Meinhardt - An Introduction to Seawater Air Conditioning (SWAC)

Isolated island resorts, such as in the Maldives and South Pacific, rely on diesel powered generators for their power supply which makes their operation and profitability highly vulnerable to fuel supply chain shocks. We can see this clearly being played out now globally due to the current geopolitical conflicts in the Gulf.

Running diesel power generation also creates other problems – smells, particulate matter and noise that detract from the guest experience. So reducing reliance on diesel power generation is obviously highly desirable.

At a minimum, resort owners should be supplementing with as much solar power as possible. However, the scope for this on island resorts is usually constrained by lack of available free land and roof area.

One workaround is floating PV (FPV) - solar panels mounted on floating structures over water. This is already a well established technology, for island resorts and also on large inland freshwater bodies, such as reservoirs and dams. For example, EGAT plan to roll out 2,725 MW of FPV in Thailand by 2030.

The 2nd strategy to limit risk is to reduce power consumption. At the design stage, the design team would typically consider:

• Passive strategies to minimise cooling loads, including thermally efficient building envelopes, natural ventilation in lieu of AC, and mitigation of heat island effects by incorporating solar shading and biophilic design.
• Active strategies to reduce energy consumption, including energy efficient equipment, smart control systems to manage energy use and avoid waste by matching supply to real time demand, proper commissioning at handover, to ensure systems operate as intended.
• Circular economy practices such as using waste heat from gensets to generate steam for the resort’s laundry and hot water for guests, and recycling sewage water to reduce the sea water reverse osmosis plant operating costs.

However, the potential to save significant energy from passive and active design measures is limited. Equipment today is already optimised for energy efficiency and effective passive design measures often clash with the need to maximise the guest experience.

SWAC

Even with the above savings, air conditioning will still typically take 40-60% of a resort’s total power consumption. It therefore makes sense to focus on air conditioning energy consumption and to consider whether there isn’t a more transformative technology that might help. For isolated island resorts near deep oceanic drops, seawater air conditioning (SWAC) may well be a feasible alternative.

SWAC leverages the naturally cold temperatures of deep ocean water, typically stable year round at around 4-6°C at depths beyond 900m. Isolated island resorts are often located near such deep water.

A SWAC system is relatively simple, comprising 3 parts, a primary loop (typically an HDPE pipe) that draws 5°C seawater from the depths, a secondary loop that circulates chilled water at around 7°C around buildings, and a land based plant room containing circulating pumps for both loops and titanium (for corrosion resistance) heat exchangers where the cooling energy from the sea water is transferred into the buildings’ chilled water system.

The seawater never enters the buildings and never mixes with the chilled water.

The seawater is then returned to the ocean at a slightly warmer temperature, typically 12°C, discharged at an appropriate depth and turbulence zone to minimise environmental disruption. This cool water resource could also have secondary uses – to feed desalination plants, and to supply aquaculture tanks or secondary cooling of greenhouses.

Since there are no chillers or condenser units there is no noise or heat discharge near the buildings, limiting heat island effects and also reducing maintenance requirements.

SWAC is not a common technology but is proven. Existing SWAC systems report savings of 80–90% of the power used by traditional air conditioning systems, because they do not rely on the energy intensive refrigeration cycle and compressors. Energy consumption is largely limited to pumping.

The examples of successfully operating SWAC systems include:

• Cornell University's site at Lake Ontario, Canada. A deep lake water cooling system installed in 2000, providing an 87% saving in power consumption.
• InterContinental Bora Bora Thalasso Resort, French Polynesia: 1,600 kW capacity SWAC system installed in 2006 with 2,300m intake pipeline from 900m depth.
• Brando Resort, Polynesia. 2,400kW SWAC system installed in 2014 with 2,600m intake pipeline from 960m depth.
• CHPF, Tahiti’s largest hospital: 6,000 kW capacity SWAC system installed in 2022 with 3,800m intake pipeline from 900m depth, providing a 93% reduction in energy consumption and a projected 10-15 year payback.

Limitations to Note

Physical feasibility depends on the bathymetry – it is limited to areas near where water at approximately 5°C is accessible.

Economic feasibility is specific to each site. CAPEX can be high due to the marine pipeline. Payback periods of 7 to 15 years have been suggested, depending on factors such as:

• Piping distance: long distances will increase the CAPEX and also the OPEX, since efficiency reduces due to higher pumping costs.
• Project’s air conditioning load. SWAC is more feasible at scale and if cooling is required throughout the year
• Complexity of the chilled water distribution system. SWAC is more feasible where buildings are large and compactly arranged.

There are also some key design issues to consider:

• Risks from biofouling, which could impair the thermal conductivity of the system
• System redundancy, in case of pipe failure. A conventional standby chiller plant will be needed in case the SWAC system fails.

Conclusion

This is a proven technology and a niche solution for suitable sites. While not always feasible, for the right location and the right scale, it offers substantial energy savings that would have a big impact on the resort’s profitability. It would also significantly increase the resilience of isolated island resorts against fuel supply chain shocks.