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Implementation of District CoolingSystem in Hong Kong:

Challenges and Experiences

Ir Patrick CHEUNG, Ir LO Siu Kuen,Ir MA Chun Yue

Electrical and Mechanical ServicesDepartment

Government of the Hong Kong SpecialAdministrative Region

ABSTRACT

The Kai Tak Development (KTD) is a

huge development project spanning a totalarea of over 320 hectares covering theex-airport and nearby areas in which therehas been a planned total of about 1.73million square metres of public and privatenon-domestic air-conditioned floor areasrequiring a large demand forair-conditioning of about 284 megawatt ofrefrigeration (MW). The Government aimsto promote energy saving and took the leadto implement District Cooling System (DCS)

which could be considered the most energyefficient air-conditioning system for thedevelopment at KTD.

The project of DCS at KTD is toconstruct a large scale centralizedair-conditioning system which would

produce chilled water at its central chiller plants and distribute the chilled water toconsumer buildings in the KTD through anunderground water piping network. Apartfrom constructing central chiller plant rooms,the laying of underground water pipes isanother challenging task in view of theuncertainty in underground conditions. Theoperation of DCS is also one of the mostchallenging tasks to be dealt with. Energyefficiency as well as reliability of servicesare both important. This paper aims to

present the challenges faced and alsoexperience gained during theimplementation and operation of the DCS.

Keywords: District Cooling System (DCS),

Kai Tak Development (KTD), EnergyEfficiency

1. INTRODUCTION

Since the operation of the Hong KongInternational Airport at Chep Lap Kok in1998, the Government started to develop theex-Kai Tak International Airport and thenearby areas into a new development area –Kai Tak Development (KTD). KTD is ahuge development spanning a total planningarea of 320 hectares. It comprises varioustypes of buildings including hospitals, hotels,schools, commercial buildings, sportfacilities, residential buildings, government

buildings, etc. With the high cooling demandand diversity of cooling load profiles, theGovernment took the opportunity toimplement District Cooling System (DCS)at KTD.

2. BENEFITS

DCS consumes 35 percent and 20 percent less electricity as compared totraditional air-cooled air-conditioningsystems and individual water-cooledair-conditioning systems using coolingtowers respectively. With its high energyefficiency, the implementation of DCS atKTD will achieve estimated annual savingof 85 million kilowatt-hour (kWh) inelectricity consumption, with acorresponding reduction of 59,500 tonnes ofcarbon dioxide emissions per annum.

Apart from energy saving, DCS wouldalso bring along the following benefits to theconsumers:

a. Reduction in upfront capital cost forinstalling chiller plants at their

buildings which account for about5-10% of the total building cost;

b. More flexible building designs forconsumer buildings as they do not

need to install their own chillers andthe associated electrical equipment

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in their buildings;

c. Mitigation of heat island effects inKTD and elimination of noise andvibration arising from the operation

of heat rejection equipment andchillers of air-conditioning plants in

buildings as such equipment will nolonger be necessary for buildingssubscribing to district coolingservices; and

d. More adaptable air-conditioningsystem to the varying demand ascompared to individualair-conditioning systems. For eachindividual building, cooling capacitycan be increased by requestingadditional cooling capacity from theDCS without carrying out extensivemodification works for the buildingin question.

3. IMPLEMENTATION

The DCS at KTD comprises two centralchiller plants, namely the North Plant andthe South Plant cum seawater pump house,underground chilled water distribution

piping network, seawater supply anddischarge pipes and consumer substationslocated in the buildings to interface with the

building’s own chilled water circulationsystems. The total cooling capacity of theDCS at KTD would be about 284 megawattof refrigeration (MWr) which could provideabout 1.73 million square metres of public

and private non-domestic air-conditionedfloor areas. The cooling capacities of the North Plant and the South Plant would be162 MWr and 122 MWr respectively. Both

plants are underground structures for thechiller installations with abovegroundfacilities at the North Plant. Uponcompletion of the project, about 39kilometres of underground chilled water

pipes would have been laid and there would be around 60 buildings in KTD connected to

the DCS.

Figure 1 - Kai Tak DCS North Plant

The project will be implemented inthree phases. The construction workscommenced in February 2011. Phase I and II

include the construction of two plant roomsand some pipeworks to enable the provisionof district cooling services to the Kai TakCruise Terminal and Ching Long ShoppingCentre in the public rental housing in 2013.Phase III includes further pipes laying worksand chiller installation to meet the coolingdemand growth in KTD. Phase I and II werecompleted in 2013 and 2014 respectively.Phase III commenced in mid 2013 and isexpected to be completed by 2022.

4. RELIABILITY

To assure the consumers of a reliabledistrict cooling services, several designfeatures have been incorporated into theDCS.

4.1 Electricity Supply

The electricity supply to the DCS plantis such a robust arrangement that eachsupply carries only 50% of the requiredelectrical load such that failure of any one ofthe cable will result in no reduction in the

power supply condition. To furtherenhance the power supply reliability forDCS, 11kV power supply fed from twosupply sources is adopted such that whenone source fails, the power supply will beautomatically switched over to the other

source.4.2 Chilled Water Piping Network

The underground chilled water pipingnetwork is designed to be in ring circuitforming a dual-feed supply so that if thesupply from one side of the distribution

pipework is not available, chilled water canstill be supplied to the consumer buildingsfrom other side. Moreover, the whole

chilled water distribution piping network isdesigned as a 3-pipe system such that when

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one of the duty supply or return pipe isdamaged or under maintenance, the standby

pipe can be put in operation to maintain thedistrict cooling services.

4.3 Chiller Plant

There are always at least one standbychiller and chilled water pump such that ifany one of the chillers or pumps fails, thestandby equipment will be put in operationto maintain full cooling capacity to serve theconsumers.

5. DCS PIPELINES ANDCHALLENGES

The DCS pipelines (including chilledwater pipes and seawater pipes) are mostlylaid along the carriageway while branch

pipes and valve chambers are located in thefootpath in order to minimize conflict withother underground utilities. Due to theirlarge sizes, the DCS pipes are normally laidat the bottom among other undergroundutilities.

The DCS chilled water pipes are pre-fabricated with a pair of leakagedetection cables secured externally at 4o’clock and 8 o’clock positions of the

pipeline, then annular insulated with polyurethane foam and protected in anextruded high density polyethylene (HDPE)outer jacket. Polyurethane insulationtogether with HDPE outer jacket areconsidered with good thermal insulation

performance, mechanically stable and closestructure which provide good resistance tomoisture penetration for direct buriedapplication.

As most of the DCS pipelines are laidunderground, water leakage detection cablesfixed on the chilled water pipes allowmonitoring the condition of pipescontinuously and give early warning of anywater leakage. On site, leakage detection

panels are installed at an interval ofapproximate 1.5 kilometres of pipe run to

closely monitor any water leakage point andidentify any fault signal due to broken cable.

Figure 2 - Prefabricated chilled water pipe

with leakage detection cables and polyurethane insulation with HDPE outer

jacket

In general, open trench excavationsecured with sheet-pile walls are adopted forthe laying of DCS pipes. However, in somelocations, there are prohibitively existingsite constraints for constructing any opentrench. To surmount such constructiondifficulties, trenchless excavation methodare adopted.

Other than the congested undergroundutilities, there are also various existing andnew structures including Kwun Tong Bypass,Kai Tak Tunnel, box culverts, Kai TakTaxiway Bridge, Shatin-Central Link,Central Kowloon Route and Kai TakApproach Channel in close proximity of theDCS pipelines which are the constraints inlaying the DCS pipes.

There are about 11 sections of DCS pipelines to be constructed with trenchlessmethod at KTD, where open trenchexcavation method is not practical or thespace required for laying of DCS pipesabove the existing structure is inadequate. Inwhich, about 1km of DCS pipes areconstructed or will be constructed byheading method or hand dug tunnel. Over

5km of DCS pipes are constructed or will beconstructed by pipe jacking method with theuse of tunnel boring machines (TBMs). Thelargest TBM size is 2,800mm in diameterwhich is the largest one ever used in pipe

jacking in Hong Kong.

In order to have smooth construction of pipe laying works, considerable pre-construction precautionary measures toidentify the actual underground conditions

and to determine the appropriate type oftrenchless excavation method are necessary.

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In view of this, the following measures will be carried out before construction.

a. Sufficient trial pits to expose andverify the existence, extent, location

and elevation of all undergroundutilities, natural or man-madeobstructions and structure; and

b. Suitable underground detectionequipment are used to locateunderground utilities; to furthercheck the underground conditionsalong the proposed pipe jackingroute; and to assess the feasibilityof the proposed route, locations andinverts of jacking and receiving

pits.

The formation level of the Kai TakDevelopment is generally at +5.0mPDwhilst the mean sea level in 50 year return

period is approximately +3.5mPD. As someof the pipelines and excavation pits are aslow as (-)2mPD to (-)4mPD, the chance ofseepage of underground water is high. Foropen trench excavation, the toe-in of thesheet-pile wall need to be carefullydetermined to prevent the inflow ofunderground water from the bottom of thetrench. Installing grout curtain to controlgroundwater inflow into the excavation, aswell as dewatering from inside thecofferdam excavation for all excavationlevels are required. For trenchlessexcavation method, ground treatment needto be carried out to control ground water

flows to stabilize ground prior excavation.

Figure 3 - Grout curtain and dewatering provided at deep open trench for pipe laying

Due to deep excavation and trenchlessexcavation construction involved,establishment of settlement control pointsand survey of the existing ground levels arerequired to be set up for close monitoring of

the underground condition. Measurements atsettlement points are carried out at least

twice a day before and after any jackingworks. The alarm-alert-action (3As)measures are adopted in ground movementmonitoring.

6. DCS OPERATION ANDCHALLENGES

The cooling energy required by eachconsumer building will be transferred fromthe DCS to the individual building’s centralair-conditioning system via plate type heatexchangers installed inside the substation ofthe consumer buildings. The primary side ofthe heat exchanger is connected to the DCSdistributing chilled water pipes and thesecondary side is connected to theconsumer’s chilled water system pipework.

Figure 4 - Distribution of district coolingservices

Under normal operating conditions, thedesigned chilled water supply and returntemperatures are as follows:-

a. At the primary chilled water side ofthe heat exchanger, i.e. DCS side:

Supply Temperature = 5 ℃ Return Temperature = 13 ℃

b. At the secondary chilled water sideof the heat exchanger, i.e. consumerside:

Supply Temperature = 6 ℃ Return Temperature = 14 ℃

It is desirable for both the DCS plantoperator and the consumers to meet theabove design conditions in order to achieveenergy efficient DCS plant operation andreliable chilled water supply to theconsumers.

Since the commencement of districtcooling services in 2013, in some

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circumstances like low load condition, it isnoted that when the chilled water returntemperature at the consumer side fall below14℃, the temperature difference (Delta-T)at DCS side and consumer side are reduced.

Accordingly, the chilled water flow rate ascompared to the design flow rate for a givencooling load demand has to be increased. Asa result, more pumping energy is consumedand the system efficiency is adverselyaffected.

In order to enhance the energyefficiency of the DCS, the consumers arerecommended to incorporate the followingdesign features in their chilled water systemof the consumer side.

6.1 Variable Flow Chilled Water

Variable flow chilled water systemtogether with two-way equal percentagecontrol valves for controlling all air handlingunits (AHU) and fan coil units (FCU) should

be adopted. Each control valve should becapable of controlling the flow throughoutthe entire range of designed operatingconditions of the equipment.

6.2 Temperature Oriented Control

Operation of the control valves forcontrolling the AHU/ FCU should makereference to the Return Air Temperature(RAT). Since RAT represents the actual heatload from the building, adjusting the controlvalve based on the RAT instead supply air

temperature or off-coil temperature couldmaintain the designed chilled water Delta-T.

Figure 5 – Return air temperature control onair handling units

6.3 Interlocking Control Mechanism

When the status of the AHU/ FCU is off,the associated control valves should also be

closed in order to save energy and enhancethe system’s efficiency. Such interlocking

control mechanism could be implemented by programmable logic controller (PLC) orsimilar mechanism.

7. CHARGING PRINCIPLES

The public and private non-domestic building owners or their authorized agents inKTD who have central air-conditioningsystem of their buildings being subscribed todistrict cooling services are required to paythe district cooling services charges to theGovernment. The District Cooling ServicesBill is being introduced to the LegislativeCouncil in 2014-15 to set the tariffmechanism and tariff rate.

The district cooling services tariff is proposed to be set out with the followingcharging principles.

a. The district cooling services tariffshould be set at a competitivelevel comparable to the cost ofindividual water-cooledair-conditioning systems (WACS)using cooling towers which is oneof the most cost-effectiveair-conditioning systems availablein the market;

b. Both the capital and operatingcosts should be recovered from theconsumers over the project lifewhich is estimated to be 30 yearsas taxpayers should not subsidizesuch air-conditioning charges;

c. Price stability could be achievedunder the proposed chargingmechanism; and,

d. The proposed charging mechanismshould be a simple chargingregime with common charge ratesfor all consumers regardless oftheir load profiles.

8. KEY TARIFF COMPONENTS

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In line with international practices, the proposed tariff of district cooling servicescomprise two major components, namely thecapacity charge and consumption charge:-

8.1 Capacity Charge

The capacity charge serves to cover thecapital cost of the DCS plant and equipmentand operation and maintenance (O&M) cost.The capacity charge will be levied accordingto the contract cooling capacity, which will

be determined by the consumer and agreed by EMSD before the commencement ofdistrict cooling services.

8.2 Consumption Charge

The consumption charge will be leviedto cover costs that will vary according to thedemand of the consumer. The major part ofthe charge is the utility cost such aselectricity used to generate chilled water

being delivered to the consumer.

9. TARIFF ADJUSTMENTMECHANISM

Having regard to the composition of thetwo charges, the capacity charge rate is

proposed to be adjusted annually based onthe Composite Consumer Price Index whilethe consumption charge rate is proposed to

be adjusted annually to take into account ofthe change in electricity tariff rate.

10. CONCLUSIONS

Subsequent to the projectcommencement in early 2011 and thecompletion of the early phases of the project,the DCS at KTD has been providingservices to consumer buildings includingKai Tak Cruise Terminal and Ching LongShopping Centre in the public rental housingsince 2013. The construction of theremaining phase of the DCS project is in

progress and will be completed along with

the growing needs of air-conditioning of thenew buildings which are coming up

progressively. It is expected that the whole project will be completed around 2022 andwill achieve an estimated annual saving of85 million kilowatt-hour (kWh) in electricityconsumption.

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