The challenge of keeping cool

 By Nicholas Newman

In many countries that suffer from hot summers, the need for air-conditioning is a necessity for making life bearable. Traditionally, electrically powered air conditioners have been used. Unfortunately, for power companies, such gadgets are very power hungry. Across the world cities are looking at new solutions, including operating district cooling systems. One such example is in Paris where the Seine water is refrigerated to 5 Celsius, transported via 71km of canalisation to cater five million square meters of Grand Hotels, offices, government buildings, theatres, and the Louvre. Air is projected on the refrigerated tubes of water hidden in ceilings to air condition Mona Lisa. That system uses 80% less chemical products than traditional air conditioning, reduces CO2 emissions by 50% and lower electricity bills by 35%. Nicholas Newman looks at the innovative ways people are using to keep cool and save energy…

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For countries in the Middle East, the Mediterranean, Latin America and the southern states of North America, keeping workplaces, shopping malls, hospitals and hotels cool in high summer is a major challenge. Traditional solutions such as electric powered ceiling fans have only been relatively recently overtaken by air conditioners, but these are often noisy and costly to run. Today’s air conditioners are power-hungry and the more an air conditioning unit is used, the less efficiently it operates and the more power is needed to meet its temperature settings.

Satisfying this demand for keeping cool is proving a real challenge for today’s power companies and has led to the application of new technology and integrating temperature control and ventilation systems to an old concept – a central cooling plant simultaneously serving multiple buildings in a distinct city district. According to historical records, the Colorado Automatic Refrigerator Company installed the world’s first district cooling scheme in Denver in 1889. In the 1930s, New York City’s Rockefeller Center and Washington DC’s capitol buildings also featured district cooling systems.

Across the world, city utilities have been looking for new, more energy-efficient, and environmentally friendly solutions for keeping cool, including combined heating, ventilation and air conditioning district systems (HVAC), that provide cooling services to a multiplicity of hotels, shopping centers, offices and factories in a discrete area. The US, Japan, Denmark and the United Kingdom have seen rapid growth in the use of district cooling systems.

District cooling systems have already been used in a number of world famous buildings including the US Pentagon and Europe’s largest business district, La Defense in Paris, France. Currently the city-state of Qatar has the world’s largest district-cooling scheme, which is operated by the Qatar District Cooling Company.

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The use of HVAC technology is not confined to modern commercial buildings. It is increasingly being employed in historical buildings and architectural gems, where individual air-conditioning units are prohibited and, or where, space is at a premium – from the heights of Rome’s most famous Chapel to the depths of a deep mine. To protect Michelangelo’s masterpieces against deterioration from the many thousands of daily visitors, the Sistine Chapel is served by HVAC technology, as are several deep South African gold and platinum mines, whose temperatures can exceed 60ºC (140ºF).

Intelligent or smart, connected “ Internet of Things” HVAC devices are now being marketed to provide real-time responsiveness to changing conditions and temperatures. Such digitally connected devices can now provide a well-regulated, ventilated airflow and temperature ambience within workplaces, enhancing both employee productivity and equipment longevity, says technology company Samsung South Africa.

Business Sweden reports, that the district cooling approach is potentially 5 to 10 times more energy efficient, has lower operating costs and is more reliable than traditional air cooling systems. A case in point is Dubai, which invested $150 million between 2009 and 2014, to construct the world’s largest district cooling network. It removes a staggering 1 million refrigeration metric tons (a measure of the amount of heat removed) per year. By 2030, Dubai expects district cooling to reduce power consumption of air conditioning use by 50 percent. A transformation on the present day, where 70 percent of Dubai’s power consumption goes on air conditioning.

Essentially, district cooling systems deliver chilled water from a central plant through an underground pipe network to multiple buildings. The water circulates through refrigeration coils, or enters the air conditioning system through absorption chillers. The energy needed to drive the chillers can come from multiple sources including waste heat from power plants or industrial processes, steam turbines or electric chillers. HVAC plants, in Kuala Lumpur, the capital city of Malaysia for example, are versatile and can be powered by a variety of sources including natural gas and solar power.

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HVAC plants can run on fresh water, seawater or treated sewage effluent (TSE) a relatively new source of cooling water. For example, cold South Atlantic seawater is used in Cape Town’s 6 MW Carrier cooling plant, which supplies chilled and hot water, to the Clock Tower, the recently completed Zeitz Museum of Contemporary Art Africa and the Radisson Red Hotel, which is currently under construction. The system works by circulating cold seawater (which is abundant) through heat exchangers and exports the unwanted heat into the local harbor rather than emitting it into the atmosphere via cooling towers. According to its operators, their 23XRV variable-speed screw chiller runs at a coefficient of 10.37 to 16.04 efficiency — very high performance indeed.

Fresh water is commonly used in Canada and Finland. For example, HVAC plants in Toronto‘s business district use water drawn from Lake Ontario, and Helsinki’s central district is supplied with fresh water from nearby lakes which is stored in a huge underground water tank located under the city’s Esplanade Park. The tank, blasted out of rock to a depth of 100 m (328 ft) can hold up to 25 million liters (6.6 million gallons) in volume. Cold water is pumped out to keep the city cool in the heat of the day and warmed water returns to the tank at night for cooling. The system is especially environmentally friendly, emitting about 80 percent less carbon dioxide and other greenhouse gases than conventional air conditioning for the same number of buildings. According to its operators, it also has an overall energy efficiency of about five times that of building-specific cooling systems .

In Paris, Europe’s largest district cooling system, La Defense, relies on water pumped from the River Seine. The system, operated by Société Urbaine de Climatisation, a subsidiary of International Group Dalkia, cools 1.5 million sq m (16 million sq ft) of prime office space. The water leaves the refrigeration units at 4.5°C (40.1°F) and after absorbing heat from the buildings, returns warmer at 14.5°C (58.1°F ). A tailor-made Bernoulli Filter model BSG 500 has been fitted in order to protect the heat exchanger (PHE) from any clogging and fouling from the dirty river water. The cooling facility has a production capacity of 76.5 MW and, despite the large size of the equipment, is housed in a discrete plain soundproofed building that fits in well with the surrounding urban landscape.

Treated sewage effluent (TSE) is another source of cooling agent, especially in water-hungry regions such as the Middle East. According to George Berbari, CEO of DC Pro Engineering, using TSE can be six times cheaper than using desalinated seawater and it is around three times cheaper than desalinated well water produced by a reverse osmosis plant. Currently, district cooling in Abu Dhabi and Qatar operates on approximately 100 percent TSE, in Dubai it’s more than 30 percent and in Saudi Arabia TSE will reach 60 percent in the next few years.

District energy in cities

According to a power-point presentation by US-based water treatment firm Nalco, besides substantial cost saving, it is TSE’s availability and sustainability that are its chief advantages. There are, however, some disadvantages, including corrosion, phosphate deposit formation, slime formation from microbes and possible health issues from using TSE.

There are also additional obstacles to widespread adoption of district cooling. In the UAE for example, regulations, badly designed buildings, metering problems and a lower flat rate tariff for conventional air conditioning inhibits uptake of district cooling. Nor does it help, according to George Berbari, a founder of Tabreed, the UAE’s first district cooling firm, that fresh water is in short supply and the oil price crash is leading to budget cuts which threaten the end of water and power subsidies in the Gulf States. These factors are likely to reduce the profitability of district cooling companies, which are already hard pressed, to achieve a 10 percent return.

More generally, whole building thermostat temperature controls have attracted complaints from women. Recent research by Dr Boris Kingma from Maastricht University Medical Centre in Holland demonstrates that a one-for-all temperature setting ignores the difference in metabolic rates between men and women. Men typically have more heat generating muscle than women and so feel comfortable at cooler temperatures. The research tested young women and men dressed in t-shirt and tracksuit bottoms doing light office work and found the optimum temperature for women was 24.5oC (75oF) and 22oC (71oF) for men. “These findings could be significant for the next round of revisions of thermal comfort standards — which are on a constant cycle of revision and public review — because of the opportunities to improve the comfort of office workers and the potential for reducing energy consumption,” said Dr Joost van Hoof lead researcher for Fontys University of Applied Sciences in the Netherlands.

District cooling systems are environmentally friendly, energy efficient and potentially cheaper than individual building and customer traditional air conditioning. And with rapid urbanization and rising business activity, Africa, where city authorities and property developers actively engage to cut energy costs and emissions, could provide a substantial marketplace in the future.

about the author
Nicholas Newman
Freelance energy journalist and copywriter who regularly writes for AFRELEC, Economist, Energy World, EER, Petroleum Review, PGJ, E&P, Oil Review Africa, Oil Review Middle East. Shale Gas Guide.