District Cooling
The opposite of district heating is district cooling. Working on broadly similar principles to district heating, district cooling delivers chilled water to buildings like offices and factories needing cooling. In winter, the source for the cooling can often be sea water, so it is a cheaper resource than using electricity to run compressors for cooling.
What is District Cooling?
Basically, a district cooling system (DCS) distributes thermal energy in the form of chilled water or other media from a central source to multiple buildings through a network of underground pipes for use in space and process cooling. The cooling or heat rejection is usually provided from a central cooling plant, thus eliminating the need for separate systems in individual buildings.
A DCS consists of three primary components: the central plant, the distribution network and the consumer system. The central plant may include the cooling equipment, power generation and thermal storage. The distribution or piping network is often the most expensive portion of the DCS and warrants careful design to optimize its use. The consumer system would usually comprise of air handling units and chilled water piping in the building.
The Helsinki district cooling system uses otherwise wasted heat from summer time CHP power generation units to run condensers for cooling during summer time, greatly reducing electricity usage. In winter time, cooling is achieved more directly using sea water. The adoption of district cooling is estimated to reduce the consumption of electricity for cooling purposes by as much as 90 per cent and an exponential growth in usage is forecast. The idea is now being adopted in other Finnish cities.
Cornell University’s Lake Source Cooling System uses Cayuga Lake as a heat sink to operate the central chilled water system for its campus and to also provide cooling to the Ithaca City School District. The system has operated since the summer of 2000 and was built at a cost of $55-60 million. It cools a 14,500 tons load.
In August 2004, Enwave Energy Corporation, a district energy company based in Toronto, Canada, started operating system that uses water from Lake Ontario to cool downtown buildings, including office towers, the Metro Toronto Convention Centre, a small brewery and a telecommunications centre. The process has become known as Deep Lake Water Cooling (DLWC). It will provide for over 40,000 tons (140 megawatts) of cooling—a significantly larger system than has been installed elsewhere. Another feature of the Enwave system is that it is integrated with Toronto’s drinking water supply
Applications
Condenser water distribution
The distribution medium of most DCS is usually chilled water. Condenser water can also be the distribution medium, while the central plant is made up of the cooling towers or heat rejection equipment. In such systems, each building has individual chiller plants but without cooling towers or heat rejection equipment. This system would be suitable for existing building clusters, as each building is already equipped with chiller plants. Supplying condenser water to these buildings would allow the building owner to get rid of the cooling towers and reduce their maintenance effort and eliminate the risk of legionnaire’s disease arising from inadequate maintenance. Poor maintenance would also lead to high potable water consumption.
The centralized heat rejection would facilitate more cost-effective operation of these plants that may become more attractive with rising potable water costs. In Singapore, there is an opportunity to exploit the sea as a huge heat sink that can supply unlimited cooling capacity to cool the condenser water without consuming potable water. Figures 1 and 2 show a typical schematic of a condenser water-based DCS with indirect seawater cooling and thermal storage that can be implemented in Singapore. In addition, the condenser water distribution system is significantly lower in cost as compared with chilled water-based systems. Condenser water pipes usually do not require insulation and can be directly buried underground with no concern about heat gain from the soil, as the condenser water temperature is usually higher than the soil temperature.
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Cogeneration, Thermal Storage and Desalination
The cogeneration of thermal energy and electric power allows for a much higher combined efficiency of energy use. The thermal energy can be used as an energy source for producing chilled water, using absorption chillers. Cogeneration can increase the efficiency of fossil fuel-based plants from an average of 40% to over 80%. The increase in efficiency can translate into lower costs and lowered emissions of pollutants than conventional methods of generating electricity. Due to the need to be in close proximity to a market for the heat produced, cogeneration power plants tend to be smaller in size and designed to emit fewer pollutants. This is most often done by using cleaner fuels such as natural gas. With the deregulation of the power industry and the adoption of demand side management by major consumers of electricity, the potential for expanded use of cogeneration in the future is great.
In addition, there is good economy in expanding the use of thermal ice storage for DCS. This would exploit off-peak power, made possible in Singapore as the power industry is being deregulated and the difference between peak and off-peak electricity cost widens. This will effectively lower the operating cost of DCS. By shifting a part of the chilling load to the night (using off-peak electricity), chiller equipment requirements can be reduced and sized closer to the average load than the peak load. This would translate into higher chiller operating efficiencies and lower the cost per unit cooling even more. In the case of cogeneration, spare generation capacities during off-peak hours can be used to produce ice for cooling during the day. This means a smaller cogeneration plant used for producing ice would not be competing for energy use during peak hours.
Cogeneration used to produce thermal ice slurry for storage can also be combined with desalination. This mode offers another avenue for harvesting drinking water from seawater, besides providing cooling using cheaper off-peak power. This concept utilizes vacuum ice technology that employs the phenomenon of the triple point of water where vapour, liquid and ice co-exist. Seawater is subjected to triple-point conditions in a tank under vacuum conditions. Evaporation of part of the water would force the formation of ice crystals. The melting of the ice slurry would then supply both fresh water and cooling.
Advantages:
The concept of district cooling is becoming more and more widespread all over the world. The idea, as for district heating, is to use one central source instead of local systems for each building. This will create both economic and environmental benefits.
The district cooling system offers operating flexibility, since each building can use as much or as little cooling as needed, without worrying about chiller size or capacity. The installation will be very comfortable and convenient for the customer, with the possibility of having the same supplier for electricity, heat and cooling. The installation of a district cooling system is greatly facilitated if it is combined with an existing district heating system, or one constructed at the same time, since the costs can be split between the two systems.
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No chiller problems
One of the benefits for the customer is the space saving effect and lower investment cost at the location because there is no chiller. Chiller replacement is never a factor as cooling towers or pumps wear out. With CFC/HCFC phase-out, the CFC/HCFC handling problem will also be taken care of.
No noise or vibrations
With centrally produced comfort cooling there will be no noise or vibrations. Maintenance and running costs will be lower, and a better level of equipment redundancy and round-the-clock expert management, which individual buildings cannot match, will be achieved.
Conclusion
The deregulation of the power industry, in concert with the trends towards outsourcing and environmental concerns are fuelling interest in DCS, thermal storage and co-generation.
DCS applies in most areas with appreciable concentration of cooling loads, such as industrial complexes, densely populated urban areas and high density building clusters, and can offer economic and environmental benefits. As the central plant of a DCS is large, there will be economies of scale and higher thermal efficiency as compared to that of many isolated small systems. A larger plant usually comprises of a number of capacity modules, which can be operated to match the combined cooling load. In addition, a centralized plant would be more optimal in terms of operation and maintenance. There is no need for individual building owners to employ operations and maintenance personnel for chiller plants. Usable space in the building would increase as large rooms for housing the cooling systems are no longer required.
Local DCS
DCS have been used in business districts and institutional settings, such as college and university campuses in many countries. Local DCS can be found in the Changi Business Park and Changi Naval Base in Singapore.
Resources:
http://www.nccc.gov.sg/building/dcs.shtm
http://en.wikipedia.org/wiki/District_heating
http://www.alfalaval.com/ecoreJava/WebObjects/ecoreJava.