## 3 Power Consumption, Heat Fluxes, and Cooling in Data Centers
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{6}{c}{**Distribution of Heat and Temperatures**} & \multicolumn{4}{c}{**Power Loads**} \\ \hline  & **Component** & **Heat Share** & **Temperature** & & **Component** & **Heat Share** \\  & Microprocessor & 30\% & 85 \({}^{\circ}\)C & & Microprocessors & 63\% \\  & DC/DC & & & DC/DC & & 13\% \\  & conversion & & & conversion & & \\  & I/O processor & 3\% & 40 \({}^{\circ}\)C & **High** & I/O processor & 10\% \\
**Standard Server** & AC/DC & & & & **performance** & \\  & conversion & & & **cluster** & Memory chips & 14\% \\  & Memory chips & 11\% & 70 \({}^{\circ}\)C & & & \\  & Fans & 9\% & 30 \({}^{\circ}\)C & & & \\  & Disk drives & 6\% & 45 \({}^{\circ}\)C & & & \\  & Motherboard & 3\% & 40 \({}^{\circ}\)C & & & \\  & Other & 3\% & - & & & \\ \hline  & \multicolumn{6}{c}{**Heat sources and stream**} \\ \hline
**Cooling** & **Parameter** & **Value** & **Cooling** & **Parameter** & **Value** \\ \hline  & Cold aisle (CRAC supply) temperature & 10–32 \({}^{\circ}\)C & & Water supply to & 60–75 \({}^{\circ}\)C \\  & Hot aisle (CRAC return) temperature & 50–65 \({}^{\circ}\)C & & Water rise from & 2–5 \({}^{\circ}\)C \\  & Temperature rise over servers & 10–20 \({}^{\circ}\)C & & Flow rate per rack & 840–1680 kg/h \\
**Air cooling** & Air flow rate per rack & 321–4013 \({}^{\circ}\)N\({}^{3}\)/h & **Water Cooling** & \(\Delta\)T from water & 5–18 \({}^{\circ}\)C \\  & Chiller water supply to CRAC & 7–10 \({}^{\circ}\)C & & Weaker heat & 840–1680 kg/h \\  & & & & flow rate & \\  & Chilled water return from CRAC & 35 \({}^{\circ}\)C & & exchanger supply & 3–5 \({}^{\circ}\)C above & ambient \\ \hline  & Coolant at evaporator & 60–75 \({}^{\circ}\)C & & Coolant supply to & 60–75 \({}^{\circ}\)C \\  & Coolant at condenser & 90 \({}^{\circ}\)C & & & evaporator & 75–80 \({}^{\circ}\)C \\
**Two-phase** & Coolant at condenser & **Two-phase** & **cooling with** & Condenser & & \\
**cooling with vapor compressor** & Condenser cooling fluid inlet & 30 \({}^{\circ}\)C & & **liquid pump** & cooling fluid inlet & 30 \({}^{\circ}\)C \\  & Condenser cooling fluid outlet & 90 \({}^{\circ}\)C & & & Condenser & \\  & & & & cooling fluid outlet & 45–90 \({}^{\circ}\)C \\ \hline  & \multicolumn{6}{c}{**Summary data**} \\  & Heat recovery & 70\% (max) & & & & \\  & Power consumption range & 8–15 kW/m\({}^{2}\) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data center power characteristics and cooling systems [12; 31; 37]. racks, as depicted in Figure 1. The cooling strategy is tailored to specific rows or directly to individual racks. The air distribution in these systems is typically 63% within the room, 23% between aisles, 12% along the rack rows, and 2% targeted at individual racks [42]. For high heat flux dissipation, around 100 W/cm\({}^{2}\), the heat removal capacity of forced air systems is estimated to be about 37 W/cm\({}^{2}\). This request necessitates more efficient cooling systems like liquid-based or two-phase cooling [43]. Efficient air cooling systems typically supply cold air at 25 \({}^{\circ}\)C, with the exhaust air returning at 40-45 \({}^{\circ}\)C to the computer room air conditioning (CRAC) unit [43; 44; 45]. In these CRAC apparatuses, the heat from the hot air is typically expelled outdoors through a chiller and cooling tower loop. Water cooling has proven more effective. An in-depth study of water-cooled high-density servers underscored the benefits of this approach [46]. For instance, in an analysis involving a 150 W dual-core chip and an 8 W memory chip, a water flow rate of 0.95 L/min maintained the chip temperature at 65 \({}^{\circ}\)C, with the maximum temperature of the incoming water being 28 \({}^{\circ}\)C [47]. On average, waste heat temperatures range between 60 \({}^{\circ}\)C and 70 \({}^{\circ}\)C [43]. Two-phase cooling has been recognized for its superior performance, removing heat fluxes ranging from 0.8 kW/cm\({}^{2}\) to 27 kW/cm\({}^{2}\), exhibiting waste heat temperatures of about 70-80 \({}^{\circ}\)C [12; 43].