Dear OGS Community,
I am working on the coupling of OGS with TESPy in a one-way architecture. In the current setup, the fluid enthalpy value at the splitter point is set to be equal to the sink point enthalpy, which ensures that all the heat consumed by the heat pump is supplied by the BHEs array. However, I have some questions regarding this approach:
- Why is enthalpy used instead of temperature?
In this case, why is the enthalpy at the splitter point set to match the sink point enthalpy, rather than simply setting the temperatures equal? What is the physical or thermodynamic reasoning behind using enthalpy for this energy balance?
- Pressure and Temperature Relationship
In this system, it seems that pressure is not directly related to temperature changes, especially when the mass flow rate is fixed. Could you clarify how pressure and temperature interact in this context, and why enthalpy is a more suitable parameter for ensuring energy balance?
Enthalpy
The enthalpy can be described as the energy of a flowing fluid, so it is more accurate than only the temperature. If you only use the temperature, than you only compare the inner energy of the fluid. The parts of the velocity and pressure are than not included, but the enthalpy recognize it.
In this example of the open loop the same enthalpy at the splitter and at the sink point it set, because there is no other heat consumer or producer in between. This example should show, how to other heat sources/sinks especially in combined energy systems can be integrated into the loop. From a physical point of view there is no difference between the open and closed loop example, but from the modeling perspective.
Pressure/Temperature Relationship
For the calculation of BHEs in OpenGeoSys only the volume flow rate and the temperature are used, the non linearities of eg. the density with temperature changes was not resolved. The mass flow rate is set constant in Tespy, but the volume flow rate changes with the nonlinear density in real applications. How to deal with this assumptions of OGS in Tespy is a bit out of scope for this forum, maybe you ask in Tespy community. For most of geothermal systems with BHE in the shallow subsurface the nonlinear interaction should be small.
Best Max
Many thanks Max！！！
Best,
Shuoshuo