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Topics: Solar Energy: Solar radiation measurements,
Solar Thermal: Flat plate and focusing collectors, solar space heating and cooling, solar pond,
Solar Photovoltaic: Solar cells and storage
7. SOLAR ENERGY
SUN
Energy received from the sun in 30 days exceeds total energy available in fossil fuels
Infrared Black spots
HIGHLY DYNAMIC
Moon’s Shadow(at the time of solar eclipse)
THE EARTH
Sun is the prime source of all renewable energy
SOLAR ENERGY
Energy from the sun reaches earth’s surface in the form ofsolar radiation.
The Sun is a sphere of intensely hot gaseous matter,continuously generating heat by thermo-nuclear fusionreactions, which convert hydrogen atoms to helium atoms.
This energy radiated from the sun in all directions and a verysmall fraction of its reaches the earth.
The maximum intensity of solar radiation known as solarconstant which is defined as the total energy received fromthe sun, per unit time on a surface of unit area keptperpendicular to the radiation, in space, just outside theearth’s atmosphere when the earth is at its mean distancefrom the sun. The value of solar constant is 1366 W/m2.
SOLAR RADIATION
DIFFERENT COLORS OF LIGHT HAVE DIFFERENT WAVELENGTHS AND DIFFERENT ENERGIES
Originates with the thermonuclear fusion reactions occurring in the sun. Represents the entire electromagnetic radiation (visible light, infrared, ultraviolet, x-rays, and radio waves).
SOLAR RADIATION SPECTRUM
Total Energy ReceivedPer unit surface area (Solar Insolation)
Outside atmosphere
Earth surface
1366 W/m2
The solar radiation received on a flat, horizontal surface at a particular location on earth at a particular instant of time is called the solar insolation and usually expressed in W/m2.
For a given flat horizontal surface, the parameters of the solar insolation are: Daily variation (Hour angle). Seasonal variation and geographical location of the
particular surface. Atmospheric clarity. Shadows of trees, tall structures, adjacent solar panels,
etc. Degree of latitude for the location. Area of surface, m2. Angle of tilt.
SOLAR INSOLATION
The angle between the incident beam(Ibn) and normal (ON) to surface (S).
If surface S is fixed, angle of incidence θ has hourly variation due to changing position of the sun.
Equivalent Incident Flux (IN) normal to the surface S = component of Ibn along ON.
IN = Ibn cos θ
Angle of Incidence θ depends on several variables such as angle of declination, tilt angle, hour angle, latitude , azimuth angleassociated with the location and orientation of the surface (S) and the direction of sun rays.
The fixed type collector surface ‘S’ should be so oriented that it collects maximum energy during the year.
ANGLE OF INCIDENCE (θ )
The angle between the collector surface plane and the horizontal plane is called the tilt angle or the slope angle and is designated by β.
For vertical surface β = 900
For horizontal surface β = 00
β is always positive.
For sun tracking collectors/reflectors, the angle β is changed automatically to track the sun.
For fixed type collectors/reflectors, angle β is constant.
TILT ANGLE OR SLOPE ANGLE (β)
ANGLE OF DECLINATION (δ)
The angle between the line joiningcenters of the sun and earth andthe equatorial plane.
The angle of declination (δ) varieswith season from maximum valueof +23.45° on June 21 to minimumvalue of –23.45° on December 21.The angle δ is zero at twoequinoxes, i.e., March 21 andSeptember 21.
The declination angle can be calculated from the following expression:
Declination angle (δ)=23.45sin{(360/365) (284 + n)}where, n = the day of year counted from first January.
Angle traced by sun in 1 hour with reference to 12 noon (Local Solar Time) and is equivalent to 15° per hour.
ω= 15×(ST-12),where ST is local solar time At 9 am ω = 15×(9-12) = - 45°At 6 pm ω = 15×(18-12) = 90°
HOUR ANGLE (ω)
The angle made by the radial line joining the given location and the center of the earth, with equatorial plane.
Tilt Angle (β) and Angle of Latitude (φ)
LATITUDE (Φ)
The Earth receives at an average of 1366 W/m2
energy (January: 1412 W/m², and July: 1321 W/m²) in the form of electromagnetic radiation from the Sun
This is equivalent to over 43 thousand times the entire power generation rate on the Earth
But… Large portion of this energy is absorbed in the
atmosphere. Not available all the time at one particular place. Needs to be collected (absorbed) before its
utilization.
SOLAR RESOURCES
SOLAR ENERGY CONVERSION
To use solar energy, some part of the electromagnetic spectrum must be converted into two other farms:
Heat (Thermal Energy)
Electricity
The amount of heat or electricity produced depends upon the technology used and its efficiency.
Solar energy is used in three different ways:
1. By converting solar energy to thermal energythrough solar heater (thermal conversion)
2. By direct conversion of solar energy to electricitythrough photovoltaic (PV) approach
3. By converting solar energy to chemical energy(photosynthesis)
TECHNIQUES FOR USING SOLAR ENERGY
Semiconductor/LiquidJunctions
Photosynthesis Photovoltaics
CO
Sugar
H O
O
2
2
2 H2O
O H22
SC
Heating
e-
SOLAR ENERGY CONVERSION OPTONS
APPLICATIONS OF SOLAR ENERGY
Water heating
Air heating for agricultural and industrial applications
Heating and cooling of buildings
Cold storage for preservation of food
Cooking of food
Green houses
Distillation of water
Water pumping
Solar furnaces
Power generation
Solar photovoltaic
ADVANTAGES AND DISADVANTAGES
Advantages All chemical and radioactive pollutants of the
thermonuclear reactions remain behind on the Sun,while only pure radiant energy reaches the Earth.
Energy reaching the earth is incredible. By onecalculation, 30 days of sunshine striking the Earth havethe energy equivalent of the total of all the planet’s fossilfuels, both used and unused!
Disadvantages Solar energy is not available round the clock. Available Solar energy is diffused. Required to be focused
at one point before using (particularly for thermal conversion).
Three step approach is required: 1) collection, 2) conversion, 3) storage.
SOLAR THERMAL TECHNOLOGIES
Solar thermal is the oldest solar energy technology – has been used for centuries
Solar thermal technologies can be divided in three types:
Passive solar building design
Thermal collectors for water heating, space heating and other uses
Solar thermal power plants
PASSIVE SOLAR DESIGN
Passive solar design is a set of practices that accommodate the local climate by: Letting the sun into the building in the
winter Keeping the sun out in the summer
The most important aspect of passive solar design are Building and window orientation Insulation and building materials Shading
Best design of a building is to act as a solar collector and storage unit for the purpose of heating. This is achieved through three elements: collection, storage, and insulation.
Efficient heating starts with proper Collection of solar energy that can be achieved by keeping south-facing windows and appropriate landscaping (location of tree, tall building, etc.).
Insulation on external walls, roof, and the floors. The doors, windows, and vents must be designed to minimize heat loss (double layer panels).
Storage: Thermal mass holds heat.
Water= 62 BTU per cubic foot per degree F.
Iron= 54, Wood (oak) = 29, Brick = 25,
concrete = 22, and loose stone = 20
HEATING OF LIVING SPACES
Passive Solar
Trombe Wall
Passively heated home
HEATING OF LIVING SPACES (contd…)
A passively heated home uses about 60-75% of the solar energy that hits its walls and windows.
The Center for Renewable Resources estimates that in almost any climate, a well-designed passive solar home can reduce energy bills by 75% with an added construction cost of only 5-10%.
About 25% of energy is used for water and space heating.
Major factor discouraging solar heating is low price of electricity!!!
HEATING OF LIVING SPACES (contd…)
Thermal collectors convert solar radiation into heat
Main uses are water heating and space heating for homes and businesses
Many different types, but they can be categorized as:
Flat plate collectors
Concentrating collectors
SOLAR THERMAL COLLECTORS
A flat-plate collector is used to absorb the sun’s energy to heat (mostly water).
Two methods of heating: passive (no moving parts) and active (using pumps).
In passive collectors water circulates throughout the closed system due to convection currents.
Tanks of hot water (insulated) are used for storage.
FLAT PLATE COLLECTOR
Flat-plate solar collector absorbs sunlight and transfer the heat to water or a mixture of anti-freeze and water
The hot fluid can be used directly or indirectly for hot water and space heating
Generally used for low temperature applications like residential hot water heating
FLAT PLATE COLLECTORS (contd…)
A flat-plate solar collector is one of three main types of solar collectors, which are key components of active solar heating systems. The other main types are evacuated tube collectors and batch solar heaters (also called integrated collector-storage systems).
Flat-plate collectors are the most common solar collectors for use in solar water-heating systems in homes and in solar space heating. A flat-plate collector consists basically of an insulated metal box with a glass or plastic cover (the glazing) and a dark-colored absorber plate. Solar radiation is absorbed by the absorber plate and transferred to a fluid that circulates through the collector in tubes. In an air-based collector the circulating fluid is air, whereas in a liquid-based collector it is usually water.
Flat-plate collectors heat the circulating fluid to a temperature considerably less than that of the boiling point of water and are best suited to applications where the demand temperature is 30-70°C and for applications that require heat during the winter months.
FLAT PLATE COLLECTORS (contd…)
Evacuated tube collectors use a “thermos bottle” type of collector that prevent freezing and can achieve higher temperatures
Used when large volumes high temperature water are needed like commercial laundries, hotels and hospitals
EVACUATED TUBE SOLAR THERMAL COLLECTORS
Active System uses antifreeze so that the liquid does not freeze if outside temperature drops below freezing point.
HEATING WATER: ACTIVE SYSTEM
FLAT PLATE SOLAR COLLECTORS PERFORMANCE
η = =
=
= 2
Useful energy gainCollector efficiency,
Solar radiation incident on collector
Collector area
Incident solar radiation on collector (kW/m )
u
c T
c
T
QA I
A
I
FLAT PLATE SOLAR COLLECTORS PERFORMANCE (contd…)
ατ
ατ
= − = − − =====
=
,
,
( )
Collector efficiency factor
Absorptivity of collector
Transmissivity of glass cover
Overall loss coefficient
Temperature of fluid in the tubes
Ambient temperat
u in out R c T L f O a
R
L
f O
a
Q E E F A I U T T
F
U
T
T ure
COLLECTORS EFFICIENCY VS (Tf,o- Tamb)/ITFLAT PLATE SOLAR COLLECTORS PERFORMANCE (contd…)
COLLECTORS EFFICIENCY VS (Tf,o- Tamb)/ITFLAT PLATE SOLAR COLLECTORS PERFORMANCE (contd…)
Efficiency of solar heating system is always less than 100% because: % transmitted depends on angle of incidence, Number of glass sheets (single glass sheet transmits 90-
95%), and Composition of the glass
By using solar water heating in place of a gas water heater, a family will save 500 kg of pollutants each year.
Market for flat plate collectors grew in 1980s because of subsidy.
While solar water heating is relatively low in the India, in other parts of the world such as Cyprus (90%) and Israel (65%), it proves to be the predominate form of water heating.
HEATING WATER—LAST THOUGHTS
S.
No.Concentrator-Receiver Combination
Concentration Ratio
1. Plane reflector – plane receiver 1 to 4
2. Conical reflector – cylindrical receiver 4 to 10
3.Parabolic cylindrical reflector-cylindrical receiver
10 to 100
4. Paraboloidal reflector-spherical receiver Up to 10000
CONCENTRATING SOLAR THERMAL COLLECTORS: CONCENTRATION RATIO
2
2
kW/m insolar radiation on surfaceConcentration Ratio
kW/m on surface of focus of collector=
CONCENTRATING SOLAR THERMAL COLLECTORS (contd…)
Parabolic dish collectors use optical mirrors to focus sunlight on a target
can achieve very higher temperatures, but are more expensive and complex Small size Collectors are more
economical in hilly area (Ladakh)
CONCENTRATING SOLAR THERMAL COLLECTORS (contd…)
INAUGURAL FUNCTION OF WORLD'S LARGEST SOLAR COOKING SYSTEM (SHRI SAIBABA SANSTHAN TRUST, SHIRDI 30-07-2009)
Parabolic trough collectors also use optical mirrors to focus sunlight on a linear target, usually a tube with a circulating fluid in it
Used for power generation
CONCENTRATING SOLAR THERMAL COLLECTORS
PARABOLIC DISHES AND TROUGHS
Because they work best under direct sunlight, parabolic dishes and troughs must be steered throughout the day in the direction of the sun.
Collectors in southern CA
Focus sunlight on a smaller receiver for each device; the heated liquid drives a steam engine to generate electricity.
The first of these Solar Electric Generating Stations (SEGS) was installed in CA by an Israeli company, Luz International. Output was 13.8 MW; cost was $6,000/peak kW and overall efficiency was 25%.
Through federal and state tax credits, Luz was able to build more SEGS, and improved reduced costs to $3,000/peak kW and the cost of electricity from 25 cents to 8 cents per kWh, barely more than the cost of nuclear or coal-fired facilities.
The more recent facilities converted a remarkable 22% of sunlight into electricity.
SOLAR-THERMAL ELECTRICITY: PARABOLIC DISHES AND TROUGHS
MIRRORS
Tracking mirrors focus sunlight on a stationery “power tower” to generate very high temperatures (~1000o F)
Used to generate electricity
CONCENTRATING SOLAR THERMAL COLLECTORS
SOLAR THERMAL POWER PLANT
1. Solar collector 2. Hot water reservoir 3. Head exchanger4. Cold water reservoir 5. NH3 Gas Turbine 6. Generator7. NH3 condenser 8. NH3 Pressuriser 9. Cooling Tower
BINARY CYCLE SOLAR THERMAL POWER PLANT
General idea is to collect the light from many reflectors spread over a large area at one central point to achieve high temperature.
Example is the 10-MW solar power plant in Barstow, CA. 1900 heliostats, each 20 ft by 20 ft a central 295 ft tower
An energy storage system allows it to generate 7 MW of electric power without sunlight.
Capital cost is greater than coal fired power plant, despite the no cost for fuel, ash disposal, and stack emissions.
Capital costs are expected to decline as more and more power towers are built with greater technological advances.
One way to reduce cost is to use the waste steam from the turbine for space heating or other industrial processes.
SOLAR-THERMAL ELECTRICITY
POWER TOWER IN BARSTOW, CALIFORNIA
Photovoltaic cells are capable of directlyconverting sunlight into electricity.
A simple wafer of silicon with wires attachedto the layers. Current is produced based ontypes of semiconductor (n- and p-types) usedfor the layers. Each cell produces 0.5 V.
Battery may be needed to store electricalenergy
No moving parts, no pollution, do not wearout, but because they are exposed to theweather, their lifespan is about 20 years.
DIRECT CONVERSION INTO ELECTRICITY
Photovoltaic (PV) converts sunlight to DC electricityusing a semiconductor cell.
The PV effect was discovered in 19th century byAlexander Becquerel
Bell labs pioneered early application, especially forsatellites, in the 1960s
Very small, remote applications emerged in the1970s and early 1980s
As cost declined, PV became more common forlarger applications in late 1980s and early 1990s
PHOTOVOLTAIC TECHNOLOGY - BACKGROUND
Because of their current costs, only rural and other customers far away from power lines use solar panels.
Subsidy is given for these panels by Central and State nodal agencies.
The costs may go down in coming years in view of ongoing R&D work worldwide
SOLAR PANELS IN USE
PHOTOVOLTAIC EFFECT Electromagnetic radiation can be viewed as photons Each photon has energy E = hν = h c/λ Photons travel at speed c = ν λ Photons having an sufficient energy can dislodge an
electron from silicon (1.12 eV = 1.794 x 10-22 kJ) The electron is accelerated by the electric field If a circuit is provided a current will flow
PHOTOVOLTAIC CELLS
When sunlight strikes the solar cell, it “knocks loose” electrons, which generates a flow of DC current
PHOTOVOLTAIC CELL EFFICIENCY
The most commonly used material crystalline silicon, absorbs energy in a small part of the spectrum. Efficiency depends on how much of the available spectrum can be converted to electricity.
MANUFACTURER’S DATA
PV CELLS IN SERIES AND PARALLEL
Series arrangement: voltages addParallel arrangement: currents add
PV CELL MATERIALS
The most common PV cells are made fromcrystalline silicon wafers
Other types of materials include thin films like Cadmium Telluride (CdTe), Copper-Indium-Gallium-Diselenide (CIGS), amorphous silicon (a-Si)
The main goals for manufacturers are to minimizethe amount of materials and maximize efficiency
Today, the best crystalline silicon cells are about 15%efficient, the best thin films are about 8% efficient.
PV CELLS, MODULES AND ARRAYS
PV cells are connected like batteries to increase voltage and current output and are assembled in to modules
Modules become part of larger arrays
HOW ARE PV SYSTEM RATED?
PV modules are rated based on themaximum power produced in Wattswhen the amount of sunlight is 1,000W/m2
PV systems are rated based on themaximum combined power output ofthe PV modules
Since the amount of sunlight changes,the power output of the system will vary
SOALR PHOTOVOLTAIC PANELS
1950 1960 1970 1980 1990 2000
5
10
15
20
25
Effi
cien
cy (%
)
Year
crystalline Siamorphous Sinano TiO2
CIS/CIGSCdTe
EFFICIENCIES OF PHOTOVOLTAIC DEVICES
Primary Batteries can store and deliver electrical energy, but can not be
recharged. Typical carbon-zinc and lithium batteriescommonly used in consumer electronic devices areprimary batteries. Primary batteries are not used in PVsystems because they can not be recharged.
Secondary Batteries can store and deliver electrical energy, and can also be
recharged by passing a current through it in an oppositedirection to the discharge current. Common lead-acidbatteries used in automobiles and PV systems aresecondary battery.
BATTERIES
Battery Type CostDeep Cycle
PerformanceMaintenance
Flooded Lead-Acid
Lead-Antimony Low Good High
Lead-Calcium Open Vent Low Poor Medium
Lead-Calcium Sealed Vent Low Poor Low
Lead Antimony/Calcium Hybrid Medium Good Medium
Captive Electrolyte Lead-Acid
Gelled Medium Fair Low
Absorbed Glass Mat Medium Fair Low
Nickel-Cadmium
Sintered-Plate High Good None
Pocket-Plate High Good Medium
SECONDARY BATTERY TYPES AND CHARACTERISTICS
Battery Capacity It is a measure of battery’s ability to store or deliver
electrical energy, commonly expressed in units ofampere-hours.
Ampere-Hour Definition It is the common unit of measure for a battery’s electrical
storage capacity, obtained by integrating the discharge orcharge current in amperes over a specific time period. Anampere-hour is equal to the transfer of one ampere overone hour, equal to 3600 coulombs of charge. Forexample, a battery which delivers 5 amps for 20 hours issaid to have delivered 100 ampere-hours.
BATTERIES FOR PV
Discharge rate affects capacity
Typical discharge times
Industrial, motive applications 10 hours
Photovoltaic applications 100-300 hours
Maximum Recommended Depth of Discharge for Lead Acid Batteries
Shallow cycling types 50%
Deep cycling types 80%
Time to fully discharge
Time to discharge (rating) = Days of reserve 24 hours(1 day)Maximum Depth of Discharge (%)
×
BATTERIES FOR PV (contd…)
Nominal system voltage Charging requirement Required capacity Ampere-hour capacity at discharge rate Daily and maximum depth of discharge Self-discharge rate Gassing characteristics Efficiency Temperature effects Size, weight and structural needs Susceptibility to freezing Electrolyte type and concentration Maintenance requirements Terminal configurations Battery life (cycles/year) Availability and servicing Cost and warranty
BATTERY SELECTION CRITERION
PV SYSTEMS
A complete PV system may also include a device to convert DC to AC power (inverter), batteries to store energy, and a back up generator
PV systems can be connected to the electric utility and can be used to reduce the amount of electricity purchased from the local utility without using batteries or generators
Efficiency is far lass than the 77% of usable solar spectrum (theoretical efficiency).
Only 43% of photon energy is used to warm the crystal. Efficiency drops as temperature increases (from 24% at 0°C
to 14% at 100 °C.) Light is reflected off the front face Internal electrical resistance are other factors. Overall, the efficiency is about 10-14%. Underlying problem is weighing efficiency against cost. Crystalline silicon-more efficient but more expensive to
manufacture Amorphous silicon-half as efficient, less expensive to
produce.
LOW EFFICIENCY AND OTHER DISADVANTAGES
FUTURE OF SOLAR ENERGY
Solar thermal energy is already very cost-effective for providing low temperature heat almost anywhere
PV is very cost effective for providing electricity in remote areas and in niche applications
As the costs of fossil fuels and electricity increase, PV is becoming more cost effective compared to electricity from conventional sources
The costs of all solar technologies are declining
SOLAR ENERGY ISSUES AND BARRIERS
‘Fuel’ is free but the systems are not. Can be costly to install compared to grid supplied electricity and fossil fuels
Certain technologies, like PV, can require large areas
Some PV technologies use toxic materials, although in very small amounts
Energy storage must be used in some cases
Argument that sun provides power only during the day is countered by the fact that 70% of energy demand is during daytime hours. At night, traditional methods can be used to generate the electricity.
Goal is to decrease our dependence on fossil fuels.
Currently, 75% of our electrical power is generated by coal-burning and nuclear power plants.
Mitigates the effects of acid rain, carbon dioxide, and other impacts of burning coal and counters risks associated with nuclear energy.
Pollution free, indefinitely sustainable.
FINAL THOUGHT