Cool Ways to Stay Cool
As summer rolls around, it is time to take advantage of longer days and warmer weather. Yet, hot months often take a major toll on our budgets. What gives? As mercury rises, so do the costs of keeping our houses and buildings cool. Our thermal comfort at home, in office, in a supermarket, or in other public establishments, can present a major expense for industrial and residential consumers alike.
Some communities, however, found an unparalleled solution to slash their cooling bills.
Cold-water cooling 101
Their approach is to tap into deep cold seawater to replace energy-intensive chillers. Renewable and sustainable, this process is referred to as “sea water cooling” or “lake source cooling”, depending on whether water is sourced from a chilly depths of an ocean or a lake. In both ways, the basic idea is to pump cold seawater to cool another water loop, which circulates through buildings and picks up heat. At some point, heat is transferred from the warm “buildings” loop to the cold “seawater” loop. Because heat naturally transfers from hot to cold fluids, no extra work is required. Therefore, unlike the conventional refrigeration systems, the cold-water cooling has appreciable energy savings.
Who’s using it
Amongst the earliest adopters was the Natural Energy Laboratory of Hawaii Authority, lying on the Big Island of Hawaii. In 1980’s the research agency built its own cooling plant and since then it has saved about several thousands a month in energy costs. A decade later, Stockholm, Sweden, set up their own renewable cooling system as a means to achieve the carbon neutrality by 2050. Elsewhere, the lake source cooling projects are implemented in Toronto, Canada, and Cornell University, Ithaca, NY. Currently, the largest yet project is being developed in Honolulu, Hawaii, where sea water cooling is planned to serve the downtown area. The district cooling is expected to save 75% of the current energy consumption. Plus, it will reduce carbon dioxide emissions by 84,000 tons each year, which is equivalent to keeping 15,000 vehicles off the road.
Sure, deep-source cooling has won its supporters worldwide. Yet still, we could ask whether the technology is desirable for all coastline communities? If so, are there potential risks or drawbacks? All these are good questions and should be considered.
Where and when
In short, a deep-water cooling project may get green lights when cold water (from 4ºC to 7ºC) is attainable. Interestingly, an important property of water is its relationship between temperature and density. As temperature decreases up to 4ºC, water density increases. Therefore, ocean or lake surface waters are always the warmest and lightest. Conversely, the coldest and heaviest waters will always sit at the bottom. Called hypolimnion, this bottom layer is ideal for cooling water intake because its temperature doesn’t wiggle too much around the year.
Where hypolimnion lies depends on a water source and its geography. For example, Hawaiian authorities draw their cold ocean water at a depth of cca 600 m, while Toronto and Cornell communities get their lake water at depth of 66-76 m. And Swedes? Well, lucky enough, Nordic residents don’t have to dive in too deep to get chilly water.
Obviously, building a piping infrastructure at such depths usually comes at the expense of high engineering costs. As a result, densely populated shoreline areas might be more keen to go forward because they recover from high initial costs soonest.
When the logistics are sorted out, the concept of using cold water for cooling has several advantages. Amongst all fluids, water has relatively high volumetric heat capacity and thermal conductivity. Basically, this means that water is a very efficient medium for heat transfer. For example, imagine you have two 1L-bottles and fill one of them with water and the other with air. When temperature changes (as it does with cooling), the water bottle can store 3 200 times more heat than the air bottle.
Secondly, it is also likely that water undergoes turbulence in heat exchanger devices. A highly turbulent flow guarantees the maximum rate of heat transfer while minimizing any fouling.
Harnessed from nature, water has played a central role in human activities. As worldwide electricity demand escalates and outstrips its supply, abundant sources of water energy surging around us in nature might become our favorite “go-to” choices.
Article by Adela Kuzmiakova
photo: Kevin Saff.
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