Seasonal snow storage for cooling applications
Abstract: Seasonal snow storage for cooling applications is an ancient technique now being revived in different parts of the world. From June 2000 the first Swedish large-scale snow cooling plant provides for comfort cooling at the hospital in Sundsvall. The snow is thermally insulated by a 0.2 m layer of wood chips and stored in a shallow pit (140´60 m) of watertight asphalt, with a capacity for 60,000 m3 (40,000 tons) of snow. Meltwater from the snow storage is cleaned and pumped to the hospital, and after cooling the heated meltwater is re-circulated to the snow storage. During the first summer the hospital required 655 MWh of cooling with a maximum cooling power of 1,366 kW. Approximately 93% of the cooling demand was attained by 19,000 m3 of snow, the rest by a cooling machine. The main part (75%) of the snow was natural while the rest was produced with snowguns. The running COP of the snow cooling system was 10.5 while the corresponding value of the chiller was 2.2. When material depreciation was included, based on lifetimes and energy input, the total COP became 8.6 for the snow cooling system and 2.2 for the chiller. The operation of the plant experienced only minor problems. Urban snow is polluted and the meltwater quality in the snow cooling plant was such that further treatment had to be considered, in particular concerning non-biodegradable compounds, phosphorus, and lead. The snow storage technique enables analysis and treatment of the meltwater. Without thermal insulation and cold extraction a 30,000 m3 snow pile in Sundsvall would be gone by mid-June. With 0.2 m of wood chips covering the snow the melting is reduced considerably. In this case 75% of the snow remains and can be utilised for cooling. In a laboratory experiment it was found that with a 0.08 m layer of wood chips the natural melt rate was reduced by approximately 85%. The thermal insulation qualities of wood chips on snow and ice depend on both evaporative cooling and heat resistance of the wood chips. Approximately 3% of the melt water was evaporated, which corresponds to 20% of the total heat transfer. The absorbtivity of light, which was 0.5-0.6 in fresh wood chips, is important for the thermal insulation qualities. Since the absorbtivity increases and water transport functions of wood chips might deteriorate by time it is of interest to study the impact of ageing. As water freezes in an open system, some water is evaporated. This mass loss is typically 2.5-3% of the mass of ice and accumulates to large quantities with repeated freezing and thawing. It should therefore be included in snow storage modelling since the snow is stored during a period when repeated melting and re-freezing occur. The total construction cost of the Sundsvall plant was about 1.3 MEUR with a running cost of 15.3 EUR MWh-1 (total 0.01 MEUR) during the first year of operation. In a more recent study it was concluded that the construction cost could be considerably reduced. Estimated pay back time is about three years for a new project with the same conditions as the Sundsvall Hospital system, decreasing with increased cooling power and size. The expected lifetime of a snow storage plant is about 40 years. The snow/ice cooling plant has a great technical potential in industrial, agricultural, and comfort cooling applications in large parts of the world. By replacing cooling machines the use of cooling media is reduced. These are often flammable, poisonous and/or environmental hazardous. A snow cooling plant means not only saving of money and energy but also environmental benefits.
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