**2. Objectives**

Doubtless to say, the implementation of waste heat recovery measures in the crypto mining industry to building or district heating, absorption cooling, Rankine Cycle, Organic Rankine Cycle (ORC), biomass co-location, and desalination for clean water production, can tremendously improve financial performance and reduce CO2 emissions. Nevertheless, the main impediment to introducing this technology in the data center is the low quality of the heat produced from electronic devices, despite the large quantity. The quality of heat is bound by the upper-temperature limits of electronics, which in most cases remain below 85°C. Air cooling is limited by the thermal properties of air which require co-located siting, making the retrofit into the existing data center impossible. The temperature difference across the inlet and outlet of the hybrid air portion is low due to the water loop removing most of the heat, thus the air cooler portion can operate at a low capacity and the water portion should be targeted for heat recovery. Two-phase immersion cooling is restricted by its characteristic of the saturated temperature-pressure curve which must be realized by a complicated evaporation-recondensation structure. As far as the grade/quality of the heat recovered by the above three approaches is concerned, air cooling typically results in outlet temperatures of 25 to 35°C, while outlet temperatures of up to 60°C are possible with single-phase immersion and hybrid liquid cooling [16]. The max. Temperature of two-phase immersion cooling relies on the selection of two-phase

coolant and max. Case temperature of chips. 3 M Novec 7100 with 61°C boiling point @ 1 atm is the most common working fluid in two-phase open bath immersion cooling. Even though Marcinichen [17] reports that high temperatures as 60–70°C for liquid-cooled systems and 70–80°C for two-phase cooling systems provide higher waste heat quality and open up a wide range of waste heat reuse opportunities. These temperatures were achieved in the complex direct on-chip evaporator by HFC134a or HFO1234ze under a working pressure of approx. 15 bar. Kuncoro [18] reports that the temperature differential between chips and coolant can be reduced up to 91.3% by the replacement of air cooling with single-phase oil cooling. It not only reveals the reason liquid cooling is a better option to harvest the high-grade thermal energy but also points out the direction to improve it continuously.

In this article, the prototype of a digital boiler has been designed and built that is helping reduce greenhouse gas emissions by repurposing the heat produced by their ASICs. The miner is cooled by spraying coolant, with intend to enhance heat transferring and consequently elevated coolant outlet temperature for high-grade heat recovery. The concept of PUE based on energy has been redefined accordingly regarding the exergy in the reclaimed heat, catering to the demands of reasonable performance evaluation of data center with thermal energy reuse. Experimental testing validated the exergy efficiency of this innovative design.
