Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

Research on a Refrigeration Dehumidification System with Membrane-Based Total Heat Recovery

An independent air dehumidification system is helpful to improve indoor air quality and decrease energy consumption by heating, ventilation, and air conditioning (HVAC). A refrigeration dehumidification system with membrane-based total heat recovery is the key equipment to realize this goal. The system comprises two subsystems: a membrane total heat recovery and a direct expansion refrigeration system. The total heat exchanger has a membrane core where the incoming fresh air exchanges moisture and temperature simultaneously with the exhaust air. In this manner, the total heat or enthalpy from the exhaust air is recovered. Then the fresh air flows through a cooling coil where it is dehumidified below the dewpoint. Finally, the cold and dry air is supplied to indoors. A prototype of practical application is designed and fabricated. Experiments are conducted under variable operating conditions in the psychrometric calorimeter chamber. The effects of varying operating conditions like temperature and air humidity on the air dehumidification rate, cooling power, coefficient of performance, and compressor power are evaluated with indoor exhaust air dry bulb 27°C, wet bulb 19°C, and fresh air flow rate 200 m³/h. In comparison with a conventional refrigeration dehumidification system, the coefficient of performance and air dehumidification rate of the prototype are 2.3 times and 3 times higher, respectively. The performance of the prototype is rather robust under a hot and humid environment.     

Experimental investigation of a multi-function heat pump under various operating modes

Heat pumps have been spotlighted as efficient building energy systems because they have great potentials for energy reduction in building air conditioning and reducing CO₂ emission. In this study, a multi-function heat pump which has the functions of heating, cooling, and hot water supply was designed and its performance was investigated according to operating modes. In the cooling-hot water mode, the capacity and COP were enhanced as compared to other modes because the waste heat from the outdoor heat exchanger was utilized as useful heat in the indoor heat exchanger. In the heating and hot water supply mode, the compressor speed should be increased to get appropriate heating and hot water capacities. For all operating modes, the system could be optimized by adjusting the superheat.     

Effect of ambient conditions on the first and second law performance of an open desiccant cooling process

An open desiccant cooling process is presented and applied to ventilation and recirculation modes of the system operation. The cooling system consists of a desiccant wheel, a rotary regenerator, two evaporative coolers, and a heating unit. Certain ideal operating characteristics based primarily on the first law of thermodynamics are assumed for each component. The system with indoor and outdoor ARI conditions has a thermal coefficient of performance (COP) of 1.17 in ventilation mode and 1.28 in recirculation mode. A second law analysis is also performed and at ARI conditions, the reversible COP of the system is determined to be 2.63 in ventilation mode and 3.04 in recirculation mode. Variation of the first and second law based COP terms and cooling load with respect to ambient temperature and relative humidity are investigated in both modes of the system operation. The results of the analysis provide an upper limit for the system performance at various ambient conditions and may serve as a model to which actual desiccant cooling systems may be compared. As an additional study, a non-ideal system operation is considered and it is determined that both the COP and cooling load decrease with increasing ambient temperature and relative humidity, and they approach zero at high values of ambient temperature and humidity.     

Control performance of a dedicated outdoor air system adopting liquid desiccant dehumidification

Liquid desiccant is energy efficient for dehumidification in air-conditioning systems. In this study, a novel dedicated outdoor air system (DOAS) adopting lithium chloride solution as liquid desiccant is proposed to process supply air. The DOAS mainly consists of a membrane-based total heat exchanger, a liquid dehumidifier, a regenerator and a dry cooling coil. It can realize independent temperature and humidity controls for supply air. Control strategies for the supply air dehumidification and cooling process as well as the desiccant solution regeneration process in the DOAS are developed and verified. The control performances of the proposed dedicated outdoor air system are investigated at different operation conditions by simulation tests. The results show that the DOAS is more suitable for hot and humid climates. The effects of the total heat exchanger on the performance of the DOAS are also evaluated. It can improve the system energy performance by 19.9–34.8%.     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

Experimental investigation of effect of ventilation strategies on the performance of cold storage

Fresh fruits and vegetables are living organisms that require fresh air to survive while being stored. Respiratory gases can build up and damage produce if there is not enough ventilation during storage. Ventilation is required to prevent an excessive build-up of respiratory gases and to maintain the freshness of produce. In hot and dry areas, air ventilation of refrigerated storage can have a negative impact on food produce quality and affect storage performance. An indoor experimental study was carried out to investigate the air ventilation of the cold room during various modes of air supply. The energy recovery device (ERD) with and without evaporative cooling arrangement is used in the air supply and the performance enhancement is compared. The ERD reduces the air ventilation load by more than 50% compared to direct ventilation of the cold room and consequently, it reduces the total power consumption of air ventilation. Furthermore, the overall cooling performance of the system is improved by up to 45.7% when the ERDs are used compared to the direct supply of fresh air during air change mode and the use of energy-saving equipment has also proven improvements in the desired air conditions for food production.     

夏季自然通风对农宅室内热环境影响的研究

以山西省运城地区农家住宅为研究对象,叙述了进行室内热环境参数现场测试的过程,分析了不同自然通风方案对室内能耗、舒适性及人工调节便捷性的影响特征,提出了运城地区夏季室内自然通风的理想调节方式。     

Performance evaluation of a radiant floor cooling system integrated with dehumidified ventilation

The radiant floor cooling system can be used as an alternative to all-air cooling systems, using the existing Ondol system (a radiant floor heating system) in Korea to save energy and maintain indoor thermal comfort. Unfortunately, a radiant floor cooling system may cause condensation on the floor surface under hot and humid conditions during the cooling season. In addition, the radiant floor system does not respond quickly to internal load changes due to the thermal storage effect of the concrete mass, which is usually present in radiant floor cooling systems.This study proposes a radiant floor cooling system integrated with dehumidified ventilation, which cools and dehumidifies the outdoor air entering through the cooling coil in the ventilator by lowering the dew-point temperature to prevent condensation on the floor surface. Furthermore, outdoor reset control was used to modulate the temperature of chilled water supplied to the radiant floor, and indoor temperature feedback control was then used to respond to the internal load changes.To evaluate the performance of the radiant floor cooling system integrated with dehumidified ventilation, both a physical experiment in a laboratory setting and TRNSYS simulation for an apartment in Korea have been conducted. As a result, it was found that the proposed system was not only able to solve the problem of condensation on a floor surface but also to control the indoor thermal environment within the acceptable range of comfort. Furthermore, the proposed system improved the responsiveness to internal load changes.     

FUTURE RESEARCH ACTIONS IN PASSIVE COOLING

The PASCOOL program was the most important European project on passive cooling of buildings. The project addressed topics included solar control, the combined effect of ventilation and thermal mass, thermal comfort during summer and the potential of natural cooling techniques. PASCOOL put in evidence also the axes towards which future research on passive cooling should be oriented. This research, giving the continuously increasing trend of energy consumption for cooling purposes, is absolutely necessary in order to take advantage of the complete potential that passive cooling can offer to buildings while maintaining the living standards, health and comfort of the occupants. This paper presents these future issues that comprise (a) research on the microclimatic scale in order to address the impact of outdoor environment on the cooling load of buildings, (b) investigation of comfort requirements under transient conditions during summer, (c) research on natural ventilation in urban environments and the impact of outdoor pollution on indoor air quality, (d) development of alternative cooling systems and techniques, (e) development of integrating design concepts optimising the use of solar heating, passive cooling and natural light in buildings.     

Performance of a coupled cooling system with earth-to-air heat exchanger and solar chimney

Buildings represent nearly 40 percent of total energy use in the U.S. and about 50 percent of this energy is used for heating, ventilating, and cooling the space. Conventional heating and cooling systems are having a great impact on security of energy supply and greenhouse gas emissions. Unlike conventional approach, this paper investigates an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector enhanced solar chimneys. By simultaneously utilizing geothermal and solar energy, the system can achieve great energy savings within the building sector and reduce the peak electrical demand in the summer. Experiments were conducted in a test facility in summer to evaluate the performance of such a system. During the test period, the solar chimney drove up to 0.28 m³/s (1000 m³/h) outdoor air into the space. The EAHE provided a maximum 3308 W total cooling capacity during the day time. As a 100 percent outdoor air system, the coupled system maximum cooling capacity was 2582 W that almost covered the building design cooling load. The cooling capacities reached their peak during the day time when the solar radiation intensity was strong. The results show that the coupled system can maintain the indoor thermal environmental comfort conditions at a favorable range that complies with ASHRAE standard for thermal comfort. The findings in this research provide the foundation for design and application of the coupled system.     

地板辐射与下送风复合式供冷系统运行特性实验研究

测量地板辐射与下送风复合式供冷系统运行过程中的室内空气温湿度、围护结构表面温度等室内环境参数,分析室内温湿度、热舒适性、系统换热量的变化规律,并对室内空气环境进行影响因素分析。实验结果表明:室内空气绝对湿度较室内空气温度达到稳定需要的时间更短;MRT(mean radian temperature)、OT(operation temperature)和PMV-PPD值在系统开启后第1.0 h减小速率最大,1.5 h后逐渐趋于稳定,此时,PMV约为0.49,PPD约为10%,在热舒适范围内;地板净辐射换热量、对流换热量和总换热量在系统开启后的1.5 h内递增,然后趋于稳定,此时,地板辐射换热量约为37 W/m~2,占总换热量的47%;室内空气温度和作用温度均随室外综合温度、室内发热量、供回水平均温度和送风温度的增加而增加,当室外综合温度较低或较高,或室内发热量较低,或供回水平均温度较低时,室内空气温度和作用温度变化梯度较小,室内空气温度和作用温度随送风温度增加而增加的速率近似呈线性。     

Variable air volume ventilation control strategies analysed in six climate zones

This paper presents eight ventilation control strategies and their annual energy and indoor air quality simulation results for an academic building as if it were situated in each of six geographic locations. The results show that without tempering at the terminal boxes, no ventilation strategy could satisfy the outdoor air requirements when the thermal loads are low. The fixed outdoor air percentage method is the worst one. From an economic perspective, strategies using optimization techniques minimize the operating energy demand and consumption. Supply air temperature (SAT) and primary airflow rate are the two proper optimization parameters on the air side of heating, ventilating, and air‐conditioning systems. In addition to control strategies, geographic locations or weather patterns influence the benefits of optimization. Generally, a mild‐dry climate intensifies the advantages of the SAT reset and encourages the primary airflow optimization. Inversely, hot‐humid weather minimizes the benefits. Copyright © 1999 John Wiley & Sons, Ltd.     

High efficiency micro trigeneration systems

This work focuses on micro polygeneration research in three areas: prime movers, cooling devices, and novel system integration options. An original analysis of matching prime mover heat sources to thermally-driven devices is presented, and waste heat for eight prime movers used in micro CHP is characterized using a T-Q diagram. “Micro” is roughly defined as 1–50 kW electric, and an emphasis is placed on systems of <5 kW electric. With regard to cooling technologies, the increasing demand for tight control of humidity and ventilation represents distinct opportunities for integrated energy systems. Polygeneration can provide efficient ways to accomplish better comfort, such as by separating sensible and latent cooling loads. The increasing legislation dealing with GWP of refrigerants for vapor compression systems also provides a driving force for thermally-activated cooling. Advances are being made both for vapor compression and thermally-activated cooling cycles, and a review is provided of research in the areas of transcritical cycles, subcooling, adsorption cooling, desiccant dehumidification, and integrated systems. Many new adsorbent working pairs and desiccant dehumidification materials have driving temperatures low enough to utilize heat from reciprocating engine coolant or even a vapor compression condenser. This enables smaller installations to benefit from combined cooling, heating and power; and opens new possibilities for separating sensible and latent cooling to extend the operating range of sorption-based heat pumps and improve solar cooling efficiency.     

吸湿供冷系统的空气处理方式分析

温度和新风量的大小在影响室内环境的舒适性方面起着重要的作用。为了改善室内空调环境，ASHRAB规定设计温度应该保持在30％-60％之间，且需要把原有的设计新风量增大2倍至4倍。显然，降低温度和增大新风会加大空调的冷负荷，传统的空调供冷系统在满足上述条件方面显示出了自身的一些缺点。吸温供冷系统把除温和冷却降温供冷解耦分开处理．不但能把空气温度处理到一个比较低的程度，而且在节能方面也显示出很大的潜力。对吸温供冷系统的几种空气处理方式进行了分析和比较，从而可根据不同的情况为吸温供冷系统的使用选择比较理想的空气处理方式，以达到节能目的。     

Model-based optimal control of a dedicated outdoor air-chilled ceiling system using liquid desiccant and membrane-based total heat recovery

This study presents a model-based control strategy for a novel dedicated outdoor air-chilled ceiling (DOAS-CC) system with the aim of optimizing the overall system performance. The DOAS-CC system incorporates liquid desiccant dehumidification and membrane-based total heat recovery technologies. Simplified but reliable models of major components in the DOAS-CC system are firstly developed to predict the system performance. A cost function is then constructed to minimize total energy consumption while properly maintaining thermal comfort reflected by indoor air temperature and relative humidity. Genetic algorithm is used to search for optimal set-points of the supply air temperature and humidity ratio of the dedicated outdoor air subsystem as well as the supply water temperature. The performance of this strategy is tested and evaluated with different control settings in a simulated multi-zone space served by the DOAS-CC system under various weather conditions. The results show that optimized control variables produced by the optimal strategy can improve the system energy performance and maintain indoor thermal comfort.     

Energy saving by integrated control of natural ventilation and HVAC systems using model guide for comparison

Integrated control by controlling both natural ventilation and HVAC systems based on human thermal comfort requirement can result in significant energy savings. The concept of this paper differs from conventional methods of energy saving in HVAC systems by integrating the control of both these HVAC systems and the available natural ventilation that is based on the temperature difference between the indoor and the outdoor air. This difference affects the rate of change of indoor air enthalpy or indoor air potential energy storage. However, this is not efficient enough as there are other factors affecting the rate of change of indoor air enthalpy that should be considered to achieve maximum energy saving. One way of improvement can be through the use of model guide for comparison (MGFC) that uses physical-empirical hybrid modelling to predict the rate of change of indoor air potential energy storage considering building fabric and its fixture. Three methods (normal, conventional and proposed) are tested on an identical residential building model using predicted mean vote (PMV) sensor as a criterion test for thermal comfort standard. The results indicate that the proposed method achieved significant energy savings compared with the other methods while still achieving thermal comfort.     

Feasibility study on applications of solar chimney and earth tube systems for BEAM/LEED assessment

Building Environment Assessment Method (BEAM) and Leadership in Energy and Environmental Design (LEED) aim to promote better environment performance of buildings in their life time. This study explores innovative solutions to achieve key requirement of Energy Use (EU) and Indoor Environmental Quality (IEQ) from BEAM/LEED assessments by Solar Chimney (SC) or Earth Tube (ET). EnergyPlus is used to perform the simulation of building's ventilation and energy usage under the typical Hong Kong's weather data. It was found that the SC performance is affected by the building's dimension and orientation and is also determined by solar availability and absorber surface temperature. In most simulation cases, SC provided sufficient natural ventilation, but it also increased cooling load to the space when ambient temperature was higher than indoor. The ET performance was affected by the pipe dimension and buried depth. It was found that a single ET could decrease cooling load to the space in the summer design day ‘July 21’ in Hong Kong, but it could not provide sufficient outdoor fresh air. The result also demonstrated that both SC and ET systems had capabilities to help achieve on‐site renewable energy requirements from BEAM/LEED because of their energy saving capacities on ventilation and thermal comfort. Because SC had higher capability to provide sufficient ventilation, SC could more significantly contribute on the sections pertinent to building ventilation in BEAM/LEED assessments. Because ET had higher capability to save cooling energy, ET could more significantly contribute on the sections pertinent to energy saving in BEAM/LEED assessments. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of window size and thermal mass on building comfort using an intelligent ventilation controller

A prototype microcomputer-controlled thermostat was developed that can manage airflow according to cooling the needs in a building and the resources in the environment. This intelligent control system measures both indoor and outdoor temperature and uses decision rules to control a whole-house fan, in addition to the furnace and air conditioner. No such residential thermostat is currently commercially available. This paper presents the controller strategy that optimizes cooling with outdoor air. This paper also quantifies the effects of modifying the amount of thermal mass and the window area on indoor comfort when using this controller. These test confirm that smaller windows and more mass performed better than larger windows and less mass, and that higher volumes of controlled ventilation outperformed fixed ventilation rates.