Here is a compilation of essays on ‘Solar Energy’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Solar Energy’ especially written for school and college students.
Essay on Solar Energy
Essay Contents:
- Essay on the Introduction to Solar Energy
- Essay on the Extraterrestrial Solar Radiation
- Essay on the Total Solar Radiation Incident on an Inclined Solar Collector
- Essay on the Solar Water Heaters
- Essay on the Thermal Storage Systems
- Essay on Heliostats
- Essay on the Heat Transport System
- Essay on the Design of Solar Thermal Plants
- Essay on the Applications of Solar Energy
- Essay on the Limitations of Solar Energy
Essay # 1. Introduction to Solar Energy:
ADVERTISEMENTS:
Solar energy is an essentially inexhaustible source potentially capable of meeting significant portion of the world’s future energy needs with a minimum of adverse environmental consequences. Solar energy is the most promising of the unconventional energy sources. In sheer size, it has the potential to supply all energy needs- electric, thermal process, and chemical, and even transportation fuels.
The sun is a sphere of intensely hot gaseous matter, continuously generating heat by thermonuclear fusion reactions converting hydrogen atoms into helium atoms. This energy is radiated from the sun in all directions and a very small fraction of it reaches the earth. The earth and its atmosphere receive continuously 1.7 × 1017 W of radiations from the sun. A world population of 10 billion with a total power need per person of 10 kW would require about 1011 kW of energy.
It is thus apparent that if irradiation on only one percent of the earth’s surface could be converted into useful energy with 10% efficiency, solar energy could fulfill the energy needs of the entire population.
The immense magnitude of direct solar energy can be illustrated by the following estimates:
ADVERTISEMENTS:
The average solar constant at sea level = 1 kW/m2
The normal surface of the earth presented to the sun
(πR)2 = π(3960 miles)2 × (1610 m/mile)2
Total solar energy at sea level = 1 × π × (3960)2 × (1610)2
ADVERTISEMENTS:
= 1.28 + 1014 kW.
If 1% of earth’s surface is used for converting solar energy into useful energy at an efficiency of 10% (a figure well below that of some available solar cells), the total energy generated per year.
1.28 × 1014 kW × 0.01 × 0.1 × 24 × 365
= 11.2 × 1014 kWh
ADVERTISEMENTS:
This is the present annual rate of consumption of the whole world. Another interesting comparison is that the total fossil energy on earth is less than the solar energy incident upon the earth in one year.
Solar energy is, however, very diffuse, cyclic and often undependable. It, therefore, needs systems and components that can gather and concentrate it efficiently for conversion to any of the uses and that can do the conversion as efficiently as possible.
Essay # 2. Extra-Terrestrial Solar Radiation:
The mean extra-terrestrial radiation normal to the solar beam on the outer fringes of the earth’s surface is called solar constant. Its value is approximately 1.353kW/m2. This varies by ± 3.4 percent during the year due to elliptical orbit of the earth. The radiant energy from the sun is distributed over a range of wave-lengths and can be approximated by the spectrum of a black body at 5762 K.
Terrestrial Solar Radiation:
Extra-terrestrial solar radiation is all of the beam-radiation type, also called direct radiation. This is received from the sun in essentially straight rays or beams that are un-scattered by the atmosphere. The solar energy falling on the earth’s surface is called terrestrial radiation. The rate of terrestrial energy falling on a unit surface area in W/m2 is variably referred to as radiation, irradiation, irradiance, insolation, or energy flux. Terrestrial radiation is not constant and varies significantly.
i. It varies daily because of earth’s rotation around the sun.
ii. It changes seasonally because of sun’s declination angle, i.e., the angle between the sun’s rays and the earth’s equatorial plane normal to the polar axis.
iii. It changes due to spatial changes between the sun and the earth due to elliptical motion of earth. During summer, the earth’s axis is tilted towards the sun and northern hemisphere receives more radiation than the southern hemisphere and vice-versa during winter.
The extra-terrestrial radiations are attenuated by the following:
1. Scattering:
A part of a radiation beam is scattered laterally and attenuated by the air molecules, water vapour and dust in the atmosphere. The scattered and diffuse radiation is mostly of shorter wavelengths.
2. Absorption:
The solar radiation is absorbed by Ozone (O3), water vapour (H2O) and Carbon dioxide (CO2)
About 25-50 percent of solar energy is lost by scattering and absorption. The mean annual horizontal surface irradiance for India is about 200 W/m2 and for Thar Desert, it exceeds 250 W/m2.
The global solar radiation incident on the earth surface comprises both beam (direct) and diffuses solar radiation. The intensity of global radiation on one square meter of the earth surface in a unit time depends on the geographical latitude, season of the year, time of the day and on weather conditions especially cloud cover of the sky.
The annual global solar radiation varies widely from 800 kWh/m2 to 2400 kWh/m2 depending upon the location. Another parameter of solar radiation at a particular location is the daily and annual sunshine hours. The value can be anything from 1400 to 3500 hours per year. Solar radiation data for various selected cities of the world have been measured & recorded.
Essay # 3. Total Solar Radiation Incident on an Inclined Solar Collector:
Solar collectors are installed either in a fixed position or track the sun’s motion in the sky.
The total solar radiation incident on an inclined flat plate collector consists of:
(1) Beam radiation;
(2) Diffuse sky radiation and
(3) Diffuse radiation reflected from the earth surface.
The flat plate collectors are installed in fixed position inclined towards the equator, i.e., to the north in southern hemisphere and to the south in the northern hemisphere.
Typically, the collector tilt angle β is selected as follows:
where,
Ibc = Hourly beam solar radiation on inclined collector surface [kWh/m2-hr]
Ib = Hourly beam solar radiation incident on a horizontal surface [kWh/m2-hr]
θ = Angle of incidence of beam solar radiation on a horizontal plane [ °]
θc = Angle of incidence of beam solar radiation on tited solar collector towards equator [ °]
β = Angle of tilt of solar collector [ °]
φ = Geographical latitude [ °]
δ = sun’s declination [ °]
ω = hour angel [ °]
The geographical latitude φ varies from 0° (equator) to +90° (north pole) in northern hemisphere and 0° to -90° (south pole) in the southern hemisphere. φ for Delhi is 28.6°N.
The sun’s declination δ varies between -23.45° and +23.45° over the year as shown by a graph in Fig. 8.2. The seasonal variation of the terrestrial radiation on a horizontal surface at any one location on the earth’s surface is accounted by the variation of δ.
The hour angle ω is equal to zero at noon (the sun is at zenith). One hour is equivalent to 15°. The hour angle is positive in the morning, 0° at noon and negative in the afternoon.
Example:
At 10 a.m., ω = +30°, and at 2pm ω = -30°.
The daily beam radiation incident on an inclined collector surface tilted towards the equator is given by the following relation. The collector surface faces true south in the northern hemisphere and true north in the southern hemisphere.
where
Ebc = Daily beam radiation [kWh/m2 per day]
Eb = Daily beam radiation incident on a horizontal surface [kWh/m2 per day]
ωsc = sunset hour angle for the inclined collector surface [ °]
ωs = sunset hour angle for the horizontal surface [ °]
The sunset hour angles can be calculated as:
ωs = cos-1 (-tan φ tan δ)
ω = cos-1 [-tan φ – β) tan δ]
The hourly total solar radiation incident on inclined collector surface can be calculated as:
where,
Ic = Total hourly solar radiation incident on unit surface area of inclined collector surface [kWh/m2 per hour]
I = Hourly global solar radiation on a horizontal surface [kWh/m2-hr]
Ib = Hourly beam radiation on a horizontal surface [kWh/m2-hr]
Id = Hourly diffuse radiation on a horizontal surface [kWh/m2-hr]
ρG = Reflectivity of earth’s ground surface
The ratio of total solar radiation per hour on the collector surface to the global solar radiation per hour on a horizontal surface.
The total solar radiation per day on a unit area of inclined collector surface is:
where,
Ec = Total solar radiation per day per unit area of inclined collector surface [kWh/m2 per day]
E = Total global solar radiation per day per unit area on horizontal surface [kWh/m2 per day]
Eb = Total beam solar radiation per unit area per day on horizontal surface [kWh/m2 per day]
Ed = Total diffuse solar radiation per unit area per day on horizontal surface [kWh/m2 per day]
The above relations have been derived by the use of spherical trigonometry.
Essay # 4. Solar Water Heaters:
Solar plants for water heating can be used for the following:
i. Swimming Pool. Solar water heaters can be used for heating water to 23 to 28°C for swimming pools.
ii. Domestic hot water at 45-60°C.
iii. Space heating: Water is heated to 30-90°C.
iv. Refrigeration and space cooling. Heat is supplied to refrigerant vapour generator at 95 to 150°C. Domestic water heating is most widely used.
There are two types of water heating plants:
1. Single Loop, Thermosyphon Circulation Solar Water Heaters:
These can be used with ambient temperature above 0°C. It consists of a solar collector, hot water storage tank and return tubes. The water is returned due to gravity and rises due to thermosyphon effect.
2. Forced Circulation Water Heater:
It is used for supply of hot water in summer and winter even with ambient temperature below 0°C. It also includes a circulation pump with automatic control and a heat exchanger. An antifreeze solution of polypropylene or polyethylene is used as collector fluid. Water is used for storage in the hot water tank. A back-up heating facility is also used in addition to solar heating.
The heat quantity for hot water per month is:
QHW = dw Nper CPw (THW – TCW) ND [J]
where,
dw = daily hot water demand in kg/day/person.
Nper = No. of hot water consumers
CPw = Specific heat of water [4187 J/kg-K]
THW = Hot water temperature [°C]
TCW = Cold water temperature [°C]
ND = No. of days in the month
Heat required per month for space heating,
Qheat = UB AB DD [J/month]
where,
UB = Average heat loss Coeff. of building [W/m2-K]
AB = Total external surface area of heated building [m2]
DD = No. of degree-days per month [K-days]
The surface area of collector required,
A = QHW/qsol.
= mHMCρw(THM – TCW)/Ec ƞsol. [m2]
where,
QHW = Amount of heat required for hot water supply [J]
qsol. = Heat output of collector [J/m2]
mHW = water required [kg]
EC = Solar radiations on collector [J/m2]
ƞsol. = Efficiency of solar plant.
Essay # 5. Thermal Storage Systems:
In order to take care of intermittency of solar energy availability and to fill up the gap between power demand and supply of the power plant, there is need for energy storage system.
There are basically two types of energy storage systems:
i. Electrical storage like batteries, superconducting coils, pumped hydro, compressed air, flywheels, etc.
ii. Thermal storage including sensible heat, latent heat and chemical reaction.
Various energy storage systems are classified in Fig. 8.15. But at present thermal single-phase energy storage system is cost effective due to location of most solar power plants.
There are two types of thermal storage systems suitable with solar systems:
1. Single-tank or thermocline storage system.
2. Dual-tank or hot-cold storage system.
1. Thermocline Storage System:
The hot primary coolant is passed through a packed bed consisting of rocks + oil. The packed bed is heated and cooled coolant is returned to solar collector for reheating. During times of need, the direction of fluid flow is reversed. The primary coolant is admitted at the bottom of storage tank which gets heated by extracted heat from the packed bed and hot fluid goes to the power plant. The storage utilization factor is 0.8 for liquid-solid systems.
2. Hot Cold System:
The storage system consists of two well insulated tanks with necessary pumps, valves and piping. The storage fluid is sodium, molten salt or oil. The tanks are made of austenitic stainless steel. During storage of extra energy cold liquid is pumped out of the cold tank, heated in the receiver and returned to hot tank. During extraction of energy, the hot liquid is drawn from the hot tank, used in raising steam in the steam generator and returned to cold tank. The amount of sensible energy stored varies by changing the fluid levels in the tanks.
The thermal energy storage capacity (density) depends upon the storage media. Some typical values of energy storage density for an operating temperature range of 550° to 565°C are given below:
Soidum – 0.08 MWh/m3
Molten salt (nitrate) – 0.22 MWh/m3
Rock (25% void fraction) – 0.15 MWh/m3
The storage utilization factor is the fraction of stored energy which can be extracted within maximum and minimum temperature limits of storage media. This value is in excess of 0.98. The storage sizing can be made to supply enough thermal energy together with direct thermal power from receiver to operate the power plant continuously 24 h/day Sizing depends upon many conflicting factors variations in daily and seasonal load demands, cloud conditions, availability of backup power and cost. The cost of storage capacity will include cost of additional heliostats and large receiver to fill the storage system.
Essay # 6. Heliostats:
1. These are reflecting mirrors to reflect the sun rays on the receiver.
2. It consists of a mirror, mirror support structure, pedestal, foundation and control and drive mechanism.
3. A glass heliostat is divided into 10 to 14 panels for ease of manufacture and transportation. The rectangular panels are 1.2 × 3.6m with 1.5 to 3 mm thickness of low iron glass sheets to minimize absorption.
4. The cost of heliostats may be 45% of total capital cost of a solar thermal power plant.
5. Plastic heliostat has lower cost, lower mass but lower in reflectance.
6. A north heliostat field is used for higher efficiency.
7. During normal operation, the heliostats are in sun-tracking mode.
8. Active reflected beam sensors are used to control the drive motors. Preprogrammed computer control can also be used.
Heliostat Losses:
There are additional energy losses between the incident energy on the heliostat field and the receiver.
1. Shadowing:
This is caused by one heliostat causing a shadow on the reflective surface of another at certain times of the day.
2. Cosine Loss:
In order to adjust the angle of beam reflected to the receiver, the heliostat may not be perpendicular to the sun rays. The area of solar flux intercepted by the heliostat is cosine of the angle between the surface and perpendicular to the beam.
3. Blocking:
Reflected light from one heliostat may be partly blocked by the back side of another.
4. Reflective Loss:
Some radiation may be absorbed by the glass and silvering and some may be scattered due to dirt on the mirror surface.
5. Attenuation:
Absorption and scattering by water vapour, haze, fog, smoke and particulates in the atmosphere between heliostat and receiver.
Receiver:
1. Central receivers sit atop tall towers and receive radiation energy fluxes from 300 to 700 kW/m2.
2. The receiver should intercept, absorb and transport this energy to a heat- transfer fluid.
3. The panels of parallel tubes with headers at each end absorb the solar energy on their outside surface and conduct it to heat transfer fluid inside. The panels are supported at top to allow thermal expansion downwards.
4. The coolant tubes are 20 to 56 mm diameter and 1.2 to 6.4 mm thickness.
The efficiency of a receiver is reduced due to following losses:
1. Spillage:
The reflected rays may miss the receiver due to heliostat tracking errors, wind effects, steering back loss, etc. These losses are less than 5 percent in a well-designed system.
2. Reflection:
The energy scattered back from the receiver heat transfer surface can be minimized by painting these surfaces with high-absorptive paint. These losses are less than 5 percent in a well-designed system.
3. Convection:
There is a loss by wind convection from the hot surfaces of the receiver.
4. Radiation:
There are radiation losses from the hot receiver surface to the environment.
The convection and radiation losses can be 5 to 15 percent.
5. Conduction:
About 1 percent energy is lost due to conduction of heat to structural members, insulation, etc.
Essay # 7. Heat Transport System:
The heat transport system is composed of primary coolant piping, pumps, etc. The primary coolant may or may not be same as power plant working fluid.
1. Water-Steam:
Steam may be generated at 70 to 140 bars and 540 to 600 °C as in a once-through boiler or drum type boiler. It is extension of conventional technology to solar system.
2. Liquid Metal:
Liquid metals have low vapour pressures and hence low operating temperatures. Sodium may be operated as a single-phase coolant up to 540°C as used in fast-breeder nuclear reactor.
3. Molten Salts:
Molten salts have also low vapour pressures, high volumetric heat capacities and high freezing temperatures. Nitrate salt mixtures can be used for solar central receiver system.
4. Gases:
Air and helium can be used up to 840°C. These can be used to generate steam for a Rankine cycle or may be used directly in a Brayston-cycle power plant gas turbine of open type (with air) or in a closed cycle (with helium) or in a combined cycle.
5. Heat Transfer Oil:
Low corrosive properties and low vapour pressures of some oils can be used as primary coolant of raising steam in a boiler. However, the danger of decomposition of oil and fouling of piping can cause operational problems.
Essay # 8. Design of Solar Thermal Plants:
There are basically two types of solar thermal plants:
1. Low Temperature Solar Plants:
These plants are used for various thermal applications such as swimming pool heating (pool water temperature of 23 to 28°C), hot water system (45 to 60°C), space heating (heating water to a temperature of 30 to 90°C) and refrigeration and space cooling (heat is supplied to the refrigerant vapour generator at 90 to 150°C). Domestic water heating is the most widely used system.
Solar air heaters are used for drying of agricultural produce, heating of green houses and buildings.
2. High Temperature Solar Plants:
These plants are used for generation of electricity by using a thermodynamic cycle Rankine cycle, Brayton cycle.
All solar thermal plants have the following systems/components:
i. Solar radiation data.
ii. Solar collector and receiver.
iii. Heat transport system.
iv. Thermal storage system.
The detailed design of each component/system depends upon the specific application.
Solar Radiation Data:
1. Radiation:
Average cumulative daily solar radiation on a horizontal surface in MJ/m2 per day is available for different locations. The available territorial solar energy at a given time and place is influenced not only by time of the day (hour angle, ω) or year (sun’s declination angle, δ), location (latitude angle, θ) and scattering but also by cloudiness. The incident radiation on the earth’s surface is influenced by the solar angles and is usually presented in terms of dimensionless air mass ma, defined as –
ma= air mass = ratio of optical thickness of the atmosphere through which beam radiation pass to the surface to its optical thickness if the sun were at the zenith, i.e., directly above (dimensionless).
In addition to three basic, angles, latitude, hour angle and sun’s declination, certain additional angles are also useful in solar radiation data-analysis.
i. Altitude Angles α (Solar Altitude):
It is a vertical angle between the projection of the sun’s rays on the horizontal plane and the direction of the sun’s rays passing through a point.
ii. Zenith Angle (θZ):
It is a vertical angle between the sun’s rays and a line perpendicular to the horizontal plane through a point. Therefore θZ and α are complementary angles.
θZ = π/2 – α
iii. Solar Azimuth Angle, γs:
It is a horizontal angle between the north and the horizontal projection of the sun’s rays.
The above solar angles can be represented in terms of basic angles.
cos θz = cosφ cosω cosδ + sin φ sin δ
... θz = π/2 – α
... cos θ = sin α
cos γs = sec α(cosφ sinδ – cosδ sinφ cosω)
and sin γs = sec α cosδ sinω
The air mass,
ma = (cos θz)-1 for θZ = 0° to 70°
For extraterrestrial radiations, ma = 0
For sun at the zenith, ma = 1 at sea level
For θz = 60°, ma = 2
Clearness Index (Ci):
All the effects of solar angles, scattering, absorption, and cloudiness may be combined in one parameter called the Clearness Index.
Ci = Clearness index = ratio of the average radiation on a horizontal surface for a given period to the average extraterrestrial radiation for the same period.
The averaging could be monthly, daily or hourly. Therefore, Ci would be a monthly, daily or hourly clearness index. It varies widely from 30% to 70% in some localities on earth. Even in the day time due to bad weather, Ci could be zero.
2. Measurements of Solar Radiations:
The total radiation is the sum of both the beam and diffuse components.
The following measurements are made:
i. Beam Radiations on a Horizontal Surface:
A Pyrheliometer is used to measure the beam radiation on a horizontal surface. It is a small telescope mounted on a drive mechanism and follows the sun throughout the day.
ii. Diffuse Radiations on a Horizontal Surface:
A Pyranometer with a shade ring is used to measure the diffuse radiation on a horizontal surface.
iii. Total Radiation on a Horizontal Surface:
A Pyranometer without a shade ring is used to measure the total or global radiations.
iv. Sunshine Duration:
A sunshine recorder is used to measure the hours of bright sunshine during the course of a day. A lens burns a trace on a card when exposed to the sun. The length of the trace directly measures the duration of bright sunshine.
v. Beam Radiation on a Normal Surface:
The total radiation received by a surface normal to the beam radiation is more than that on a horizontal surface. Therefore, a radiation collecting surface would be more effective when held perpendicular to the direction of the sun rays. The collector surface should track the sun by changing angle of installation.
Essay # 9. Applications of Solar Energy:
Most of developing countries including India are located in those regions of the world (between 35°N and 35°S) which are most blessed with sunshine. The relative high population densities in the rural areas in these regions, and the high exposure to the sun therefore make it natural that the applications of solar energy, which lends itself to decentralized power of small size, should be given high priority when trying to solve the energy problems in the developing countries.
Although solar energy may be used in many markets such as active and passive space heating and cooling, industrial process heating, desalination and water heating and in electric generation where, power requirements are small and in remote areas.
The development, welfare and prosperity of rural villages depend upon the availability of cheap and abundant energy for domestic cooking, heating and lighting, community development, irrigation water, mechanised agriculture and small industrial parks based on solar energy.
If the energy and power needs of the villages can be met from solar energy, the central/zonal power grids connecting thermal, nuclear and hydro power plants can work in more efficient and reliable manner at high load factors feeding power only to industrial belts and big commercial centres. Tremendous investments and huge power losses in the long and unwieldy power lines to remote and scattered villages can be saved.
Therefore, there is an urgent need to use/develop the following solar appliances for villages:
i. Solar cookers which must fit well with the age-old habits of conventional cooking and should be accepted by rural population without any reservations.
ii. Small solar driven refrigeration units for preservation of food, storage of milk and other products in the rural areas where little electric power is available.
iii. Solar water irrigation pumps to boost the agricultural production which is vitally important in view of growing population.
iv. Solar water heaters and dryers for agricultural products.
v. Small power plants to produce electricity for domestic and street lighting and cottage industries.
Solar Cookers:
Eighty per cent of the basic energy needs of rural India are the provision of cooking energy, which amounts to 0.5 to 1 KWh/person/day with a peak power of 0.2 to 1.2 kW per capita. It is mostly met from traditional fuels like fire-wood and cow-dung cakes. The Government of India is trying to introduce box-type solar cookers with financial subsidies but with poor success due to many technical drawbacks of the design.
The temperature obtained is less than 100° C and is insufficient for chapati making and frying which are very important cooking processes. There is no provision for storage of heat and cooking has to be carried out while the sun is shining, which is an odd time for preparing breakfast and dinner. Cooking has to be carried out in the open without privacy and in the sun which is very inconvenient to the housewife.
Hot-Plate Solar Cooker:
A small family size solar cooker has been developed to overcome the above drawbacks of solar cooking. It consists of a flat plate collector with mirror boosters. It is coupled to an oil storage insulated tank placed under the cooking platform inside the kitchen through a thermo syphon to the outside collector. The schematic diagram is shown in Fig. 8.18. It can provide sufficient flux of heat energy at high enough temperature to a hot plate through a heat pipe.
The main components with specifications are as follows:
i. Solar collector (1m2)
ii. Thermo syphon (32 mmNB)
iii. Hot oil storage tank (100 l Dowtherm)
iv. Thermometer (0—300°C)
v. Hot plate (2 kW)
vi. Heat pipe (2 kW)
vii. Control valve
viii. Non-return Value.
The system would be suitable for cooking of conventional food at convenient time for breakfast, lunch and dinner inside the kitchen. It would just replace a bottled gas cooking range and would be fitted permanently It will fit in with the age-old habits and practices of rural households. It is coupled to a thermal storage and can last for a day without sunshine.
The heat flux rate can be conveniently controlled by turning a small control valve suitably placed at the hot plate. There are no operating and maintenance expenses and lifelong oil fill is provided. The prototype cost of Rs. 3000 can be substantially reduced during commercial design and manufacture.
Solar Refrigeration:
A solar driven refrigerator is needed for preservation of food in rural areas where little electric power is available. Figure 8.19 depicts the flow diagram of a solar domestic refrigerator. Solid absorption zeolite refrigeration cycle can ensure long and trouble free operation life because of absence of moving parts, toxic and corrosive chemicals.
The main components are:
i. 1.5 m2 zeolite filled solar collector.
ii. 10 NB Non-return valve (Ball type).
iii. 4.2 m2 panel coil condenser.
iv. 20l water storage tank.
v. 0.5mm × 1.4m capillary tube.
vi. 0.3 m2 panel coil evaporator and ice tank.
vii. 32 NB Non-return valve (Ball type).
It consists of 1.5 m2 flat plate collector with 50 mm thick zeolite and can be fitted to 165l domestic refrigerator. Water vapour is driven out of collector by solar heating during the day. The vapour are condensed and stored in a tank until evenings. As the collector cools at night due to sky radiation, a vacuum in the system causes the water in the evaporator to boil constantly at a temperature of -2°C at a pressure of 4mm of Hg.
The frozen surface can make 9 kg of ice in 8 hours. The water vapour returns to the collector for re-absorption into zeolite and the system is ready for the next day. The ice can be removed for use or left to cool the refrigerator at times of no sunshine. The system can be scaled up into small cold storage for preservation of food in rural areas.
Solar Milk Cooler:
The total milk production per day per village on an average may be taken as 2000 litres out of which the milk producers can consume 25% for self-use and convert about 45% into milk products like ghee, paneer and khoa. Therefore, 500-600 litres of milk per villages is available for sale as liquid milk per day.
Rural processing of milk can add value to the product and increase rural income. The zeolite solar refrigeration system can be attached to a cooling-cum- storage tank made of single-embossed panel coils of stainless steel, the inside being plain sheet and outside made of embossed sheet.
The water refrigerant circulates by thermosyphon effect in the walls and acts as a very efficient heat exchanger. The temperature difference between milk and water vapour is reduced to less than 0.5°C. As water storage is in the bottom, it makes possible to cool even small quantities of milk.
Solar Water Pumps:
Organic vapour Rankine cycle system with rotary machines or screw expanders or reciprocating engine or spiral expander has been used to drive water pumps. Schematic diagram of such a pump is shown in Fig. 8.20.
The main specifications are:
i. Pump rating: 1 kW
ii. Refrigerant: R114
iii. Collector area: 10m2
iv. Total cost: Rs. 12,500
The solar heat can operate the pump for 4 hours a day. When not pumping, it can be used to drive a generator or thresher or a lathe.
Essay # 10. Limitations of Solar Energy:
Immense solar energy is available free of cost. Its potential is huge. But the nature of solar energy has certain limitations which affect the engineering design of the equipment and system and reduces its cost effectiveness.
1. Low Flux Density:
Large surfaces are needed to collect solar energy of low flux density for large-scale utilization resulting in increased cost of the delivered energy. When the sun is directly overhead on a cloudless day, 10 m2 of surface could theoretically provide energy at 10% efficiency of collection at the rate of 1 kW.
Most of the man-made solar collectors can convert only direct energy efficiently and the position of the sun, thick clouds and anthropogenic pollution let less energy pass and scatter the energy back into space and percentage of diffuse energy increases.
Almost total absorption of solar energy by Ozone at wavelengths below 300nm and by carbon dioxide at wavelengths beyond 2500nm, the irradiance on the earth’s surface is effectively limited to wavelengths between 300 to 2500nm.
2. Intermittency of Solar Energy:
Solar energy has a regular daily cycle due to the turning of the earth around its axis, a regular annual cycle due to inclination of the earth axis with the place of the elliptic and due to the motion of the earth around the sun, and is also unavailable during periods of bad weather. The diffuse and cyclic nature of the source introduces special problems in storage and distribution of the energy, restrict the areas of application and need supplementation from other sources.
3. Global Distribution of Solar Energy:
The global distribution of solar energy does not favour the industrial parts of the world, but may be of help to industrially developing countries located in the favourable radiation belt. The greatest amount of energy is available in the continent desert areas around 25°N and 25°S of the equator and falls off towards both the equator and the poles.
Mostly, these are flat deserts that are practically unusable for agriculture or any type of industrial development. These regions have little or no water which can create special problems in the generation of thermal electric power.
4. Cost Effectiveness:
Although solar energy is essentially free, there is definite cost effectiveness associated with its utilization whereas for conventional energy sources, the processing cost has traditionally been borne by large industries which borrow money from a bank and then charge the consumer for each unit of energy used. Solar energy installations, especially for low temperature applications in home heating and cooling, are the total responsibility of the user.
As a result of this shift in the economic risk, a solar energy installation appears to the users as a huge additional investment that they personally must make in advance before driving any benefit from it. Solar energy must be viewed as a long-term investment, and its cost must be prorated just like cost of fossil fuels. Acceptance of this view is necessary to assess the cost effectiveness of solar energy.
5. Energy Intensive:
Much of the construction materials used for collecting and converting solar radiations are they very energy intensive. For example, large quantities of aluminium, steel, copper, concrete, glass and plastic are needed, all of which require large quantities of energy for conversion from ore to finished products.
The energy consumed in the processes for producing the above finished products and in constructing the systems is a large fraction of the net energy generated during the life of the plant. This fraction is in most cases bordering on the unacceptable limits, except in special cases where solar energy is uniquely suited.
6. Environmental Pollution:
Solar energy systems are not pollution free as usually expected. The mining of large quantities of mineral ores, the processing of these ores into the finished material, the manufacturing of equipment and construction of the plants are the processes that generate their own pollution and health and occupational safety risks. In addition, the low efficiency of these plants, results in large heat rejection, hence the thermal pollution.