Study on finned pipe performance as a ground heat exchanger

The GHEs (ground heat exchangers) is an important element that determines the thermal efficiency of the entire ground-source heat-pump system. The aim of the present study is to clarify thermal performance of a new type GHE pipe, which consists straight fins of uniform cross sectional area. In this paper, GHE model is introduced and an analytical model of new type GHE pipe is developed. The heat exchange rate of BHEs utilizing finned pips is 40.42 W/m, which is 16.3% higher than normal BHEs, based on simulation analyses.


Introduction
In the recent decades, energy consumption for building sector has increased in multifold around the world [1]. Efforts are being made to develop alternate energy sources for meeting the demand of building heating and cooling loads. One of the best alternate ways is the use of nature source energy.
Geothermal energy is regarded as one of the most efficient forms of energy, and it has great potential as it is directly usable. At deeper layers, the ground temperature remains almost constant throughout the year and is usually higher than that of the ambient air during the cold months of the year and lower during the warm months, as shown in Fig.1.
A ground-source heat-pump (GSHP) system combined with GHEs (ground heat exchangers) absorbs and extracts geothermal energy for space heating and cooling. The GHE is an important element that determines the thermal efficiency and initial construction cost of the entire GSHP system. It is important to lower initial construction cost by improving thermal performance of GHE.
Heat transfer through GHE pipe is closely related to the heat transfer between the fluid that circulates within the GHE pipe and the complex medium (grout/ground).It is well known that the ground thermal conductivity and borehole thermal resistance are among the most important parameters in the design of a GSHP system.
The borehole thermal resistance can be decreased by increasing the thermal conductivity of the grout and the GHE pipe, and by optimizing the type of pipe used and the pipe configuration [2][3].
In the present study, the thermal performance of a new  Table.1 Simulation models type GHE pipe is clarified to determine its applicability as a ground heat exchanger. In this paper, the configuration of new type GHE and analytical model are introduced.

Configuration in borehole and model description
The new type GHE pipe is polybutylene (PB) consisting straight fins of uniform cross sectional area, and double Utubes BHE is considered as shown in Fig.2. The depth of borehole is 80m.

Model description
Quasi-Three-Dimensional model and finite line-source models are used in this study.In addition, fin efficiency methods is used to calculate heat conduction through fins of PB pipe, as shown in Table1.
-The temperature distribution along the vertical direction has a negligible influence.
-There is no contact resistance between the boreholes and the ground.
-The fluid temperature in the BHEs is determined as average of inlet and outlet temperature.
-A uniform initial temperature of 18.2 Degree C is equal to the undisturbed ground temperature. Table.2 shows the Geometrical parameters and properties of materials and so on. Fig.3 shows the fin efficiency of PB pipe at various fin length h and thickness δ. Out radius of Pipe r o is16 mm, λ p =0.05 W/mK. It is clearly seen that the fin length affects the fin efficiency is stronger than fin thickness. It is noticed that increment of fin length causes rapid decrement on the fin efficiency of pipe. Fig.4 shows the relationship between the thermal resistances of finned pipe Rp_f and the number The fin length 5mm,fin thickness 3mm and the fin's number 20 are selected, the fin efficiency of pipe is 0.57, considering manufacturing process requirements, in this paper. Fig.5 shows the temperature distribution for 2 BHEs with finned pips in the ground at the end of 2, 5,7days under condition of inlet temperature 35 degree C and ground temperature 18.2 degree C. It is obvious from these figures that the ground temperature is raised by the time. It can also be noticed that the temperature response at any location keeps rising. The heat exchange rate is 40.42 W/m. Fig.5 (b) shows the temperature distribution in the ground for 2 BHEs with finned pipes after 5days. Comparing to Fig.6, It is clearly seen that the configurations which utilize finned pipes with the same distance have more thermal interaction between boreholes. The reason is that the heat exchange rate of BHEs with finned pips is 40.42 W/m, which is 16.3% higher than normal BHEs.