Ground source heat pumping is an energy efficient technology for space heating, space cooling, and water heating. But a large-scale market introduction, at least in the United States, is hindered by high installation costs of the connected heat exchanger. And the cost-reduction efforts tested so far have restricted potential due to the sizing of the borehole.

Recently, a new type of ground heat exchanger was developed, which could change this. It is an underground thermal battery, showing promising results in lab tests. According to them, this type of heat exchanger could achieve the same performance as the conventional type, to a cost that is lowered with 39% according to preliminary cost analyses.

The underground thermal battery could have a positive effect also on the stability of the electric grid. With an increased part of the power supply coming from renewable sources, it becomes difficult to steer. Thus, there will most likely be discrepancies between supply and demand, something that is currently challenging for the grid. But as a large part of the electricity demand in households in the United States is used for heating, cooling and domestic hot water, the renewable electricity could be stored in a thermal battery when the supply is high, to be used when the supply is low and demand is high. In this way, it provides a new solution for both efficient air-conditioning and active demand side management without sacrificing the desired comfort condition and convenience in the building.

A prototype of an underground thermal battery has been built. It is a tank filled with water, buried in the shallow subsurface of the ground, with a phase-change material suspended in the water, increasing thermal storage capacity. In a conventional heat exchanger for ground source heat pumps, heat is transferred through conduction between the heat exchanger and the surroundings. In comparison, the thermal battery design leads to heat transfer through convection within the water tank. The result is an almost homogenous water temperature throughout the tank.

Another prototype enables both thermal energy storage and ground heat exchange. This has a smaller tank within the original water tank, wrapped with a blanket filled with phase-change material for energy storage. There is one heat exchanger in each tank. Cold water or ice can be stored in the inner tank, providing direct cooling during electricity peak hours. Such integration of thermal energy storage and ground heat exchanger into one device makes it less costly than using multiple individual components for the same purpose.

A prototype of an underground thermal battery has been built. It is a tank filled with water, buried in the shallow subsurface of the ground, with a phase-change material suspended in the water, increasing thermal storage capacity. In a conventional heat exchanger for ground source heat pumps, heat is transferred through conduction between the heat exchanger and the surroundings. In comparison, the thermal battery design leads to heat transfer through convection within the water tank. The result is an almost homogenous water temperature throughout the tank.

Another prototype enables both thermal energy storage and ground heat exchange. This has a smaller tank within the original water tank, wrapped with a blanket filled with phase-change material for energy storage. There is one heat exchanger in each tank. Cold water or ice can be stored in the inner tank, providing direct cooling during electricity peak hours. Such integration of thermal energy storage and ground heat exchanger into one device makes it less costly than using multiple individual components for the same purpose.

Xiaobing Liu, Mingkan Zhang and Kaushik Biswas, USA (Oak Ridge National Laboratory)
Joseph Warner, USA (University of Tennessee)
Ming Qu and Liang Shi, USA (Purdue University)


The text has been shortened by the HPC team


Read the full article here.

Read the full text in the HPT Magazine