# Solar Radiation. ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner

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2 Solar Radiation Outline Properties of radiation: Summary of equations, terms, concepts Solar Spectra Terrestrial Solar Radiation: Effects of atmosphere, angular dependence of radiation, variation of solar radiation Calculation of Solar Radiation: Estimate of intensity of solar radiation Angular Dependence Solar Noon calculations Time-based calculations Solar Radiation Data Sets Radiation hitting a panel Caution: Watch out for units!!! Make sure you pay attention to radians, degrees. Also pay attention to details - which hemisphere, difference between solar noon and local time, etc.

3 Solar Radiation Goals Calculate power density, photon flux, photon energy, photon wavelength & relationships between them. Calculate properties of arbitrary light source given its spectral irradiance Calculate properties of black body radiation: power density, peak emission wavelength. Convert local time to solar time Determine angles of incident sunlight, both at solar noon and as a function of time. Determine light normal to a surface, at solar noon and at an arbitrary time

4 Photovoltaic Effect First observed in 1839! (Becquerel) Seen when some combinations of different materials are exposed to light we see a current flow Current is extracted with a potential difference due to different materials hence it is a voltaic effect

5 Nature of light Light acts as both wave and particle basis of quantum theory Einstein (1905) explanation of the photoelectric effect calls for quantized particles of light - photons Energy of photon determined by the wavelength or frequency (they are equivalent) of the light Only get emission when hυ > Φ hυ= Φ + ½mv 2 h is Planck s constant Φ is work function υ is frequency of light v m is maximum velocity of ejected electron m

6 Properties of Radiation Summary of equations & concepts: Wave/energy relationship: Burn into brain! Common units of energy: electron-v (ev) 1 ev = 1.602x10-19 J Power density (H, W/m²) and monochromatic photon flux (Φ, photons/sec m²): Power density (in W/m²) is photon flux (# photons/m² sec) multiplied by energy per photon. W hc 1.24 H = Φ ( J) = qφ 2 m λ λ ( µ m)

7 Paint me a spectrum Rarely have single wavelength, more usual to have a spectrum Sun provides a spectrum of light we need to know how to handle this Need some way of conveying the spectra in a sensible format Spectral irradiance, F, is standard way to specify the properties of a light source Gives the power density at a particular wavelength.

8 Spectral Irradiance W 1 Units are. m² µ m The W/m² refers to the power density at a given wavelength, the µm refers to the wavelength at which the power density is specified. W H m² = = 0 F( λ) dλ N F i i = 1 ( λ) λ continuous spectral irradiance Discrete spectral irradiance, N is # points in F(λ) more usual The spectral irradiance of xenon (green), halogen (blue) and mercury (red) light bulbs (left axis) are compared to the spectral irradiance from the sun (purple, which corresponds to the right axis).

9 Examples: Energy of light with wavelength 1000 nm? 1 x photons/s on 1 cm². Energy of photons is 2 ev. What is power density?

10 Black Body Radiation Blackbody: emits based on its temperature; absorbs all light incident on it. Power density for blackbody: H = σt Spectral Irradiance for black body: 4 Only explained by quantum theory Depend on temperature of blackbody source

11 Blackbody Radiation Wavelength at peak spectral irradiance: λ ( µ m) = p 2900 T T -» power density, shifts spectrum more to blue

12 Solar Radiation Sun approximates a black body at ~6000K, radiating with a power density of H sun 63 MW/m². Total power emitted is 4πR 2 sun, where the radius of the sun, R sun = km. Power density at a distance D from sun is given by: W m ² R D 2 sun H = 2 H sun

13 Atmospheric Effects Earth s atmosphere has several impacts on radiation: Scattering Scatters light so that some hits the earth s surface at a wide range of angles and from a large area of the sky. So two components to light hitting the earth s surface: Direct light is the light from the sun which reaches the earth without scattering. Diffuse light is scattered by the atmosphere. This light CANNOT be concentrated. Most effective for higher energy (blue) photons.

14 Atmospheric Effects Earth s atmosphere has several impacts on radiation: Absorption in the atmosphere Changes both the power density and the Spectral distribution of terrestrial solar spectrum. Ozone absorbs at high photon energies. Water vapor, CO 2, absorb in infra-red. Other Clouds, other local variation in atmosphere introduce variability (both locally and temporally) into terrestrial solar radiation.

15 Atmospheric Effects ASTM G Reference Spectra Etr W*m-2*nm-1 Global tilt W*m-2*nm-1 Direct+circumsolar W*m-2*nm-1 Spectral Irradiance W m -2 nm Wavelength nm Absorption bands

16 Terrestrial Radiation Since the impact of the atmosphere on the terrestrial solar radiation is substantially determined by the path length of the light through the atmosphere, the terrestrial solar radiation is characterized by the Air Mass. Air Mass (AM) is defined as: Because of the variability in terrestrial radiation, standard spectra are defined, e.g. AM1.5G. (G stands for global, or the direct + diffuse radiation). Can you guess the name given to the AM1.5 spectrum with only a direct component?

17 Terrestrial Radiation The standard AM1.5 spectra consists of a standard spectral irradiance for terrestrial solar radiation. The solar radiation outside the Earth s atmosphere is called AM0. Direct radiative power can be approximated from AM value. Note the order! Rule of thumb: I D is direct irradiance, I G is global irradiance (direct + indirect (diffuse)).

18 (W)Here Comes The Sun Determining the angle between the sunlight and a surface is critical in finding the usable power since only the normal component of the incident sunlight upon a surface matters. The angle between the sun and the Earth s surface is continually changing because of the rotation of the Earth about the sun and the rotation of the Earth on its axis. The position of the sun at any one time is specified by three angles: Declination angle Elevation angle Azimuth angle Let s find it at solar noon when the sun is either due South (NH) or due North (SH). Only need Declination and Elevation angles.

19 Rotation of Earth When calculating solar angles, more convenient to express the day of the year (d) as the number of degrees which the earth has rotated around the sun denoted by Ω. Day 81 is equinox 360 Ω = ( d 81) 360 hour Daily variation Tilt of the Earth s axis and rotation around the sun causes a variation in the angle at which the sun strikes the Earth Declination angle represents this tilt Maximum value is the tilt of the earth s axis (23.45 ) Ω = d Hourly 24

20 Declination Angle Declination angle: angle between the sun the earth The same everywhere on earth at a given time, changes with the seasons.

21 Declination Angle Rotation of the Earth changes the angle at which the sun strikes the Earth and also through AM the power density of sunlight on the Earth s surface.

22 Declination Angle Declination angle The declination angle, denoted by δ, varies seasonally due to the tilt of the Earth on its axis of rotation and the rotation of the Earth around the sun. Maximum and minimum occur at summer and winter solstice Declination angel is 0 at equinoxes. δ = sin 1 [ sin( ) sinω] sinω d is the number of day of the year. Summer solstice Equinox Winter solstice

23 Elevation Angle The elevation angle (used interchangeably with altitude angle) is the angular height of the sun in the sky measured from the horizontal. Note: both altitude and elevation are also used to describe the height in meters above sea level this is NOT the same parameter. Zenith angle is measured from vertical rather than horizontal. The elevation angle varies throughout the day it is 0 at sunrise and achieves its maximum value at solar noon. The elevation angle at solar noon is largest at Summer Solstice (where it equals latitude plus maximum declination (23.45 )) and lowest at Winter Solstice, (where it equals latitude minus maximum declination (23.45 )). For most many latitudes (greater than 23 ), the sun is never directly overhead.

24 Elevation angle Maximum and minimum elevation depend on latitude and season. Maximum altitude angle (solar noon) can be found from declination and latitude. α = 90 φ + δ Northern α = 90 φ - δ Southern δ is the declination angle at a particular day of the year φ Is the latitude angle α Is the altitude angle.

25 More on Declination Equations and models for declination: If the Earth s orbit is assumed circular, then: δ ε sin Ω 2π = ( NDY 81) Ω 365 Holds for small values of ε, ε = 23.45º NDY is number of the day of the year, and 81 is the day of the year when the equinox occurs (which is defined from UTC, where it occurs on day 80 ) Earth s orbit around the sun is elliptical, not circular. The earth precesses, and therefore the tilt of the Earth is not constant. Parallax error: the angle are determined from the center of the earth, not its surface. Other smaller errors: The sun appears to be in a different position in the sky due to refraction of light in the atmosphere only significant at higher latitudes. Difficulties in defining the position of the sun in the sky due to the size of the sun s disc (which is 0.27 º).

26 and still more More complicated model of declination: Including some of these factors (elliptical orbit, procession) gives: δ = cosΩ sin Ω cos 2Ω sin 2Ω cos3Ω sin 3Ω Where (in radians) Ω = 2 π NDY 1 + hour

27 Solar Noon Calculate the suns altitude at solar noon on June 21 in Sydney (latitude 34S) What about San Francisco (latitude 34N)

28 Hourly Sun Calculations Hourly sun position calculations To perform system analysis on an hourly level and to use TMY data, need to calculate (1) the sun s position (declination, azimuth, elevation) for each hour of the day and (2) the normal component of the solar radiation on a tilted surface. There are numerous equations for declination, varying in what non-idealities they take into account. The accuracy of the calculations depends on the use to which they will be put flat plate PV systems are generally not oriented very accurately. The accuracy required for positioning tracking increases as the level of concentration increases. Notes on angular calculations and accuracy As the sun crosses the sky, its variation from one position to another is described in degrees of arc. Subsets of degrees are minutes, and then seconds, with 1 º = 60 minutes and 1 minute = 60 sec. For solar energy calculations, need accuracy of calculations to ~ 3 minutes, much less for flat plate systems. Generally do not need the corrections on the order of seconds needed in astronomical calculations. Convert from degrees to radians π deg = 180 rad

29 Path of the Sun Net path of sun throughout the day calculated based on azimuth and altitude angle. Represent on polar plots or elevation angle plots. Atlanta, Feb 19 th

30 Elevation and Azimuth Daily and seasonal angular dependence at a particular location given by elevation (or zenith or altitude) and azimuth angle. Both elevation and azimuth vary through the day. Maximum and minimum value depend on latitude and season. Azimuth is due East/West at sunrise/sunset on equinoxes.

31 Azimuth Angle The azimuth angle is the compass direction from which the sunlight is coming. At solar noon, the sun is always directly south in the northern hemisphere and directly north in the southern hemisphere. At the equinoxes, the sun rises directly east and sets directly west regardless of the latitude, thus making the azimuth angles 90 at sunrise and 270 at sunset

32 Hourly Calculations Hourly calculations for solar radiation Need to (1) convert from local time to solar time and (2) convert time-based inputs (time of day) into angles. Solar time and local time The solar calculations use solar time, in which the sun is at its highest spot in the horizon at solar noon. Two basic corrections for local time and solar time: (1) Time zones and (2) Equation of time. Time Zones Time zones introduced to make coordination of activities easier. There are 24 time zones, each 15º apart. Each time zone had its own meridian (LSTM for local solar time meridian), and at the meridian the local time is also solar time (ignoring non-idealities). The Prime Meridian is the meridian from which time and other astronomical features are defined, which is located in Greenwich, England.

33 What time is it? Time Zones Ideally, a time zone spans 7½º on either side of the meridian, but often substantially different due to political boundaries. LSTM =15 T UTC Need to determine how many minutes need to be added/ subtracted to account for the difference between the LSTM and the longitude of a particular location. The earth rotates 1º every 4 minutes, so time correction (TC) in minutes to account for location within a time zone is: TC 1 = 4 ( LSTM Longitude) Blue = LSTM, red = time zone boundaries

34 Equation of Time Equation of time* Accounts for variations between solar time and measured time due elliptical orbit of the earth around the sun and variation of declination angle. EoT Ω = = 9.87sin 2Ω 7.53cosΩ 1.5sin Ω NDY 81+ hour Been used to calibrate timepieces since 1665! (Huygens) * Not useful for building a time machine

35 Solar Time Total time correction Total time correction is the sum of the Equation of Time and the time correction for longitude. LocalSolar Time = Local Time(minutes) + TC1 + EOT Hour Angle Final part of the time part of the solar radiation calculations is to convert the solar time to an angle One day has 24 hours, and the earth rotes 360º in this time, so the earth rotates 15º every hour. The hour angle is 0 at solar noon, negative in the morning, positive in the afternoon. ( ) HRA = 15 LST 12

36 So.at last Altitude or elevation angle sinα = sinφ sinδ + cosφ cosδ cos ( hra) Azimuth angle cosφ sinδ sinφ cosδ cos cosθ = cosα ( hra) Can define where the sun will be on an hourly basis. Phew!!!

37 Solar Radiation Solar Radiation and location dependent parameters General parameters Ambient temperature Average wind speed Presence of a microclimate Solar Radiation: Full description of solar radiation involves measurements of direct and diffuse radiation several times an hour for a statistically significant time period (usually at least 10 years). This level of data is expensive to obtain, or exists only for limited locations, and due to its volume also inherently requires a computer program to analyze and use. Full data sets used for statistical processing of data used in worst-month type calculations to determine relationship between availability and battery size. Reduced data sets include monthly averages and TMY

38 Measuring Solar Radiation Most commonly use a pyranometer Can use a pyrheliometer for greater accuracy

39 Measured solar radiation Actual solar radiation varies from ideal due to effects of local weather. Goal is to determine: (1) Total expected power during year; (2) Variation throughout the year. Typical Metrological Year (TMY) data Measures radiation (and several other parameters such as temperature) at hour intervals for multiple years. Calculates average for each month, and then picks the month with the average value closest to the average. Keeps realistic day-to-day variation of solar radiation. Used for simulation of systems. Requires long measurement history.

40 Let s get some numbers Typical Metrological Year (TMY) TMY data maintains the variability in the solar resource, while reducing multiple year s worth of data to a single year. TMY solar radiation data determined by averaging the solar radiation for each month of the year over multiple years. The monthly data averaged over the entire data set is compared to the average solar radiation for that month from every year. The month in a particular year with the average closest to the average over the entire data set becomes the data used for that month. TMY data consists of global and direct (and in some sets diffuse) radiation measurements every hour of the day. The angle at which the global and direct radiation measurements are taken must be verified in the specification of data, as there is more than a single TMY standard (TMY2 also exists). Global generally measured on a horizontal surface.

41 TMY Typical Metrological Year (TMY) TMY data generally needs to be processed by computer to extract information. Sample from TMY2 data: PORTLAND OR -8 N W ?00000?00000?00000?00000?00000?00000?010A710A70050A70039A7093A71031A7000B8000A70016A700213A F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010B810B80048B80038E7093B81032B8190B8005B89999?099999? F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010B810B80046B80036E7093B81032B8240B8010B89999?099999? F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010A710A70044A70033A7093A71032A7300A7015A70002A700061A F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010B810B80042B80030E7092B81032B8360B8019B89999?099999? F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010B810B80041B80026E7090B81032B8050B8022B89999?099999? F8068F8000A703E ?00000?00000?00000?00000?00000?00000?010A710A70039A70022A7089A71032A7110A7026A70097A700396A F8068F8000A703E F50000F40002F50000I50000I40000I50000I610B810B80041B80026E7090B81032B8120B8028B89999?099999? F8068F8000A703E F50000F40034F50037I50000I40037I50104I610B810B80042B80030E7092B81032B8120B8029B89999?099999? F8068F8000A703E E50000E40062E50070I50000I40070I50224I610A710A70044A70033A7093A71032A7130A7031A70040A700427A F8068F8000A703E7 Each file contains 8670 lines of data. Instructions for use of data are included in the description of most formats. TMY data also includes more information than solar radiation, including temperature, wind speed and direction, cloud conditions, and other atmospheric conditions

42 TMY Cloud Cover Wind Direction Wet Bulb Temp (x10 C) Wind Speed (x10 m/s) Temp (x10 C) Tracking Beam Irradiation x100mj/h/m 2 Global Irradiation x100mj/h/ m 2 H o u r Day M o n t h

43 Peak Sun Hours Average daily insolation can also be given in terms of peak sun hours Take total insolation and express as the equivalent number of hours of being under 1kWh/m 2 /day Simple way of presenting data and good for simple design of a system

44 Sunshine Hours Highly accurate measurements difficult to obtain sunshine hours much easier and gives good design information Sunshine hours data marks when the solar radiation is above 200 W/m 2 Can be combined with knowledge of location to estimate how much radiation will fall according to a and b are location dependent constants take into account atmospheric conditions H 0 is average solar radiation for that month on clear day N is calculated sunshine hours, n measured sunshine hours Very handy for system design when away from location with detailed data available

45 Variation in solar radiation Variation due to (1) Variations in length of day and transmission through atmosphere and (2) Variation in local climate (ie cloud cover). Equatorial regions have more uniform solar radiation Cloud cover can be measured by probability of # sunny days in a row

46 Cloud Cover Give minimum flux per day in units of kwh/m 2 /day Not generally available and used only rarely

47 Measured solar radiation Average solar radiation per month: Gives average power density per day in a particular month for a particular angle (usually either horizontal or tile=latitude). Contour plots for solar radiation

48 How much is enough Statistical analysis of radiation provides an indication of the amount of storage needed. Common statistical analysis is to analyze the probability of having a given number of cloudy or less that 50% radiation days in a row. Example: probability of having 5 cloudy days in a row is 3%: This means that if the battery is sized for 5 days, then the availability should be 97%. This is an approximation: Assumes that array will recharge battery to a full level before another cloudy period comes (tends to underestimate battery size needed). Ignores the power available to the array during the cloudy days (tends to underestimate the power available). Some monthly-average climate data also includes probability of a given number of < 50% days in a row

49 When are you available? Statistical analysis of solar radiation allows analysis of system performance as a function of availability. LOLP = Loss Of Load Probability Shows how for a given array size, the availability and its standard deviation varies with storage.

50 Solar Radiation Statistical approach can work for a range of locations, as appearance of cloudy days is not random, but is correlated with deviation from average expected for that latitude. Means that many locations will behave relatively similarly on a statistical analysis. Locations with distinct weather patterns (ie weather patterns which consistently have cloud cover probabilities not well correlated to their averages) cannot use generalized graphs, but need to be calculated based on their location. Example: Monsoon climates with high averages and high distinct rainfall patterns.

51 Power on a tilted surface The amount of solar radiation incident on a tilted module surface is the component of the incident solar radiation which is perpendicular to the module surface. In solar radiation data sets, the sunlight is often specified as the component normal to a horizontal surface (S horizontal )

52 More generally Normal component of solar radiation on an tilted surface, tilted both northsouth (variation in azimuth) and elevation of surface. S [ cosα sin β cos( ψ Θ) sinα cos β ] module = Sincident + From A. Sproul, ANZES Conference, 2002

53 Effect of Module Tilt Maximum power into a module occurs when the surface is normal to the incident sunlight, but since the angle of the sun is continually changing, a module with fixed tilt captures only a fraction of the total power over a year. Optimum often to set module tilt = latitude, but usually get equal amounts of sunlight in summer and winter. For maximum power: module tilt = Latitude - 15 For more uniform production of power: module tilt = Latitude + 15

54 Power on a tilted surface. The tilt of a surface will determine the power from that surface throughout the year. Array angle = latitude gives maximum overall power in ideal location Array = latitude + 15 deg often used as optimized summer use.

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