UNIT - II

Solar Energy

PRINCIPLES OF SOLAR RADIATION:

The sun is a hydrodynamic sphere of intensely hot ionized gaseous matter (Plasma) continuously generating energy by thermonuclear fusion reaction. Most of the energy is generated by the fusion or hydrogen to helium. This energy produced in the core is transferred to the surface and subsequently radiated to space.

Fig: Sun-Earth Geometry.

The solar constant Isc is the energy from the sun, per unit time, received on a unit surface are, perpendicular to the direction of propagation of the radiation and at the earth’s mean distance from the sun, outside the earth atmosphere.

The constant has an universally accepted value of 1367 W/m2.

EXTRATERRESTRIAL AND TERRESTRIAL REGION SOLAR RADIA TION:

Extraterrestrial solar radiation: it the measure of solar radiation that would be received in the absence of atmosphere. The distance between the sun and earth varies due to elliptical motion of the earth. Accordingly, the extraterrestrial flux also varies, which can be calculated (on any day) by the equation

Where n= is the day of the year, Isc = Solar Constant.

Terrestrial solar radiation: the radiation received on the earth surface is called terrestrial radiation and is nearly 70% of the extraterrestrial radiation.

Fig: Solar radiation atmospheric mechanisms.

Beam Radiation (Ib): Solar radiation received on the earth’s surface without change in direction.

Diffuse Radiation (Id): The radiation received on a terrestrial surface (Scattered) from all parts of the sky dome.

Total Radiation (IT): The sum of beam and diffuse radiations (Ib+Id) is referred to as total radiation. When measured at a location on the earth’s surface, it is called solar insolation at the place. When measured on a horizontal surface, it is called global radiation (Ig)

Irradiance (W/m 2): The rate of incident energy per unit area of a surface is termed irradiance.

Albedo: The earth reflects back nearly 30% of the total solar radiation energy to the space by reflection from clouds, by scattering and by reflection at the earth’s surface. This is called the albedo of the earth’s atmosphere system.

SOLAR RADIATION GEOMETRY:

Fig: Latitude , hour angle ω and sun’s declination δ.

Latitude ( ): The latitude of a place is the angle subtended by the radial line joining the place to the centre of the earth, with the projection of the line on the equatorial plane. Conventionally, the latitude for northern hemisphere is measured positive.

Declination (δ): Declination δ is the angle subtended by the line joining the centers of the earth and the sun with its projection on the earth’s equatorial plane. Declination occurs as the axis the earth is inclined to the plane of its orbit at an angle 66 ½ o, as shown below.

Fig: Tropics and northern hemisphere.

The declination angle changes from a maximum value of +23.45O on June 21 to a maximum of -23.45O on December 22. The declination is zero on two equinox days, i.e. March 22 and September 22.

Where n= the total number of days counted from January till date of calculation.

Fig: Variation of declination angle.

Hour angle (ω): Hour angle ω is the angle through which the earth must rotate to bring the meridian of the point directly under the sun. it is the angular measure of time at the rate of 15O per hour. Hour angle is

measured from noon, based on local apparent time being positive in the afternoon and negative in the forenoon.

Fig: Sun’s zenith, altitude and azimuth angles (northern hemisphere).

Altitude angle (α): It is a vertical angle between the direction of the sun’s rays and its projection on the horizontal plane.

Zenith angle (θz): It is the vertical angle between the sun’s rays and the line perpendicular to the horizontal plane through the point. It is complimentary angle of the sun’s altitude angle.

θz + α = π/2

Surface azimuth angle (γ): It is an angle subtended in the horizontal plane of the normal to the surface on the horizontal plane. By convention, the angle is taken positive if the normal is west of south and negative when east of south in northern hemisphere, and vice versa for southern hemisphere.

Slope (β): It is an angle made by the plane surface with the horizontal surface. The angle is taken as positive for a surface sloping towards south, and negative for a surface sloping towards north.

SUNRISE, SUNSET AND DAY LENGTH:

The times of sunrise and sunset and the duration of the day – length depend upon the latitude of the location and the month in the year. At sunrise and sunset, the sunlight is parallel to the ground surface with a zenith angle 900. The hour angle pertaining to sunrise or sunset (ωs) is

Cos ωs = - tan φ tan δ

ωs = Cos-1( - tan φ tan δ)

The value of hour angle corresponding to sunrise is positive, and negative corresponding to sunset. The total angles between sunrise and sunset is given by

2ωs = 2Cos-1(- tan φ tan δ)

Since 150 of hour angle corresponds to one hour, the corresponding day – length (Td) in hour is given by

Td = 2/15 Cos-1(- tan φ tan δ)

LOCAL APPARENT TIME (LAT):

LAT = Standard time ± 4 (Standard time longitude – longitude of location) + (Time correction)

The positive sign in the first correction is for the western hemisphere while negative sign is applicable for the eastern hemisphere. In India we use negative sign.

COMPUTATION OF COS θ FOR ANY LOCATION HAVING ANY ORIENTATION:

Fig: drawing shoeing angle incidence, zenith angle, solar altitude angle, slope and surface azimuth angle for a tilted surface.

The basic angle for a location P on a tilted surface are shown in above figure.

To compute the beam energy falling on a surface having any orientation, the incident beam flux Ib is multiplied by Cos θ, where θ is the angle between the incident beam and the normal to the tilted surface as shown n above figure. The angle θ is depends on the position of the sun in the sky.

A general equation showing the relation of angle is

Cosθ = Sinφ (Sinδ Cosβ + Cosδ Cosγ Cosω Sinβ ) + Cosφ (Cosδ Cosω Cosβ - Sinδ Cosγ Sinβ ) + Cosδ Sinγ Sinω Sinβ - Eq. 1

Use of eq.1 can be demonstrated as:

(i) For a vertical surface, β = 900, therefore

Cosθ = Sinφ Cosδ Cosγ Cosω - Cosφ Sinδ Cosγ + Cosδ Sinγ Sinω - Eq. 2

(ii) For a horizontal surface, β = 00, therefore

Cosθ = Sinφ Sinδ + Cosφ Cosδ Cosω - Eq. 3

In this case, the angle θ is the zenith angle θz

(iii) In northern hemisphere the sun during winter is towards south. For a surface facing due south, γ = 00, therefore

Cosθ = Sinφ (Sinδ Cosβ + Cosδ Cosω Sinβ ) + Cosφ (Cosδ Cosω Cosβ - Sinδ Sinβ )

= sinδ sin(φ – β) + cosδ cosω cos(φ – β) - Eq. 4

(iv) For a vertical surface facing due south, β = 900, γ = 00, therefore

Cosθ = sinφ cosδ cosω - cosφ sinδ - Eq.5

EMPIRICAL EQUATION FOR ESTIMATING THE AVAILABILITY OF SOLAR RADIATION:

The measurement of solar radiation at every location is not feasible, so engineers have developed empirical equations by utilizing the meteorological data like the number of sunshine hours, the days – length and the number of clear days. For accurate calculations, the hourly, the daily and the monthly time scales are used. Angstrom (1924) suggested a linear equation as follows for determining the amount of sunshine at a given location.

���� � � � � �

� �� - Eq. 6

Where

Hg = Monthly average of daily global radiation on a horizontal surface at a given location, in MJ/m2/day Hc = Monthly average of daily global radiation on a horizontal surface at the same location on a clear sky day, in MJ/m2/day DL = Monthly average measured solar day length, in hours Dmax = Monthly average of the longest day – length, in hours a, b = Constants for the location. It is difficult to define a clear sky day, so it was proposed that Hc in eq. 6 should be replaced by Ho is the monthly average of daily extra terrestrial radiation that would fall on a horizontal surface at the given location.

���� = � � � �

� �� - Eq. 7

The value of Ho can be obtained by the following empirical equation

�� � ��� ��� 1 � 0.033 cos !"#

!$ � - Eq. 8

Where Isc = Solar constant per hour = 1367 W/m2 in SI Units, ωs = sunset hour angle, n = day of the year.

MONTHLY AVERAGE DAILY DIFFUSE RADIATION:

�%�� � 1.390 ' 4.027 +, � 5.531 +,� ' 3.108+, - Eq.9

Hd = Monthly average for daily diffuse radiation on a horizontal surface, in KJ/m2/day

KT = ���� = monthly average clearness index.

It was indicated by kreith that eq.9 was obtained with a value of 1394 W/m2 for the solar constant. When the Indian data was analyzed, two linear equations were finalized.

�%�� � 1.411 ' 1.696 +, - Eq.10

�%�� � 1.354 ' 1.570 +, - Eq.11

The above equations provide similar results, and are valid for

0.3 0 1�2�34 0 0.7

Solar radiation on an inclined surface:

BEAM RADIATION:

56 � 789 :789 :; =

9<= > 9<=?:@ ABC789 > 789 D 789?E@ AB9<= E 9<= >C789 E 789 > 789 D - Eq. 12

DIFFUSE RADIATION:

52 � FC789 A� -Eq. 13

REFLECTED RADIATION:

5G � H?F@789 AB� -Eq. 14

TOTAL RADIATION:

The total radiation flux falling on an inclined surface at any instant is expressed as:

�, � �656 � �252 � ?�6 � �2B5G - Eq. 14

SOLAR RADIATION MEASUREMENTS:

Pyranometer:

The Pyranometer measures global or diffuse radiation on a horizontal surface. It covers total hemispherical solar radiation with a view angle of 2π steradian.

It was designed by the Eppley laboratories, USA, operates on a principle of thermopile. It consists of black surface which heats up when exposed to solar radiation. Its temperature rises until the rate of heat

gain from solar radiation equals the heat loss by conduction, convection and radiation. An electric output voltage (0 to 10mV range) generated by the temperature difference between the black and the white surfaces indicates the intensity of solar radiation.

Pyrheliometer:

A pyrheliometer is an instrument which measures beam radiation on a surface normal to the sun’s rays. The sensor is a thermopile and its disc is located at the base of a tube whose axis is aligned in the direction of the sun’s rays.

Sunshine Recorder:

The duration in hours of bright sunshine in a day is measured by a sunshine recorder. It consists of a glass sphere installed in a section of spherical metal bowl, having grooves for holding a recorder card strip.

SOLAR RADIATION DATA FOR INDIA:

India lies within the latitudes of 70 N and 370N, with annual intensity of solar radiation between 400 and 700 cal/cm2/day. Most parts of india receive 4 – 7 kWh/m2/day of solar radiation with 250 – 300 sunny days in a year. The annual average daily global solar radiation in India (in kWh/m2/day) is shown in fig. below.

The highest annual radiation energy is received in the western Rajasthan while the north-eastern region receives the lowest annual radiation.

Fig: Solar radiation in India

PERFORMANCE ANALYSIS OF A LIQUID FLAT – PLATE COLLE CTOR:

The performance of a solar collector can be improved by enhancing the useful energy gain from incident solar radiation with minimum losses. Thermal losses have three components, namely conductive loss, convective loss and radiative loss.

Conductive loss is reduced by providing insulation on the rear and sides of the absorber plate. Convective loss is minimized by keeping the air gap of about 2cm between the cover and the plate. Radiative losses from the absorber plate are lowered by applying a spectrally selective absorber coating.

The energy balance of the absorber can thus be represented by a mathematical equation

IJ � KL M ' IN -Eq.15

Where

IJ � OPQRST UQ�V WQTXYQZQW �[ VUQ \]TTQ\V]Z ?^�VVPB S = Solar heat energy absorbed by the absorber plate (W/m2) KL � KZQ� ]R VUQ ��P]Z�QZ _T�VQ ?`�B IN � 5�VQ ]R UQ�V T]PP �[ \]aYQ\VX]a �aW ZQZ�WX�VX]a RZ]` VUQ V]_, �[ \]aWS\VX]a �aW \]aYQ\VX]a RZ]` VUQ �]VV]` �aW PXWQP ?c�VVPB From eq.14, the flux absorbed is obtained if the eq is multiplied by the transmissivity – absorptivity product (deB, therefore

M � �656?deB6 � f�252 � ?�6 � �2B5Gg?deB2 - Eq. 16

Where

?deB6 is the transmissivity – absorptivity product for the beam radiation falling on the collector and ?deB2 is the transmissivity – absorptivity product for diffuse radiation impinging the collector It is necessary to define two terms – instantaneous collector efficiency and stagnation temperature which are required to indicate the performance of the collector. The instantaneous collector efficiency is defined as the ratio of useful heat gain to radiation falling on the collector. It is expressed by

hi � jklmno - Eq.17

Depending on the given data, the collector aperture area Aa or the collector gross area Ac is used in place of Ap.

In case the flow of liquid through the collector is stopped, the useful heat gain and the efficiency become zero. At this stage the absorber attains a temperature so that ApS = QL. This is the maximum temperature

that the absorber plate can attain and is called as stagnation temperature. Accordingly Qu useful heat gain in one hour becomes KJ/h and IT the energy falling on the collector face in one hour becomes KJ/m2h

TOTAL HEAT LOSS COEFFICIENT AND HEAT LOSSES:

The total loss coefficient is a relevant parameter as it is a measure of all the losses. Its value ranges from 2W/m2. K to 10W/m2. K

Top Loss Coefficient (Ut):

Top loss coefficient can be determined by considering convection and reradiation losses from the absorber plate, in the upwards direction. Four assumptions are made for the determination of Ut.

(i) Transparent covers and the absorber plate create a system of infinite parallel surfaces (ii) The flow of heat is steady and one – dimensional (iii) There is negligible temperature drop across the thickness of cover and (iv) For long wavelengths, the transparent glass cover acts to be opaque.

Convective heat transfer coefficient at top cover:

The convection heat transfer coefficient hw at the top cover is evaluated by an empirical correlation put forward by and engineer AcAdams as

hw = 5.7 + 3.8υ -Eq.26

where hw is in W/m2.K and υ is wind speed in m/s

Bottom loss coefficient (Ub):

Side loss coefficient (Us):

Thus from eq.21

-Eq.30

PERFORMANCE ANALYSIS OF CONCENTRATING COLLECTOR:

The useful heat output of a concentrating collector (Qc) is given by

I� � pq KJr f�6�h�Ls ' t��u � ?Vi# ' VrBg -Eq.31

Where

FR = heat removal factor of the collector, Aua = Unshaded aperture area (m2) Ibc = intensity of beam radiation (W/m2) hopt = optical efficiency of the collector Uoc = overall / total heat loss coefficient (W/m2oC) C = correction factor Tin = the inlet temperature of the collector (oC) and Ta = ambient temperature. The optical efficiency hopt of a concentrating collector is define as the ratio of the solar radiation absorbed by the absorber to the beam solar radiation on the concentrator. It is given by

hopt = hopt0o Copt = ργταa . Copt -Eq.32

Where

hopt0o = optical efficiency of the collector at 0o – incident angle of beam radiation

Copt = correction factor for deviation of incidence angle from 0o ρ = mirror reflectivity γ = intercept factor τ = transmissivity of transparent cover of the absorber, and αa = absorptivity of the absorber. Therefore efficiency of concentrating collector (hc) is given by:

h� � I�Kr�6� ?Vi# ' VrB Or h� � pqh�Ls ' vwt��

unx� ?Vi# ' VrB -Eq.33

The outlet enthalpy (hout) of heat transfer fluid in a concentrating collector with a phase change of fluid is given by:

U�Js � Ui# � j�yz (J/Kg) -Eq.34

hout = inlet enthalpy of heat transfer fluid (J/Kg) Qc = useful heat output of collector (W), and z̀ = mass flow rate of heat transfer fluid (Kg/s)

COMPARISON BETWEEN FLAT PLATE AND CONCENTRATING COL LECTOR: S.No Aspects Flat Plate Collector Concentrating Collector 1 Absorber area Large Small (Comparatively) 2 Insolation intensity Less More 3 Working fluid

temperature Low temperatures attained

High temperature attained

4 Material required by reflecting surface

More Less

5 Use of power generation Cannot be used Can be used 6 Need of tracking system No Yes 7 Flux received on the

absorber Uniform Non-uniform

8 Collection of beam and diffuse solar radiation components

Beam as well as diffuse solar radiation components collected

Only beam component collected (because diffuse component cannot be reflected and is thus lost)

TYPES OF CONCENTRATING COLLECTORS:

Plane receiver with plane collector:

It is simple concentrating collector having up to four adjustable reflectors all around, with a single collector as shown in fig. the CR caries from 1 to 4 and non-imaging operating temperature can go up to 140oC

Fig: Plane receiver with plane collector

Compound parabolic collector with plane receiver:

Reflectors are curved segments that are parts of two parabolas. The CR varies from 3 to 10. For CR 10 acceptance angle is 11.5o.

Fig: Compound parabolic collector with plane receiver

Cylindrical parabolic collector:

The reflector is in the form of trough with parabolic cross section in which the image is formed on the focus of the parabola along a line as shown in fig. The basic parts are (i) an absorber tube with a selective coating located at the focal axis through which the liquid to be heated flows, (ii) a parabolic concentrator, and (iii) a concentric transparent cover. The aperture area ranges from 1 m2 to 6m2, where the length is more than the aperture width. The CR ranges from 10 to 30.

Fig: Cylindrical parabolic collector

Fresnel lens collector:

This are refraction type focusing collector is made of an acrylic plastic sheet, flat on one side, with fine longitudinal grooves on the other as shown in fig. The angles of grooves are designed to bring radiation to a line focus. The CR ranges between 10 to 80 with temperature varying between 150oC to 400oC

Fig: Fresnel lens collector

Parabolic dish collector:

To achieve high CRs and temperature, it is required to build a point – focusing collector. A paraboloid dish collector is point-focusing type as the receiver is placed at the focus of the paraboloid reflector. The CR ranges from to 100 to a few thousands with maximum temperature up to 2000oC.

Fig: Parabolic dish collector

Central receiver with heliostat:

To collect large amounts of heat energy at one point, the central receiver concept is followed. Solar radiation is reflected from a field of heliostats (an array of mirrors) to centrally located receiver on a tower as shown in fig. Heliostats follow the sun to harness maximum solar heat. Water flowing through the receiver absorbs heat to produce steam which operates a Rankin cycle turbo generator to generate electrical energy.

Fig: Central receiver with heliostat

SOLAR THERMAL ENERGY STORAGE

Solar energy is available only during sunshine hours. Consumer energy demands follow their own time pattern and the solar energy does not fully match the demand. As a result, energy storage is a must to meet the consumer requirement.

Sensible Heat Storage:

Heating a liquid or a solid which does not change phase comes under this category. The quantity of heat stored is proportional to the temperature rise of the material. It T1 and T2 represent the lower and higher temperature, V the volume and ρ the density of the storage material and cp the specific heat, the energy stored Q is given by

Latent Heat Storage (Phase change heat storage):

In this system heat is stored in a material when it melts and heat is extracted from the material when it freezes. Heat can also be stored when a liquid changes to gaseous state, but as the volume change is large, such a system is not economical.

Phase change materials such as sodium sulphate decahydrate (Glauber’s salt) melt at 32oC, with a heat of fusion of 241KJ per kg

Thermochemical Storage:

With a thermochemical storage system solar heat energy can start an endothermic chemical reaction and new products of reactions remain intact. To extract energy, a reverse exothermic reaction is allowed to take place. Actually the thermochemical thermal energy is the binding energy of reversible chemical reaction.

A schematic representation of thermochemical storage reaction is shown in fig. Chemicals A and B react with solar heat and through forward reaction are converted into products C and D. The new products are stored at ambient temperature.

Solar Distillation:

Sade drinking water is scare in arid, semiarid and coastal areas, through an essential requirement for supporting life. At such places, saline water is available underground or in the ocean. This water can be distilled utilizing abundant solar insolation available in that area. A device which produces portable water by utilizing solar heat energy is called solar water still as shown in fig.

Fig: cross section of a solar still

Solar Wax Melter:

It consists of a flat plate collector connected to a water storage tank built around a wax chamber, as shown in fig. Hot water from the flat plate collector circulates in the storage tank that transfers heat to the solid wax. When temperature reaches the melting point, phase change occurs and the liquid wax is collected.

Fig: Liquid bath solar wax melter.

THERMODYNAMIC CYCLES AND SOLAR PLANTS:

Solar thermal energy can be converted into mechanical power by using any of the thermodynamic conversion cycles: the Rankin Cycle, the Brayton Cycle and the Striling Cycle. All steam power stations operate on rankin cycle, where the working fluid, i.e. water is heated in the boiler and vapours thus produced are explained in the turbine to perform mechanical work. The brayton cycle is used to operate gas turbine plants working with a gaseous medium. The stirling power cycle operates on a gaseous working substance and can use an external heat sources like solar, biogas or biomass.

The Carnot Cycle:

An ideal cycle was proposed by Sadi Carnot which operates at maximum efficiency within the purview of the second law of thermodynamics. The cycle constitutes four reversible processes as detailed in fig. Water at point 1 is evaporated in the boiler at constant pressure to form steam at point 2 due to heat input. Steam is then expanded adiabatically doing work in turbine to be at point 3. After performing work steam is partly condensed as heat is rejected to reach point 4. The cycle is completed as steam is compresses adiabatically to reach point 1.

Fig: ideal carnot cycle on T – S and P – V diagrams.

Rankine Cycle:

The cycle is used to operate steam power plants, where the water is heated in the boiler to produce steam (540oC). in the T-S diagram, 4-1a represents the compression of condensed water by BFP to boiler drum pressure, while 1a-1b is sensible heat, 1b-1c is latent heat and 1c-2 is super heat addition into the cycle. Superheated steam is expanded performing work in turbine by 2-3. Finally 3-4 is the condensation of steam at constant pressure and temperature. The efficiency of rankine cycle is lower than that of carnot cycle for the same temperature range.

Fig: Ideal rankine cycle with T-S and P-V diagrams.

Stirling Cycle:

The stirling cycle operates with hot air, where heat addition and rejection take place at constant temperature. It uses a definite mass of gas in a fully – sealed system, putting heat in and out from the working fluid (air or gas). The cycle carries four heat transfer process as shown in fig.

Fig : Ideal stirling cycle on P-V and T-S diagram.

1-2 heat supplied to the working fluid at high temperature TH with isothermal expansion. 2-3 heat transfer at constant volume from working fluid to the regenerator. 3-4 heat transfer from working fluid to sink at low temperature TL 4-1 heat transfer from regenerator to working fluid at constant volume. Brayton Cycle: This is a gas turbine power cycle which operates as brayton cycle wherein the heat addition and rejection processes happen at constant pressure shown by (1-2), (3-4) as shown in fig.

Fig: ideal brayton cycle on P-V and T-S diagram.

Hot compressed gas at point 2 is allowed to expand through a turbine to perform work, represented by 2-3. Exhaust gas from the turbine enters the heat exchanger and heat is rejected. Then the cool gas is compressed and the cycle is completed. The efficiency of the brayton cycle can be improved by adding a regenerator after the turbine exhaust for preheating the compressed gas before the heater.

Solar Ponds:

The concept of solar pond was derived from the natural lakes where the temperature rises (of the order of 45oC) towards the bottom. It happens due to natural salt gradient in these lakes where water at the bottom is denser. In salt concentration lakes, convection does not occur and heat loss from hot water takes place only by conduction.

Fig: schematic diagram of solar pond.

This technique is utilized for collecting ans storing solar energy. An artificially designed pond filled with salty water maintaining a definite concentration gradient is called a solar pond. The top layers remain at ambient temperature while the bottom layer attains a maximum steady-state temperature of about 60oC – 85oC

Solar Pumping System:

Water pumps can be driven directly by solar heated water or fluid which operates either a heat engine or a turbine. For low heads the pump driven by vapour of a low-boiling point liquid heated by a flat plate collector is used as shown in fig. For larger heads, a parabolic trough concentrator or parabolic bowl concentrator is installed to drive a steam turbine.

Solar flat plate collector arrays are installed to heat water or an organic fluid. Hot fluid then flows to mixing tank/storage tank and then to a heat exchanger to convert the working fluid of the heat engine from liquid to vapour. It may be noted that R-115 is an acceptable working fluid as it gives high cycle efficiency besides its low costs.

Fig: Schematic diagram of a solar pump.

Hot transport fluid or water is fed again into the collector circuit by a circulating pump. With heat engine cycle, discharged vapour from the turbine flows into the condenser where the vapour gets condensed. Working liquid is fed into the heat exchanger by a feed pump to complete the cycle. Pumped water is used as a coolant in the turbine condenser.

A higher temperature in heat exchanger or boiler provides high engine efficiency. An optimum range of operating temperature is used for a solar pumping system to attain maximum efficiency. Practically, energy efficiency i.e., the percentage of solar energy collected with the quantity converted into useful work, is about 14%.

Solar Cooker:

Cooking is a common application of solar energy in India. Several varieties of solar cookers are available to suit different requirements.

Box Solar Cooker:

Fig: box solar collector

Dish solar cooker

Fig: dish solar cooker

SOLAR AIR CONDITIONING AND REFRIGERATION:

One of the thermal applications of solar energy is for cooling buildings (known as air conditioning) or for refrigeration needed for preserving food. Solar cooling is advantageous in tropical countries where the cooling demand is the highest when the sunshine is the strongest. There are three modes of solar energy cooling: (i) evaporative cooling, (ii)absorption cooling, and (iii) passive desiccant cooling.

Evaporative cooling

Evaporative cooling is a passive cooling technique, generally used in hot and dry climate. It works on the principle that when warm air is used to evaporate water, the air itself becomes cool and then it cools the living space of a building. Common techniques used for cooling are vapour absorption and vapour compression. Between these two, the absorption cooling system is considered to be more practical, since there is a seasonal matching between the energy needs of refrigeration system and the availability of solar radiation. The vapour absorption cooling system is discussed here.

Absorption cooling system

A simple solar-operated absorption cooling system is shown in fig. Water is heated in a flat-plate collector array and is passed through a heat exchanger called the generator. Suitable chemical solutions for absorption cooling are: (i) NH3-H2O where NH3 is used as the working fluid, and (ii) LiBr-H 2O solution, where H2O operates as the working fluid.

Fig: solar absorption cooling system

1000 KW SOLAR FURNACE WITH MULTIPLE HELIOSTATS

The first 1000 kw solar furnace started operation in 1973 at Odeillo, France. Solar intensity was 1000w/m2, with bright sunshine for about 1200 hours a year. It consisted of 63 heliostats installed at 8 elevations which reflected sun rays to the concentrator parallel to its optical axis as shown in fig. The paraboloidal concentrator was 40m high with a focal length of 18m, effective mirror area 1920 m2, aperture ratio nearly 2.8, with input solar energy of 1800kw. For obtaining different temperatures, the receiver diameter was changed, i.e., the smaller the area the higher the temperature.

Fig: 1000kW solar furnace with heliostat

SOLAR PHOTOVOLTAIC SYSTEM

Photovoltaic power generation is a method of producing electricity using solar cells. A solar cell converts solar optical energy directly into electrical energy. A solar cell is essentially a semiconductor device fabricated in a manner which generates a voltage when solar radistion falls on it.

In semiconductors, atoms carry four electron in the outer valence shell, some of which can be dislodged to move freely in the materials if extra energy is supplied. Then, a semiconductor attains the property to conduct the current. This is the basic principle on which the solar cell works and generates power.

SEMICONDUCTOR MATERIALS AND DOPING

A few semiconductor materials such as silicon (Si), cadmium sulphide (CdS) and gallium arsenide (GaAs) can be used to fabricate solar cells. Semiconductors are divided into two categories (i) Intrinsic pure and (ii) extrinsic. The process of adding impurity atoms is called Doping.

PHOTON ENERGY

Sunlight is composed of tiny energy capsules called photons. The number of photons present in solar radiation depends upon the intensity of solar radiation and their energy content on the wavelength band. The solar spectrum constitutes three main regions.

1. Ultraviolet region (λ < 0.4µm); 9% irradiance

2. Visible region (0.4 µm < λ < 0.7µm); 45% irradiance

3. Infrared region ( λ > 0.7 µm); 46% irradiance

p-n JUNCTION

A semiconductor when doped by a donor impurity increases electrons in the conduction band and become n-type material. When a semiconductor is doped by an acceptor impurity, it becomes the p-type material with excess holes. When these n-type and p-type materials are joined, a junction is formed as detailed in fig. The number of electrons in the n-type material is large; so when an n-type material is brought into contact with p-type material, electrons on the n-side flow into holes of the p-material. Thus in the vicinity of the junction, the n-material becomes positively charged and the p-material negatively charges. The process of diffusion of carriers continues till the junction potential reaches an equilibrium value at the time of equal flow of electrons and holes from both directions as shown in fig. This is known as the unbiased condition of the p-n junction. In this condition, v is the contact potential (i.e., not an externally imposed potential) developed between the p-n junctions. The contact potential so developed is a property of the junction itself.

Fig: (a) p-n junction, (b) p-n junction with applied voltage Vf in forward bias, and (c) p-n junction with reverse bias.

When there is no illumination (dark) the flow of the junction current Ij with imposed voltage V in a p-n junction is expressed by

PHOTOVOLTAIC EFFECT

When a solar cell (p-n junction ) is illuminated, electron-hole pairs are generated and the electric current obtained is the difference between the solar light generated current IL and the diode dark current Ij, i.e.

I = IL - Ij

I = IL – I0[exp (eV/kT) – 1]

This phenomenon is known as the photovoltaic effect.

HOW PHOTOVOLTAIC CELLS WORK:

A typical silicon PV cell is composed of thin wafer consisting of an ultra-thin layer of phosphorus-doped (n-type) silicon on top of a thicker layer of boron-doped (p-type) silicon. An electrical field is created neat the top surface of the cell where these two materials are contact, called the p-n junction as shown in fig.

Fig: diagram of a photovoltaic cell.

When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load. Regardless of size, a typical silicon PV cell produces about 0.5 – 0.6 volt DC under open-circuit with no-load conditions. The current output of a PV cell depends on its efficiency and size, and is proportional to the intensity of sunlight striking the surface of the cell.

EFFICIENCY OF SOLAR CELLS

Electrical characteristics of a solar cell are expressed by the current-voltage cures plotted under a given illumination and temperature conditions as shown in fig.

Fig: current (I) – voltage (V) characteristic of solar cell

The significant points of the curve are short-circuit current Isc and open circuit voltage Voc. Maximum useful power of the cell is represented by the rectangle with the largest area. When the cell yields maximum power, the current and voltage are represented by the symbols Im and vm respectively. Leakages across the cell increases with temperatures which reduces voltage and maximum power. Cell quality is maximum when the value of fill factor approaches unity where the fill factor (FF) is expressed as

Maximum efficiency of a solar cell is defined as the ratio of maximum electric power output to the incident solar radiations. So,

Where Is = incident solar flux, Ac = cell’s area

SEMICONDUCTOR MATERIALS FOR SOLAR CELL:

Solar cells are fabricated from semiconductor materials prepared in three physical states – single multicrystal, many small crystals (polycrystalline) and amorphous (noncrystallin).

Single Crystal Silicon (SCS):

Silicon solar cells are commonly used for both terrestrial and space applications. The basic raw material is sand (SiO2) from which (Si) is extracted and purified repeatedly to obtain the metallurgical grade silicon. A single crystal ingot is about 6cm to 15cm in diameter. Crystalline cells basically require 300µm to 400µm of absorber material; the ingot is sliced in wafer of 300µm thickness as shown in fig.

Fig: cross section of a silicon cell.

Solar cells are fixed on a board and connected in series and parallel combinations to provide the required voltage and power to form a PV module as shown in fig.

Fig: solar cell, module, array and array field.

To protect the cells from the damage a module is hermetically sealed between a plate of toughened glass and layers of ethyl vinyl acetate. Single PV modules of capacities ranging from 10Wp to 120Wp can provide for different loads. The size of individual cell varies from 10cm2 to 100cm2 and a module contains about 20 cells to 40 cells. A standard module constituting 30 cells, each of 7.5 cm in diameter can provide electrical parameters of 12 volts, 1.2 ampere and 18 watt peak power.

Polycrystalline silicon cells (PSC):

The production cost of SCS is higher than PSC. It is obtained in thin ribbons drawn from molten silicon bath and cooled very slowly to obtain large size crystallites. Cells are made with care so that the grain boundaries cause no major interference with the flow of electrons and grains are larger in size than the thickness of the cell as shown in fig.

Fig: cross section of a polycrystalline silicon cell.

This are fabricated in three designs, (i) p – n junction cells, (ii) metal insulator semiconductor cells, and (iii) conducting oxide – insulator semiconductor cells. A nicely developed cell with chromium metal base with SiO2 insulation over it, the p – type crystalline silicon can give efficiency up to 12% at AM-1 conduction with cell dimension of 0.2 cm2.

Amorphous silicon cells (ASC):

Amorphous silicon is pure silicon with no crystal properties. It is highly light absorbent and requires only 1µm to 2µm of material to absorb photons of the incident light. Hydrogenated amorphous silicon (a-Si: H) is a suitable material for thin film solar cells, mainly due to its high photo – conductivity, high optical absorption of visible light with optical band gap of 1.55eV. thin film of nearly 0.7µm can produce solar cells comparatively at low cost. This can be fabricated in four structures: (i) metal, insulator – semiconductor (MIS), (ii) p – I – n devices, (iii) hetrojunction, and (iv) schottky barriers. Theoretical efficiency obtained is up to 24%.

TYPES OF PV SYSTEMS:

Grid-connected photovoltaic system:

Grid connected PV systems are design to operate parallel with electric utility grid. It is also called as grid commutated inverter which transforms the DC power from PV arrays into AC power at a voltage that is accepted by the grid. The bidirectional interface is made b/w PV system AC o/p circuit and electric utility net work at on site of distributed panel as shown in fig. This allows the PV system to supply either electrical load or feed back to PV system when PV system o/p is greater than on site load demand.

Fig: grid-connected photovoltaic system.

Stand-Alone photovoltaic system:

These systems are designed to operate independent of the electric utility grid and are design to supply DC and /or AC electrical loads. The simplest stand-alone PV system is direct coupled to system where the DC o/p is directly connected DC load as shown in fig. Since there is no batteries in direct couple systems the load only operate during sunlight hours making this design used for applications like ventilation fans, water pumps, and small circulating solar thermal water heaters.

Fig: stand-alone PV system.

V-I characteristic of PV module:

The V-I characteristic of PV module is non linear graph b/w current and voltage generated by PV module as shown in fig for different temperatures. Maximum power point have also shown to represent the point at which max power can be drawn from PV module. This MPP constitute the Max power line, which represented the track path of max power point tracker.

Fig: V-I characteristic of PV module

P-V characteristic of PV module:

P-V characteristic curve of a PV module is also a non linear curve plotted b/w power and voltage of PV module for different densities in W/m2. the MPP is shown in graph.

Fig: P-V characteristic PV module.

APPLICATIONS OF SPV SYSTEMS:

SPV Lighting System:

SPV lighting is one of very important application. Residential lighting or street lighting employs the SPV lighting system usually. SPV lighting system is compact size, highly durable, highly efficient and low maintenance.

Fig: SPV lighting system

Solar water pumping system:

Water pumping is another important application of SPV system. These systems are mainly, employed in rural areas for agricultural applications. Where power is not available easily and economically available. These systems acts as a boon for irrigation application. For improving the efficiency and performance of SPV water pumping system, some MPPT’s may also employed.

Fig: Solar water pumping system.

Satellite Solar power plant:

Most of the fanciful proposal for grid connected PV plant is the satellite solar power system. This concept was suggested in 1972. The basic concept was to construct huge PV arrays each over 50km2 in area and produce several GW of power in geostationary orbit around the earth. The power would be converted to microwave radiation and beamed form 1km diameter transmitting antennas in space to 10km2 receiving antennas on earth. The receiving power would then converted to alternating current and fed into national electricity grid. The advantages of SSPS are the arrays would receive a full 1365W/m2 of solar power instead of the 1000w/m2 that is the max available at the earth’s surface.

Fig: solar power plant carried by a manmade satellite.

ADVANTAGES OF SPV:

• Systems are durable • Low maintenance • Systems are eco-friendly • Highly reliable. • No operational cost • Long effective life

LIMITATIONS OF SPV

• Weather dependent • Low efficiency • High installation cost.