**1. Introduction**

Bitcoin, the world's largest cryptocurrency, currently consumes an estimated 150 terawatt-hours of electricity annually-more than the entire country of Argentina, with a population of 45 million [1]. Acknowledging that Bitcoin mining is a high energy consuming and high energy density industry, correspondingly, miners generated large quantities of heat during the continuous hashing process, which conventionally has been dissipated into the ambient by air cooling circulation. Regarding the financial and environmental benefits from the heat reuse in mining industry, competitive entities are explored an economical approach to elevate the revenue performance and cut the carbon footprint at initial stage of project plan and make effort to maximize these benefits further by advanced heat exchanging technologies, which can partially

or completely replace the traditional fossil fuels in the field of low temperature heating. Due to the resilience of spatial arrangement, miners can be deployed in the pattern of centralized and decentralized models. Centralized mining farms are more suitable to heat the green house, vertical farm, district heating network, i.e., work as a heat resource in a large scale. On the contrast, decentralized mining utilities can provide the space heating or hot water in the small-scale projects, such as in residential or light commercial deployment.

Heat recovery is a low-carbon technology that cuts across multiple sectors of the technologies. From the perspective of the first law of thermodynamics, the primary challenge is to identify effective ways to capture and transfer heat, along with sound economic use cases. From the perspective of the second law of thermodynamics, the second challenge is to elevate the grade of heat expelled by miners to extend the scope of waste heat application and provide feasibility to seamless connection with the current heat system driven by electricity or fossil fuels. Regarding the approaches of capture and transferring heat from miners, current technologies can be categorized into three types, i.e. air cooling, liquid immersion cooling (single phase and two phases), and hybrid (air/liquid) cooling [2]. Air cooling utilizes fans to pass air over the miner heat sinks. Hampus [3] reports that 5.5–30.5% of the electrical input to a 1 MW air-cooled mining farm could be recovered, which can fill the heating demand of a 2000m<sup>2</sup> greenhouse by 89.7–97.9%, and a 10,000 m<sup>2</sup> greenhouse by 50.0–61.5% respectively. Up to 94.5% of thermal energy has been wasted due to the poor thermal properties of air in the process of thermal transmission. Enachescu [4] pointed out that waste heat from a 45 MW data centre is sufficient to provide a year-round heating to a 8.34-acre greenhouse for commercial cannabis growth. The total annual avoided emissions were calculated at 70,000 tonnes of CO2 by economizer cycles and waste heat reuse in Alberta, Canada. Agrodome [5] is the first retail facility that uses servers as heat sources distributing residual heat through a cascading set of greenhouse applications. Blockchain Dome, each dome has an input of 1.5 MW and produces 5,000,000 BTU/h of heated air, requires no additional electricity to maintain the preferred temperature range. In July of 2018, United American Corp. [6] announced their intention to deploy 25 Blockchain Domes across Quebec, filing a power license request at 5 MW at the large power preferential rate. Data center heat reuse colocation with an associated greenhouse minimizes losses to electricity distribution, heat transportation, and associated heat loss, which is a primary scenario for heat recovery from the air-cooled data center.

Immersion cooling is an IT cooling practice by which IT components and other electronics, including complete servers and storage devices, are submerged in a thermally conductive but electrically insulating dielectric liquid or coolant (single-phase or two-phase). Heat is removed from the system by circulating relatively cold liquid into direct contact with hot components, then circulating the now heated liquid through cool heat exchangers. The advantages of using liquid cooling over air cooling include liquid's higher specific heat capacity, density, and thermal conductivity. This allows liquid to transmit heat over greater distances with much less volumetric flow and reduced temperature difference. Regarding the higher coolant temperature, inlet water temperatures [7] in the order of 45–70°C can cool server rack chips and CPUs to approximately 80–90°C to avoid triggering the auto frequency throttling protection, while the maximum permissible temperature range of processors is 100–120°C. According to [8], the inlet temperatures can range between 30 and 60°C. The solution presented by Ernest Orlando Lawrence Berkeley National Laboratory [9] has an inlet temperature between 15 and 45°C, whereas the case study presented in [10] has an

#### *Heat Recovery from Cryptocurrency Mining by Liquid Cooling Technology DOI: http://dx.doi.org/10.5772/intechopen.107114*

inlet temperature of 50°C. Other advantages are the considerable fan energy saving and a lower noise level. The main drawback of liquid-cooled systems is the introduction of liquid within the data center and the potential damage that a failure can cause. In 2022, Bitcoin mining company Mint Green to deliver an innovative low-carbon mining waste solution to heat the City of North Vancouver, BC [11]. Production of both bitcoin and usable thermal energy positions the Digital Boilers to be the costleading low-carbon heating technology. In MintGreen design, the hash boards have been placed radially in a bell month-like chamber, submerged fully in the single-phase dielectric fluid. The cold coolant will flow in the chamber along the central line, be heated by the radial located hash boards, and then be collected circumferentially. Wisemining's digital boiler designed for residential mining and heating [12] includes a tank container design for ASIC and a 200-liter water tank with two heat exchangers. This product was designed based on two-phase immersion cooling concepts. The drawbacks of two-phase liquid are the coolant is expensive and highly leakable; The condensation of two-phase fluid in the inner tube driven by thermosiphon has a comparatively lower efficiency than that driven by forced convective heat transfer; The position of the miner must be lower than the coil in the water tank, which needs more footprint of this product.

Hybrid liquid-cooled systems are defined by the integration of the direct-to-chip liquid cooling of some high heat density components such as CPUs and DIMMs by microchannel flow [8, 13] or cold-plate heat exchangers [9, 14], with the air cooling of the rest of low heat density components. A 15 kW IBM rack as a typical example of a hybrid liquid cooled system has been tested [3, 15] under different ambient temperatures. This chiller-less rack can work under inlet coolant temperature of up to 45°C. In the summer season, its IT load of 13.16 kW can be maintained under 0.442 kW cooling power consumption. The average cooling power for a full year could be expected to be below 3.5%.
