29/01/2019 | 23:24 GMT
By Julian Hunt |
Regardless of the causes of global warming, we can be sure that the Earth is warming and so air-conditioning demand. It is expected that air-conditioning demand will rise 30% by 2050 due to increase in population, wealth and world temperatures. Conventional air-conditioning is the most common technology used for cooling and represent a considerable share in the energy demand of low latitude countries. They function as heat pumps, cooling the interior of buildings, but warming up outside.
One alternative that is not frequently considered for cooling is Seawater Air Conditioning (SWAC). SWAC is a renewable alternative for cooling and consists of pumping seawater from ocean depths of around 700 to 1200 meters and temperatures of 3°C to 5°C to the coast, exchange this heat with a district cooling system and return the warmer water to the ocean. It should be noted that seas, which are not connected to the deep sea cannot be used for SWAC, such as the Mediterranean and Red seas, and the Persian Golf.
Deep Sea-Water as a Cooling Alternative
The economic aspect is the opposite of conventional AC systems, where the initial costs are low (10%) and the energy costs are high (90%). In SWAC systems, the investment costs are high (80%) and the energy costs are low (20%). In other words, it is appropriate only for base-load cooling demand and long-term investments of up to 20 years. Recent research has mapped the global viability of SWAC, gives a review of the current work involving the technology and presented its advantages and disadvantages when compares with other technologies .
“The SWAC solution cost is $ 0.042/KWht, which is 48% less than conventional technologies”
The main potential for SWAC is in small islands in tropical regions, where the distance from the coast to the deep ocean is small, energy costs are high and average temperatures are high throughout the year. For example, in Puerto Plata, where the electricity costs are $0.32/KWh, the cost of conventional AC cooling is around $0.08/KWht. The SWAC solution would cost $ 0.042/KWht, which is 48% less than conventional technologies. In Nauru, assuming the same electricity cost, the SWAC solution would be $0.0185/KWht, which is 77% lower compared with conventional technologies. There are already a dozen SWAC projects implemented and under implementation around the world. These can be found in Hawaii, French Polynesia, Bahamas, Aruba, Curaçao, Reunion Island and Mauritius.
Seawater air conditioning compared with other renewable energy for cooling 
Comparing with other renewable sources of energy, 1 m³ of seawater in a SWAC plant can provide the same cooling energy than energy generated in 21 wind turbines, a solar power plant with 488,000 m² area or a hydropower plant with a 100 m³/s flow and 19 meters generation head.
SWAC projects cost estimate in dollars per KWh of cooling (©JH)
SWAC district heating systems could also be considered for large-scale district cooling due to considerable gains in scale. Ice-pumping technology would improve the efficiency and viability of district cooling systems. Also, intermittent sources of energy such as wind and solar energy could be stored with the production of ice during moments of an excess generation. This could be a viable energy storage approach for locations with high intermittent renewable generation and cooling demand.
The advantages of the technology are reliability as a non-intermittent renewable source of cooling, reduction of greenhouse gas emissions from cooling processes, the maturity of the technology, energy and cost savings for base-load cooling processes, reduction of around 80% in electricity consumption, and reduction of water consumption in cooling systems. Another benefit is that SWAC does not contribute to warming the outside of the building or cities. It cools not only the building but also the surroundings, resulting in a better and more efficient approach for cooling.
The challenges are: the return of the seawater should be handled with care to minimize its impact on coastal wildlife, the heat losses in the intake pipelines should be minimized, district cooling and buildings retrofit demand high capital costs, risk of thermal shock and increased nutrient loading in the deep seawater outlet. Cooling demand requirements should be as high as possible, which is not always the case before project construction due to the risks involved in the investment.
Seawater Air Conditioning is an innovative and sustainable technology that has great potential for expanding into a benchmark technology for cooling in tropical locations close to the deep sea and will help keep us chilled in a warming world.
About the Author
Julian Hunt is a contributor to the UCERGY Insight Journal and a member of the Editorial Board. He is a Research Scholar at the International Institute of Applied Systems Analysis (IIASA), Austria, and a Managing Director of Stanhope Energy Brasil, an energy consulting company. His interests include analysis of energy systems, climate change, energy price modelling, energy storage and security. Dr. Hunt worked at the National Commission for Nuclear Energy in Brazil and the Energy and Climate Change branch at the United Nations Industrial Development Organization (UNIDO). He holds a D.Phil in Engineering Science from the University of Oxford and a B.Eng degree in Chemical Engineering from the University of Nottingham
 Hunt, J., Byers, E., Sánchez, A., Technical potential and cost estimates for seawater air conditioning. Energy, vol. 11, pag. 979-988, 2019.
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