CHAPTER 4 RENEWABLE ENERGY I - SOLAR ENERGY

1
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 rain
Oceans Temperature gradient Potential
for waves
Biomass Potential for plant growth
Economics Extent of underprivileged countries
Politics The worlds oil resources
2
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

3
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
4
Solar Renewable Energy (2)

• Indirect
• Hydro electricity
• Wind
• Biomass
• Waves
• Ocean temperature gradient
• These will be considered later in the section on
  alternative energy

5
Renewable Energy

• Note
• Renewable sources only 8
• Of these Solar related energy sources make up
  96
• Direct solar energy is only 1

6
Solar Energy at Top of Atmosphere

• Spectra of solar electromagnetic wave emission
  over wavelength range 0.2 - 3.0 mm
• Energy emitted at different wavelengths
• Wavelength band associated with UV, visible, IR
• Broken line at top of atmosphere
• Solid line at earths 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

7
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

8
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.

9
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

10
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.

11
Solar Energy CollectorsThe Greenhouse Effect (1)

• The greenhouse effect depends on two physics
  principles
• Stefans Law
• Wiens Law

12
Solar Energy CollectorsThe Greenhouse Effect (2)

• Stefans Law
• This describes how much power an object emits at
  a given temperature when all wavelengths are
  considered
• P/A e s T4
• e emissivity between 0 and 1.0 (shiny at low
  end, dull black at high end, 1.0 is called a
  black body)
• s Stefans 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

13
Solar Energy CollectorsThe Greenhouse Effect (3)

• Wiens Law
• This describes the wavelength at which the peak
  emission occurs
• lmax 2898 / T mm
• Thus for solar energy from the sun
• lmax 2898/6000 0.48 mm
• While for the collector
• lmax 2898/360 8.1 mm

14
Solar Energy CollectorsThe Greenhouse Effect (4)

• The effect occurs because the greenhouse glass
  is transparent to 0.48 mm but is opaque to 8.1
  mm. 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 earths temperature increase
  causing global warming to be discussed later.

15
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)

16
Schematic Solar Heating System (System)
17
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)

18
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!)

19
Passive Solar Collection (1)

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

20
Passive Solar Collection (2)

• 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

21
Passive Solar Collection (3)

• 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

22
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.

23
Passive Solar CollectionPractical systems
24
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)

25
Parabolic trough collectors
26
Heliostat Collector
27
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

28
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

29
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 Einsteins 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/l
• Where l 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.

30
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.

31
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

32
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 lt0.50/peak watt to compete with
  fossil fuel power.

33
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

34
Solar Cells (4)
35
Solar Cells (5)
36
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?

37
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

38
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
  earths surface.
• Be familiar with the major processes in the
  atmosphere which reduce the total solar energy at
  the earths surface compared to that above the
  atmosphere.
• Know approximately what percentage of the suns
  radiant energy reaches the earths surface
  compared to that above the atmosphere.
• Know the approximate power per unit area from
  solar radiation at the earths surface on a sunny
  day near noon (1000W/m2).

39
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
  Stefans Wiens 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

40
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.