ENERGYPHYX 1020USU 1360 CHAPTER 4 RENEWABLE - SOLAR 2002 1 CHAPTER 4 RENEWABLE ENERGY I - SOLAR ENERGY What do we see related to solar energy? Light Electromagnetic.

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CHAPTER 4RENEWABLE ENERGY I - SOLAR ENERGY

What do we see related to solar energy?

Light Electromagnetic wave energy Latitude effects Seasons

Weather Wind Clouds Potential for rainOceans Temperature gradient Potential for waves

Economics Extent of underprivileged countries

Politics The world’s oil resources

Biomass Potential for plant growth

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The Source of Solar Energy• Inside the sun (as in other stars) the pressure and temperature is sufficient for

nuclear fusion reactions to occur• Part of the high energy release in the form of heat energy converted from the

mass energy of the fusion is used to maintain the high temperature for fusion to occur (c.f. Burning)

• The rest is conducted and convected to the surface of the sun where an equilibrium temperature of ~6000K results

• This is much less than the temperature in the interior needed to maintain fusion

• The hot surface of the sun radiates electromagnetic wave energy • This form of energy travels through the near vacuum of space in all

directions• A small fraction of this is intercepted by the earth and provides almost all of

our energy either directly or indirectly• Bottom line the source of most of our energy is Nuclear

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Solar Renewable Energy (1)

• Channel of energy flow– Direct– Indirect

• Direct– Active heating panels

• Electromagnetic wave energy

– Passive heating systems• Electromagnetic wave energy

– Solar boilers• Electromagnetic wave energy

– Solar photoelectric conversion• Electromagnetic wave energy

Heat energy

Heat energy

Heat energy

Electrical energy

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Solar Renewable Energy (2)

• Indirect– Hydro electricity– Wind– Biomass– Waves– Ocean temperature gradient

• These will be considered later in the section on alternative energy

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Renewable Energy• Note:

– Renewable sources only 8%

– Of these Solar related energy sources make up 96%

– Direct solar energy is only 1%

DOE Annual Energy Review, 1999

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Solar Energy at Top of Atmosphere• Spectra of solar electromagnetic

wave emission over wavelength range 0.2 - 3.0 m

– Energy emitted at different wavelengths

– Wavelength band associated with UV, visible, IR

– Broken line at top of atmosphere– Solid line at earth’s surface

• Broken line characteristic of emission from a black body at ~6000K with superimposed line emissions

• Solid line shows strong absorption bands by atmospheric atoms and molecules

• Area under the curve is the total energy in the wavelength range

• Over all energies it is called the SOLAR CONSTANT

Solar Spectrum

Above atmosphere

Earth’s Surface

UV VIS Infrared

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Atmospheric Modification of Solar Energy

• Absorption and scattering are wavelength dependent– Causes the dips in the previous curve

• Above the atmosphere the power from the sun is ~1300W/m2– At the earth the average over a full (24 hr) day is ~164W/m2

• Mid-latitude, av. cloudiness, accounts for rotation of earth, horizontal surface

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Distribution of Annual Insolation in USA

• Regions depend on clear skies, latitude, low humidity.• Energy figures are for an 8-hour day while sun is up.

Daily energy averaged over one year

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Geometrical Effects of Solar Energy Plate Collectors

• Total energy collected per m2 are areas under curves• Note peak power ~1000W/m2, but average 500W/m2 or less for 8-hour day• Slanted stationary plate is inclined to point at sun at noon• Actual angles depend on day of year• Steering is an expensive option for domestic use

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Flat Plate Collectors• This is an example of an ACTIVE solar energy

conversion system.– Active because energy is used to distribute the heat from

where the conversion occurs to where it is needed (forced convection)

• Heat energy at surface of sun is transferred to a collecting plate by electromagnetic radiation.– Surface of collecting plate absorbs the electromagnetic wave

energy and converts it to heat energy– Heat energy is conducted through the plate to a fluid in

contact with the plate and increases the heat energy of the fluid

– A pump keeps the fluid moving past the plate to a HEAT EXCHANGER where the energy is extracted and stored in another fluid for future use.

– The original fluid with a lower level of heat energy is then returned to the collector plate to gain more energy.

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Solar Energy CollectorsThe Greenhouse Effect (1)

• The greenhouse effect depends on two physics principles:

– Stefan’s Law

– Wien’s Law

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– Stefan’s Law• This describes how much power an object

emits at a given temperature when all wavelengths are considered

• P/A = T4

= emissivity between 0 and 1.0 (shiny at low end, dull black at high end, 1.0 is called a “black body”)

= Stefan’s constant (5.76 x 10-8 W/m2.K4)– T is the temperature of the object on the Kelvin

scale• Note the very strong dependence on

temperature - often determines the steady temperature of objects

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– Wien’s Law• This describes the wavelength at which the

peak emission occursmax = 2898 / T m

– Thus for solar energy from the sunmax = 2898/6000 = 0.48 m

– While for the collector

max = 2898/360 = 8.1 m

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• The effect occurs because the “greenhouse” glass is transparent to 0.48 m but is opaque to 8.1 m. Thus little energy can escape by radiation and the heat energy (temperature) increases in the enclosure compared to no enclosure

• This is the basis for solar energy collectors and has relevance to the earth’s temperature increase causing global warming to be discussed later.

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Schematic Solar Heating System (Collector)

• Insulation at base to reduce heat loss by conduction

• Black collector on absorber – low loss by reflection

– High absorption of solar radiation

• Water pumped through collector (Active System)

– Heat transfer by forced convection

• Double paned glass– Reduced loss by

conduction– Blocks loss by radiation

(greenhouse effect)

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Schematic Solar Heating System (System)

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Calculation for Active Solar Heating (1)

• What area of flat plate solar collector oriented to point at the sun at would be needed to maintain the temperature at a comfortable level in an average house on a cold winter day in northern Utah?– Assume the heat energy needed to be supplied to the

house is 106 Btu/day– Referring to fig 4.3 the average daily solar energy input

in N. Utah is 4.4 kWh/m2

– The flat plate collector will not be 100% efficient - assume an efficiency of 0.7 (70%)

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Calculation for Active Solar Heating (2)• Calculation

– Solar energy needed accounting for efficiency• Eff = Heat energy to house / Heat energy from sun• Substituting known values: 0.7 = 106 / solar heat energy• Thus by rearranging: solar heat = 106 / 0.7 = 1.4 x 106 Btu

– Convert kWh/m2 to Btu/m2

• Using table in book we find 1 kWh = 3413 Btu• Thus 4.4 kWh/m2 = 4.4 x 3413 = 15,017 Btu/m2

– Calculate area of solar collector needed to supply the house thermal energy• Area = Solar heat energy needed / solar heat energy/m2

• = 1.4 x 106 / 15,017 = 93.2 m2 ~1000 ft2

• This is could be constructed as a panel 50 x 20 ft (quite large!)

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Passive Solar Collection (1)

• Three components to passive solar heating of buildings– Collection– Storage– Insulation

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• Collection– Solar energy must penetrate the building and

will be trapped due to the greenhouse effect.– Provision must be made to reduce solar energy

input during the summer.• Use of roof overhangs to shield windows from the

sun in summer and around noon

• Use of sun blinds on S and W windows

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• Storage– The heat energy from the absorbed solar

radiation must be used to heat up a large mass to provide heat energy after sunset or when cloudy.

– Heat energy storage varies with material• Water 62 Btu per cubic foot per °F• Iron 54 “ “• Brick 25 “ “• Concrete 22 “ “

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Passive Solar Collection (4)• Insulation

– The windows, walls, roof and floors must be constructed to reduce the loss of heat energy from the building by conduction.

– Use of layers of low thermal conductivity material• Glass wool• Styrofoam sheets• Shredded paper• Air (as in double/triple glazed windows)

– Make layer as thick as possible (except air gaps)– Reduce inevitable heat loss from outside walls by

windbreaks to reduce the forced convection of the air flow associated with the wind.

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Passive Solar CollectionPractical systems

Trombe Wall passive solar heating system

Direct CollectionPassive solar heating system

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Solar Boilers• Solar thermal electric power generation

– Solar energy is used to generate the heat source for a heat engine to subsequently rotate an alternator

– High temperatures are required for high thermodynamic efficiency of the heat engine.• Hard to do with direct solar energy - even in a car the temperature is below

boiling point of water

– The solar radiation energy need to be concentrated in a small volume• Use of curved (parabolic) mirrors• Use of arrays of plane mirrors (heliostats)

– Water pumped through the region of solar energy concentration and the high heat flow turns it to steam used as the heat source for a heat engine. • Boiler temperatures of 1000 - 2000 °C are achievable now• Depends on ratio of mirror collecting area to boiler surface area (concentration

ratio)

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Parabolic trough collectorsBarstow CA

Colorado

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Heliostat Collector

10 MW installation at Barstow CA

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Heliostat Calculation (1)

• A University decides to convert its football field to a heliostat array to generate electricity by producing steam from the collected and directed solar energy. Assume all of the solar energy falling on the field is concentrated on the solar boiler and calculate the peak watts of power input to the boiler and electrical power generated.– Size of football field (including end zones) 160 x 360

ft– Peak solar power at site 1000 W/m2

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Heliostat Calculation (2)• Calculation

– Convert area of field to m2 1m = 3.28 ft• Field area = (160 / 3.28) x (360 / 3.28) = 5354 m2

– Calculate total power at 1000 W/m2 peak power• Total power at boiler = 1000 x 5354 = 5.354 x 106 or ~

5.4 MW– Account for efficiency of a modern heat

engine/generator ~ 0.35• Electric power delivered = 0.35 x 5.4 = 1.9 MW

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Solar Photo-Electricity (1)• It is possible to cause charge separation (and hence

electrical energy) directly from electromagnetic waves by the wave-electron interaction in materials.– This was discovered as the photo-electric effect in which light

was found to cause the emission of electrons from surfaces.– A threshold effect was seen in the wavelength of the

electromagnetic radiation necessary for the emission to occur– This led to Einstein’s explanation of the effect that the light

could be considered as a stream of particles called photons– The energy of each photon is proportional to 1/

• Where = wavelength of the light– Note the shorter the wavelength the more energy carried by

the photon.– The photon energy must then exceed the minimum energy to

extract the electron from the surface accounting for the wavelength threshold.

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Solar Photo-Electricity (2)

• The photo electric effect requires a vacuum to allow the emitted electrons to be captured and maintain the charge separation.

• In 1954 another method of employing the energy of photons to energize electrons in material was developed as the solar cell.

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Solar Cells (1)• Solar cells are semi-conductor devices formed into a p-n junction in

silicon.• p refers to silicon doped with a substance which results in the absence of a

chemical bond electron• n refers to and excess electron to chemical bonding requirements• When these doped mixtures in silicon are brought into contact charge

migrates from one side of the junction to the other and sets up a voltage of ~0.5 volts

• The effect of the light photons is to release bound electrons in the junction so that the 0.5 V can drive a current in an external circuit.– The energy comes from interaction with light photons– There is again a threshold set by the minimum energy to release a bound

electron– The more photons/second (brighter) the more current can be driven by the

solar cell

Schematic Solar Cell

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Solar Cells (2)• Practical information

– Solar cells are fabricated on slices of crystals of silicon– Mounted in glass they are typically 2 inches by 1/16 inch thick– Each cell generates about 0.5 volts

• To produce high voltages they are connected in series• To produce high currents they are connected in parallel• In practice large arrays are a combination of series/parallel connection

– Power output related to solar power input• Expressed in peak watts out for 1000W/cm2 solar energy input• Typical efficiency is 10%• Rest of solar power input results in increased heat energy of cells

– i.e. higher temperature increases which reduces efficiency of electrical power generation

– Costs in 1997 were $4.16/peak watt in a module ($2.78 for cell)• Equivalent to $0.50 - $1.00 per unit of electricity (but no anti-pollution

costs)• Needs to fall to <$0.50/peak watt to compete with fossil fuel power.

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Solar Cells (3)• What will bring down costs?

– Price reductions in fabrication• Extract a thin ribbon of crystallized silicon from a molten mass• Amorphous silicon deposited as a film of very small crystals

– Increased efficiency• Use of gallium arsenide instead of silicon• Concentrating solar energy using mirrors

Production of Solar Cells 1982-98

DOE annual Energy Review, 1999

51,000kW peak

SHIPMENTS OF SOLAR CELLS TRADE IN SOLAR CELLS

EXPORTS

IMPORTS

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Solar Cells (4)

DOE annual Energy Review, 1999

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Solar Cells (5)

Solar Cell modules

Solar CellPower Station Solar Cell

Street lamp

Solar Cell powered buildings

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Solar Cell calculation (1)

• Suppose the household consumption of electrical energy is an average of 25kWh per day. What area of solar cells would be needed to supply this amount of energy during 8 hours of daylight if the solar power received during those 8 hours is 600W/m2 and the solar cells are 10% efficient at converting solar energy to electrical energy? What is cost of cells?

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Solar Cell calculation (2)• Calculation

– Calculate the solar energy received / m2 during the 8 hours• Energy = power x time• = 600 x 8 = 4,800 Wh/m2 or 4.8 kWh/m2

– Calculate energy from cells at 10% (0.1) efficiency• Energy out/Energy in = 0.1• Thus: Energy out/4.8 = 0.1 or Energy out = 4.8 x 0.1 = 0.48 kWh/m2

– Calculate area required• Area = Electrical energy needed / Electrical energy from cells/unit

area• = 25 / 0.48 = 52 m2

– Calculate cost of cells at $4.20 per peak watt • The cells being considered produce 100 peak watts ($420) per square

meter (based on peak solar power input of 1000 watts/m2)• Cost = 420 x 52 = $21,840 at 1997 prices for solar cell modules

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Learning Objectives (1)• Understand that renewable solar energy may be used directly or indirectly• Be aware of four techniques used in the direct conversion of solar energy.• Know that renewable energy provides only 8% of the US energy requirements.• Be aware that solar energy accounts for 96% of renewable energy.• Be aware that only 1% of renewable energy is direct solar energy.• Understand what is meant by the solar spectrum• Be familiar with the form of the solar spectrum and the approximate wavelength

at which it peaks• Know what is meant by the the term “Solar Constant”• Be aware of the effect of the atmosphere on the spectrum as the solar energy

penetrates it to the earth’s surface.• Be familiar with the major processes in the atmosphere which reduce the total

solar energy at the earth’s surface compared to that above the atmosphere.• Know approximately what percentage of the sun’s radiant energy reaches the

earth’s surface compared to that above the atmosphere.• Know the approximate power per unit area from solar radiation at the earth’s

surface on a sunny day near noon (1000W/m2).

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Learning Objectives (2)• Understand that the flat plate is an example of active, direct solar energy

conversion.• Know the principal of converting solar energy using flat plate collectors.• Know the principal of the “Greenhouse Effect”.• Be aware of the relevance and predictions of Stefan’s & Wien’s laws in rlation to

the “Greenhouse Effect”• Know the parts of a flat plate solar energy collector and their purpose.• Understand what is meant by passive, direct solar energy conversion.• Know the three components that are used in buildings to facilitate passive solar

energy conversion to heat the buildings.• Be aware of practical examples of passive solar heating such as the Trombe wall

and construction techniques for south windows.• Understand how the collecting area for radiant solar energy can be enhanced by

focussing collecting mirrors.• Be aware of parabolic trough collectors to generate steam from solar radiation• Know what is meant by a heliostat used to generate steam from solar radiation

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Learning Objectives (3)

• Know what is meant by the effect known as photo electricity.• Understand that electromagnetic radiation has an energy associated with its

wavelength.• Know that short wavelengths are more energetic than long wavelengths• Be aware of the development of the photo sensitive semiconductor junction

to form the solar cell.• Know that photo electronic effects have a threshold effect with respect to

the wavelength of the radiation.• Know the general shape and size of a solar cell• Understand that series connections can be used to increase the voltage of a

solar cell array.• Understand that the current which can be drawn can be enhanced by parallel

connection.• Know that the voltage of one cell is ~0.5V and the power it can deliver is

~10% of the incident solar radiation power.• Be aware of the increasing production of solar cells in recent years.• Know some uses of solar cells as commercial electrical energy sources.

ENERGYPHYX 1020USU 1360 CHAPTER 4 RENEWABLE - SOLAR 2002 1 CHAPTER 4 RENEWABLE ENERGY I - SOLAR ENERGY What do we see related to solar energy? Light Electromagnetic.

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