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.     

The analysis of a solar‐assisted desiccant evaporative cooling system and its application

Abstract

The direct application of an evaporative cooling system is impractical in Taiwan due to local hot and humid weather conditions. In this study, a hybrid cooling system utilizing an evaporative cooler coupled with a chemical dehumidifier is investigated. The solid desiccant, or silica gel, which could be regenerated by solar energy, dehumidifies the incoming air while the evaporative cooler effectively cools it down to the indoor comfort condition. An optimal design approach including a sensitivity study was performed during the computer simulation process. A systematic result was obtained providing design information, such as the desiccant consumption rate and solar collector area needed for indoor comfort air‐conditioning.     

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。     

Effect of desiccant isotherm on the performance of desiccant wheel

Numerical simulation has been conducted for the desiccant wheel, which is the crucial component of a desiccant cooling system. As the key operating/design parameters, the rotation speed and the area ratio of regeneration to dehumidification have been examined for a range of regeneration temperature from 50 °C to 150 °C. Optimization of these parameters is conducted based on the wheel performance evaluated by means of Moisture Removal Capacity (MRC). Simulations are focused on the effect of desiccant isotherm on the optimal conditions of these operating/design parameters. Also the effects of the outdoor air temperature and humidity on the optimum design parameters are examined.     

Exergoeconomic performances of the desiccant-evaporative air-conditioning system at different regeneration and reference temperatures

This paper presented the exergoeconomic evaluation of the developed desiccant-evaporative air-conditioning system. The developed system was evaluated based on the steady-state conditions at different regeneration and reference temperatures. The exergoeconomic evaluation method was implemented to the system components and the whole system to evaluate the exergy efficiency, exergy destruction ratios, cost rates, relative cost differences and exergoeconomic factors. The regeneration and reference temperatures affected the exergy efficiencies, exergy destruction ratios, cost rates, relative cost differences and exergoeconomic factors. The desiccant wheel, heating coil and evaporative cooler had a high cost rate (investment cost, operation and maintenance cost, and exergy destruction cost). The exit air fan, outdoor air fan and evaporative cooler had a high relative cost difference. The exit air fan, outdoor air fan and secondary heat exchanger had a high exergoeconomic factor. Replacement of the desiccant wheel with a higher dehumidification performance could decrease the high cost rate. A higher efficiency evaporative cooler and heating coil were needed. Cheaper air fans (outdoor air fans and exit air fans) were needed.     

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.     

Indoor temperature variations resulting from solar desiccant cooling in a building without thermal backup

The performance of a once-through solar desiccant cooling system, for air-conditioning a commercial office space, was modelled using the TRNSYS computer simulation software package. The study particularly focused on the potential for designing and operating a desiccant cooling system, without any thermal backup provided to mitigate for intermittent solar availability in three Australian cities.Increasing (i) indirect evaporative cooler effectiveness, (ii) air flow to the office space, and (iii) solar collector area were all shown to reduce the frequency of high temperature events inside the building occupied space. In the warm temperate climate of Melbourne (and to a lesser extent Sydney), high ventilation rates enabled comfort conditions to be maintained at or near acceptable levels in the occupied space, without the use of a backup thermal source.The synergy between evaporative cooling and solar desiccant cooling, observed in the warm temperate climates, was not evident in the tropical Darwin climate, suggesting that the selected ventilation desiccant cooling cycle is not appropriate for tropical climates.     

Evaluation of solid-desiccant-based evaporative cooling cycles for typical hot and humid climates

This paper presents an evaluation of various solid desiccant cycles for air conditioning in hot and humid climates. Psychometric evaluation of potential cycles for 16 typical Indian cities has been carried out with the objective of achieving standard comfort conditions in the room. Computer simulation is based on constant effectiveness of heat exchangers and evaporative coolers and actual performance data of a commercially available desiccant wheel dehumidifier. The influence of various outdoor conditions, the effectiveness of heat exchangers/evaporative coolers on the cooling coefficient of performance and volumetric air flowrate per unit cooling capacity have been investigated. It is found that amongst ventilation, recirculation and Dunkle cycles, the Dunkle cycle is better for a wide range of outdoor conditions. However, cycles using wet surface heat exchangers give even higher performance.     

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.     

基于露点板式间接蒸发冷却器空调机组特性的探讨

通过对目前国内外露点间接蒸发冷却技术的发展进行介绍,提出一种基于露点板式间接蒸发冷却器的空调机组—露点间接—直接蒸发冷却空调机组。并对此空调机组的原理和特性进行了分析,结果表明这种空调机组与传统露点间接蒸发冷却器相比,温降幅度大,可以进一步逼近室外空气的露点温度,是露点间接蒸发冷却技术的一个新发展。同时还对这一机组的性能测试提出了一些研究方法。     

Analysis of an air cycle refrigerator driving air conditioning system integrated desiccant system

This study presents theoretical investigation on the performance of air cycle refrigerator driving air conditioning system integrated desiccant system. Total system performance is evaluated and the system feasibility is examined. The system has such characteristics that (1) safe material of air and water are used as a refrigerant, (2) waste heat from air cycle refrigerator performs the regeneration of desiccant material for energy saving. It has been clarifying that (1) controlling the evaporative cooling process in air washer, the system can operate for a wide range of cooling loads, (2) the total coefficient of performance on air conditioning system is better than the conventional vapor compression system with reheating coil, and (3) the system performance highly depends on the ratio of the amount of outdoor intake air to the supply air.     

直接蒸发冷风机在西安地区应用的性能测试及方案改进

为了解决直接蒸发冷风机在西安地区应用中存在的问题,对冷风机进行现场测试,获得其温度、相对湿度、冷却效率、风速、流量、阻力及噪声等技术参数。根据对测试结果的分析,认为需大力开发间接蒸发冷却器,才能弥补直接蒸发冷风机的缺陷。在要求降温幅度大的场所,可以将直接蒸发冷却与间接蒸发冷却相结合,作成多级蒸发冷却机组,为蒸发冷却技术的大力推广奠定基础。同时指出应对填料进行优化设计以降低填料的阻力,改进风机、风管以降低噪声。     

Description and experimental results of a semi-indirect ceramic evaporative cooler

Evaporative cooling is used in industrial and air conditioning processes to reduce temperature in different fluids. Direct evaporation systems can lead to environmental problems such as Legionnaire's disease, and indirect systems reduce system efficiency.This work presents the manufacture, test bed set up and trials carried out on a ceramic evaporative cooling system which acts as a semi-indirect cooler. Depending on air characteristics, it may act as a sensible or enthalpic exchanger. The water cooled in a cooling tower, using the return air coming from the conditioned room (22 °C and 50% comfort conditions) goes through the ceramic pipes, exchanging sensible and latent heat with a current of outdoor air.The use of this recovery system is mainly in climates with a high temperature and humidity such as tropical environments where the system yields a decrease in supply air humidity, using the cooling power of return air.The tests presented show the system behaviour for various supply air conditions.     

Performance analysis of a solar cooling plant based on a liquid desiccant evaporative cooler

Summer air conditioning represents a growing market in buildings worldwide, with a significant growth rate observed in European commercial and residential buildings. Available heat driven cooling technologies can be used in combination with solar thermal collectors to reduce the load caused by air conditioning on the electric utilities and to reduce the environmental impact. This work reports a performance analysis of an open cycle solar cooling plant. The plant, installed in Northern Italy, is based on a liquid desiccant evaporative cooler coupled with a solar field. Experimental tests run during summer show average primary energy ratio and primary energy saving index of about 1.6 and 30%, respectively. Though this performance is satisfactorily, the regeneration unit always operated near the lower bound of the nominal temperature range. Therefore, optimization of the solar system design could lead to higher performance.     

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.     

Mathematical model and performance analysis of a novel outside evaporative cooling liquid desiccant dehumidifier

A numerical model of a novel outside evaporative cooling liquid desiccant dehumidifier (OECD) was developed and the effects of inlet parameters, including the inlet temperature and relative humidity of dehumidified air and evaporative cooling (EC) air, as well as the inlet mass flow rate of solution and so on, on the device performances were investigated in this paper. The results show that as the inlet temperature of solution increased from 31 to 42 °C, the moisture removal rates of OECD were increased by 14.0–18.0% and 31.1–101.5% compared to the non-evaporative cooling dehumidifier (NECD) and the adiabatic dehumidifier respectively, whereas the dehumidification rate was only decreased by about 1.6% with increase in the inlet temperature of LiCl solution from 24 to 44 °C. All these results can provide guidance for the structural design and performance analysis of the dehumidifier in the future.     

基于溶液除湿潜能释能的制冷系统的构建与研究

基于溶液除湿潜能的蓄能模式，构建一种新型除湿潜能释能冷水机组，以给空调系统提供冷源，特别是为辐射供冷的空调系统提供冷源途径。介绍了新构建系统的流程形式与工作过程，阐述了潜能蓄能机理，从理论上计算分析了该系统的单位理想制冷量随蒸发温度、冷却水温度和溶液浓度的变化关系，数据结果表明该系统在一定工况下能获取空调用冷水，并具有较大的单位理想制冷量，单位理想制冷量能达到10kW，该系统所能获取冷冻水的理论最低温度为除湿干燥后湿空气的露点温度。     

微型直吹式空调研究

微型直吹式空调体积较小、便于携带,且可以达到环保、节能目的。既可用于户外也可用于室内进行小范围内的调温工作。通过对半导体制冷系统不同工作电压、环境温度及外部散热条件的研究,找出了影响半导体制冷性能的主要因素。确定了该微型直吹式空调在不同环境温度下的最佳工作电压为12V、最佳工作电流为4.5～4.8A,并认定存在最佳热端散热强度等。另外,通过实验得出了在不同环境温度下,工作在最佳工作参数条件时制冷器内部所能达到的最低温度。     

蒸发冷却式热管与蒸气压缩复合数据中心空调系统性能实验研究

为解决传统数据中心空调系统能耗高和冷却效率低等问题,本文提出了带有蒸发式冷凝器的制冷剂泵驱动热管与蒸气压缩复合数据中心空调系统,实验分析了不同室外温度与冷凝器风速下系统的运行性能.结果表明:在热管模式下,当室外温度低于0℃时,降低冷凝器风速能够提升系统COP;当室外温度高于0℃时,增大室外机风速能够提高系统节能性.降低...     

Thermodynamic analysis and optimization of a novel N2O–CO2 cascade system for refrigeration and heating

In this study, a natural refrigerant based cascaded system, with nitrous oxide as the low temperature fluid and carbon dioxide as the high temperature fluid, is analyzed for simultaneous cooling and heating applications. Effects of significant design and operating parameters on system performance are studied. Optimization of intermediate pressure for maximum COP for various design and operating parameters are presented as well. Results show that use of internal heat exchanger has marginal influence on system performance. Due to similar thermodynamic properties of nitrous oxide and carbon dioxide, the optimized intermediate temperature turns out to be independent of the performance of gas cooler and evaporator for a given operating condition. Due to the same reason, N₂O as low temperature fluid and CO₂ as high temperature fluid in a cascade arrangement exhibit similar behavioural trends in a system where the fluids are swapped.