In view of the non-availability of potable water and the limited availability of treated sewage effluent for district cooling, should we intensify our gaze seawards? If so, what are the inherent challenges… and solutions?

With Government mandated non-availability of potable water for district cooling in Abu Dhabi and Dubai, and with questions being raised about the adequate availability of treated sewage effluent (TSE) and also the costs associated with polishing the water to acceptable standards, many experts have been advocating the use of seawater for a while now.

The question about the availability of TSE is a pertinent one, given that there is a demand for it for agricultural and horticultural uses. And then there is the cost. It is possible to purchase TSE at one-sixth of the cost of potable water, but there is a cost associated with treating it. A third issue involving TSE is drainage, or rather the lack of it, from a blow-down point of view. When viewed against the backdrop of these three factors, seawater is regarded as a strong option. Says Craig Thomas of High Performance Tube company, “Using seawater to reject heat, when feasible, will free up potable water and TSE for other more important uses.” Adds Greg Cox of Mott McDonald, who has had extensive experience with a successful seawater cooling regimen in Hong Kong: “There is a higher demand on TSE for irrigation, so seawater, although having lower cycles of concentration, could be the correct choice, considering that it has the lowest overall energy usage.”

The concept of seawater is not a new one. As Thomas says, while it is new to district cooling, it is not to other applications. Indeed, seawater has been used commonly in refineries and the petrochemical industries, LNG, power generation and thermal desalination. One of the success stories for seawater cooling is Jubail in Saudi Arabia, where the volume of water used equals two-third of the flow of the Tigris and the Euphrates rives combined. And as for the UAE, Fujairah has been using seawater since 1993, George Berbari of DC Pro Engineering, points out. “In Fujairah, we did district cooling with 2,000 TR that used seawater cooled with titanium and fibreglass tubes,” he says. Amar Farjo of JCI backs Berbari’s observation. “We need seawater cooling towers, because they can save 57,600 gallons of potable water per kilo of cooling every day,” Farjo says.

A GAMUT OF ISSUES

While there is a growing favour for seawater cooling in some circles, there are several aspects to consider. The various components – be they cooling towers, chillers, heat exchangers or intake systems – come with their attendant issues and challenges.

Cooling towers, for instance, come with a gamut of sub-issues. Typical questions revolve around the impact of seawater on the sizing of the cooling towers, the materials needed in building the towers and the environment around them.

Seawater impacts thermal performance in three ways, says Kent Martins of SPX Cooling Technologies. If seawater has 70,000 ppm of TDS, it lowers vapour pressure by five per cent, which is a negative impact, Martins says. The same seawater characteristic reduces specific heat (0.92 compared to 1 for freshwater). “The cooling tower,” he says, “must be sized three to seven per cent larger to compensate for the reduced heat transfer characteristics of seawater at 70,000 ppm TDS in circulation.”

When it comes to material selection for cooling towers, corrosion of compounds can occur. Fibreglass, Martins says, is a good material for seawater exposure. Likewise, concrete towers also hold good, he says, but they would require a specialty mix and rebar design. Generally speaking, he recommends premium hardware materials and coatings.

Another sub-issue is the drift from the cooling towers. Circulating water is distributed as droplets or films to maximise surface area. Exit water for cooling towers contains water vapour, drift droplets and condensate droplets. It, therefore, becomes essential to reduce drift, and for this, careful installation is of paramount importance. “It is crucial to reduce drift at the source with the best available drift eliminator,” Martins says. “That way, salt deposition will not be a major concern. Also, drift eliminator data should be specific to the distribution system used. Further, it would help to site the tower downstream from the prevailing wind direction and away from high-rise structures.”

Heat exchangers and chillers also come with a string of challenges, considering the corrosive nature of seawater. Newer plants use titanium to overcome the challenge of the aggressive nature of seawater. Earlier, coppernickel was widely prevalent, Thomas says, but it came to be found that copper is prone to erosion and washing away. Also, ammonia does not go well with any copper alloy, observes Farjo. In that context, he says, titanium is the “gold standard”. Though high on costs, it is immune to erosion and corrosion and has the longest life expectancy. Besides a corrosion point of view, there is also the conductivity point of view. Most other materials simply do not match up to titanium when considering resistance to corrosion and conductivity. For instance, the thermal conductivity of stainless steel is less than that of titanium.

Intake systems also have their attendant sub-issues, ranging from costs of installation and maintenance to environmental acceptability (safety of fish and fry, which are prone to getting sucked into the system). Screens and strainers are integral parts of intake systems. Says Daniel Bewg of GLV-EIMCO Water Technologies, “We put a screen before the cooling water pump and a strainer after it. Bewg says there are several factors to consider, be it to determine the method of filtration or the size of the intake channel. For instance, an important aspect to consider for filtration is the flow rate. A passive screen would work with a low flow rate (less than 0.1 m3/s). In the case of a medium flow rate (0.1 to 5 m3/s) or large flow rate (greater than 10 m3/s), Bewg says, bar filtration, and bar and screen filtration would serve the purpose.

Passive screens are prone to buildup of marine matter, mainly in the form of zebra mussels, because it is not possible to chlorinate the screens. To prevent the occurrence, it becomes necessary to coat them with a copper-nickel material.

The pipework walls are prone to mollusc and crustacean infestation due to the settlement of larvae. If not properly addressed, says Cox, they will dislodge when mature and collect in the condenser tubes.

In view of such factors, maintenance is of critical importance in a seawatercooling regimen, be it for intake systems or condenser tubes. Cox talks of the need for a proper maintenance framework and for involving a girth of experts, be it sustainability advisors, marine biologists, water treatment specialists, operators, manufacturers, cost consultants and legal professionals.

Seawater is the future, agree many in the district cooling fraternity, and is, perhaps, integral to ambitious coastal developments in the region. In view of that, they recommend prudent planning in developments to later accommodate a seawater-cooling regimen. Says Jamie Saunier of TAS: “If district cooling plants are built to accept TSE today, at some point in the future, if the Government implements seawater, then you would be faced with huge capital costs to convert or upgrade to seawater. So we are better off designing to accommodate later.”

Note: This article is based on the Sea Water District Cooling Symposium, on November 14, 2009, at Atlantis, Dubai. Despite extracting information from an event, held a few months ago, the author of this article, after ascertaining from the organiser, IMEC, guarantees that the statements and observations by the participants in the symposium are not outdated.

Story and pictures: B Surendar