Innovative Ground Storage

Project Description:

Ground temperature after a certain depth into the earth is constant over all the year. This is because that the soil mass is very large and its thermal capacity is large as well. For this reason, the idea to use the soil as a heat exchanger to preheat a working fluid in winter and precool it in summer has been developed.
To exploit the ground thermal capacity it is necessary a heat exchanger. Basically, it consists of buried pipes in the ground. To exploit the deep zone is necessary to drill the ground with very large vertical borehole and only if the geothermal gradient of temperature is significant. To exploit the shallow zone horizontal pipes are buried. The system ground pipes behave like a heat exchanger. The classification is between open and closed systems. In an open system, the working fluid that is exchanging heat with the ground is the same that after can be used for space heating or cooling. In the closed system, the media is used as a source to exchange heat with another fluid.
The air can be recirculated in the loop with a large influence on the overall performance of the system. By recirculating air in the loop during winter, low inlet temperature in the system is avoided, and then it is realized the possibility to make the system able to supply air at interesting temperature for a longer period.

Project Implementation:

The analysis of a ground storage involve a big volume in the ground, therefore, it is very expensive from a computational point of view to make an analysis in the time domain. And if it is considered that basically, the heat transfer in the ground is by conduction, the partial differential equation that describes the phenomenon is linear.
1. LTI Systems: Linear Time-Invariant i) linearity: the system satisfies the superposition property. ii) Time invariance: it means that, if an input signal is applied at the instant 0 or T second later the output will not be affected and will be identical except for the time delay.
2. Fourier analysis: The Fourier analysis is the decomposition of a signal as a sum of sinusoidal waves with different frequency. The Fourier analysis can be applied to continuous and discrete signals, both periodic and aperiodic. In the present work, the analyzed signals are periodic and discrete. Therefore, we will concentrate on it but it is useful to briefly introduce the continuous signals as well.
3. Multiphysics: Multiphysics has been used for the cross-section study. The geometry is bi-dimensional and the drawing of the geometry has been done with a CAD integrated with Multiphysics.
4. Solar Collector Model: The solar collector of the present case is an air solar collector. The idea is to integrate it in the south facade of the building and in the roof. The data should be collected are hourly direct solar radiation, hourly diffuse solar radiation, hourly outdoor temperature. The useful radiation is the direct solar radiation that strikes perpendicularly the surface of the collector. We assume that all the diffuse radiation is striking the surface of the collector.
Various finite element analysis is done and the test is conducted for various assumed example variables and air as working fluid the system can have good performances and can be used as a good source for a heat pump and also for a preheating system supplying directly part of the heat needed from the house.

• solar panel
• Multiphysics

• groundwater
• Linear time invariant
• Fourier analysis