THE NEGEV SOLAR RADIATION SURVEY - TMY V.4
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Negev Radiation Survey, TMY v.4 3 THE NEGEV SOLAR RADIATION SURVEY - TMY v.4 David Faiman, Daniel Feuermann, Peter Ibbetson and Amos Zemel The Center for Energy and Environmental Physics The Jacob Blaustein Institute for Desert Research Ben-Gurion University of the Negev Sede Boqer Campus, 84990 Israel and Amiran Ianetz, Vera Liubansky and Ilan Setter Research Development and Engineering Branch The Meteorological Service P.O. Box 25, Bet Dagan, 50250 Israel ABSTRACT This report summarizes the first 14 years of data (1989 - 2002) that have been processed from the Negev Radiation Survey. It accompanies a CD-ROM containing Typical Meteorological Year files (TMY v.4) based on the direct beam component, and the archived hourly data upon which they are based. The average annual direct beam total for all the stations is 2196 kWh m-2 year-1. For purposes of simulating the performance of solar-concentrator power plants the nine stations fall naturally into two groups: those having direct beam totals up to 14% higher than the mean (Mitzpe Ramon, Sede Boqer, Eilat, Arad, Yotvata) and those having annual direct beam totals up to 11% lower than the mean (Beersheba, Sedom, Hatzeva, Besor Farm,). Mitzpe Ramon is the highest and Besor Farm the lowest, on average. Annual fluctuations in direct beam radiation may, however, be considerable. For example, during the 14 years of data that are discussed in the present report, both Beersheba and Sede Boqer reveal differences of more than 33% between their best and worst years. It is concluded that: (1) sufficient data probably now exist in order to enable one to identify the best places for locating solar power stations; (2) several more years of data will be necessary before a sufficiently reliable data base will exist for the purpose of simulating solar-concentrator power plant performance and determining their economic benefit. The average annual global horizontal radiation for all stations is 2017 kWh m-2 year-1. For purposes of simulating solar power plants of the non-concentrator variety the nine stations divide up in a somewhat different manner: those having global horizontal totals up to 5% higher than the mean (Mitzpe Ramon, Yotvata, Sede Boqer, Arad) and those having global horizontal totals up to 6% lower than the mean (Eilat, Beersheba, Hatzeva, Besor Farm, Sedom). Mitzpe Ramon and Yotvata are highest and Sedom is lowest. We note that for non- concentrator purposes, Eilat changes “leagues” compared with its superior status for solar- concentrator power plants, it here being slightly below the average. We note also that the year- to-year fluctuations in global horizontal radiation are very much smaller than those among the direct beam components. We conclude, therefore, that for non-concentrator purposes we probably now have enough data for reliable simulations. 4 Negev Radiation Survey, TMY v.4 Negev Radiation Survey, TMY v.4 5 THE NEGEV SOLAR RADIATION SURVEY - TMY v.4 David Faiman, Daniel Feuermann, Peter Ibbetson and Amos Zemel The Center for Energy and Environmental Physics The Jacob Blaustein Institute for Desert Research Ben-Gurion University of the Negev Sede Boqer Campus, 84990 Israel and Amiran Ianetz, Vera Liubansky and Ilan Setter Research Development and Engineering Branch The Meteorological Service P.O. Box 25, Bet Dagan, 50250 Israel 1. INTRODUCTION A Typical Meteorological Year (TMY) consists of twelve monthly files of actual hourly meteorological data selected in a particular manner [1]. The months will not, in general, have come from the same year. Instead, each will have been chosen as being a “typical” representative of the month in question and, ideally, the choice for each will have been made from very many years of accumulated data. The reason for taking actual months of data rather than averaged files is that the former preserve correlations (both known and unknown) that exist among the different measured parameters (e.g. solar radiation and ambient temperature) and also correlations that exist over a period of several days among values of any given parameter. These correlations are often of great importance when one tries to simulate the performance of an energy conversion system that has characteristic time-scales of several days (e.g. passive solar buildings [2], swimming pools [3], etc.). The term “typical” is somewhat subjective in that its choice depends, to a large extent, on how the data are to be used. For agricultural purposes it might, for example, be sensible to select typical months on the basis of rainfall and ambient temperature. The present study, however, is part of the Negev Radiation Survey, the aims of which are to provide data of relevance to the performance of solar power stations. Clearly, therefore, the relevant criterion for this purpose is solar energy. There are in fact two solar radiation components that are measured: the global horizontal radiation and the normal direct beam component. For any given Negev site the former is found to vary by approximately ± 5% from year to year. On the other hand, year to year variations of more than 30% have been observed in the direct beam component, over the comparatively few years that this study has been in progress. Since, moreover, solar power plants will have to be designed in a manner that will enable them to meet contractual obligations - irrespective of weather fluctuations - it is the direct beam component of the solar radiation that is the critical parameter on which the term “typical” must be based. 6 Negev Radiation Survey, TMY v.4 We expect, therefore, that simulation of a solar power station using one of these TMY files will provide quantitative information about how the system should behave in a typical year. It will not, however, provide any indication as to how the system would behave in an “unusually poor” (from the standpoint of weather) or an “unusually good” year. This information can only come from using data from such years. Again, in an ideal situation where a great many years of data exist, one could construct “unusually poor” and “unusually good” meteorological years in order to meet the needs of such system simulations but this is not yet the situation in which the Negev Radiation Survey finds itself. Fig.1 shows the locations of the meteorological stations involved in the Negev Radiation Survey. At the present stage, the Survey contains up to fourteen years of data from the nine stations, Arad, Beersheba, Besor Farm, Eilat, Hatzeva, Mitzpe Ramon, Sede Boqer, Sedom and Yotvata. One of the stations (Hatzeva) has recently had to be closed down owing to a lack of available personnel to maintain the instrumentation. 2. TMY CONSTRUCTION As already indicated, our TMY files have been chosen for typicality of the direct beam component of the solar radiation. For sites at which we have an odd number of years of data we “tried to choose” the year whose monthly direct beam average rendered it the median year of the available set. On the other hand, where an even number of years of data were available we “tried to choose” the year whose monthly mean was closest to the average taken over all of the years in the set. The words “tried to choose” have been used because sometimes this was not possible owing to large amounts of missing data in the desired file. In such cases a “second best” choice was made. Regarding small quantities of missing data, even relatively complete day files occasionally have a few hours when one or more instruments were not working properly. In such cases our practice is to look for a nearby qualitatively similar day, copy the relevant data sequence and use it to patch the hole. Table 1 shows the number of data years we presently have for each month at each site. Tables 2-10 display the monthly average values of direct beam radiation recorded by each of the stations during the fourteen-year period 1989-2002. Values in bold characters indicate the specific months that were ultimately chosen for each site as the basis for TMY v.4. 3. THE DATA FORMAT The raw data provided by the Israel Meteorological Service include hourly average values of: direct beam radiation; global horizontal radiation; shadow-band pyranometer data; dry-bulb temperature; relative humidity (or alternatively, from some stations, the wet-bulb temperature); wind speed and wind direction. Of the solar radiation data only direct beam and global horizontal are archived, the shadow-band data having been used for consistency checks only. Humidity data have been processed using algorithms given in the ASHRAE chapter on psychrometrics [4] in order to compute the humidity ratio. The present report is accompanied by a CD-ROM which contains, in addition to the TMY v.4 files for each of the 9 stations in the Negev Radiation Survey, a complete set of all of the historic data files from which the TMYs have been constructed. The actual data files, although Negev Radiation Survey, TMY v.4 7 using an identical format to the TMYs, differ in that they frequently contain gaps in the measured data. These are indicated by the entries “-999” (or “-9” in the case of wind data). Thus, any algorithm which has been designed to access the TMY files will also be able to access the files corresponding to individual years provided appropriate caution lines have been inserted to prevent the program from misinterpreting missing data as negative input numbers. The present study employs a format which the University of Wisconsin originally established for the US SOLMET TMY diskettes so that the latter could interface with their TRNSYS simulation program. One diskette was used for each site.