Solar Thermal EnergySolar Thermal Energy

Unit -IIUnit -II

Originates with the thermonuclear fusion reactions occurring in the sun.

Represents the entire electromagnetic radiation (visible light, infrared, ultraviolet, x-rays, and radio waves). Out of them, approx 8% UV radiations, approx 46% Visible light, approx 46% infra red radiations and rest are in very small fraction.

Diameter of sun = 1.39 106 kmDiameter of earth = 1.27 104 km

Solar Constant (ISC)

The amount of solar energy received in unit time on a unit area perpendicular to the sun’s direction at the mean distance of the earth from the sun. The currently accepted value is 1367 W/m2.

Solar Azimuth Angle (s)

The angle between the horizontal direction (of the sun) and a reference direction (usually North, although some solar scientists measure the solar azimuth angle from due South).

Angle of Latitude (l)

The angular distance from the equator to the pole. The equator is 0°, the North Pole is 90° North, and the South Pole is 90° South.

Lines of latitude Lines of Longitude or

  “Meridians"

Angle of Declination ()The angular displacement of the sun from the plane of earth’s equator.

It is positive when measured above equator and negative when below the equator.

Maximum value +23.5Minimum value -23.5

It may be calculated as:

(where n is the number of days in the year)

Hour Angle ()It is angular displacement of the sun east or west of the local meridian,

due to the rotation of the earth on its axis at 15o per hour. = 15 (ST - 12 hours)

(ST= Standard Time)

360365

= 23.45 sin (284+ n) degrees

Altitude Angle ()It is a vertical angle between the projection of the sun rays on the horizontal plane and the direction of the sun rays (passing through the point).

Zenith Angle (z)

It is complimentary angle of sun’s altitude angle. It is a vertical angle between the sun’s rays and a line perpendicular to the horizontal plane through the point, i.e, the angle between the sun rays and perpendicular to the horizontal plane.

z = /2 -

N S

Normal to surface

The slope (s)It is the angle made by the plane surface with the horizontal.

Surface Azimuth Angle ()It is the angle of deviation of the normal to the surface from the local meridian, the zero point being south, east positive and west negative.

Surface azimuth angle and slope

Altitude angle , Zenith angle z, and solar azimuth angle s can be expressed in terms of latitude angle l , declination angle and hour angle . The expressions are:

cos z = cos cos cos + sin sin cos s = sec (cos sin cos sin cos )sin s = sec cos sin

(These equations allow calculation of the sun’s Zenith angle, altitude angle and azimuth angles, if the

declination, hour angles and latitude angles are given)

Airmass – The relative path length of the direct solar beam radiance through the atmosphere. When the sun is directly above a sea-level location the path length is defined as airmass 1 (AM 1.0). When the angle of the sun from zenith (directly overhead) increases, the airmass increases approximately by the secant of the zenith angle. A better calculation follows:

  m = 1.0 / [cos(Z) + 0.50572 * (96.07995 - Z)-1.6364] (where Z is the solar zenith angle)

   

Air mass is a representation of the amount of atmosphere radiation that must pass through to reach Earth’s surface.

Solar Radiation

Solar Radiation

• Solar 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.

Solar Radiation

• Atmospheric Effects: Solar radiation is absorbed, scattered and reflected by components of the atmosphere

• The amount of radiation reaching the earth is less than what entered the top of the atmosphere. We classify it in two categories:

1. Direct Radiation: radiation from the sun that reaches the earth without scattering

2. Diffuse Radiation: radiation that is scattered by the atmosphere and clouds

Solar Radiation

Direct or Extraterrestrial Solar Radiation

Solar radiations that has not been absorbed or scattered and reaches the ground directly from the sun is called “Direct radiations” or “Beam radiation” or “Direct radiations” or “Beam radiation” or “Extraterrestrial solar radiations”“Extraterrestrial solar radiations”. These are the radiations which produces a shadow when interrupted by an opaque object.

The energy flux (irradiance) is called solar constant Isc, and its averaged value is estimated as 1367 W/m2.

The value of solar constant remain constant throughout

the year and taken at mean sun-distance (changes seasonally with the time).

Diffused or Terrestrial Solar Radiation

The radiations received on the earth’s surface atmosphere after absorption (molecules of the air, water etc. e.g., O3 layer absorbed nearly all the UV radiations) and changing its direction by reflection and scattering are called “Diffuse radiation” or “Terrestrial solar radiations”“Diffuse radiation” or “Terrestrial solar radiations”.

The energy flux (irradiance) is called solar constant Isc, and its averaged value is estimated as 1000 W/m2. This is because large part of radiations are absorbed, reflected back while propagating through the earth.

The total radiations received at any point of the earth’s surface is the sum of direct and diffuse radiations, referred to as SOLARSOLAR INSOLATION.INSOLATION.

Solar Radiation• Zenith 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•

Solar Radiation

• Solar spectral distribution is important to understanding how the PV modules that we’re going to utilize respond to it

• Most Silicon based PV devices respond only to visible and the near infrared portions of the spectrum

• Thin film modules generally have a narrower response range

Solar Radiation

Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy.

Solar radiation is the total frequency spectrum of electromagnetic radiation produced by the sun.

This spectrum covers visible light and near-visible radiation, such as x-rays, ultraviolet radiation, infrared radiation, and radio waves.

The visible light and heat of the sun makes life possible, and is called daylight or sunshine.

Solar radiation is the basis for all life on earth. Autotrophs, organisms that produce their own food from the sun (mainly plants), use solar energy along with carbon dioxide and water to produce simple sugars in a process called photosynthesis. Heterotrophs, organisms that eat other organisms (like animals and fungi), depend on autotrophs to form the bottom level of the food chain. Heterotrophs couldnt exist without autotrophs, and autotrophs couldnt exist without the sun, so life as we know it depends on electromagnetic radiation.

The amount of incident energy per unit area and day depends on a number of factors, e.g.:

◦ Latitude◦ local climate◦ season of the year◦ inclination of the collecting surface in the direction of the sun.◦ TIME AND SITE

The solar energy varies because of the relative motion of the sun. This variations depend on the time of day and the season. In general, more solar radiation is present during midday than during either the early morning or late afternoon. At midday, the sun is positioned high in the sky and the path of the sun’s rays through the earth’s atmosphere is shortened. Consequently, less solar radiation is scattered or absorbed, and more solar radiation reaches the earth’s surface.

The amounts of solar energy arriving at the earth’s surface vary over the year, from an average of less than 0,8 kWh/m2 per day during winter in the North of Europe to more than 4 kWh/m2 per day during summer in this region. The difference is decreasing for the regions closer to the equator.

The availability of solar energy varies with geographical location of site and is the highest in regions closest to the equator.

Seasons and Climate

The Earths seasonal climate variation occurs as a result of minute changes in our planets distance from the sun during orbit. Solar radiation is also a contributing factor to the process of global warming. Sunlight affects different parts of the Earth in different ways, with extremes manifesting in equatorial regions and the poles.

What is the Reason For Seasons?What is the Reason For Seasons?1. The Tilt or Obliquity of Axis of rotation relative to

the plane of the Earth’s Orbit about the Sun

The equatorial plane is tipped 23.5° from the ecliptic plane. As Earth revolves around the sun, this

orientation produces a varying solar declination.

The ecliptic plane is formed by Earth’s elliptical orbit The ecliptic plane is formed by Earth’s elliptical orbit around the sun.around the sun.

December 21

June 21

March 20

September 21

Solstice Solstice

Occurs two times of the year when the sun is at its greatest distance from the celestial equator. The summer solstice in the Northern Hemisphere occurs about June 21, when the sun is in the zenith at the tropic of Cancer; the winter solstice occurs about December 21, when the sun is over the tropic of Capricorn. The summer solstice is the longest day of the year and the winter solstice is the shortest.Summer Solstice is at maximum solar declination (+23.5º) and occurs in the Northern Hemisphere around June 21st – The sun is over the tropic of Cancer.

Winter Solstice is at minimum solar declination (-23.5º) and occurs about December 21 - The sun is over the tropic of Capricorn.

EquinoxEquinox

Occurs two times during a year when the solar declination is zero. At that time, the length of day and night are approximately equal;

The summer solstice occurs when the Northern Hemisphere is tipped towards the sun.

The winter solstice occurs when the Northern Hemisphere is tipped away from the sun.

The fall and spring equinoxes occur when the sun is directly in line with the equator.

Seasonal Declination

What is the Reason For Seasons?What is the Reason For Seasons? Eccentricity of Earth’s Orbit is a secondary factor

Earth’s orbitis not perfectlycircular, buthas an ellipticalshape

Orbit shaped bythe gravitationalpull of nearbyplanets

Effect of Increased Tilt on Poles Larger tilt moves summer-hemisphere pole more towards the Sun and winter

season away from Sun Decreased tilt does the opposite decreasing seasonality

Standard time organizes regions into time zones, where every location in a time zone shares the same clock time.

The solar window is the area of sky containing all possible locations of the sun throughout the year for a particular location.

Solar Radiation Intensity (or solar flux)

It is given by using Stefan-Boltzmann law (it states that a blackbody radiates electromagnetic waves with a total energy flux E (at the surface of sun) is directly proportional to the fourth power of the Kelvin temperature T of the object:

E = T4

(where = 5.67 10-8 W/m2K4, Stefan-Boltzmann constant & T = 5762 o K, Temperature of the object )

E = 6.25 × 107 W/m2

Total radiant power emitted from the sun:

P = E P = E Surface area of the sun Surface area of the sun P = E P = E 4 4 R R2 2 (R = 6.955 (R = 6.955

10 108 8 m)m)

P= 3.8 1026 W

Solar Radiation

The inverse square law is used to calculate the decrease in radiation intensity due to an increase in distance from sun:

I = E where I = Irradiance at the surface of outer space

or Solar ConstantE = Total Energy Flux

or

R = 6.955 ×108 m r = 1.5 ×1011 m

E = 6.25 × 107 W/m2

4R2

4r2

R2

r2I= E

Three Temperature Scales

Solar Radiation MeasurementSolar Radiation Measurement

Measurement of Solar Radiation The global solar radiation has two components namely direct and diffuse radiation. The global radiation is measured with the Pyranometers, and the direct radiation with Pyrheliometer.

The devices use two types of sensors: thermal and photovoltaic.

A pyranometer measures total global solar

irradiance from the whole sky.

Pyranometers• The instruments are used in

meteorological research, solar energy research, material testing, climate control in greenhouses, building physics, science and many other applications.

To make a measurement of irradiance, it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence

◦ so that there will be a full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 degrees angle of incidence)

◦ zero response when the sun is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle)

◦ and 0.5 at 60 degrees angle of incidence.

Therefore, a pyranometer should have a so-called “directional response” or “cosine response”.

A pyranometer’s main components are:

A thermopile sensor with a black coating. This sensor absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nm range, and has a near-perfect cosine response.

A glass dome: limits the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800 nm), while preserving the 180 degrees field of view. Another function of the dome is that it shields the thermopile sensor from convection.

The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates a voltage output signal that is proportional to the solar radiation.

(1) sensor, (2, 3) glass domes, (5) cable, standard length 5 m, (9)

desiccant.

Diffuse solar irradiance can be measured by adding a shadowing device to a pyranometer, which blocks the direct component of total irradiance.

Handheld pyranometers use less precise sensors

than precision pyranometers but are more suitable for field

measurements.

Pyrheliometers

It is an instrument designed specifically to measure the direct beam solar irradiance with a field of view limited to 5°.

This is achieved by the shape of the collimation tube, with precision apertures, and the detector design

Pyrheliometer: (1) protection cap, (2) window with heater, (3) sight, (5) sensor, (7) humidity indicator, (10) cable for heater

Physical Meteorology II 51

Example of a pyrheliometer on a solar tracker which keeps the instrument pointed at the sun. A black shadow band keeps the pyranometer shaded, so that it measures diffuse radiation only. The global solar radiation is then calculated from direct and diffuse radiation.

Instruments for measuring solar radiation components

Solar Radiation Collectors