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

SOLAR CYCLE VARIATIONS

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