Understanding the Science behind Solar Energy

Presented by:Arturo Pelayo

Western Illinois UniversityMacomb, 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.

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

• 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).

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.