Desiccant solar air conditioning in tropical climates: II—Field testing in Guadeloupe

This paper presents the results of the experimental investigation of a solar desiccant air conditioning device exposed to the sun in Guadeloupe to test the adaptability of a silicagel compact bed, the most simple technology, in a tropical climate.     

Dynamic modelling and simulation of a new air conditioning prototype by solar energy

This work presents a new design of an air conditioning prototype by solar energy developed at the Laboratory of Electromechanical Systems of the National Engineering School of Sfax, Tunisia. The new conception permits to produce heat or cold by using solar energy without polluting the environment. The installation, composed of four compartments, consists of three functioning modes according to the season of the year and according to the climatic conditions.A numerical model is developed to study the behaviour of the unit. This model uses real meteorological data to predict the performance of a thermal solar driven system. The dynamic modelling and simulation of only two modes of functioning (winter mode and summer mode without pre-cooling of air) are presented in this paper. This theoretical model is expected to help in predicting the behaviour of the installation in various climatic conditions. Besides, it would enhance the performance of such installation.     

Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate

This paper presents an indoor and analytical study to evaluate the performance of a desiccant cooling system that uses silica gel as desiccant, electric light bulbs to simulate solar radiation, and forced flow of air through an IDC (integrated Desiccant/Collector). In the regeneration process, the rate at which water is removed from the desiccant increases with irradiation and decreases with air flowrate. In the air dehumidification process, the adsorption rate decreases with irradiation and increases slightly with flowrate. Comparisons between analytical calculations and experimental data show good agreement, and the calculations show that it should be possible to operate this system in tropical humid climates using the regeneration process in the day and the air dehumidification in the night time.     

Analysis of monthly time series of solar radiation and sunshine hours in tropical climates

This paper analyses the interannual variability of solar radiation and sunshine hours for a large tropical region (Brazil), located between latitude 0°S and 30°S, in order to improve knowledge on solar resources, generate statistical parameters for model checking or to be used as input data of synthetic time series generation. The statistics for the daily, monthly average solar radiation deviations, and daily, monthly average sunshine hours, for the various localities in Brazil, tested with the Kolmogorov–Smirnov method, show that they are random variables, normally distributed. On the other hand, the sequential properties analysis shows that the auto-correlation coefficients with lag 1 are statistically significant only for a few locals: Fortaleza, São Luís, Manaus and Belém. But it is necessary to emphasize that the auto-correlation coefficients with lag 1, though not usually statistically significant, are positive for almost all the locals. The AR-1 is the suggested procedure for monthly solar radiation synthetic time series generation, with auto-correlation coefficients varying from 0.30 to 0.47 for the localities in the north of Brazil and zero for other regions.     

Dynamic response of a packed bed thermal storage system—a model for solar air heating

A mathematical model for simulating the dynamic temperature response of a packed column to an arbitrary time-dependent inlet air temperature is developed. The model includes axial thermal dispersion as well as intra-particle conduction, features that have usually been neglected but can be important in solar energy applications. Solutions, presented in terms of moments of the temperature response to an impulse of heat at the inlet, can be evaluated by simple numerical quadrature. Results of the model compare favorably with experimental data found in the literature. The model is used to optimize heat storage in a rock bin system subject to a realistic transient inlet temperature.     

Analytical and experimental studies on the thermal performance of cross-corrugated and flat-plate solar air heaters

The cross-corrugated heaters consist of a wavelike absorbing plate and a wavelike bottom plate, which are crosswise positioned to form the air flow channel. Two types of these heaters are considered. For the type 1 heater, the wavelike shape of the absorbing plate is along the flow direction and that of the bottom plate is perpendicular to the flow direction, while for the type 2 heater it is the wavelike shape of the bottom plate that is along the flow direction and that of the absorbing plate is perpendicular to the flow direction. The aims of the use of the cross-corrugated absorbing plate and bottom plate is to enhance the turbulence and the heat-transfer rate inside the air flow channel, which are crucial to the improvement of efficiencies of solar air-heaters. To quantify the achievable improvements with the cross-corrugated absorbing and bottom plates, flat-plate solar air-heaters which have both a flat absorbing plate and a flat bottom plate, are also considered. The thermal performance of these three types of solar air-heaters are analyzed, measured and compared under several configurations and operating conditions. All the analytical and experimental results show that, although the thermal performance of the type 2 heater is just slightly superior to that of the type 1 heater, both of these cross-corrugated solar air-heaters have a much superior thermal performances to that of the flat-plate one. It is also found that the use of selected coatings on the absorbing plates of all the heaters considered can substantially enhance the thermal performances of the heaters, whereas such a selected coating on the bottom plates or/and the glass covers does not have such a significant effect on the thermal performances of the heaters.     

A feasibility study of a solar desiccant air‐conditioning system—Part II: Transient simulation and economics

In this paper, a solid desiccant cooling system with a backup vapour compression system is simulated using TRNSYS and the performance of the system is evaluated in four cities in the United States with different climates. Economic analysis is performed in order to assess the feasibility of these systems and to determine the relevant economic parameters such as life cycle costs, life cycle savings and payback periods. Results show that the system has higher COP values for the locations with more latent loads. The air conditioner was able to meet the cooling demand in all four regions, but it needed more auxiliary energy in the Eastern and Mountain regions than in the Central region, because of the higher solar fraction in the Central region. The simulation also showed that the desiccant cooling system by itself was capable of meeting the cooling demand and hence the requirement of a backup system may be eliminated. Thermal and economic parameters were analysed for varying solar subsystem sizes which proved helpful in optimizing the design of the solar system. Recommendations to minimize the auxiliary energy costs using different methods for supplying the thermal energy for desiccant regeneration are described. Copyright © 1999 John Wiley & Sons, Ltd.     

Bringing simulation to implementation: presentation of a global approach in the design of passive solar buildings under humid tropical climates

In early 1995, a DSM pilot initiative was launched in the French islands of Guadeloupe and La Reunion through a partnership between several public and private partners (the French Public Utility EDF, the University of Reunion Island, low cost housing companies, architects, energy consultants, etc…) to set up standards to improve thermal design of new residential buildings in tropical climates. This partnership led to defining optimized bio-climatic urban planning and architectural designs featuring the use of passive cooling architectural principles (solar shading, natural ventilation) and components, as well as energy efficient systems and technologies. The design and size of each architectural component with regard to internal thermal comfort in buildings has been assessed with validated thermal and airflow building simulation software (CODYRUN). These technical specifications have been edited in a reference document which has been used to build over 800 new pilot dwellings through the years 1996–2000 in Reunion Island and in Guadeloupe. Monitoring experiments were held in these first ECODOM dwellings in 1998 and 1999. This resulted in experimental validation of the impact of the passive cooling strategies on the thermal comfort of occupants leading to the modification of specifications when necessary. The paper presents all the methodology used for the application of ECODOM, from the simulations to the experimental results. This follow up is important, as the setting up of the ECODOM standard will be the first step towards the introduction of thermal regulations in the French overseas territories, by the year 2002.     

A feasibility study of a solar desiccant air‐conditioning system—Part I: psychrometrics and analysis of the conditioned zone

A desiccant dehumidifier in conjunction with evaporative coolers can reduce air conditioning operating costs significantly since the energy required to power a desiccant cooling system is small and the source of this required energy (solar, waste heat, natural gas) can be diverse. Such a solid desiccant cooling system with a backup vapour compression system is simulated and the performance of the system is evaluated to study its feasibility in four cities in the United States. This paper describes the relevant psychrometric calculations and analyses of the conditioned zone required for simulating the transient performance of the system. Copyright © 1999 John Wiley & Sons, Ltd.     

Entropy production and exergy destruction: Part I—hierarchy of Earth''s major constituencies

Earth does not consume energy. Neither do we nor does our energy system. Rather, we consume exergy. So using an exergy optic should be preferred when we plan future hydrogen-electricity systems. In this article, we provide a perspective on the exergy optic from the viewpoint of Earth's major energy-transaction constituencies. We first consider the Earth as a whole and then consider the following constituencies: the biosphere, people and civilization's energy system. Our objectives are to understand better the nature and sources of thermodynamic losses and to provide a background against which future hydrogen opportunities may be evaluated. As a codicil, we hope our results can improve our understanding of how people, civilizations and nature operate, and are connected. In a companion article, representative components within the energy system hierarchy (e.g., Niagara Falls, home heating systems and fuel cells) are considered.     

空调水系统管路间耦合特性的分析及试验研究

为了探讨压差控制方式的可靠性 ,提高该控制方式的节能效果 ,通过控制领域中相对增益的概念 ,阐述了相对增益值与管路间耦合程度的关系 ,根据试验对一空调水系统的相对增益进行了计算 ,并分析了压差控制方式下变流量空调水系统中压差设定值及定压点位置对各支路间耦合特性的影响。最后得出结论 :压差控制回路对支路间的耦合起一定的改善作用 ,压差设定值大小对支路间的耦合程度没有影响 ,对耦合程度严重的回路 ,有必要采取解耦控制     

Utilization of cool ceiling with roof solar chimney in Thailand: The experimental and numerical analysis

The objective of this paper is to study the benefits of application of solar chimney on the south roof and cool metal ceiling on the north roof through the experiment in a detached building called a controlled cell, and the related numerical model constructed from a computational fluid dynamics (CFD) program. The experimental results are used for calculation of values of heat transfer coefficient of the cool ceiling and evaluation of the mean cooling potential of the combined passive cooling system. The two-dimensional numerical models generated by the CFD program use the mean values of wall temperatures in the application of solar chimney in the controlled cell as the boundary conditions. The effects of cool ceiling on the temperature, velocity and airflow rate in the controlled cell are investigated through the numerical model in which the north ceiling temperature is reduced by 2–4 °C from the measured value of 32.8 °C. The mean cooling potential of the application of combined system is found to be two times higher than the application of the solar chimney. Good agreements between the predicted and experimental results are obtained from the comparison of temperature and volume flow rate at the middle section of the controlled cell. The reduction of north ceiling temperature in the free-convection numerical model shows the decrease of air temperature in the upper region of the room by 0.5–0.7 °C from the original value of 33.3 °C, and the increase of volume flow rate by 12%.     

A novel hybrid heat-pipe solar collector/CHP system—Part II: theoretical and experimental investigations

A theoretical analysis has been carried out to investigate the thermodynamic and heat transfer characteristics of a hybrid heat pipe solar collector/CHP system based on the assumption that the system operates on a typical Rankine cycle. Experimental testing of the prototype was also carried out using two types of turbine units. The variation of refrigerant pressures and temperatures, hot water temperatures in the collector and boiler systems, as well as chill water temperatures were recorded. The results were used to estimate the heat from the boiler and the solar collectors, the electricity and hot water generation (indicated as kW energy) from the CHP operation and the gas consumption of the system. The modelling and experimental results were compared for the impulse-reaction turbine system, and a simple analysis of the energy and environmental benefits of the system was carried out. The analysis indicated that the proposed system would save primary energy of approximately 3150 kWh per annum compared to the conventional electricity and heating supply systems, and this would result in reduction in CO₂ emission of up to 600 tonnes per annum. The running cost of the proposed system would also be lower than conventional heating/power systems.     

Performance of a solar liquid desiccant air conditioner – An experimental and theoretical approach

In this paper the results of testing a solar liquid desiccant air conditioner (LDAC) in the tropical climate of Queensland, Australia have been presented. The system uses polymer plate heat exchanger (PPHE) for dehumidification/indirect evaporative cooling, and a cooling pad as the direct evaporative cooler for the dry air leaving the PPHE. Lithium chloride, which is an effective desiccant in air dehumidification, was used in the experiments and a scavenger air regenerator concentrates the dilute solution from the dehumidifier using hot water from flat plate solar collectors. The data obtained from performance monitoring of the solar LDAC operating on a commercial site in Brisbane was compared with a previously developed model for the PPHE. The comparison reveals that good agreement exists between the experiments and model predictions. The inaccuracies are well within the measuring errors of the temperature, humidity and the air and solution flow rates. The above tests further indicate a satisfactory performance of the unit by independently controlling the air temperature and humidity inside the conditioned space.

In order to prevent carryover of the solution particles into the environment, eliminators are used at outlet of the absorber unit and the regenerator. An alternative method in preventing the carryover is the use of indirect cooling, in which the supply air does not contact the solution. The method can be used to produce potable water from the atmospheric air in remote areas.

The liquid desiccant system can be used in the HVAC industry, either as a packaged roof-top air conditioner, or as an air handler unit for commercial applications. The system could also be used for space heating in winter due to the property of desiccants to provide heat when wetted.     

Numerical studies on the reaction of carbon particles in a vacuum residue—air flame

A computational work was carried out for the study of one‐dimensional, laminar, premixed, flat, atomized vacuum residue (VR) particle–air flames. The mathematical model includes the specified pyrolysis scheme, soot and char oxidation scheme. With some experimental works, the product composition and kinetic parameters of VR pyrolysis were determined and used for the present computational work. The computed results show that the oxidation of VR carbon char and soot occurs mainly in the reaction zone and the oxidation rate of soot is much higher than that of VR carbon char. The oxidation rates of carbon char and soot can be increased with the decrease in particle diameter, and it might be accomplished by the more effective atomization and mixing of solid particles with combustion air. Copyright © 2005 John Wiley & Sons, Ltd.     

Dynamic and thermal study of air flow control by chicanes with inclined upper parts in solar air collectors

A dynamic and thermal simulation for two-dimensional model is developed on air flow and heat transfer control by chicanes in solar air collectors. New chicane form is adopted with two parts: the first is orthogonal to the air flow direction and the second is titled (α=60°). It is apparent that the turbulence created by introducing chicanes, resulting in greater increase in heat transfer inside the dynamic air vein with a rise of 23%. The effect of the variation of the Reynolds number in the range of 100<Re<4500, on the convective heat transfer coefficient, the pressure drop and Nusselt number are analysed and have shown good agreements with the literature results. Therefore, the mass flow rates effect on the velocity magnitude, temperature and the turbulent intensity is analysed. The Reynolds number variation showed a substantial effect on the mechanism of vortex development and separation phenomenon.     

不同地区住宅能耗与太阳能吸收式空调系统匹配性分析与模拟研究

为了满足农村住宅清洁用能的需求,多种形式的能源系统逐渐开始应用于广大的农村地区。随着太阳能集热器集热效率的提高,热驱动机组各项性能不断改善,这样有利于太阳能吸收式空调系统在农村地区的应用。为了研究太阳能吸收式空调系统与农村住宅全年能耗的匹配问题,文章首先建立了DeST住宅模型,然后利用TRNSYS软件建立了太阳能吸收式空调系统模型,最后根据模拟结果对国内不同气候区内农村住宅供热季、供冷季的平均热负荷值,以及全年的能耗进行分析。此外,文章还分析了典型日太阳能吸收式空调系统的运行策略与效果。分析结果表明:在无辅助热源的条件下,太阳能集热器的集热温度会大于80℃,满足空调机组的热驱动温度,因此可以作为太阳能吸收式空调系统的的热源;当启动温度为85℃时,空调机组的制冷量可以达到8 kW,性能系数COP为0.733。     

Dynamic analysis and control of a solar power plant—II. Control system design and simulation

The paper (splitted in two parts) describes the main results of the study concerning dynamic analysis and control of the EEC solar power plant, coupled to the grind for the first time in April 1981.High temperature solar energy systems require quite sophisticated receivers, able to work with considerable radiation flux, and well designed and controlled steam generators, to meet the severe constraints imposed by the turbine on the produced steam quality.In the case of the EEC solar power plant using a water cooled solar receiver, receiver and steam generator are, actually, the same component, which indeed is subject to a number of stringent somewhat conflicting requirements, concerning, in particular, its transient behavior.This latter aspect plays a fundamental role in view of the environmental conditions, i.e. of the irregularity of the power source due, for instance, to passing of clouds.In this Part I of the work, it is shown how a carefull dynamic analysis of the process is necessary for the verification and the assessment of the receiver design, the precise formulation of the plant operation procedures and safety conditions, the specification of the control system requirements. In particular, it is recognized that such kinds of processes can be adequately simulately by means of accurate partial derivatives models, based on suitable simulation codes.     

An experimental and numerical study of thermal stratification in a horizontal cylindrical solar storage tank

The thermal behaviour of a horizontal cylindrical storage tank has been investigated both experimentally and numerically. Four sets of experiments have been carried out where cold water is injected into the bottom of the tank with three different initial thermal fields. The first one is the tank with initial thermal stratification with bottom temperature the same as the inflow temperature. The second set is the tank with the initial thermal stratification, the bottom being at a relatively higher temperature than the inflow temperature. The third set is an initially heated isothermal tank and the fourth is the same as the first set of experiments except that the straight tube inlet nozzle is replaced by a 30° downward bent divergent conical tube. The above experiments show that better thermal stratification can be obtained using the divergent conical tube as the inlet nozzle due to the diffusion effect of the nozzle. Also a slight improvement in the tank performance has been achieved in the second set of experiments when the initial bottom temperature of the tank is higher than the injected cold water temperature. To check the accuracy of the experimental results two different types of one-dimensional numerical models, namely Turbulent Mixing Model and Displacement Mixing Model have been developed and the results are compared with the experiments. This comparison indicates that the numerical results are in good agreement with the experiments especially at the top of the tank.