0.005
Diesel and Otto
Engines
0.005 - 4.0 25-42 70-85 150-700 0.003-0.01
Microturbines 0.025 – 0.30 15-31 60-85 400-800 0.002-0.005
Fuel Cells 0.001 - 10 30-60 75-90 450-3800 <0.005
Stirling Engines
0.003- 0.1 40 65-85 2000
?
.
3.2 Fuel Cells
A fuel cell is an electrochemical energy device that
converts hydrogen (fuel) and oxygen (air) into
electricity and heat. The hydrogen can be obtained
from a variety of sources but the most common is
through reforming of natural gas or other gaseous or
liquid fuels.
Figure 9. Principle of operation of fuel cell [13]
A fuel cell (Figure 9), consists of two electrodes, an
anode and a cathode, separated by an electrolyte.
Power is produced when ions (charged particles)
formed at one end of the electrodes with the aid of
catalysts pass through the electrolyte. The current
produced can be used for electricity. The electrolyte
plays a key role as it must permit only the appropriate
ions to pass between the anode and cathode. Passage
of free electrons or other substances through the
electrolyte, would disrupt the chemical reaction.
Fuel cells are categorized by the kind of electrolyte
they use and include:
• Solid Oxide Fuel Cells (SOFC)
• Polymer Electrolyte Fuel Cell (PEFC)
• Proton Exchange Membrane Fuel Cell
Figure
8
.
Bowman TG80RCG microturbine CHP
system
Stack
Recuperator
Fuel Gas
Compressor
Combustion
Chamber
Alternator
Compressor
Turbine
Figure
7
.
Schematic diagram o
f a microturbine
Table 2. Characteristics of fuel cells
(PEMFC)
• Phosphoric Acid Fuel Cells (PAFC)
• Alkaline Fuel Cells (AFC)
Apart from high electrical generation efficiencies,
fuel cells offer advantages of low noise and
greenhouse gas emissions and high reliability.
Disadvantages include their relatively high cost, and
short life span, typically 10 years. PAFCs are the
most widely deployed fuel cells and more than 250
units of 200 kWe capacity have been installed
worldwide since the early 1990s.
The characteristics of these fuel cells are summarised
in Table 2. Fuel cells have a typical electrical
efficiency of between 30 and 60 % and an overall
efficiency, if using the heat in a CHP arrangement of
70-90 %.
3.3 Stirling Engines
The Stirling Engine (Figure 10),is an emergent
technology in the realm of CHP systems, despite the
fact it has been invented in the 19th century.
Combustion takes place outside the engine and the
heat generated is used to heat the operating gas in the
cylinder of the engine (Figure 11).
Figure 10. 35 kW Stirling engine [14]
Figure 11. Schematic of a Stirling based CHP system
[15]
The advantage of Stirling CHP systems over internal
combustion based systems is that they can be
powered by biomass, solar, and fossil fuels. This
flexibility and the high efficiency of the engine offer
significant potential for further development in the
future.
3.4 Thermally Driven Refrigeration
Technologies
The most common thermally driven refrigeration systems
are based on the sorption technology where the
mechanical compressor of the common vapour
compression cycle is replaced by a ‘thermal compressor’
and a sorbent. The sorbent can be either solid in the case
of adsorption systems or liquid for absorption systems.
When the sorbent is heated, it desorbs the refrigerant
vapour at the condenser pressure. The vapour is then
liquefied in the condenser, flows through an expansion
valve and enters the evaporator. When the sorbent is
cooled, it reabsorbs vapour and thus maintains low
pressure in the evaporator. The liquefied refrigerant in the
evaporator absorbs heat from the refrigerated space and
vaporises, producing the cooling effect.
Electrolyte Operating
Temperature
(oC)
Electrical
Efficiency
(%)
Typical
Electrical
Power
(kW)
Fuel Type Possible Applications
AFC 60-90 40-60 20 kW Pure hydrogen Spacecraft,
Submarines
PAFC
100
-
220
35
-
40
>50 kW
Pure
hydrogen
Buses, trucks, large
stationary applications
MCFC
550
-
700
45
-
60
>1.0 MW
Most hydrogen
based fuels
Power Stations
PEFC/
PEMFC
80 30-35 <250 kW Pure hydrogen Passenger cars and
mobile applications
SOFC 450-1000 45-65 >200 kW Most hydrogen
based fuels
Small to large
stationary applications
3.4.1 Absorption Refrigeration Systems
The most common sorption technology is absorption
refrigeration. Absorption refrigeration systems are
characterised by the refrigerant fluid pair they use.
The most common pairs are Lithium Bromide
(refrigerant)-Water (absorbent) and Ammonia
(refrigerant)-Water (absorbent). LiBr-H2O water
systems can only be used for cooling temperatures
above 0 oC. The technology is well established and
packaged systems are readily available from a
number of manufacturers in the USA, Japan, India
and China which include Carrier, York, Sanyo,
Hitachi, Yazaki, Thermax, Broad and many others.
Ammonia-water systems can provide refrigeration at
temperatures down to -60 oC. Commercial systems
are available from only a very small number of
manufacturers which include Colibri bv and
Transparent Energy Systems. Robur supplies
packaged systems able to provide 12 kW of
refrigeration at brine flow temperatures down to – 12
oC [16]. A schematic diagram of a single stage
ammonia-water system is shown in Figure 12.
The condenser, expansion valve and evaporator
operate in exactly the same way as for the vapour
compression system. In place of the compressor,
however, theabsorption system uses a number of
other components: a generator, an absorber a solution
pump and a regenerating heat exchanger. A liquid
solution weak in ammonia in the absorber absorbs
ammonia vapour exiting the evaporator. The process
is exothermic and so cooling is required to carry
away the heat of absorption. The solution which
becomes rich in ammonia is then pumped through a
heat exchanger to the generator.
Heat supplied to the generator evaporates the
ammonia from the solution. The solution which
becomes weak in ammonia returns to the absorber
through the regenerating heat exchanger where it
preheats the solution supplied to the generator. The
ammonia after passing through a rectifier where any
water present in the refrigerant is removed travels to
the condenser where it condenses by rejecting heat to
a cooling medium. The ammonia liquid from the
condenser flows through the expansion device before
entering the evaporator where it evaporates and
produces refrigeration.
Figure 13. Transparent Energy Systems PVT Ltd
absorption refrigeration system [18]
Figure 13 shows an ammonia-water system employed
in a trigeneration application in a dairy factory. The
system provides 280 kW of refrigeration at a brine
flow temperature of – 5.0 oC.
Figure 14 shows typical performance characteristics
of ammonia-water absorption refrigeration systems.
The refrigeration capacity and COP of the system is a
function of the evaporating temperature, the
temperature of heat input to the generator and the
temperature of the condenser cooling medium. The
COP of these systems will be 0.5-0.6 at evaporating
temperature of -10 oC and 0.25-0.4 at evaporating
temperature of -50 oC.
3.4.2 Adsoprtion Refrigeration Systems
Adsorption refrigeration unlike absorption and
vapour compression systems, is an inherently cyclical
process and multiple adsorbent beds are necessary to
provide approximately continuous capacity. A
schematic diagram of a simple adsorption system is
shown in Figure 15. Adsorption systems inherently
require large heat transfer surfaces to transfer heat to
and from the adsorbent materials which automatically
increases their size and cost.
Absorber
Figure 12. Schematic of Ammonia-Water
absorption refrigeration system [17]
Heat input
Heat
Exchanger
Generator
Evaporator
Condenser
COP
Figure 14. Performance characteristics of ammonia-water absorption refrigeration system [18]
Figure 15. Schematic diagram of adsorption chiller
[17]
High efficiency systems require that heat of
adsorption be recovered to provide part of the heat
needed to regenerate the adsorbent. These
regenerative cycles consequently need multiples of
two-bed heat exchangers and complex heat transfer
loops and controls to recover and use waste heat as
the heat exchangers cycle between adsorbing and
desorbing refrigerant.
Adsoprtion systems for air conditioning applications
are already commercially available from a small
number of manufacturers . "MYCOM", Mayekawa
Mfg. Co., Ltd. are producing Silica-gel/water
adsorption chiller (ADREF-models) with ranges
between 35 and 350 kW for use in the air-
conditioning industry. NISHIYODO KUCHOUKI
CO. LTD, produce Silica-Gel/Water adsorption
chillers (ADCM models) with capacities between 70
kW and 1300 kW capable of being driven by low
grade heat 50 – 90 °C and able to give COPs of
around 0.65 (Figure 16).
Figure 16. Nishiyodo adsorption chiller [19]
Research and development is also underway to produce
systems for refrigeration applications and prototypes
for temperatures down to – 25 °C are currently in
operation.
4. Research and Development on Trigeneration
Systems at Brunel University
The Centre for Energy and Built Environment
Research (CEBER) at Brunel University has been
involved in research on the development of
trigeneration systems for food engineering
applications since 2001. Research projects that have
been funded by Defra and a number of companies in
the retail food and refrigeration industries have resulted
in the development of experimental test facilities and
tools for the evaluation of the economic and
environmental performance of trigeneration systems.
4.1 Experimental Test Facilities
The trigeneration test facility incorporates three main
modules; CHP module, absorption refrigeration
system module, and a refrigeration load module. A
schematic diagram of the facility is shown in Figure
17.
Figure 17. Schematic diagram of test facility
4.1.1 CHP Module
The CHP module is based on a 80 kWe recuperated
microturbine generation package with in-built boiler
heat exchanger (exhaust heat recovery heat
exchanger). The microturbine consists of a single
stage radial compressor, single radial turbine within an
annular combustor and a permanent magnet rotor
(Alternator) all on the same rotor shaft. Other systems
in the engine bay include the fuel management system
and the lubrication/cooling (oil) system.
Heat recovery from the exhaust gases is performed in
a flue-gas/water heat exchanger. The heat exchanger
consists of stainless steel coils imbedded in parallel
flue gas streams to reduce pressure drop and the back
pressure on the turbine.
The fuel system comprises an external gas boost
compressor which compresses the gas supplied to the
combustor to 5.0 bar, and an internal fuel system that
provides fine control of the gas fed to the burners.
Figure 18 shows the variation of the electrical
generation efficiency and exhaust temperature of the
microturbine CHP with power output. Maximum
efficiency and exhaust gas temperature is obtained
when the turbine delivers the maximum electrical
power output of 80 kW.
4.1.2 Absorption Refrigeration Module
The refrigeration capacity of the test facility is
depended on the type of thermally driven refrigeration
system used. For ammonia-water refrigeration systems
providing refrigeration at -10 oC, the unit is able to
provide up to 50 kW of refrigeration.
The absorption refrigeration system currently
employed, is a packaged gas fired chiller of specified
refrigeration capacity of 12 kW at ambient
temperature of 35 oC and chilled fluid (brine) inlet and
outlet temperatures of 0 oC and –5 oC respectively.
The gas fired unit was modified to operate with heat
recovered from the exhaust gases of the microturbine.
A number of different designs were investigated: a)
using the exhaust gases directly on the generator and
b) using a heat transfer fluid to transfer heat from the
heat recovery heat exchanger (boiler) of the
microturbine to the generator. The latter arrangement,
was found to be more effective and the arrangement is
shown in Figure 17.
Figure 19. Performance of absorption refrigeration
system driven by heat transfer fluid from gas turbine.
Figure 19 illustrates the performance of the absorption
refrigeration system with the heat transfer fluid for a
range of glycol delivery temperatures. It can be seen
that the COP of the unit varies from around 0.58 to
0.67 as the brine flow temperature is increases from -
10.0 oC to -2.0 oC. If the heat transfer fluid pump
power is taken into consideration, the system COP
drops by approximately 0.08 over the whole range of
brine flow.
2
6
10
14
18
22
26
30
0 10 20 30 40 50 60 70 80 90
Electrical power ou tput (kW
e
)
Electr ical eff iciency (%)
200
210
220
230
240
250
260
270
280
290
Exhau st g as tem peratu re (
o
C)
Electrical efficiency
Exhaust gas temperature
Figure 18. Variation of electrical efficiency and
exhaust gas temperature with electrical power
output
4.1.3 Integration of trigeneration and CO2
refrigeration systems in supermarket applications
The energy savings potential of tri-generation
systems in supermarket applications can be
increased through the use of CO2 as a secondary
refrigerant to replace conventional secondary
fluids such as propylene glycol, potassium
formate and others.
The viscosity of liquid CO2 is approximately 100
times less than the viscosity of common
secondary fluids and thus the power that will be
required to circulate CO2 from the tri-generation
system to the display cabinets will be very small
and insignificant compared to the power required
for conventional secondary refrigerants. Other
advantages of this system include:
• Smaller pipe sizes and lower piping and
insulation costs.re uniformity in the cabinet.
• The use of only a single working fluid to
satisfy both the frozen food and chilled food
refrigeration requirements in a retail food
store.
• The use of only a single working fluid to
satisfy both the frozen food and chilled food
refrigeration requirements in a retail food
store.
Figure 20 shows a schematic diagram of an integrated
trigeneration and CO2 refrigeration system for
supermarket applications. The CO2 refrigeration
system is of a cascade system with cooling generated
by the absorption system used to condense the CO2
refrigerant at a temperature of around -10 oC.
5. Energy analysis and comparison with
conventional systems
Figure 21 shows the energy flow diagram for the
proposed supermarket energy system. The
supermarket considered in the study has a sales
area of 2800 m2. Annual electricity consumption
of the supermarket was 3495 MWh with peak
and average demand of 662 kWe and 399 kWe
respectively while gas consumption was 988
MWh with peak demand during the winter of 385
kWth. Average demand of thermal energy was
113 kWth. There was also a significant variation
between daytime and night time electrical and
gas energy demand.
Figure 22 shows the variation of the MT
(medium temperature) and LT (low temperature)
refrigeration system electrical energy demand
and the thermal energy demand for the
supermarket during a whole year. The total
electrical energy demand for refrigeration was
Figure 20. Model of integrated CO2 refrigeration and trigeneration with ammonia-water
absorption chiller
2830 MWh of which 20% was for LT
refrigeration. Peak refrigeration demand of the
supermarket was 536 kW of which 447 kW for
MT and 89 kW for LT refrigeration. Annual
heating demand of the supermarket was 791
MWh, with peak heating demand of 308 kW.
Figure 22 shows the variation of the MT and LT
refrigeration system electrical energy demand
and the thermal energy demand for the
supermarket during a whole year.
Figure 22. Daily average energy demand of the supermarket
Simulation results of the conventional
refrigeration system show the COP of the MT
refrigeration system to vary between 1.42 in the
summer and 3.08 in winter with average annual
COP of 2.79. The COP of the LT refrigeration
system varies from 0.25 in the summer to 1.29 in
winter with average annual COP of 1.04. The
overall average seasonal COP of the conventional
refrigeration system was found to be 2.02.
Figure 21. Energy flow diagram for conventional and proposed system in a supermarket
0
100
200
300
400
500
0 30 60 90 120 150 180 210 240 270 300 330
360
MT Refrigeration Heating LT Refrigeration
Time (days)
Refrigeration and heating
demand (kW)
Figure 23 shows daily average efficiencies of the
conventional supermarket energy system. It can
be seen the overall efficiency in winter fluctuates
between 60% and 70% and then drops to about
52% in the summer due to higher outdoor
temperatures giving an overall seasonal
efficiency of 61%.
Refrigeration and heating efficiency also vary
throughout the year. Annual average efficiency
of refrigeration is 48% and heating 13%
respectively. Primary fuel required by
conventional system is 11580 MWh per year of
which 10592 MWh is electricity and 988 MWh
gas.
Figure 23. Daily average efficiencies of conventional supermarket energy system
Daily average efficiency of the integerated
trigeneration-CO2 refrigeration system is shown
in Figure 24. It can be seen that the overall
efficiency of the system can reach 75% in the
winter and drops to 51% in the summer giving an
overall seasonal efficiency of 64.6%. The
efficiency of refrigeration fluctuates in the range
24% to 38% with annual average of 29.4%. The
average electrical efficiency is 27.5%. The Figure
also shows that the efficiency of heating is
relatively low particularly in the summer with a
seasonal average of only 7.7%.
Proposed Supermarket Energy Systems Figure 4 shows the energy flow diagram for the
proposed supermarket energy systems.
Figure 24. Efficiency of the integrated trigenration-CO2 refrigeration system
0
10
20
30
40
50
60
70
80
0 30 60 90 120 150 180 210 240 270 300 330 360
Time (days)
Efficiency (%)
Overall Refrigeration Heat ing
0
10
20
30
40
50
60
70
80
0 30 60 90 120 150 180 210 240 270 300 330 360
Time (days)
Efficiency (%)
Overall Refrigeration Electrical Heating
Table 2. Results of fuel saving analysis of the
proposed supermarket energy systems
Fuel utilization Proposed
System
Trigeneration fuel (MWh) 8511
Auxiliary boiler fuel
(MWh)
3
78
Imported electricity
(MWh)
313
Fuel required for imported
electricity (MWh) 948
Total fuel required
(MWh)
9
838
Fuel savings
(
MWh/year)
1742
Fuel energy saving ratio (FESR) (%) 15.1
Table 1 summarises the energy performance of
the proposed system compared to the
conventional system of power heating and
refrigeration in the supermarket. It can be seen
that the proposed system can provide significant
energy savings over the conventional system and
a fuel energy saving ratio of 15.1%.
Table 2 shows a comparison between the CO2
emissions of the conventional energy system and
the proposed integrated trigeneration and CO2
refrigeration system in the supermarket. The
results were determined for two annual
refrigerant leakage rates of 15% and 30%
respectively. It can be seen that for a leakage rate
of 15% of refrigerant charge per annum the
proposed system will lead to 1453 tonnes of CO2
emissions per year which equates to 43.5% of
annual emissions. For a refrigerant leakage rate
of 30% the GHG emissions savings are even
higher at 56%.
Table 3. CO2 emissions of conventional and proposed energy system for case study supermarket
6. Conclusions
From the review presented and research on the subject
by the authors it can be concluded that:
• Trigeneration is a very efficient way of generating
simultaneously electrical power, heating and
refrigeration. It can produce substantial energy
and greenhouse gas emission savings over
separate production of electricity, heat and
refrigeration.
• A number of different fuels, such as biofuels, and
reliable technologies can be used in trigeneration
applications.
• The initial investment costs in trigeneration
systems can be relatively high, but payback
periods of between 3 and 5 years can be achieved
under certain operating conditions.
• The payback period of trigeneration installations
is a strong function of the difference between the
fuel price and the purchase price for electricity. A
ratio of gas to electricity prices of less than 0.3 is
required to obtain reasonable payback periods.
• GHG emission from energy consumption and
refrigerant leakage of chilled and frozen food
products are responsible for over 90% of the
carbon footprint of the distribution and retail
phase of their life cycle. This carbon footprint can
be reduced significantly by the use of natural
refrigerants and trigeneration.
• An innovative integrated trigeneration and CO2
refrigeration system provides enhanced
opportunities for application of trigeneration
systems in the retail food sector and reducing
GHG emissions.
• Models developed and used to investigate the
energy efficiency of conventional HFC based
refrigeration systems and integrated trigeneration
and CO2 refrigeration systems have shown that
the latter system can offer fuel energy savings of
the order of 15% and carbon emission reductions
of over 44% compared to conventional systems.
• Further refinement of system design is expected
to increase fuel energy savings to ver 30%
compared to conventional systems and increase
significantly the sustainability of the food
industry.
CO2 emissions
Annual leakage 15% of
charge
Annual leakage 30% of charge Units
Conventional
System
Proposed
System Conventional
System
Proposed
System
Indirect CO
2
emissions 2094 1807 2094 1807 tCO
2
/year
Direct CO
2
emissions:
Refrigerant leakage 1097 68 2194 136 tCO
2
/year
Refrigerant recovery losses 146 9 146 9 tCO
2
/year
Total annual emissions 3337 1884 4434 1952 tCO
2
/year
Net emission savings 1453 2482 tCO
2
/year
CO
2
emissions reduction
43.5
56.0
%
Acknowledgements
The authors acknowledge the financial support
received from the Food Technology Unit of
DEFRA (Department of Environment Food and
Rural Affairs) for research reported in this paper
and the contribution of a large number of
industrial collaborators: Tesco Stores Ltd, A&N
Shilliday & Company Ltd, ACDP (Integrated
Building Services) Ltd, Apex Air Conditioning Ltd,
Bock Kältemaschinen GmbH, Bond Industries Ltd,
Bowman Power group, Cambridge Refrigeration
Technology, Cogenco, CSA Consulting Engineers
Ltd, Danfoss, Doug Marriott Associates, George
Baker & Co (Leeds) Ltd and Somerfield Property
Co Ltd.
References
1. Robert, K-H., Schmidt-Bleek, B., Aloisi de
Larderel, J., Basile, G., Jansen, J. L., Kuehr,
R., Price Thomas, P., Suzuki, M., Hawken,
P.,Wackernagel, M. Strategic sustainable
development — selection, design and
synergies of applied tools, Journal of Cleaner
Production 10 (2002) 197–214.
2. BSI Group, PAS 2050:2008, Specification for
the assessment of the life cycle greenhouse
gas emissions of goods and services, 43 pgs,
downloadable from:
http://www.bsigroup.com/en/sectorsandservic
es/Forms/PAS-2050-Form-page/Thank-you/
last accessed 17/03/2010.
3. BSI Group, Guide to PAS2050, How to assess
the carbon footprint of goods and services, 58
pgs, downloadable
from:http://www.bsigroup.com/en/sectorsands
ervices/Forms/PAS-2050-Form-page/Thank-
you/ last accessed 17/03/2010.
4. Lillywhite, R., Collier, R. (2009), Why carbon
footprinting (and carbon labelling) only tells
half the story, Aspects of Applied Biology 95,
2009, http://www.aab.org.uk/
5. Defra (2008). PAS2050 Case Study. Applying
PAS2050 to a complex product: Cottage pie
ready meal, www.defra.gov.uk.
6. FDF (2008). Carbon management, best
practice in food and drink manufacturing,
www.fdf.org.uk.
7. Defra, 2008. Environmental Impact of food:
Life Cycle Greenhouse Gas Assessments,
http://www.defra.gov.uk/foodfarm/food/envir
onment/gas.htm.
8. Bassols, J., Kuckelkorn, B., Langreck, J.,
Schneider, R., Veelken, H. (2002). Trigeneration
in the food industry, Applied Thermal
Engineering, Volume 22, Issue 6, Pages 595-602.
9. Maidment, G. G., Zhao, X., S. B. Riffa S. B.,
Prosser, G. (1999). Application of combined heat-
and-power and absorption cooling in a
supermarket, Applied Energy Volume 63, Issue 3,
July 1999, Pages 169-190.
10. OPTIPOLYGEN (2007). Optimum Integration of
Polygeneration systems, www.optipolygen.org/
11. Tassou, S. A., I Chaer, I., Sugiartha, N., Ge, Y-T.,
Marriott, D (2007) Application of tri-generation
systems to the food retail industry, Energy
Conversion and Management, Volume 48, Issue
11, Pages 2988-2995
12. Sugiartha, N., Tassou S. A., Chaer, I., Marriott, D.,
Trigeneration in food retail: An energetic,
economic and environmental evaluation for a
supermarket application Applied Thermal
Engineering 29 (2009) 2624–2632.
13. Fuel cell Today (2007), http:// www.
fuelcelltoday.com/media/pdf/
14. Stirling Danmark (2007) http://www.stirling.dk/
15. OPET (2004) Micro and small scale CHP from
Biomass, http://akseli.tekes.fi/
16. Robur (2010). http://www.robur.it/
17.Tassou, S. A., Lewis, J. S., Ge, Y. T., Hadawey,
A., Lewis, J., Chaer, I., Review of Emerging
Refrigeration Technologies for Food Refrigeration
Applications, Applied Thermal Engineering 30
(2010) 263–276.
18. TES PVT Ltd (2010) http://www.tesplaarp.com/.
19.HIJC INC (2006) USA,http://www.
adsorptionchiller.bigstep.com/
