Seawater Cooling and the Central Chiller Plant — The "Natural Air Conditioner" of HKUST's Seaside Campus

Bottom line: Since the university opened in 1991, the Hong Kong University of Science and Technology (HKUST) has pumped Clear Water Bay seawater through its central chiller plant to cool the campus; in 2013–2015 the system was remodelled into a district cooling network, winning the 2016 Asia-Pacific Energy Project of the Year Award, while the Cheng Yu Tung Building records 20–35% annual energy savings.

A campus built on a hillside, facing the sea on three sides, handed its single biggest utility bill to the water at its feet. Most people at HKUST know only that classrooms stay cool in summer and halls stay cold enough to sleep in; few notice that the constant temperature of the whole campus is not maintained by rows of energy-hungry outdoor condenser units, but by seawater pumped up from the bay, run through heat exchangers, and sent back. This article traces, along the project's engineering timeline, the full story of this "natural air conditioner" — from the founding of the university to the recent cooling-tower upgrade — with every figure and its source noted.

---

# Why does HKUST cool with seawater instead of ordinary air conditioners?

The HKUST Clear Water Bay campus came close to being tailor-made for water-cooled air conditioning. The campus sits on a slope at Tai Po Tsai in the Sai Kung Peninsula, surrounded by the sea on three sides, with the main buildings only a few hundred metres from the coastline — making the piping cost of drawing and discharging seawater far lower than for an inland campus. The central problem of any air-conditioning system is "where do you dump the heat from indoors?" and seawater is a nearly infinite heat sink with a relatively stable year-round temperature.

To appreciate the system's value, it helps to set out the two mainstream heat-rejection paths. The first is air-cooled: the chiller uses outdoor air directly to reject heat from the condenser, giving simple equipment whose efficiency is hostage to air temperature — at its worst precisely in high summer. The second is water-cooled: water carries the heat away, and a cooling tower or a body of water then disposes of it; efficiency is generally well above air-cooled systems. HKUST chose a further variant of the water-cooled route — using seawater directly as the heat-exchange medium on the condenser side.

Hong Kong's Electrical and Mechanical Services Department (EMSD) has long promoted Water-cooled Air Conditioning Systems (WACS) precisely for their energy-saving potential over air-cooled systems. According to the EMSD's WACS page^※, water-cooled air conditioning can cut electricity consumption significantly relative to conventional air-cooled systems, and is one of the efficiency measures promoted territory-wide. HKUST's direct seawater exchange sits at the most favourable end of this spectrum — it does away with most of the evaporative water loss of a cooling tower, entrusting heat rejection directly to the sea. That is why a science-and-engineering university treats "cooling with seawater" as a matter serious enough for its sustainability reporting.

The three mainstream heat-rejection paths can be set side by side, to make clear the inherent advantages and costs of seawater cooling:

The "high-summer efficiency" column is a qualitative comparison, based on EMSD's general conclusion that water-cooled systems are more efficient than air-cooled ones. Direct seawater exchange is most advantageous on a coastal campus precisely because it avoids both the air-cooled system's high-temperature degradation and the cooling tower's evaporative water loss; its only requirements are proximity to the sea and careful management of the warmed discharge. The HKUST Clear Water Bay campus happens to satisfy the coastal precondition — a geographical dividend no inland institution can replicate.

---

# How does seawater carry heat out of the classrooms? The mechanics of exchange and return

The Central Chiller Plant's operation can be broken down into a clear chain of energy transfer. Heat from the academic buildings, halls and laboratories is first absorbed by chilled water; the chilled water passes through the chiller, where the heat is transferred to the condenser side; the condenser-side heat is then passed, via heat exchangers, to seawater pumped up from the bay; the warmed seawater is finally discharged back into the sea under temperature control. In this whole chain, seawater serves as the "ultimate heat sink", and the campus buildings are separated from the sea by a series of heat exchangers — seawater itself never enters the buildings' pipework.

One environmental checkpoint in this chain is easily overlooked: thermal pollution. Seawater drawn in, warmed, and discharged raises water temperature near the outfall; prolonged, concentrated warm discharge can disturb nearshore marine ecology. Such systems therefore require temperature control on the discharge — keeping the temperature rise within permitted limits, and choosing the outfall location and diffusion design so the warmed seawater is rapidly diluted and mixed into the wider body of the bay. When HKUST writes this system into its sustainability narrative, it emphasises the positive value of "seawater heat exchange" and "reducing the carbon footprint"; temperature-controlled return flow is the precondition for a seawater-cooled system to operate over the long term.

The system also holds a piece of ingenuity that turns waste into resource. According to HKUST's official announcement, the waste heat from the condensing water side of the central chiller plant is not all discarded — it is recovered to supply hot water to campus facilities. The official wording:

"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 cools the buildings on one side while capturing otherwise-wasted residual heat to produce hot water on the other — both the cold and the heat find a use, which is a key reason the system's energy ledger looks so good.

The significance of this waste-heat recovery only becomes clear within the framework of energy conservation. Cooling is not about "destroying heat" but "moving heat from indoors to outdoors" — and in the course of that move, the condenser side inevitably accumulates a great deal of residual heat that would otherwise be thrown away. If that heat simply went out with the seawater, the campus would then have to burn more electricity or gas elsewhere to heat domestic hot water — the same campus would be "spending effort cooling in one place and spending effort heating in another", two energy bills with nothing to do with each other. Intercepting the condenser waste heat for hot water means the by-product of cooling directly offsets part of the heating load: one unit of energy does two jobs. For a campus with substantial hot-water demand from halls and laboratories, the absolute energy saved by this "combined cooling and heating" approach accumulates with building scale.

---

# The 2012 expansion: how one new building anchored the cost case for "expanding by borrowing"

HKUST's seawater cooling was not built once and left untouched. The campus kept growing; new buildings came on stream; cooling demand rose; and the original Central Chiller Plant reached a crucial expansion point.

According to HKUST's official account, the seawater cooling system was expanded in 2012 to serve the newly completed Cheng Yu Tung Building (CYT); the system was later further extended to cover the multi-purpose auditorium under construction at the same time, expected to be completed by end-2018^※ — today's Shaw Auditorium (邵逸夫礼堂). Hidden inside this is a key engineering decision: facing the new load, the university did not build a separate air-cooled system, but expanded the original chiller plant.

This decision — "expand by borrowing, don't start from scratch" — rewrote the cost ledger directly. According to HKUST's 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 building new air-cooled capacity: first, greater efficiency and lower carbon; second, avoiding the capital expenditure of a standalone chiller plant.

The following table sets out the key figures for this expansion, with their scope and timing:

These figures need to be read with their scopes straight: the HK$1.5 million is an annual saving in operating costs; the HK$7 million is a one-off capital cost avoided — the two cannot be added to claim "HK$8.5 million saved per year". The 20–35% energy saving refers specifically to the Cheng Yu Tung Building, not the campus as a whole; the 18% efficiency gain refers specifically to the three replaced chillers. Conflating "a percentage for one building" with "the campus-wide total" is the most common misreading in this kind of energy-saving publicity.

---

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

Around this expansion, HKUST carried out a systematic remodelling of the Central Chiller Plant during 2013–2015, upgrading it into a district cooling system. District cooling means a single central plant supplying chilled water through a distribution network to a whole cluster of buildings, replacing each building's separate, self-contained air-conditioning system — the scale effect of centralised dispatch is precisely the source of the efficiency gain.

The remodelling earned the university recognition from the industry. According to HKUST's official announcement^※, for the project upgrading the central chiller plant into a district cooling system, the university received the 2016 Energy Project of the Year Award for the Asia Pacific Region from the Association of Energy Engineers (AEE). The AEE is an international professional body in energy management and efficiency, and its annual regional awards carry considerable standing in the field.

Mark Hodgson, then Vice-President of HKUST, commented on 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's engagement with energy-efficiency management began remarkably early. According to the same official announcement^※, as early as 1997 the university introduced Southeast Asia's first energy-saving performance contract — a contract type that ties investment in efficiency retrofits to the actual energy saved, with the contractor sharing the risk and the rewards, still a novelty in 1990s Asia. The district-cooling upgrade of the seawater system can be read as a high point in this nearly three-decade thread of efficiency work, not as an isolated engineering project.

The logic of the district cooling remodelling can be summarised in three layers. First, use seawater rather than outdoor air as the heat sink, sidestepping the high-summer toll on air-cooled efficiency. Second, concentrate cooling in one central plant supplying the whole campus, capturing scale efficiency. Third, recover condenser-side waste heat for hot water, so neither the cold nor the heat is wasted. Together, these three layers produced the Cheng Yu Tung Building's 20–35% energy saving and the industry award.

---

# What does district cooling do better than each building having its own air conditioning? The economics of central dispatch

To understand the 2013–2015 remodelling, the key question is what "district cooling" actually changes. Set against the decentralised model of "each building with its own air conditioning", the differences come down to three points.

First, scale efficiency. In the decentralised model, each building runs its own small chiller, sized to its own load, and the units spend much of their time in inefficient part-load operation. District cooling, by contrast, uses one large central plant with flexible dispatch of large chillers against the campus's total load, keeping the units in efficient operating conditions far more often. Centralising in one place and dispatching as a whole is the fundamental source of district cooling's efficiency advantage.

Second, the potential for load diversity. Academic buildings peak during the day and halls peak in the evening; in principle, a centralised system lets daytime academic demand and evening hall demand fill each other's valleys, flattening equipment utilisation — there is no need to size redundant capacity for each zone's individual peak. It should be said that this "load diversity" is a general principle of district cooling; the HKUST official announcement does not disclose specific dispatch data, and this page presents it only at the level of mechanism.

Third, concentrated retrofitting and maintenance. With equipment gathered in one plant, replacement, servicing and upgrading are far more efficient than dealing with dozens of buildings one by one — the 2013–2015 replacement of 3 of 9 old chillers with new models, lifting chiller efficiency by around 18%, is a direct demonstration of that concentration advantage. Had the university instead built a separate air-cooled system, it would not only have foregone the inherent efficiency of seawater exchange, but would also have paid an extra roughly HK$7 million in capital costs. By expanding the old plant, HKUST captured both efficiency and cost benefits in one move.

# Why add cooling towers on the library roof? New developments for the seawater system in the 2020s

If the mid-2010s were defined by "district cooling", the 2020s are defined by the cooling tower. This step does not overturn seawater cooling; it adds a supplementary layer to cope with climate change.

According to the HKUST Sustainability Unit's cooling-tower project page^※, global warming has pushed up seawater temperatures, and rising seawater temperature directly erodes the efficiency of the seawater cooling plant — if the heat sink itself gets warmer, the system must work harder to achieve the same cooling effect. To counter the resulting rise in energy use from the existing seawater cooling plant, HKUST installed cooling towers on the rooftop of the Lee Shau Kee Library (University Library), using evaporative cooling to share the heat-rejection load. The project page was published on 31 October 2023.

Cooling towers work differently from seawater exchange: water falls through the tower, comes into contact with ambient air passing through, and a portion of the water evaporates, carrying heat away. According to the same official page, the library roof houses four cooling towers, each with a heat-rejection capacity of roughly 4,900 kilowatts (kW). Working alongside the existing seawater chiller plant, they form a new combination for the main campus's cooling heat rejection.

To maintain water quality, cooling towers require continuous bleed-off — constantly discharging a small stream of concentrated recirculating water to control total dissolved and suspended solids, as required by EMSD's code of practice. HKUST turned this into a story of water circularity: the bleed-off water from the cooling towers is recovered and used for the library's toilet flushing. The table below sets out the project's key water figures:

Here again, pay attention to the scopes: 8,724 m³ is the 2022 calendar-year recovery figure, 16,366 m³ is the 2022/23 academic-year figure — different periods, not two contradictory numbers for the same period. Beyond the cooling-tower bleed-off, the official account notes that condensate from the library's air-handling units (AHUs) is also collected and reused, further reducing fresh-water consumption.

HKUST's sustainability team has also run more cutting-edge experiments on these towers. According to the HKUST "Sustainable Smart Campus" project page^※, the team tried applying radiative cooling paint to the cooling-tower water tanks, to draw heat out of the water without consuming extra electricity and hold a lower water temperature — so each cycle carries away more heat and cooling uses less power. Experiments of this kind turn a functional heat-rejection facility into an open-air laboratory on campus.

---

# How much of HKUST's electricity does cooling actually use? An energy ledger

To gauge the weight of the seawater cooling system, it has to be placed in the campus's overall electricity picture. The carbon footprint of the HKUST Clear Water Bay campus comes overwhelmingly from electricity.

According to the HKUST "Sustainable Smart Campus" project page^※, roughly 95% of the campus's carbon footprint comes from electricity, and cooling accounts for about 20% of total campus electricity use — in other words, the single task of cooling buildings consumes about a fifth of all the campus's electricity and indirectly constitutes a substantial share of its carbon emissions. That explains why HKUST keeps investing effort in its cooling systems: even a few percentage points of efficiency gain, multiplied against this electricity base, becomes a sizeable absolute saving.

In aggregate, according to HKUST's SDG-related accounts^※ and sustainability pages, in the 2022/23 academic year HKUST consumed roughly 99 million kilowatt-hours (kWh) of energy, around 95% of it electricity, powering academic buildings, student and staff residences, the chiller plant and laboratories. The table below sets the key proportions side by side:

Read these numbers together: cooling is around 20% of campus electricity, and electricity is around 95% of the carbon footprint, so "the efficiency of the cooling system" almost directly determines the campus's decarbonisation success. Seawater cooling keeps being written into HKUST's sustainability reports not because it is novel, but because it sits on the heaviest item in the campus energy ledger. With the university's target of cutting energy use by 15% by 2028, continuous optimisation of the cooling system is an unavoidable part of the job.

---

# What does seawater cooling cost? The other side — intake, biofouling and maintenance

Seawater cooling looks good on paper, but it is not a free lunch. Any system that pumps seawater through heat exchangers faces engineering challenges that inland cooling-tower systems never encounter, and this is a side of the "natural air conditioner" worth not skipping.

The first is biofouling: seawater carries algae, shellfish larvae and micro-organisms; flowing long-term through intake pipes and heat exchangers, it tends to build up scale and growth on pipe walls, weakening heat transfer and increasing pumping resistance, requiring periodic cleaning or the addition of inhibitory measures. The second is corrosion: seawater is saline and highly conductive, far more corrosive to metal pipework and heat-exchange equipment than fresh water, demanding higher-grade materials and corrosion-protection maintenance. The third is intake and outfall siting and temperature control: the intake must sit clear of the outfall's thermal plume to avoid "short-circuiting" (drawing back the hot water just discharged), while the discharge itself must control temperature rise and choose diffusion points carefully to avoid nearshore thermal pollution.

As to HKUST's specific biofouling and corrosion-control practices and maintenance frequency for this seawater cooling system, this page found no detailed disclosures in official public sources, and so does not elaborate or speculate. These engineering costs are laid out plainly here to make one point: the efficiency advantage of seawater cooling rests on continuous operational and maintenance investment — "saving electricity" and "saving trouble" are not the same thing. The library-roof cooling towers described earlier are, in a sense, a hedge against the long-term variable of rising seawater temperatures — when the heat sink itself deteriorates, the system needs new heat-rejection capacity to compensate.

# A timeline of seawater cooling: from 1991 to 2028

Arranging the key milestones of HKUST's seawater cooling in chronological order makes clear that this is not a one-off project but a continuous thread of energy-efficiency management running over three decades. The table below lists only the points verifiable from official sources:

Two anchors stand out in this timeline: the 1997 energy-saving performance contract and the 2012–2016 district cooling remodelling. The former shows that HKUST's work on cooling efficiency began long before "sustainability" became a buzzword; the latter is the most brilliant engineering culmination of that thread so far.

# Does "saving about 1.8 million kWh a year" hold up? An honest verification

Popular and secondary accounts describing HKUST's seawater cooling remodelling often cite a specific figure such as "saving about 1.8 million kWh a year". Having checked every official source, this page must state plainly: what the official HKUST announcement publishes is the amount saved in money and percentages, not a specific kilowatt-hour figure.

The verifiable official figures are: annual energy savings of 20%–35% for the Cheng Yu Tung Building; annual operating-cost savings of around HK$1.5 million; and about HK$7 million in capital expenditure avoided — all per HKUST's official announcement^※. As for "1.8 million kWh", this page found no directly corresponding statement in the official announcement, the Sustainability Unit's pages, or the SDG reports. The figure may derive from some conversion of the percentages or from second-hand retelling, but it lacks primary official support, and this page therefore does not treat it as confirmed data — it records only that the claim exists and the result of checking it.

This approach of "prefer saying 'not found' to forcing a strained connection" is precisely what makes the seawater cooling topic worth taking seriously: the value of an engineering system should rest on verifiable scopes — for one building or the whole campus, in money or in kilowatt-hours, calendar year or academic year — not on an appealing number of unverifiable origin. The tables above have tried to mark the scope and timing of every figure, so readers can judge its weight for themselves.

---

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

On the long timeline, seawater cooling is the earliest and most "hard-core" piece of infrastructure in HKUST's Clear Water Bay sustainability narrative. It predates the solar panels and the smart microgrid; it has run quietly since the university opened in 1991, linking the campus and the sea beneath it into a single energy cycle.

In recent years this system has been folded into a larger framework. HKUST has set an interim target of cutting energy use by 15% by 2028, and has further developed a net-zero roadmap towards mid-century; seawater cooling, the library-roof cooling towers, waste-heat recovery, bleed-off reuse for flushing, and the radiative-cooling-paint trials together form the technology lineage of this ever-iterating "natural air conditioner". Read as a sequence, they all point to the same logic: since cooling consumes about a fifth of the campus's electricity, and electricity almost entirely determines its carbon footprint, every ounce of effort spent on cooling efficiency is multiplied in the decarbonisation ledger. Seawater cooling is not an ornament on the sustainability narrative — it is the heaviest weight, and the first one HKUST took hold of. For the university's overall sustainability plans, carbon-neutrality targets and recent expansion projects (including the medical school building and the Academy of Innovation), this site has a separate article: Green Campus, Ecology and Expansion. This page focuses on the engineering history and the numbers of the cooling system itself; the two are best read together.

Back to the contrast we began with: a hillside campus facing the sea on three sides handed its most electricity-hungry task — keeping the campus at a constant temperature — to the water at its feet. The sea charges no electricity bill, but using it well — pumping it up, running it through heat exchangers, returning it warm but controlled so as not to harm the sea, and adding cooling towers when the sea itself warms, recycling every stream of bleed-off water — has taken over three decades of sustained work by successive teams of engineers. That, in all likelihood, is the true appearance of the "natural air conditioner" that people at HKUST enjoy every day and almost never notice.

---