Indirect Evaporative Cooling Availability and Thermal Effectiveness Characteristics in Air-Water Systems of Hot and Dry Climates

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 4 Indirect Evaporative Cooling Availability and Thermal Effectiveness Characteristics in Air-Water Systems of Hot and Dry Climates Waleed A. Abdel-Fadeel Assistant professor, Faculty of Energy Engineering, Aswan University, Egypt Tel: , Fax: , address: wfadeel@yahoo.com, P. O.: 828 Aswan, Egypt. Abstract-- The success of evaporative cooling technology 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 (AST). Evaporative cooling, 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 evaporative cooling was done. Also a comparison between previous work and present study carried out which gave the same trend. Finally this research quantifies evaporative cooling availability and thermal effectiveness in depth for southern Egypt (Aswan city) which has hot and dry climates that suitable for evaporative cooling. 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 (2 22) o C during months May, June, and July, and at range of ( 8) o C in March and April months for 87% availability during these months. Index Term-- Evaporative cooling ; Cooling tower ; Indirect evaporative; Hot and dry climate; Availability; Thermal effectiveness. IEC DEC T as T pa T pr T ps T sa T sr T ss WBT NOMENCLATURE Indirect evaporative cooling Direct evaporative cooling Ambient adiabatic saturation temperature (AST) o C primary approach temperature (PAT) o C primary loop return temperature o C primary loop supply temperature o C secondary approach temperature (SAT) o C secondary loop return temperature o C secondary loop supply temperature o C Wet bulb temperature GREEK LETTERS η tp primary thermal effectiveness η ts secondary thermal effectiveness as pa pr ps pa pr ss tp ts SUBSCRIPTS adiabatic saturation primary approach primary return primary supply secondary approach secondary return secondary supply thermal primary thermal secondary. INTRODUCTION Today s high cost of energy together with its environmental impact are reasons enough to warrant a reduction in energy consumption in current air conditioning systems, or those at the design stage. Any study of an air conditioning system in a building should focus mainly on thermal comfort, energy saving and environmental protection. The use of indirect evaporative cooling has a high potential for meeting air conditioning needs at low energy cost. Buildings, which have significant latent loads and which require high rates of air supply for ventilation purposes are often treated with all air-air conditioning systems, in which all conditioning equipment is confined to a central location and from which the treated air supply to the building is distributed. This system require chilled water at low temperatures typically -8 C, to produce dehumidification on the coils. These systems generally rely on vapor compression refrigeration to generate the required chilled water temperatures. Not all buildings, however, need high rates of air supply for ventilation purposes or have significant latent loads or require close control of humidity. Such buildings are often successfully treated with air-water systems in which a smaller central air system (the primary air) is used to supply ventilation air and offset latent gains. A cooling water distribution system is also used (the secondary water) to supply local sensible cooling equipment in each zone. The temperatures required for the secondary cooling water circuit also depend on the system employed. For example, dry mode fan coil units require a supply at -4 o C, while chilled ceilings require water at 4-8 o C. The sensible IJET-IJENS April 23 IJENS

2 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 4 cooling output of these systems is reduced as the cooling water temperature rises; however this reduction can be compensated for, if required, by increasing the area of the heat transfer surface. A crucial feature of the feasibility of the evaporative cooling in temperate climates is the achievement of a low temperature difference between the water exiting from the cooling tower and the ambient WBT (the primary approach temperature, PAT). This is necessary, in order to ensure a significant level of cooling water availability, especially in summer. To separate the tower water circuit from the building cooling circuit by means of a heat exchanger. Hence, the crucial design parameter becomes the temperature difference between the water exiting from the heat exchanger and the ambient WBT (the secondary approach temperature, SAT ). B. Costelloe and D. Finn [, 2, 3] confirm a major potential for the generation of cooling water by evaporative means, which can be used to provide effective cooling of deep plan buildings by means of contemporary water based sensible cooling systems, such as fan coil, radiant chilled ceiling panels and chilled beams. Also the thermal effectiveness of water side, of open indirect evaporative cooling has been presented. Finally the experimental performance of an open industrial scale cooling tower, utilizing small approach temperature difference has examined. The performance and energy reduction capability of combined system has been evaluated by Shahram Delfani et al [4] through the cooling season. The results indicate IEC can reduce cooling load up to 7 % during cooling season. Ala Hasan [] presents a method to produce air at sub-wet bulb temperature by indirect evaporative cooling without using a vapor compression machine. Cooling performance of two stage indirect/direct evaporative cooling system is experimentally investigated by Ghassem Heidarinejad et al [6] in the various simulated climatic condition. An experimental system of two stage evaporative cooling was constructed and tested in Kuwait environment by Hisham El-Dessouky et al [7]. The system is formed of an IEC unit followed by DEC unit. Results show that the efficiency of IEC/DEC varies over a range of 9-2%. An analytical evaluation using the field performance results of 8 L/s IEC unit and the recorded weather data in coastal and interior location in Kuwait was presented by G.P. Maheshwari et al [8]. Several types of materials, namely metals, fibers, ceramics, zeolite, and carbon was investigated by X. Zhao et al [9], which have potential to be used as heat and mass transfer medium in the indirect evaporative system, and the results show that thermal properties of the materials have little impact on system heat/mass transfer. A new heat and mass transfer model based on basic principles has been developed by J.Fsan et al [] for thermal calculation of an indirect evaporative cooler. A simplified model for indirect cooling towers behavior is presented by Pascal Stabat and Dominique Marchio [] the model is devoted to building simulation tools and fulfills several criteria such as simplicity of parameterization, accuracy, and possibility to model the equipment under different operation condition. An analysis was carried out by Francisco Javier et al [2] for the influence of factors such as temperature, flow, relative humidity, water flow rate, etc. on the basic characteristics defined by the mixed system, heat flow, heat efficiency and COP. Chilled ceiling panels can operate with a s upply water temperature as high as 8-2 o C [3]. These elevated secondary cooling water temperatures raise the possibility of generating the required cooling in cooling towers. Hence, the view has developed that tower based evaporative cooling ought now to be the subject of major review as a practical and low energy means of cooling modern buildings. At present, cooling systems generally use conventional vapor compression refrigeration to generate all cooling water at temperature suitable for primary air dehumidification and subsequently raise the water temperature to the required secondary temperature by means of a mixing arrangement or a heat exchanger. While water side evaporative cooling arrangements are occasionally used with air water systems, particularly in more arid climates, the use of the technique falls far short of its potential. This is particularly the case in west Egypt Aswan city of hot and dry climates where high difference between dry and wet bulb temperature compared to Egyptian cities as in table I that make it suitable for evaporative cooling, many opportunities to benefit from evaporative cooling technique are often overlooked. Also from previous work carried out on evaporative cooling availability and effectiveness no author touched this point especially for hot and dry climate. So our present research was directed to study indirect evaporative cooling availability and thermal effectiveness characteristics for hot and dry climates. 2. EXPERIMENTAL TEST RIG At present, however, there is little in depth research and analysis of the performance, energy efficiency and year round availability of this alternative form of cooling, especially in dry and hot climates at very low approach. To address these issues, an experimental research program has been established. So laboratory scale of indirect evaporative cooling was designed and erected at refrigeration and air condition laboratory at Aswan faculty of energy engineering Aswan University. The test rig as in Fig. is consists of an open counter-flow cooling tower, optimized for close approach conditions and incorporating a shell and tube heat exchanger which designed also for close approach conditions with two shell and two tube pass. The heat exchanger length of m and a diameter of.6 m.. The tower is upset truncated pyramid in shape as in Figure. Its lower base dimensions are 82 cm wide and 62 cm deep and its upper base dimension are cm wide and 8 cm deep and the tower height is 2 m. The tower has a forced draft fan of a power.37 kw and a revolution per minute of 42. The cooling load is provided by two electric immersion heaters each of which 2 w connected parallel to give a load of 2 W and 24 W that put in a vertical cylindrical container of. m length and.32 m diameter. A key issue in this research is the detailed evaluation of the extent of cooling availability which IJET-IJENS April 23 IJENS

3 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 42 can be expected, a range of cooling water supply temperatures, and thermal effectiveness. The test rig was instrumented to measure both the water supply and return temperatures of primary and secondary loops using thermometer. 3. OPERATING PROCEDURE -check the cooling tower water and make sure that it sufficient for whole operation 2-turn on the cooling tower pump and fans 3-check the boiler water level and adjust the electric heater at required load Loads (2w-24w) 4-turn on the secondary pump and electric heater -check air valve of the secondary pump to release vapor from boiling water to protect pump from failure 6-recording the results from measuring devices and plot results curves 4. EXPERIMENTAL RESULTS In this part, the obtained experimental results will be comprehensively discussed. The results include diurnal variation, availability of cooling water, and thermal effectiveness of primary and secondary loops of water side. 4- diurnal variations In this section the results of climate graph through all year and diurnal variation in conditions during May, June, and July will be discuss. Table II confirms that a PAT of no greater than 2.8 K is feasible for no load. Also a PAT of no greater than 3.6 K is feasible for 2 W load. This research confirms that open cooling towers in cooperating modern high surface density packing designs and operating under very close approach considerations. Also it could be noted from table II that PAT depends on the ambient AST or WBT. From features of the test results shown in table II that using PAT of 2K will be approximated value for most runs. Figures 2-4 show the diurnal variation in conditions during May, June, and July month 2. The figures 2-4 could be considered to represent a typical design day in Aswan (Egypt) with a maximum ambient dry bulb temperature reaching 39 o C and minimum WBT reaching 7. o C. Nevertheless, it was possible under these conditions, to produce cooling water temperatures of 2-22 o C, which could provide or contribute towards building cooling depending on the thermal conditions considered acceptable. In figure 2 a primary cooling water temperature of 2 o C were produced in June, while in figure 3 a primary cooling water temperatures of 2 o C were produced in July a condition which would suit a dry mode fan coil application. While in Fig. 4 cooling water temperature of 22 o C were produced in May. 4-2 Availability analysis 4-2- Determination adiabatic saturation temperature In order to quantify the evaporative cooling availability which can be expected for any given location it is, therefore, necessary to establish the typical yearly record pattern of the ambient AST. This can be achieved by using, either a meteorological test reference weather year or alternatively, where such is available. The method has been used in this research as it is now readily available in [4]. For wide range of world wide locations. The available data based on record of hourly weather data for 2 typical months. The hourly weather records typically include data on dry bulb temperature, relative humidity, wind speed and solar radiation. By using the dry bulb temperature and relative humidity a new data has been developed which lists a hourly record of standard psychometric properties included the WBT a long Aswan city Evaporative Cooling Potential Having established the hourly psychometric data for the site (Aswan Egypt), the data can now be analyzed to determine the evaporative cooling potential using the percentage annual availability of cooling water A where it could be defined as A= () where Σ(Htas) is the statistically typical total number of annual hours, during which, the ambient AST is less than or equal to (Tpf _ Tpa). Tpf is the primary flow temperature ( C), Tpa the PAT (K), 876 the number of hours in a year. In the first instance the occurrence of the ambient WBT is calculated and then, using a 2K PAT the potential for cooling water generation is determined. Figure shows the percentage occurrence of the cooling water temperature for Aswan based on the hourly prediction of the ambient condition. The results show that the WBT of 3 o C required to supply cooling water at o C (as in Figure ) is statistically available for 3% of the year in Aswan. The annual occurrence is calculated on a 24 h day basis and is defined as the percentage of the total annual hours (876 h) during which a temperature at or below a particular temperature, occurs. 4-3 Calculation of cooling water temperature through year 2 In this section a daily cooling water temperature possible for all year month at Aswan will be calculated based on [4] and using 2 k PAT, monthly possible average cooling water temperature, and finally a comparison between calculated and measured cooling water temperature for some days will be discuss. Figure 6 shows calculated cooling water temperature versus days from November to April through 2 year. It could be seen that cooling water temperature fluctuating with days for all months, also it could be seen that the lowest values in temperature are (second half of January and First half of February). Finally the higher values of cooling water are through November and December. Figure 7 shows calculated cooling water temperature versus days from May to October through 2 year. It could be seen that cooling water temperature fluctuating with days for all months, also it could be seen that the lowest values in IJET-IJENS April 23 IJENS

4 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 43 temperature are through May month. Finally the higher values of cooling water are through July and August. Figures 8 and 9 show comparison between measured and possible cooling water temperature versus time for July and May 7. It could be seen that there is a good agreement between measured and calculated cooling water temperature for May and July. Also the maximum deviations between measured and calculated cooling water temperature is less than 2%. 4-4 Thermal effectiveness A suitable means of assessing the thermal performance of the process is the thermal effectiveness ( η t ). This is defined as the cooling achieved, expressed as a fraction of the maximum possible cooling which could have been achieved in the ambient conditions pertaining. For the secondary circuit this parameter is defined by equation ( ) as in [2]; a similar equation defines the primary circuit. As this parameter involves both the approach and range condition. The two key determinants of energy performance, it is also a suitable parameter from this point of view. In particular the secondary thermal effectiveness (STE) is an important parameter as it assesses the performance of the indirect system as a whole, as distinct from the performance of the tower. It is also an important indicator of availability of cooling water generation potential. The STE can be defined with reference to figure in terms of the following equation: η t = Tsr-Tss / Tsr-Tas = Tsr-Tss / [ (Tsr-Tss) + (Tss-Tas)] () which can be expressed qualitatively as secondary range / (secondary range) +(secondary approach) Tests were conducts to investigate the impact of a range of operating variables on the thermal effectiveness achieved. These variables are (time, ambient dry bulb temperature, and wet bulb temperature ). For testing purposes the parameter being examined was varied while the other test rig variables were maintained constant. As there is no control over the ambient dry bulb temperature and ambient wet bulb temperature so a large number of tests were conducted and those tests with different values was selected. Figure shows secondary effectiveness versus time in Aswan for June It could be seen that the secondary effectiveness fluctuating with time. Also the average secondary effectiveness increase with time up to a certain time after that it decrease. As this could be attributed to change of dry bulb temperature with time which affect the cooling water temperature produced and affect secondary effectiveness as well Figure shows comparison between primary and secondary thermal effectiveness versus ambient dry bulb temperature. It can be seen that the primary thermal effectiveness has higher values than secondary thermal effectiveness. Also it could be seen that both primary and secondary thermal effectiveness increase with dry bulb temperature. And this could be attributed to increasing dry bulb temperature increase the range between dry bulb and wet bulb which affect secondary effectiveness by increasing it. Figure 2 shows thermal effectiveness versus wet bulb temperature through May, June, and July 2 at Aswan. It could be seen from the figure that thermal effectiveness at wet bulb temperature of (8-2 o C) increase with wet bulb temperature. Also it could be seen that primary effectiveness has a higher values than that the secondary effectiveness. And this could be explained as primary effectiveness is occur as a first exchange between air and water and occur at relative high temperature. At the opposite side secondary effectiveness occurs at relative low temperature and as we know that the efficiency is increase with temperature. So primary effectiveness is higher than secondary effectiveness as in figure Comparisons with previous work A comparisons of Measured diurnal variation and annual availability between reference (a) and present study (b) was shown in Figures 4-. It could be seen that the result of reference and present study has the same trend although the result different in values according to different conditions which could be attributed to changed meteorologic conditions such as ambient temperature, solar radiation, and wind velocity where reference carried out at temperate climates condition where present study carried out at hot and dry climates. Also comparisons of variation in primary and secondary approach temperatures and thermal effectiveness versus wet bulb temperature for both reference 2 and present study was shown in figures 6-7. It could be seen that the result of reference 2 and present study has the same trend although the result different in values according to different conditions which could be attributed to changed meteorologic conditions such as ambient temperature, solar radiation, and wind velocity where reference 2 carried out at temperate climates condition where present study carried out at hot and dry climates. CONCLUSIONS The results of a detailed meteorological analysis of evaporative cooling availability for south Egypt Aswan city have been presented and discussed. The results confirm a major potential for the generation of cooling water, which can be used to provide effective cooling of modern building by means of contemporary water based sensible cooling system, such as fan coils and chilled ceiling panels and beams. While the technique offers most potential in location as Aswan where the difference between dry bulb and wet bulb temperature is the highest over all Egypt. Also this paper outlines how the thermal effectiveness can be used as a measure of the degree to which the evaporative cooling system has succeeded in exploiting the cooling potential of the ambient air. The following specific conclusions can be drawn:- -Cooling water could be generated at range of (2 22) o C during months May, June, and July, which implies that buildings, such as educational institutes which are occupied IJET-IJENS April 23 IJENS

5 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 44 during these months may be successfully sensibly cooled through the year. 2- Cooling water could be generated in March and April month at range of ( 8) o C for chilled ceiling panel and beams for 87% availability during these months. 3-The test results indicate that thermal effectiveness is not affected by change in load. 4-The results of the tests indicate that both primary and secondary thermal effectiveness are significantly affected by both dry and wet bulb temperature. - Comparison between previous work and present study carried out which gave the same trend. REFERENCES [] B. Costelloe and D. Finn Indirect evaporative cooling potential in air water system in temperate climates Energy and building, 3 (23) 73-9 [2] B. Costelloe and D. Finn Thermal effectiveness characteristics of low approach indirect evaporative cooling systems in buildings Energy and building, 39 (27) [3] B. Costelloe and D.P. Finn Heat transfer correlations for low approach evaporative cooling systems in buildings Applied Thermal Engineering [4] Shahram Delfani, Jafar Esmaeelian, Hadi.P,and Maryam Karami Energy saving potential of an indirect evaporative cooler as a precooling unit for mechanical cooling systems in iran Energy and buildings [] Ala Hasan Indirect evaporative cooling of air to a sub-wet bulb temperature Applied Thermal Engineering [6] Ghassem Heidarinejad, Mojtaba Bozorgmehr, Shahram Delfani, and Jafar Esmaeelian Experimental investigation of two stage indirect/direct evaporative cooling system in various climatic conditions Building and Environment [7] Hisham El-Dessouky, Hisham Ettouney, and Ajeel Al-Zeefari Performance analysis of two stage evaporative coolers Chemical Engineering Journal [8] G.P. Maheshwari, F.Al-Ragom, and R.K. Suri Energy saving potential of an indirect evaporative cooler Applied Energy [9] X. Zhao, Shuli Liu, and S.B. Riffat Comparative study of heat and mass exchanging materials for indirect evaporative cooling systems Building and Environment [] J.FSan Jose Alonso, F.J.Rey Martinez, E.Velasco Gomez, M.A.Alvarez-Gurra Plasencia Simulation model of an indirect evaporative cooler Energy and buildings [] Pascal Stabat and Dominique Marchio Simplified model for indirect contact evaporative cooling tower behaviour Applied Energy [2] Francisco Javier, Mario Antonio, Eloy Velasco, Fernando Varela, and Ruth Herrero Design and experimental study of a mixed energy recovery system, heat pipes and indirect evaporative equipment for air conditioning Energy and Buildings [3] J. Fa Cao, A.C. Oliveira, Thermal behavior of closed wet cooling towers for use with chilled ceilings, Applied Thermal Engineering [4] T ABLE I AVERAGE EXTERNAL CONDITIONS FOR SOME EGYPTIAN CITIES THROUGH MAY TO JULY 2 CITY DBT C RH% WBT C DBT-WBT ALEXANDRIA CAIRO PORT SAID ASUIT LOXUR ASWAN IJET-IJENS April 23 IJENS

6 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 4 T ABLE II SUMMARY OF SOME EXPERIMENTAL TEST RESULTS FROM COOLING TOWER TEST RIG.(ASWAN) THROUGH MAY TO JULY 2 PAT Nominal load AST o C Primary flow temperature Secondary flow temperature SAT No load No load No load T ABLE III ANNUAL AVAILABILITY OF COOLING WATER TEMPERATURE IN ASWAN AT 2 K PAT Mont Cooling water temperature, o C h Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Des Annu al Open counter cooling tower Primary circuit Tpr Lood Tss Fan Tps Secondary Circuit Shell & tube Heat exchanger Tsr Air Fig. Simplified schematic of indirect evaporative cooling syst em IJET-IJENS April 23 IJENS

7 Temperature Co International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 46 Temperature C o Tps Tpr Tss Tsr TDB TWB Time hr Fig. 2. Measured diurnal variation for June 27 2 in Aswan at constant.2 kw load Tps Tpr Tss Tsr TDBT Twbt Time hr Fig. 3. Measured diurnal variation for July 3, 2 in Aswan at constant 2.4 kw load IJET-IJENS April 23 IJENS

8 Temperature Co International Journal of Engineering & Technology IJET-IJENS Vol:3 No: Time hr Fig. 4. Measured primary supply loop temperature for May 8, 2 in Aswan at no load. Fig.. Percentage annual occurrence of cooling water temperature in Aswan IJET-IJENS April 23 IJENS

9 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 48 Fig. 6. Calculated cooling water temperature for November to April At 2 k PAT in Aswan. Fig. 7. Calculated cooling water temperature for May to October At 2 k PAT in Aswan IJET-IJENS April 23 IJENS

10 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 49 Fig. 8. Comparison between measured and calculated cooling water temperature using 2K PAT for July, 2. Fig. 9. Comparison between measured and calculated cooling water temperature using 2K PAT for May 7, 2. Fig.. Thermal effectiveness versus time for June 27, IJET-IJENS April 23 IJENS

11 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 Fig.. Primary and secondary thermal effectiveness versus ambient dry bulb temperature through (May, June, and July) 2. Fig. 2. Primary and secondary thermal effectiveness versus wet bulb temperature through (May, June, and July) IJET-IJENS April 23 IJENS

12 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 Fig. 3. Primary and secondary approach temperature versus wet bulb temperature through (May, June, and July) 2. a Measured diurnal variation in conditions for 6 September b Measured diurnal variation for July 3 2 2, in Dublin at a constant 2 kw and maximum fan power. in Aswan at 2.4 constant kw load. Fig. 4. comparison of Measured diurnal variation between reference (a) and present study (b) a Comparison of annual availability with direct and b Percentage annual occurrence of cooling water temperature in indirect system for Dublin and Milan. Aswan IJET-IJENS April 23 IJENS

13 International Journal of Engineering & Technology IJET-IJENS Vol:3 No:2 2 Fig.. comparison of annual availability between reference (a) and present study (b) a Variation in primary and secondary approach temperature with annual. b Primary and secondary approach temperature range of AST in Dublin (load 2 kw, flow rates: tower water flux 2.9 kg/s m2, versus wet bulb temperature through (May, June, tower air flux 4. kg/s m2, secondary water.6 kg/s). The mean AST of. 8C and July)2 is shown with associated 3 K SAT Fig. 6. comparison of variation in primary and secondary approach temperatures between reference 2 (a) and present study (b) a Variation in thermal effectiveness with typical annual range of AST in b Primary and secondary thermal effectiveness versus wet Dublin (load 2 kw, flow rates: tower water flux 2.9 kg/s m2, tower air bulb temperature through (May, June, and July) 2 flux 4. kg/s m2, secondary water.6 kg/s). Fig. 7. comparison of thermal effectiveness between reference 2 (a) and present study (b) IJET-IJENS April 23 IJENS