Presentation on theme: "Solar Radiation Solar Radiation"— Presentation transcript:
1 Solar RadiationSolar RadiationThe sun is a gaseous body composed mostly of hydrogenGravity causes intense pressure and heat at the core initiating nuclear fusing reactionsThis means that atoms of lighter elements are combined into atoms of heavier elements, which releases enormous quantities of energyEven when planet Earth is 93 million miles away, we still received an amazing quantity of usable energy from the sun.Considering 25% efficient PV modules, if we used 1% of the surface of the earth we could meet 29 times our current total energy demand –These some rough calculations I did, but I’ll be glad to discuss your numbers if you happen to get something different.
2 Solar RadiationSolar RadiationSolar irradiance is the intensity of solar power, usually expressed in Watts per square meter [W/m^2]PV modules output is rated based on Peak Sun (1000 W/m^2).Since the proportion of input/output holds pretty much linearly for any given PV efficiency, we can very easily evaluate a system performance check by measuring irradiance and the PV module output.The amount of radiation received is proportional to the inverse of the square of the distance from the source –that is, twice the distance ¼ of the energy, four times the distance 1/16 and so onSolar irradiation is simply the solar irradiance multiplied by time. It is measured in Watt-hours per square meter [Wh/m^2]
5 Solar RadiationSolar RadiationSolar Spectrum most the energy received from the sun is electromagnetic radiation in the form of waves.Electromagnetic Spectrum is the range of all types of electromagnetic radiation, based on wavelength.
6 Solar RadiationSolar RadiationAtmospheric Effects: Solar radiation is absorbed, scattered and reflected by components of the atmosphereThe amount of radiation reaching the earth is less than what entered the top of the atmosphere. We classify it in two categories:Direct Radiation: radiation from the sun that reaches the earth without scatteringDiffuse Radiation: radiation that is scattered by the atmosphere and clouds
8 Solar RadiationSolar RadiationAir Mass represents how much atmosphere the solar radiation has to pass through before reaching the Earth’s surfaceAir Mass (AM) equals 1.0 when the sun is directly overhead at sea level. AM = 1/ Cos ӨzWe are specifically concerned with terrestrial solar radiation –that is, the solar radiation reaching the surface of the earth.At high altitudes or in a very clear days, Peak Sun may be more than 1000 W/m^2 but it is a practical value for most locationsPeak Sun Hours is the number of hours required for a day’s total radiation to accumulate at peak sun condition.
9 Solar RadiationSolar RadiationZenith is the point in the sky directly overhead a particular location –as the Zenith angle Өz increases, the sun approaches the horizon AM = 1/ Cos Өz
10 Solar Radiation Example problem of Peak sun hours per day: If during the day we have 4 hours at 500 Wh/m^2 and 6 hours at 250 Wh/m^2 we should compute the peak sun hours per day as follow:First, multiply 4hs x 500 W/m^2 and add to it 6hs x 250 W/m^2 – This will equal 3500 Wh/m^2Second, we know that by definition Peak Sun is 1000 W/m^2, so if we divide the total irradiation for the day by Peak Sun we will obtain Peak Sun hours. – That is,Peak Sun Hours = Total Irradiation [Wh/m^2] / Peak Sun [W/m^2] = Peak Sun hoursIn our specific problem:Peak Sun Hours = 3500 Wh/m^2 / 1000 W/m^2 = 3.5 Peak Sun hoursNote: most solar irradiation data is presented in Peak Sun Hours units
11 Solar RadiationSolar RadiationInsolation; this is an equivalent term for solar irradiation and can be expressed in KWh/m^2/day or peak sun hours
12 Solar RadiationSolar RadiationSolar spectral distribution is important to understanding how the PV modules that we’re going to utilize respond to itMost Silicon based PV devices respond only to visible and the near infrared portions of the spectrumThin film modules generally have a narrower response range
14 Solar RadiationSolar RadiationLong-term solar irradiation measurements are the basis for developing databases, which help us to calculate output.Being able to predict the output of our PV system, and this will allow us to know whether it is working adequately or notPredicting output will help us to calculate the cost of the energy generated over a given time periodPyranometers measure irradiance. Typically, you will use a handheld pyranometer that uses a silicon cell or photodiodes and you will set it adjacent to the array, in the same plane as the array –not as precise but appropriate for constructionPyrheliometers measure direct solar radiation (and ignore diffuse) and I’ve never ran into a situation where I had to use one
16 Solar RadiationSolar RadiationTwo major motions of Earth affect the apparent path of the sun across the sky:Its yearly revolution around the sunIts daily rotation about its axisThese motions are the basis for solar timescale and the reason why we have seasons, days and nightsEcliptic Plane is the plane of Earth’s orbit around the sunEquatorial Plane is the plane containing Earth’s equator and extending outward into space
18 Solar RadiationSolar Declination is the angle between the equatorial plane and the ecliptic planeThe solar declination angle varies with the season of the year, and ranges between –23.5º and +23.5º
19 Solar RadiationSummer Solstice is at maximum solar declination (+23.5º) and occurs around June 21st –Sun is at Zenith at solar noon at locations 23.5º N latitudeWinter Solstice is at minimum solar declination (-23.5º) and occurs around December 21stAt any location in the Northern Hemisphere, the sun is 47º lower in the sky at noon on winter solstice than on the summer solstice – Days are significantly shorter than nights
20 Solar RadiationEquinoxes occur when the solar declination is zero. Spring equinox is around March 21st and the fall equinox occurs around September 21st –Sun is at Zenith at solar noon on the equator.Around the equinoxes the daily [rate of] change is at maximum as oppose to change of declination during the solstices when it is at its minimum
21 Solar RadiationSolar RadiationStandard meridian is located at a multiple of 15º east or west of zero longitude –located at Greenwich.15º represents one hour of change, so we can infer the 1º will be 4 minutesBoth of these facts are interesting to know; however, you will not apply either of these things to design or install PV systems.You’re concerned with the average peak sun hours for your location and your shading analysis tool will be reading in solar time –but this irrelevant to you.
23 Solar RadiationSolar RadiationSolar Altitude Angle is the vertical angle between the sun and the horizon –added to the Zenith angle is equal to 90ºAzimuth Angle is the horizontal angle between a reference direction.In the solar industry we call south 180º and this angle will range between 90º (east) and 270º (west)
24 Solar RadiationSolar Window is the area of sky between sun paths at summer solstice and winter solstice for a particular location
25 Solar RadiationSolar RadiationIncidence Angle is the angle between the direction of direct radiation and a line exactly perpendicular to the array angle
26 Solar Radiation Array orientation is defined by two angles: Tilt angle is the vertical angle between the horizontal and the array surface
27 Solar Radiation2. Array Azimuth Angle is the horizontal angle between a reference direction –typically south- and the direction an array surface faces
28 Solar RadiationSolar RadiationMaximum energy gain will be achieved by orienting the array surface at a tilt angle close to the value of the local latitude –In high latitudes arrays should be very steep and vice versaFor optimal performance the tilt angle should be adjusted from the latitude angle by an amount equal to the average declination during that timeDuring the summer the average declination is +15º, so we should have a tilt of latitude minus 15º to make the array perpendicular to the average solar path –during the summerArray Azimuth angle will be optimal when that array is due southSun trackers allow the PV array to change the tilt angle, the azimuth angle, or both –generally is not considered cannot be made cost effective
30 Solar RadiationSolar RadiationComputer models and the average climate conditions are used to calculate an optimal tilt angle factor –aka correction factor we have to subtract from the latitude.In our area we use an optimal tilt angle factor of 15ºBy making our tilt angle equal to the latitude angle minus this angle factor, we will improve the performance of our PV arrayExample for our area: If we look at dataset provided by NREL, we can see that for a 0º tilt we would have 4.7 peak sun hours and for a latitude - 15º tilt we would have 5.3 peak sun hoursIn other words, we would have an output roughly 13% higher by using this correction factorYou can calculate this percentage as follows:(5.3 – 4.7) / 4.7 = or 12.7%
32 Solar RadiationSolar RadiationWith NREL dataset we can find the most convenient tilt for our system and use the average peak sun hours of this tilt to calculate the annual production of our system.Annual energy production = Avg peak sun hours per year [hr/day] x 365 Days x system size [Kw] = [KWh/year]Example:For a 4 KW system, located in San Francisco where we can expect 5.3 peak sun hours per day the annual production of the system will be equal to 7738 KWh/yr –(4KW x 5.3 peak sun hours per day x 365 days)If we multiply this result by the number of years that we expect the system to be producing energy and we divide the cost of the PV system by this number, we will know how much it cost each KWh produced.In our example: 7738 KWh/yr x 25 years = KWh . Let’s say that this system cost $26500 after rebate, then $26500/ KWh =13.5 ¢/KWh
33 Solar RadiationTypical Inclination/Orientation Factors for Parallel Roof Mounted PV Systems in CaliforniaWOW! Don’t worry about it and match the roof line!If the bad news was how traumatic hard shadows were, the good news is how little imperfect orientation and inclination affect output.Remember that if you have the freedom (without extra costs, hassles or poorer aesthetics), point the modules due south (not magnetic south) at an angle somewhere between latitude minus 15 degrees and latitude.If the perfect angle costs you in money, hassle or aesthetics, don’t worry about it! Look at how little it matters.On flat roofs, go more horizontal than the ideal if there is any inter-row shading.For maximum value on TOU rate structures, point south west since afternoon power will be more valuable than morning power.Likewise, if given the choice of an east facing roof or west facing roof with neither shaded, go for the western face and get on a time of use rate structure (consider TOU for any PV system). You will lose 4 or 5%, but you will shift almost three hours of near peak day production into the afternoon, and since afternoon power is worth roughly 3 times morning power, the 5% decrease in production is well worth it.
34 Solar RadiationSolar RadiationTypical Inclinations
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