Western Illinois University
Macomb, IL. March 15, 2004
• Every day, the Earth
receives an amount of
solar energy equal to 30
years of world fossil fuel
• 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.
• 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.
• 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
• 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
• The amount of energy involved is 26 MeV (or
26 x 10^6 eV) each time the abovereaction
• 90% of the energy generated by the sun comes
from this fusion reaction.
• The sun’s energy reaches the earth as
solar radiation, which is composed of
discrete 'packets' of energy known as
• The energy of a photon is dictated by
E = hv
• E = is the photon energy (J),
• h = Planck’s constant (6.62 x 10^-34
• 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).
• 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
• 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
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
• 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
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.
• Photovoltaic materials produce electrical
power from sunlight. The basic component
of photovoltaic power conversion is the
• 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
• 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).
• Four types of silicon semiconductor devices are in use:
• thin film polycrystalline
• 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
• 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
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
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.
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