The Azimuth Project
Thermal energy storage



To have page that lists and compares Thermal Energy Storage (TES) systems and processes. Wikipedia defines it as:

Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) that of the ambient environment. The applications today include the production of ice, chilled water, or eutectic solution at night, or hot water which is then used to cool / heat environments during the day.

Thermal energy is often accumulated from active solar collector or more often CHP Plants, and transferred to insulated repositories for use later in various applications, such as space heating, domestic or process water heating.


TES is a generic term for several different storage technologies and depending on the source it can be designed as underground TES (UTES) either in the for of Aquifier TES (ATES) or Borehole TES (BTES). Deep lake water cooling (DLWC) - utilizing the constant temperature of 4 degrees C in areas with cold winters or if its thermal energy from the sun we have Solar TES (STES or SES).

It also uses several different materials as phase change material (PCM). As examples of deployed or investigated materials are: water, inorganic PCM like salts and salt hydrates, organic PCM as paraffins, sugar alcohol and fatty acids. Also combinations of inorganic and organic materials have been investigated, see Paksoy et. al pp 261-265 for details. There are over 50 commercially available PCM’s today.

Deep lake water cooling

It has been deployed at Cornell University, USA in a a project called Lake Source Cooling that started in 2000 and has enabled cooling savings up to 80%.

Underground thermal energy storage

UTES can be deployed and typically its decided which technology to use depending on the geophysical conditions of the place. ATES uses aquifiers - water carrying layers below groundwater level:


For the BTES you need several parallel boreholes 50-200 m deep.

Environmental risks

Bonte et. al argues that there is a lack of knowledge how UTES interacts with ground water and what environmental dangers it has. They also propose pre-coordination between all interested parties - eg water companies and energy companies, and standardization.


ABSTRACT. We present an overview of the risks that underground thermal energy storage (UTES) can impose on the groundwater system, drinking water production, and the subsurface environment in general. We describe existing policy and licensing arrangements for UTES in the Netherlands, as well as the capability of the current and future Dutch policy and legal framework to minimize or mitigate risks from UTES on groundwater resources. A survey at the European Union member state level indicates that regulation and research on the potential impacts of UTES on groundwater resources and the subsurface environment often lag behind the technological development of and evergrowing demand for this renewable energy source.

The lack of a clear and scientifically underpinned risk management strategy implies that potentially unwanted risks might be taken at vulnerable locations such as near well fields used for drinking water production, whereas at other sites, the application of UTES is avoided without proper reasons. This means that the sustainability of UTES as a form of renewable energy is currently not fully understood, and the technology may be compromising the natural resilience of the subsurface environment. We recognize three main issues that should be addressed to secure sustainable application of UTES: Scientific research is required to further elucidate the impacts of UTES on groundwater; cross-sectoral subsurface planning is required to minimize negative conflicts between UTES and other subsurface interests; and EU-wide guidelines and standards are required for quality assurance and control when installing UTES systems.