Seawater Cooling and the Central Chiller Plant – the "Natural Air-conditioning" of HKUST's Coastal Campus

# Seawater Cooling and the Central Chiller Plant – the "Natural Air-conditioning" of HKUST's Coastal Campus

In a sentence: Since opening in 1991, The Hong Kong University of Science and Technology (HKUST) has drawn seawater from Clear Water Bay to exchange heat for its central chiller plant. Between 2013 and 2015 the system was remodelled into a district cooling scheme, winning the 2016 AEE Asia-Pacific Energy Project of the Year Award; the Cheng Yu Tung Building alone now saves 20–35% in annual energy consumption.

A campus built into a hillside, facing the sea on three sides, outsources its largest utility bill to the water at its feet. Most people on campus know only that the lecture theatres are cool in summer and the dorms are frigid; few notice that the constant temperature across the entire Clear Water Bay campus is maintained not by rows of power-hungry outdoor condenser units, but by seawater pumped from the bay, run through a heat exchange, and returned to the sea. This article traces the engineering timeline of this "natural air-conditioning" from the university's founding to the recent cooling-tower upgrade, with every figure and claim sourced and annotated.

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# Why does HKUST use seawater for air-conditioning instead of ordinary chillers?

The physical setting of HKUST's Clear Water Bay campus is practically purpose-built for water-cooled air-conditioning. The campus sits on a hillside in Tai Po Tsai on the Sai Kung Peninsula, ringed by the sea on three sides, with its main buildings a few hundred metres from the shoreline; the cost of seawater intake and outfall piping is therefore far lower than for an inland campus. The core challenge of any central air-conditioning system is "where to dump the indoor heat," and seawater offers a heat sink that is both virtually infinite and relatively stable in temperature year-round.

To understand the value of this system, one must first distinguish between the two dominant heat-rejection pathways. One is air-cooled: the chiller uses outdoor air to dissipate heat at the condenser side. The plant is simple, but its efficiency is hostage to air temperature and drops off precisely when it is hottest. The other is water-cooled: water serves as the heat-rejection medium, carrying heat away via a cooling tower or a seawater loop, normally achieving noticeably higher energy efficiency than air-cooled systems. HKUST chose the most advanced branch of water cooling – using seawater directly as the heat-exchange medium on the condenser side.

The Electrical and Mechanical Services Department (EMSD) of the Hong Kong government has long promoted Water-cooled Air Conditioning Systems (WACS) precisely for their energy-saving potential relative to air-cooled alternatives. According to the EMSD WACS programme page^※, water-cooled systems can achieve markedly lower electricity consumption than conventional air-cooled ones and are among the city-wide energy-efficiency measures the department advocates. HKUST's direct seawater exchange belongs to the most favourable category within this approach – it eliminates most of the evaporative water loss associated with cooling towers, handing the heat-rejection task straight to the sea. This is why a science-and-technology university treats "cooling with seawater" as something worth documenting in its sustainability reports.

The three main heat-rejection pathways can be compared side by side to highlight the innate advantages – and the trade-offs – of seawater cooling:

"Peak-summer efficiency" above is a qualitative comparison, based on the EMSD's general conclusion that water-cooled chillers outperform air-cooled ones. Direct seawater exchange holds the greatest advantage for a coastal campus because it sidesteps both the high-temperature inefficiency of air cooling and the evaporative water consumption of cooling towers. Its only prerequisite is immediate proximity to the coast coupled with proper management of discharge temperature rise. HKUST's Clear Water Bay campus meets that coastal prerequisite; it is a geographical dividend that inland institutions cannot replicate.

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# How exactly does the sea carry heat out of a lecture theatre? The mechanics of heat exchange and return flow

The operation of the Central Chiller Plant can be broken down into a clear energy-conveying chain. Heat from the university's academic buildings, halls and laboratories is first absorbed by chilled water; inside the chillers, the chilled water surrenders that heat to the condenser side. The heat on the condenser side is then transferred, via heat exchangers, to seawater drawn from the bay. The now slightly warmer seawater is finally discharged back into the sea under temperature control. Across this entire chain, seawater plays the role of the ultimate heat sink; the campus buildings and the sea are separated by multiple layers of heat exchangers, so seawater itself never enters building pipework.

Embedded in this chain is an environmental gate that is easy to overlook: thermal pollution. The seawater absorbs heat and is discharged, raising the water temperature near the outfall. Prolonged, concentrated discharge of warmed water can disturb nearshore marine ecosystems. Such systems therefore require temperature-controlled discharge – holding the temperature rise within permitted limits, and selecting appropriate outfall locations and diffuser designs so the warmed water mixes rapidly and dilutes with the bulk seawater. When HKUST weaves this system into its sustainability narrative, it emphasises the positive values of "seawater heat exchange" and "reduced carbon footprint"; temperature-controlled return flow is the prerequisite that makes a long-term seawater cooling operation possible.

The system also harbours a "waste-into-resource" touch. According to an official HKUST announcement, the waste heat on the condenser-water side of the Central Chiller Plant is not entirely discarded but is recovered to supply hot water to campus facilities. The official text reads:

"Waste heat from the plant's condensing water is also reused for providing campus facilities with hot water." (HKUST official announcement)^※

In other words, the same system simultaneously supplies cooling to buildings and intercepts heat that would otherwise be dumped, using it to heat water. Both the cold and hot ends are utilised, which is a significant factor in the system's favourable energy arithmetic.

The significance of this waste-heat recovery only becomes fully visible through the lens of energy conservation. The essence of cooling is not "destroying heat" but "moving heat from indoors to outdoors." During that process, a large volume of residual heat inevitably accumulates on the condenser side, waiting to be discarded. If that heat were simply discharged with the seawater, the campus would have to burn a separate amount of electricity or gas somewhere else to produce hot water – effectively paying for cooling and heating inside the same building independently. Intercepting condenser waste heat for hot water allows a by-product of cooling to directly displace a portion of the heating energy load; one unit of energy performs two functions. For a campus with substantial hot-water demand from both halls of residence and laboratories, the absolute energy savings from this kind of "combined cooling and heating" approach grow in step with the building footprint.

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# The 2012 capacity expansion: how one new building underwrote a "piggyback" financial case

HKUST's seawater cooling system did not stand still after the initial installation. The campus has expanded continuously, new buildings have been completed, and cooling demand has risen accordingly, bringing the original Central Chiller Plant to a critical capacity-inflection point.

According to official HKUST accounts, the original seawater cooling system was expanded in 2012 to serve the newly completed Cheng Yu Tung Building (CYT). The system was subsequently extended to cover a multi-purpose hall built in the same period, which was expected to be completed by the end of 2018^※ – the facility now known as the Shaw Auditorium. Here lies a key engineering decision: faced with the additional load, the university chose not to build a separate, independent air-cooled system but to expand the capacity of the existing chiller plant.

This decision to "piggyback on capacity rather than build afresh" directly rewrote the cost calculation. According to the HKUST official announcement^※, the university chose to "expand the capacity of the original plant, instead of building a new air cooling system, to maximize efficiency and reduce carbon footprint." Expanding the old plant rather than constructing a new air-cooled one simultaneously improved efficiency, reduced carbon, and saved the capital expenditure that a separate cooling station would have entailed.

The table below sets out the key figures from this expansion alongside their precise scope and reference period:

These figures need to be read with their precise scope in mind: the HK$1.5 million is an annual O&M saving; the HK$7 million is a one-off capital-expenditure avoidance. The two cannot be added to claim "HK$8.5 million saved in a year." The 20%–35% energy saving refers specifically to the Cheng Yu Tung Building, not to the whole campus; the 18% efficiency improvement refers specifically to the three chillers that were replaced. Conflating a percentage that applies to a single building with a campus-wide total is the commonest misreading in this sort of energy-efficiency publicity.

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# The 2013–2015 remodelling and the 2016 AEE Asia-Pacific award: the birth of a district cooling system

Building on the capacity expansion, between 2013 and 2015 HKUST carried out a systematic remodelling of the Central Chiller Plant, upgrading it into a district cooling system. In a district cooling scheme, a single centralised chiller station supplies chilled water to an entire cluster of buildings through a pipe network, replacing the scattered, ad‑hoc air-conditioning plant in each individual block. The economies of scale produced by that centralised dispatch are the source of the efficiency gain.

This remodelling earned the university industry recognition. According to the official HKUST announcement^※, for the project to upgrade the Central Chiller Plant into a district cooling system the university was awarded the 2016 Energy Project of the Year Award for the Asia Pacific Region by the Association of Energy Engineers (AEE). The AEE is an international professional body in energy management and efficiency; its regional annual awards carry significant industry standing.

The then Vice-President of HKUST, Mark Hodgson, said of the award:

"It is a testament to the University's commitment to be a global leader in sustainability education and to put this vision into practice." (HKUST official announcement)^※

It is worth noting that HKUST embarked on energy-efficiency management very early. According to the same official announcement^※, as far back as 1997 the university introduced the first energy-saving performance contract in Southeast Asia. Such contracts tie the investment in energy-efficiency retrofits to the actual energy savings achieved, with the contractor sharing the risk and the returns – a novel concept in 1990s Asia. The district cooling upgrade of the seawater system can be seen as a high-water mark on a nearly three-decade trajectory of energy-efficiency work, not an isolated one-off project.

The core logic of the district cooling remodelling can be distilled into three layers: first, reject heat into seawater rather than outdoor air, circumventing the efficiency penalty that high summer temperatures impose on air-cooled plant; second, centralise chilled-water production in a single station to capture economies of scale; third, recover condenser waste heat for hot-water supply, so neither the cold side nor the hot side is wasted. The 20%–35% energy saving at the Cheng Yu Tung Building and the industry award are the combined result of those three layers.

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# Why is district cooling better than separate chillers in every building? The arithmetic of centralised dispatch

To grasp what the 2013–2015 remodelling actually changed, one must understand what the term "district cooling" altered in practice. Compared with the decentralised model of "a chiller for every block," the differences boil down to three.

The first is economies of scale. In a decentralised arrangement, each building has its own small chiller plant, each sized for that building's peak load. Most of the time these units run at part load, operating well below their most efficient point. A district cooling system, by contrast, uses a single large central station to produce chilled water for the whole campus; it can flexibly dispatch large chillers against the total campus load, keeping them running in their high-efficiency band more of the time. Centralising production and dispatch in one location is the fundamental source of district cooling's efficiency advantage.

The second is the potential for load diversity. Academic buildings peak during the day; halls of residence peak at night. A single centralised system can, in principle, let the daytime demand from the academic zone and the evening demand from the residential zone fill each other's troughs, flattening overall plant utilisation and avoiding the need to install redundant capacity tailored to each zone's separate peak. It should be noted that this "load diversity" argument is a general principle of district cooling; the official HKUST announcement does not disclose specific dispatch data, and this page states it only at the mechanism level.

The third is economies of scope in retrofits and maintenance. With plant concentrated in a single location, replacement, overhaul and upgrade are far more efficient than dealing with dozens of buildings one by one. The decision in 2013–2015 to replace three of the nine old chillers with new models in a single sweep, lifting chiller efficiency by about 18%, is a direct expression of that consolidation advantage. Had the university instead chosen the path of building a separate air-cooled system back then, it would not only have missed the innate efficiency of seawater heat exchange but also incurred roughly HK$7 million in extra capital expenditure; by expanding the existing plant, HKUST captured both the energy-efficiency and the cost-avoidance benefits in a single step.

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# Why were cooling towers added to the library rooftop? A new variable in the seawater system

If the keyword of the mid-2010s was "district cooling," the keyword of the 2020s is cooling tower. This step is not a repudiation of seawater cooling but an extra layer of capability added to cope with climate change.

According to the HKUST Sustainability Unit cooling-tower project page^※, global warming has pushed seawater temperatures higher, and warmer seawater directly erodes the efficiency of a seawater-cooled plant: when the heat sink itself becomes warmer, the system has to work harder to achieve the same cooling effect. To offset the resulting increase in energy consumption at the existing seawater cooling station, HKUST installed cooling towers on the rooftop of the Lee Shau Kee Library (University Library), using evaporative cooling to share the heat-rejection load. This project page was published on 31 October 2023.

A cooling tower works differently from seawater heat exchange: water cascades inside the tower, contacting the ambient air stream; a fraction of the water evaporates, carrying heat away. According to the same official page, a total of four cooling towers have been installed on the library rooftop, each with a heat-rejection capacity of approximately 4,900 kilowatts (kW). These towers work alongside the existing seawater chiller plant, forming a new combined heat-rejection set-up for the main campus air-conditioning system.

To maintain water quality, a cooling tower must continuously "bleed off" a small stream of highly concentrated circulating water to control total dissolved solids and suspended solids – a requirement under the EMSD Code of Practice. HKUST has taken this bleed-off and closed the loop: the water discharged from the cooling towers is recovered and used for toilet flushing in the library. The table below sets out the key water-quantity figures for this project:

Again, scope matters: 8,724 m³ is the reuse volume for the 2022 calendar year; 16,366 m³ is for the 2022/23 academic year. These are different statistical intervals, not two contradictory figures for the same period. In addition to the cooling-tower bleed-off, the official account notes that condensate from air handling units (AHUs) inside the library is also collected and reused, further reducing fresh water consumption.

HKUST's sustainability team has also carried out more cutting-edge experiments on these cooling towers. According to the HKUST "Sustainable Smart Campus" project page^※, the team has trialled applying radiative cooling paint to the cooling-tower water tanks. The idea is to carry heat away from the water without consuming additional electricity, maintaining a lower water temperature so that each cycle removes more heat and the chillers use less power. An experiment of this kind turns a functional piece of heat-rejection plant into an open-air campus laboratory.

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# How much of HKUST's electricity actually goes to cooling? A look at the energy ledger

To gauge the weight of the seawater cooling system, one must place it within the overall campus electricity picture. The carbon footprint of HKUST's Clear Water Bay campus is overwhelmingly dominated by electricity.

According to the HKUST "Sustainable Smart Campus" project page^※, approximately 95% of the campus carbon footprint comes from electricity, and cooling accounts for about 20% of total campus electricity consumption. In other words, keeping buildings cool on its own eats roughly one-fifth of the campus's power and, indirectly, accounts for a commensurate share of its carbon emissions. This explains why HKUST has repeatedly invested effort in its cooling system: even a few percentage points of efficiency gain, multiplied by such a large electricity base, translate into appreciable absolute savings.

In aggregate terms, according to HKUST SDG-related records^※ and sustainability pages, in the 2022/23 academic year the campus consumed roughly 99 million kilowatt-hours (kWh) of energy, of which about 95% was electricity, used to run academic buildings, student and staff quarters, the chiller plant and laboratories. The table below brings together the key proportions in this energy ledger:

Link these numbers: cooling accounts for about 20% of campus electricity consumption; electricity accounts for roughly 95% of the carbon footprint. The energy efficiency of the cooling system therefore almost directly determines the university's success or failure in carbon reduction. Seawater cooling features repeatedly in HKUST's sustainability reports not because it is a novelty but because it sits squarely on the heaviest slice of the campus energy ledger. The university has set a target of reducing energy consumption by 15% by 2028; continued optimisation of the cooling system is an unavoidable part of that trajectory.

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# What price does seawater cooling extract? The other side: intakes, fouling and maintenance

The financial case for seawater cooling looks elegant, but it is not a zero-cost free lunch. Any system that pumps seawater for heat exchange faces a set of engineering challenges that inland cooling-tower systems do not encounter, and this is a side of the "natural air-conditioning" that should not be glossed over.

The first is marine fouling (biofouling): seawater carries algae, bivalve larvae and micro-organisms. Flowing continuously through intake pipes and heat exchangers, these organisms can form deposits and growths on internal surfaces, impairing heat-transfer efficiency and increasing pumping resistance; the system requires periodic cleaning or chemical dosing to suppress them. The second is corrosion: seawater is saline and highly conductive; it corrodes metal pipework and heat-exchange equipment far more aggressively than fresh water, demanding more stringent material selection and anti-corrosion maintenance. The third is intake and outfall siting and temperature control: the intake must be positioned to avoid the thermal plume from the outfall (to prevent "short-circuiting," where newly discharged warm water is immediately drawn back in), while the discharge must control temperature rise and employ well-chosen diffuser locations to avert nearshore thermal pollution.

Regarding the specific anti-fouling and anti-corrosion measures HKUST applies to its seawater cooling system, and their maintenance frequency, this page has found no detailed disclosures in official public sources; it therefore does not expand upon them or speculate on the details. These engineering costs are presented here candidly to make the point that the efficiency advantage of seawater cooling rests on sustained operational and maintenance investment: "saves electricity" is not synonymous with "saves hassle." The cooling towers on the library rooftop mentioned earlier can, in a sense, also be seen as a hedge against the long-term variable of rising seawater temperatures – when the conditions of the heat sink itself deteriorate, the system needs to supplement its heat-rejection capacity.

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# A seawater cooling timeline: from 1991 to 2028

Laying out the key milestones of HKUST's seawater cooling on a timeline makes it clear that this has been not a one-off project but a continuous energy-efficiency operation spanning more than three decades. The table below uses only nodes verifiable in official sources:

On this timeline, the 1997 energy-saving performance contract and the 2012–2016 district cooling remodelling are two anchor points worth particular attention: the former shows that HKUST was managing cooling energy efficiency long before "sustainability" became a buzzword; the latter is the most public culmination of that lineage to date.

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# Does the claim "saves roughly 1.8 million kWh a year" stand up? A candid piece of source-checking

Unofficial accounts and some second-hand introductions describing HKUST's seawater cooling retrofit often cite a specific figure along the lines of "annual saving of roughly 1.8 million kWh." After checking official sources one by one, this page must state plainly: the official HKUST announcements publish savings in monetary amounts and percentages, not in absolute kilowatt-hours.

The verifiable official figures are: 20% to 35% annual energy saving at the Cheng Yu Tung Building, annual O&M cost saving of roughly HK$1.5 million, and roughly HK$7 million in avoided capital expenditure – all from the HKUST official announcement^※. As for a specific kilowatt-hour figure of "1.8 million kWh," this page has found no directly corresponding wording in HKUST's official announcements, Sustainability Unit pages or SDG reports. It may originate from a secondary conversion of a percentage or from a second-hand retelling, but it lacks backing from a primary official figure. This page therefore does not accept it as verified data and merely records its existence and the result of the check.

This "better to record an absence than force a connection" approach is precisely why the seawater cooling subject deserves serious treatment: the value of an engineered system should be supported by verifiable scope – is it for a single building or the whole campus? In money or in kilowatt-hours? In a calendar year or an academic year? – not propped up by a neat number of unknown provenance. The tables above have attempted to label the scope and reference period of every figure; readers can weigh their significance accordingly.

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# From seawater to net-zero: where this system sits in HKUST's sustainability map

Zooming out, the seawater cooling system is the earliest and most "hardcore" piece of infrastructure in the HKUST Clear Water Bay campus sustainability story. It predates the solar panels and the smart microgrids; it has been running quietly since the campus opened in 1991, connecting the university and the sea at its feet in a single energy loop.

In recent years the system has been drawn into larger frameworks. HKUST has set a medium-term target of cutting energy consumption by 15% by 2028, and has further laid out a trajectory towards net-zero around mid-century. Seawater cooling, the library rooftop cooling towers, waste-heat recovery, bleed-off toilet-flushing reuse, and radiative-cooling-paint trials together form a technical genealogy in which this "natural air-conditioning" is continuously iterating. String these pieces together and they all point to the same logic: if cooling consumes about one-fifth of the campus electricity, and electricity in turn determines nearly all of its carbon footprint, then every increment of effort spent on cooling efficiency is amplified on the carbon ledger. Seawater cooling is not an ornament on the sustainability story; it is the heaviest counterweight, and the one HKUST got its hands on earliest. For the university's overall sustainable development plan, carbon-neutrality targets and recent expansion projects (including the medical school building and the InnoHub), this site has a dedicated article – see Green Campus, Ecology, and Expansion History. The present page focuses specifically on the engineering history and numerical detail of the cooling system itself; the two pieces are complementary reading.

And so we return to the original contrast: a hillside campus facing the sea on three sides entrusts the most electricity-hungry task – maintaining a steady indoor temperature – to the water at its feet. The sea does not send a utility bill, but putting it to work well – drawing it up, exchanging heat, returning it under temperature control without harming marine life, and then topping up with cooling towers when the sea itself warms, while recycling every stream of bleed-off – demands more than three decades of sustained stewardship across successive engineering teams. That, in substance, is the real picture of the "natural air-conditioning" that the HKUST community enjoys every day but barely notices.

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# Sources

• HKUST, "HKUST Wins Asia Pacific Region Award for Remodeling Central Chiller Plant that Saves Millions of Dollars and Over 20 Percent of Energy Consumption" (official announcement, covering 2012 expansion, 2013–2015 remodelling, 20–35% energy saving, HK$1.5M/HK$7M, replacement of 3 of 9 chillers, 18% efficiency gain, waste heat reused for hot water, 1997 first Southeast Asian energy-saving performance contract, 2016 AEE Asia-Pacific Energy Project of the Year Award, Mark Hodgson quote). https://hkust.edu.hk/news/recognition/hkust-wins-asia-pacific-region-award-remodeling-central-chiller-plant-saves
• HKUST Sustainability, "The Cooling Tower Project: Promoting Circularity of Water in A Hong Kong University" (four cooling towers on library rooftop, ~4,900 kW each, evaporative cooling, bleed-off reused for toilet flushing, 8,724 m³ in 2022, EMSD Code of Practice, published 31 Oct 2023). https://sust.hkust.edu.hk/news/cooling-tower-project-promoting-circularity-water-hong-kong-university
• HKUST Sustainability, "SDG 6: Clean Water and Sanitation 2022/23" (total water consumption 507,161 m³ for 2022/23, cooling tower bleed-off reuse 16,366 m³). https://sust.hkust.edu.hk/progress-and-performance/SDGs/2023/SDG-6
• SSC HKUST, "Passive Radiative Cooling for Solar PV Frames + Cooling Tower Water Tank" (cooling ~20% of campus electricity, ~95% carbon footprint from electricity, radiative cooling paint on cooling tower water tanks). https://ssc.hkust.edu.hk/projects/protecting-our-scarce-resources/passive-radiative-cooling-for-solar-pv-frames-cooling
• EMSD, "Water-cooled Air Conditioning Systems (WACS)" (water-cooled AC more energy-efficient than air-cooled; city-wide energy-efficiency measure). https://www.emsd.gov.hk/en/energy_efficiency/water_cooled_air_conditioning_system/
• HKUST Sustainability, "Energy and GHG | 2028 Sustainability Challenge" (15% energy reduction target by 2028). https://sust.hkust.edu.hk/2028-sustainability-challenge/energy-and-ghg

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This page is fact-based documentary research; figures are drawn from official primary sources. Unofficial claims involving specific kilowatt-hour figures have been noted in the main text with the results of verification; any figure not backed by an official source is not included as confirmed content.

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