발전소 폐열원 열펌프 시스템의 시설원예 적용

Abstract

This study has been conducted with the purpose to demonstrate the economic value of utilizing the enormous heat energy carried in the hot waste water from power plants by comparing the benefit from using hot waste water against that from using geothermal energy in widespread application under the Korean government's policy. It is expected that the findings from this study would lead to drastic savings in the heating/cooling expenses of the facility horticulture and help meet the government's green-growth policy by reducing the CO2 emission. In Korea, power generation of 1 Twh emits, on average, 138 million tons of hot waste water, annually totalling to upwards of 47.73 billion tons. On Jeju's coast, 230 million tons of hot waste water, equivalent to the energy of about 143,000 TOE, is discharged every year. Temperature of the hot waste water from the Namjeju Thermal Power Plant was observed to be 25～28℃ in Winter(lowest in February), 32～35℃ in Summer(highest in September). Annual sum of hot waste water discharged from all the power plants in Korea is estimated at over 388,000GWh. Energy of 13,217GWh is used every year on heating greenhouses(13,000ha of facility horticulture, light oil consumption of 1,256,000kl), accounting for 3.4% of the waste heat from power generation. The acreage of facility horticulture potentially benefitted by hot waste water has been estimated at 1,028ha for 20℃ heating, and at 631ha for 24℃ heating on jeju. Considering the corrosion caused by seawater, the heat retrieval system used metal pipes of SUS 316 quality, weighing 1,950kg, with a capacity of 300,000 Kcal/hr, and was manufactured for the use with 90RT. Test plant was constructed on July 27, 2010 and was test run for 6 months until February 28, 2011, turning out to show a smooth operation and a high efficiency in heat use and economy. But some troubles have been identified in that the maintenance is not handy because the heat retrieval system is installed in the hot water tank, and that the chain hooks holding, and the mount supporting, the heat retrieval system are corroded by seawater and thus dislocated off the fixed part. A PE-pipes heating and cooling system for facility horticulture was developed and tested for efficiency, compared to the existing metal-pipes system, in order to retrieve the heat in the waste water from a power plant and to use it efficiently. Total length of the used PE pipes was 2,800m. Temperature measurements of the water at the inlets of the evaporator and the condenser in the system of 30RT scale. The experimentation was carried out through the help from the watt-hour meters, flow meters and temperature sensors installed on the 115kw-unit heat pump system. Analysis of the obtained data disclosed that the evaporated heat was equivalent to 108.1kw, condensed heat was 144.9kw, electricity consumption was 37.2kw and the heating efficiency coefficient was 3.9 when the temperature of the heat media was 21.3℃ at the inlet to the evaporator and 45.3℃ at the inlet to compressor. Efficiency was thus validated. The use of PE pipes solved the corrosion problem, and reduced the manufacturing cost by more than 27%. It is also believed that the hot water tank, if installed in a farm, can enlarge the heat retrieval system and facilitate frequent maintenance work. During the period of study from Nov 1, 2010 to Jan 31, 2011, the lowest outside temperature was -5.1℃ in Jan 2011 around Hwasoon-ri, Andeok-myeon. The lowest temperature inside the heat storage was 43.6℃ in November 2010, 45.0℃ in December 2010, and 44.2℃ in January 2011. It thus stayed stable regardless of the fluctuation in the exterior temperature and the amount of energy used in the mandarine or mango greenhouses. When the outside temperature was -5.1℃, the lowest temperature inside the mango greenhouse stood at around 22℃, the same as on the other days. This point demonstrates that the system is able to hold a stable temperature in the greenhouse regardless of that outside. The result of this experimental study showed that the system using hot waste water has many strengths resulting from easy control of the temperature in the green house unrestricted by the high or low outside temperature. It enables to keep the inside temperature at an adequate level to the floral differentiation of the plant and growth phase of the fruit, to control harvest time, to better the artificial coloration with low-temperature treatment in a relatively hot season. It proved to be a system which can lighten the farmers' burden by enabling them to operate the heating system with 13% of the expense needed by the use of light oil, and thus yield high income though the quickened-growth shipment. The growth diary in year one of the mandarine in the greenhouse heated with hot waste water goes like this. Started heating on November 15, began to flower on November 27, was in full bloom on December 2, and mature ready for harvest late June. 13 tons of early satsuma mandarine were harvested from an acreage of 3,050㎡. An experimental test was run on the mango greenhouse which contained 2-year-old apple mango trees of Irwin variety planted in the 200ℓ plastic boxes on an acreage of 2,215㎡. New shoots shot up 4.9 times a year, began to bud on Dec. 11, began to flower on Nov 3, and got mature on Nov 15. Harvest time has thus been accelerated by more than a year compared to that in normal farming. Electricity of 226,641 kwh was consumed, leading to the electric bills of 9,975,124 won, by the heating and cooling system using hot waste water for the greenhouse of 5,265㎡(3,050㎡ for early satsuma mandarine, and 2,215㎡ for apple mango) for 5 months from Dec 15, 2010 to Feb 28, 2011. By contrast, the consumption of tax-free light oil amounted to 76,013 L in the system providing the same amount of energy in the form of hot air. The energy bill was calculated to be 76,013,000 at the price of 1,000 won per litter. This calculation offers 87% of cost-saving rate reaped by this heating system, compared to that of a system consuming tax-free light oil. The study of cost/benefit analysis and pay-back period for the facility cultivation of paprica showed the pay-back period to be 3.19 years for the system using hot waste water. This figure is 1.34 years (25%) shorter than that gained by the system using geothermal energy. It has been estimated that CO2 emission can be reduced by 306.3 tons every year if hot waste water replaces light oil for the facility horticulture in a greenhouse of 1ha.