Photovoltaic CellsPhotovoltaic Cells

Paula WarrenPaula Warren

What is a photovoltaic cell?What is a photovoltaic cell?

It converts solar light into electricity and heat. Some materials exhibit a It converts solar light into electricity and heat. Some materials exhibit a property known as the photoelectric effect that causes them to absorb property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity. captured, an electric current results that can be used as electricity.

PV cells made of a form of silicon. Could be single crystalline/mono-crystalline, polycrystalline, ribbon and sheet crystalline, or thin-layer crystalline silicon. Silicon is the most highly developed PV material and is the one most likely to succeed in large scale applications.

If you can remove the heat from the PV modules, it is able to improve the If you can remove the heat from the PV modules, it is able to improve the electricity generation.electricity generation.

How it WorksHow it Works

Semi-conductors are solids with electrical properties between metals Semi-conductors are solids with electrical properties between metals and insulators.and insulators.

Conduction BandConduction Band

Valence BandValence Band

e- e- valence band, results in a hole, (+) chg; e- move freely via valence band, results in a hole, (+) chg; e- move freely via motions of e- to fill vacancies motions of e- to fill vacancies

e- e- conduction band, moves freely since all orbitals in the band are conduction band, moves freely since all orbitals in the band are emptyempty

An e- in a conduction band and a hole in the valence band are called An e- in a conduction band and a hole in the valence band are called mobile carriersmobile carriers. .

Band gapEnergy

Flow of ElectronsFlow of Electrons Need to produce a flow of e- so that after the sunlight hits the silicon, and e- are Need to produce a flow of e- so that after the sunlight hits the silicon, and e- are

promoted to conduction band, they have a direction to go and do not fall back into promoted to conduction band, they have a direction to go and do not fall back into the valence band. the valence band.

If some of the semi-conductor’s atom are replaced with other atoms that either If some of the semi-conductor’s atom are replaced with other atoms that either have fewer or more valence e-, then you produce a built-in electrical potential. have fewer or more valence e-, then you produce a built-in electrical potential.

Arsenic (5 e-) is inserted in the valence band, and that extra e- has high energy Arsenic (5 e-) is inserted in the valence band, and that extra e- has high energy and only takes a little energy to enter the conduction band. A (+) chg is left on the and only takes a little energy to enter the conduction band. A (+) chg is left on the As atom. This produces an n-type semi-conductor.As atom. This produces an n-type semi-conductor.

Gallium (3 e-) is inserted in the conduction band, and the missing e- can be Gallium (3 e-) is inserted in the conduction band, and the missing e- can be supplied by the surrounding Si lattice, creating a hole in the valence band. A fixed supplied by the surrounding Si lattice, creating a hole in the valence band. A fixed (-) chg is left on the Ga atom. (-) chg is left on the Ga atom.

Electron PathElectron Path After a photon makes it's way through the encapsulate (glass or After a photon makes it's way through the encapsulate (glass or

clear plastic) it encounters the antireflective layer. The clear plastic) it encounters the antireflective layer. The antireflective layer channels the photon into the lower layers of the antireflective layer channels the photon into the lower layers of the solar cell. solar cell.

Once the photon passes the AR coating, it will either hit the silicon Once the photon passes the AR coating, it will either hit the silicon surface or the contact grid metallization. The metallization, being surface or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet The contact grid must be large enough to collect electrons yet cover as little of the solar cell's surface, allowing more photons to cover as little of the solar cell's surface, allowing more photons to penetrate.penetrate.

The photon's energy transfers to the valance electron of an atom The photon's energy transfers to the valance electron of an atom in the n-type Si layer. That energy allows the valance electron to in the n-type Si layer. That energy allows the valance electron to escape its orbit leaving behind a hole. escape its orbit leaving behind a hole.

a. Encapsulateb. Contact Gridc. The Antireflective Coating (AR Coating)d. N-Type Silicone. P-Type Siliconf. Back Contact

PV FormationPV Formation

Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced.

2/3 of sun’s wavelengths= 1140nm

Silicon band gap= 1.09 electron volts (which is smaller than 1140 nm)

Silicon can capture most of solar photons.

How do you remove heat?How do you remove heat?

In PV/T technology, the system removes In PV/T technology, the system removes and uses the waste heat through a and uses the waste heat through a convective airflow behind the PV panels. convective airflow behind the PV panels. It can either be free air convective It can either be free air convective cooling or a forced flow scheme, but the cooling or a forced flow scheme, but the latter consumes energy and reduces the latter consumes energy and reduces the net electrical gain of the system. net electrical gain of the system.

China Hotel StudyChina Hotel Study

260m260m22 (2340 ft (2340 ft22) wall proposed, ) wall proposed, on west side of buildingon west side of building

Multi-framed mono-crystalline Multi-framed mono-crystalline silicon modules mounted onto silicon modules mounted onto a 3m wide steel structure from a 3m wide steel structure from 55thth-28-28thth level. Overall level. Overall height=86.4m. height=86.4m.

Each module= 36 silicon cells, Each module= 36 silicon cells, yielded 18V at max. power yielded 18V at max. power pointpoint

Grouping of 30 panels/2 floor Grouping of 30 panels/2 floor levelslevels

Fig. 2. Hotel building under study: (a) hotel tower with PV wall on west-facing facade, (b) layout of guest room floor.

Study ResultsStudy Results

The annual outputs from PV/C, PV/T, The annual outputs from PV/C, PV/T, and BiPV differences are less than and BiPV differences are less than 1%, being at 83,680, 83,584, and 1%, being at 83,680, 83,584, and 83,205 MJ, respectively. There’s no 83,205 MJ, respectively. There’s no significant difference in electricity significant difference in electricity output. output.

This is equal to about 2.32x10This is equal to about 2.32x104 4 WW

The overall electrical efficiencies of all The overall electrical efficiencies of all three in the year are ~10.2%three in the year are ~10.2%

In terms of annual electricity output, In terms of annual electricity output, PV/C is marginally the best. PV/C is marginally the best.

PV/C has a simple design, greater PV/C has a simple design, greater effectiveness in reducing the space effectiveness in reducing the space cooling energy consumption, and cooling energy consumption, and the resulting small improvement in the resulting small improvement in the PV cooling performance. the PV cooling performance. Therefore, would be the better Therefore, would be the better choice.choice.

Fig. 4. Comparison of space heat gain via the PV wall for the period from May to October for four different designs.

Costs of PVCosts of PV Unit energy costs: Unit energy costs: P= {(CP= {(CTT *[R/1 – (1-R)] *[R/1 – (1-R)]-t-t) + AC) + ACO&MO&M}/Q}/QAA

where: P=unit electricity cost, Cwhere: P=unit electricity cost, CTT=total capital costs, R=discount rate, t=lifetime of =total capital costs, R=discount rate, t=lifetime of the system, ACthe system, ACO&MO&M=annual costs of operation and maintenance, and Q=annual costs of operation and maintenance, and QAA=annual =annual output of the system.output of the system.

A BiPV has an energy payback time of about 5.5 years. And are expected to have a lifetime of about 25 years.

Building-integrated systems offer cost reductions in both energy and economic, Building-integrated systems offer cost reductions in both energy and economic, especially if the costs of avoided building materials are taken into account.especially if the costs of avoided building materials are taken into account.

At current energy prices, PV power is uncompetitive with electric power by a factor At current energy prices, PV power is uncompetitive with electric power by a factor of 3. But in 1972, PV electricity costs were 100x higher than they are now. of 3. But in 1972, PV electricity costs were 100x higher than they are now.

Worldwide PV = more than Worldwide PV = more than 1 GW (1 Billion Watts) in 19991 GW (1 Billion Watts) in 1999 Use is increasing at 17% annual rate Use is increasing at 17% annual rate (4 yr doubling time)(4 yr doubling time) US Dept. of Energy forecasts 18% decrease in cost per doubling US Dept. of Energy forecasts 18% decrease in cost per doubling

period.period.

M. Oliver and T. JacksonM. Oliver and T. Jackson

West Lafayette West Lafayette IrradianceIrradiance 7 yr ave.= 14.31MJ/m7 yr ave.= 14.31MJ/m22 Clear sky ave.=13.67 MJ/mClear sky ave.=13.67 MJ/m22

Ave. % penetrated through clouds=92%Ave. % penetrated through clouds=92% Actual ave. solar E penetrated (irradiance)=13.16 MJ/mActual ave. solar E penetrated (irradiance)=13.16 MJ/m2 per yr per yr

W.L. pop= 28,778 people, PU=38,847 total= 67,625 W.L. pop= 28,778 people, PU=38,847 total= 67,625 U.S. pop=2.98x10U.S. pop=2.98x1088

W.L. % of US pop= W.L. % of US pop= .000227.000227

(.000227)(3.5x1012 W)= 7.95x108 W This is the amount of energy used by West Lafayette

7.95x108 W / (153 W/m2)(.10)= 5.2x105 m2 The area we would need to supply energy for W.L. via solar power

5.2x105 m2= 52 km2= 12.96mi2 = 4.5x4.5mi

ConclusionsConclusions

Would not be able to supply all of our Would not be able to supply all of our electricity needs via photovoltaics, but electricity needs via photovoltaics, but could supply a fraction of the energy. could supply a fraction of the energy.

Is not yet competitive with primary energy Is not yet competitive with primary energy sources, but price continues to lower. sources, but price continues to lower.

Could definitely be a larger source of Could definitely be a larger source of energy use in the future. energy use in the future.

SourcesSources Chow, T. T.; Hand, J. W.; Strachan, P. A. Chow, T. T.; Hand, J. W.; Strachan, P. A. Building - integrated Building - integrated

photovoltaic and thermal applications in a subtropical hotel photovoltaic and thermal applications in a subtropical hotel building.building. Applied Thermal Engineering (2003), 23(16), 2035-2049. Applied Thermal Engineering (2003), 23(16), 2035-2049. CODEN: ATENFT ISSN:1359-4311. CAN 140:148974 AN 2003:647578CODEN: ATENFT ISSN:1359-4311. CAN 140:148974 AN 2003:647578

M. Oliver and T. Jackson, M. Oliver and T. Jackson, Energy and economic evaluation of Energy and economic evaluation of building-integrated photovoltaics.building-integrated photovoltaics. Energy 26 (2001), pp. 431–439. Energy 26 (2001), pp. 431–439.

S&S pp. 80-84S&S pp. 80-84

http://www.census.gov/http://www.census.gov/

Pictures: Pictures: NASA- NASA- http://science.nasa.gov/headlines/y2002/solarcells.htmhttp://science.nasa.gov/headlines/y2002/solarcells.htm US Dept. of Energy- US Dept. of Energy-

http://www.eere.energy.gov/RE/solar_photovoltaics.htmlhttp://www.eere.energy.gov/RE/solar_photovoltaics.html http://www.eere.energy.gov/solar_photovoltaics.htmlhttp://www.eere.energy.gov/solar_photovoltaics.html