Aquifer thermal energy storage (ATES) is a form of underground thermal energy storage technology to can store intermittent wind and solar power for seasonal thermal storage and provide heating and cooling for district heating-cooling or large buildings.
Doubts about its technical and economic feasibility, however, still prevent this storage technology from reaching commercial maturity, although several pilot projects are currently being developed worldwide.
One of these is the ATES system that Mitsubishi Heavy Industries Thermal Systems, Ltd., a unit of Japanese conglomerate Mitsubishi Heavy Industries (MHI), has begun testing this week together with the Osaka Metropolitan University (OMU) at the Osaka City Maishima Sports Center for the Disabled.
The proposed ATES system is intended to store renewable energy and produce cooling for air conditioning in the summer or heating for the winter. It utilizes, for the first time worldwide according to Mitsubishi, an energy absorption and release technology that allows short-cycle thermal storage and discharge functionality, enabling flexible switching between cold and heat storage.
In ATES systems, renewable electricity from wind and solar is used to pump water up from underground reservoirs and heat or cool this water at the surface during the summer or winter, respectively. Mitsubishi utilized, in particular, gravel and groundwater stored in aquifers deep underground as a heat storage tank. Heat or cooling can be pumped out with a small amount of stored renewable electricity, according to the company.
“The newly developed control system features a mode that automatically switches to the optimal operation depending on the surplus energy availability and underground water temperature,” the company said in a statement. “Furthermore, owing to the incorporation of a function that optimizes cold storage operation in real time based on fluctuations in electricity market prices and the volume of surplus power. This enables energy-saving operation that reduces electricity consumption during cooling while ensuring effective use of renewable energy.”
The demonstration test began yesterday to directly use 10,000 m3 of groundwater stored at 5 C in a cooling well. This was made possible by a multi-layered seasonal storage function that allows lower-temperature chilled water to be stored within existing underground cold-water masses.
The OMU explained that the system is expected to achieve a charge/discharge efficiency target of 70% when the upper limit of pumped water temperature used for direct cooling is 13 C. “This system adds two new functions to the conventional ATES: storing surplus renewable energy electricity in cold wells at 5 C using a heat pump heat source based on real-time information on the amount of electricity; and directly cooling the electricity using groundwater stored at 5 C,” it further explained.
MHI Thermal Systems is responsible for operating the system and its heat-pump type centrifugal chillers. It is also currently designing a control system that efficiently utilizes surplus energy.
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