The artificial ground freezing (AGF) is one of the most popular ground-support methods. Driven by its reliability, compatibility with a wide range of ground types, and low impact on the environment, the AGF became one of the most favorable geotechnical-support methods in various mining, civil, and environmental projects. Over the last few decades, there has been a growing interest in the AGF. 

The main purpose of AGF is twofold : (i) to prevent water seepage into underground occupied zones and (ii) to enhance and strengthen the mechanical structure of ground strata, thus increasing the stability of the working area. The main principle of any AGF is to circulate a sub-freeze coolant fluid in a network of freezing pipes to freeze the ground. As the coolant flows through the pipes, it extracts heat from the soil, and groundwater is transformed, gradually, into ice. Compared to other geotechnical support methods, the AGF has many comparative advantages: compatible with wide range of soil types; lower effect on the ground soil during and after freezing process; low impact on the environment; and more reliable method for high-risk applications. One of the main drawbacks, however, is that the system requires a continuous and intensive energy supply, ranging from several months in civil applications, to more than 20 years in underground mines. Hence, finding an effective and practical method to reduce the energy consumption of the AGF system is of great interest to engineers and specialist in this field. 

For instance, the Giant Mine in Yellowknife, NWT was one of the richest gold mines in Canadian history. 200 tonnes of gold was mined from the site between 1948 and 2004. The gold extraction over five decades has created a massive environmental burden; it produced 237,000 tonnes of a highly toxic by-product, a dust called arsenic trioxide. The dust, which contains up to 60% arsenic, is highly lethal even in small doses; it is odorless, tasteless, and water-soluble. The 2,300 acres footprint site holds, in underground chambers, one of the largest amounts of stored arsenic trioxide dust in the world. Besides, the mine is located next to the Great Slave Lake, one of the largest freshwater bodies in Canada. Therefore, the Giant Mine Remediation plan was initiated as a blueprint to contain the arsenic trioxide. Exploiting the impenetrable nature of ice and the strength of frozen ground, the Giant Mine Remediation team decided to implement the frozen block method (FBM) to seal-off the toxic waste, by employing the AGF. The team declared that the FBM is the safest approach to isolate the waste from the surrounding environment. The associated AGF system, however, has to operate continuously to isolate the arsenic chambers by maintaining a sufficient thickness of the frozen shell. This operation mode requires intensive energy input, which leads to immense operational and maintenance costs – not to mention a large carbon footprint! As reported by the Indigenous and Northern Affairs Canada (INAC), the Giant Mine Remediation plan is expected to cost about a billion dollars, with an extra two million dollars per year to maintain the system forever. These expenses could put great pressure on the available resources and arouse cost-effective concerns regarding the AGF systems, especially for such a long-term project. Therefore, new reliable, cost-effective AGF operational techniques are critical to the future development of natural resources in the Giant Mine remediation, which, in turn, support the government of Canada’s long-term plan for sustainable mineral extraction and environmental protection. 

This project aims to develop, demonstrate, quantify, and optimize a novel concept of a sustainable AGF system comprising a closed thermosyphon and a phase-change material (PCM) that serves as a "cold" energy storage. The fundamental notion behind this idea is to creates a unique secondary evaporator and condenser cycle inside thermosyphon which has never been explored before. In winter, a pressurized carbon dioxide with a sub-zero temperature extracts the heat from the ground, at the evaporator, and diffuses it to the cold ambient, at the condenser. The novel and unique feature is the involvement of the PCM storage, which will create secondary evaporation-condensation loops. The PCM area acts as a secondary evaporator, where the working fluid cools down and freezes the PCM in order to store the "cold" energy, as illustrated in the schematic diagram. In summer, on the other contrary, the ambient temperature could rise beyond certain limits where the temperature gradient across the condenser is not large enough to condensate the vaporized fluid. In such scenarios, an automated control valve above the PCM cold storage area should close, forcing the working fluid to disperse its energy into the PCM.