
THERMAL PROPERTIES OF A BOREHOLE 

1. 
Convective resistance for the fluid. 
2. 
Thermal resistance of the fluid/pipe. (Laminar sub layer) 
3. 
Short circuit for upwards and downwards. Space between pipes. 
4. 
Conductive resistance of the pipe. 
5. 
Contact resistance of the pipe and the backfilling material. 
6. 
Conductive resistance of the backfilling material. 
7. 
Contact resistance of the backfilling material and the ground. 

CONDUCTIVITY / RESISTANCE
The amount of heat that can be transferred between surrounding ground and heat carrier fluid depends on the two thermal properties: The thermal conductivity of the soil and the thermal resistance of the borehole. The soil quality is usually related to geological situation which cannot be changed by planer but borehole thermal resistance can be engineered and must be kept as low as possible.Borehole thermal resistance Rb, consists of the convective resistance of the fluid, thermal resistance of the fluid/pipe, shortcircuiting effect between the shanks, the conductive resistance of the pipes, contact resistance of pipe and back filling material, conductive resistance in the backfilling material and contact resistance of backfilling material and soil. The first three parameters can be reduced by increasing the flow rate of the fluid but increasing the flow rate has some side effect on the efficiency of the heat pump. Therefore there is a need to have a verified, scientifically based design tools to warrant of an optimal system that works prefect in long term.
Borehole thermal resistance (K m W1) 

Reynolds Number 

Carrier fluid flow rate (L min1) 

A. Laminar flow
B. Traditional / Turbulent flow
C. Reynolds Number
1. The dependence of borehole thermal resistance and fluid flow rate.2. Calculations were made using the program EED. 3. Based on a 127 mm borehole diameter with a single U pipe 32 mm SDR11 and carrier fluid 25% ethylene glycol. 
DECREASE THERMAL RESISTANCE
The picture up to the right shows that higher flow rate can improve the performance of the system by decreasing the borehole thermal resistance. However this advantage is tradeoff by higher energy consumption of the circulation pump. The circulation pump must overcome the system’s pressure drop. It means a bigger pressure loss, a larger circulation pump is required and greater pump power consumption. 
SMALL CHANGES IN THE FLOW RATE CAN CAUSE A RATHER BIG CHANGE IN PRESSURE DROP
The picture illustrates that small changes in the flow rate can cause a rather big change in pressure drop, in fact the pressure drop is proportional of power 2 of flow rate. At the same time, pump consumption is linear dependent on both pressure drop and flow rate. An important practical conclusion is that, the energy consumption of the circulation pump in the heat pump is roughly proportional to the third power of the flow rate. Pumping power is essentially important and plays a big role in the coefficient of performance known as COP. Here it can be concluded so far that flow rate in a GSHP system is an important factor; COP would be maximized when the flow rate is in an optimal set up. 
Pressure Drop (kPa) 


Flow rate (L/min) 
Borehole 200m with single U pipe 40 mm diameter with SDR11. Water as fluid at 10°C.

A GOOD GSHP SYSTEM HAS THE FOLLOWING CHARACTERISTICS:
• Has high heat exchange efficiency with the surrounding heat source (with low borehole thermal resistance) articularly at the peak load.
• Has a turbulent flow regime in the system for the higher heat transfer.
• Has an acceptably low pressure drop to minimize circulation pump power consumption.
RESEARCH STUDIES
MuoviTech has been conducting a number of research studies to improve the quality of the system through achieving a low value of Rb. Based on these research studies a geothermal collector with a turbulator mechanism (passive) known as the TurboCollector® has been developed. TurboCollector® is the state of the art technology and patent pending which has an internallytwisted fin that can apply for different U pipe collectors in the GSHP systems. These fins disturb the laminar sublayer adjacent to the internal pipe surface and enhance the heat transfer.

