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Presented by:
      Arturo Pelayo

Western Illinois University
Macomb, IL. March 15, 2004
Important Facts:
• Every day, the Earth
  receives an amount of
  solar energy equal to 30
  years of world fossil fuel
  energy use.

• In half a day, the US
  receives the same amount
  of energy from the sun
  that it consumes for all
  purposes in an entire year.
Important facts:
• If concentrated, the sunlight that falls on the hood of a car
  would be enough power to boil a pot of water in minutes.
• Solar energy can heat buildings, heat water,
  cook food, drive pumps and refrigerators,
  and make electricity.
• Passive solar, which uses
  little or no mechanical
  devices, is the easiest form
  to use. It can supply all or
  most of the energy required
  by a conventional home.
  Larger buidings like schools
  and apartments that use
  passive solar energy may
  use less than half the
  electricity, oil or gas of a
  similar conventional building
  and can often be built at
  little or no additional cost.
• Solar energy can be utilized almost anywhere in one form or
  another. Even in places that are considered cloudy like New
  England or Europe, passive solar energy can be readily
  harnessed to warm buildings economically.

• Many other parts of the world, like the Mediterranean and
  Africa, receive months of endless sunshine.
Understanding the Science behind Solar Energy
Understanding the Science behind Solar Energy
Solar Energy.

• Although the solar surface and atmosphere comprise regions of
  very different temperatures, the sun is often equated to a black
  body (i.e. a perfect radiator) at a temperature of 5,762K

  The energy released from the sun comes about due to a fusion
  reaction in which hydrogen nuclei combine to from helium,
  releasing energy in the process. The overall reaction is:

          4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
Understanding the Science behind Solar Energy
• In this reaction, the final nuclei (Helium) have
  less internal energy than the starting particles

  (Hydrogen). This difference is released as

  energy of motion of the nuclei and electrons in

  the solar gas, low energy photons and high

  energy neutrinos.

• The amount of energy involved is 26 MeV (or
  26 x 10^6 eV) each time the abovereaction
  take place.

• 90% of the energy generated by the sun comes
  from this fusion reaction.
www.stanford.edu/~rachelm/ diagrams/geology/
• The sun’s energy reaches the earth as
  solar radiation, which is composed of
  discrete 'packets' of energy known as
  photons.

• The energy of a photon is dictated by
  its frequency:

               E = hv


• E = is the photon energy (J),
• h = Planck’s constant (6.62 x 10^-34
  Js) and
• The sun radiates photons over a range of frequencies;
  • These frequencies are related to the radiation wavelength
    (l ) by the equation

                            l = c/ v

• c=   speed of light in a vacuum (3 x 10^8 m/s).
www.stanford.edu/~rachelm/ diagrams/geology/
• Solar radiation reaching the surface of the earth has two
    components – direct or beam radiation and diffuse radiation.

•    As the name suggests beam radiation arrives directly from
    the sun diffuse radiation is the portion of solar radiation,
    which is scattered in the Earth’s atmosphere. On a clear day
    beam radiation makes up about 90% of the total reaching the
    earth’s surface.

    • The ratio of direct and diffuse radiation changes with the
      quantity of cloud and haze in the atmosphere (atmospheric
      turbidity): e.g. on heavily overcast days the beam
      component of solar radiation will be 0%. The total solar
      irradiance G (W/m^2) at a point on the earth’s surface is
      therefore the sum of the diffuse and beam radiation:

      •             G = G beam + G diffuse
• Opaque materials such as
  concrete will absorb and

  reflect solar radiation,




• Transparent materials
  such as glass will reflect,

  absorb and transmit solar

  radiation.
The preceding information can be used to estimate the solar radiation

falling on a surfaces of different orientations and of different properties.



This information can be used when designing a solar collector system.
• By far the most common type of solar
  collector is the flat plate solar collector,
  these are often found on the roofs of
  buildings throughout the US and in
  southern Europe.

• In these collectors solar energy is used
  to heat water, which can then be used
  inside the building.
•   The rate at which heat is absorbed by the collector (W) is given by:

                                    Qp = GsAt a

•    where Gs is the incident radiation (W), A the area of the collector (m^2), t the
    transmission factor of the cover and a the absorptance of the back plate. The
    losses from the collector are calculated from

                                   QL = UA(Tc-Ta)

     where U is the collector U-value (W/m^2 °C) , Tc is the average collector plate
    temperature and Tais the air temperature. The useful rate of energy recovery from
    the collector is therefore

                               QR = GsAt a - UA(Tc-Ta)

    The temperature rise in the water flowing through the collector is given by:

                                     D T = Qs/mC

    where m is the water flow rate to the collector (kg) and C is the water specific heat
    (J/kg °C).
The simplest solar collector is a
window, which admits heat and
light into a building, reducing
both fossil fuel consumption for
heating and electrical energy
consumption for lighting.
http://www.esru.strath.ac.uk/Courseware/Solar_energy/
Photovoltaics.
• Photovoltaic materials produce electrical
  power from sunlight. The basic component
  of photovoltaic power conversion is the
  solar cell.

• The history of photovoltaic materials goes
  back to 1839 when Edmund Becquerel
  discovered the photo galvanic effect: where
  electric currents were produced from light
  induced chemical reactions. However it was
  not until 1954 that the first solar cell was
  developed with an efficiency of 6%:

• efficiency =     power output      .

                 available solar power
Solar cells found their
first use in powering
satellites, however their
use for terrestrial power
production has been
growing rapidly.
• The most common solar cell is a p-n junction, where the
  p-type (positive) and n-type (negative) materials are
  doped semiconductor(s). The p-n junction is a boundary
  in a semiconductor material where a region of electron
  depletion neighbours a region of electron surplus.
• Solar cells are most commonly fabricated from silicon,
  however other materials such as cadmium and gallium may
  also be used. Silicon is a semiconductor material that is
  tetravalent, i.e. group IV of the periodic table. If silicon is
  doped with ions from a group III material it becomes an
  acceptor (p-type), when doped with a group V material it
  becomes a donator (n-type). The p-type material is said to
  have a surplus of holes (rather than a deficit of electrons).
Understanding the Science behind Solar Energy
Semiconductors
• Four types of silicon semiconductor devices are in use:

  • monocrystalline,
  • polycrystalline,
  • thin film polycrystalline
  • morphous.
Silicon semiconductors.
• Monocrystalline silicon has a highly ordered atomic
  structure and cells made from it have the highest
  photovoltaic conversion efficiencies (18%).

• Polycrystalline silicon consists of many crystalline
  grains; the conversion efficiency of a solar cell
  manufactured from polycrystalline silicon is around
  13%.

• A standard solar cell is typically cut from a large ingot
  of polycrystalline silicon and is typically between 200
  and 400 microns thick.
• In order for a current to flow in the
  semiconductor material, electrons in the
  valence orbitals (which form the bonds
  between the atoms) must be promoted
  to a higher energy level so that they are
  capable of conduction.

• The energy required for this is achieved
  by the absorption of photons of light.

• The amount of energy required for a
  valence electron to jump to this higher
  energy level is known as the band gap
  energy, Eg. This is an intrinsic property
  of the material (e.g. crystalline silicon
  has a band gap energy of 1.12 eV).
The liberation of an electron from the valence band
creates a corresponding vacancy in the valence band
known as a hole. Electrons and holes are the charge
carriers in the semiconductor material (i.e. the source
of electrical current). In p-type materials the holes are
the majority carriers, while in n-type materials
electrons are the majority carriers.
• The liberation of an electron from the valence band can
  be achieved by the interaction of a photon with the
  electron. The jump from the ground state to the excited
  state liberates one (and only one) electron-hole pair
  and requires the absorbed photon to have energy of

                       hv > Eg

• h = Planck’s constant: 6.626 x10^-34 Js
• v is the frequency (Hz).

• If a photon has an energy greater than Eg, it creates an
  electron-hole pair with an energy of greater than Eg,
  however the excess energy is soon dissipated as heat
Photons with a frequency less than Eg/h will not liberate an
electron-hole pair. This creates a fundamental efficiency
limitation in all photovoltaic conversion devices: only a
fraction of the photons absorbed in the photovoltaic material
will have a frequency greater than Eg/h (so-called above-
band-gap photons) and much of the energy from the above-
band-gap photons is wasted as heat. The silicon cell has
metallic grids deposited on each side, which act as electrical
contacts and allow electrons liberated by sunlight to flow: an
electrical current will flow from the cell. Under standard test
conditions of 1000W/m^2 irradiance and a cell temperature
of 25°C a good solar cell will generate a potential difference
of 0.5V and supply a current of up to 5A.
The output of a solar cell depends
upon several factors: the
properties of the semi-conductor
material, the intensity of insolation,
the cell temperature and the nature
of the external loads the cell
supplies.

The combination of these factors
gives rise to the characteristic
operating curves, of generated
current against the output voltage
for the solar cell.
•   Isc is the short circuited output of the cell, while Voc is the open circuit voltage. The
    maximum power of the cell occurs at the maximum power point (the knee of the
    curve in figure 6) where voltage is Vmpp and the current is Impp. The quality of a cell
    is indicated by its fill factor (FF):

    FF = Vmpp Impp./ Isc Voc

    The closer the fill factor to 1 the better the quality of the PV cell. The power output
    of the solar cell is related to the incident solar radiation and the cell temperature.
    The power output will vary linearly with incident solar radiation (when kept at the
    same temperature):

    P max 25 = PSTC G/1000

     where P max 25 is the power output at 25°C, PSTC the power output at standard test
    conditions (25°C and 1000W/m^2) and G the value of irradiance incidental on the
    module (W/m^2). Increasing temperature has a detrimental impact on the output of a
    solar cell: the hotter the cell operating temperature the poorer the efficiency of the
    cell. Typically efficiency will drop off by around 0.5% per °C increase in operating
    temperature. The following equation relates the cell temperature to its power output:

    P max T = Pmax25 [1-b (T-25)]

    where P max T is the power output at temperature T (°C), P max 25 the power output
    at 25°C and b the temperature coefficient of the cell (e.g. for a 0.5% drop of in
    efficiency this = 0.005). Substituting the previous expression gives the power output
    for the PV at any particular value of irradiance and temperature:

    P max = PSTC G/1000 [1-b (T-25)]
A major barrier to the uptake of PV materials into the building
structure is their low efficiency and high cost. Due to the impact
of low radiation levels and high temperatures real of efficiencies
of 12% have been calculated for crystalline silicon panels

(compared to flash test efficiencies of over 18% in some cases).
The result of the lower operational efficiencies of PV

arrays is that their payback period becomes longer and

they become less financially attractive.



One method of boosting the operational efficiency of PV

(when incorporated into a building façade) is to recover

heated air from the rear of the panels. This boosts the

efficiency in two ways.
Understanding the Science behind Solar Energy
Understanding the Science behind Solar Energy
Understanding the Science behind Solar Energy

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Understanding the Science behind Solar Energy

  • 1. Presented by: Arturo Pelayo Western Illinois University Macomb, IL. March 15, 2004
  • 2. Important Facts: • Every day, the Earth receives an amount of solar energy equal to 30 years of world fossil fuel energy use. • In half a day, the US receives the same amount of energy from the sun that it consumes for all purposes in an entire year.
  • 3. Important facts: • If concentrated, the sunlight that falls on the hood of a car would be enough power to boil a pot of water in minutes.
  • 4. • Solar energy can heat buildings, heat water, cook food, drive pumps and refrigerators, and make electricity.
  • 5. • Passive solar, which uses little or no mechanical devices, is the easiest form to use. It can supply all or most of the energy required by a conventional home. Larger buidings like schools and apartments that use passive solar energy may use less than half the electricity, oil or gas of a similar conventional building and can often be built at little or no additional cost.
  • 6. • Solar energy can be utilized almost anywhere in one form or another. Even in places that are considered cloudy like New England or Europe, passive solar energy can be readily harnessed to warm buildings economically. • Many other parts of the world, like the Mediterranean and Africa, receive months of endless sunshine.
  • 9. Solar Energy. • Although the solar surface and atmosphere comprise regions of very different temperatures, the sun is often equated to a black body (i.e. a perfect radiator) at a temperature of 5,762K The energy released from the sun comes about due to a fusion reaction in which hydrogen nuclei combine to from helium, releasing energy in the process. The overall reaction is: 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
  • 11. • In this reaction, the final nuclei (Helium) have less internal energy than the starting particles (Hydrogen). This difference is released as energy of motion of the nuclei and electrons in the solar gas, low energy photons and high energy neutrinos. • The amount of energy involved is 26 MeV (or 26 x 10^6 eV) each time the abovereaction take place. • 90% of the energy generated by the sun comes from this fusion reaction.
  • 13. • The sun’s energy reaches the earth as solar radiation, which is composed of discrete 'packets' of energy known as photons. • The energy of a photon is dictated by its frequency: E = hv • E = is the photon energy (J), • h = Planck’s constant (6.62 x 10^-34 Js) and
  • 14. • The sun radiates photons over a range of frequencies; • These frequencies are related to the radiation wavelength (l ) by the equation l = c/ v • c= speed of light in a vacuum (3 x 10^8 m/s).
  • 16. • Solar radiation reaching the surface of the earth has two components – direct or beam radiation and diffuse radiation. • As the name suggests beam radiation arrives directly from the sun diffuse radiation is the portion of solar radiation, which is scattered in the Earth’s atmosphere. On a clear day beam radiation makes up about 90% of the total reaching the earth’s surface. • The ratio of direct and diffuse radiation changes with the quantity of cloud and haze in the atmosphere (atmospheric turbidity): e.g. on heavily overcast days the beam component of solar radiation will be 0%. The total solar irradiance G (W/m^2) at a point on the earth’s surface is therefore the sum of the diffuse and beam radiation: • G = G beam + G diffuse
  • 17. • Opaque materials such as concrete will absorb and reflect solar radiation, • Transparent materials such as glass will reflect, absorb and transmit solar radiation.
  • 18. The preceding information can be used to estimate the solar radiation falling on a surfaces of different orientations and of different properties. This information can be used when designing a solar collector system.
  • 19. • By far the most common type of solar collector is the flat plate solar collector, these are often found on the roofs of buildings throughout the US and in southern Europe. • In these collectors solar energy is used to heat water, which can then be used inside the building.
  • 20. The rate at which heat is absorbed by the collector (W) is given by: Qp = GsAt a • where Gs is the incident radiation (W), A the area of the collector (m^2), t the transmission factor of the cover and a the absorptance of the back plate. The losses from the collector are calculated from QL = UA(Tc-Ta) where U is the collector U-value (W/m^2 °C) , Tc is the average collector plate temperature and Tais the air temperature. The useful rate of energy recovery from the collector is therefore QR = GsAt a - UA(Tc-Ta) The temperature rise in the water flowing through the collector is given by: D T = Qs/mC where m is the water flow rate to the collector (kg) and C is the water specific heat (J/kg °C).
  • 21. The simplest solar collector is a window, which admits heat and light into a building, reducing both fossil fuel consumption for heating and electrical energy consumption for lighting.
  • 23. Photovoltaics. • Photovoltaic materials produce electrical power from sunlight. The basic component of photovoltaic power conversion is the solar cell. • The history of photovoltaic materials goes back to 1839 when Edmund Becquerel discovered the photo galvanic effect: where electric currents were produced from light induced chemical reactions. However it was not until 1954 that the first solar cell was developed with an efficiency of 6%: • efficiency = power output . available solar power
  • 24. Solar cells found their first use in powering satellites, however their use for terrestrial power production has been growing rapidly.
  • 25. • The most common solar cell is a p-n junction, where the p-type (positive) and n-type (negative) materials are doped semiconductor(s). The p-n junction is a boundary in a semiconductor material where a region of electron depletion neighbours a region of electron surplus.
  • 26. • Solar cells are most commonly fabricated from silicon, however other materials such as cadmium and gallium may also be used. Silicon is a semiconductor material that is tetravalent, i.e. group IV of the periodic table. If silicon is doped with ions from a group III material it becomes an acceptor (p-type), when doped with a group V material it becomes a donator (n-type). The p-type material is said to have a surplus of holes (rather than a deficit of electrons).
  • 28. Semiconductors • Four types of silicon semiconductor devices are in use: • monocrystalline, • polycrystalline, • thin film polycrystalline • morphous.
  • 29. Silicon semiconductors. • Monocrystalline silicon has a highly ordered atomic structure and cells made from it have the highest photovoltaic conversion efficiencies (18%). • Polycrystalline silicon consists of many crystalline grains; the conversion efficiency of a solar cell manufactured from polycrystalline silicon is around 13%. • A standard solar cell is typically cut from a large ingot of polycrystalline silicon and is typically between 200 and 400 microns thick.
  • 30. • In order for a current to flow in the semiconductor material, electrons in the valence orbitals (which form the bonds between the atoms) must be promoted to a higher energy level so that they are capable of conduction. • The energy required for this is achieved by the absorption of photons of light. • The amount of energy required for a valence electron to jump to this higher energy level is known as the band gap energy, Eg. This is an intrinsic property of the material (e.g. crystalline silicon has a band gap energy of 1.12 eV).
  • 31. The liberation of an electron from the valence band creates a corresponding vacancy in the valence band known as a hole. Electrons and holes are the charge carriers in the semiconductor material (i.e. the source of electrical current). In p-type materials the holes are the majority carriers, while in n-type materials electrons are the majority carriers.
  • 32. • The liberation of an electron from the valence band can be achieved by the interaction of a photon with the electron. The jump from the ground state to the excited state liberates one (and only one) electron-hole pair and requires the absorbed photon to have energy of hv > Eg • h = Planck’s constant: 6.626 x10^-34 Js • v is the frequency (Hz). • If a photon has an energy greater than Eg, it creates an electron-hole pair with an energy of greater than Eg, however the excess energy is soon dissipated as heat
  • 33. Photons with a frequency less than Eg/h will not liberate an electron-hole pair. This creates a fundamental efficiency limitation in all photovoltaic conversion devices: only a fraction of the photons absorbed in the photovoltaic material will have a frequency greater than Eg/h (so-called above- band-gap photons) and much of the energy from the above- band-gap photons is wasted as heat. The silicon cell has metallic grids deposited on each side, which act as electrical contacts and allow electrons liberated by sunlight to flow: an electrical current will flow from the cell. Under standard test conditions of 1000W/m^2 irradiance and a cell temperature of 25°C a good solar cell will generate a potential difference of 0.5V and supply a current of up to 5A.
  • 34. The output of a solar cell depends upon several factors: the properties of the semi-conductor material, the intensity of insolation, the cell temperature and the nature of the external loads the cell supplies. The combination of these factors gives rise to the characteristic operating curves, of generated current against the output voltage for the solar cell.
  • 35. Isc is the short circuited output of the cell, while Voc is the open circuit voltage. The maximum power of the cell occurs at the maximum power point (the knee of the curve in figure 6) where voltage is Vmpp and the current is Impp. The quality of a cell is indicated by its fill factor (FF): FF = Vmpp Impp./ Isc Voc The closer the fill factor to 1 the better the quality of the PV cell. The power output of the solar cell is related to the incident solar radiation and the cell temperature. The power output will vary linearly with incident solar radiation (when kept at the same temperature): P max 25 = PSTC G/1000  where P max 25 is the power output at 25°C, PSTC the power output at standard test conditions (25°C and 1000W/m^2) and G the value of irradiance incidental on the module (W/m^2). Increasing temperature has a detrimental impact on the output of a solar cell: the hotter the cell operating temperature the poorer the efficiency of the cell. Typically efficiency will drop off by around 0.5% per °C increase in operating temperature. The following equation relates the cell temperature to its power output: P max T = Pmax25 [1-b (T-25)] where P max T is the power output at temperature T (°C), P max 25 the power output at 25°C and b the temperature coefficient of the cell (e.g. for a 0.5% drop of in efficiency this = 0.005). Substituting the previous expression gives the power output for the PV at any particular value of irradiance and temperature: P max = PSTC G/1000 [1-b (T-25)]
  • 36. A major barrier to the uptake of PV materials into the building structure is their low efficiency and high cost. Due to the impact of low radiation levels and high temperatures real of efficiencies of 12% have been calculated for crystalline silicon panels (compared to flash test efficiencies of over 18% in some cases).
  • 37. The result of the lower operational efficiencies of PV arrays is that their payback period becomes longer and they become less financially attractive. One method of boosting the operational efficiency of PV (when incorporated into a building façade) is to recover heated air from the rear of the panels. This boosts the efficiency in two ways.