The Component Geothermics
The component 
Geothermics models the heat supply through a geothermal heat exchanger. The geothermal heat exchanger can be a Borehole heat exchanger (geothermal probe) or a Horizontal ground heat exchanger (ground heat collector). Firstly, in the Technical input data form of the component under the heading Design parameters, choose the heat exchanger type – Borehole heat exchanger or Horizontal ground heat exchanger in the drop-down menu.
In addition to the amount of heat transferred, the required electrical power of the circulation pump is also modeled for all heat exchanger types.
In a typical application, the component Geothermics in combination with a Heat Pump covers the Heat Demand of a residential building.
Component Template
The Geothermics.e-ctpl component template is located in the Component template library folder Renewable Energy Sources.
Integrating the Component Geothermics into the Scheme
How the Component Geothermics Works
Regardless of the heat exchanger type, the component Geothermics serves as a heat source. The maximum heat output of the component is calculated depending on the input data. The electrical power of the circulation pump is also modeled.
For the type of Borehole heat exchanger (geothermal probe), the Maximum specific extraction rate is given as input parameter.
For the heat exchanger type Horizontal ground heat exchanger, two calculation methods can be selected. On the one hand, as with the Borehole heat exchanger, a calculation based on Maximum specific extraction rate is possible. On the other hand, a quasi-static calculation method is possible by means of heat exchanger and soil parameters, such as the Soil heat conductivity or the Installation depth of the collector.
Calculation of the Maximum Thermal Capacity
When calculating the Maximum thermal capacity, a distinction is made – as described in the previous section – between the system types geothermal probe and geothermal collector:
The Maximum thermal capacity is determined by multiplying the input parameters Maximum specific extraction rate, Probe length, and Number of probes.
As described in the following two sections, the Maximum thermal capacity can be determined by two different calculation methods.
Calculation With Maximum Extraction Rate
The Maximum thermal capacity (heat output) is determined by multiplying the input parameters Maximum specific extraction rate and collector Area.
Calculation With Heat Exchanger Parameters and Soil Type/Soil Parameters (quasi-static calculation method)
In addition to the collector properties, you can either specify the soil parameters individually (see following figure) or select a soil type from the drop-down list.
The following soil types are available for selection from the drop-down list:
The heat transfer between the heat transfer medium and the ground is modeled quasi-statically. The Maximum thermal capacity of the component is proportional to the mean logarithmic temperature difference between the heat transfer medium and the surrounding soil with \(\dot{Q}_{max} \propto \Delta T_m\). The heat transfer coefficient is calculated by means of a form factor from the input parameters Soil heat conductivity, Installation depth, Distance between pipe strings, Pipe diameter, and Number of parallel pipe strings. The temperature of the surrounding soil is calculated on the basis of the Local ambient temperature, the Installation depth and soil parameters, such as Soil density, Soil heat conductivity, and Soil heat capacity.
Calculation of the Circulation Pump
Electrical power of the circulation pump is required for pumping the heat transfer medium. It is calculated using the following formula \(P_{el} = \frac{ \Delta p \dot{V} }{\eta}\), with Efficiency circulating pump \(\eta\), depressurization \(\Delta p\) and volume flow rate \(\dot{V}\). The volume flow rate depends on thermal capacity \(\dot{Q}_{th}\), Estimated temperature spread \(\Delta T\) and material parameters of the heat transfer medium by \( \dot{V} = \frac{\dot{Q}_{th}}{\Delta T c_p \rho}\). Depressurization depends on the type of flow of the heat transfer medium and is generally defined by \(\Delta p = \lambda \frac{l}{D} \frac{\rho}{2} w^2\), in which \(w\) is flow velocity of the heat transfer medium, \(\lambda\) is the pipe friction coefficient and \(\frac{l}{D}\)is the ratio of the Length of single pipe string to the Pipe diameter. A turbulent flow rate is assumed to determine the coefficient of pipe friction, since in this area there is usually an economic optimum between improved heat transfer and energy consumption of the circulation pump. Consequently, the coefficient of pipe friction for turbulent flow can be calculated on the assumption of technically smooth pipe surface according to Blasius by \(\lambda = 0.3164 Re^{-1/4}\), in which \(Re\) is the Reynolds number. Flow velocity \(w\) is determined from the ratio of volume flow rate to pipe cross-section.
The calculation of the electrical power demand of the circulation pump shown above is non-linear and is approximated in TOP-Energy by a linear relationship based on the operating state with maximum thermal power, as shown in the following diagram:
Specification of the Outlet Temperature
The Outlet temperature of the heat transfer medium is used when selecting Heat exchanger parameters and Soil type/Soil parameters of the Horizontal ground heat exchanger to determine the Maximum thermal capacity (see also above the section Calculation of the Maximum thermal capacity). In addition, the Outlet temperature is used to calculate the mixing temperatures for all types of heat exchangers after optimizing the energy balance. This behavior is described in the article Special Features of Cold and Heat.
