The Design and Operation of Ice-Slurry based District Cooling Systems
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Introduction
Since the oil crisis of the 1970's, interested in district heating and cooling has increased dramatically. The Scandinavian countries particularly, are continuously applying the latest improvements in district heating technology. There are a number of excellent reasons for this renewed interest. In addition to improved performance due to economy of scale, a wide range of heat sources can be applied to DHC systems including waste heat and combined heat and power (CHP).
Interest in DHC and community integrated energy systems (CIES) is now increasing markedly in North America as well. One important element of CIES is the summer air conditioning load.
Environmental concerns about the ozone depletion potential of some CFCs used today have prompted a search for alternative cooling technologies. With the recent improvements in low-temperature absorption chillers, waste heat from any process stream of CHP plan may be used to satisfy a chilling load, without the use of CFCs. This also improves the overall efficiency of the process of which the residual energy is derived.
In a typical air conditioning installation, cold water is supplied at about 7 °C and returned at about 13 °C. The small amount of sensible heat corresponding to this temperature difference of 6 °C means that a large volume of water has to be circulated in the DHC system. If the temperature of the supply water could be decreased, or if phase change materials were incorporated in the circulating heat transport liquid, the system mass flow and pipeline diameters could be reduced, thereby improving the economics of the system. Smaller pipes reduce the installation and capital costs, and the reduced surface area decreases heat gains and therefore operating expenses. Also, existing district cooling systems can be expanded using ice slurry without increasing pipeline capacity.
The advantages of low temperature air distribution (i.e. less than 10 °C) are now being recognized. Ice slurry based air conditioning is of course very suitable for low temperature air distribution. The advantages include less air transport, lower horsepower fan motors, less voluminous duct work (the height of buildings can thus be reduced) and extra comfort from lower relative humidity, without terminal reheat (Tamblyn, 1992).
This report describes the advantages and disadvantages of using ice slurry in district cooling systems. Some existing ice slurry air conditioning projects, and some that are under construction are described to demonstrate the feasibility of the concept and to highlight current practice. The more important physical properties and behaviour of ice slurries are presented in a form that will help the practising engineer or consultant to develop a DHC system design.
Summary
The district heating and cooling (DHC) industry is enjoying renewed interest in North America. The successful implementation and operation of district heating systems in Northern Europe proves that significant advances have been made in the areas of fuel efficiency and self-sufficiency. Impressive reductions of harmful emissions have been achieved at the same time. With respect to district cooling, scientific breakthroughs have been made recently with friction reducing additives for cold liquids and phase change slurries.
Interest in district cooling is propelled by the increasing cooling load in commercial buildings due to factors such as the widespread use of personal computers, other electronic office equipment and by the need to phase out the use of CFCs in accordance with the Montreal Protocol.
Ice slurry has the potential to achieve considerable benefits for district cooling systems. The cooling capacity of ice slurries can be four to six times higher than that of conventional chilled water.
In this report, current practices and possibilities of ice slurry utilization are reviewed. The pressure drop properties of ice slurries are presented. Several commercially available ice slurry chillers are compared in terms of performance and costs.
The distribution network is one of the high cost items in DHC systems. The advantages of ice slurry with respect to cold water are presented. The effect of storage facilities on the capacity of the chillers is discussed. Possible consumer substation configurations are compared. The report concludes with a look at controls, energy metering possibilities and heat exchanger design for ice slurry use.
Ice slurry technology can significantly decrease the operating cost of a district cooling system. Although the capital cost of ice slurry chillers is at present higher than that of conventional chillers, significant progress is being made to reduce this cost.
