Effect on Energy Savings andReduction in CO 2 Emissions
through Application of a HighEfﬁciency Centrifugal Chiller
JUNNOSUKE NAKATANI*1

WATARU SEKI*2

MASAHIRO ISHII*2

FUTOSHI NISHIZAKI*2

Air-conditioning accounts for a large portion of the energy consumed by industrial and commercial facilities. More speciﬁcally,according to the Energy Conservation Center, Japan (ECCJ), air-conditioning amounts to approximately 40% of the energy consumed
in commercial buildings such as shopping malls. Energy-efﬁcient air-conditioning should therefore make an important contribution
to the reduction in CO2 emissions. In response to such needs, Mitsubishi Heavy Industries, Ltd. (MHI) provides energy-saving
solutions based on centrifugal chillers that produce an excellent level of efﬁciency. At the core of these solutions is the achievement ofcomplete optimization by providing consistent heat source solution services commensurate with the life cycle of products and systems
to promote more effective energy-savings. Here we introduce MHI’s involvement in this ﬁeld.

open impellers that allow high-precision machining andeliminate the breakaway and stagnation of refrigerantgas flow. The compressor is designed with flow analysis
including static channels, and it is more efﬁcient than earlier
models. An additional capacity control mechanism installedat the inlet of the second stage impellers has improved
the efficiency in low-load operations. Furthermore, highcapacity machines, with inverter specifications, are alsoavailable so that the entire series of products successfully
achieves the optimal rotational speed control and expands
the zones of operation based on the operating conditions. Asa result, the load tracking control remains efﬁcient even if thecooling water inlet temperature falls to 12°C, contributing
to dramatically improving the partial load performance(coefﬁcient of performance, COP) as shown in Fig. 1.

1. Features of centrifugal chillers with exceptional
levels of efﬁciency 1
The following section describes features of the latest
cent r i f uga l chil ler models developed by M H I , which
represent the key technology to energy-saving solutions.
The refrigerant used in the chillers is HFC-134a, which does
not destroy the ozone layer. Moreover, modifications andimprovements to the compressors, heat exchangers, andother units achieve higher performance over the entire rangeof operation.
The centrifugal compressor is equipped with two-stage

Major improvementin characteristics
Cooling water inlet
temperature

Comparison index(100 = value before the solution
was implemented)

COP

Inverter machine (AART-I series)
2220181614121086420: AART-I 12°C
: AART-I 20°C
: AART-I 25°C
: AART-I 32°C
: NART-I 13°C
: NART-I 20°C
: NART-I 25°C
: NART-I 32°C

2040 60
Load (%)

80 100: Estimated reduction before the solution
was implemented: Actual reduction after the solution
was implemented

Reductionof about 70%

Runningcosts

Fig. 1 Partial load characteristics of the variable speed drive
centrifugal chiller (chilled water outlet temperature: 7°C)
The diagram shows a major improvement in the par tial load
characteristics. The coefficient of performance (COP) can be
expressed by the following equation.
COP = output energy/input energy

Reductionof about 60%

Primary energy
consumption

Reductionof about 65%

CO2emissions

Fig. 2 Effects of reducing energy consumption and CO2emissions
The diagram shows that implementing the solution greatly reduces
the yearly running costs and CO2 emissions.

*1 Takasago Research & Development Center, Technical Headquarters*2 Air-Conditioning & Refrigeration Systems Headquarters

73

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

After introducing the solution
Variable speed drive centrifugal chillers (360USRt × 2 units)- Introducing high-efficiency variable speed drive centrifugal chillers(major reduction in power consumption under partial load operation during
intermediate seasons and winter)- Using the high-capacity cooling tower and cooling water pump of
the absorption chiller(lowering cooling water temperatures and variable flow control of cooling water)

Steam

Steam-type absorption
6°C
chiller 360USRt
chilledwater
Steam

6°C
Steam-type absorption chilledchiller 360USRt
water

Chilled water reservoir tank

6°C
chilledwater

Electricity

Electricity

Variable speed drive
centrifugal chillers
360USRt

6°C chilled water

6°C chilled water

Variable speed drive
centrifugal chillers
360USRt

6°C chilled water

Load heat exchanger

Chilled water reservoir tank

Before introducing the solution
Steam-type absorption chiller(360USRt × 2 units)

Load heat exchanger

Fig. 3 Example of an introduced solution
An example in which the centrifugal chiller replaces the steam-type absorption chiller.

chiller. This contributes to a major reduction in power
consumpt ion dur ing pa r t ia l load operat ions in t heintermediate seasons and winter.
(2) By effectively using the existing high-capacity cooling
tower and the cooling water pump of the steam-type
absor ption chiller, the resulting lower temperature
cooling water and variable f low control of the coolingwater help save more energy than was previously possible.

2. Case study of a solution for saving energy andreducing CO2 emissions
T his sect ion int roduces a solut ion proposed to theTakasago plant of Suntory Ltd. Heating and cooling takeplace at various temperatures in this food and beveragefactory, which requires a heat source capable of copingwith loads that vary considerably. The proposed energysaving solution satisﬁed these requirements while achieving
6 0 % or more in energ y savings and reductions in CO 2emissions (Fig. 2). Figure 3 provides a schematic of theproposal. The main points of the proposal are as follows.
(1) A high- ef f iciency, variable speed drive centrif ugal
chiller has replaced the existing steam-type absorption

3. Solution proposal scheme
Figure 4 shows the schematic f low of events in the
proposal in which the latest system technology improves
the chiller, and its peripheral equipment and air conditioner
reduce energy consumption. The ﬂow of events is as follows.

Accuratelyinterpret
current
conditions

21

3
CO2
DOWN

Assess client
requirements

6Measure anddiagnose

Plan and studyProvide a complete solution toeliminate client concerns
ReduceEnergyCost
CO2 emissions
savings reduction

Propose and implementa variety of solutions fromthe service menu, including
remote monitoring service,operation, maintenance, andafter-sales service

4

Total
optimization

5
Proposesolution plans
Design andinstallation

Fig. 4 Energy-saving solution proposal scheme showing the event ﬂow of the solution proposalcommensurate with the life cycle of products and systems

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)74

Remote monitoring system
ClientMonitoringcenter• Periodical maintenance• Mobilization in an emergency

Internet orpublic phone line
monitoring• Monitoring
operation data at monitoring center• In an emergency, the manufacturer’s
service engineer diagnoses data tooffer appropriate instructions to local
service personnel• Emergency measures and disposition report• Reporting periodical operation data
• Recommending appropriate maintenancebased on dataFig. 5 Outline of the remote monitoring service

Air
conditioner

Chilled water
1360m /h
34747kW
1350RT8°C- t 3°C

User, air conditioner
Supplier, heat source

5°C

Chilled water pump
Cooling water pump Cooling tower
Centrifugal chiller 1350RT
30kW
110kW
454m3/h
Coping with a chilled water flowrate that exceeds the rated flowrate reduces the number ofchillers in operation.

Excess operation

Chilled water pump
Cooling water pump Cooling tower
Centrifugal chiller 1350RT30kW
110kW
454m3/hChilled water pump
Cooling water pump Cooling tower
Centrifugal chiller 1350RT
30kW
110kW
454m3/hAir
conditioner

Chilled water
1360m3/h

4747kW
1350RT
8°C

User, air conditioner- t 3°C

Supplier, heat source

5°C

Cooling water pump Cooling tower
30kW
110kW
Group of chilled water pumps,
pumping a maximum of 1360m3/h

Centrifugal chiller 1350RT

Fig. 6 Outline of the variable over water volume system (patented)
Coping with a chilled water ﬂow rate that exceeds the rated ﬂow rate reduces the number of chillers in operation, which in turn reduces energy
consumption.

(1) C ol lec t i n for mat ion f rom t he cl ient on t heconfiguration of the facility and its operating conditions,energy consumption, and other details. (2) If necessary,measure the operat ion parameters of the facilit y anddiagnose them to study the items requiring energy-saving.

(3) Based on the study results, use simulation or anothermethod to calculate the energy-saving effect. (4) At a stage
when the effect is conﬁrmed, compile the study results intoa solution proposal for submission to the client. (5) Discuss
the proposal with the client. Upon securing the client’s
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)75

approval, begin detailed design and installation. (6) Afterinstallation, provide the client with solutions to the chiller
operation, including support for energy-saving operations
through remote monitoring and equipment diagnosis 24
hours a day, 365 days a year (Fig. 5) and 15-year operation
support for functions, stable operation, and performance
maintenance. Given this after-sales service menu, checking
the operating conditions of the equipment and implementing
appropriate maintenance enables the improvements to beevaluated and confirmed. This helps the client to better
appreciate the major energy savings compared to a singleunit energy-saving proposal.

of the entire facility. To improve these situations, Mitsubishi
Jisho Sekkei Inc. and MHI have codeveloped a Variable overwater volume system. Figure 6 shows an outline of thissystem. To reduce the number of chillers in operation, thesystem can handle the ﬂow rate of chilled water that exceeds
the rated flow rate, enabling the system to reduce energy
consumption.
4.2 Stealth Turbo ® system
Facilities equipped with two or more absorption chillers tocope with peak loads in summer must operate fewer chillers
under partial load during the intermediate seasons andwinter when the load is generally reduced. In many cases like
this, the entire facility is likely to operate at low efficiency.
We suggest that clients in this situation install a smallcapacity centrifugal chiller, the Stealth Turbo ®, to realize
energ y savings without investing money by increasing
the contract power capacity. Mitsubishi Jisho Sekkei Inc.,SANKI Engineering Co., LTD and MHI have codeveloped
this system, which is outlined in Fig. 7.

4. System examples applying chillers
The solution proposals involving the centrifugal chiller
are divided into a variety of aspects covering the variable
ﬂow control of chilled water and cooling water, free cooling,
and reuse of waste heat. The following sections introduce
actual energy-saving proposals employing systems aroundchillers.
4.1 Variable over water volume system
Genera lly, t her mal loads should be reduced in t heintermediate seasons and winter. However, dependingon the control on the load circuit, the heat exchanger
characteristics, and other factors, the reduction in thethermal loads may not be proportional to the reduction in
the flow. This makes the temperature difference smaller
between the supply water temperature and return watertemperature, and is more likely to lead to nonreduction of theload ﬂow. In facilities where the system controls two or morechillers, more than the necessary number of chillers arelikely to operate in many cases, which lowers the efﬁciency

5. Conclusions
Reducing energy consumption and CO2 emissions is oneof the most critical issues facing the world. Given thesecircumstances, we have introduced MHI’s involvement inproviding consistent heat source solutions from facility
investment planning and installation to after-sales service.
T hese solutions optimize the air conditioners and heatsource facility to effectively reduce energy consumption andCO2 emissions. MHI’s home page at the URL below provides
valuable information for clients.
Air-Conditioning & Refrigeration Systems Headquarters
URL: http://www.mhi.co.jp/en/aircon/index.html
Reference

Cooling
tower

Cooling
tower

Absorption
chiller

Absorption
chiller

Absorption
chiller

Small-capacity,
high-efficiency
centrifugal chiller

Stealth Turbo® system

Cooling
tower

1. Seki, W. et al., New Model Turbo Chiller, AART Series, Contributes
to the Reduction of Energ y Consumption under Year-RoundOperation, Mitsubishi Heavy Industries Technical Review Vol.43
No.2 (2006) p.41

Junnosuke Nakatani

Wataru Seki

Masahiro Ishii

Chilled water header (return)
Chilled water header (supply)
Fig. 7 Outline of the Stealth Turbo® system (patent pending)
A bypass circuit is hooked up to the existing chilled water andcooling water pipe lines to install a small-capacity, high-efﬁciency
centrifugal chiller.

Futoshi Nishizaki

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)76