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Potential Of Using A Solar Liquid Engineering Essay

This experimental and simulation study investigates the potential of using a solar liquid desiccant dehumidification system(LLDS) for building ventilation applications in Karachi. A review of literature reveals that compared to the conventional dehumidification system in vapor compression cycle, this cycle could offer saving in overall primary energy. Since the cycle is based on supply of conditioned ventilation air it has significantly lower environmental impact.

A solar liquid desiccant dehumidification test rig has been installed at the Energy Conservation Laboratory NED to study the performance of various components. These experiments enabled the understanding of the system and helped developed effect of individual component on the system performance. Important parameter of system components established from the experiment were used as input to the TRNSYS simulation model.

The simulation studies have been based on a well established transient energy software TRNSYS. Specific to this work a building model was prepared from TRNbuild and added as module to TRNSYS. Addition of this component resulted in evaluating the performance of a dehumidifier in a building located at Karachi.

The source of thermal energy needed for regeneration plays an important part in the overall success of the desiccant dehumidification scheme. For that purpose a solar still has been included in the study.

The success of the liquid desiccant dehumidification system is strongly associated with the cost of the thermal energy. Combined heat and power systems offer an opportunity of providing this energy at no additional cost. The latter study has been suggested to be taken up as future work.

Terminology

Throughout this report, the terms component model, model, module, and component will be used interchangeably. These terms all describe the TRNSYS representation of a piece of equipment or module. For TRNSYS purposes, a model is represented by a subroutine or sub-program, usually written in FORTRAN, describing its operation. Examples of TRNSYS component model include a heat exchanger, dehumidifier, weather processor, and a printer.

The terms project refer to a set of component models which are interconnected in such a way as to perform a set task. For example, the interconnection of the weather processor model, the dehumidifier model, and the building model in such a way as to simulate the dehumidification of a building by the dehumidifier is considered to be a project or assembly. For TRNSYS purposes, assemblies of component models are represented by a TRNSYS input file (the deck), a file listing the component models and their interactions.

Test rig, desiccant dehumidification system and test plant refer to the same desiccant dehumidification system installed in the energy conservation lab

Dehumidifier, rotary solid desiccant wheel or desiccant wheel all refer to same component which

dehumidifies process air.

Process air or supply air or conditioned air refer to the outdoor air treated by the liquid desiccant

dehumidification system.

Desiccant dehumidification system, liquid desiccant dehumidification system and solar liquid desiccant dehumidification system have same meaning when they are referring to the present system under study.

Conventional cooling system, chiller, cooling coil and auxiliary cooler refer to vapor compression based air condition system.

Chapter 1: Background of the Present Study

1.1. Introduction

Space cooling is important to achieve healthy and productive living conditions in a building.

Temperature, humidity and ample amount of fresh air among other things determine the quality of indoor healthy and wholesome living conditions. Recent research indicates a direct association

between indoor air quality and fresh air ventilation rates which supports requirements for building ventilation standards calling for continuous supply and increased amounts of ventilation to help assure safe and healthy interior air environments [Seppanen, 2002]. Most commercial and public buildings utilize packaged heating and cooling equipment, designed to provide inexpensive, efficient heating and cooling, with minimal outdoor air [Westphalen, 2001]. This type of equipment was not designed to handle the continuous supply or increased volume of outdoor air necessary to comply with minimum ventilation standards. Attempts to meet these ventilation and makeup air recommendations using conventional packaged HVAC units often result in poor indoor humidity control and wide temperature fluctuations [Sand, 2005]. Realizing the importance of healthy indoor conditions American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) has increased the recommended ventilation air flows for almost all buildings [1].

As the standard of living improves in warmer climatic areas of the developing world building cooling is becoming more common. High rise commercial buildings with high fenestration have large solar gains; such buildings tend to have a high cooling load. Office and other commercial building have to be cooled in summer even in the colder areas of the world due to widespread use of computers, printers and artificial lightings, which add to the cooling load of the building. Keeping other things constant the building cooling load is directly proportional to the ventilation flow.

Summer in Pakistan is hot and humid with about seven month cooling season. The weather

conditions of Karachi are characterized by high humidity and high temperatures. Figure 1.1 shows a one year psychometric representation of Karachi. Each dot on the figure representing the humidity value for one hour corresponding to a temperature value.

Figure 1.1: Psychrometric representation of Karachi’s climate

Increased use of air conditioning has resulted in appreciable demand for electricity that is produced in central power plants. In case of Karachi almost all of the electricity is generated by thermal power plants burning fossil fuel [PEYB, 2005] [2]. The vapor compression air-conditioning systems, which constitute a large portion of air conditioning plants, consumes high grade electric power. Under high ventilation loads or low sensible heat ratio (SHR) conditions buildings have a large latent cooling load [Daou, 2004] [3]. Buildings which need high ventilation rates are essentially those with high occupant density such as theaters, convention centers and supermarkets.

The availability of a more economic, flexible and environmentally friendly cooling technology that could supply ample amount of ventilation air economically is poised to be adopted. During the past two decades active desiccant systems have become a common component of HVAC systems in commercial buildings needing lower than usual humidity levels [Lewis, 1999] [4]. Desiccant dehumidification systems use low temperature solar heat as primary energy source and some electric power to take care of parasitic power demand. This system will certainly be welcome in present scenario. To achieve this goal desiccant air conditioning systems must be evaluated for the prevailing environmental conditions. Significant research has been done for their use in temperate climatic conditions of Europe and North America but South Asian region has not been fully explored for the adaptation of LDDS. Climatic conditions of Pakistan are very different from high latitude countries and wholesale transplantation of ideas from there without rigorous study of the same technology for the local conditions will be counterproductive.

A literature review revealed that no studies have been done to investigate the energy saving potential of solar liquid desiccant dehumidification system for the weather conditions of Pakistani cities. Consequently there is a shortage of experimental and simulation data concerning the use of this system for space cooling. Availability of the relevant data will help planners and builders make better choice about the use ofdehumidification technology. Similarly no study has been undertaken to evaluate the use of solar energy for desiccant dehumidification system in this part of the world. The main argument in favor of solar desiccant dehumidification systems is that the cooling demand peak occurs during the peak solar energy availability [Hoefker, 2001] [5].

A number of newly constructed buildings in Pakistan are using desiccant dehumidifiers for reducing the humidity of ventilation air but of solar energy in conjunction with dehumidifier as important air conditioning components is nonexistent. This project evaluated the potential of a solar liquid desiccant dehumidification system in order to identify the system ability to utilize solar energy to minimize the gas consumption in heating the regeneration air.

1.2. Aim and Scope of Project

Using a natural refrigerant for air conditioning is recognized as extremely important from the stand point of environmental protection [Watanabe, 1998] [6] solar liquid desiccant dehumidification is one dehumidification method which uses Calcium Chloride and water for dehumidification purpose with low overall primary energy consumption. Many studies have been carried out in the past few decades to prove the usefulness of this technology for space dehumidification along with cooling [Matsuki, 1992] [7], [Camargo, 2003] [8], [Carpinlioglu, 2005] [9], [Hirunlabh, 2005] [10]. These efforts have inspired this study.

Present study through testing and computer simulation offers a holistic strategy for the adaptability and utility of a solar liquid desiccant dehumidification system for the climatic conditions of Pakistan. This study involved desiccant dehumidification plant installation and data collection from the experiments conducted on the plant. It was concerned with the determination of interaction of individual components constituting the desiccant dehumidification test plant thereby facilitating the assessment of the performance of the complete system. The study moreover involved preparation of TRNSYS simulation model of the desiccant dehumidification systems and the validation of the model from test results.

This investigation was also involved in the conceptual formulation of the design of the single-zone building using TRNBuild for incorporation in the TRNSYS simulation model. The simulation of modified building file module in TRNSYS desiccant dehumidification project showed that this component can be used to study the performance of the system in a room.

Finally based on validated model of the desiccant dehumidification system suggestions were made regarding its use in space cooling. Hence the present study aims to evaluate the presumption that a solar liquid desiccant dehumidification system can supply dehumidified building ventilation air in the climatic conditions of Pakistan at lower overall primary energy consumption.

1.3. Project Objectives

Having outlined the aims of this study it is possible to state its main objectives in a nutshell, as

follows:

To improve the understanding of solar liquid desiccant dehumidification system behavior in the context of Pakistani climatic conditions for one city.

To determine the energy saving potential of liquid desiccant dehumidification system for Pakistani weather conditions.

To assess the energy, environmental and economical impact of the solar still supplying thermal energy for the regeneration of desiccant in the dehumidifier.

To recommend further research issues emanating from present study.

1.4. Formulation of Problem

The Energy Conservation Laboratory was established at the NED University of Engineering &

Technology - Karachi with a view to search and develop innovative methods to help conserve scarce and costly energy. Among other things one scheme was to investigate new methods which could allow users to switch from conventional air conditioning systems to heat driven cycles. This was envisaged to ease pressure on the electric utilities due to phenomenally growing air conditioner use in Pakistan [personal communication with Mr. Sajid of ASHRAE, Pakistan Chapter]. Pakistan has sizeable national natural gas reserves furthermore it is also in serious negotiations with its neighboring countries for import of natural gas through an extensive pipe line. Plans are afoot to develop liquefied natural gas (LNG) import facilities from Middle East. Natural gas is the cleanest fossil fuel with least amount of carbon dioxide (CO2) and production per joule of energy released. With this national energy scenario and an extensive gas distribution system covering all major cities of the country a heat based air conditioning system would be welcomed by users and utility companies. Desiccant dehumidification system which runs on thermal cycle partly obtained from renewable sources can certainly be a good alternative to conventional electric driven system. It is hoped that research in the area of desiccant dehumidification will eventually result in the dissemination of know-how of this dehumidification technique and its use in this part of the world.

Many studies have been undertaken to evaluate the benefits of liquid desiccant dehumidification system for the temperate climatic regions of the world however very few investigations have been done to investigate the energy saving potential of this innovative air conditioning system for the hot and humid regions of the world. There is a dearth of experimental and simulation data concerning the use of solar assisted desiccant dehumidification system. Presently a number of buildings are using desiccant dehumidifiers for reducing the latent load of ventilation air but use of solar energy for regeneration of the desiccant is very limited or non existent in this part of the world. This study aims to fill this gap and attempts to find out the potential of a solar assisted liquid desiccant dehumidification system.

1.5. Project Methodology

This project work has been carried out in two stages: literature review and the experimental and

modeling work. To realize the proposed aims and objectives, an experimental test rig was set up and testing of the liquid desiccant dehumidification system was performed in the summer cooling season of 2012. The experimental set up gave an insight into the working and interaction of different system components. The modular nature of the test plant allowed parametric studies to be performed on the dehumidification system. In order to establish a systematic and comprehensive approach to the experimental study, ventilation cycle performance of the desiccant dehumidification system has also been investigated. Few parametric studies helped in determining the best configuration of system and pinpointing the important contributory factors in the efficient performance of the system. As result of working on the liquid desiccant dehumidification system sufficient amount of data was generated that was used in setting the input parameters of the simulation model of the system.

A building file was prepared with the help of TRNbuild. This model was linked to TRNSYS software for simulation of different dehumidification system configurations. Prior to simulation of this component model it was necessary to ensure the reliability of the code; accordingly a validation study was carried out which proved the reliability of the component code and encouraged further use of this component in the modeling of solar liquid desiccant dehumidification system. The validation of the model was performed by analytical method.

For the simulation of liquid desiccant dehumidification system TRNSYS16, transient energy system simulation software was used [TRNSYS16, 2004]. Standard TRNSY component module were used to perform seasonal simulations of the system. Simulation results were obtained for different configuration of this system and for different months of the year. Input of the TRNSYS DCS model included the yearly weather files for the city modeled. The weather files were generated using METEONORM [Meteonorm, 2005] software. These weather files commonly known as Typical Meteorological Year (TMY) files, have a suitable format for use on TRNSYS program. Other inputs included the air flow rates, temperature and relative humidity. The outputs of the simulation, among other variables, were temperature and humidity of the process air in the room. Knowledge of these variables eventually made it possible to calculate the coefficient of performance of the system and other important parameters that ultimately led to determine system energy saving potential in Pakistan’s scenario.

1.6. Structure of the Report

This report covers both experimental and computational studies performed on desiccant cooling

system. The thesis is divided in eight chapters.

Chapter 1, the present chapter, discusses the background of the project work it outlines the factors which led to start this investigation. It also summarizes the goal of the project work.

Chapter 2 is mainly concerned with review of relevant project work done in the general area of

desiccant dehumidification. The project works reviewed the area of desiccant dehumidification, research articles discussing different aspects such as energy saving potential, system configuration, economics, component and system modeling and simulation. A discussion of current passive dehumidification and active dehumidification technology has also been included.

Chapter 3 outlines the fundamentals of adsorption process with reference to silica gel, an important solid desiccant material. In this connection isotherms of desiccant materials are discussed in some detail and the dehumidification cycle is described. The theory of desiccant wheel based on Jurinak’s [Jurinak, 1983] iso-potential function model used in the design of TRNSYS standard module is explained in some details here. It also outlines the basic principles of absorption process taking place in the desiccant.

Chapter 4 discusses the experimental work performed on the solar desiccant dehumidification test plant. This chapter begins with the introduction of test plant design its constituting components and data acquisition system. Performance curves of system components such as dehumidifier are discussed.

Chapter5 gives an account of the design of room on TRNbuild. The detailed design of the single-zone building and the input parameters set are given in this chapter.

Chapter 6 elucidates the comprehensive desiccant dehumidification modeling and simulation work. Simulation case studies for Karachi are discussed here which give important results of TRNSY simulation.

Chapter7 gives an account of the use of solar still to supply regeneration heat for the dehumidifier. Different types of solar stills used with regard to this have been discussed here and the parameters that effect the performance of solar still are also discussed in detail.

Chapter 8 discusses the energy and economic potential of solar liquid desiccant dehumidification system. Finally, the results and conclusions of the present study is discussed in detail

After the completion of empirical chapters conclusions and suggestions for further work have been presented. There includes some guidelines about using desiccant dehumidification system in conjunction with other component to achieve acceptable comfort conditions at minimum energy and monitory cost.

Reference

[1] ASHRAE Standard 62-1981, Ventilation for acceptable indoor air quality, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, USA, 1981

[1]ASHRAE Standard 62-2001, Ventilation for acceptable indoor air quality, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, USA, 2001

[2] (PEYB) Pakistan Energy Year Book, Hydro Carbon Development Institute of Pakistan, Ministry of Petroleum and Natural Resources, Government of Pakistan, 2005

[3] Westphalen, D., Koszalinski, S., Energy consumption characteristics of commercial building HVAC systems,

[4] Lewis, G.H., Michael, J.W., Marek, C., and Douglas, R.K., “Evaluating Active Desiccant Systems for Ventilating Commercial Buildings” ASHRAE Jr. No 41, Vol. 10, pp 28-33, 1999

[5] Hoefker, G., “Desiccant Cooling with Solar Energy.” 2001, PhD thesis, University of De montforte, UK,

[6]Watanabe, K., “A general overview on natural working fluids” Refrigeration, vol. 73, pp 961-967, 1998

[7]Matsuki, K., Saito, Y., Desiccant cooling R&D in Japan, Desiccant cooling and dehumidification, ASHRAE, Atlanta, GA, USA, pp. 134–143, 1992

[8] Camargo, J. R., Ebinuma, C.D. and J. Silveira, J., “Thermoeconomic analysis of an evaporative desiccant air conditioning system”, Appl Thermal Engng Vol. 23, pp1537–1549, 2003

[9] Carpinlioglu, M. O., Yildrim, M., “A Methodology for the Performance Evaluation of an

Experimental Desiccant Cooling System”, Intr Comm H and M Transfer, Vol. 32, pp 1400-1410,

2005

[10] Hirunlabh, J., Charoenwat, R., Khedari, J., Teekasap, S., Feasibility Study of Desiccant Air-

Conditioning System in Thailand”, Building and Environment, Article in press,

Chapter 2: Overview of the Relevant Literature

This chapter endeavors to introduce the study with a summarized presentation of earlier works done in the field of desiccant space dehumidification systems. This chapter consists of two parts, first part deals with in depth review of research literature in this field. The second part evaluates the current dehumidification technologies discussing the pro and cons of traditional dehumidification method.

2.1 Previous Works

Variety of configurations of desiccant dehumidification system has been proposed by different researcher for attaining maximum possible energy savings and coefficient of performance (COP). A number of modeling techniques have been tried by investigators to assess the efficiency of this system. Detailed literature review of the research work done in the area around the world is given below.

Maclaine-Cross and Banks [Maclaine–Cross, 1972] [11] investigated a desiccant cooling system under Australian weather conditions. Their study revealed that the gas fired desiccant cooling systems for large air conditioning projects are feasible both from the economic and energy saving point of view. Through the analysis of their proposed desiccant cooling system they showed that up to 50% saving in energy can be achieved if the waste heat from an engine driven vapor compression system was used to regenerate the dehumidifier.

Benjamin [Benjamin, 1979] [12] undertook a study to evaluate the possibility of saving energy by using solar desiccant cooling system (DCS) in the buildings. It was also the objective of the study to ascertain the optimum size of the combination of a solar DCS and auxiliary vapor compression cooler. He adopted TRNSYS software for the seasonal simulation job. For simulation purpose climatic conditions of five American cities i.e., Charleston, Dodge City, Fort Worth, Phoenix and Washington D.C were used. Two desiccant cooling system configurations were analyzed for the performance modeling; first one was the ventilation cycle and the second one was recirculation cycle. Benjamin concluded that recirculation desiccant cooling cycle was superior on the basis of its energy saving potential.

In his research study Jurinak [Jurinak, 1982] [13] simulated a desiccant dehumidification system by first modeling the system components; foremost of which was the rotary dehumidifier or the desiccant wheel. He determined the approximate numerical solution for rotary dehumidifier by proposing a new combined “potential” technique. This solution scheme was afterward used to simulate the dehumidifier alone as well as incorporated in a comprehensive model of desiccant cooling system. The desiccant cooling systems simulated were ventilation cycle and recirculation cycle. Operating aspects such as purging and variation of rotation speed of the dehumidifier were integrated in the solution scheme. Simulation also was performed by incorporating the component models into TRNSYS software. Several TRNSYS simulations were carried out based on the models developed for desiccant wheel. Through his work he showed that the performance of these cooling systems can be predicted more precisely by the intersection point method and little less accurately by the uncorrected and corrected analogy solutions. The analogy method is also fast and takes smaller amount of computer time than other methods. After analyzing the solution and comparing it with experimental results he concluded that analogy method was a very powerful tool for quantitatively analyzing a rotary dehumidifier.

Pesaran et al [Pesaran, 1992] [14] after initially outlining the basic principles of desiccant cooling system, claimed that in 1990 about US$ 22 billion were spent in air conditioning of residential and commercial buildings. A substantial part of that money could be saved by switching to desiccant cooling systems as these systems required low grade heat. According to them there were notable advantages of using desiccant system for cooling, such as improved indoor air quality (IAQ), reduction in the use of CFC’s and better control of indoor humidity. They claimed that although the cycle efficiency of system was less than one but it used low grade heat which could be obtained from solar or waste heat and offer a better solution than the vapour compression or vapour absorption cycle using electricity or steam respectively.

They claimed that the thermal COP of desiccant cooling cycle was about 0.5 which although low was still comparable with COP of vapor absorption cycle. They mentioned that as result of research in this area a lot of improvement in the cycle performance was reported. The research in the US and elsewhere in the world had, according to them matured into a $50 million desiccant cooling industry.

Economic analysis was also performed for the desiccant cooling system to assess the impact of wide spread use of this technology on the fossil fuel replacement and savings to the users. The present worth of the life cycle costs of the both solar desiccant cooling system and conventional system was calculated. They concluded from their analysis that solar desiccant cooling system had the potential to be economically very attractive for humid climates with high fuel costs.

Using their own developed software Jain et al [Jain, 1995] [15] simulated and analyzed a number of solid desiccants cooling cycles for the hot and humid cities of India. For the purpose of simulation 1% summer outdoor design data for sixteen major Indian cities was used. The analysis showed varied coefficient of performance (COP) for various cities of India under different weather conditions. Good performance criteria were high COP and low air flow rate to kW of refrigeration ratio. Extensive simulation of these cycles showed that desiccant cooling cycle based on Wet Surface Heat Exchanger (WSHE) performed the best among the cycles analyzed. This was exclusively due to high performance of the WSHE however, according to them, the high cost of WSHE made this cycle the most expensive among the cycles analyzed.

Jekkel’s [Jekkel, 1997] [16] research work consisted of two parts first one was the experimental

determination of heat and mass transfer in a commercial rotary dehumidifier and the second part was the performance simulation of rotary desiccant wheel. Using the governing equations for a rotary dehumidifier Jekkel developed a mathematical model of the system. Basically two types of simulations were carried out. First one was the equilibrium performance simulation of a rotary

desiccant wheel with infinite heat and mass transfer coefficients. Combined potential theory was use to achieve this goal. This study enabled him to determine the upper performance limit of the

desiccant wheel. The second simulation was done to ascertain the performance of the wheel for the non equilibrium conditions of operation for this purpose, simulation of the model using finite

difference method was performed. The effects of isotherm shape, total water capacity and matrix

specific heat were incorporated in the simulation model. A parametric study was also carried out

which attempted to predict the effect of desiccant properties and regeneration temperature on the

optimal performance of the wheel enabling determination of the size of the wheel. Experiments were conducted to determine heat and mass transfer coefficients needed in the non equilibrium modeling and simulation of the rotary desiccant wheel. Two types of experimental setups were used to determine the heat and mass transfer coefficients. The mass transfer coefficient was determined by conducting a single blow experiment on a commercially available desiccant wheel. The wheel had an aluminum core matrix of triangular passages with polymer desiccant coating. During experiment the core geometry was subjected to step changes in inlet humidity ratio. The heat transfer coefficient was ascertained by doing the same experiment on a dehumidifier passage without desiccant coating on its surface. Jekkel showed that resistance to mass transfer was large for a step increase in humidity ratio at the same temperature.

Hofker [Hofker, 2001] [17] performed both computer simulation and experiment in his PhD research work on a solar desiccant cooling test plant. Through his analysis of the system he showed that dehumidification is a function of rotation speed of the dehumidifier. He also demonstrated that relatively high COPs could also be achieved if regeneration is done with outdoor air. He studied different aspects of a desiccant cooling system in his model. The simulated desiccant cooling system was operated in different modes to observe the behavior of the system. Simulations were performed for the climates of Stuttgart, Phoenix, Seville and Djakarta. Predictions by the model for Stuttgart were compared with result from actual measurements of the plant and found to agree well. From the results of the extensive simulation he concluded that a solar driven desiccant cooling system performed best for the mild Stuttgart and worst for tropical Djakarta. For Seville and Phoenix solar energy was found to be not enough to comfortably cool the building and for the hottest days of the year and additional cooling was needed. Djakarta having a tropical humid weather had very few hours in a year when the solar powered desiccant cooling system could meet the comfort conditions. Simulation results showed that most of the time a mixture of these modes could successfully cool buildings located in all cities except buildings located in Djakarta. Djakarta being a very humid and hot in cooling season can not be cooled by single wheel DCS. They showed that it can however be cooled by a system using two desiccant and two heat exchanger wheels. Their analysis showed that this method was capable of meeting 29% of the cooling demand.

Khalid and Nabeel [Khalid, 2002] [18] performed computer simulation of a solar powered heating and solar desiccant cooling system for Baghdad. Transfer function method approach was used to develop the simulation model for estimating building cooling load. For heating load variable base degree day method was employed. Solar collector array model incorporating the shading effects was part of the comprehensive model. The results from the computer simulation showed that the major portion of cooling load could be met by the solar heater connected desiccant cooling system. They also demonstrated that desiccant cooling system can provide comfort conditions in cooling season in Baghdad.

Freund [Freund, 2003] [19] modeled an enthalpy wheel for a building air conditioning energy recovery system. He studied two rotary enthalpy exchangers for developing his model. He incorporated his model in TNSYS software for the simulation of the HVAC system. One problem which Freund tried to address to was the ice formation in the flow channels of the wheel. This situation may arise in winter heating mode of the desiccant wheel when the outside temperature was in the range of -25 to - 10ºC. To rectify this problem he considered two solutions, either to decrease the speed of the wheel or to preheat the outside cold air before allowing it to enter the wheel. He adopted the first solution considering it to be more energy efficient. The uncertainty of predicting the performance of the enthalpy wheel by the governing equations led Freund to use experimental data to model his enthalpy wheel in TRNSYS. This technique helped develop a model which was of general in nature and could predict the performance of any enthalpy wheel. From the simulation results the author concluded that

the use of a desiccant wheel in the HVAC system could save significant energy and improve indoor air quality.

2.2. Overview of Dehumidification Technologies

Building dehumidification in Karachi is important from the point of view of comfort and improved work efficiency. The comfort conditions are met when the building temperature and humidity are within certain limit. A comfort chart, according to ASHARE [ASHRAE, 1983] [1], showing these limits is given in Figure 2.1. Space cooling needs could be met by using natural means to create cool and comfortable condition in a dwelling. The use of natural means of space cooling depends very much on the climatic conditions of an area. A building can also be dehumidified by using artificial means; this of course can be done at the expense of energy. The first method which depends on nature for dehumidification a space and hence requires little or no energy input is called passive dehumidification method and the other which needs energy expenditure to operate is called active dehumidification method. Both passive and active dehumidification techniques are discussed in this section.

Figure 2.1 Thermal comfort chart [Sustainability Victoria, 2005] [20]

2.2.1. Passive dehumidification Techniques

In hot humid regions, air dehumidification is considered to be an issue of increasing concern

as an important aspect of indoor environment. A good air dehumidification is crucial to moisture

related problems, e.g. human health and comfort, building energy performance and building durability and maintenance. At the present, an air conditioning system plays an important part in decreasing air humidity and has become a necessity for almost all buildings. However, it often

consumes a great deal of energy and releases CFCs of HCFCs, which is harmful to environment

To produce an alternative solution to this issue, desiccant dehumidification applications

have been studied by several researchers whose studies were reviewed [Waugaman et al., 1993] [21].

Of particular interest are those with passive approach, which depends on the transfer of air moisture by natural means from a building to environment sinks such as clear skies, atmosphere, ground or water. The essentials of passive design were developed and used through the centuries by many civilizations across the globe. In fact many of these early civilizations dwellings were better suited to the climatic surroundings than those built today. This has been largely due to the advent of cheap fossil fuels that allowed for artificial humidity control at the cost of natural cooling. A substantial share of world energy resources is therefore being spent in dehumidification of such buildings. The International Institute of Refrigeration has estimated that approximately 15% of all the electricity produced in the whole world is employed for refrigeration and air-conditioning processes of various kinds. It has also been estimated that air conditioning consumes about 45% of the electric power consumed by all domestic and commercial buildings [Wimolsiri, 2005] [22]. And since most of the energy in air conditioning is spent on the dehumidification process taking place, so therefore it has become a necessity to explore passive dehumidification techniques so that we do not rely on our valuable energy resources in future.

The use of passive measures for dehumidification that are renewable and environment friendly can considerably bring down the costs as well as the energy needs of the building. Passive methods have no deteriorating effect on environment. These methods usually can reduce the building latent load appreciably and help reduce the number of hours when active dehumidification is required. However passive dehumidification alone cannot completely meet the dehumidification needs of a building.

Many passive dehumidification techniques are available; their implementation depends on the local conditions and design limitation. Under favorable conditions all of the given the dehumidification method can be utilized. Usually in humid conditions nocturnal cooling is not feasible because of small day and night time temperature difference.

2.2.1.1. Solar Regenerated Desiccant Dehumidifier

With regard to the fundamentals of desiccant, it initially attracts moisture from surrounding air until equilibrium state with the surrounding moisture is reached. In order to attract moisture once again, the desiccant needs to be dried or so technically called regeneration process. A typical regeneration process includes heating and exposure to scavenger air stream as shown in Figure 2.2. As this cycle repeats, the moisture in air absorbed by the desiccant could be defined as the level of air dehumidification. In these systems, the chamber of the system consists of netting solid desiccant bed. During nighttime, the desiccant bed dehumidifies air and releases sensible heat due to an absorption process through nocturnal radiation. During daytime, the bed is regenerated by solar energy and exhaust ventilation and ready again for air dehumidification in night time .

C:\Users\baqai\Documents\FYP Report\Solar Regenerated Desiccant Dehumidifier.png

Figure 2.2: Schematic of Solar Regenerated Desiccant Dehumidifier [23]

2.2.1.2. Passive Dehumidifying System Using Wood as Desiccant

In Figure 2.3, the operation of the system during daytime and nighttime is illustrated. An airtight

wooden attic space plays a role as a chamber while wood compositions of the attic, especially

plywood underneath roof material, work as desiccant material. During daytime, high temperature

in the attic space regenerates wood and moist air is to be vented out by exhaust ventilation.

Inversely, during nighttime, low moisture content generated during daytime induces wood

to adsorb moisture from humid air, supplied from the bedroom while sensible heat of the

absorption process relieved through nocturnal radiation. Here, the moisture load from the

bedroom represents the latent heat production of human's activity. After the air is dried by the

absorption process of the attic space, it is subsequently fed back into the bedroom. This cycle

is repeated throughout nighttime. The decrease in air humidity of the bedroom during nighttime

due to the moisture absorption of the attic space is taken into consideration as the quantity of

dehumidification.

Figure 2.3: Schematic view of the operation of the system during day and nighttime [24].

2.2.2. Active Dehumidification Techniques

In extreme heat and humidity conditions passive dehumidification methods are unable to dehumidify a building to comfort level, under such conditions active dehumidification techniques are employed for space cooling. These methods by their very nature need energy in the form of heat or electricity to operate; such systems are therefore classified according to the type of energy they use i.e., work driven cycles and heat driven cycles. Some active dehumidification methods are given below.

2.2.2.1. Dehumidification of Ventilation Air with Cooling Coils

The air conditioning cooling load can be divided into two parts – sensible load and the latent load. The sensible load is due to temperature difference between the indoor air and the outside ventilation air while latent load is due to moisture present in the air. Moisture is introduced in the room air by direct evaporation from wet objects, human, infiltration. A significant part of the sensible and latent cooling load comes from the ventilation air if it is warmer than and more humid than the room air. Conventional air conditioning systems use air for both cooling and ventilation purpose and as a result the cooling and dehumidification processes are coupled. If the building supply ambient air is of high humidity or a building needs low humidity air than all the above mentioned systems have to use under cooling and reheating method to remove extra moisture from air. Under cooling and reheating, shown in Figure 2.4, is a wasteful form of dehumidifying supply air. During this process air is cooled down below its dew point temperature to condense water. The under cooling is shown by line 1-2, at point 2 condensation of water vapor just starts further cooling along saturation line 2-3, results into desired condensation of water vapor. The overall result of this process is a drop in the absolute humidity of air and its temperature. At state point 3 air is cold and can not be sent into the building at this low temperature hence it is heated along line 3-4 which brings the temperature within comfort range. Under cooling increases compressor work because larger temperature difference between condenser and evaporator is needed. Reheating, if done by resistance heating, consumes electric

power. The outcome of these two processes is an increased use of energy by the air conditioning

system - consequently the COP of reheat cycle is low.

Figure 2.4 under-cooling and reheating method used in conventional air conditioning systems to dehumidify air

2.2.2.2. Desiccant Dehumidification System

An important heat operated cooling cycle is desiccant cooling cycle (DCS). This cycle is very

attractive for cooling of buildings since it uses the ventilation air as the cooling media and is regarded as an open cycle since air passes once through the system and is then sent in the building for space cooling purpose. This system essentially works at atmospheric pressure consequently the construction of the system is simple. DCS decouples the dehumidification and cooling hence the system is more flexible and part load operation is better managed. These systems are especially competitive in situations where large quantity of outdoor ventilation air is needed and latent load is large. Regeneration air for desiccant wheel can be obtained from a variety of sources such as engine cooling water and exhaust gases, CHP, exhaust gases from different industrial sources, solar energy etc. Wide spread use of this method of cooling eventually will result into switching air conditioning from electric grid to natural gas and renewable sources such as solar energy.

There are basically two types of desiccant cooling systems - liquid desiccant systems and solid

desiccant systems.

2.2.2.2.1. Liquid Desiccant Dehumidification System

The liquid desiccant system uses a concentrated solution of water and hygroscopic compounds like lithium chloride, calcium chloride and triethylene glycol. Ventilation air which is to be dehumidified is passed through packing material kept wet by a thin film of salt solution Figure 2.5 shows a liquid desiccant based air dehumidification system. Low vapor pressure at the film surface causes water vapor to migrate toward the salt solution where it is absorbed. The absorption process releases heat of condensation resulting in rise of air and solution temperature. One way to overcome this heating is to use cooled plates over which liquid absorbent flow this will remove heat of absorption and dehumidified air will remain cool. The solution gaining water from air becomes weaker with accompanying increase in its vapor pressure and has to be regenerated by heating. Regeneration of desiccant solution can be done by passing hot air through the solution which drives the vapor out of weak salt solution bringing the solution back to its original concentration. This liquid desiccant solution is then cooled and is used again in a cyclic fashion [Alizadeh, 2002] [25]. Solar operated liquid desiccant cooling system was first suggested by Löf [Löf, 1955] who used triethylene glycol as absorbent to dehumidify ambient air. Many such systems were used in large installation in USA in the past.

Liquid desiccant systems have a number of features that make them very attractive air conditioning systems [NREL, 2005] [26]. In the context of waste-heat recycling, they can make effective use of lowtemperature waste streams that other technologies can not use effectively. Their air-conditioning and regeneration functions can be physically separated, so the humidity-absorbing solution can be regenerated at any onsite heat source and conveniently piped throughout a building to where dehumidification is needed. Liquid desiccant can also be regenerated when heat is available and stored until there is a need for dehumidification. Storing cooling potential as chemical energy in this way also minimizes losses relative to thermal storage approaches.

Liquid desiccant cooling systems suffer from a number of shortcomings however including corrosive nature of absorbent salts, carryover problem of salt spray in the building. Absorbent such as lithium chloride reacts with certain chemicals in polluted air and loses its absorbing power [Yadav, 1995] [27]. Triethylene glycol is volatile and may by carried in the building by the ventilation air.

Figure 2.5 Liquid desiccant dehumidification system [ASHSRAE, 2003] [28]

2.2.2.2.2. Solid Desiccant Dehumidification System

The solid desiccant dehumidification system consists of four distinct processes happening in different system component to achieve dehumidification. These processes are sorptive dehumidification, heat recovery, evaporative cooling and heating. The cycle mentioned here is ventilation cycle which was first proposed by Pennengton in 1955 [Waugaman, 1993] [29]

Ventilation desiccant air conditioning cycle has two counter flowing air streams one process fresh air and the other building exhaust air. Warm moist ambient air is passed through a rotary desiccant wheel. As the ventilation air passes through the passages of desiccant wheel it losses moisture and the heat of adsorption raises its temperature. The emerging hot and dry air is cooled in a rotary sensible heat exchanger by the building exhaust air. The supply air can be further cooled in an evaporative cooler where its temperature is lowered and humidity is increased to comfort level. The exhaust air is heated in a heater to the regeneration temperature and flows through the desiccant wheel. The hot air removes the moisture from the desiccant wheel and is exhausted to atmosphere.

The performance of desiccant cooling system can be substantially improved if only part of the

regeneration air is used for desorption purpose and part is bypassed. Usually the warmer and moister the ambient condition the smaller is the bypass.

2.2.2.3. Hybrid Desiccant Dehumidification System

A hybrid desiccant dehumidification cycle combines a conventional vapor compression system and a solid desiccant dehumidification system to get a better temperature and humidity control for the hot and humid climatic conditions and achieve an improved coefficient of performance. Figure 2.6 shows the schematic of a hybrid desiccant dehumidification system and Figure 2.7 shows a solar hybrid desiccant dehumidification system. A hybrid desiccant dehumidification system combines following three sub systems:

1. A rotary wheel solid desiccant cooling system

2. A conventional vapor compression system

3. Indirect/Direct evaporative coolers (IEC)

A desiccant cycle is regarded as an open cycle since the system not only exchanges energy with the surroundings but mass also. In this system ventilation air acting as refrigerant passes once through the system and is then sent in the building for space cooling purpose.

Desiccant cooling systems are complete air handling units in which the cooling effect of evaporating water is used.

The need for low temperature regeneration air for desiccant wheel is an important feature related to the success of the cycle. Heat can be obtained from a variety of sources: diesel engine exhaust gases and jacket water, exhaust gases from different industrial sources, solar energy, air conditioner condenser reject heat and hot waste water from the textile industry Table 2.1 shows some of the waste heat sources for a desiccant dehumidification system with dehumidification possible for Karachi weather conditions.

Desiccant cooling and dehumidification technology offers new opportunities for commercializing waste heat and other systems powered by renewable energy. Applications for the desiccant air conditioner are as a thermally activated cooling system for processing ventilation air in humid climates. Adding solar thermal collectors can reduce operating costs. In rooftop installations, the collectors can be located near the desiccant cooling system to simplify installation and reduce costs.

Following advantages of desiccant dehumidification cycle makes it superior as compared to conventional vapor compression system used in the air conditioners.

1. A desiccant dehumidification system allows larger flow rates of ventilation air which permits to maintain a healthy environment in the building.

2. Air which is non toxic and none polluting and non corrosive is the working fluid in a

desiccant dehumidification system apart from water, the system remains at atmospheric pressure hence leakage can not be a problem. Because of this favorable flow situation cheap material of

construction can be employed.

3. The use of air as refrigerant in a desiccant based air conditioning system is environment

friendly. Chlorofluorocarbons and Hydrofluorocarbons used in vapor compression based air

conditioning systems are responsible for the ozone layer depletion and are a contributor to

greenhouse effect.

4. The desiccant dehumidification system shifts the power need from high grade and high priced electrical power to low grade low priced thermal energy.

5. It is also a desirable system in situations where latent cooling load is high such as in high

humidity climatic conditions furthermore it is preferable air conditioning system where the

need for low dew point temperature air exists.

6. Desiccant dehumidification extends the use of evaporative cooling to hot and humid climatic

zones.

7. A desiccant dehumidification system can be used in co-generation systems where the needed heat for regeneration can be obtained from the gas turbine exhaust, diesel engine exhaust or diesel engine jacket water etc.

8. Desiccant systems allow independent control of temperature and humidity in a zone. As a

result of this added control, energy savings are possible by raising the temperature set point

while maintaining low relative humidity without causing any discomfort to building

occupants.

9. Desiccant cooling system essentially operates at ambient pressure. This makes the fabrication

and maintenance of the plant much easier than vapor absorption and vapor compression

systems. These two systems need high pressure and low pressure to operate. The maintenance

of vacuum in a vapor absorption plant is must. If air leaks into the system the evaporator

operation is affected and cooling capacity suffers and eventually plant has to be shut down.

No such high pressure and low pressure area exist in the desiccant cooling system and hence

it is free from problems associated with it.

The applications of desiccant air conditioning are many, some are given below:

Super Markets

Theatres

Hospitals

Hotels

Office Buildings

Indoor Swimming Pools

Pharmaceutical Manufacturing Plants

Table 2.1 Sources of waste heat for heating dehumidifier regeneration air

Figure 2.6 Hybrid desiccant dehumidification system and its process representation on psychrometric chart. This system uses direct evaporative cooler and vapor compression cooling unit for post cooling of air

Figure 2.7 A solar assisted hybrid desiccant cooling system

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