1. Introduction
With the development of information technology, the scale of data centers (DCs) is increasing daily, and the mass heat dissipation affects the stability and reliability of the circuits [
1,
2]. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) proposed that the indoor temperature of DCs should be controlled at 28 °C, and the air cleanliness should reach grade three [
3]. The air conditioning systems of DCs run continuously for 8760 h throughout the year, which results in a huge energy consumption [
4,
5]. Due to the low outdoor environment temperatures in the transition season and winter, the natural cold energy can be fully utilized to reduce the cooling system energy consumption [
6]. Therefore, using natural cold energy efficiently has great significance on the research of DCs.
The technologies making use of natural cold energy in data centers are mainly divided into three conventional DC cooling architectures [
7]: (a) Room-based cooling throughout the space, (b) row-based cooling between servers, (c) chip and rack-based cooling. Because of the simple design and high efficiency of the first architecture, it is suitable for energy-saving reconstruction in small-sized and medium-sized DCs, which has been widely studied by scholars worldwide. According to the direct air-side free cooling system, the results showed that the energy consumption of free air cooling system reduced by 20%–47.5% compared to conventional the air-conditioning system in DCs [
8,
9]. However, this type is always limited by the cleanliness, temperature and humidity of the outdoor environment. Other researchers have analyzed the applicability and energy saving potential of various types of air–air heat exchangers system in data centers. The energy saving effect is remarkable, and the structure is simple compared to a conventional air conditioning system [
10,
11,
12,
13]. However, the air–air heat exchanger systems of various types, along with the heat transfer rate, are also limited by the high quality requirements of air flow, narrow space and large heat transfer area.
Many scholars [
14,
15,
16] have applied desiccant cooling system (DCS) and also phase-change materials to DCs, and as the free cold air and indoor exhausted–air can be dehumidified by solid (silicone), liquid (LiCl) and other desiccants, the dry air can then be cooled by evaporative cooling equipment to reach the indoor air supply requirements. In a sensible heat state, the cooling efficiency of DCS is higher, reaching more than 80%. This technology replaced CFCs with no pollution to the environment, and in the process of regeneration can be combined with solar energy and other clean resources. However, the integrated desiccant cooling system has many shortcomings and problems which hinder its development and application; for example, the stability of desiccants, the pressure drop of air flow in desiccant devices, power consumption in the regeneration link and complexity, etc. To overcome these disadvantages, different types of air–water heat exchanger systems have been studied. An integrated water side economizer (IWSE) adopted by Le Bot [
17] was simulated to investigate the efficiency of the server room temperature, and a new temperature adaptive control strategy was proposed and tested. To enhance the heat transfer performances, Wang et al. [
18,
19,
20] studied the heat transfer and resistance characteristics of AWHEs with different kinds of fins. The results showed that different plate spacing, numbers of rows, and arrangement modes had certain effects on the comprehensive performance, and the louver fin showed the best performance amongst the flat fins, perforated fins, corrugated fins, and louver fins of air–water heat exchanger. The results of Hsieh [
21] showed that when the Reynolds number (
Re) was in the range of 300–2000, the heat transfer factor
j of the louver fins was 18.6%–29.8% higher than flat fins, and the friction resistance factor
f increased by 39.7%–58.9%. Based on the above research, it was concluded that the heat transfer performances of air–water heat exchangers were better than those of air–air heat exchangers. However, common air–water heat exchangers have the disadvantages of large pressure drops and thermal resistances [
22].
To enhance heat transfer and uniformity of the temperature distribution, a growing number of scholars are applying heat pipe cooling systems to DCs with the development of heat pipe technology. Zhu et al. [
23] proposed a separate heat pipe heat exchanger system and a simulation method to estimate the operating performance and the energy efficiency, which reduced the mismatch degree by increasing the heat pipe series, and the maximum heat transfer efficiency of a three-stage heat pipe heat exchanger was 65%. Yue et al. [
24] developed a parallel micro-channel separate heat pipe system, where the maximum cooling capacity reached 9.6 kW when the corresponding optimal refrigerant filling ratio was 65.27%. Moreover, the heat flux was greatly affected by the temperature distribution of the inlet and outlet and different filling ratios. There are also some studies that use gravity heat pipes to improve the performance of heat exchanger system [
25,
26,
27]. However, the separated and the gravity heat pipe systems were directly affected by many factors, such as (a) the selection and filling rates of the working fluids, (b) the ambient temperature and pipeline length, and (c) the limitation of the shape of round heat pipes, which are difficult to fit with fins. There are several limitations in the heat transfer rate, system stability, and spatial distribution.
Based on these limitations, Zhao et al. [
28] proposed a flat micro-heat pipe array (MHPA). It possessed a large heat transfer coefficient and a large contact area with a flat shape which showed good heat transfer performances [
29,
30,
31,
32,
33]. Diao et al. [
34] used an air–air heat exchanger based on MHPA to achieve heat recovery. The surface of the MHPA was composed of serrated fins to enhance the heat transfer, and the highest heat transfer efficiency was 75%. From these previous studies, it can be concluded that a heat exchanger with MHPA as its core component would exhibit a relatively better performance than the above mentioned heat exchangers.
In this study, a new type of indoor air–water heat exchanger based on MHPA (MHPA-AWHE) is proposed to apply in DCs, and the software DeST-c developed by Tsinghua University [
35] was adopted to simulate the hourly environment temperature and cooling load of typical DCs in Beijing all the year round, and the energy saving and equipment investment payback period were analyzed. This work has the following advantages:
- –
With regard to the theoretical calculation and actual design of MHPA-AWHE, there are several steps as follows: (a) The comprehensive performance based on heat transfer and pressure drop characteristics of serrated fins shows the best performance of different kinds of fins [
19,
20]. (b) The heat transfer equations are established from the air side to water side of MHPA-AWHE, and the optimal solution is obtained by simultaneous equations. Under the design condition of certain heat transfer rates, the size of the heat exchanger with the minimum number of MHPAs and the minimum length, width, height is obtained. (c) On this basis, the experimental platform is established, and the experimental results are verified with theoretical calculation values.
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With regard to the structure aspect, there are several advantages: (a) The flat-plate appearance of MHPA facilitates the combination with the heat transfer enhancement structure. (b) On the water side, the parallel flow tube with tiny porous channels enlarges the heat transfer area, and the flat-plate appearance can easily fit with MHPA, (c) The serrated fins are used to increase the disturbance of air flow and enlarge the convective heat transfer area to enhance heat transfer.
- –
With regard to the operating effect, (a) compared with traditional heat exchangers, the proposed configuration has a compact structure, small footprint, and low heat loss. (b) The MHPA-AWHE system is flexible and reliable and uses water or antifreeze as the circulating medium between indoor and outdoor sides instead of refrigerant pipelines.
- –
With regard to the energy saving aspect, the most economical operation condition and matching number of modules are suggested in this work, and the critical environment temperature for the opening and closing of MHPA-AWHE system is obtained. The power consumption of DCs after the transformation is lower than that before the transformation.
According to these, the Enthalpy Potential Method Laboratory was used to simulate the ambient temperature of the data center at 28 °C. The heat transfer process and thermal performance of MHPA-AWHE under different conditions were investigated, factors j and f were obtained to analyze the heat transfer and resistance characteristics of MHPAs, and the energy saving characteristic of the MHPA-AWHE system showed good performance and is useful for the application in DCs.
4. Conclusions
In this study, the thermal performance and energy efficiency of the MHPA-AWHE system were analyzed and studied comprehensively. The main conclusions were as follows:
(1) The air flow rate had a greater influence on the performance of the MHPA-AWHE than the water flow rate. The maximum heat transfer efficiency was 81.4%, analyzed by the ε-NTU method when Cmin/Cmax was at 0.25. The maximum heat transfer rate was 9.29 kW, when the maximum air flow rate was 3000 m3/h.
(2) The MHPA components showed excellent performances. The temperature difference between the evaporation and condensation sections was within 1.3 °C. The equivalent thermal conductivity reached 5.01 × 104 W/(m·K) when the temperature difference between the air and water side was 13 °C.
(3) The maximum pressure drop of the air side was 339.8 Pa, and the maximum pressure drop of the water side was 8.86 kPa. The pressure drops of both sides were at a low level in the experiment.
(4) The comprehensive evaluation index j/f1/2 represented the comprehensive performance of heat transfer and resistance characteristics, which increased by 10.8% compared to the plate–fin heat exchanger.
(5) The MHPA-AWHE modules were proposed to be applied to a small DC in Beijing, and the method of energy saving analysis can be adopted for further application in DCs. During the annual hours analyzed, the shortest investment payback period was 2.3 years, and the control strategy was carried out. The critical temperature of the MHPA-AWHE system suitable for operation is 18.4 °C in Beijing. The annual power consumption after the transformation was reduced by 28.32% compared with that before the modification.