Experimental and numerical study of earth pipe cooling performance in a subtropical climate

The energy crisis and environmental issues are the major obstacles for human development. There is ongoing research occurring in order to overcome these obstacles through energy saving initiatives and alternative energy options for a sustainable environment and economy. A passive air cooling strategy is seen as a viable option to save energy for all hot and humid subtropical climatic zones. An earth pipe cooling technique is one of them which utilises the earth’s near constant underground temperature for cooling a space in a passive process without using any mechanical units, and thus saves energy in buildings with less or no environmental impact. It has mainly been used or studied in Europe and America and never been studied before in any subtropical climate of Australia. The earth pipe cooling system involves long buried pipes in which intake air comes through one end and passes through buried underground pipes, then the air is cooled by the cold soil and finally the cooled air is released into the room through the outlet end. In this study, the buried pipes were aligned in horizontal and vertical trenches and thermal performance of the horizontal and vertical earth pipe cooling systems was investigated for a hot and humid climatic zone, Rockhampton, Australia using a new and novel approach with experimental and numerical modelling and parametric investigation. The earth pipe cooling experiment was conducted on two rooms that were fabricated from two shipping containers and installed in the sustainable precinct of Central Queensland University, Rockhampton, Australia. One of the rooms was connected to a horizontal earth pipe cooling (HEPC) system and the other to a vertical earth pipe cooling (VEPC) system. Effects of air velocity, air temperature, relative humidity and soil temperature on room cooling performance were investigated through a series of experiments. An integrated thermal model was developed for both the horizontal and vertical earth pipe cooling systems using the simulation program ANSYS Fluent to compare and validate their cooling performance. A temperature reduction of 1.13oC was found for the HEPC system while 1.87oC was obtained for the VEPC system. The temperature reductions saved an average energy cost of $122.82 (maximum of $210.87) in 3 months of summer 2013-2014 using the HEPC system and $131.84 (maximum of $275.58) in 3 months of summer 2014-2015 using the VEPC system for a 27.23m3 room. These savings contributed to return the initial investment in 4.59 years and 4.27 years (payback period) for the HEPC and VEPC system respectively. To measure the optimum efficiency of the earth pipe cooling system, a parametric analysis was carried out in ANSYS Fluent. The parameters affecting the earth pipe cooling performance, namely air velocity, pipe length, pipe diameter and thickness, pipe material, and pipe depth were taken into consideration for this analysis. Since the VEPC system exhibited better performance than the HEPC system, the effects of these parameters on the thermal performance of the earth pipe cooling system were examined and analysed using the VEPC model. The parametric study showed that the VEPC system is capable of reducing the room temperature by a further 3.50oC. The results and outcomes of this study provide new knowledge and information which will assist to develop optimum passive air cooling product guidelines and the product unit(s) for consumers and builders to design energy efficient buildings.