UNIT 1Solar Energy Technologies

Solar RadiationAnnual solar radiation on a horizontal surface at the equator is over 2000kWh/m2In Northern Europe this falls to about 1000kWh/m2 (per annum)The tilt between the sun and the land reduces the intensity of the midday sunUltraviolet 0.20 - 0.39Visible0.39 - 0.78Near-Infrared0.78 - 4.00Infrared4.00 - 100.00Energy from the sun in the form of ultra-violet, visible and infra-red electromagnetic radiation is known as solar radiation

Orientation Flux of solar radiation incident on a surface placed at the top of the atmosphere, depends on time t, geographical location (latitude , longitude , and on the orientation of the surfaceZPHorizonEquatorz is the declination of the sun is the hour angle of the sun is the angle between the incidentsolar flux and the normal to the surface

E(t, , ) = S(t)cos (t , , )

S(t) is known as the solar constantThe solar constant is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit (AU) (roughly the mean distance from the Sun to the Earth).

Solar radiation spectrum for direct light at both the top of the Earths atmosphere and at sea level The sun produces light with a distribution similar to what would be expected from a 5525 K (5250 C) blackbody, which is approximately the sun's surface temperature

Radiation interacts with matter in several ways:AbsorptionTransmissionScattering Reflection

Solar QuantitiesThe sun generates approximately 1.1 x 10 E20 kilowatt-hours every second. The earths outer atmosphere intercepts about one two-billionth of the energy generated by the sun, 1.5 x 10 E18 kilowatt-hours per year. Because of reflection, scattering, and absorption by gases and aerosols in the atmosphere, only 47% of this, (7 x 10 E17 ) kilowatt-hours, reaches the surface of the earth. In the earths atmosphere, solar radiation is received directly (direct radiation) and by diffusion in air, dust, water, etc., contained in the atmosphere (diffuse radiation). The sum of the two is referred to as global radiation.The amount of incident energy per unit area and day depends on a number of factors, e.g.:Latitudelocal climateseason of the yearinclination 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 suns rays through the earths atmosphere is shortened.

Consequently, less solar radiation is scattered or absorbed, and more solar radiation reaches the earths surface.

The amounts of solar energy arriving at the earths 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.

Solar Corrections[Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2) per day.In the case of photovoltaics it is commonly measured as kWh/(kWpy) (kilowatt hours per year per kilowatt peak rating). ]

Direct normal solar radiation is the part of sunlight that comes directly from the sun. This would exclude diffuse radiation, such as that which would through on a cloudy day. Iindication of the clearness of the sky. Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky.

Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation. Global Horizontal Radiation total solar radiation; the sum of direct, diffuse, and ground-reflected radiation; however, because ground reflected radiation is usually insignificant compared to direct and diffuse, for all practical purposes global radiation is said to be the sum of direct and diffuse radiation only.

SUN PATHS DIAGRAM Sun path diagrams are a convenient way of representing the annual changes in the path of the Sun through the sky on a single 2D diagram.Their most immediate use is that the solar azimuth and altitude can be read off directly for any time of the day and month of the year.There are quite a few different types of sun path diagrams, however, we will only examine two main forms.The Stereographic Diagrams Stereographic diagrams are used to represent the suns changing position in the sky throughout the day and year.

Azimuth lines and Altitude Lines.Data lines and Hour lines.CYLINDRICAL DIAGRAMS:A cylindrical projection is simply a 2D n position in Cartesian coordinates.

Extraterrestrial characteristics of RadiationThe energy in solar irradiation comes in the form of electromagnetic waves of a wide spectrum. Longer wavelengths have less energy (for instance infrared) than shorter ones such as visible light or UV. Intensity and EnergyFor the purpose of solar power, the most significant measures are the intensity and energy delivered one measure at a point in time, the other over a period of time.At a point in timeIrradiance [W/m2]: The intensity of solar radiation hitting a surface, which is the sum of the contributions of all wavelengths within the spectrum, expressed in units of Watts per m2 of a surface.

Power [W]: Momentary total irradiance incident on a particular area.Over a period of TimeEnergy per unit area [kWh/m2]: Energy per unit area is a measure of irradiance incident on a surface over a period of time. It is often expressed

Surface OrientationAs sunlight is smoothly distributed over whole areas, a mere figure for intensity is never sufficient without knowledge of the orientation of the surface in question. Typically, the orientation of a surface is described by the zenith angle, the angle between the sunbeam and the normal of the area. If the surface area is not perpendicular to the sunbeam (i.e. zenith angle is not zero), a larger area is required to catch the same flow as the cross section of the sunbeam.If I0 denotes the intensity on a surface with the sun in its zenith, the intensity, I, on an area where the sun is observed under the zenith angle (see figure) the intensity is reduced to

Values for range from 0 to 90. Turning the face of the area away from the sun means less energy is flowing through that area.

Mean intensity on horizontal surface on earth without atmosphereAssuming the atmosphere has no impact on the incoming light, we can easily calculate a mean intensity on earth by dividing the total irradiance on the cross section of the earth by its surface area.

Radiation on tilted surfaceIn addition to direct beam and diffuse light, a tilted surface will also be struck by rays reflected off the ground.Accordingly, the radiation on a tilted surface has three components:

Beam RadiationIf RB denotes the ratio of the average daily beam radiation on a tilted surface to that on a horizontal surface, then the direct beam part can be written as

RB is a pure geometric parameter, dependent on the horizontal tilt, surface azimuth, declination angle and latitude.

Diffuse RadiationAssuming an isotropic distribution of the diffuse radiation over the hemisphere, the diffuse part is only dependent on the horizontal tilt angle and the diffuse radiation of the horizontal surface:This takes into account that the tilted slope sees only a portion of the hemisphere.Reflected LightThe energy of the reflected light is dependent on the grounds ability to reflect, a property which is expressed by thealbedo factor. The albedo ranges from 0.1 (asphalt paved road) to 0.9 (snow). Given the albedo, the reflected term can be calculated from:

ALBEDO FACTOR FOR VARIOUS SURFACEThe reflected light itself has parts of diffuse and parts of direct light.

Albedo Factor ()Lawn0.205Untitled Field0.26Naked Ground0.17Weather-beaten concrete0.3Asphalt0.15Fresh snow0.85Old snow0.58

Radiation on tilted surface in relation to horizontal surfaceIt can be shown with the help of the above formulas that tilting up a surface can increase the irradiance incident.

The actual amount depends on numerous factors such as latitude, day in the year, albedo and clearness index as well as both the tilting angle and the surface azimuth.

FLAT PLATE COLLECTOR THERMAL ANALYSISTypes of collectorsStationarySun trackingThermal analysis of collectorsPerformanceApplicationsSolar water heatingSolar space heating and coolingRefrigerationIndustrial process heatDesalinationSolar thermal power systems

Types of solar collectors

Flat-plate collector

Flat-plate Collectors

Schematic diagram of an evacuated tube collector

Evacuated tube collectors

Stationary collectorsConcentrating

Flat plate collector with flat reflectors

Sun tracking collectorsConcentrating

Schematic of a parabolic trough collector

Schematic of a parabolic dish collector

Thermal analysis of collectors

Useful energy collected from a collectorGeneral formula:

by substituting inlet fluid temperature (Ti) for the average plate temperature (Tp):

Where FR is the heat removal factor

Collector efficiencyFinally, the collector efficiency can be obtained by dividing qu by (Gt Ac). Therefore:

Overall heat loss coefficientThe overall heat loss coefficient is a complicated function of the collector construction and its operating conditions and it is given by the following expression:UL=Ut+Ub+Ue(for flat plate collector)i.e., it is the heat transfer resistance from the absorber plate to the ambient air.

ConcentrationThe concentration ratio (C) is defined as the ratio of the aperture area to the receiver/absorber area, i.e.:

For flat-plate collectors with no reflectors, C=1. For concentrators C is always greater than 1. For a single axis tracking collector the maximum possible concentration is given by:

and for two-axes tracking collector:

where m is the half acceptance angle limited by the size of the suns disk, small scale errors and irregularities of the reflector surface and tracking errors.

Maximum concentrationFor a perfect collector and tracking system Cmax depends only on the suns disk which has a width of 0.53 (32). Therefore:For single axis tracking: Cmax = 1/sin(16) = 216For full tracking: Cmax = 1/sin2(16) = 46,747

Concentrating collectorsThe useful energy delivered from a concentrator is:

Where no is the optical efficiency given by:

And Af is the geometric factor given by:

Concentrating collectors efficiencySimilarly as for the flat-plate collector the heat removal factor can be used:

And the collector efficiency can be obtained by dividing qu by (GbAa):

Note C in the denominator

PERFORMANCE OF SOLAR COLLECTORSThe thermal performance of the solar collector is determined by: Obtaining values of instantaneous efficiency for different combinations of incident radiation, ambient temperature, and inlet fluid temperature. Obtaining the transient thermal response characteristics of the collector (time constant). Determining the incidence angle modifier.

1. Collector Thermal EfficiencyIn reality the heat loss coefficient UL in previous equations is not constant but is a function of collector inlet and ambient temperatures. Therefore:

Applying above equation we have:For flat-plate collectors:

and for concentrating collectors:

Flat plate collector efficiencyTherefore for flat-plate collectors the efficiency can be written as:

and if we denote co=FR and x=(Ti-Ta)/Gt then:

Concentrating collector efficiencyFor concentrating collectors the efficiency can be written as:

and if we denote ko=FRno, k1=c1/C, k2=c2/C and y=(Ti-Ta)/Gb then:

Incidence Angle ModifierFlat-plate collectorsThe above performance equations assume that the sun is perpendicular to the plane of the collector, which rarely occurs. For the glass cover plates of a flat-plate collector, specular reflection of radiation occurs thereby reducing the () product. The incident angle modifier is defined as the ratio of at some incident angle to at normal radiation ()n:

For single glass cover, a single-order equation can be used with bo equal to -0.1 and b1=0

Efficiency equation by considering incidence angle modifierWith the incidence angle modifier the collector efficiency equation can be modified as:

Incidence Angle ModifierConcentrating collectorsFor off-normal incidence angles, the optical efficiency term (no) is often difficult to be described analytically because it depends on the actual concentrator geometry, concentrator optics, receiver geometry and receiver optics which may differ significantly. Fortunately, the combined effect of these parameters at different incident angles can be accounted for with the incident angle modifier. It describes how the optical efficiency of the collector changes as the incident angle changes. Thus performance equation becomes:

Typical Schematic of SEGS plants

Parabolic Trough System

Parabolic trough collectors

Parabola detail

Receiver detail

Central receiver system

Tower detail

Heliostat detail

Central receiver-1

Central receiver-2

Central receiver-3

Central receiver-4

Central receiver-5

Central receiver-6

Central receiver-7

Central receiver-8

The largest solar thermal-electric installation of its kind in the world, the Luz project in Californias Mojave Desert, has a peak output of some 350 megawatts and occupies several square kilometers of land Parabolic TroughSolar DishStirling Energy Systems, solar dish technology is the worlds most efficientdevice for the conversion of solar energy to grid-delivered electricity,nearly twice as efficient as any alternative solar technology.

Plant LocationsDirect normal solar radiationLand OwnershipRoad AccessLocal transmission infrastructure capabilities and loadingsState-level policies and regulations

Economic and Environmental BenefitsCreation of jobs for both construction and operationIncrease in state and local tax revenuesIncrease in gross state output

Market DevelopmentThe Southwest CSP has set a goal of achieving 1,000 MW of CSP systems in the southwestern US by 2010.US DOEs goal is to develop 30,000 MW of new clean and diversified generation by 2015.SolarPaces plans to deploy 5,000 MW of CSP by 2015.

Residential ApplicationsSolar HeatingSolar CoolingSolar Hot WaterSolar Lighting