Performance of closed loop systems
Rules of thumb of design of borehole-based closed-loop systems
The performance of a closed-loop systems can be affected by: (1) The specific capacity, thermal conductivity and temeprature of the ground. ???, (2) The operation pattern of the systems, e.g. the the number of full-load operational hours in different seasons and the duration of peak operation on a daily to weekly basis, (3) The ground loop operating temperature. In heating scheme, this is typically around 0 oC . However, in cooling mode, there is no upper limit. For example, a higher operating temperature can result in a shorter ground loop, (4). Thermal interference between boreholes. It can influence the steady state of carrier fluid and is disscused in ?????. (5) Complexity of heating and cooling loads. In a single day, there may have heating demand and cooling demand in different period of time. Coefficience of performance and carrier fluid temperature are differential in heating mode to those in cooling mode. Shorter borehole may be applied in seasonally reversible(complex heating and cooling) schemes compared to heating only scheme as the heat energy in the subsurface are much more balanced, e.g. Heat energy in the subsurface is extracted in winter and replenished in summer. Thus schemes of large scale GSHP systems have to cope with unambiguous assumptions and require a sophiticated understanding of heat storage and transfer, site investigation and more fine distinction design tools.
Indirect circulation systems
Closed-loop systems have two types of schemes: direct circulation and indirect circulation. Indirect circulation is more favorable and far more widespread than direct circulation systems, its features will be discussed here. In a indirect system, carrier fluid circulates in the closed loop of U-shaped pipes, which travel into the ground subsurface, along with the structural piles. In a heating mode, the cooler carrier fluid aborbs heat from the subsurface by conduction and carrys back to the heat pump, where extraction of heat is undertaken. The carrier fluid is chilled again and start another circulation through the earth. In a cooling mode, the warmer carrier fluid conveys to the subsurface and conduct heat to the relatively cooler earth. The fluid is cooled and re-enter to the heat pump. The closed loop functions as a subsurface heat exchanger and it is commonly used to describe this kind of heat exchange tools. A seperate heat distributed system within the building is driven by the heat pump. The carrier fluid is a refrigerant and circulate around the building where heating or cooling is required. The earlier refrigerants used on heat pump systems include chlorofluorocarbons (CFCs) and Freon. However, the release CFCs is found to be resulting in destruction of the ozone layer. Nowadays, permitted refrigerants have to be non toxic and non-flammable. They include fluorinated hydrocarbons, hydrocarbons and ammonia.
Flows in the closed-loop pipes
The carrier fluid is typically a waster-based antifreeze solution, thus it can be chilled to below 0oC when the subsurface temperature is ultimately low in winter. Generally, they have a freezing points of lower than -10 oC, some may low to -21oC. The antifreeze can be a solution of salt, ethanol or ethylene glycel. These alcohol-based antifreeze are permitted under regulatory authorities because thay are environmentally friend and biodegradable, therefore leakages of these solution to the environment is less concerned. The fluid flow rate and pipe diameter are designed such that (1) turbulent flow are archieved in the closed loops as this can facilitate the heat transfer from the ground to the fluid solution, the flow is said to be turbulent if the Reynold number of the flow, Re = beyond 4000 where Re is the Reynolds Numberber (non-dimensional), (kg/m3) is the density, U(m s-1) is the velocity, (m) is the distance between two points, and (Pa s-1) is the dynamic viscosity, (2) required amounts of heating load is archieved. In a heating scheme, an average carrier fluid temperature between -1.5 oC to -2 oC with flow rates of 3 -3.5 L min-1 per 1kW of heat transfer is commonly used. Fluid viscosity, which increases with decreasing temperature, depend on antifreeze type as well and can affect the flow rate and must be considered carefully. The fluid flow rate (L min-1), in a heating mode, can be estimated by the equation: where COPH is the coefficient of performace and Svccar (xxxxx)is the volumetric heat capacity, and is the temperature drop during heat exchange in heat pump.
Design of Piles
In a closed loop scheme, a U-shaped closed loop(U-tube) is usually placed down the length of the borehole. U-tubes are usually made of high-density polyethene tubing of diameter 32-40 mm. The number of vertical boreholes is dependent and proportional to the heating load, heat pump capacity and length of piles, In energy piles installation, these have to be specifically deliberated as the borehole is filled with concrete which has a lower thermal conductivity, thus descending the heat transfer. Figure ? demonstrates the relationship between the number of closed-loop boreholes and the installed heat pump capacity. It shows that the installed heat pump capacity per borehole(40m) is around 2kW. It then equates to an average of thermal output is 50W per pile meter. If we assume the coefficient of perfomance is 4, a heat aborption rate of 73 Wm-1 can be obtained. The antifreeze flow rate, nature and thermal conductivities of the ground, patterns of heat usuage are also important in the decision making. The number and depth of the ground loop must base on the structural pile design as the structural part of the piles is more expensive than the heat exchanger system. Moreover, it is not econimical to build extra energy piles to meet the required heating load.
Borehole thermal resistance
The equations described for the conduction of heat, we assume that the closed loop is in direct contact with the ground surface. However, in reality, there will be thermal resistances (Km-1 W-1) within the borehole. It imparts a temperature loss between the subsurface and the carrier fluid . Thermal resistance is realated to (1) the conductivity backfill of the borehole, for energy pile, it is the concrete. The selection of concrete is majorly based on the vertical dead and live load of the whole building and the number of piles to withstand the axial force applied to them, different concrete has different strength and thermal conductivity. (2) Short circuit of thermal transfer. It is caused by heat leakage, conduction between the downflow(cooler) and upflow(warmer) shanks(spacing between the tubes) of the U-tube,(3) thermal resistance when the heat transfer from one medium to another medium. It usually start from the reinforced concrete, through the U-tube, then to the carrier fluid. The thermal resistance(Km-1 W-1) of borehole can be measured in the in-situ field test or by analytical programs such as EED.
Figure ??? shows the dependence of borehole thermal resistance on filling material. The borehole thermal resistance decreses with higher conductivity. It shows that concrete has a borehole thermal resistance of around 0.18 K m W-1 and a thermal conductivity ranges from 1 to 2 W m-1 K-1. Relationship of thermal resistance with disscussed parameter are shown in figure.??? ,The thermal resistance can be minimized by (1) producing turbulent flow in the carrier fluid, (2) ensure a large shank spacing between the upward and downward flow to avoid thermal short-circuiting, (3) backfill the borehole with material that has a high thermal conductivity.