Optimisation of a desiccant cooling system design with indirect evaporative cooler

Solar desiccant-based air-conditioning has the potential to significantly reduce cost and/or greenhouse gas emissions associated with cooling of buildings. Parasitic energy consumption for the operation of supply fans has been identified as a major hindrance to achieving these savings. The cooling performance is governed by the trade-off between supplying larger flow-rates of cool air or lower flow-rates of cold air. The performance of a combined solid desiccant-indirect evaporative cooler system is analysed by solving the heat and mass transfer equations for both components simultaneously. Focus is placed on varying the desiccant wheel supply/regeneration and indirect cooler secondary/primary air-flow ratios. Results show that for an ambient reference condition, and 70 °C regeneration temperature, a supply/regeneration flow ratio of 0.67 and an indirect cooler secondary/primary flow ratio of 0.3 gives the best performance with COP_e > 20. The proposed cooling system thus has potential to achieve substantial energy and greenhouse gas emission savings.     

Use of compound desiccant to develop high performance desiccant cooling system

The paper is aimed to develop a high performance rotary solid desiccant cooling system using a novel compound desiccant wheel (DW). The unique feature of the desiccant wheel is that it can work well under a lower regeneration temperature and have a higher dehumidification capacity due to the contribution of the new compound desiccant materials. Experimental results indicate that the novel desiccant wheel under practical operation can remove more moisture from the process air by about 20–40% over the desiccant wheel employing regular silica gel. A mathematical model that is used to predict the system performance has been validated with the test results. By integrating the desiccant wheel with evaporative cooling, heat recovery and heating for regeneration sections, a solid desiccant cooling system can be formed. Simulation results show that because of the use of the new compound desiccant, the desiccant cooling system can work under much lower regeneration temperature and have a relative high COP, thus low grade thermal energy resources, such as solar energy, waste heat, etc., can be efficiently utilized to drive such a cooling cycle.     

Thermodynamic performance assessment of a novel air cooling cycle: Maisotsenko cycle

This study presents energy and exergy analyses and sustainability assessment of the novel evaporative air cooling system based on Maisotsenko cycle which allows the product fluid to be cooled in to a dew point temperature of the incoming air. In the energy analysis, Maisotsenko cycle’s wet-bulb and dew point effectiveness, COP and primary energy ratio rates are calculated. Exergy analysis of the system is then carried out for six reference temperatures ranging from 0 °C to 23.88 °C as the incoming air (surrounding) temperature. The specific flow exergy, exergy input, exergy output, exergy destruction, exergy loss, exergy efficiency, exergetic COP, primary exergy ratio and entropy generation rates are determined for various cases. Furthermore, sustainability assessment is obtained using sustainability index method. As a result, maximum exergy efficiency is found to be 19.14% for a reference temperature of 23.88 °C where the optimum operation takes place.     

Potential of a desiccant-evaporative cooling system performance in a multi-climate country

In this paper, a desiccant-evaporative cooling system introduced and applied to a Ventilation and Makeup mode operating cycle. Desiccant part of hybrid cooling system is a heat driven component and effective in area where the use of thermal energy is more economical than electrical power. First, mathematical model of desiccant component based on transient and coupled heat and mass transfer derived. Then the hybrid system model applied to predict the system performance under various operational conditions. The numerical results validated using experimental measurements. The effects of various outdoor design conditions on COP and output of hybrid cycle temperature presented in contour plot forms. Based on these contour plots, COP and output cycle temperature can easily obtain under various ambient conditions. In addition, the potential of presented hybrid desiccant-evaporative cooling system to provide thermal comfort in various outdoor design conditions evaluated and compared with direct and direct-indirect evaporative cooler. The results show these systems are more effective than direct and direct-indirect evaporative cooling systems and provide a better thermal comfort even in hot and humid area. Moreover, introduced systems successfully provide better thermal comfort condition in a multi-climate country (Iran) especially in the area where the evaporative coolers are not applicable.     

开式循环吸附制冷过程的实验研究

本文对基于开式循环的吸附除湿空调制冷系统进行了实验研究。实验证明,该空调制冷系统可用８５℃以上的热源作动力,制冷效率可以超过４０％ 。这样,可利用太阳能或工业余热驱动该制冷系统, 减少常规蒸汽压缩式制冷产生的氟里昂及热岛对空气的污染。本文取得的实验数据为开式吸附空调制冷系统的进一步完善及中试设计提供了可靠依据。     

Experimental evaluation and performance enhancement prediction of desiccant assisted separate sensible and latent cooling air-conditioning system

CO₂ and R410A desiccant wheel (DW)-assisted separate sensible and latent cooling (SSLC) air-conditioning systems were tested under the AHRI standard. At a 50 °C regeneration temperature, the coefficient of performance (COP) of the vapor compression cycles improved only 7% from the respective baseline systems for both refrigerants. This paper proposed the idea of applying divided condensers (or gas coolers) to the R410A (or CO₂) SSLC system to enhance its performance. It was found that the application of divided heat exchangers to the SSLC system provided sufficiently hot airflow for regenerating the desiccant wheel at both a reduced high side pressure (from 10.4 MPa to 9.7 MPa for CO₂, from 3.46 MPa to 3.45 MPa for R410A) and a reduced discharge temperature from the condenser (gas cooler) (4 K lower for both refrigerants). The COP improvement is 36% and 61% to R410A and CO₂ baseline systems, respectively.     

Performance analysis of desiccant dehumidification systems driven by low-grade heat source

If a desiccant dehumidification system can be driven by a heat source whose temperature is below 50 °C, exhaust heat from devices such as fuel cells or air conditioners can be used as its heat source, thereby saving energy. Therefore, in this study, we used a previously validated simulation model to determine the minimum heat source temperature for driving a desiccant dehumidification system. We considered four desiccant dehumidification systems that can be driven by waste heat—conventional desiccant-type systems (wheel type and batch type with only desiccant), a system with a precooler, double-stage-type systems (a type with two desiccant wheels and a four-partition desiccant wheel type), and a batch-type system with an internal heat exchanger. We found that among these systems, the last system can be driven by the lowest heated air temperature—approximately 33 °C—which is considerably lower than that of the conventional system.     

Design and performance of an internal heat exchange desiccant wheel

A fundamental limitation in the dehumidification performance of adiabatic desiccant wheels occurs due to heating of the air stream to be dried. This results from both the carry-over of heat stored in the wheel and the release of the adsorption heat. Previous authors have identified an isothermal dehumidification process as theoretically superior, though the practicalities of constructing such a wheel have meant that demonstrating the benefit is difficult. Recently experimental data from testing of an internally cooled wheel was published. Here we use this data to calibrate a mathematical heat and mass transfer model of the internally cooled heat exchange desiccant wheel. The model is then used to estimate the performance for selected modifications to the design and materials. The results show that more than a 40% improvement is possible relative to the previously tested cooled wheel. The results have application to the development of desiccant air-conditioners.     

Influence of the atmospheric pressure on the mass transfer rate of desiccant wheels

The performance of a desiccant wheel is evaluated by modelling a representative channel. The hypothesis of negligible resistances to heat and mass transfer in the cross-direction is assumed in the thin porous desiccant wall of the channels and the airflow is treated as a bulk flow. Parametric studies were conducted to investigate the influence of the atmospheric pressure decrease from 101,325 Pa to 60,000 Pa (0-4217 m of altitude) on the mass transfer rate of desiccant wheels considering distinct channel lengths and different inlet airflow rates, a large range of values of the rotation speed, as well as three alternative ways to specify the inlet conditions of the regeneration and of the process airflows. A procedure to derive correlations based on the numerical results is presented for the correction factor of the mass transfer rate when a desiccant wheel is operating at non-standard atmospheric conditions. Four parametric studies were performed, the derived correlations were tested and a good agreement was found between the estimated correction factor and the correction factor calculated after the numerical results.     

Investigation on capacity matching in liquid desiccant and heat pump hybrid air-conditioning systems

Liquid desiccant and heat pump (LDHP) hybrid air-conditioning system provides a promising independent air dehumidification solution. Capacity matching among the four major heat and mass transfer components, i.e. dehumidifier, regenerator, evaporator and condenser, is essentially important for energy efficiency of the hybrid system. In this paper, the configuration of the hybrid system is firstly studied. Novel matching indices are proposed to evaluate the matching effect. The results show that a LDHP hybrid system with double-condenser, one solution-cooled and one air-cooled, is a feasible configuration for achieving capacity matching. To achieve dynamic capacity matching under real changing operating conditions, the effects of three critical operating variables, including solution flow rate, revolution of the compressor and air flow rate in the air-cooled condenser, on capacity matching and energy performance are studied. Simulation results show that dynamic capacity matching can only be achieved by regulating these three operating variables simultaneously.     

电主轴冷却系统设计与仿真优化

为解决电主轴因内部温度场复杂而造成冷却效果差的问题,设计了一种用于电主轴冷却的水冷机系统。根据电主轴热特性分析结果,提出了水冷机冷却方案,计算了相关的传热参数,并建立了电主轴温度-流速控制模型。然后,利用ANSYS Fluent软件对电主轴进行了流体冷却有限元仿真,并通过电主轴冷却实验对仿真结果进行了验证。通过对比仿真结果和实验结果可知,冷却后电主轴电机定子最高温度约下降了60%,转轴的形变量约降低了70%。结果表明：利用水冷机系统对电主轴进行冷却具有良好的冷却效果,这可为高精密机床主动热控制技术的研究提供一定的借鉴和参考。     

In-orbit performance of a helium dewar for the soft X-ray spectrometer onboard ASTRO-H

ASTRO-H was an X-ray astronomy satellite that the Japan Aerospace Exploration Agency (JAXA) developed to study the evolution of the universe and physical phenomena yet to be discovered. The primary scientific instrument of ASTRO-H was the Soft X-ray Spectrometer (SXS). Its detectors were to be cooled to 50 m K using a complex cryogenic system with a multistage adiabatic demagnetization refrigerator (ADR) developed by the National Aeronautics and Space Administration (NASA), and a cryogenic system developed by Sumitomo Heavy Industries, Ltd. (SHI). SHI’s cryogenic system was required to cool the ADR’s heatsink to 1.3 K or less in orbit for three years or longer. To meet these requirements, SHI developed a hybrid cryogenic system consisting of a liquid helium tank, a 4 K Joule-Thomson cooler, and two two-stage Stirling coolers.ASTRO-H was launched from Tanegashima Space Center on February 17, 2016. The initial operation of the SXS cryogenic system in orbit was completed successfully. The cooling performance was as expected and could have exceeded the lifetime requirement of three years.This paper describes results of ground tests, results of top-off filling of superfluid liquid helium just before launch, and cooling performance in orbit.     

Flight model performance test results of a helium dewar for the soft X-ray spectrometer onboard ASTRO-H

ASTRO-H is a Japanese X-ray astronomy satellite, scheduled to be launched in fiscal year 2015. The mission includes a soft X-ray spectrometer instrument (SXS), which contains an X-ray micro calorimeter operating at 50 mK by using an adiabatic demagnetization refrigerator (ADR). The heat sink of the ADR is superfluid liquid helium below 1.3 K. The required lifetime of the superfluid helium is 3 years or more. In order to realize this lifetime, we have improved the thermal performance from the engineering model (EM) while maintaining the mechanical performance. Then, we have performed a thermal test of the flight model (FM). The results were that the heat load to the helium tank was reduced to below 0.8 mW in the FM from 1.2 mW in the EM. Therefore, the lifetime of the superfluid helium is more than 3 years with 30 L of liquid helium.In this paper, the thermal design and thermal test results are described.     

Neural model for forecasting temperature in a distribution network of cooling water supplied to systems producing petroleum products

This article presents the application of a neural model of heat transfer for the purpose of forecasting temperature at selected points of a circulating water ring network. The purpose of a circulating water system is to lower the temperature of petroleum products manufactured on numerous petrochemical lines at a Polish petrochemical plant. Temperature forecasting at 96 nodes of the circulating water system, significant from the point of view of system operation, is carried out using SVM neural networks. Neural networks learn based on archival data recorded in the process parameter monitoring system. Thermal, hydraulic and control parameters of the cooling process, as well as weather variables, constitute crucial input data for the neural model. The temperature forecasting algorithm has been implemented in a computer program that was then applied and remains in use for temperature forecasting in a maintenance department of an industrial plant.     

大功率半导体激光器全固态风冷散热系统

设计并制作了一种全固态大功率半导体激光阵列恒温散热系统。它利用半导体制冷器对大功率半导体激光阵列吸热,然后经由风冷散热。经测试,单bar激光阵列连续输出功率达到15.28W,双 bar 阵列输出达 27.8W 时,全部达到风冷散热控温精度±0.1 ;当环境温度达到 45 时,仍然能够保证激光阵列的正常使用。     

The potential of solar energy use in desiccant cooling cycles

The use of heat produced by solar thermal collectors is an interesting option for thermal driven air conditioning processes. A thermal driven cooling technique which fits well to non-tracking solar collectors is the desiccant cooling technique. Recently several projects have been carried out which focus on the connection of desiccant cooling systems with solar thermal energy for regeneration of the sorbents. This communication deals with three main topics: (1) experiences achieved in a realized system which is coupled to a solar collector are discussed, (2) a new concept is presented, in which a solar air collector is integrated into the desiccant cooling cycle as the only heat source and (3) a comparative study is presented which compares system performance for different system configurations and different climatic situations.     

回热式制冷机产冷量不足的原因及改进措施

针对制氮设备回热式制冷机产冷量不足的现象,分析了原因,并采取了相应措施,从而保证了制氮设备的液氮产量。     

太阳能/余热固体除湿冷却系统研究

提出并研制一种太阳能／余热驱动除湿冷却系统。系统包括2台内冷却紧凑式固体除湿器、热交换器、蒸发冷却器等部件。在不同工况下对系统的性能进行模拟计算，分析再生温度、热交换器效率及蒸发冷却器效率对系统性能的影响。     

Performance analysis of air cycle refrigerator integrated desiccant system for cooling and dehumidifying warehouse

In this paper, the performance of air cycle refrigerator integrated desiccant system used to cool and dehumidify warehouse is analyzed theoretically. Simulation analysis is carried out to calculate the system coefficient of performance, cooling effects and the humidity change under different values of pressure ratio, storage zone temperature inside dock and outdoor air conditions. Also, the effect of the air cycle and the rotor parameters on the system performance is evaluated. From the simulation result it is found that, the desiccant system has the ability to supply air to the dock area at very low humidity. The system coefficient of performance increases due to the exhaust heat recovery on the desiccant system, and this enhancement can be more than 100%. The coefficient of performance of the proposed system is greater than that of a conventional system under the same operating conditions.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.