LECTURE 3 - SOLAR ENERGY AND SOLAR RADIATION -

Transcription

1 LECTURE 3 - SOLAR ENERGY AND SOLAR RADIATION - Prof. Marco Perino DENER Politecnico di Torino C.so Duca degli Abruzzi Torino marco.perino@polito.it Tel If not otherwise specified, figures are taken from Solar Engineering of Thermal Processes Duffie & Beckman

2 THE SOLAR ENERGY Solar energy can be used by three technological processes: heliochemical, helioelectrical heliothermal. Heliochemical process, through photosynthesis, maintains life on earth by producing food, biomass and converting CO2 to O2. Helioelectrical process, using photovoltaic converters, provides direct conversion of radiative energy to electric energy. They are used to power spacecraft and in many terrestrial applications. Heliothermal process, can be used to provide the thermal energy required for domestic hot water (DHW) production and space heating.

3 THE SUN- 1 The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 x 109 m and, on an average, at a distance of 1.5 x 1O11 m from the earth. With an effective black body temperature Ts of 5777 K, the sun is effectively a continuous fusion reactor. This energy is produced in the interior of the solar sphere, at temperatures of many millions of degree. The produced energy must be transferred out to the surface and then be radiated into the space (Stefan Boltzmann law: T4) It is estimated that 90 % of the sun's energy is generated in the region from 0 to 0.23R (R being the radius of the sun).

4 THE SUN- 2 At a distance of about O.7R from the center, the temperature drops to about 1.3 x 105 K and the density to 70 kg/m3. Hence for r > 0.7 R convection begins to be important and the region 0.7R < r < R is known as the convective zone. The outer layer of this zone is called the photosphere. The edge of the photosphere is sharply defined, even though it is of low density. Above the photosphere is a layer of cooler gases several hundred kilometers deep called the reversing layer. Outside that, is a layer referred to as the chromosphere, with a depth of about 10,000 km. This is a gaseous layer with temperatures somewhat higher than that of the photosphere and with lower density. Still further out is the corona, of very low density and of very high temperature (about 106 K).

5 THE SUN- 3 The photosphere is the source of most solar radiation. Solar Energy Fundamentals, Design, Modelling and Applications G.N. Tiwari

6 THE SUN- 4 The simplified picture of the sun, its physical structure, temperature and density gradients indicate that the sun, in fact, does not function as a blackbody radiator at a fixed temperature. Rather, the emitted solar radiation is the composite result of the several layers that emit and absorb radiation of various wavelength. The photosphere is the source of most solar radiation and is essentially opaque, as the gases, of which it is composed, are strongly ionized and able to absorb and emit a continuous spectrum or radiation.

7 THE SUN: SPECTRAL IRRADIANCE - 1 The maximum spectral density occurs at about 0.48 m wavelength ( ) in the green portion of the visible spectrum. About 6.4% of the total energy is ultraviolet ( < 0.38 m), 48% is in the visible spectrum and the remaining 45.6% is in the infrared region ( > 0.78 m) Extra-atmosferic Solar Energy Fundamentals, Design, Modelling and Applications G.N. Tiwari

8 THE SUN: SPECTRAL IRRADIANCE Extra-atmosferic

9 THE SUN: SPECTRAL IRRADIANCE - 3 Solar Energy Fundamentals, Design, Modelling and Applications G.N. Tiwari

10 THE SUN: EXTRA-ATMOSPH. IRRADIANCE 1 The eccentricity of the earth's orbit is such that the distance between the sun and the earth varies by 1.7%. The solar constant, Gsc, is the energy from the sun per unit time [W/m2] received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth-sun distance outside the atmosphere. The World Radiation Center (WRC) has adopted a value of Gsc = 1367 W/m2, with an uncertainty of the order of 1%.

11 THE SUN: EXTRA-ATMOSPH. IRRADIANCE 2 Two sources of variation in extraterrestrial radiation must be considered: a)the variation in the radiation emitted by the sun. For engineering purposes, in view of the uncertainties and variability of atmospheric transmission, the energy emitted by the sun can be considered to be fixed. a)the variation of the earth-sun distance, that leads to variation of extraterrestrial radiation flux in the range of ± 3.3%. A simple equation with accuracy adequate for most engineering calculations is: (1) where Gon is the extraterrestrial radiation incident on the plane normal to the radiation on the nth day of the year

12 DEFINITIONS - 1 Beam Radiation b (e.g. Gb): is the solar radiation received from the sun without having been scattered by the atmosphere (beam radiation is often referred to as direct solar radiation; to avoid confusion between subscripts for direct and diffuse, we use the term beam radiation, subscript b ). Diffuse Radiation d (e.g. Gd): is the solar radiation received from the sun after its direction has been changed by scattering by the atmosphere (diffuse radiation is referred to, in some meteorological literature, as sky radiation or solar sky radiation; the definition used here will distinguish the diffuse solar radiation from infrared radiation emitted by the atmosphere. Diffuse radiation, subscript d ). Total Solar Radiation (e.g. G): is the sum of the beam and the diffuse solar radiation on a surface (the most common measurements of solar radiation are total radiation on a horizontal surface, often referred to as global radiation on the surface. When we will make reference to total solar radiation we will not use any subscript). G = Gb + Gd

13 DEFINITIONS - 3 Irradiance, G [W/m2]: is the rate at which radiant energy (energy flux) is incident on a surface per unit area of surface. The symbol G is used for solar irradiance, with appropriate subscripts for beam (b), diffuse (d), or spectral radiation ( ). Irradiation or Radiant Exposure, I or H [J/m2]: is the incident energy per unit area on a surface, found by integration of irradiance over a specified period (usually an hour or a day). Insolation is a term applying specifically to solar energy irradiation. The symbol H is used for insolation for a day. The symbol I is used for insolation for an hour (or other period if specified). The symbols H and I can represent beam, or total and can be on surfaces of any orientation (with their corresponding subscripts). Subscripts on G, H, and I are as follows: o refers to radiation above the earth's atmosphere, referred to as extraterrestrial radiation, b and d refer, respectively, to beam and diffuse radiation; T and n refer to radiation on a tilted plane and on a plane normal to the direction of propagation. If neither T nor n appears, the radiation is on a horizontal plane.

14 DEFINITIONS - 4 Radiosity or Radiant Exitance, [W/m2]: is the rate at which radiant energy leaves a surface per unit area by combined emission, reflection, and transmission. Emissive Power or Radiant Self-Exitance, [W/m2]: The rate at which radiant energy leaves a surface per unit area by emission only. Any of the above defined radiation fluxes, except insolation, can apply to any specified wavelength range (such as the solar energy spectrum) or to monochromatic radiation. Insolation refers only to irradiation in the solar energy spectrum. Solar Time: is the time based on the apparent angular motion of the sun across the sky, with solar noon defined as the time the sun crosses the meridian of the observer. Standard time: is the time given by local clock.

15 SOLAR TIME STANDARD TIME - 1 Solar time is the time used in all of the sun-angle relationships; it does not coincide with local clock time (standard time). It is necessary to convert standard time to solar time by applying two corrections: First, there is a constant correction for the difference in longitude between the observer's meridian (local longitude, Lloc) and the meridian on which the local standard time is based (longitude of the standard meridian for the local time zone, Lst). (to find the local standard meridian, in degree, multiply the time difference in hour between local standard clock time and Greenwhich Mean Time GMT, by 15. In fact, the sun takes 4 min to transverse 1 of longitude). The second correction is from the equation of time, which takes into account the perturbations in the earth's rate of rotation which affect the time the sun crosses the observer's meridian. The difference in minutes between solar time and standard time is: (all equations must be used with degrees and NOT radians)

16 SOLAR TIME & EQUATION OF TIME PM day i Sun path swept in 24 h 12 PM day i+1 Sun 12 PM day i+1 Sun path swept in 24 h 12 PM day i

17 SOLAR TIME & EQUATION OF TIME - 2

18 LATITUDE ( ) & LONGITUDE (L)

19 SOLAR TIME & EQUATION OF TIME - 3 (3) Lst is the standard meridian for the local time zone (in ), Lloc is the longitude of the location in question (in ), longitudes are in degrees west, that is 0 < L < 360. The parameter E is the equation of time (in minutes) (4) (all equations must be used with degrees and NOT radians)

20 SOLAR TIME & EQUATION OF TIME - 4 Where B is given by (and n is the day of the year, 1 n 365) (5) In using eq. 3 it has to be remembered that: the equation of time, E, and displacement from the standard meridian (i.e. first term of right hand side of equation 3) are both in minutes, there is a 60-min difference between daylight saving time and standard time. Time is usually specified in hours and minutes, Care must be exercised in applying the corrections between standard time and solar time, which can total more than 60 min.

21 ANGLE DEFINITIONS 1 The geometric relationships between a plane of any particular orientation relative to the earth at any time and the incoming beam solar radiation, that is, the position of the sun relative to that plane, can be described in terms of several angles:

22 SUN EARTH POSITION - 1

23 SUN EARTH POSITION - 2

24 SUN EARTH POSITION - 3

25 ANGLE DEFINITIONS 2, Latitude: is the angular location north or south of the equator, north positive: , Declination: is the angular position of the sun at solar noon (i.e., when the sun is on the local meridian) with respect to the plane of the equator (e.g. is the angle between the equator plane and a line drawn from the centre of the earth to the centre of the sun), north positive: , Slope: is the angle between the plane of the surface in question and the horizontal ( > 90 means that the surface has a downward-facing component): 0 180, Surface azimuth angle: is the deviation of the projection on a horizontal plane of the normal to the surface from the local meridian, with zero due south, east negative, and west positive: , Angle of incidence: is the angle between the beam radiation on a surface and the normal to that surface.

26 ANGLE DEFINITIONS 3, Hour angle: is the angular displacement of the sun east or west of the local meridian due to rotation of the earth on its axis at 15 per hour; morning negative afternoon positive (solar time must be in hours, hour angle in degrees): (Solar Time - 12) 15 (6) Additional angles are defined that describe the position of the sun in the sky: z, Zenith angle: is the angle between the vertical and the line to the sun, that is, the angle of incidence of beam radiation on a horizontal surface (0 z 90 when the sun is above the horizon) s, Solar altitude angle: is the angle between the horizontal and the line to the sun, that is the complement of the zenith angle ( s = 90 - z ) s Solar azimuth angle: the angular displacement from south of the projection of beam radiation on the horizontal plane. Displacements east of south are negative and west of south are positive (-180 s +180 )

27 ANGLES SCHEME 1 Zenith Sun N z s Earth s S α s 90 θ z

28 ANGLE DEFINITIONS DECLINATION, - 4 The declination can be found from the approximate correlation of Cooper (for more precise relations see Duffie & Beckman) : (7) Variation in sun-earth distance, the equation of time, E, and declination,, are all continuously varying functions of time of year. It is customary to express the time of year in terms of n, the day of the year, and thus as an integer between 1 and 365. All equations could also be used with non integer values of n, but the use of integer values is adequate for most engineering calculations (the maximum rate of change of declination is about 0.4 per day).

29 ANGLE DEFINITIONS DECLINATION, - 5

30 ANGLE DEFINITIONS Monthly Mean Days & n Tabular data (see table below) help the assessment of n and supply information about which day of the month must be used as the representative average day of the month (to be used in some formulas - to be seen later)

31 ANGLE RELATIONS Angle of incidence, To assess, angle of incidence of beam solar radiation on a surface whatever oriented and tilted at a certain time during the year, the following equations can be used: (8) and (9) The angle may exceed 90, which means that the sun is behind the surface. When using equation (8), it is necessary to ensure that the earth is not blocking the sun (i.e., that the hour angle,, is between sunrise and sunset).

32 ANGLE RELATIONS Solar Zenith angle, z The assessment of the Zenith angle of the sun, z, can be done using the following equation: (10) The value of z must be between 0 and 90. It has to be remembered that for horizontal surfaces ( = 0) the angle of incidence of the beam radiation is equal to the solar zenith angle, that is z = and eq. 8 reduces to:

33 ANGLE RELATIONS Solar Azimuth angle, s The solar azimuth angle s, can be assessed by menas of: (10a) s can have values in the range of 180 to s is negative when the hour angle,, is negative and positive when the hour angle is positive. The sign function in the above equations is therefore equal to +1 if is positive and is equal to - 1 if is negative. For north or south latitudes between and 66.45, s will be between 90 and -90 for days less than 12 h long; for days with more than 12 h between sunrise and sunset, s, will be greater than 90 or less than -90 early and late in the day when the sun is north of the east-west line in the northern hemisphere (or south of the east-west line in the southern hemisphere).

34 ANGLE RELATIONS Special cases ( = 0) Tilted surfaces sloped due south (typical for north hemisphere) For this case = 0 and the angle of incidence of surfaces sloped due south (or due North) can be derived from the fact that surfaces with slope to the north or south have the same angular relationship to beam radiation as a horizontal surface at an artificial latitude of ( - ) :

35 ANGLE RELATIONS Sunset hour angle, s Equation 10 can be solved for the sunset hour angle s, that is when z = 90 : (11) the sunrise hour angle is the negative of the sunset hour angle. From this it follows the number of daylight hours, N : (12)

36 ANGLE RELATIONS Profile angle, p An additional angle of interest is the profile angle, p, of beam radiation on a receiver plane R that has a surface azimuth angle of. It is the angle through which a plane that is initially horizontal must be rotated about an axis in the plane of the surface in question in order to include the sun. The profile angle is useful in calculating shading by overhangs and can be determined from: (13)

37 SHADING - 1 Three types of shading problems typically occur: a)shading of a collector, window, or other receiver by nearby trees, buildings or other obstructions. The geometries may be irregular and systematic calculations of shading of the receiver in question may be difficult. Recourse is made to diagrams of the position of the sun in the sky, for example plots of solar altitude, s, versus solar azimuth s, on which shapes of obstructions (shading profiles) can be superimposed to determine when the path from the sun to the point in question is blocked. b)the second type includes shading of collectors in other than the first row of multi-row arrays by the collectors on the adjoining row. c)shading of windows by overhangs and wingwalls. When the geometries are regular, shading problems can be assessed through analytical calculation, and the results can be presented in general form.

38 SHADING - Solar plots Solar plots are a 2D representation of the sun paths over the sky dome. These paths are plotted for different periods of the year and are the projection of the sun orbits over the horizontal plane. Each solar plot is draw for a specific location (that is, for a certain latitude, ) and allows to assess the sun position for every hour of the day and for every day of the year, by means of the solar altitude angle, s, and of the solar azimuth, s. Solar plots may be plotted in either polar or rectangular coordinate charts.

39 SHADING - Type a) Use of Solar plots (rectangular coordinate plot) Solar position plot of z and s, versus s, for latitudes of ± 45 is shown in Figure. Lines of constant declination,, are labeled by dates of mean days of the months (see Table 1.6.1). Lines of constant hour angles,, are labeled by hours. (plots for latitudes from 0 to ± 70 are included in Appendix H of Duffie & Beckman). The angular position of buildings, wingwalls, overhangs, or other obstructions can be entered on the same plot (the angular coordinates corresponding to altitude and azimuth angles of points on the obstruction - the object azimuth angle, o, and object altitude angle, o) can be calculated from trigonometric considerations and drawn on the plot). For obstructions such as buildings, the points selected must include corners or limits that define the extent of obstruction. It may or may not be necessary to select intermediate points to fully define shading.

40 SHADING - Type a) Use of Solar plots (rectangular coordinate plot) s (that is day and month) s Solar Energy Pocket Refernce C.L. Martin, D.Y. Goswami ISES

41 (rectangular coordinate plot) C A B The shaded area represents the existing building as seen from the proposed collector site. The dates and times when the collector would be shaded from direct sun by the building are evident

42 USE OF SOLAR PLOTS - Example (rectangular coordinate plot) percorso solare: latitudine 37 N giugno maggio - luglio aprile - agosto marzo - settembre febbraio - ottobre gennaio - novembre dicembre altezza zenitale azimuth

43 SHADING - Type a) Use of Solar plots Implicit in the preceding discussion is the idea that the solar position at a point in time can be represented for a point location. Collectors and receivers, in reality, have finite size, and what one point on a large receiving surface sees may not he the same as what another point sees. The hypotheses of a point collector/receiver can be made if the distance between collector and obstruction is larger compared to the size of the collector itself (this also implies that the collector is either completely shaded or completely lighted). For partially shaded collectors (that is no point hypotheses ), it can be considered to consist of a number of smaller areas, each of which is shaded or not shaded. Besides solar plots in Rectangular Cartesian coordinates, quite common are also solar plots in polar coordinates.

44 SHADING - Solar plots in polar coordinate The concentric circumferences represent point at the same solar eight (same solar eight angle, s). For the circumference of max radius it is s = 0 (horizon), for the circumference centre s = 90 (sun at zenith). Each circle is spaced of 10. Radiuses represent points having the same azimuth (again interval between radius is 10 ) (Note: In this chart the sign of the azimuth angle is reversed)

45 SHADING - Solar plots in polar coordinate example Results: Linea dell ora: 15 Linea del mese: marzo - settembre s = 30 s = 55 S = 30 S = -55 (= 305 ) Assess the sun position at o clock of 23rd September at a latitude of 46 N

46 SHADING - Type c) Solar plots and overhangs or wingwalls The solar position charts can be used to determine when points on the receiver are shaded. The procedure is identical to that of the previous example; the obstruction in the case of an overhang and the times when the point is shaded from beam radiation are the times corresponding to areas above the line. This procedure can be used for overhangs of either finite or infinite length. The same concepts can be applied to wingwalls. Alternatively the concept of shading planes and profile angle, p, can be used: If the profile angle, p, is less than (90 - ) the receiver surface will see the sun and it will not be shaded).

47 SHADING - Type b) - Collectors in row Shading calculations are needed when flat-plate collectors are arranged in rows. Normally, the first row is unobstructed, but the second row may be partially shaded by the first, the third by the second, and so on. For the case where the collectors are long in extent so the end effects are negligible, the profile angle provides a useful means of determining shading. As long as the profile angle is greater than the angle CAB, no point on row N will be shaded by row M (with M = N-1). If the profile angle at a certain time is CA B and is less than CAB, the portion of row N below point A will be shaded from beam radiation.

48 EXTRATERRESTRIAL RADIATION ON A HORIZONTAL SURFACE, Go Several types of radiation calculations are done using normalized radiation levels, that is, the ratio of radiation level to the theoretically possible radiation that would be available if there were no atmosphere. G o G o,n cos z Go,n z Go Atmosphere (Go,n is assed by means of eq. 1) And substituting cos z (that is eq. 10): (15) where Gsc, is the solar constant and n is the day of the year.

49 BEAM RADIATION ON HOR. AND TILTED SURFACE Angle scheme Sun Gb,n n z Gb n Gb,n

50 DAILY EXTRATERRESTRIAL RADIATION ON A HORIZONTAL SURFACE, Ho It is often necessary for calculation of daily solar radiation to have the integrated daily extraterrestrial radiation on a horizontal surface, Ho. This is obtained by integrating Go over the period from sunrise to sunset. If Go is expressed in W/m2 and Ho, in J/m2, it is possible to write: (16) where s, is the sunset hour angle (in )

51 MONTHLY MEAN DAILY EXTRATERRESTRIAL RADIATION ON A HORIZONTAL SURFACE, Ho The monthly mean daily extraterrestrial radiation: Ho for latitudes in the range +60 to -60 can be calculated, with good approximation, with the same equation (16), using n and corresponding to the mean day of the months from Table An overbar is typically used to indicate a monthly average quantity. The monthly mean day is a day which has the Ho closest to H o

52 HOURLY EXTRATERRESTRIAL RADIATION ON A HORIZONTAL SURFACE, Io It is also of interest to calculate the extraterrestrial radiation on a horizontal surface for an hour period. Integrating equation 15 for a period between hour angles 1 and 2 which define an hour (where 2 is the larger): (17) The limits 1 and 2 may define a different time other than an hour.

53 RATIO OF BEAM RADIATION ON TILTED SURFACE TO THAT ON HORIZONTAL SURFACE 1 It is often necessary to calculate the hourly radiation on a tilted surface of a collector from measurements or estimates of solar radiation on a horizontal surface. The most commonly available data are total radiation for hours or days on the horizontal surface, whereas the need is for beam and diffuse radiation on the plane of a collector. The geometric factor Rb, the ratio of beam radiation on the tilted surface to that on a horizontal surface at any time, can be calculated exactly as: (14)

54 BEAM RADIATION TRANSMISSION - scheme Sun Atmosphere GO,n n' n Gb Gb,n

55 RATIO OF BEAM RADIATION ON TILTED SURFACE TO THAT ON HORIZONTAL SURFACE 2 The symbol G is used to denote rates [W/m2], while I [J/m2], is used for energy quantities integrated over 1 hour (and H over one day). Rb G b,t Gb Rb I b,t Ib The typical development of Rb, was for hourly periods; in such case to assess angles with the previous equations angles assessed at the midpoint of the hour must be used (e.g. for the assessment of Rb for the hour comprised between 10 and 11 am the evaluation of the angles must be done at the time 10.30). The optimum azimuth angle for flat-plate collectors is usually 0 in the northern hemisphere (or 180 in the southern hemisphere). Thus it is a common situation that = 0 (or 180 ).