Conference Paper

Proceedings of ASim Conference 2012: 1st Asia conference of IBPSA-China, Japan, Korea

Optimization of ground source air-conditioning system configuration and operation

Rongling LI, Ryozo OOKA

Abstract: Geothermal is a fast-growing alternative heat source for HVAC systems due to higher energy efficiency than conventional heating and cooling systems. There are two ways to use geothermal sources: direct utilization of geothermal sources (DUGS) and ground source heat pumps (GSHP). In order to predict the performance of these two different geothermal systems under Tokyo weather conditions, computer modelling and simulation was carried out using the TRNSYS software package. Models of borehole heat exchangers (BHE), a water-to-water heat pump, a fan coil unit (FCU) and chilled ceiling panels were implemented in TRNSYS. Modelling of both DUGS (BHE + FCU/Panel), and GSHP system (U-tube + GSHP + FCU) was carried out. A case study is presented to show the cooling performance of these systems during summer. A model room was built as one classroom in a university building with one external wall. The external wall and window were set according to Japan Energy Saving Standard 1999. The results indicate that the BHE + Panel system cannot meet the cooling load of the model room with the supplied 22℃ water. We attribute this to the chilled panels only covering 45% of the ceiling. However, DUGS connected to a FCU cooling system is able to handle the sensible cooling load on most summer days, except during the hottest period in August. To improve DUGS performance on the hottest days, we adjusted the air volume of the FCU. The cooling capacity successfully met the cooling load when the air volume was set to 160% of the FCU's rated value. Further simulations were conducted to evaluate the energy performance of both DUGS and GSHP cooling systems. The analysis showed that DUGS connected to a FCU system performs better than a traditional GSHP cooling system. The electricity consumption of the DUGS system was found to be less than 1/2 of the GSHP system. * Corresponding author email: lirl@iis.u-tokyo.ac.jp Lastly, the electricity consumption of the GSHP system was calculated in two cases with different temperature of supplied cold water: 7℃, 15℃. The result showed that when the heat pump supplies 15℃ water, the system is 4% more energy efficient than when it supplies 7℃ water. Although the heat pump water chiller is 12% more efficient when supplying 7℃ water, the pump and fan use more electricity as the volume of water and air increases.
Pages: 235 - 242
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