Analysis of Typical Meteorological Year for Seeb/Muscat, Oman

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1 Analysis of Typical Meteorological Year for Seeb/Muscat, Oman Y. H. Zurigat 1, N. M. Sawaqed 2, H. Al-Hinai 3 and B. A. Jubran 4 1 Department of Mechanical Engineering, University of Jordan, Amman-Jordan zurigat@ju.edu.jo 2 Department of Mechanical Engineering, Mu tah University, Mu tah, Jordan nsawaqed@mutah.edu.jo 3 Department of Mechanical and Industrial Engineering, Sultan Qaboos University, Al-Khod, Sultanate of Oman hilal@squ.edu.om 4 Department of Aerospace Engineering, Ryerson University, Toronto, Ontario, Canada bjubran@ryerson.ca Abstract In this study the Typical Meteorological Year (TMY) data for the Seeb/Muscat area of the Sultanate of Oman is presented and analyzed. The analysis shows that diurnal variations in dry-bulb temperature are relatively small. However, seasonal variation indicates two distinct seasons: the hot season covering the months of April through to October (ambient temperature exceeds 30 C for most of the hours) and the relatively cool season covering the months of November through to March (ambient temperature less than 27 C for most of the time). Air conditioning (AC) is required over the seven-month hot season period. During the cool season the outdoor conditions are favourable for natural ventilation. The outdoor design temperature for the hot season may be deduced from the cumulative probability distribution calculated for the season. It is around 40 C based on 2.5% summer frequency level. The high level of solar radiation flux available in Seeb (exceeding 500 W/m 2 for March through October) makes it attractive for solar energy based technologies. The information presented in this paper is essential for designers, architects, planners and contractors for proper design and selection of energy systems and application of energy-related projects in the most populated region in the Sultanate of Oman. Keywords typical meteorological year; data analysis; building energy systems Introduction Global warming, air pollution and the continuous depletion of nonrenewable energy resources are attributed to the present energy use patterns. The building sector (commercial, residential, and industrial) consumes 30 50% of the total energy requirements of a society [1]. In many parts of the Sultanate of Oman where cooling and dehumidification is required over a large part of the year, over 70% of the electricity bill in residential and public buildings goes into AC [2]. Typical cooling load profiles indicate that this load is essentially steady for seven months (April through to October) and falls to an almost steady rate for the remainder of the year. CFC-based vapour compression cooling which is the cause of the ozone layer damage is the most widely used AC technique. For sustainable development, a reduction in energy demand is essential. This could be achieved through improving energy efficiency, using renewable environmentally-friendly energy sources and effective energy conservation and management.

2 324 Y. H. Zurigat et al. The weather conditions of a given region are the most important considerations for the proper design of space AC systems. They are essential for the application of different passive cooling techniques. For example, in dry hot climates where large temperature differences exist between day and night, massive wall structures may be used successfully to cool the indoors during the day. Also, evaporative cooling can be employed in such weather conditions. However, these same two techniques cannot be used under hot and humid conditions with little difference between day and night temperatures. A study conducted by Zurigat et al. [3] showed that as high as 43% reductions in peak cooling load can be achieved using a combination of well-established passive cooling techniques and technologies such as roof and wall insulation, internal and external shading and double glazing. In their study a single year weather data was used. It must be noted that careful analysis of weather data is essential for successful selection and implementation of passive cooling techniques and proper sizing of cooling equipment in general. Normally, Typical Meteorological Year (TMY) data are used to facilitate proper performance comparisons of energy systems whose performance depends on weather conditions. Recognizing the importance of TMY, Sawaqed et al. [4] have recently developed TMYs for seven different locations in Oman. The TMY for a given location is a data set of actual hourly meteorological parameters for one year constructed from measured data of several past years. A TMY month is selected from the months of the period being considered based on statistical analysis and the months selected are assembled to form the TMY. The TMY is commonly used in building energy systems simulations and in assessments of wind and solar energy systems performance including PV systems. Also it is used in the modeling of agricultural systems and the dispersion of air pollutants and in simulations of microenvironment greenhouses in agriculture and solar desalination systems. The objective of this paper is to present analysis of TMY for Seeb/Muscat which is the largest populated region in Oman. This is essential for designers, architects, planners and contractors for proper selection of energy systems and application of energy-related projects. A preview of Sultanate of Oman weather and energy consumption The Sultanate of Oman lies between latitudes and North and Longitudes and East. The coastline extends 1700 kilometers from the Strait of Hurmuz in the north to the borders of the Republic of Yemen and overlooks three seas: the Arabian Gulf, Gulf of Oman and the Arabian Sea. The Sultanate of Oman has a variety of topographical features consisting of plains, valleys and mountains. The climate differs from one area to another. It is hot and humid in the coastal areas in summer but hot and dry in the interior with the exception of the higher mountains, which enjoy a moderate climate throughout the year. Rainfall is generally light and irregular; although rare heavy rain occurs and thunderstorms can cause severe flooding. In the south, the Dhofar region has a moderate climate and the pattern of the rainfall is more predictable with the monsoon influenced rains occurring regularly between May and September.

3 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 325 Since the launching in 1970 of a comprehensive development program through successive five-year development plans, steady progress has been achieved in the Sultanate of Oman in different areas such as education, health, housing and electricity. The increased construction within the country has resulted in a rapid rise in energy consumption during the last three decades. This is reflected in the rapid growth in the number of power plants constructed and the level of power produced. Power capacity has risen from 4.5 MW in 1970 to 1520 MW in 2000 [5]. Considering the fact that over 70% of the electricity consumption in residential and commercial sectors goes into vapour-compression air conditioning, the search for energy conservation measures and alternative cooling techniques is justified. Weather data analysis constitutes a first step towards this objective. Seeb TMY data analysis The TMY for Seeb [4, 6] was developed based on 17 years of data ( ) using the Sandia method [7] and presented without analysis, hence the present contribution. The weather parameters considered in the development of TMY were the dry bulb temperature (mean, min, max and range), relative humidity (mean, min, max and range), wind velocity (mean, max gust and max sustainable-10 minutes) and global solar flux data. Figures 1 and 2 show the monthly averages of global solar flux and mean temperature for different months of the TMY along with the long term monthly means. Figure 1. Average global solar fl ux for Seeb TMY and long term monthly averages (Sawaqed et al., 2004).

4 326 Y. H. Zurigat et al. Figure 2. Monthly mean temperature for Seeb TMY and long term monthly averages (Sawaqed et al., 2004). The latter were computed by averaging the monthly means over the 17 years data considered. It is seen that the TMY data closely represent the long term data. Considering the TMY, in general, solar radiation flux data indicate monthly averages between 400 and 725 W/m 2 with December and January having the minimum and May having the maximum. The monthly mean solar flux exceeds 600 W/m 2 for 5 months (April through to August). Figure 2 indicates that the hottest months are May through to July which have monthly mean temperature values of over 34 C while the coolest months are December through to February where the monthly mean temperature does not exceed 22 C. To visualize the severity of the weather conditions over the 17 years, the extreme values of several indicators were extracted from the data for Seeb. Figure 3 shows the yearly maximum wind indicators; mean wind speed (Ws,mean), max gust (Ws,gust) and max sustained-10 minutes (Ws,sust). Also shown are the minimum and maximum temperatures (Tmin, Tmax, respectively). The maximum temperature reaches as high as 49.5 C and the minimum temperature as low as 10 C. The wind speed indicators (see Fig. 3) show that the maximum mean wind speed ranges between 10 and 20 knots (1 knot = kilometer/hour) whereas the max gust reaches as high as 46 knots and ranges between 30 and 46 knots. The yearly average values of the indicators discussed above along with the relative humidity (RH) are shown in Fig. 4. It is observed that the yearly average maximum and minimum temperatures are almost constant at 33 and 24 C respectively, whereas

5 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 327 Figure 3. Extreme values of the wind speed and temperature indicators for the 17 years considered. Figure 4. Yearly average values of mean relative humidity, min and max temperatures, and wind speed indicators (mean, max gust, and max sustainable 10 min.).

6 328 Y. H. Zurigat et al. Figure 5. Cumulative distribution function of daily global solar fl ux for Seeb TMY and for each month of the TMY (labeled 1 to 12). the yearly average mean relative humidity is between 46 and 52%. The yearly average values of wind indicators are seen to vary slightly over the years considered, knots for gust, 7 12 knots for sustained, and the mean wind speed is essentially constant at approximately 5 knots. Fig. 5 for the TMY daily-averaged global solar flux indicates solar flux values higher than 500 W/m 2 for over 70% of the days whereas it exceeds 700 W/m 2 for only 10% of the days. For individual months, clearly the months of April through to August have the highest solar flux exceeding 600 W/m 2 for over 80% of the days (123 days). For December and January the solar flux exceeds 400 W/m 2 for 50 and 60% of the days, respectively. The solar flux values can be viewed best when considering the TMY daily global solar flux data as well as the TMY total hourly horizontal solar flux (beam and diffuse) as shown in Fig. 6. The cumulative probability distributions (CPD) are calculated over the sunshine hours. The hourly values were generated from measured daily global solar radiation data using the model given in Duffie and Beckman [8] and presented in details in Sawaqed et al. [4]. It is seen that the hourly horizontal solar flux exceeds 500 W/m 2 for 38% of the sunshine hours of the year and exceeds 700 W/m 2 for over 15% of the sunshine hours of the year. As shown in Fig. 6 the hourly solar flux is the lowest for the months of December and January for which it is less than 500 W/m 2 for over 97% of the time (sunshine hours). Also, for the months of November and February the solar flux is less than

7 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 329 Figure 6. Cumulative distribution function of hourly total horizontal solar fl ux for the TMY and each month of the TMY (labeled by numbers 1 to 12). 500 W/m 2 for 75% and 63% of the sunshine hours, respectively. The rest of the months (March through to October) have solar flux exceeding 500 W/m 2 for about 50% of the time. This indicates the potential in using solar-based technologies such as solar desalination, photovoltaic power generation, solar absorption cooling, solar drying and the like. Regarding the hourly temperature (dry bulb T db, wet bulb T wb, and dew point T dp ) and hourly mean wind speed of the TMY, the CPDs are presented in Fig. 7. Except for the dry bulb temperature daily range parameter (T db,max T db,min ) the distributions shown in Fig. 7 are based on hourly data. The hourly mean wind speed is below 10 knots for 90% of the time. The mean wind speed daily range is above 10 knots for 60% of the time. Also plotted in Fig. 7 are the CPDs of the hourly values of wet bulb temperature, dew point temperature and the dry bulb temperature at 80% saturation. The wet bulb and the dew point temperatures were calculated from the dry bulb temperature and the relative humidity using psychrometrics equation [9]. These parameters are important in many engineering applications. For example, the wet bulb temperature indicates the maximum achievable cooling effect in evaporative cooling. For Seeb TMY this temperature is below 20 C for 40% of the time and it is below 25 C for 70% of the time. In practice, however, full saturation is unachievable due to limitations in the effectiveness of evaporative cooling equipment. A value of 80% saturation is common with commercially available evaporative coolers. The dry bulb temperature with 80% saturation indicates that it is below 20 C for 30% of the time and it is below 30 C all the time.

8 330 Y. H. Zurigat et al. Figure 7. Cumulative probability distribution of several temperature and wind indicators for Seeb TMY. The potential of evaporative cooling is frequently assessed using the evaporative cooling degree hours (ECDH) method. The method consists of summing up the product of wet bulb temperature depression WBD = (T db T wb ) and the number of hours of its occurrence in a particular day for all days of the month. For example, assuming for a particular day the following wet bulb depressions and their frequency (time duration of occurrence) have been calculated based on psychrometrics of the hourly weather data: 10 C for 5 hours, 7 C for 10 hours and 5 C for the rest of the day i.e., 9 hours. Then the CDH number for that day is calculated as: ( ) = 165 CDH. Summing up these numbers for each month and for the whole year the monthly and yearly CDH numbers for the site in question may be obtained. Table 1 shows the results calculated for Seeb TMY. This method was used by Zurigat et al. [10] to assess the feasibility of gas turbine inlet air cooling by fogging in which a 100% wet bulb temperature approach is assumed. The dew point temperature is another important indicator in applications such as gas turbine inlet air cooling used for boosting the power output. In order to avoid condensation at the gas turbine inlet which causes erosion of the compressor blades, the temperature to which one can cool the inlet air must be limited to a value above the dew point temperature. The TMY hourly dew point temperature plotted in Fig. 7 indicates that it assumes values above 10 C for over 90% of the time. For the TMY the hourly dry bulb temperature (see Fig. 7) is higher than 30 C for 40% of the time and it is above 25 C for over 70% of the time. It reaches a maximum

9 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 331 Table 1. ECDH values (in C.hr) and Frequency (in hours) of wet bulb depression values for the ranges shown for different months of the year Month ECDH (C.hr) <1 C 1 5 C Annual Twb yearly mean = C Wet bulb depression (Twbd) yearly mean = 6.97 C Twbd yearly mean for Twb d > 1.0 C = 7.25 C of 46 C. The daily range of a given parameter indicates the swing in daily values, i.e. (max min). For the daily mean temperature Fig. 7 shows that the temperature daily range is below 10 C for 80% of the time which indicates that passive cooling using thermal mass would not be effective. Considering the ambient temperature data Fig. 8 shows the CPDs of the daily mean data for Seeb TMY and for the months of the TMY. Clearly, the hottest months are May through to July with June being the hottest. The temperature exceeds 35 C for 50% of the days in June, 43% in July and 33% in May. The months of December through to February experience daily mean temperatures below 23 C for all the days. The distribution for the whole TMY (the curve labeled TMY) indicates mean temperatures in excess of 25 C for 67% of the days of the year. The months of November through to March seem to be the coolest months. For these months the mean daily temperature does not exceed 30 C, and 23 C for December through to February for which the temperature variation is very small. The cumulative probability distributions of the hourly mean dry bulb temperature is shown in Fig. 9 and exhibit similar trends as observed for the mean daily data shown in Fig. 8. Clearly, the months of May through to July experience the highest temperature exceeding 35 C for 42% of the hours of the month. For the months of December through to February the hourly ambient temperature does not exceed 28 C and it is below 25 C for 80% of the time. This indicates that a natural cooling technique is potentially applicable, especially in buildings with small internal thermal loads. In air conditioning applications the design of air conditioners requires a selection of outdoor design temperatures to avoid oversizing which normally results in a high

10 332 Y. H. Zurigat et al. Figure 8. Cumulative distribution function of daily mean temperature for Seeb TMY and TMY months. Figure 9. Cumulative distribution function of hourly temperature for the months of Seeb TMY.

11 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 333 Table 2. Cooling season design temperature (in C) based on Seeb TMY for different frequency values and two cooling periods (numbers in brackets indicate the number of hours the indicated temperature is exceeded) Cooling period Total hours 0.4% 1% 2% 2.5% April through October (21) 42 (51) 40.8 (103) 40.4 (129) May through July (9) 43 (22) 42.3 (44) 41.8 (55) frequency of the on and off cycles which reduces the moisture removal capability of the conditioner. For summer cooling, a 2.5% design temperature (i.e. the outside temperature, which will be exceeded only 2.5% of the hours of the summer season) is frequently selected. The ASHRAE Handbook of Fundamentals [11] lists 2%, 1% and 0.4% summer design temperatures. Figure 9 indicates two groupings of hot months, namely May through to July and April through to October. This is done because of the nature of the weather conditions having distinct seasons. Table 2 shows the summer design temperatures calculated for the TMY based on 2.5%, 2%, 1% and 0.4% design temperature for the two groupings. Their corresponding hours, out of the total hours of the season, that exceed the design temperature are also shown in brackets. Although similar design values for other weather parameters may be deduced similarly, the figures presented in this work give a comprehensive picture of the variations of these parameters. The second parameter that affects human comfort and influences the latent cooling of the equipment is the relative humidity. The cumulative distribution for the relative humidity and its daily range is shown in Fig. 10. It is seen that for Seeb the relative humidity is below 70% for 73% of the TMY hours (i.e hours). It reaches over 80% for 10% of the time. The daily range indicates quite a large swing reaching at times 85% and it is above 60% for 8% of the time. As done with the mean temperature (see Fig. 9) it is preferable to view the cumulative probability distributions of the daily mean relative humidity for the months of the TMY. As shown in Fig. 11, August is seen to be the most humid month of the year. The daily mean relative humidity exceeds 70% for 73% of the days. In contrast, April and May are the driest months. Figure 12 shows the CPD for hourly relative humidity for the months of the TMY which shows similar variation in relative humidity to that shown in Fig. 11 for the daily mean relative humidity. As stated earlier knowing the dry bulb temperature and the relative humidity a number of other parameters can be calculated (i.e., wet bulb and dew point temperature and vapour pressure). Figure 13 shows the CPD for the wet bulb temperature which indicates that except for May and June it is less than 25 C for all the time. Since the wet bulb temperature is the temperature achievable by a 100% efficient evaporative cooler one can evaluate the potential of evaporative cooling in the Seeb area as given in Table 1. Even when evaporative cooling is 80% efficient (resulting in dry bulb temperature at 80% saturation), except for June and July, the resulting

12 334 Y. H. Zurigat et al. Figure 10. Cumulative distribution function of hourly relative humidity and its daily range for the Seeb TMY. Figure 11. Cumulative distribution function of daily mean relative humidity for Seeb TMY and each month of the TMY (labeled 1 to 12).

13 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 335 Figure 12. Cumulative distribution function of hourly relative humidity for the months of Seeb TMY. Figure 13. Cumulative probability distribution of hourly wet bulb temperature for the months of Seeb TMY.

14 336 Y. H. Zurigat et al. Figure 14. Cumulative probability distribution of hourly dry bulb temperature at 80% saturation for the months of Seeb TMY. dry bulb temperature does not exceed 30 C (see Fig. 14). The difference between the dry bulb and wet bulb temperatures (see Fig. 15) indicates the magnitude of the potential reduction in the dry bulb temperature as a result of evaporative cooling. Fig. 15 shows that April to July have the highest. The temperature difference exceeds 10 C for over 50% of the time while for the months of August and December through to March the difference is minimal as it stays below 10 C almost all the time. The last temperature parameter is the dew point temperature shown in Fig. 16. The dew point temperature governs the condensation rate of water moisture in the ambient air. The higher the dew point temperature the higher the potential of condensation as the mean temperature daily range is increased. Examining Fig. 16, the months of May through to October have the highest potential. Summary and conclusions The typical meteorological year data (TMY) for Seeb/Muscat are presented and analyzed. The data show little diurnal variation in dry-bulb temperature (less than 10 C for most of the time). However, seasonal variation indicates two distinct seasons: the hot season (ambient temperature, T a, exceeds 30 C for most of the time) and the winter pleasant season (T a less than 27 C for most of the time). The air conditioning is required over a seven-month period. During the winter season

15 Analysis of a Typical Meteorological Year for Seeb/Muscat, Oman 337 Figure 15. Cumulative distribution function of hourly dry bulb-wet bulb temperature difference for the months of Seeb TMY. Figure 16. Cumulative distribution function of hourly dew point temperature for the months of Seeb TMY.

16 338 Y. H. Zurigat et al. (November March) the outdoor conditions are favorable for natural cooling/ventilation. Although the weather conditions may be characterized as being hot and humid during the hot season the evaporative cooling degree hours (ECDH) calculated shows that about 58,800 ECDH is achievable indicating the potential of this technique in such a location. The small diurnal variation in dry bulb temperature indicates that a massive structure is not recommended as a means of passive cooling measure. Results were also obtained for the outdoor design temperature, mean daily ranges for dry-bulb temperature and relative humidity. Acknowledgements The funding provided by the Petroleum Development of Oman is gratefully acknowledged. We also thank Mr Sami Al-Salti for his assistance with the figures. References [1] S. C. M. Hui, Energy efficient buildings-practical design guide, Report: HKU Arch 1998/99, Department of Architecture, The University of Hong Kong, (1999). [2] ESCWA, Thermal insulation of buildings envelope and energy-efficient lighting: Choices of priority for energy conservation in building sector in ESCWA countries, Proceedings of Expert Group Meeting on Energy for Sustainable Development in ESCWA Member States: The Effi cient Use of Energy and Greenhouse Gas Abatement, 8 11 October, 2001, Document No (Beirut). [3] Y. H. Zurigat, H. Al-Hinai, B. A. Jubran and Y. S. Al-Masoudi, Energy efficient building strategies for school buildings in Oman, Int. J. of Energy Research, 27 (2003). [4] N. M. Sawaqed, Y. H. Zuirgat and H. Al-Hinai, A step-by-step application of Sandia method for developing typical meteorological years for different locations in Oman, Int. J. of Energy Research, 29 (2005), 6. [5] Ministry of Water and Electricity of Oman, Statistics of Energy Consumption, Annual Report (2000). [6] Y. H. Zurigat, N. Sawaqed, H. Al-Hinai and B. Jubran, Development of Typical Meteorological year for Different Climatic Regions in Oman, Final Report, Petroleum Development Oman (2003). [7] I. Hall, R. Prairie, H. Anderson and E. Boes, Generation of Typical Meteorological Years for 26 SOLMET Stations, SAND , 1978 (Albuquerque, NM: Sandia National Laboratories). [8] J. A. Duffie and W. A. Beckman, Solar Engineering of Thermal Processes, John Wiley (1982). [9] W. P. Jones, Air Conditioning Engineering, 4 th Edition, Arnold (1994). [10] Y. H. Zurigat, B. Dawoud, J. Bortmany and S. Al-Shehabi, Technical and economic feasibility of gas turbine inlet cooling using evaporative fogging system for two different locations in Oman, in ASME Turbo Expo, Power for Land, Sea, and Air, Vienna, June, 2004 (Vienna, Austria, Paper GT ). [11] ASHRAE Handbook of Fundamentals (2001).