• Energy Research

Free cooling based on loop thermosyphon is an ideal method for energy-saving of data centers. In order to promote its future application, it is necessary to investigate the performance of utilizing environment friendly working fluids. In this paper, an experiment based on enthalpy difference method and thermal imaging is conducted on a CO2 loop thermosyphon with microchannel parallel-flow evaporator and condenser. The results show that the effect of filling ratio on CO2 loop thermosyphon is more significant and the optimal filling ratio is higher, compared with R22 loop thermosyphon. Thermal image of the evaporator shows that superheated region shrinks and then disappears with the increase of filling ratio. Moreover, to analyze the relationship between the phenomenon and the internal flow state, a distributed-parameter model is built and validated. The simulation results show that 150% is the optimal filling ratio for no superheating or subcooling occurs along the loop. The effects of height difference and pipe length are more significant for low filling ratio. Superheating is easier to occur at the evaporator outlet when height difference is smaller or pipe length is larger, and subcooling is easier to occur at the condenser outlet when height difference or pipe length is smaller.

Abstract Integrating evaporative cooling with loop thermosyphon can significantly improve the free cooling ability. In this paper, a loop thermosyphon with evaporative condenser is investigated experimentally. The mist water flow out of a single nozzle is observed and analyzed. The performance and annual free cooling potential of the system are investigated, compared with conventional loop thermosyphon. The results show that the evaporative cooling effect of the locations below the horizontal level of the nozzle is more significant due to gravity. With the increase of horizontal distance from the nozzle, the temperature decreases and then increases and the optimal distance is 200–400 mm. The heat capacity of LTEC increases with the increase of indoor and outdoor temperature difference while it decreases with the increase of humidity. Evaporative cooling can enhance the heat transfer of LTEC by 7%–33% compared with loop thermosyphon with conventional condenser, and this value is larger for smaller indoor and outdoor temperature difference and higher indoor temperature. LTEC can expand the annual free cooling time by 7%–14% compared with LTCC and the effect is more significant in regions with drier weather.

Abstract Nanofluids show clearly a very interesting alternative for working fluids of microchannels. This paper presents an experimental study on heat transfer and flow features of Al 2 O 3 –water nanofluids through a circular microchannel with the 0.5 mm inner diameter. Al 2 O 3 –water nanofluids with particle volume concentrations of 0.25%, 0.51% and 0.77% are prepared. The effect of particle concentration and Reynolds number on the heat transfer and flow features of nanofluids-cooled microchannel are examined and then compared with the data for water-cooled microchannel. The results show that Nusselt number of Al 2 O 3 -water nanofluids is higher than that of deionized water, and increases with increasing Reynolds number and particle concentration. The maximum heat transfer enhancement achieves 10.6% in the test conditions. Meanwhile, the enhancement effect is more apparent at higher Reynolds number in the range of laminar flow. A Nusselt number correlation for nanofluids flowing in microchannels is proposed, and the predicted Nusselt numbers agree with the experimental data with deviations of −4% and +5%. The friction factor of the nanofluids has an increase within 7.9% compared with that of deionized water. A friction factor correlation is proposed, and the predictions agree with the experimental values with deviations of −3% and +5%.

AbstractMulti-unit air conditioners (MUACs) keep fast increasing for its high efficiency and thermal comfort. A novel method, performance chart, is presented for the performance analysis of MUACs. The performance chart is generated by superimposing the performance of indoor units and outdoor unit, which can be obtained by simulations and/or experiments. The performance chart is validated by the experiments and detailed simulation models. Based on the performance chart, the performance of MUACs is analyzed at conditions of variable indoor temperatures. The performance chart can give the reasonable and quantitative trends of MUAC's performance and guide the system design, indicating that it can be used as an effective and convenient tool for performance analysis and system optimization of MUACs.

Abstract Free cooling based on loop thermosyphon is ideal energy-saving method for data centers. Most working fluids presently used in this field will cause greenhouse effect or destroy the ozone layer. It is necessary to investigate the applicability of environment friendly fluids. A distributed-parameter model of a CO2 loop thermosyphon in an integrated air conditioning system for free cooling is built and validated. The performance is compared with traditional working fluids and the effects of some key geometric parameters are evaluated. The results show that the optimal filling ratios for CO2, R22 and R134a are 120%, 100% and 90%, respectively. The circulation flow rate of CO2 is much smaller than those of R22 and R134a. For CO2 loop thermosyphon, the heat transfer rate decreases with the increase of pipe length, while increases with the increase of riser diameter and height difference. The heat transfer rate increases with the increase of tube number and length of the evaporator, while the increasing rate decreases gradually. The optimal tube number and length are 80 and 0.6 m, respectively. The relationships between the above phenomenon and the internal flow state are also analyzed.

Abstract Free cooling based on thermosyphon is a promising technology for energy-saving in data centers. Integrated system of mechanical refrigeration and thermosyphon does not need auxiliary refrigeration system, therefore it is an ideal way of free cooling based on thermosyphon. To study its free cooling performance and potential, it is built that a distributed-parameter model of the thermosyphon mode in an integrated system of mechanical refrigeration and thermosyphon and validated by experimental data. The simulation results show that the cooling capacity and circulation flow rate increase with the increase of height difference mainly due to higher driving force of gravity, while the increase is little when the height difference is higher than 0.5 m. The cooling capacity and circulation flow rate decrease with the increase of connection pipe length due to higher flow resistance, therefore the connection pipe should be as short as possible. The cooling capacity increases rapidly with the temperature difference and reaches 5.3 kW when the temperature difference is 15 °C, with an EER of 14.3. Also with the performance model and weather data of China, annual free cooling potential in China and corresponding energy-saving and economic benefits are analyzed. For the studied cities in China except those in the hot summer & warm winter zone, the free cooling percentages are approximately 30–70%, the annual energy-saving rates are 16–49% and the payback period is 1.7–4.3 years. Therefore, free cooling based on integrated system of mechanical refrigeration and thermosyphon has great application potential for energy-saving of data centers.

AbstractFree cooling is one of the most potential ways of reducing the energy consumption of data centers. Thermosyphon has superior heat transferability thus it is suitable for application in free cooling. However, a separate thermosyphon free cooling system usually cannot provide sufficient cooling capacity in warm seasons. In order to overcome this weakness and avoid two sets of equipment, hybrid system of thermosyphon free cooling and vapor compression refrigeration for data centers has been developed by researchers. The cooling performance of this kind of system depends greatly on the climate outside therefore its applicability in different time and zones needs to be studied. In this paper, the climate data from five cities of different climate zones in China is collected. The performance model of the hybrid system is built up. Based on the climate data and performance model, the annual energy consumption is then calculated and compared with traditional air conditioner. The result shows that the energy-saving potential of the hybrid system varies in different climate zones, and when the mode switching temperature is relatively high, it is distinct in most climate zones of China.

Abstract Loop thermosyphon with multiple evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.

Abstract Hybrid energy source heat pump system is one of the best solutions to achieve efficient and stable utilization of renewable energies such as solar energy, geothermal energy, etc. The Three-fluid heat exchanger can realize heat transfer from the refrigerant to the outside air and the inside water simultaneously or independently, which will keep the refrigerant well distributed under difference working modes. The simulation model is built and validated for performance analysis of the three fluid heat exchanger based on Matlab and REFPROP. Three heat exchangers with different flow path designs are investigated for the heat transfer and pressure loss. The results show that the heat exchanger can give better performance by optimization of heat transfer coefficient and the pressure loss. Moreover, the influence on the heat exchanger from the variation of the air flow speed, air temperature, water flow speed and water temperature are also been analyzed.