Typical Meteorological Year Data (TMY)

The most common data for describing the local solar climate is through what is called Typical Meteorological Year data (TMY). To determine TMY data, various meteorological measurements are made at hourly intervals over a number of years to build up a picture of the local climate. A simple average of the yearly data underestimates the amount of variability, so the month that is most representative of the location is selected. For each month, the average radiation over the whole measurement period is determined, together with the average radiation in each month during the measurement period. The data for the month that has the average radiation most closely equal to the monthly average over the whole measurement period is then chosen as the TMY data for that month. This process is then repeated for each month in the year. The months are added together to give a full year of hourly samples.

There is no strict standard for TMY data so the user must adjust the data to suit the application. Considerable care must be taken with sample periods. An example of a raw TMY data file is given below for January 1st in Melbourne, Australia. The comments on the right hand side of the file describe the measurements taken and the data format.

TMY2 and TMY3

Due to the inconsistencies in TMY data, the data sets were updated for the US in 1994  (http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/tmy2/) and again in 2008 (http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/)

          1 1 1  0  0130  0 99007   MELBOURNE TMY: 75 71 75 73 76 78

	  1 1 2  0  0127 10 98040                  78 72 78 70 69 68

	  1 1 3  0  0118  7 94040

	  1 1 4  0  0109  3 90041   -----------

	  1 1 5  0  0100  0 86001   PARAMETERS:

	  1 1 6 33106117  3 96141   -----------

	  1 1 7110260133  7106140   MONTH

	  1 1 8186311150 10115140   DAY

	  1 1 9257342183  7133140   HOUR

	  1 110317351217  3151140   HORIZONTAL GLOBAL IRRADIATION, MJ*100/HOUR

	  1 111356353250  0167000   SUN TRACKING BEAM IRRADIATION, MJ*100/HOUR

	  1 112381357272 14166120   AMBIENT TEMPERATURE, DEG.C*10

	  1 113387362293 27165120   WIND SPEED, M/S*10

	  1 114365359315 41164120   WET BULB TEMPERATURE, DEG.C*10

	  1 115324355314 43164130   WIND DIRECTION, COMPASS POINTS

	  1 116267348314 45163130   CLOUD COVER, OCTAS

	  1 117198336313 46162140   --------

	  1 118120277307 38162140   FORMAT :

	  1 119 44147300 29160150   --------

	  1 120  2 12294 21159150   1X,3I2,5I3,I2,I1

	  1 121  0  0274 26154150

	  1 122  0  0253 31148160

	  1 123  0  0233 36143160

	  1 124  0  0235 36142160

The description at the side of the above data is formatted into a more readable format, and is shown below:

TMY data is used for a wide variety of meteorological applications and therefore a large amount of data is usually irrelevant for photovoltaic applications. Of the parameters given, usually only the time and irradiation figures are used. However, more advanced models also use the temperature and wind speed.

• Month is the month of the year with January = 1 and December = 12.
• Day is the number of days within a month. Not all months are of equal value.
• Hour is the hour of the day in 24 hour time, so the table above covers one full day. The data is usually an average for the hour and covers ½ an hour before the sample to ½ an hour after the sample. Thus the first row on the table is for 1^st January and covers 12:30am to 1:30am.
• The Global Irradiation is the amount of energy striking a horizontal surface during the hour. To convert from the above units of x100MJ/hr/m² to the typical photovoltaic units of kW/m², divide by 360. The greatest irradiance is at midday and falls to 0 at night.
• The Direct Beam Irradiation gives the irradiation striking a plate perpendicular to the sun's rays but does not include diffuse radiation. A tracking concentrator only sees the direct beam irradiation
• The Temperature and Wind Speed are averaged over the hour. Note the x10 multiplication factor.
• The Wet Bulb Temperature is the temperature shown by a thermometer bulb surrounded by a damp wick. The WBT varies from the dry bulb temperature according to the amount of evaporation and thus the humidity. This detail is generally not used for PV applications.
• The Wind Direction shows the compass points from which the wind is coming. North is 00, East is 04 etc. Again, this detail is generally not used for PV applications.
• The Cloud Cover is a visual estimate of the proportion of cloud covering the sky. In this case it is in eights so 0 is no cloud and 8 is full cloud cover. And, once again this detail is generally not used for PV applications.

One additional unit often required in PV applications is the amount of diffuse radiation. The diffuse irradiation (D_h) falling on a horizontal surface can be calculated from the global irradiation (G_h), the direct beam tracking radiation (I_t) and the elevation (el) from:

The diffuse radiation is not uniformly spread across the sky. For instance, the area just around the sun (circumsolar) is considerably brighter than the rest of the sky. A commonly used model is the Perez model 1 which divides the sky into a circumsolar component, horizon component,and the rest of the sky. The size of each of these regions and the relative intensities of the regions are adjusted to closely model the measured radiation.

• 1. R. Perez, Ineichen, P., Seals, R., Michalsky, J., and Stewart, R., “Modeling daylight availability and irradiance components from direct and global irradiance”, Solar Energy, vol. 44, pp. 271 - 289, 1990.