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Heating Performance Analysis of the Heat Pump System for Agricultural Facilities using the Waste Heat of the Thermal Power Plant as Heat Source
A numerical analysis of a single U-tube of borehole heat exchanger (BHE) in a ground source heat pump (GSHP) system containing ground soil and mortar is required to derive expectations for greenhouse cooling and heating. Compared with the 400-m-long ground heat exchanger (GHE), the 200-m-long GHE requires less installation cost and produces stable thermal energy because it only extracts energy where the temperature is stable, making the thermal energy easier to predict. In this study, numerical analysis was performed on a single BHE U-tube at a depth of 200 m in a GSHP system including soil and grout to be used for greenhouse heating and cooling.
A 3D model of the BHE was created using ANSYS Design Modeler 21.1 software. The BHE consists of a single U-tube, working fluid, grout, and surrounding soil. The fluid flow and heat transfer process within a single U-tube GHE follows the laws of mass, momentum, and energy. ANSYS Fluent 21.1 software, a commercial computational fluid dynamics (CFD) program, was used for the numerical analysis.
In the cooling condition with the inlet temperature of 308.15 K, the outlet temperatures of 30, 32, 34, and 36 LPM for each volume flow are 301.93, 302.24, 302.52, and 302.77 K, respectively, and the temperature difference is 3.22, 5.91, 50.63, and 5.38 K, respectively. In the heating condition with the inlet temperature of 274.75 K, the outlet temperatures of 30, 32, 34, and 36 LPM for each flow rate are 279.6, 279.35, 279.14, and 278.94 K, respectively, and the temperature difference is 4.85, 4.6, 4.39, and 4.19 K, respectively. The lower the velocity, the higher the inlet and outlet temperature difference. This is because when the flow rate is low, the velocity decreases, and the temperature variation increases. In addition, when the flow rate is high, the velocity increases, and the temperature variation decreases. The heat transfer of 30, 32, 34, and 36 LPM was 12.985, 13.162, 13.319, and 13.468 kW under cooling conditions and 10.109, 10.246, 10.369, and 10.798 kW under heating conditions, respectively. The pressure drops for 30, 32, 34, and 36 LPM are 23.9, 25.872, 27.871, and 31.626 kPa, respectively. As the flow rate increases, the heat transfer increases and the pressure drop also increases.
In this paper, the thermal energy supply plan for a smart farm complex in Dangjin area was analyzed. First, the dynamic energy load was calculated by time-series weather data and 3D venlo-type glass greenhouse model with the TRNSYS program. In addition, the boiler and power plant waste water source heat-pump, stream water source heat-pump, and the ambient air source heat-pump was configured as the heating facility model with optimum capacity through dynamic energy load value. The heating simulation to maintain growth conditions of crops familiar with low-temperature and high-temperature was also performed. Through these models, the heating performance and economic evaluation in each cases were carried out. As a result, it was evaluated that low-temperature crops (strawberry) has a better economic performance by 400 % than high-temperature one (tomato), and 50 % or more of government subsidies should be required for high-temperature crops. It was analyzed that the heat pump heating cases showed better economic efficiency because the operating cost could be reduced by about 40 % compared to the boiler.
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