Effect of regeneration temperatures in the exergetic performances of the developed desiccant-evaporative air-conditioning system

The developed desiccant-evaporative air-conditioning system was evaluated using the exergetic method under controlled environmental conditions to determine the performances of the whole system and its components.Percentage contributions of exergy destruction of system components at different regeneration temperatures and reference temperatures were determined. Exergy destruction coefficient of different components at different regeneration and reference temperatures were presented. It was shown that exergetic performances varied with respect to the regeneration and reference temperatures. The exergetic performances based on thermal, electric, total exergy input, first definition and second definition efficiencies were shown.Based on the results, reference and regeneration temperatures affected the determination of the system performances and its components. It was shown that air-heating coil, air fans and desiccant wheel contributed to large percentage of exergy destruction. Hence, the mentioned components should be given attention for further improvement of the system performances.     

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.     

一种新型前置预冷转轮除湿复合空调系统

提出了一种新型前置预冷转轮除湿复合空调系统,除湿转轮采用第Ⅲ类吸湿剂。对利用该系统用于独立新风系统的可行性进行了性能分析。结果表明,在华南地区夏季两种典型工况下,该转轮除湿采用再生温度为45℃就可以满足室内湿度的要求;对于相对湿度为90％且温度低于36.7℃的高温高湿工况,该转轮除湿采用再生温度为60℃仍可以满足室内湿度的要求;但与除湿转轮采用第Ⅰ类或第Ⅱ类吸湿剂相比,不能达到更低的送风露点温度。因此,该系统方案所需再生温度较低,较适用于高品质/舒适性空调方面。     

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.     

Contributions of system components and operating conditions to the performance of desiccant cooling systems

This study systematically analyzes the effect of various kinds of design parameters on the performance of a desiccant cooling system under two different system configurations. The considered parameters include system components such as the sensible heat exchanger, regenerative evaporative cooler and desiccant wheel, as well as operating conditions of outdoor conditions, regenerative temperature and rate of outdoor influx. Numerical simulation has been conducted for these 11 design parameters with 3 levels. The orthogonal array L₂₇(3¹³) is adopted for the analysis of variance. In the range of the parameters considered, the regenerative temperature is found to be the most dominant parameter of contribution ratio of 31.9% and 23.9% for each system configuration. In the case of confined interest of the applications such as a district cooling system or a solar system using medium-temperature collectors, the cooling performance of the regenerative evaporative cooler is the most crucial for the system performance.     

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.     

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.     

Characterization of desiccant wheels with alternative materials at low regeneration temperatures

A number of new desiccant materials have been proposed which have the potential to improve the performance of desiccant wheels being regenerated at low temperature. Desiccant wheels containing two such desiccant materials (zeolite and superadsorbent polymer) were compared with a conventional silica gel desiccant wheel. The superadsorbent polymer desiccant wheel achieved greater dehumidification than the silica gel wheel when dehumidifying high relative humidity air with low temperature (50 °C) regeneration air. The temperature of dehumidified air exiting the polymer wheel was also lower. The zeolite desiccant wheel was generally less effective at dehumidifying air and had a higher pressure drop.     

中湿度地区蒸发冷却复合式空调系统与传统集中(半集中)式空调系统的比较分析

以西安地区某办公楼为代表,通过对西安地区气象参数的统计,对中湿度地区蒸发冷却与机械制冷相结合的集中式空调系统、蒸发冷却与风机盘管相结合的半集中式空调系统,分别与传统的集中式和半集中式空调系统进行了对比分析。结果表明,采用蒸发冷却与传统空调方式的结合,可以大大减少冷水机组装机冷量和运行时间,从而节约了运行成本,提高了系统的能效比,为中湿度地区蒸发冷却工程应用提供了依据。     

A comparison of refrigerants in a two-stage ejector-expansion transcritical refrigeration cycle based on exergoeconomic and environmental analysis

A comprehensive energy, exergy, exergoeconomic and environmental comparison between carbon dioxide, ethane and nitrous oxide as the refrigerants of a two-stage ejector-expansion transcritical refrigeration cycle is carried out. All of the obtained results are attained by optimizing COP subject to gas cooler and intercooler pressures in different values of gas cooler and evaporator temperatures. It is observed that the compressors operating pressure and temperature levels in the cycle for ethane are lower than other refrigerants, which leads to higher system safety and lifetime. Furthermore, the highest COP and exergy efficiency in a wide range of gas cooler temperature belongs to the ethane. The nitrous oxide refrigerant has the lowest product unit cost, which is about 4.2% lower than that of the ethane refrigerant with the highest product unit cost. Therefore, ethane is the most preferable refrigerant from energy and exergy aspects and nitrous oxide is suitable based on exergoeconomic viewpoint.     

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.     

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.     

蒸发式冷风扇在干燥地区的性能分析

针对室内蒸发式冷风扇加排气扇的应用方式,风量与排气扇排风量的理论计算公式。计算结果表明,性要求,并将其与传统的蒸发式空调进行对比分析。研究了其室内空气处理过程,得出了蒸发式冷风扇在干燥地区,蒸发式冷风扇完全可以达到室内舒适     

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.     

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。     

基于CFD方法的间接蒸发冷却器(火用)分析

采用CFD方法模拟分析间接蒸发冷却器的传热传质过程，讨论通道间距、空气流速、温度、相对湿度等因素对换热效率和[火用]效率的影响。结果表明基于湿空气[火用]分析理论的评价方法更合理，为间接蒸发冷却器设计提供依据。     

独立新风空调系统(DOAS)的节能分析

由于SARS疫情、恐怖主义活动及病态建筑综合症等影响着建筑环境,故为解决这些问题而提出的独立新风系统逐渐受到重视.本文论述分析了独立新风系统的节能特性;比较了相同负荷下,风机盘管加新风系统和风机盘管式独立新风系统的不同能耗,结果表明独立新风系统是一种既节能又安全的空调系统.     

Analytical modeling of a desiccant wheel

A simple integral model is presented for a desiccant wheel. The original governing equations for a desiccant wheel were simplified to a set of linear ordinary differential equations and an analytical solution was obtained. A brief analysis is given about the solution regarding the non-dimensional numbers that decide the behavior of a desiccant wheel. From the solution, algebraic expressions were obtained for time-averaged heat and mass transfer rates and the results were compared with a numerical model and a set of experimental data in the literature. In comparison with the numerical model, relative error was found less than 12% at 120 °C regeneration temperature and 10% standard deviation was observed with the experimental data. The analytical model is considered capable of describing a symmetric desiccant wheel realistically.     

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.     

利用蒸发式冷凝器改善高温环境下R410a空调器性能的研究

R410a空调器在高温环境下制冷能力显著下降.本文通过凝结水量的计算及喷淋装置的设计,利用凝结水、喷淋管、冷却盘管、冷却风机构成的蒸发式冷凝器来改善环境高温下R410a空调器的性能.