2.1. Direct evaporativecooling (DEC)
The working principle scheme of the DEC equipment and a simplified flow scheme are presented in figure 1. The warm inlet air (1) enters in a pad which is sprayed with water at the wet bulb (WB) temperature of the inlet air. The heat transfer is realised from the warm air to the cold water. The heat is transferred by the air stream as sensible heat and is absorbed by the water as latent heat. Corresponding to the value of latent heat, a part of the water is evaporated being embedded by diffusion into the flowing air, increasing the moisture content of this air. The temperature of the outlet air (2) decreases due to the sensible heat transferred by the air, but the enthalpy of the outlet air will be the same with the enthalpy of inlet air as effect of the latent heat recovered into the air as moisture.
The dehumidified air temperature is increased due to the heat of adsorption (process 1–2) as shown in Fig. 3 which depends on the type of adsorbent as discussed earlier. The dehumidified air is then cooled at constant humidity using heat exchanger (process 2–3) and meanwhile the recovered heat is used for the desiccant regeneration (process 6–7) which increases the system COP. Then the following cooling techniques are used to push the process air (stage 3) into the thermal comfort zone (stage 4 or 5) that includes: (i) direct/indirectevaporativecooling, (ii) cold coil, (iii) M-cycle cooling, and (iv) evaporator of the VAC. In case of DEC cooling the water vapours are added as an isenthalpic process as shown in process 3–4 whereas the rest of cooling is performed at constant humidity that can be noticed from process 3–5. M-cycle cooling technique is a kind of IEC technique which can cool the air near to the dew point rather than wet bulb temperature which is shown in Fig. 4. The process 8–9 in Fig. 3 shows the desiccant regeneration which can be made by heating the desiccant or reducing its pressure. However, heating is preferred in commercial applications whereas pressure reduction is adopted in industrial processes. A typical DAC system consists of a desiccant unit, heat recovery component, a cooling unit and a regeneration unit. The available solid DAC systems can be categorized as: (i) adsorbent bed or rotating wheel type, (ii) single-stage or multi-stage dehumidification type, (iii) single-rotor or multi-rotor type, (iv) hybrid system assisted by VAC, and (v) electricity/gas/solar/waste energy operated DAC system [17-22].
Abstract—Air-conditioning plays an essential role in ensuring occupants thermal comfort. However, building’s electricity bills have become unaffordable. Yet the commercially dominant cooling systems are intensively power-consuming ones, i.e. vapor compression systems. This paper aims to review the recent developments concerning evaporativecooling technologies that could potentially provide sufficient cooling comfort, reduce environmental impact and lower energy consumption in buildings. An extensive literature review has been conducted and mapped out the state-of-the-art evaporativecooling systems. The review covers direct evaporativecooling, indirectevaporativecooling and combined direct-indirectcooling systems. The indirectevaporative coolers include both wet-bulb temperature evaporative coolers and dew point evaporative coolers have been of particular interest because of high thermal performance. The dew point evaporative coolers have shown great potential of development and research opportunity for their improved efficiency and low energy use.
For any technology to be successive at grassroots level in Africa, it must: have no power grid sources, be easily duplicated by farmers with readily available materials, be on an appropriate scale in operation and economics for individual farmer/family or village Saving and Credit Co-operative (SACCO), have more than one use (year-round utility), be inexpensive and efficient and preserve organoleptic properties of the food. Technologies centered on evaporativecooling as method for preserving vegetables in developing countries are discussed. This is because such technologies do not require any special skill to operate and are simple to construct in rural areas. According to Liberty et al. (2013a) the biggest challenges that occurs during storage of vegetables are the change in quality parameters such as the color, texture, and freshness which determine price and consumers perceptions. During storage, vegetables undergoes chemical and physical changes due to evaporation of water from the tubers, respiration, sprouting, and spread of diseases due fungal invasion resulting in storage losses. However, to prevent storage losses favorable storage conditions within the store should be maintained. According to Liberty et al. (2013b), deteriorative reactions in vegetables can be
Comprehensive review covering basics and potentials of thermoelectric devices has been presented. It showed that the technology can be used practically in different areas and applications. The application of small capacity thermoelectric coolers are in wide spread while applications of large capacity ones and power generators are limited by their low efficiency. However, concern for the environment to be free of chlorofluorocarbons (CFC s ) and energy costs have revived interests in the use of thermoelectric systems. Efficient, clean energy conversion for high value added applications such as space, defence, etc is needed now and in the future. Thermoelectric devices should be a good choice. (Riffat et al, 2002). Research in to this area is still very much desired. Equations for the maximum rate of heat pumping capacity and the maximum coefficient of performance (COP) (Chen et al, 1997) which can help in analysing the performance of the thermoelectric system have been established. (Yamanashi et al, 1996) investigated an optimum way to maximise the coefficient of performance (COP) for a thermoelectric cooling system using thermoelectric cooler and heat exchangers at the cold side and hot side.
Indirectevaporative coolers (IEC) are components that can be effectively installed in air handling units to increase energy efficiency of air conditioning systems. In particular, such devices can be used in summer conditions to reduce chiller load in both existing and new buildings. In this paper, an IEC system based on a cross flow heat exchanger has been tested, evaluating its cooling capacity in different operating conditions. Performance is evaluated in terms of wet bulb effectiveness, primary air temperature reduction and fraction of evaporated water. Results put in evidence that a significant cooling capacity can be achieved in many operating conditions. Therefore, IECs are a promising technology that can be effectively used to reduce primary energy consumption of conventional systems.
This paper studied a case for applying RIEC technology to an all-fresh-air A/C system of a wet market in Hong Kong. The results show that the RIEC has the largest annual energy saving potential among the RIEC system and two kinds of rotating heat recovery wheel system, with a payback period of 2.9 years. Annually, HK$79232 can be saved by the fresh air pre-cooling RIEC system, which is 45% more than the enthalpy heat recovery wheel system, showing a great potential of application in Hong Kong.
EAC control type#4, Unit powered individual microprocessor based thermostat, is essentially like type #3 except there are no batteries to change. Some control systems will feature a portable memory transfer device that can be used to transfer programming from a PC to the individual stand- alone controllers. Maintenance consists of a periodic (seasonally) review of the programmed on/off schedules, time change and clock accuracy check, and sometimes reprogramming on power loss. EAC control type#5, Central energy management control systems (EMCS), is the most versatile of all these control systems. These systems have temperature, humidity and other sensors located in the EAC (and heating) units, and are also wired into the damper and/or valve motors. A personal computer is used to monitor and control all functions, and to alert you when values stray from preset values. Additional programming can perform temperature setback/setup, optimum start/stop, and control of various arrangements for the optimal sequencing of combination direct/indirect with refrigerated air conditioning. Additional software can help track, assign personnel, audit maintenance logs and more. Maintenance required on these systems consists of periodically replacing or repositioning sensors and backing up local data files.
Traditional vapour compression systems (VCS) employing vapour compression cycle have been used for air-conditioning & refrigeration in both industrial and residential applications. Thus, these systems are quite popular and well known. But some limitations of the VCS technology are that the refrigerant gases used are harmful for the environment and also these systems have high energy consumption in the form of electricity. An alternative to these systems are the evaporativecooling systems (ECS). These systems achieve cooling effect through evaporation of water. The ECS can be categorized into direct contact and indirect contact devices. Since the working fluid is water, there are no harmful by-products of this system. Major advantage of ECS is that it blows fresh air into the desired location instead of recirculating same air like VCS. Only a fan or blower is required to circulate cool air. Also, pumping device is required to circulate water throughout the circuit. Hence, ECS have comparatively less electricity consumption than VCS. Despite these advantages, the ECS can be employed only in environments having less humidity. This is because ECS work on the principle of evaporation of water. As warm air comes in contact with cool water, the water at the contact evaporates.
There are two basic approaches to this form of indirectcooling system (i) the closed wet cooling tower and (ii) the open tower with external plate heat exchanger. Each arrangement has advantages in particular circumstances and locations (Costelloe and Finn, 2000). While much research has been done on the closed tower in this application (Facao and Oliveira, 2000) there is a need to investigate the thermal performance of the open tower in operating conditions well outside those encountered in refrigeration condenser heat rejection - range and approach conditions as low as 1-4 K, cooling water temperatures of 14- 18˚C and ambient conditions of < 20˚C AST. These conditions result in much smaller levels of enthalpy difference, the key driving force in the tower, and therefore smaller associated heat and mass transfer rates with, crucially, resulting higher air and water flow rates. To address these issues an experimental research facility has been developed at the Dublin Institute of Technology and is described elsewhere (Costelloe and Finn, 2000). The thermal efficiency (η t ) of the process is defined as the cooling achieved as a fraction of the maximum possible cooling which could have been achieved in the ambient conditions pertaining.
Abstract -- The success of evaporativecoolingtechnology as a significant means of a cooling in modern application is the ability to generate cooling water, in an indirect circuit, at a temperature which closely approaches the ambient adiabatic saturation (AS T). Evaporativecooling, can be used to provide effective cooling in building by means of contemporary water based sensible cooling system, such as fan coil systems and ceiling cooling convertors (chilled beams). In this research a diurnal variation for May, June, and July was measured .A comparison between measured and calculated cooling water temperatures result from evaporativecooling was done. Also a comparison between previous work and present study carried out which gave the same trend. Finally this research quantifies evaporativecooling availability and thermal effectiveness in depth for southern Egypt (Aswan city) which has hot and dry climates that suitable for evaporativecooling. The results of this research confirm a major potential for the generation of cooling water by evaporative means. Where Cooling water could be generated at range of (20 – 22) o C during months May, June, and July, and at range of (15 – 18) o C in March and April months for 87% availability during these months.
This study was thus intended to define what problems PDEC systems currently have, to understand the fundamentals of evaporativecooling, and to present viable solutions so that PDEC towers can be more widely utilized as a cooling system in buildings. In the literature review, the key barriers that must be overcome to advance these systems were defined. A computational analysis was performed to understand the main physical phenomena that take place in the passive down-draft cooling processes. With this computational model, the influence of each critical parameter that significantly affects the cooling performance of PDEC towers was studied. To support further study on the actual impact of these systems, a mathematical model that allowed the accurate predictions of PDEC temperature and velocity exit conditions was developed using regression analysis with the forward selection method. These models were then implemented into the whole building energy simulation program EnergyPlus in order to investigate the potential problems and the impact of a typical PDEC system on a typical summer day. As a result of this study, an alternative control method that can resolve some of the problems with PDEC towers was presented. The overall impact of PDEC systems in various building types and climatic conditions was finally investigated for a 6-month period during cooling season. The main findings obtained from the results of the computational analysis as well as dynamic energy simulations are summarized below.
The indirectevaporativecooling (IEC) represents old water based coolingtechnology which is rediscovered in our days, being still relatively unknown . In this study IEC is evaluated from the energy efficiency point of view in fresh air cooling systems.
The most important particularity of IEC is represented by the use of water in the cooling process. Different reviews are presenting technical, technological, economical and scientific aspects about IEC [2-4].
One of the world’s most energy consuming sector is heating, ventilation and air conditioning. A lot of energy is consumed by traditional cooling systems based on the conventional vapour compression refrigeration cycle. The VCR system which contains refrigerants in circuits produced a harmful effect on environment and also dangerous to human life if leakages occur. These factors were mainly responsible for the development of evaporative coolers. In direct evaporativecooling system, it will give adequate temperature drop of air with increase in humidity which is not desirable for human health. So for overcoming these difficulties, indirectevaporativecooling system will be used for air coolers in which heat mass exchanger is used. To get higher temperature drop and wet bulb effectiveness above 100%, Scientist Valeriy Maisotsenko developed Maisotsenko cycle. The aim of our study is to find the best alternative of existing conventional cooling systems.
Heat and mass transfer process is the core of evaporativecooling that utilizes the evaporation of water for air conditioning process. In this process, high volume of heat is exchanged from air to water which simultaneously decreases the air temperature. There are three main types of evaporative coolers. First, direct evaporative coolers, indirectevaporative coolers and combination of direct and indirectevaporative coolers . Evaporativecooling is known to be a efficient option in thermal management and controlling the temperature across a wide scope of natural and artificial applications . Existing air conditioning technology such as thermoelectric refrigeration, vapour compression, and absorption or adsorption when compared with evaporativecooling shows that it is a better choice. Aspects such as cheaper cost and possibility of higher efficiency is what evaporativecoolingtechnology is made of regardless of dry and hot weather or milder climate .
Department of Mechanical Engineering, University College Dublin, Ireland
Recent developments in enhancing heat transfer in cooling towers, together with the success of chilled ceilings, have prompted a review of the evaporativecooling technique. in temperate maritime climates. The thermal efficiency of such systems is a key parameter, as a measure of the degree to which the system has succeeded in exploiting the cooling potential of the ambi- ent air. This paper presents the results of ex- perimental research into the thermal efficiency of a water-side open indirectevaporative cool- ing test rig designed to achieve low (1-4 K) ap- proach conditions. Secondary efficiencies in the range 0.24-0.76 have been achieved.
dimensions of the visible plumes that would potentially occur from the water vapor emissions associated with the cooling towers. Required physical parameters such as exit cell dimensions, exit velocity, tower heat rejection rates, and downwash dimensions for the cooling tower were estimated and modeled to determine the frequency of occurrence of visible plumes as well as the associated plume lengths and heights. Seasonal heat input and ambient air intake associated with the plume-abatement system were also included. The CALPUFF model predicts the potential for cooling tower plume visibility from the proposed Facility by referencing the actual hourly meteorological conditions that existed during the 5-year period between 2005 and 2009. According to U.S. Environmental Protection Agency’s (EPA’s) Guideline on Air Quality Modeling (40 Code of Federal Regulations [CFR] Part 51 Appendix W), five consecutive years of hourly meteorological data is an adequate sample of meteorological data to capture a long-term distribution of potential conditions representative of the project site.
5.4.4 The effect of cooler configuration on humidity ratio
The operating properties of the conditioned air in the sub-wet bulb temperature evaporative cooler are further shown on a psychometric chart of Figure 5.9. It can be seen that at design conditions of T di =35 o C and relative humidity of 35%, the conditioned air supply has a constant water vapour content of 12.3 g/kg d and a temperature lower than the wet bulb temperature, with the ultimate cooling temperature that can be achieved equal to the dew point temperature of 17.7 o C. This led to the dry channel side of the porous ceramic panel being sealed with a thin non- permeable membrane so no latent heat transferred from the porous ceramic panel to the intake air dry channel; heat only transferred from the intake air to the porous ceramic panel. However, the working air circulated in the wet channel is rejected at saturation conditions when its characteristic coincides with that of the saturation line on the psychometric chart and exits the evaporative cooler at 26.3 o C and water vapour content increases from 12.3 g/kg d to 22 (g/kg d ) because of the direct contact between the working air and the ceramic panel.
Direct evaporativecooling adds moisture to the supply air. Indirectevaporativecooling occurs in the heat exchanger without adding moisture. A High-Pressure System is shown here. Direct and indirectevaporativecooling can function similarly when using a Wetted Media System.
Fig. 1: Basic principle of evaporating cooling 
Evaporative pads were made from different materials such as Aspen , metal [8–10], cement , ceramic materials , coconut coirs [8,10,12–14], wood wool fibers [10,12,15], jute fiber [8,16], date palm fibers , khus fibers [4,9,10,15], cellulose paper pad [12,18], plastic , and glass. Cellulose pads are mainly superior these days for their light weight, low cost, and high saturation efficiency, typically, greater than 80 % . They have an excellent cooling efficiency, but they also increase the humidity. So, this increased humidity is the major problem today in case of these direct evaporative coolers. Besides, manufacturing of commercial pads made of these materials are complicated and costly. Aspen pad also is widely used, but it very sensitive to algae infestation that could lead to decay and compaction, which makes it difficult to maintain its efficiency without frequent and costly pad replacement . Therefore, evaluating the locally available cheap materials for use as pads, particularly in rural agricultural areas is essential.