**3.3 Digital boiler-design**

In **Figure 3**, a modular heat recovery system by the name of Digital Boiler has been developed for hot water heating. This modular can be extended or combined to be an array providing higher heat capacity. This modular digital boiler includes four major kits, i.e. miner liquid cooling chassis, water tank kit, water replenishing kit, and dry cooler. The thermal energy generated from the miner including three hash boards (WhatsMiner M30S) will heat the dielectric coolant (7.3 L coolant charged) in the enclosure and then be suctioned into the pump (a DC permanent magnet pump). The heated dielectric coolant out of the pump will flow into the line connected with the

#### **Figure 2.**

*Schematic diagram of coolant spraying design. (a) Stack of hash board, (b) singular hash board, and (c) plain view of miner.*

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

#### **Figure 3.**

*Typical configurations of a digital boiler. 1 mining liquid cooling chassis, 101 miner, 102enclosure, 103 pump, 104 Filter,105 sprayer, 2 water tank kit,201 Sprial coil,202 relief valve,203 air purger, 204 liquid level controller, 205 supply water line,3 water replenishing kit, 4 dry cooler.*

spiral coil in the water tank (a 190 L insulated tank). The internal volume of the water tank would be filled with fresh water from the water replenishing kit. Hot water in the water tank will be heated by coil and pushed out to the supply water line. In the supply water line, the air purger will discharge the air released from water into the ambient. The thermostat can maintain the water temperature in the water tank by controlling the bypass flow through the dry cooler partially or fully in response to the load requests. Thermocouples connected with the datalogger have been put at the positions shown in **Figure 3** for testing purposes. **Figure 4** indicates the main components and their installation of a prototype. **Figure 5** is the image of a real digital boiler, which has the exact compatible geometrical size and connections with the current electric/gas water heater. **Table 1** shows the main parameters of Digital Boiler.

#### **4. Performance of digital boiler**

As a key parameter of heat recovery, the max. Water temperature has been studied under the steady, reliable, and continuous operation of miners. **Figure 6** presents the variation of power, average temperature of hash boards and frequency of hash board with different spraying coolant temperature. It can be found in **Figure 6** that under a certain spraying flowrate, the miner power consumption, hash rate, and frequency have a very limited rising with the increase of dielectric coolant temperature (T4 in **Figure 3**). It implies that the miner can deliver a constant hashing rate in parallel with

#### **Figure 4.** *Drawing of digital boiler.*

stable thermal output. The average temperature of hash boards read from Whatsminer-M3x&M5x firmware has a linear trend with the dielectric coolant temperature. At the 45°C spraying dielectric coolant temperature (T5 in **Figure 3**), the temperature differential between the hash board and dielectric coolant is 10.17°C; At the max. Spraying dielectric coolant temperature of 65°C, the temperature differential between the hash board and the coolant is 8.50°C. Lower temperature differential at higher dielectric coolant temperature can be attributed to the viscosity reduction under higher spraying dielectric coolant temperature, which is a positive factor to elevate the dielectric coolant temperature further. To avoid triggering the auto frequency throttling mechanism for internal temperature protection [29], spraying coolant temperature is set as 65°C with safety tolerance for this miner without sacrificing its reliability.

**Figure 7** reveals the trend of spray coolant temperature (T5 in **Figure 3**) and hash board temperature as the function of coolant flowrate. The temperature differential between spraying coolant and hash board will reduce with the increase of coolant

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

**Figure 5.** *Real image, and sections of digital boiler.*


#### **Table 1.**

*Digital boil performance as a residential water heater.*

flow. For example, the temperature differential between spraying coolant and the hash board no.2 is 10°C at the flow rate of 25 L/min and decreased to 6°C at the flow rate of 45 L/min. It can be attributed to the compound effects of viscosity reduction under higher temperatures and the flush velocity increase with more coolant. This trend implies a possible approach to elevate the coolant temperature further. In addition, it can be found that with the increased coolant flow to 45 L/min, max. 70°C

**Figure 6.**

*Variation of power, average temperature of hash boards and frequency of hash board with different spraying coolant temperature.*

**Figure 7.**

*Variation of hash boards' temperature with different spraying coolant flow rate.*

coolant temperature can be achieved with a hash board temperature under 80°C due to the minimized temperature differential by enhanced spraying momentum. This high coolant temperature can not only provide high exergy output for high-grade heat recovery but also provide the capability to kill the legionella in the water system within minutes. To minimize the legionellosis risk for building water systems has been defined compulsorily by ASHARE and CDC as a national code [30–33]. Reviewing the maximum temperature achieved in previously published single-phase dielectric liquid cooling solutions [7–9, 34, 35], 50°C can be considered a record that cannot meet the primary safety requirements of a building water system.

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

Reviewing the evolution of district energy technologies [36], the operation temperature categorized by International Renewable Energy Agency is dropping from approx. 200°C of First-generation district heating which was based on the steam system, and transferred by steam pipes in concreted ducts during Y1880–Y1930, to the range of 50–70°C of Fourth generation district heating which is based on the smart energy during Y2020–Y2050, including an optimum interaction system of sustainable energy sources, intelligent distribution system, two-way energy reservoir, and end consumption. From Marco and quantitative perspective, the renewable share in global district heat will be increased from 8% (30.64EJ) in Y2017 to 77% (270.27EJ) in Y2050, renewable share in electricity will be increased from 25% (95.75EJ) in Y2017 to 86%(301.86EJ) in Y2050 respectively. Refer to global trends in internet traffic, data center workloads, and data center energy use [37, 38], the global energy consumption by the data center in Y2030 will be 11.52EJ, which could provide 9.4% district heat load to Forth generation district heating directly as renewable energy, if the medium temperature reclaimed from datacenter into district heating network could catch up the range of 50–70°C. Regarding the transmission network losses, output medium temperature from the data center higher than 60°C can be considered as the bottom line to ensure the seamless integration with the Fourth general district heating network. The infrastructure of district heating can be merged with that of the data center as an integrated energy complex. From the micro perspective, the heat pump is one of the most energy-efficient and environment-friendly options to boost the low-grade thermal energy from the data center, COP of the heat pump booster under temperature lift 45°C can exceed 5.6 with some low GWP synthetic refrigerants, i.e. R1234ze and R1234zd. Their optimal working range to achieve high COP is 55–65°C evaporation temperature [39]. ORC is a thermal-electrical recovery for low-grade waste heat. The higher the water temperature is, the higher the cycle efficiency can be achieved. From either macro or micro perspectives, elevating the outlet temperature from the data center is a key to not only determining the system efficiency technically but also impacting the capital investment and revenue return economically. 60°C as the medium outlet temperature of data center heat recovery can be considered the reference temperature technically and financially.

### **5. Exergy efficiency analysis**

The exergy [40] refers to the availability or quality of a thermodynamic system to a specified reference and is related to the first and second laws of thermodynamics. The availability of a thermal system is zero when in balance with the reference conditions. The physical exergy definition is given in Eq. (4) where *hi* and *ho* refer to the specific enthalpies and *si* and *s*<sup>0</sup> refer to the specific entropies of *i th* point and dead state conditions and *T*<sup>0</sup> is the dead state or ambient temperature. The exergy factor of mechanical energy and electrical energy is 1.0 [41].

$$e\mathbf{x}\_i = h\_i - h\_0 - T\_0 \times (\mathbf{s}\_i - \mathbf{s}\_0) \tag{4}$$

The curve in **Figure 8** is the specific exergy of hot water under the condition of 1 bar pressure and variable temperature. The specific exergy at 50°C hot water (Point 1) is 4.19 kJ/kg, which is half of the specific exergy at 61°C (Point 2). It means the exergy recovered in this design is twice as much as the exergy in the current commercial liquid immersion system. The slope of this curve indicates the increasing ratio

**Figure 8.** *Exergy in hot water under different temperatures @ 1 atm.*

of exergy at the higher temperature is much larger than that at the lower temperature. This characteristic reveals the essence to pursue high-grade heat recovery.

Power usage effectiveness (PUE), a concept based on the first law of thermodynamics, is a ratio that describes how efficiently a computer data center uses energy; Specifically, how much energy is used by the computing equipment in contrast to cooling and other overhead that supports the equipment, which was published in 2016 as a global standard under ISO/IEC 30134-2:2016. An ideal PUE is 1.0, given that Non-IT Facility Energy is zero, refer to Eq. (5). Anything that is not considered a computing device in a data center (e.g., lighting, cooling, etc.) falls into the category of facility energy consumption.

$$PUE\_{en} = \frac{\text{TotalFacidityEnergy}}{\text{ITEupimentEnergy}} = 1 + \frac{\text{NonITFacidityEnergy}}{\text{ITEupimentEnergy}} \tag{5}$$

Since Total facility energy and IT equipment Energy are supplied in the form of electrical energy, the exergy factor of electrical energy is 1. Eq. (5) can be recast as follows:

$$PUE\_{\text{ex}} = \frac{\text{TotalFacidityExp}}{\text{ITEupimentExergy}} = 1 + \frac{\text{NonITFacidityExp}}{\text{ITEquiredExergy}} \tag{6}$$

Regarding the exergy in the reclaimed thermal energy, the Non-IT Facility Exergy can be offset partially. The definition of *PUEex* in Eq. (6) involving the heat recovery will be

$$\begin{split} \text{PUE}\_{\text{cx}} &= \frac{\text{TotalFacidity} - \text{Exerginreclained} \text{heat}}{\text{ITE equipmentExp} \text{y}} \\ &= 1 + \frac{\text{NonITFacidity} - \text{Exerginreclained} \text{heat}}{\text{ITE equipmentExp} \text{y}} \end{split} \tag{7}$$

The exergy efficiency of the heat reclaim system will be defined as

$$E\_{re} = \frac{\text{Exergyinreclainede} \text{heat}}{\text{Total[faciticy: } \text{energy}]} \tag{8}$$

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


#### **Table 2.**

*Exergy calculation of heat reclaim.*

**Table 2** lists the testing results and derivative calculation of this prototype. The miner consumes 3.3 kW electrical energy at its nominal hash rate. The energy consumption of the circulation pump is 100 W. PUE refer to Eq. (5) would be 1.03. Total input exergy from electrical energy within 1 hour is 12225.6 kJ. Regarding the heat recovered from the miner, the specific exergy in the hot water at 63.23°C@1 atm is 9.62 kJ/kg obtained in **Figure 8**. The total reclaimed exergy at the hot water supply flow of 103.5 kg/h during its first hour of running. Based on Eqs. (6) and (8), *PUEex* and *Ere* will be 0.95 and 8.16% respectively. The conventional *PUEen* is 1.03, in contrast*PUEex* is less than one with consideration of reclaimed exergy. As far as the rate of energy reuse is concerned, *PUEex* can be considered as a better index than *PUEen* due to its characteristic energy-quality measures.

## **6. Conclusions**

A water heater with the heating element as a bitcoin miner has been designed and built, in which the single-phase dielectric coolant has been sprayed on the hash boards and coupled heat sinks of the miner in steady conventional submerge. By avail of the spray momentum, the resultant heat transferring has been enhanced and validated by increased *Nu* in theory and up to 70°C outlet coolant temperature in the test. The temperature differential between coolant and hash board is controlled as lower as 10°C, by which the energy grade to be recovered from either centralized or decentralized mining operations has been elevated largely. From the perspective of the first law of thermodynamics, the liquid spray cooling circulation can extract the heat from miners and transfer the energy to the exterior with minimal losses. From the perspective of the second law of thermodynamics, the quality of thermal energy reclaimed in the miners, as the monotonically increasing function of the output medium temperature, has been elevated to the extent that the district heating system can be integrated seamlessly with datacentre cooling system without extra infrastructure investments, and critical hygienic codes have also been fulfilled completely. PUE, a metric used to determine the energy efficiency of a data center, has been reconsidered and redefined in the exergy flow analysis, rather than the energy flow previously. From the testing results of the prototype, *PUEen* is 1.03 and *PUEex* is 0.95. The discrepancy between them tells the influence of reclaimed useful energy, which must be considered with the prevalence of energy-saving thinking in the data center industry.

This study is the initial part of the synthesis energy system integrating digital energy (energy reclaimed from extensive digital industry), fossil energy, and sustainable energy. Regarding the stable and unidirectional output characteristics of 7x24h operation in the datacentre, the reclaimed thermal energy can work as the basic component of the district heating load, which can ease the tensions in the network caused by the fluctuating sustainable energy like wind, and solar energy. With the

quickly developing electronic industry, the upper-temperature limits of chips tend to rise continuously. The electrical power generator based on ORC would be a good solution to realize the electrical-thermal-electrical close loop in the data center. The advantage of this thermal-electrical transition is the saving of access pipework to the current heating system. However, this thermal-electrical recovery is more applicable to large-scale scenarios. Considering the high start temperature of ORC, more studies are required to suit the 60–70°C heat sources.
