Impact of Fan Airflow of IT Equipment on Thermal Environment and Energy Consumption of a Data Center

Energy-saving in regard to heating, ventilation, and air-conditioning (HVAC) in data centers is strongly required. Therefore, to improve the operating efficiency of the cooling equipment and extend the usage time of the economizer used for cooling information-technology equipment (ITE) in a data center, it is often the case that a high air-supply temperature within the range in which the ITE can be sufficiently cooled is selected. In the meantime, it is known that when the ambient temperature of the ITE rises, the speed of the built-in cooling fan increases. Acceleration of the built-in fan is thought to affect the cooling performance and energy consumption of the data center. Therefore, a method for predicting the temperature of a data center—which simply correlates supply-air temperature with ITE inlet temperature by utilizing existing indicators, such as air-segregation efficiency (ASE)—is proposed in this study. Moreover, a method for optimizing the total energy consumption of a data center is proposed. According to the prediction results obtained under the assumption of certain computer-room air-conditioning (CRAC) conditions, by lowering the ITE inlet temperature from 27 ◦C to 18 ◦C, the total energy consumption of the machine room is reduced by about 10%.

learning [21] are necessary. However, with these solutions, it takes a huge effort to consider changes in recirculation flow due to differences in the availability of flow. Therefore, in the present study, we propose a method for predicting the recirculation flow in an arbitrary machine room by simple numerical calculation by utilizing existing indicators, such as air-segregation efficiency (ASE) proposed by Tozer et al. [18]. As the proposed prediction method can simply correlate supply-air temperature with ITE inlet temperature, by incorporating the characteristics of the ITE and HVAC, the optimum points of the total power consumptions of the ITE and HVAC with respect to supply-air temperature or ITE inlet temperature can be estimated. First, the characteristics of the ITE with variable built-in-fan speed are clarified. Next, the characteristics of the ITE (determined by using a heat generator that can set airflow rate) are reproduced, and whether the thermal environment of the machine room can be predicted by the proposed prediction method is confirmed experimentally. The characteristics of the ITE (which have been determined) are reproduced by using a heat generator that can set the airflow rate, and an experiment is conducted to confirm whether the thermal environment of the machine room can be predicted by the prediction method. Finally, a method for finding the optimum point of total energy consumption of the machine room-by using the clarified ITE characteristics and the power-consumption characteristics of the HVAC (assumed to be computer-room air-conditioning, CRAC)-is proposed.

Overview of Airflow Performance
To correlate supply-air temperature with ITE inlet temperature, the metrics for the air performance of data centers and air-segregation efficiency (ASE) proposed by Tozer et al. [18] is further developed in this study. As for these metrics, by using mass-flow balance equations and weighted average temperatures of the air supply and return air of the air conditioner in the machine room, as well as intake and exhaust temperatures of the ITE, it is possible to obtain the average bypass airflow and recirculated airflow, and the efficiencies of air supply and demand can be calculated.
The air-flow model assumed for these metrics are explained as follows. The supply airflow of the CRAC (m c ) merges with the negative-pressure flow (NP) (airflow attracted from the hot aisle into the raised floor of the machine room by the Venturi effect) in the raised floor. The airflow after this merging is divided into the airflow (m f ) that contributes to cooling the ITE and the bypass airflow (BP) that does not. The amount of cooling air (m f ) that reaches the ITE is sucked into the ITE as the ITE airflow rate (mi) together with the circulation flow (R) of the ITE exhaust gas. Tozer et al. excluded NP flow from the evaluation because it does not exist in all data centers and is difficult to measure; however, according to the air-performance metrics, NP is included in R [18].
Bypass flow (BP) is the air supply that returns to the air conditioner without contributing to the cooling of the ITE. It commonly occurs in the gaps between hot-aisle raised-floor opening panels and double-floor fixed panels. It is also caused by the oversupply of cold air to the cold aisle. Recirculation flow (R) is air that returns the exhaust air of the ITE to the air-supply side of the ITE before it is cooled by the air conditioner; as a result, the inlet temperature of the ITE increases. Recirculation flow occurs in the machine room (e.g., in the racks) owing to the local lack of air supply. BP and R are defined as follows: where T ci is the CRAC inlet (return) air temperature, T co is the CRAC outlet (supply) air temperature, T ii is the ITE inlet-air temperature, and T io is the ITE outlet-air temperature. Bypass flow (BP) and recirculation flow (R) affect the amount of cold air reaching the ITE. "Air performance" can therefore be defined by the performance of the supply and demand air streams.
Energies 2020, 13, 6166 4 of 27 "Supply performance" (η supply ) is the ratio of the amount of cooling air (m f ) that reaches the ITE to the amount of air-conditioned air supply (m c ). If η supply equals 1, it is taken that no bypass flow exists in the room. "Demand performance" (η demand ) is the ratio of the amount of cooling air (m f ) that reaches the ITE to the ITE airflow (m i ). If η demand equals 1, it is taken that recirculation does not occur in the machine room. η supply and η demand are defined as follows: "Availability of flow" (A f ) is the ratio of the CRAC air supply (m c ) to airflow required by the ITE (m i ). A f greater than 1 indicates an oversupply of cold air, and A f less than 1 indicates an undersupply of cold air. To prevent hot air from flowing back into the cold aisle, A f should be set slightly higher than 1, and it is calculated as follows: Air-segregation efficiency (ASE)-an index related to the efficiency of separating cold air and hot air-is introduced as an index of "air management" that is independent of IT load and fan speed of the air conditioner. ASE is given as follows:

Method of Predicting R by Using ASE
Substituting Equations (3) and (4) into Equation (5) and rearranging them gives the following equation: Substituting Equation (7) into Equation (6) and rearranging them gives the following equation: Equation (8) indicates that recirculation flow (R) can be simply obtained from the availability of flow (A f ) and air-segregation efficiency (ASE). It should be noted here that ASE is an index independent of the power consumption of the ITE and air-conditioner fan speed. In other words, unless the air-leakage path that affects BP and R (such as the gap between the raised-floor fixed panels and the installation status of the blanking panels) is changed, ASE takes a constant value even if the power consumption of the ITE, airflow rate of the ITE, and air-supply rate of the HVAC fluctuate. Therefore, if ASE is considered a known value, R for a certain A f can be obtained. The relationship expressed by Equation (8) is illustrated in Figure 1.

Prediction Procedure
The prediction procedure, starting from ITE inlet temperature (Tii), is shown in Table 1. For airsupply amount (mc) and air-segregation efficiency (ASE), which are additional given conditions to ITE inlet temperature, in the case of an existing machine room, actual measurement results are used, and in the case of a new machine room, design values are used. It has been reported that the ASE for a machine room with containment is about 0.65 to 0.8, the ASE for a machine room without containment is about 0.4 to 0.7, and the average ASE is 0.6. [18]. In addition to the given conditions, necessary information is the characteristics of the ITE and the characteristics of the HVAC.
As described later, as the relationship between ITE inlet temperature and ITE airflow varies according to CPU usage rate, ITE outlet temperature (Tio) may be used as a given condition. In this study, to fix CPU usage rate in the prediction, ITE inlet temperature, which is often handled, is used as a given condition.

Prediction Procedure
The prediction procedure, starting from ITE inlet temperature (T ii ), is shown in Table 1. For air-supply amount (m c ) and air-segregation efficiency (ASE), which are additional given conditions to ITE inlet temperature, in the case of an existing machine room, actual measurement results are used, and in the case of a new machine room, design values are used. It has been reported that the ASE for a machine room with containment is about 0.65 to 0.8, the ASE for a machine room without containment is about 0.4 to 0.7, and the average ASE is 0.6. [18]. In addition to the given conditions, necessary information is the characteristics of the ITE and the characteristics of the HVAC.
As described later, as the relationship between ITE inlet temperature and ITE airflow varies according to CPU usage rate, ITE outlet temperature (T io ) may be used as a given condition. In this study, to fix CPU usage rate in the prediction, ITE inlet temperature, which is often handled, is used as a given condition.

Overview of Experimental Verification Room
With the verification room, which reproduces an actual machine room of a data center, the characteristics of the ITE are revealed through reproduction experiments. The verification room is shown schematically in Figure 2, and the experimental equipment is outlined in Table 2. The target rack with 12 servers, which will be described later, is shown in the figure. The verification room forms a cold aisle and a hot aisle, and either of the two cooling methods-namely, "raised-floor supply room return" or "raised-floor supply ceiling return"-can be selected. Raised-floor opening panels (with width of 300 mm and aperture ratio of 25% or 50%) are alternately arranged in the cold-aisle section. In the case of ceiling inlets, an inlet port with an aperture ratio of 33% is fitted in the hot-aisle portion of the ceiling. Either "cold-aisle containment" (CAC) or "hot-aisle containment" (HAC), which are airflow management and design methods introduced in the EU Code of Conduct for Data Center Energy Efficiency [22], can be configured. As for the verification room, the CAC is formed so that the cold aisle is covered by horizontal members passed over the top of the racks. HAC is formed so that the hot aisle is surrounded by vertical members that extend from the top of the racks to the ceiling. The inside of the verification room at the center (hot-aisle section) (with width of 9.6 m and depth of 11.0 m), the inside of the raised floor, and the inside of the ceiling are partitioned by temporary partitions (antistatic flameproof film), and the range of the experiment covers a width of 5.0 m and depth of 11.0 m. Of the 22 19-in. (48.26 cm) racks, 16 racks are operated, the rack to be tested is fitted with 12 servers or one heat generator (with a 2 kW heater), and one heat generator is installed in 15 racks. A shielding plate (a blanking panel) for suppressing the recirculation flow generated through these spaces can be installed on the nonmounted portion of the mounting rail or at the side of the mounting rail in the rack. For the air conditioning, a computer-room air-handler (CRAH) (with a rated sensible cooling capacity of 48.6 kW) and a computer-room air-conditioner (CRAC) (with a rated sensible cooling capacity of 45.0 kW) are used. The heat-source system for producing chilled water consists of air-cooled chillers and chilled-water pumps.
Energies 2020, 13, x FOR PEER REVIEW 6 of 27 conditioning (HVAC), return-air wet-bulb temperature or supply air temperature (dry bulb, wet bulb) may be used.

Overview of Experimental Verification Room
With the verification room, which reproduces an actual machine room of a data center, the characteristics of the ITE are revealed through reproduction experiments. The verification room is shown schematically in Figure 2, and the experimental equipment is outlined in Table 2. The target rack with 12 servers, which will be described later, is shown in the figure. The verification room forms a cold aisle and a hot aisle, and either of the two cooling methods-namely, "raised-floor supply room return" or "raised-floor supply ceiling return"-can be selected. Raised-floor opening panels (with width of 300 mm and aperture ratio of 25% or 50%) are alternately arranged in the cold-aisle section. In the case of ceiling inlets, an inlet port with an aperture ratio of 33% is fitted in the hot-aisle portion of the ceiling. Either "cold-aisle containment" (CAC) or "hot-aisle containment" (HAC), which are airflow management and design methods introduced in the EU Code of Conduct for Data Center Energy Efficiency [22], can be configured. As for the verification room, the CAC is formed so that the cold aisle is covered by horizontal members passed over the top of the racks. HAC is formed so that the hot aisle is surrounded by vertical members that extend from the top of the racks to the ceiling. The inside of the verification room at the center (hot-aisle section) (with width of 9.6 m and depth of 11.0 m), the inside of the raised floor, and the inside of the ceiling are partitioned by temporary partitions (antistatic flameproof film), and the range of the experiment covers a width of 5.0 m and depth of 11.0 m. Of the 22 19-in. (48.26 cm) racks, 16 racks are operated, the rack to be tested is fitted with 12 servers or one heat generator (with a 2 kW heater), and one heat generator is installed in 15 racks. A shielding plate (a blanking panel) for suppressing the recirculation flow generated through these spaces can be installed on the nonmounted portion of the mounting rail or at the side of the mounting rail in the rack. For the air conditioning, a computer-room air-handler (CRAH) (with a rated sensible cooling capacity of 48.6 kW) and a computer-room air-conditioner (CRAC) (with a rated sensible cooling capacity of 45.0 kW) are used. The heat-source system for producing chilled water consists of air-cooled chillers and chilled-water pumps.

Understanding Characteristics of ITE
The behavior of the fan of the ITE itself is determined first. As for the ITE, a general-purpose 1U server of a certain vendor is used. To understand the behavior of the built-in fan, the ambient temperature of the server is varied. Moreover, to understand the effect of different airflow environments surrounding the servers, the presence or absence of airflow-separation technology such as CAC and blank panels is taken as a parameter.

Experimental Conditions (Experiments A and B)
Twelve servers (with specifications as listed in Table 3) are installed in the target rack. One server is installed in row "6U" (the 6th row from the bottom of the rack, which can mount 42 units) and in every third row above that. The uppermost server is installed in row 39.

Understanding Characteristics of ITE
The behavior of the fan of the ITE itself is determined first. As for the ITE, a general-purpose 1U server of a certain vendor is used. To understand the behavior of the built-in fan, the ambient temperature of the server is varied. Moreover, to understand the effect of different airflow environments surrounding the servers, the presence or absence of airflow-separation technology such as CAC and blank panels is taken as a parameter.

Experimental Conditions (Experiments A and B)
Twelve servers (with specifications as listed in Table 3) are installed in the target rack. One server is installed in row "6U" (the 6th row from the bottom of the rack, which can mount 42 units) and in Energies 2020, 13, 6166 8 of 27 every third row above that. The uppermost server is installed in row 39. The characteristics of the server when the ambient temperature of the server is varied are determined by using the temperature setting of the air-conditioning supply air as a parameter. The specifications of the measurement items and measurement equipment in the experiment are listed in Table 4, and the installation status and measurement points of the servers in the rack to be tested are shown in Figure 3. The mean of the measured values taken over five minutes when the conditions are stable is used. The server inlet temperature and outlet temperature are average values of the five measured temperatures, and the server airflow is calculated from the average value of the difference between server outlet temperature and server inlet temperature and the total power consumption of the 12 servers. The temperature-measurement points are arranged as evenly as possible, including the top and bottom ones, so that the average values for 12 servers can be covered. Under the conditions listed in Table 5, an experiment using the CRAH and CAC presence or absence, BP presence or absence, CPU utilization rate, and setting of air-conditioning supply-air temperature as parameters was conducted. The CPU usage rate is defined as (real time-idle-state time/real time), and to generate a CPU load, a stress-test tool based on multiple technical calculations was used. Checking the CPU load factor (i.e., percentage of actual computing power to maximum computing power) with respect to the CPU usage rate with the management tool installed in the server revealed that when the CPU usage rate was 100%, the CPU load rate was about 100%; when the CPU usage rate was 50%, the CPU load rate was about 51%; and when the CPU usage rate was 0%, the CPU load rate was about 1%.   As mentioned above, it is known that the speed of the internal fan of the server fluctuates with varying ambient temperature. Therefore, as shown in Table 5, changing the air-conditioning supplyair temperature changes the ITE airflow; however, it is a concern that changing the balance between ITE airflow and supply-air volume will affect the measurement results. Therefore, a preliminary experiment under certain conditions, namely, with CAC, blanking panels, a CPU usage rate of 50%, and a constant CRAH air volume, was conducted, and the server airflow (i.e., total airflow of 12 servers)-corresponding to the temperature setting of the CRAH air supply (15,19,23,27, and 31 °C)-was obtained. At each supply-air-temperature setting, the airflow of the heat generator was set so that the differences between the inlet and outlet temperatures of the server and the heat generator determined in a preliminary experiment matched. The CRAH supply-air volume was also set so that the ratio of the air-conditioning air-supply volume (mc) to ITE air volume (mi) (i.e., availability of flow (Af)) was close to 1.4. The airflow setting for each determined supply-air temperature setting is shown in Figure 4.  As mentioned above, it is known that the speed of the internal fan of the server fluctuates with varying ambient temperature. Therefore, as shown in Table 5, changing the air-conditioning supply-air temperature changes the ITE airflow; however, it is a concern that changing the balance between ITE airflow and supply-air volume will affect the measurement results. Therefore, a preliminary experiment under certain conditions, namely, with CAC, blanking panels, a CPU usage rate of 50%, and a constant CRAH air volume, was conducted, and the server airflow (i.e., total airflow of 12 servers)-corresponding to the temperature setting of the CRAH air supply (15,19,23, 27, and 31 • C)-was obtained. At each supply-air-temperature setting, the airflow of the heat generator was set so that the differences between the inlet and outlet temperatures of the server and the heat generator determined in a preliminary experiment matched. The CRAH supply-air volume was also set so that the ratio of the air-conditioning air-supply volume (m c ) to ITE air volume (m i ) (i.e., availability of flow (A f )) was close to 1.4. The airflow setting for each determined supply-air temperature setting is shown in Figure 4.

Air-Flow Characteristics of Server
The results of experiment A are plotted in Figure 5, which shows the relationship between server inlet and outlet temperatures and server fan speed (airflow) with server CPU usage as a parameter. As shown in Figure 5a, when the CPU usage rate is 0%, the fan speed (airflow) corresponding to the server inlet temperature is lower than those values for the other CPU usage rates. It can be said that the relationship between server inlet temperature and server airflow differs according to CPU usage rate. On the contrary, it can be seen from Figure 5b that server outlet temperature and server-fan speed (airflow) have the same correlation regardless of CPU usage rate. As mentioned above, ITE controls the built-in fan speed to keep the temperature of the components at the target value [14]. As the CPU usage rate increases, the CPU load increases, so the server-fan speed (airflow) is increased to maintain the target temperature of the CPU. Therefore, it is considered that there is no difference in the server-outlet temperature after heat exchange depending on the CPU usage rate. Server-outlet temperature (°C) Figure 5. Relationships of server-fan speed and server airflow (experiment A) with (a) server-inlet temperature and (b) server-outlet temperature for each CPU usage rate at server ambient temperature.
The results of experiment B are plotted in Figure 6, which shows the correlation between server inlet and outlet temperatures and server-fan speed (airflow) obtained with the use (or no use) of airflow-separation technology (i.e., with or without CAC and with or without blanking panels) as a parameter. Although the relationship between server inlet temperature and server-fan speed, shown in Figure 6a-1, varies slightly, the server airflow in Figure 6a-2 does not vary at all. In Figure 6b, server-fan speed and sever airflow also do not vary, and it can be said that the relationship between server ambient temperature and server-fan speed (airflow) is not significantly affected by the usage (or not) of the airflow-separation technology. The maximum fan speed confirmed in the experiment was 94%, and the minimum fan speed was 29%.
(a-2) (b-2) Figure 5. Relationships of server-fan speed and server airflow (experiment A) with (a) server-inlet temperature and (b) server-outlet temperature for each CPU usage rate at server ambient temperature.
(a-1) (b-1)  Moreover, to confirm whether the measurement-target server can represent all the servers, the average value of fan rotation speeds of the measurement target server was compared with those values of which all the servers were compared. As shown in Figure 7, the fan speeds of the server to be measured and all the servers are about equal, so it is reasonable to represent the temperatures of all twelve servers by the average value of the measured temperatures of five servers. Average fan speed (%) of server to be measured Moreover, to confirm whether the measurement-target server can represent all the servers, the average value of fan rotation speeds of the measurement target server was compared with those values of which all the servers were compared. As shown in Figure 7, the fan speeds of the server to be measured and all the servers are about equal, so it is reasonable to represent the temperatures of all twelve servers by the average value of the measured temperatures of five servers.  Moreover, to confirm whether the measurement-target server can represent all the servers, the average value of fan rotation speeds of the measurement target server was compared with those values of which all the servers were compared. As shown in Figure 7, the fan speeds of the server to be measured and all the servers are about equal, so it is reasonable to represent the temperatures of all twelve servers by the average value of the measured temperatures of five servers.

Server-Power-Consumption Characteristics
Plotting the results of experiments A and B, Figure 8 shows the correlation between server airflow rate and server power consumption of all 12 units. Clearly, as server airflow rate increases, server power consumption increases. In addition, server power consumption increases with CPU

Server-Power-Consumption Characteristics
Plotting the results of experiments A and B, Figure 8 shows the correlation between server airflow rate and server power consumption of all 12 units. Clearly, as server airflow rate increases, server power consumption increases. In addition, server power consumption increases with CPU usage rate. In Figure 8b, the server power consumption at the highest and lowest server airflow rates for each CPU usage rate is compared as follows: When the CPU usage rate is 100%, the server power consumption is about 18% lower at the latter airflow rate; when the CPU usage rate is 50%, it is about 22% lower; and when the CPU usage rate is 0%, it is about 48% lower. On the other hand, it can be seen from Figure 8b that the relationship between server airflow rate and server power consumption is not significantly affected by the usage (or not) of the airflow-separation technology. As a means such as lowering the supply-air temperature or suppressing recirculation by applying airflow-separation technology, lowering the ambient temperature of the server and keeping the server airflow rate low not only reduce the power consumption of the server itself but also reduce the energy consumed by the air-conditioning system that cools the server. On the contrary, it should be noted that even a server with a CPU usage of 0% (that is, an idle server) will still consume about half the power consumption of that with a CPU usage of 100%, so it cannot be ignored as a cooling target.
Energies 2020, 13, x FOR PEER REVIEW 13 of 27 airflow rate low not only reduce the power consumption of the server itself but also reduce the energy consumed by the air-conditioning system that cools the server. On the contrary, it should be noted that even a server with a CPU usage of 0% (that is, an idle server) will still consume about half the power consumption of that with a CPU usage of 100%, so it cannot be ignored as a cooling target.

Validity of Prediction Method
A heat generator was installed in each rack, and the characteristics of a general-purpose 1U server in the presence of the heat generator were reproduced. It was then confirmed whether the thermal environment of the machine room can be predicted by the proposed prediction method.

Method to Reproduce ITE
The relationship between Figure 6a-2,b-2 is reproduced in an experimental environment in order to clarify the effect of changes in the ITE airflow rate on cooling. The server is substituted by a heat generator that can adjust the fan airflow rate, and the heat generator reproduces the slope of the plot of server airflow rate versus server ambient temperature. As the inlet temperature of the heat generator depends on the rack position, it is difficult to reproduce the slope with an individual heat generator. In this experiment, therefore, the airflow rates of all the heat generators were set to the same setting, and the supply-air temperature was adjusted so that the average ambient temperature and average airflow rate of all the heat generators accorded with the slopes of the plots in Figure 6a-2,b-2. Moreover, it was determined that the temperature zone did not affect the prediction of air performance to which the temperature zone was set so that the experiment was easy.

Experimental Conditions (Experiments C and D)
All 16 racks are equipped with heat generators. The CRAC is used as the air conditioner. The

Validity of Prediction Method
A heat generator was installed in each rack, and the characteristics of a general-purpose 1U server in the presence of the heat generator were reproduced. It was then confirmed whether the thermal environment of the machine room can be predicted by the proposed prediction method.

Method to Reproduce ITE
The relationship between Figure 6a-2,b-2 is reproduced in an experimental environment in order to clarify the effect of changes in the ITE airflow rate on cooling. The server is substituted by a heat generator that can adjust the fan airflow rate, and the heat generator reproduces the slope of the plot of server airflow rate versus server ambient temperature. As the inlet temperature of the heat generator depends on the rack position, it is difficult to reproduce the slope with an individual heat generator. In this experiment, therefore, the airflow rates of all the heat generators were set to the same setting, and the supply-air temperature was adjusted so that the average ambient temperature and average airflow rate of all the heat generators accorded with the slopes of the plots in Figure 6a-2,b-2. Moreover, it was determined that the temperature zone did not affect the prediction of air performance to which the temperature zone was set so that the experiment was easy.

Experimental Conditions (Experiments C and D)
All 16 racks are equipped with heat generators. The CRAC is used as the air conditioner. The simulated heating element has a height of 6U, and it is mounted in the target rack at 23U to 28U from the bottom. As shown in Table 6, the experiment was conducted under two conditions: (i) Raised-floor outlet/horizontal inlet method with CAC and BP and (ii) raised-floor outlet/ceiling inlet method with HAC and BP. As mentioned in the previous section, the ambient temperature and airflow rate of the heat generator are matched with the slopes of the plots in Figure 6a-2,b-2. The airflow rate of the heat generator at five points is set for each experimental condition ((i) and (ii) above), and the CRAC supply-air temperature is adjusted to match the slopes of those plots while the inlet air temperature and outlet temperature are monitored. Note that the setting of the CRAC supply-air amount is constant. The fan of the heat generator is voltage-controlled, and the voltage settings at the five points and the assumed airflow rate (measured separately) are listed in Table 7. In addition, the measured items and measurement equipment used in this experiment are detailed in Table 8. Mean values (averaged over five minutes under stable conditions) were used. Note that in this experiment, the "side blanking panels" shown in Figure 3b were installed even under the "no blanking panel" condition shown in Table 6.

Confirmation of Reproduction Status
To confirm whether the heat generator can reproduce the relationship between temperature and airflow rate plotted in Figure 6a-2,b-2, the ambient temperature of the equipment and airflow rate per rack are plotted in Figure 9. As mentioned above, 12 servers were installed in each rack, and one heat generator was installed in each rack. When the results were confirmed, although the variation range of airflow was narrow, because the variable speed range of the built-in fan of the heat generator was narrow, the heat generator was able to roughly reproduce the "tilt" of the 12 servers in terms of both inlet temperature and outlet temperature.

Confirmation of Given Conditions
To obtain the values of the given conditions used for prediction, ITE inlet temperature (Tii), CRAC supply-air amount (mc), ASE, and ITE-related empirical equations are confirmed hereafter. The experimental results for Tii, mc, and ASE under the respective experimental conditions are shown in Figure 11. As given conditions, Tii uses the respective measured values, and mc and ASE use the average values of C-1, C-2, D-1, and D-2. As the airflow setting of the CRAC is fixed, mc is almost constant under each airflow scheme. However, mc of the ceiling-inlet method was smaller than that of the horizontal-inlet method. It is probable that mc of the ceiling-inlet method was small due to increased pressure losses such as those due to the ceiling inlet port, the inside of the ceiling, and the The relationship between airflow rate and power consumption of the heat generator is shown in Figure 10. The results of experiment B were used as the numerical values of the servers to be compared. As mentioned above, although fluctuation in the airflow rate of the heat generator was in the low airflow-rate range of the server, it can be seen that the power consumption tended to increase as the built-in fan of the heat generator accelerated.

Confirmation of Given Conditions
To obtain the values of the given conditions used for prediction, ITE inlet temperature (T ii ), CRAC supply-air amount (m c ), ASE, and ITE-related empirical equations are confirmed hereafter. The experimental results for T ii , m c , and ASE under the respective experimental conditions are shown in Figure 11. As given conditions, T ii uses the respective measured values, and m c and ASE use the average values of C-1, C-2, D-1, and D-2. As the airflow setting of the CRAC is fixed, m c is almost constant under each airflow scheme. However, m c of the ceiling-inlet method was smaller than that of the horizontal-inlet method. It is probable that m c of the ceiling-inlet method was small due to increased pressure losses such as those due to the ceiling inlet port, the inside of the ceiling, and the return-air duct (from the ceiling to the CRAC inlet port).  Figure 11. Information-technology equipment (ITE) inlet temperature (Tii), supply airflow rate (mc), and air-segregation efficiency (ASE) for each experimental condition.

Comparison of Measurement Results with Prediction Results
Using the given conditions and empirical equations determined as described in the previous section, recirculation flow (R) (step 5 in Table 1), supply-air temperature (Tco) of the CRAC, and return-air temperature (Tci) (steps 6 and 7 in Table 1) are predicted. Experimental measurement results and prediction results for the recirculation flow and temperature of each measurement point are shown in Figures 12 and 13, respectively. The experimental measurement results are considered first. The experimental measurement results shown in Figure 12 reveal that as the availability of flow (Af) decreases, recirculation flow (R) tends to increase. In other words, it can be said that the amount of air supplied from the CRAC was insufficient to meet the required airflow rate of the ITE, so the recirculation flow increased. This result is explained as follows. As the supply-air amount is roughly fixed, the ITE inlet temperature (Tii) becomes high, and the ITE airflow rate increases. In Figure 13, the predicted recirculation flow (R) values are used to predict supply-air temperature (Tco) and returnair temperature (Tci) with respect to ITE intake air temperature (Tii). As can be seen from these graphs, as Tii increases, the temperature difference from Tco to Tii increases. For example, in the case of D-1, in which R is largest, the temperature rise is 1.5 °C when the inlet temperature is 27 °C; however, when the inlet temperature is 30 °C, the temperature rise is bigger, i.e., 2.1 °C. In each case, the prediction results reproduce the experimental measurement results well, so the proposed temperatureprediction procedure is considered appropriate.
Supply airflow rate (m The ITE-related empirical equations are obtained by approximating the results for each heat generator fitted per rack shown in Figures 9a and 10. The approximate equation for each rack is given as follows:

Comparison of Measurement Results with Prediction Results
Using the given conditions and empirical equations determined as described in the previous section, recirculation flow (R) (step 5 in Table 1), supply-air temperature (T co ) of the CRAC, and return-air temperature (T ci ) (steps 6 and 7 in Table 1) are predicted. Experimental measurement results and prediction results for the recirculation flow and temperature of each measurement point are shown in Figures 12 and 13, respectively. The experimental measurement results are considered first. The experimental measurement results shown in Figure 12 reveal that as the availability of flow (A f ) decreases, recirculation flow (R) tends to increase. In other words, it can be said that the amount of air supplied from the CRAC was insufficient to meet the required airflow rate of the ITE, so the recirculation flow increased. This result is explained as follows. As the supply-air amount is roughly fixed, the ITE inlet temperature (T ii ) becomes high, and the ITE airflow rate increases. In Figure 13, the predicted recirculation flow (R) values are used to predict supply-air temperature (T co ) and return-air temperature (T ci ) with respect to ITE intake air temperature (T ii ). As can be seen from these graphs, as T ii increases, the temperature difference from T co to T ii increases. For example, in the case of D-1, in which R is largest, the temperature rise is 1.5 • C when the inlet temperature is 27 • C; however, when the inlet temperature is 30 • C, the temperature rise is bigger, i.e., 2.1 • C. In each case, the prediction results reproduce the experimental measurement results well, so the proposed temperature-prediction procedure is considered appropriate.
(Af) decreases, recirculation flow (R) tends to increase. In other words, it can be said that the amount of air supplied from the CRAC was insufficient to meet the required airflow rate of the ITE, so the recirculation flow increased. This result is explained as follows. As the supply-air amount is roughly fixed, the ITE inlet temperature (Tii) becomes high, and the ITE airflow rate increases. In Figure 13, the predicted recirculation flow (R) values are used to predict supply-air temperature (Tco) and returnair temperature (Tci) with respect to ITE intake air temperature (Tii). As can be seen from these graphs, as Tii increases, the temperature difference from Tco to Tii increases. For example, in the case of D-1, in which R is largest, the temperature rise is 1.5 °C when the inlet temperature is 27 °C; however, when the inlet temperature is 30 °C, the temperature rise is bigger, i.e., 2.1 °C. In each case, the prediction results reproduce the experimental measurement results well, so the proposed temperatureprediction procedure is considered appropriate.

Proposed Method for Optimization of Total Energy of a Machine Room
In the previous section, it was confirmed that the thermal environment of the machine room can be predicted by the proposed prediction procedure; in particular, the temperatures of the 20

Proposed Method for Optimization of Total Energy of a Machine Room
In the previous section, it was confirmed that the thermal environment of the machine room can be predicted by the proposed prediction procedure; in particular, the temperatures of the environments around the CRAC and ITE can be linked. It is therefore possible to predict the power consumption of the HVAC with respect to ITE inlet temperature. In this section, a method for optimizing the power consumption of the machine room, under the assumption that ITE characteristics are understood, is proposed.

Predetermined Conditions
It is assumed that 12 ITEs are mounted on each of the 16 racks. The cooling device is a single CRAC. ASE is set from 0.5 to 1.0 in 0.1 increments with indoor airflow performance as a parameter. Further, ITE inlet temperature (T ii ), which is taken as the starting point of the prediction, is predicted in 1 • C increments.

Characteristics of ITE
The characteristics of the ITE (servers) were determined by the least-squares method from the values measured in experiment B. The results of experiment B and the characteristics of the assumed ITE are shown in Figure 14. The ITE airflow rate shown in Figure 14a was set to the upper limit at 34 • C or higher and the lower limit at 13 • C or lower. It was determined by estimating the ITE inlet temperature (T ii ) for which server-fan speed has maximum and minimum values, as shown in Section 4.2.1.

Characteristics of HVAC
Various methods for assuming the characteristics of the HVAC are available; however, as the main focus of this section is to propose a total-energy optimization method, a simple method is used. We interviewed Japanese air-conditioning manufacturers about two characteristics, namely, the (i) maximum cooling capacity with respect to the indoor wet-bulb temperature of a CRAC (rated sensible cooling capacity of 63 kW) used for data centers and the (ii) power consumption under partial load, and organized those characteristics.
The characteristics of the ITE (servers) were determined by the least-squares method from the values measured in experiment B. The results of experiment B and the characteristics of the assumed ITE are shown in Figure 14. The ITE airflow rate shown in Figure 14a was set to the upper limit at 34 °C or higher and the lower limit at 13 °C or lower. It was determined by estimating the ITE inlet temperature (Tii) for which server-fan speed has maximum and minimum values, as shown in Section 4.2.1.

Characteristics of HVAC
Various methods for assuming the characteristics of the HVAC are available; however, as the main focus of this section is to propose a total-energy optimization method, a simple method is used. We interviewed Japanese air-conditioning manufacturers about two characteristics, namely, the (i) maximum cooling capacity with respect to the indoor wet-bulb temperature of a CRAC (rated sensible cooling capacity of 63 kW) used for data centers and the (ii) power consumption under partial load, and organized those characteristics.
The specifications of the CRAC are listed in Table 9, and example characteristics of the CRAC are shown in Figure 15. The wet-bulb-temperature range of inlet air under which the CRAC is operable is 12 to 24 °C WB. Table 1 exemplifies the fact that the dry-bulb temperature of the return air is used for calculating the power consumption of the CRAC. However, in this section, the wetbulb temperature of the return air is used in consideration of the characteristics of the CRAC. For the simulation, the absolute humidity was fixed at 20 °C and 50% RH, and the wet-bulb temperature was calculated from the dry-bulb temperature of the return air. The specifications of the CRAC are listed in Table 9, and example characteristics of the CRAC are shown in Figure 15. The wet-bulb-temperature range of inlet air under which the CRAC is operable is 12 to 24 • C WB. Table 1 exemplifies the fact that the dry-bulb temperature of the return air is used for calculating the power consumption of the CRAC. However, in this section, the wet-bulb temperature of the return air is used in consideration of the characteristics of the CRAC. For the simulation, the absolute humidity was fixed at 20 • C and 50% RH, and the wet-bulb temperature was calculated from the dry-bulb temperature of the return air. To estimate total energy consumption of the CRAC, annual power consumption is calculated. The power consumption in 5 °C increments is calculated, and duration times of outside-air temperature in increments of 5 °C are added up. The duration time used is meteorological data for Tokyo [23] published on Energy Plus. It is plotted in Figure 16.  Figure 15. Examples of characteristics of CRAC: (a) Change in maximum sensible cooling capacity with respect to outside-air temperature (return-air conditions, airflow, and external static pressure (ESP) are as listed in Table 9) and (b) change in power consumption with respect to partial load (return-air conditions and outdoor-air condition, airflow, and ESP are as listed in Table 9), where Ec,actual is actual power consumption of the CRAC, Ec,rated is rated power consumption of the CRAC, Qsen,actual is actual sensible cooling capacity, Qsen,max is maximum sensible cooling capacity, and Qsen,rated is rated sensible cooling capacity.   Table 9) and (b) change in power consumption with respect to partial load (return-air conditions and outdoor-air condition, airflow, and ESP are as listed in Table 9), where E c,actual is actual power consumption of the CRAC, E c,rated is rated power consumption of the CRAC, Q sen,actual is actual sensible cooling capacity, Q sen,max is maximum sensible cooling capacity, and Q sen,rated is rated sensible cooling capacity.
To estimate total energy consumption of the CRAC, annual power consumption is calculated. The power consumption in 5 • C increments is calculated, and duration times of outside-air temperature in increments of 5 • C are added up. The duration time used is meteorological data for Tokyo [23] published on Energy Plus. It is plotted in Figure 16.
(a) (b) Figure 15. Examples of characteristics of CRAC: (a) Change in maximum sensible cooling capacity with respect to outside-air temperature (return-air conditions, airflow, and external static pressure (ESP) are as listed in Table 9) and (b) change in power consumption with respect to partial load (return-air conditions and outdoor-air condition, airflow, and ESP are as listed in Table 9), where Ec,actual is actual power consumption of the CRAC, Ec,rated is rated power consumption of the CRAC, Qsen,actual is actual sensible cooling capacity, Qsen,max is maximum sensible cooling capacity, and Qsen,rated is rated sensible cooling capacity. Outside air temperature (°C) Figure 16. Durations of outside-air temperature in 5 • C increments (data for Tokyo).

Prediction Results
The results of predicting the total energy consumption of the machine room by using the prediction procedure described in Section 2.3 are shown hereafter.

Prediction Results for Inside Environment
The results of calculating the availability of flow (A f ) and circulation flow (R) in regard to ITE inlet temperature (T ii ) for each ASE by performing steps 1 to 5 shown in Table 1 are shown in Figure 17. As shown in Figure 17a, A f decreases with T ii . In this prediction, A f ranges from 1.2 to 3.4. Moreover, when A f decreases, the amount of air supplied to meet the required airflow rate to the ITE shifts to the insufficient side, so the recirculation flow tends to increase. Actually, as shown in Figure 17b, R increases as T ii increases. When ASE is 1, recirculation does not occur (R = 0), but when ASE is 0.5, R is 0.46 at maximum, and nearly half of the exhaust-air volume of the ITE is recirculated. In addition, as shown in Figure 14a, the airflow rate of the ITE does not increase nor decrease in the ranges in which the ITE inlet temperature (T ii ) is respectively 13 • C or lower and 34 • C or higher. Consequently, R does not fluctuate in those ranges.

Prediction Results
The results of predicting the total energy consumption of the machine room by using the prediction procedure described in Section 2.3 are shown hereafter.

Prediction Results for Inside Environment
The results of calculating the availability of flow (Af) and circulation flow (R) in regard to ITE inlet temperature (Tii) for each ASE by performing steps 1 to 5 shown in Table 1 are shown in Figure  17. As shown in Figure 17a, Af decreases with Tii. In this prediction, Af ranges from 1.2 to 3.4. Moreover, when Af decreases, the amount of air supplied to meet the required airflow rate to the ITE shifts to the insufficient side, so the recirculation flow tends to increase. Actually, as shown in Figure  17b, R increases as Tii increases. When ASE is 1, recirculation does not occur (R = 0), but when ASE is 0.5, R is 0.46 at maximum, and nearly half of the exhaust-air volume of the ITE is recirculated. In addition, as shown in Figure 14a, the airflow rate of the ITE does not increase nor decrease in the ranges in which the ITE inlet temperature (Tii) is respectively 13 °C or lower and 34 °C or higher. Consequently, R does not fluctuate in those ranges. The results obtained by executing steps 6 and 7 and determining the temperature of each part around the ITE and around the CRAC are shown in Figure 18. In the machine room with an ASE in the range of 0.5 to 0.7, the return-air wet-bulb temperature in the low-temperature range fell below the operable range of the CRAC; therefore, only the results within the operable range are shown. It The results obtained by executing steps 6 and 7 and determining the temperature of each part around the ITE and around the CRAC are shown in Figure 18. In the machine room with an ASE in the range of 0.5 to 0.7, the return-air wet-bulb temperature in the low-temperature range fell below the operable range of the CRAC; therefore, only the results within the operable range are shown. It can be seen from the graph that the higher ITE inlet temperature (T ii ), the larger the temperature rise from supply-air temperature (T co ) to T ii , except in the case of the machine room with almost no recirculation (i.e., ASE of 0.9 or 1.0). At the upper limit of the ITE airflow rate, the temperature-rise range was 7.3 • C at an ASE of 0.5, 4.7 • C at an ASE of 0.6, 2.8 • C at an ASE of 0.7, and 1.4 • C at an ASE of 0.8. Furthermore, although the amount of air supplied to the CRAC was constant, the power consumption of the ITE (E i ) (i.e., cooling load) increased with T ii ; thus, the temperature difference between return-air temperature (T ci ) and supply-air temperature (T co ) of the CRAC tended to increase.

Prediction Results for Power Consumption of CRAC
The wet-bulb temperature of the return air is calculated from the return-air (dry-bulb) temperature (Tci) of the CRAC (shown in Figure 18), which is plotted in Figure 19, and the power consumption (Ec) of the CRAC is calculated for each outside air temperature (step 8 in Table 1). Then, Ec is time-accumulated to calculate the annual power consumption of the CRAC (ΣEc) in Tokyo. The cooling load of the CRAC is equivalent to the power consumption of the ITE (Ei) (for 16 racks) obtained from Figure 14b. As described in the previous section, Figure 19 only shows the results that fall within the operable range.

Prediction Results for Power Consumption of CRAC
The wet-bulb temperature of the return air is calculated from the return-air (dry-bulb) temperature (T ci ) of the CRAC (shown in Figure 18), which is plotted in Figure 19, and the power consumption (E c ) of the CRAC is calculated for each outside air temperature (step 8 in Table 1). Then, E c is time-accumulated to calculate the annual power consumption of the CRAC (ΣE c ) in Tokyo. The cooling load of the CRAC is equivalent to the power consumption of the ITE (E i ) (for 16 racks) obtained from Figure 14b. As described in the previous section, Figure 19 only shows the results that fall within the operable range. The annual power consumption (ΣEc) of the CRAC with respect to ITE inlet temperature (Tii) is plotted in Figure 20. In the temperature range below 34 °C, where ITE airflow rate varies, when the curve is such that Tii is from 22 to 24 °C, ΣEc reaches its lowest point in the cases of all the machineroom airflow performances. On the contrary, in the range above 34 °C, where ITE airflow rate is constantly at its maximum, as Tii increases, ΣEc decreases. In the range of this prediction, ΣEc becomes minimum in the range in which the ITE airflow rate is maximum (Tii = 40 °C) only when the ASE equals 0.5. As shown in Figure 21, and as generally known, as Tii increases, the coefficient of performance (COP) of the CRAC increases, and its operation becomes more efficient. However, as ITE power consumption (which is equivalent to cooling load) also increases, ΣEc tends to increase in the ITE-inlet-temperature range from the lowest point to 34 °C.  The annual power consumption (ΣE c ) of the CRAC with respect to ITE inlet temperature (T ii ) is plotted in Figure 20. In the temperature range below 34 • C, where ITE airflow rate varies, when the curve is such that T ii is from 22 to 24 • C, ΣE c reaches its lowest point in the cases of all the machine-room airflow performances. On the contrary, in the range above 34 • C, where ITE airflow rate is constantly at its maximum, as T ii increases, ΣE c decreases. In the range of this prediction, ΣE c becomes minimum in the range in which the ITE airflow rate is maximum (T ii = 40 • C) only when the ASE equals 0.5. As shown in Figure 21, and as generally known, as T ii increases, the coefficient of performance (COP) of the CRAC increases, and its operation becomes more efficient. However, as ITE power consumption (which is equivalent to cooling load) also increases, ΣE c tends to increase in the ITE-inlet-temperature range from the lowest point to 34 • C.

Optimization of Total Energy
The predicted annual total power consumption (ΣE total ), which comprises total CRAC annual power consumption (ΣE c ) and ITE annual power consumption (ΣE i ), is shown in Figure 22. As ΣE i is larger than ΣE c , the range in which CRAC power consumption varies is relatively small, and the effect of a decrease in ITE power consumption due to a decrease in ITE inlet temperature (T ii ) is greater. Therefore, in the range of T ii from 14 • C to 33 • C (in which ITE airflow rate varies), as T ii drops, ΣE total decreases. As ΣE i does not fluctuate below 14 • C, as T ii drops, ΣE total increases due to the influence of ΣE c . As a result, except for the machine room with an ASE of 0.5, which is outside the operable range of the CRAC, ΣE total reaches its optimum point at 14 • C. As for the machine room with an ASE of 0.5, the optimum point is to operate at the lower limit of the operable range. Moreover, if T ii is 33 • C or higher, ΣE total starts to decrease due to the influence of ΣE c .
In this study, ITE power consumption was used as the cooling load of the CRAC; however, other loads (such as the thermal conduction) also affect the cooling load. In particular, when the ITE inlet temperature is lowered, as the case in this study, the inside temperature is lowered, and the difference between inside and outside temperatures is increased. The inflow of heat from outside therefore becomes larger than that when the ITE inlet temperature is high. As a result, the lowest point of annual total energy consumption might shift to the higher ITE-inlet-temperature side. In this study, the ITE was assumed to have the characteristics revealed in Section 4; however, some ITE used around the world have airflow rates that do not change with ambient temperature, and other ITE have airflow rates that change in steps. In an actual machine room, it is considered that different kinds of ITE are combined at the rack level. That is also one of the factors that change the optimum point of the total energy of the machine room. annual total energy consumption might shift to the higher ITE-inlet-temperature side. In this study, the ITE was assumed to have the characteristics revealed in Section 4; however, some ITE used around the world have airflow rates that do not change with ambient temperature, and other ITE have airflow rates that change in steps. In an actual machine room, it is considered that different kinds of ITE are combined at the rack level. That is also one of the factors that change the optimum point of the total energy of the machine room. In reality, the data-center operator and the customer often conclude a "service level agreement" (SLA) to define the temperature conditions inside the machine room. Commonly quoted is the "recommended environmental range" in ASHRAE's thermal guidelines [1]. This envelope requires the dry-bulb temperature of the ITE inlet port to be kept at 18 to 27 °C. The energy-saving effect when ITE inlet temperature (Tii) is changed within that range is verified hereafter. That is, the annual power consumption is calculated under Tii within that dry-bulb-temperature range for minimizing ΣEc and under Tii within that range for minimizing ΣEtotal with Tii of 27 °C as a reference, and the energy-saving effect is verified as follows.
The results of the calculation and verification are shown in Figure 23. First, if Tii for minimizing ΣEc is focused on, it becomes clear that ΣEc hardly changes (99% to 100%) compared to the standard. This is because the standard (Tii = 27 °C) is close to the bottom of the curves shown in Figure 20. On the contrary, ΣEi decreased by 3 to 7% for each Tii. Although the reduction effect of ΣEtotal depends on In reality, the data-center operator and the customer often conclude a "service level agreement" (SLA) to define the temperature conditions inside the machine room. Commonly quoted is the "recommended environmental range" in ASHRAE's thermal guidelines [1]. This envelope requires the dry-bulb temperature of the ITE inlet port to be kept at 18 to 27 • C. The energy-saving effect when ITE inlet temperature (T ii ) is changed within that range is verified hereafter. That is, the annual power consumption is calculated under T ii within that dry-bulb-temperature range for minimizing ΣE c and under T ii within that range for minimizing ΣE total with T ii of 27 • C as a reference, and the energy-saving effect is verified as follows.
The results of the calculation and verification are shown in Figure 23. First, if T ii for minimizing ΣE c is focused on, it becomes clear that ΣE c hardly changes (99% to 100%) compared to the standard. This is because the standard (T ii = 27 • C) is close to the bottom of the curves shown in Figure 20. On the contrary, ΣE i decreased by 3 to 7% for each T ii . Although the reduction effect of ΣE total depends on the ASE, it ranged from 3 to 6%. All Tii values for minimum ΣE total were at the lower limit of the recommended range (T ii = 18 • C). However, as T co was low in regard to the CRAC, ΣE c increased compared to the standard. In particular, ΣE c increased by 4% in the case of the machine room with an ASE of 0.5 (i.e., a large recirculation flow). However, ΣE i was greatly reduced (by 11%), and ΣE total was reduced by 8 to 9%.
Note that care must be taken when evaluating the optimization results by using power-usage effectiveness (PUE). Partial PUE (pPUE) is defined by the following equation, and pPUE for ITE inlet temperature (Tii) is plotted in Figure 24.
According to Equation (11), as Tii rises, the ITE annual power consumption (ΣE i ) increases, so pPUE decreases. The original desirable state is to minimize the annual total power consumption (ΣE total ); however, if pPUE is evaluated and the policy is set, ΣE total may increase.
Energies 2020, 13, x FOR PEER REVIEW 24 of 27 the ASE, it ranged from 3 to 6%. All Tii values for minimum ΣEtotal were at the lower limit of the recommended range (Tii = 18 °C). However, as Tco was low in regard to the CRAC, ΣEc increased compared to the standard. In particular, ΣEc increased by 4% in the case of the machine room with an ASE of 0.5 (i.e., a large recirculation flow). However, ΣEi was greatly reduced (by 11%), and ΣEtotal was reduced by 8 to 9%. Note that care must be taken when evaluating the optimization results by using power-usage effectiveness (PUE). Partial PUE (pPUE) is defined by the following equation, and pPUE for ITE inlet temperature (Tii) is plotted in Figure 24.
According to Equation (11), as Tii rises, the ITE annual power consumption (ΣEi) increases, so pPUE decreases. The original desirable state is to minimize the annual total power consumption (ΣEtotal); however, if pPUE is evaluated and the policy is set, ΣEtotal may increase.

Concluding Remarks
In this study, existing "air performance" indicators, such as air segregation efficiency (ASE), were expanded, and a method for predicting a thermal environment by simply correlating supplyair temperature with ITE inlet temperature was proposed. Experiments were conducted in a verification room simulating a data center, and the experimental results were compared with the predicted results. These results demonstrate that the indoor thermal environment can be predicted

Concluding Remarks
In this study, existing "air performance" indicators, such as air segregation efficiency (ASE), were expanded, and a method for predicting a thermal environment by simply correlating supply-air temperature with ITE inlet temperature was proposed. Experiments were conducted in a verification room simulating a data center, and the experimental results were compared with the predicted results. These results demonstrate that the indoor thermal environment can be predicted accurately by the proposed method. In this manner, the supply-air and return-air temperatures of HVAC can be predicted from the ITE inlet temperature, and from those predicted temperatures, the power consumption of HVAC can be predicted. Based on this prediction method, a method for optimizing the total energy consumption of the data center-by using ITE inlet temperature as a parameter-was devised.
The findings from the predictions are as follows: • The results of the trial calculation assuming a machine room with an ASE in the range of 0.5 to 1.0 and using the characteristics of a certain CRAC as an example revealed that (i) to minimize the annual power consumption of the CRAC, the ITE inlet temperature should be selected in the range of 22 to 24 • C, and (ii) to minimize the annual total energy consumption of the machine room, it is better to select the highest ITE inlet temperature among those that give the lowest rotation speed of the ITE's built-in fan.

•
Within the recommended environmental range of the ASHRAE thermal guidelines (ITE inlet temperature of 18 • C to 27 • C), the total energy consumption was lowest when the ITE inlet temperature was 18 • C. It was shown that by lowering the ITE inlet temperature from 27 • C to 18 • C, the annual power consumption of the CRAC will increase; however, annual power consumption of the ITE will be significantly reduced, so the total energy consumption will be reduced by 8 to 9%.
Various methods for assuming the characteristics of the HVAC are available; however, as the main focus of this section is to propose a total-energy optimization method, a simple method is used. We believe that the proposed method can also be applied for predicting the power consumption of complicated heat sources such as economizers.
In this study, it was assumed that ITE power consumption and cooling load are equal, and the annual power consumption of the CRAC was calculated; however, in reality, other loads such as the thermal conduction also have an effect. In particular, it should be noted that when the inside temperature is lowered, the temperature difference between the inside and outside widens and the heat inflow increases. Moreover, although the temperature and power consumption were predicted by unifying the ITE (servers) with the same characteristics, some ITE in the world have airflow rates that do not change with ambient temperature and other ITE have airflow rates that change in steps. The presence of these kinds of ITE also contributes to changing the optimum point of total power consumption, etc. Therefore, it is also important to correctly understand the ITE group implemented in the data center that is targeted for prediction. When these factors are taken into consideration, it becomes clear that the lowest point of annual total energy consumption of the CRAC may shift to the side where ITE inlet temperature is higher.