Seasonal snow storage for space and process cooling
Sammanfattning: The world’s cooling demand has increased considerably during the last decades due to increased population, industrialisation, comfort demands, electronic equipment usage and new building technologies. Conventional cooling is often produced by electrically driven devices. One less prime energy-consuming alternative is to use stored winter cold in snow and ice for cooling during the summer. This ancient technique is feasible in large parts of the world. Different systems for seasonal snow and ice storage exist, i.e. the snow/ice can be stored indoors, on the ground, in the ground and underground. This study focuses on in ground storage, in an open pond, where the cold energy is extracted by water that is cooled by direct contact with the snow. Open pond snow storage must be thermally insulated; hence, different insulation alternatives were discussed. Cutter shavings were studied in laboratory experiments and numerical modelling. The surface melt rate of snow covered with cutter shavings increased with increasedsolar radiation, air velocity, air temperature, and decreased insulation layer thickness. The evaporation rate contributed significantly to the energy balance. The surface melt rate was similar with initially wet and initially dry wood chips. It was concluded that evaporative cooling is an important part of the thermal insulation qualities of wood chips. It was also found that heat transfer from the rain and ground is usually relatively small. The heat transfer from the ground depends on soil and groundwater properties. The Sundsvall Regional Hospital snow cooling plant in Sweden has successfully operated since 1999. Natural and artificial snow is stored in a slightly sloping, shallow pond of watertight asphalt. During these years, the plant has delivered the main part (77-93%) of the cooling, totalling 655-1,345 MWh. The snow was thermally insulated by a 0.1-0.2 m layer of wood chips. The total coefficient of performance, including construction energy, was 2.0-6.6 times greater than that of a conventional chiller system. The environmental impact of a snow cooling plant and a chiller system was compared, for both existing and “environmentally optimised systems”. Of the existing systems, the chillers had the largest impact concerning climate change, acidification and nitrification, while the snow cooling system meant more photochemical ozone emissions. The dominating impact sources of the snow cooling system were fuel and electricity. In the construction phase, ground insulation had the greatest impact. In future open pond storage, a more compact design (deeper storage) is suggested to reduce maintenance and melt loss. Total cooling costs were estimated to be 0.29-0.47 SEK kWh-1 for a new open pond storage, i.e. lower than that of district cooling. The study also comprised mass loss of freezing water, since repeated freezing and thawing during the spring will evaporate large amounts of water. This was estimated to have little effect on Swedish snow storage, though the loss might be considerable at other locations.
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