UNIT 5
Solar Energy
Q1) Write a short note on Solar Radiation at the Earth’s Surface.
A1)
- Solar radiation is received at the earth’s surface in an attenuated form because is subjected to the mechanism of absorption and scattering as it passes through the earth’s atmosphere.33
- Absorption occurs primarily because of the presence of ozone and water vapors in the atmosphere and to a lesser extent due to other gases (like ).
- It results in an increase in the internal energy of the atmosphere.
- The scattering occurs due to all gaseous molecules as well as particulate matter in the atmosphere.
- The scattered radiation is distributed in all directions same going to back into space and some reaching at the earth’s surface.
- Solar radiation received at the earth’s surface cut in out change of direction i. e. Line called beam or direct radiation.
Fig 1: Schematic representation of
i) The mechanism of absorption and scattering and
Ii) Beam and diffuse radiation received at earth’s surface.
- The radiation received at the earth’s surface from all parts of the sky’s hemisphere. (After being subjected to scattering in the atom) is called diffused radiation.
- The sum of the beam and diffuse radiation is referred to as total or global radiation.
Q2) Explain the term in details Solar Constant.
A2)
The rate at which solar energy arrives at the top of the atmosphere is called Solar constant ().
This is the amount of 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.
Because of the sun’s distance and activity vary throughout the year; the rate of annual of solar radiation varies accordingly.
The so called solar constant, actual values vary up to about 3% in either direction.
Solar constant for earth is 1.353 kilowatts per square meter or 1357 watt per square meter.
Planet | Distance ( ) | Solar constant |
Mercury | 57 | 9228 |
Venus | 108 | 2586 |
Earth | 150 | 1357 |
Mars | 227 | 586 |
Jupiter | 778 | 50 |
The table below gives standardized values of solar constant for various planets.
Solar Constant -
The solar constant is defined as the energy released from the sun per unit time on the unit are of the earth surface, perpendicular to the direction of propagation of the radiation (at earths mean distance) from sun outside the atmospheric.
The latest value of is 1395 w/.
World radiation center (WRC) has adopted the value of I(on)= 1367w/ Based on the solar constant the total solar energy incident on the entire globe of earth can be calculated.
An approximate equation relating the variation of sinusoidal solar constant with time of the year is given as
Where,
Ion=Actual sinusoidal radiation measured on the plane normal to radiation
Isc=Solar constant taken as 1353w/.
n = days of the year.
Q.3 Explain the following terms a. Concentration Ratio b. Cloudy index
Answer-
- Concentration ratio (c):
w = Aperture.
L = Length
r =Rim angle
= Outer diameter of absorber tube
b. Cloudy index
Q.4 Explain in detail Pyrheliometer the instrument used for measurement of solar radiation
Answer-
Measurement of Solar radiation is important because of the increasing number of solar heating and cooling applications and the need for accurate solar irradiation data to predict performance.
Two basic types of instruments are employed for solar radiation measurement: -
i) Pyrheliometer-which collimates the radiation to determine the beam intensity as a function of incident angle.
Ii) Pyranometer-which measures the total hemispherical solar radiation. The Pyranometer measurements are the most common.
1. Pyrheliometer-
A Pyrheliometer is an instrument which measures beam radiation.
The Sensor disc is located at the base of a tube whose axis is aligned with the direction of the sun’s rays. Thus differs radiation is essentially blocked from the sensor surface.
The black absorber plate (with the hot junction of a thermopile attached to it) is located at the base if a collimating tube.
The tube is aligned with the direction of the tube sun’s rays with the help of a two axis tracking mechanism and an alignment indicator.
Thus the black plate receives only beam radiation and a small amount of diffuse radiation falling within the ‘acceptance Angle ‘ of the instrument.
Three types of Pyrheliometer have been use of measure normal incident beam radiation:
a) Angstrom Pyrheliometer
b) Abbot Silver disc
c) Eppley Pyrheliometer
Q.5 Explain in detail Pyranometer the instrument used for measurement of solar radiation
Answer-
A Pyranometer is an instrument which measures total or global radiation over a hemispherical field of view.
If a shading ring is attached, the beam radiation is prevented from falling on the instrument sensor and then measures only the diffuse radiations.
In most Pyranometer, the sun’s radiation is allowed to fall on a black surface to which the hot junction of a thermopile are attached.
Pyranometer consists of a black surface which heats up when exposed to solar radiation.
Its temperature increases until the rate of heat gain by solar radiation equals the rate of heat loss by convection, conduction and irradiation.
The hot junctions of a thermopile are attached to the black surface, while the cold junctions are located under a guard plate so that they do not receive the radiation directly.
As a result, an emf is generated. This emf which is usually in the range of 0 to 10mV can be read, recorded or integrated over a period of time and is a measure of the global radiation.
Fig – Pyranometer with alternate black and white sensor segments.
- The Pyranometer shown in Fig is used commonly in India. It has its hot junctions arranged in the form of a horizontal circular disc of diameter 25mm and coated with a spherical black lacquer having a very high absorbituity in solar wavelength region.
- The Pyranometer can also be used for the measurement diffuse radiation. This is done by mounting it at the centre of a semicircular shading ring.
- The shading ring is fixed in such a way that its plane is parallel to the plane of the path of the sun’s daily movement across the sky and it shades the thermopile element.
Q. 6 Compare Pyranometer and Pyrheliometer
Answer-
Pyrheliometer | Pyranometer |
Pyrheliometers measure beam radiation. | Pyranometer total radiations and can be modified to measure only diffuse radiation. |
The sensor disc is placed at the bottom of the tube whose axis is aligned to the sun rays. | The Sensor disc is placed with hemisphere field view. |
Pyrheliometers must track the sun to obtain beam radiations. | They do not require sun tracking. |
Collimators tubes are installed in pyrheliometers avoid diffuse radiation. | There is no Collimator tube to restrict the entry of radiations. |
The radiations falling on the sensor have a regular cone angle of about only 5%. | However, a shading ring may be installed to measure diffuse radiations only. |
The different types of pyrheliometers are: a) Angstrom b) Abbot Silver c) Eppley | The different types: a) Eppley, b) Yellow Solarimeter c) Moll-Gorczyneski d) Bimetallic action graph e) Thermoelectric, etc., |
Q. 7 Explain in detail solar collectors.
Answer-
It is a device which is used for collecting and absorbing the solar radiations on the surface called as absorber.
It transfers the part of gradient energy to the fluid like water or air in contact with it.
The surface of the collector (absorber) is defined for high absorption and low emissions.
Types of solar collectors
- Flat plate solar collectors
These collectors are used for low temperature applications i.e up to 100.
If the working fluid for absorbing heat energy is liquid such collectors are called as liquid flat plate collectors.
Flat-Plate collectors are used for temperature application
Examples: Space heating and cooling Drying, low temperature, power generators.
II. Concentrating type collectors;
These are also called as focusing collectors.
These are used for medium to high temperature applications.
- Cylindrical Parabolic collectors:
It is suitable for the applications in the range of 100 to 300.
Vapour engine and turbines.
Process heating in industries.
Cooking
Refrigeration
b. Paraboloid:
This is high temperature arrangement (gives temperarture above 200 ). It can be used or steam engines and turbines, sterling engines and thermo-electric generator.
Q. 8 Explain with diagram
Answer-
Solar distillation:
- The device which is used to convert saline water into pure water by using solar energy is called as solar, still generally known as “basin type solar still”.
- It is a shallow basin with blackened surface called basin liner.
- The saline water is supplied to the basin by filler.
- A overflow pipe allows excess water to flow out from the basin. The top of the basin is covered with a sloping air tight transparent cover that encloses the space above the basin.
- The cover is made of glass or plastic the cover is roof like and the slope is provided toward a collection on trough.
- Solar radiations are passed through the glass cover and these radiations are absorbed and converted into heat by the basin liner.
- The saline water is then heated and water vapour is produced inside the solar still.
- The water vapours come in contact with cooler vapour interior surface of the transparent cover and it gets condensed.
- The condensed water vapour flows down the slopping roof and it is collected in a tray trough as distilled water.
The efficiency of solar still can be expressed as
Efficiency,
Where,
m = mass of distilled water produced day.
Enthalpy change of inlet cold water to water vapour
H = Intensity of solar radiations per area/day, ks.
Q. 9 Compare between Flat Plate Collector and Concentric Solar Collector.
Answer-
Flat – Plate Collector | Concentric Solar Collector |
Fluids temperatures upto 100 can be obtained. | Fluid temperature can reach upto 500 . |
Suitable for low temperature applications like water and space heating | Suitable for power generation
|
Cost is low | Cost is high |
Heavy in weight | Lighter in weight |
Collector efficiently is low. | Collector efficiently is comparatively high |
Heat losses are high due to high collector are | Heat losses are low due to less collector area. |
It has negligible maintenance cost | High maintenance cost. |
Q. 10 Explain with diagram Low temperature solar power plant using flat-plate collector solar power system
Answer-
Low temperature solar power plant using flat-plate collector solar power system
- Since the water can be only heated 80 in flat plate collectors the system needs to use a working fluid having low temperature like a butane gas.
- The system consists of an array of flat plate collectors. The cold water is circulated into the collector with the help of a circulating pump.
- The water gets heated in the solar collector due to solar radiations upto a temperature of about 80.
- The heated water is circulated in a heat exchanger called butane boiler, where it generates the butane gas at high pressure
- The butane gas is supplied to butane turbine to produce mechanical power due to expansion of butane gas.
- The vapour coming out of the turbine is condensed in a condenser and sent back for recirculation with the help of butane feed pump.
Q. 11 Explain with diagram Medium temperature solar power plant
Answer-
Medium temperature solar power plant
- This systems employ an array of parabolic trough concentrating collectors spread over a large area. The general range of working temperature are between 250 to 400.
- This system work on Rankine cycle
- This system consist of an array of large parabolic trough collector duty installed with sun tracking device to collect the solar radiations which is used to heat the fluid.
- This heat is transferred to storage tank and finally to feed water where the steam is generated in the steam generator
- This steam is utilized to run a turbine coupled to a electric generator. The turbine is converted into electric power by the generator.
- The exhaust of steam turbine is condensed in a condenser with the help of cold water circulated in the condenser.
- The hot water leaving the condenser is either sent to cooling tower or its heat energy can be used as process heat if the plant is used in an industry.
Q. 12 Consider a 100 photovoltaic cell with reverse saturation current. In full sun, it produces a short-circuit current of at 25◦ C. Find the open-circuit voltage at full sun and again for 50 % sunlight.
Answer-
The reverse saturation current I0 is .
At full sun is
The open-circuit voltage is
Since short-circuit current is proportional to solar intensity, at half sun and the open-circuit voltage is
Q. 13 A PV module is made up of 36 identical cells, all wired in series. With 1-sun insolation (), each cell has short-circuit current and at 25◦ C its reverse saturation current is . Parallel resistance is and series resistance.
a. Find the voltage, current, and power delivered when the junction voltage of each cell is 0.50 V.
Answer-
Using Vd = 0.50 V in along with the other data gives current:
Under these conditions the voltage produced by the 36-cell module:
Power delivered is therefore
Q. 14 Explain in detail Impacts of Temperature and Insolation on I–V Curves-
Answer-
As insolation drops, short-circuit current drops in direct proportion. Cutting insolation in half, for example, drops Isc by half. Decreasing insolation also reduces Voc, but it does so following a logarithmic relationship that results in relatively modest changes in Voc.
As cell temperature increases, the open-circuit voltage decreases substantially while the short circuit current increases only slightly. Therefore Photovoltaic performs better on cold, clear days than hot ones.
Cells vary in temperature not only because ambient temperatures change, but also because insolation on the cells changes. Since only a small fraction of the insolation hitting a module is converted to electricity and carried away, most of that incident energy is absorbed and converted to heat.
The system designers take in to account for changes in cell performance with temperature. Therefore manufacturers provide an indicator called the NOCT, which stands for nominal operating cell temperature.
The NOCT is cell temperature in a module when ambient is 20◦ C, solar irradiation is 0.8 , and wind speed is 1 m/s.
Q. 15 Explain with necessary equivalent circuits the Shading Impacts on I–V curves.
Answer-
The output of a PV module can be reduced dramatically when even a small portion of it is shaded. Even a single shaded cell in a long string of cells can easily cut output power by more than half.
External diodes, purposely added by the PV manufacturer or by the system designer, can help preserve the performance of PV modules.
The main purpose for such diodes is to mitigate the impacts of shading on PV I –V curves. Such diodes are usually added in parallel with modules or blocks of cells within a module.
In Figure a, all of the cells are in the sun and since they are in series, the same current I flows through each of them.
In Figure b, the top cell is shaded and its current source ISC has been reduced to zero. The voltage drop across as current flows through it causes the diode to be reversed biased, so the diode current is also zero. That means the entire current flowing through the module must travel through both and in the shaded cell on its way to the load. That means the top cell, instead of adding to the output voltage, actually reduces it.
Consider the case when the bottom n − 1 cells still have full sun and still somehow carry their original current I so they will still produce their original voltage. This means that the output voltage of the entire module with one cell shaded will drop to
With all n cells in the sun and carrying I, the output voltage was V so the voltage of the bottom n − 1 cells will be
The drop in voltage at any given current I, caused by the shaded cell is given by,
Since the parallel resistance is so much greater than the series resistance
At any given current the I –V curve for the module with one shaded cell drops by.