1

Feasibility study of a cogeneration plant in a supermarket

Tiago Ribeiro Franco tiago.franco@ist.utl.pt

Instituto Superior Técnico, Lisboa, Portugal

November 2014

Abstract

We live on a Planet that suffers daily with a high dependence on fossil fuels and with the increasing consumption often associated with low energy efficiency. Investment in more efficient and less polluting technologies is essential to prosperity. In this sense, the combined production of electricity, heat and cold contributes positively towards this objective by reducing primary energy consumption and therefore emissions of greenhouse gases. This study examines the feasibility of installing a trigeneration plant in a supermarket located in western Portugal. The suggested plant is evaluated taking into account economic, technical, environmental and legal issues in order to investigate the option of parallel operation with the existing refrigeration plant. Different technologies and operation strategies were evaluated according to the plant size, the operating schedule and the equipment load. Among several scenarios, the most favourable technology was the internal combustion engine with a total output of 40 kWelectric along with an absorption chillers pack with a total of 40 kWcoling running from 9am to 21pm during the weekdays and with half load option enable. This scenario resulted in a net present value of € 44 304 with a payback period of 10.1 years and an absolute primary energy savings of 9.8%. Keywords: Cogeneration, CHP, trigeneration, CCHP, internal combustion engine, Energy Efficiency, absolute primary energy savings.

1 Introduction

Due to its geographical location and geology, Portugal is

limited in energy resources such as oil, natural gas and

coal, but rich in renewable resources such as hydro, wind

and solar. In order to take advantage of those green

resources, Portugal has strengthened the investment in

renewable technology, according to Eurostat, in 2012

took fourth place in the European ranking, with 47.6% of

electricity generated from renewable sources [1]. These

indicators suggest Portugal is in a good way to meet the

European commitment which set strategic goals for

climate and energy changes to be met by Member States

until 2020.

CHP plays an important role in the goals to be achieved

since its concept is based on the localized production of

energy. This allows a greater independence from the

power generating plants which provides greater

efficiency by avoiding grid distribution losses.

CHP and CCHP technologies have evolved in recent years

with more manufacturers developing solutions for

different scales and application areas, this progress

coupled with "green" energy policies contribute to the

feasibility of installing this technology in areas which until

then was impossible [1].

Supermarkets are intensive consumers of electricity; in

developed countries they consume about 3% to 4% of

the total available energy. A breakdown of the energy

used in a supermarket reveals 50% is used for

refrigeration, with the remaining half divided between

equipment and lighting [2,3].

The aim of this study is to examine the feasibility of

installing a CCHP plant in a supermarket. The proposed

2

system is evaluated taking into account the economical,

technical, environmental and legal constraints in order to

investigate its suitability for parallel working with the

existing refrigeration plant.

This study evaluates the proposed scenarios by changing

the plant size and operation schedule in order to perform

an economic analysis of the trigeneration system.

2 Case Study

The proposed building is located in the Midwestern

region of Portugal. It has about 1000m2 of covered area,

of which 854m2 are dedicated to sales. The supermarket

opens to the public at 9:00 am and closes at 8:30 pm,

seven days a week.

2.1 Conventional system

Employees and vendors arrive from 7am which raises the

electricity consumption by opening the refrigeration

chambers doors and other acclimatized areas such as

loading dock.

The cooling production is divided into three areas:

Low temperature cooling plant, -30⁰C – Produces

Cooling for two frozen chambers.

Medium temperature cooling plant, -10⁰C – Produces

cooling for 11 frozen chambers, 5 acclimatized areas

and 12 cooled cabinets.

10 units of standalone frozen cabinets, -18⁰C

Due to the small size and low temperature operation,

both the low temperature plant and the standalone units

are not considered. The aim of this study is a

trigeneration plant that is able to work in parallel with

the already existing cooling plant. Heating, ventilation

and air conditioning, HVAC, is not covered by the

proposed plant, however its power consumption is

included in the building electrical loads.

2.2 Thermal and electrical loads

As first approximation building heat requirements are

neglected, this happens because both the space heating

demand and the need to heat water are low. The

continuous equipment running, lighting systems and

occupancy contributes to the building heating [3]. Thus

the heat demand of the building is not included in the

proposed solution however it can be met by the existing

HVAC equipment. This approach results in all the heat

created in cogeneration used for cooling production.

The total energy consumption is separated in electricity

for conventional use and electricity used for cooling. The

electricity used in compression chillers we can estimate

50% for day-time and 68% for night-time. The remaining

50% during day-time and 32% during night-time is

consumed by equipment and lighting [4] and [2]. This

way is possible to obtain the electrical and thermal

curves, figure 1, and also the cumulative load duration

curves of the total energy consumption, figure 2.

Figure 1 – Annual consumption of electricity for cooling.

Figure 2 – Cooling load duration.

0

10

20

30

40

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60

Co

olin

g P

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Cooling Demand during the year

Day-time Night-time Average Peak

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60

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wer

kW

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Cooling Load Duration

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Figure 3 – Electricity demand profile.

Figure 3 shows the average profile of electricity

consumption for cooling production and for equipment

and lighting use. Obtaining these curves is extremely

important because they allow estimating prime mover

and cooling plant size, as well as the operation schedules.

Tables 1 and 2 and show the environmental impact and

the costs breakdown of electricity bill in 2013.

Table 1 – Environmental impact.

Designation Value

Primary Energy [toe/year] 116

CO2 Equivalent Emissions [tCO2eq/year] 253

Table 2 – Energy costs.

Designation Cost [€/year]

Cost of Electricity without cooling 27 221

Cost of Electricity consumed by the Compression Chiller

26 823

Cost of Electricity consumed by the Compression Chiller (Peak cost)

4 189

Total (Tax Excl.) 58 233

Total (Tax Inc.) 71 627

2.3 Proposed system

The proposed plant, figure 4, operates in parallel with

the existing cooling plant at strategic periods in order to

meet the cooling needs. In this operating scheme, the

generator produces steam and hot water which is used

to drive the absorption chillers. If the cooling demand is

higher than the amount of cooling produced by the

absorption chillers the compression chillers are activated

to fulfil the gap. The electricity produced by the prime

mover will power the absorption chiller and compressor

plus all the equipment and lighting installed in the

building. Whenever the electricity demand exceeds the

generator capacity, electricity from the grid will be

imported and whenever there’s an excess, energy will be

sold to the grid.

Figure 4 – Proposed trigeneration plant layout.

0

5

10

15

20

25

30

35

40

45

50

Po

wer

[kW

e]

Energy Demand Profile

Cooling Electricity

4

From the consumption maps and cooling plant design we

obtain values of 45 kWe, or 75 kWc for maximum loads,

then is possible to establish relationships between the

power necessary for the absorption chillers equipment

and for the prime mover. To define the absorption chiller

size equivalent electric power is used, as if it was a

compression chiller. In case A power for the minimum

load values is established, in case B a base load is defined

taking into account the average consumption in the night

time and the cases C and D are set to 70 and 80% of the

maximum load, respectively. The power is allocated in

each case taking also into account the size of the

equipment available. From the relation between kWc

kWe we obtain the minimum values for the cooling

demands of each case. Knowing the absorption units

COP, typically 0.5 on this technology scale, we find the

heat thermal power required to activate the absorption

chillers, i.e. twice the cooling power needs of the plant.

Knowing this value and the technical specifications of

generator sets is possible to reach the minimum

electrical power required in each scenario, table 3 and

Figure 5.

The plant operation schedule is divided into four options

presented in table 4 which were selected taking into

account the daily consumption profiles and billing

periods. This way the CCHP plant is activated when

electricity costs are higher, leaving the conventional

system to operate at night taking advantage of the "off

peak" costs [3].

Table 3 – Minimum power to satisfy cooling demand.

Case Power Chiller [kWe]

Chiller [kWc]

ICE [kWe] ICE [kWc] Turbine [kWe]

Turbine [kWc]

A Min. Load 15 25 25 50 21 50

B Base Load 25 40 40 80 33 80

C 70% Máx. 30 50 50 100 42 100

D 80% Máx. 35 60 60 120 50 120

Table 4 – Schedule options.

Schedule

1 00:00 – 24:00 7 days a week 2 07:00 – 21:00 7 days a week 3 09:00 – 21:00 7 days a week 4 09:00 – 21:00 Business days

Figure 5 – Different scenarios regarding electricity demand for cooling production.

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50

Po

we

r [k

We

]

Hours

Cooling Demand in 24h Period

Case B

Case A

Case C

Case D

Schedule 1

Schedule 2 Schedule 3, 4

5

2.4 Absorption Chiller

Single effect absorption chillers water-ammonia was the

technology selected for cooling production. Due to the

cooling plant small size and lack of equipment with the

exact power presented in Table 3, several units must be

installed in parallel depending on the case. To perform

the study two types of absorption chillers were

considered. The equipment selected was designed to

work with direct combustion of natural gas, but it can be

re-engineered to take advantage of the exhaust gases

heat from the engine or turbine [5]. table 5 shows the

technical specifications of those two devices and Table 6

shows the number of units of each model to be installed

as well as power and total cost for each case.

Table 5 – Absorption Chiller specifications.

Specifications Robur

ACF-60LB Chillii ACC50

Cooling Power [kWc] 13.0 50.0

Electric load [kWe] 0.9 3.0

Fluid Temperature [⁰C] -10 -10

Price [€] 9 100 29 500

Price [€/kwc] 700 590

O&M [€/Kwh/year] 0.008 0.008

Table 6 – Equipment to consider in each case.

Case Equipment Total

Power [kWc]

Electrical Load

[kWe]

Price [€]

A 2 X Robur 26 1.8 18 200

B 3 X Robur 39 2.7 27 300

C 1 X Chillii 50 3.0 29 500

D 5 X Robur 65 4.5 45 500

2.5 Prime Mover

The prime mover choice lies in its ability to produce

electricity and steam. The power generator size is

defined according to the heat required to activate the

absorption chillers as suggested in literature [2]. At these

small sizes the variety of equipment is low and prices

high. The research made indicates two technologies to

be considered for the prime mover: internal combustion

engines, ICE, and microturbines, both using natural gas

as fuel.

Internal Combustion Engine

In the ICE technology, we decided to seek for complete

solutions with ready-to-install units. Thus the cost of the

installed technology is closer to reality instead of

assuming indicative values of €/kWe that commonly

duplicate when installing the equipment. Several devices

were analysed and selected those that satisfy the

requirements set at the lowest price, see table 7. The

number of units to be installed in each case, the total

power of the plant and its cost is shown in table 8.

Table 7 – ICE specifications [6].

Specifications EC Power XRGI 15

EC Power XRGI20

Electric load [kWe] 15 20

Electric efficiency [%] 30 32

Thermal efficiency [%] 60 64

Price [€] 27 750 32 750

Price [€/kwe] 1 850 1 638

O&M [€/Kwh/year] 0.0165 0.0165

Table 8 – Equipment to consider in each case.

Case Equipment Total Power

[kWe] Price [€]

A 2 x XRGI15 30 55 500

B 2 x XRGI20 40 65 500

C & D 3 x XRGI20 60 98 250

Microturbine

If ICE market variety is reduced at this level of power, in

microturbines options are further restricted. It matters

only to point out a brand, Capstone which produces

microturbines and has some documentation available on

the internet. This technology has also some ready-to-

install solutions. The power associated to the analysis of

each case is described in Table 9 and the technical

specifications and the estimated costs provided by the

manufacturer are shown in Table 10.

Table 9 – Equipment to consider in each case.

Case Equipment Total Power

[kWe] Price [€]

A & B Capstone C30 28 60 000

C & D Capstone C65 65 97 500

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Table 10 – Microturbines specifications [7].

Specifications Capstone

C30 Capstone

C65

Electric load [kWe] 28 65

Electric efficiency [%] 25 29

Thermal efficiency [%] 60 62

Price [€] 60 000 97 500

Price [€/kwe] 2 000 1 500

O&M [€/Kwh/year] 0.006 0.006

3 Results

This section presents the results obtained through

simulations of the cases considered (A, B, C and D),

combined with the possible schedules (1, 2, 3 and 4), in a

total of sixteen different scenarios to analyse.

The scenarios considered allow both technologies to

operate with or without the partial load option. There

will be presented the most favourable solutions for each

prime mover, ICE operating with partial load mode

enable and microturbine at maximum load as the results

are the same with or without partial load mode due to

the optimal size of the plant.

3.1 ICE with Partial load mode enable

The ICE technology was analysed, using the partial load

option to 50% in order to optimize the operation of the

trigeneration plant, as shown in Figure 6. To perform a

pre-selection, indicators such as net present value and

savings in energy bills were selected.

Figure 6 – ICE economic analysis with half load enable.

Columns with values less than zero are not visible in

Figure 6, so all scenarios that do not exhibit both

columns are excluded. In the previous figure we can see

that A4, B3, B4, C4 and D4 scenarios had positive results

in both indicators. This way can show in Table 11 the

results for the best three scenarios, which contains other

economic indicators and relevant energy analysis.

Table 11 shows that the most energetic and financially

attractive scenario is B4, i.e. case B with the operation

schedule 4. It should be noted a payback of 10.1 years,

an internal rate of return of 7.7 % and a net present

value of 44 304 euros (values obtained with an interest

rate of 3%). The energy parameters are also favourable

considering primary energy savings greater than 20% and

an absolute primary energy savings of 9.8%.

Table 11 – ICE results with partial load enable.

Parameters B4 C4 D4

EBS [€/year] 9 216 10 501 11 687

Investment [€] 92 800 127 750 143 750

Payback [Years] 10.1 12.2 12.3

IRR [%] 7.7 5.3 5.2

NPV [€] 44 304 28 472 30 120

PES [%] 22.7 22.7 22.7

PESAbs [%] 9.8 11.6 12.1

TOE savings [toe/year]

35 31 31

GHG savings [tCO2eq/year]

60 47 46

EEE [%] 110.8 110.8 110.8

Electrical load [%] 22.4 27.3 29.1

Thermal Load [%] 25.9 31.1 35.7

Values obtained by the model calculation are obviously

theoretical, and in some parameters, based on data

provided by manufacturers which not always correspond

to reality. The value of the equivalent electrical

efficiency, for example, is directly influenced by the

efficiency indicated by manufacturers which results in

values greater than 100% in some scenarios.

3.2 Microturbine

Although there are differences in some scenarios in the

energy savings values for the case of the turbine

operating at maximum or partial load, the net present

value, and results are the same. Only one scenario is

favourable on both indicators and the results are the

K€

10K€

20K€

30K€

40K€

50K€

K€

2K€

4K€

6K€

8K€

10K€

12K€

14K€

A1

A2

A3

A4

B1

B2

B3

B4

C1

C2

C3

C4

D1

D2

D3

D4

Scenarios

ICE - Partial Load Enable

EBS NPV

7

same with or without load control, which is why we show

just the microturbine case at full load, see figure 7.

Figure 7 – Microturbine economic analysis at max load.

Through Figure 7 it can be concluded that B4 is the only

scenario that presents favourable results in both

economic indicators. Detailed results are represented in

Table 12. The operation result is negative in all other

scenarios because the initial investment in this

technology is high and the energy saving is reduced

which leads to a high payback and a negative NPV.

Table 12 – Microturbine analysis summary.

Parameters B4

EBS [€/year] 6 850

Investment [€] 87 300

Payback [Years] 12.7

IRR [%] 4.7

NPV [€] 14 608

PES [%] 10.8

PESAbs [%] 4.0

TOE savings [toe/year] 36

GHG savings [tCO2eq/year] 62

EEE [%] 75.0

Electrical load [%] 16.9

Thermal Load [%] 25.9

3.3 Comparative Analysis

Following the analysis in the previous sections, the most

favourable scenario for each technology can now be

elected, trough Table 13.

Table 13 – Technologies summary.

Parameters ICE Microturbine

EBS [€/year] 9 216 6 850

Investment [€] 92 800 87 300

Payback [Years] 10.1 12.7

IRR [%] 7.7 4.7

NPV [€] 44 304 14 608

PES [%] 22.7 10.8

PESAbs [%] 9.8 4.0

TOE savings [toe/year]

35 36

GHG savings [tCO2eq/year]

60 62

EEE [%] 110.8 75.0

Electrical load [%] 22.4 16.9

Thermal Load [%] 25.9 25.9

From the table above it is possible to conclude that the

ICE with load control, corresponding to the scenario B4,

is the most attractive choice.

The option includes an initial investment of € 92 800

which is estimated to be recovered in about 10 years,

and an IRR of 7.7%. At the end of equipment life, i.e.

after 20 years, the NPV of the investment is € 44,304

with an interest rate of 3% per year.

Summing up the scenario B4:

ICE with partial load mode

o 2 X Ec Power XRGI 20 – Total power 40 kwe.

Single effect Absorption Chiller water-ammonia

o 3 X Robur ACF-60LB – Total cooling power ≈40

kWc or 25 kwe

Operating schedule – Business days from 9 to 21h

Figure 8 shows the supermarket operation in a working

day of January, clearly highlighting the cooling demand,

the operating schedule of the absorption chillers pack

and the period in which the compression chillers are

active.

K€

2K€

4K€

6K€

8K€

10K€

12K€

14K€

16K€

K€

1K€

2K€

3K€

4K€

5K€

6K€

7K€

8K€

A1

A2

A3

A4

B1

B2

B3

B4

C1

C2

C3

C4

D1

D2

D3

D4

Scenarios

Microturbine Máx Load

EBS NPV

8

Figure 8 – Cooling production in January weekdays

Figure 9 shows the supermarket electricity demand, the

electricity produced by the generator and one that is

imported from the grid. Although not showed here, the

energy demand in the summer is higher than winter

because the power consumed by the compression

chillers is higher, as expected.

Figure 9 – Electricity Demand and production during August

weekdays.

Due to the selected prime mover size the amount of

energy sold to the grid is reduced, however it occurs in

the transition from daytime to night-time, representing

around 596 kWh which translates into approximately 66€

of cash flow. When analysing scenarios that consider

higher power generators and running during the entire

year, more electricity is produced in excess and sold to

the grid, generating higher cash flow however the higher

fuel costs lead to a negative balance.

4 Conclusion

When analysing a building of this type, geographically

located in a Mediterranean climate zone and taking into

account the equipment size, it is possible to draw some

conclusions.

Comparing the results obtained it appears that the most

viable solution in operating strategy is schedule 4,

weekdays from 9 to 21 hours, contributing to a

significant reduction of conventional plant operation

during the peak period. Thus the CCHP plant runs about

3048 hours or 35% of the year. Other combinations of

operation schedules and sizes were investigated in this

study, however not showing better results to the

previously presented and therefore not shown.

The ICE option achieved the best results with a total

installed capacity of 40kwelectric along with a cooling plant

with three single effect absorption chillers water-

ammonia, working in parallel to provide approximately

40kWcooling. The most favourable operating strategy

consists in running the plant from 9 AM to 21 PM during

the week days with the partial load (50%) option enable.

The economic and energetic results indicate a net

present value of € 44 304, a payback period of 10.1 years

and an absolute primary energy savings of 9.8%.

The study carried out also shows that it is not profitable

to operate the trigeneration plant during full time

because the cost of electricity during night time is about

25-30% less than the cost in daytime, as such the

proposed system should be operated only during the

daytime.

From the economic point of view and regarding the

current economic climate a payback period of about 10

years is not attractive to potential investors thus this

investment is not considered economically viable. As

stated in the literature, the most favorable scenarios for

0,05,0

10,015,020,025,030,035,040,045,050,0

Co

olin

g P

ow

er

[kW

e]

January - Weekdays

Absorption Chiller Compression Chiller

Cooling Demand

0,0

20,0

40,0

60,0

80,0

100,0

Ele

ctri

c P

ow

er

[kW

e]

August - Weekdays

Electricity imported from gridElectricity Generated by ICE unitElectric Demand

9

this type of investment reach payback periods of 4-5

years, about half the time compared to the best scenario

presented by this study.

The trigeneration technology may become profitable in

this area of application if we consider larger

supermarkets and/or placed in residential or commercial

parks with higher energy needs. Thus the equipment size

will be greater which will benefit from significant

reduction in the technology cost per electrical kilowatt. It

is also possible to obtain more economically attractive

results if the building is located in a colder climate region

such as northern Portugal where the demand for heating

is higher than the Midwest Region. Other option to make

this a profitable investment is to include in the analysis a

high school situated next to the supermarket. The school

schedule is similar to the supermarket and has both

thermal and electrical demand.

Acronyms

CCHP – Combined Cooling, Heat and Power

CHP – Combined Heat and Power

COP – Coefficient of Performance

EBS – Energy Bill Savings

EEE – Equivalent Electric Efficiency

GHG – Green House Gas

HAVAC – Heat, Ventilation and Air Conditioning

ICE – Internal Combustion Engine

IRR – Internal Return Rate

NPV – Net Present Value

O&M – Operation and Maintenance

PES – Primary Energy Savings

PESAbs – Absolute Primary Energy Savings

TOE – Tone of Oil Equivalent

5 References

[1] M. Jradi e S. Riffat, “Tri-generation systems: Energy

policies, prime movers, cooling technologies,

configurations and operation strategies.,” Renewable

and Sustainable Energy Reviews, vol. 32, p. 396–415,

2014.

[2] M. Marimón e e. al., “Integration of trigeneration in

an indirect cascade refrigeration system in

supermarkets.,” Energy and Buildings, vol. 23, p.

1427–1434, 2011.

[3] G. Maidment e e. al., “Application of combined heat-

and-power and absorption cooling in a

supermarket.,” Applied Energy, vol. 63, pp. 169-190,

1999.

[4] G. Maidment e R. Tozer, “Combined cooling heat and

power in supermarkets,” Applied thermal

engineering, vol. 22, pp. 653 -665, 2002.

[5] I. Suamir e S. Tassou, “Performance evaluation of

integrated trigeneration and CO2 refrigeration

systems,” Applied Thermal Engineering, vol. 50, pp.

1487 -1495, 2013.

[6] Sav Systems - EC Power, “LoadTracker CHP operating

efficiencies & Data Sheet,” 2012.

[7] Capstone Turbine Corporation, “Capstone Data Sheet

& specifications,” 2010.