A SHORT COURSE ON SUSTAINABLE ENERGY -- Part IImatecss.org/sites/default/files/Part_II_Short course new materials.pdf · Energy Conversion: Photovoltaics Overcome the Shockley-Queisser

A SHORT COURSE ON SUSTAINABLE ENERGY -- Part II

UNESCO Chair MATECSS

Materials and Technologies for Energy Conversion, Saving and Storage

Orsen Zamor,

Jennifer MacLeod, Federico Rosei,

INRS & Ecole Polytechnique, Montreal

What are some ways in which new materials and technologies can address

issues in energy conversion, saving and

storage?

New materials and technologies

Conversion

• Photovoltaic

• Thermoelectric

• Piezoelectricity

Saving• Solid-State lightning

Storage

• Capacitors and supercapacitors

• Hydrogen generation

• Batteries

Energy Conversion: Photovoltaics

A photovoltaic cell is a device that converts photons into an electric current. This technology is seen as a sustainable replacement or complement to the current fossil fuel centered electric grid because it doesn’t generate any GHGs and have

great energy potential thanks to the abundance of the sun. However factors such as price, efficiency and reliability have limited its integration.


With constant technology advances and cost reductions, Photovoltaics are now remarkably emerging in the world’s energy market with over 140 GW of cumulative PV systems installed

globally in 2013. These installations have the capacity of producing at least 160 TWh of electricity every year.

The semiconducting property of

the material is able to

respond to incident waves

Light absorption

The absorbed photon knocks out electrons at the ground state and the

semiconductors generate free

carriers

Charge generation

The dissociated charges are carried to electrodes

through charge

transporting pathways

Charge transport

The transported charges are

collected from the

semiconductor to the

electrode (cathode or

anode) at the interface

Charge collection


The photovoltaic effect has four main steps

Energy Conversion: PhotovoltaicsThe Shockley-Queisser efficiency limit

• One semiconductor material is used per solar cell.

• One p-n junction per solar cell.

• The sunlight is not concentrated.

• All energy is converted to heat from photonsgreater than the bandgap.

Assumptions for limit

calculation

•47% of the solar energy gets converted to heat because they havean excess of bandgap energy.

•18% of the photons pass through the solar cell because they don'thave enough energy to free semiconductor’s electrons and createcurrent.

•2% of the energy lost from local recombination of newly createdholes and electrons.

•33% of solar energy can theoretically be converted to electricity.

Energy Loss

Energy Conversion: PhotovoltaicsOvercome the Shockley-Queisser limit

• Use more than one semi conductormaterial per cell.

• Use multi-junction cells.• Sunlight can be significantly

concentrated using lenses in orderto generate more power per unit ofsurface area.

• Combine PV semiconductor withanother heat based technology tomaximize the solar energy or usequantum dots to harvest some ofthe excess photon energy forelectricity.


Energy Conversion: PhotovoltaicsSilicon solar cells

• Crystalline silicon is the oldest and most popular photovoltaic technology andcurrently accounts for about 85-90% of the PV market share.

• Its popularity is largely due to its lifetime and its efficiency of 13-25%.

• A module efficiency of 10% corresponds to about 100 W/m2.

• Silicon module prices have dropped more than 60% over the past 3 to 4 yearsmaking them competitive with residential electricity retail prices in countries withhigh cost of electricity and good solar resources.

Energy Conversion: PhotovoltaicsSilicon solar cells

• The cell is made using a slab of silicon with the topdiffused with an “n” dopant (ex: phosphorous)giving it an excess of free electrons and the basediffused with a “p” dopant (ex: boron) giving it atendency to attract electrons.

• The n-type (n => negative) and the p-type (p =>positive) silicon meet to form a point of contactcalled the p-n junction.

• The absorbed photon hits the electrons in the n-type silicon (cathode), leaving behind and emptyspace called hole.

• The free electron travels through a conducting wireto reach the p-type silicon (anode) due to thematerial`s electronegativity. There will be arecombination of the electron-hole pair at the backelectrical contact to restore electrical neutrality.

• This flow of electrons from the n-type to the p-type silicon creates an electric current.

Energy Conversion: PhotovoltaicsOrganic solar cells

• An organic photovoltaic (OPV) is a solar cell typically consisting of two polymeric semiconducting layers.

• When incident photons hit the electrons in the polymer atom, an empty space (hole) is left behind and an exciton (boundelectron hole pair) is formed.

• The strongly bound pairs are then diffused inside the organic semiconductors and are dissociated at the interface.

• The electron can then move independently to a hole created by another absorbed photon and the electric current isproduced by that continuous electron movement.


• One of the main advantages of organic solar cells is that they have very low costcompared to traditional silicon cells.

• Flexibility and lightweight are also important deciding factors in adopting OPVs.

• Although they have a relatively low efficiency and short life, they can still becomeeconomically viable because production cost is so low.


• A lot of research are being put into improvingthe efficiency of OPVs.

• The multilayers of donor-acceptor materialsand the composite materials containing bothdonor and acceptor molecules are two formsof active materials involved in OPVresearches.

• Low band gap polymers as the donormaterials and acceptors with high electronsaffinities and charge carrier mobility areusually used to improve efficiency.

• Variables such as the physical properties ofpolymers and the morphology of the activelayer are also very important.

Energy Conversion: PhotovoltaicsDye-sensitized solar cells

Dye-sensitized solar cell is an excitonic photovoltaic in which the organic molecule (dye) is used to collect photons and generate photoexcited electrons and electricity

is produced from the electron transfer.


• When the dye absorb a photon, one of its electron goes from a ground to an excited state.

• The photogenerated electron then diffuses across the titanium dioxide layer to reach the conductive layer(typically Tin dioxide) and travel through the wire to the counter electrode.

• There, the excited electron is collected by the liquid Iodide electrolyte which had lost an electron to theoriginally oxidized dye.


• Dye molecules have poor absorption in the red part of the spectrum (20mA/cm2) compared tosilicon (35 mA/cm2) which affects their efficiency meaning that fewer sunlight photons are usedfor electrical current generation.

• They have the particularity of being able to work effectively in low light conditions and are lesssusceptible to losing energy to heat because their thin front layer allow them to operate at lowerinternal temperature .

• The use of liquid electrolyte make them vulnerable to cold temperatures as the electrolyte canfreeze and damage the cell. Researchers are now exploring the idea of a solid electrolyte.

• Despite their low efficiency they are still very competitive thanks to their low manufacturing cost,plus there has been promising advances in the efficiency area in the past years.


Energy Conversion: PhotovoltaicsQuantum-dot solar cells

Quantum dot solar cells are photovoltaic solar cells that use quantum dots to harvest energy from light. They are third generation solar cells that can potentially deliver high efficiency at economically viable cost.

Energy Conversion: PhotovoltaicsQuantum-dot properties

• Quantum dots are nonoparticules made of semiconducting materials. Their excitons are particularlyconfined in all spatial directions.

• Quantum dots can generate multiple charge carriers with a single photon.

• The energy bandgap is inversely proportional to the size of the dots so by controlling the particle size ofquantum dots, one can vary the energetics of the particle.

• Increased band energies of quantum dots can be utilize to promote, suppress, or rectify the electron transferbetween two semiconductors nanostructures.

Energy Conversion: PhotovoltaicsQuantum-dot solar cells efficiency

The conversion efficiency record for quantum dot based solar cells is currently held at 9% by researchers at MIT. However it is important to

point out that this new energy has a theoretical efficiency of 45%.

Energy Conversion: Thermoelectric

Thermoelectric power generation is an energy conversion method based on thethermoelectric effect.

The thermoelectric effect is the direct conversion of temperature differences toelectric power (The Seebeck effect) and vice versa (The Peltier effect).

Thermoelectric devices can play an important role in energy management and inwaste heat harvesting since more than 60% of chemical energy from fossil andrenewable fuels today is lost during the conversion process, primarily as wasteheat.

During one of his experiments on the early 1800s, Thomas Seebeck observed thatwhen two similar materials are joined together and the junctions are held atdifferent temperatures, a voltage difference develops that is proportional to thetemperature difference.

The ratio of the voltage developed to the temperature gradient is related to theSeebeck coefficient.

The Seebeck coefficient is an intrinsic property of the materials. It is typically lowfor metals and significantly larger for semiconductors.

Energy Conversion: Thermoelectric

Energy Conversion: ThermoelectricThe Seebeck coefficient

Energy Conversion: ThermoelectricThermoelectric generators

• Thermoelectric generators usually have 2semiconductors (p-type and n-type)junctions and require a heat source tocreate current.

• The heat source drive the electrons in then-type element toward the cooler region,thus creating a current through thecircuit.

• Holes in the p-type element will then flowin the current’s direction.

• Charge flows through the n-type element,crosses a metallic interconnect, andpasses into the p-type element.

Energy Conversion: ThermoelectricEfficiency

• The efficiency of thermoelectricgenerators depends on thedimensionless figure of merit (ZT) ofthe materials used.

• We can conclude that the relationshipbetween electrical conductivity,thermal resistance and operatingtemperature range is a key factor inimproving the efficiency ofthermoelectric devices.

• ZT=𝜎𝑆2

𝜅𝑇

• 𝜎 : electrical conductivity

• k: thermal conductivity

• S: Seebeck coefficient or thermopower,in μV/K

• T: (T2 + T1)/2

Energy Conversion: ThermoelectricRecorded efficiencies

Energy Conversion: Piezoelectric

A piezoelectric generator converts vibrational energy into electrical energy using crystals that exhibit the piezoelectric effect.

The piezoelectric effect is the separation of charge within a material as a result of an applied strain which creates an electric field within the

material.

Energy Conversion: Piezoelectric

Piezoelectric crystals typically lack a center of symmetry and have ionic or partly ionic bonds. They exhibit an electric behavior and act as a dipole only below a certain temperature called Curie temperature.

Their efficiency can be determined by the electric displacement.

Energy Conversion: PiezoelectricMaterials’ properties

Energy Conversion: PiezoelectricTypical applications

• They can be used as both sensors andactuators because the effect is reversible.

• Piezoelectric elements can vibrate togenerate a sound wave when applied with avoltage and vice versa

• Piezoelectric vibration cancellation is alsovery common. The system work by sensingfloor vibration, then expanding andcontracting piezoelectric actuators to filterout floor motion.

• Piezoelectric motors create motion when thepiezoelectric element receives an electricalpulse, and then anplies directional force to anopposing ceramic plate, causing it to move inthe desired direction.

Energy savingSolid-State Lighting


Lighting is a big contributor of greenhouse gas emission in the world as it accounts for close to 20% of the world’s energy consumption. Thanks to new policies and the

use of efficient-lighting technologies, The energy consumption for lighting has slightly decreased in recent years.


Solid-state lighting is the use of solid state devices, like light-emitting diodes (LEDs) or organic light-emitting diodes (OLEDs), to produce white light for general illumination.

LEDs use crystalline materials like GaN and OLEDs use disordered materials based on conjugated hydrocarbons like PPV.

The light is produced when a small electric current occurs within a diode, which is an electronic component made of a conductive and insulating material. This effect is called electroluminescence.


• LEDs consist of a N-type and a P-type semiconductor placed in direct contact.

• When current passes through the semiconducting device, the excessive electronsin the N-type material move toward the P-type material, while the holes from theP-type are also moving toward the N-type materials.

• These opposite charges meet at the P-N junctions and the energy is released inthe forms of photons with very little heat output.


Solid-state lighting is a highly energy efficient technology and has the potential of being ten times as efficient as incandescent lighting and

twice as efficient as fluorescent lighting.

Energy savingComparing SSL to other technologies

Energy savingComparing SSL to other technologies

Energy storageElectrical: Capacitors

One of the main device used in storing electrical energy is the capacitor. This electronic component can store, filter and regulate

electrical energy and current flow and is one of the essential passive components used in circuits boards.


Capacitors are formed with two parallel metal electrodes separated by a non-conductive material called dielectric.

When a voltage exists between these conductive parallel plates, an electric field is present in the dielectric. This field stores energy and produce a mechanical force

between the plates.


• The amount of capacitance C for a parallelplate capacitor is determined by theequation:

• 𝐶 = 𝜀𝐴

𝑑

• The formula shows that factors such asplate area, distance between the platesand the dielectric constant are critical fora good capacitance value.

• The capacitance is expressed by the unitFarad, which represents the storage of 1coulomb of charge when 1 volt is applied.

• 1 farad = 1 coulomb / 1 volt

• The energy stored in a capacitor isdirectly proportional to its capacitance.

• 𝐸 =1

2𝐶𝑉2

Energy storageElectrical: Dielectric materials for Capacitors

Typical dielectric materials and their constant value

Energy storageElectrical: Properties of capacitors

Environmental properties

• Factors such as temperature, humidity, DC or AC voltage, signal frequency, piezoelectric effect, or age of thecapacitor are to be taken into consideration when evaluating the capacitance.

Insulation resistance and

energy dissipation

• Overtime, small DC current can leak through the dielectric material for various reasons and it will slowly loseits charge. Generally the insulation resistance is lower for materials with higher values of capacitance.

• Some of the current passing through the capacitor is dissipated because there is a small amount ofresistance of the dielectric material and the conductive parts.

Dielectric strength

• When voltage is continuously increased over the capacitor, the dielectric material will at some point breakdown and that may damage the capacitor permanently.

Supercapacitors are a unique electrical storage device, which can store much more energy (up to 10000 times) than conventional capacitors

and have a much larger life expectancy.

Energy storageElectrical: Supercapacitors

Energy storageElectrical: Double-layer Capacitors

• The electrical double-layer capacitors (EDLCs)have a pair of polarizable electrodes withcollector electrodes, an ion-permeableseparator to prevent electrical contact, andan electrolyte solution.

• By applying voltage to the facing electrodes,ions are drawn to the surface of the electricaldouble layer and electricity is charged.Conversely, they move away when dischargingelectricity.

• Capacitance is proportional to the surfacearea of the electrical double layer.

• Activated carbon, which has large surfacearea for electrodes, are generally applied tothe electricity collector of the electrodes toenable EDLC to have high capacitance.

Energy storageElectrical: Pseudocapacitors

• Pseudocapacitors have a similar structurethan EDLCs but the energy is storedelectrochemically instead ofelectrostatically.

• Combinations of porous metal oxides,carbon composite, conducting polymers,are generally used as electrodes.

• The fast and reversible faradaic redoxprocesses at the electrode surfaces, incombination with the nonfaradaicformation of the electric double layer,allow pseudocapacitors to store muchmore energy that EDLCs.

• Capacitance value depends on the surfacearea, the structure of the electrodes andthe material.

Energy storageElectrical: Materials for Pseudocapacitors’ electrodes

Hydrogen is one of the most abundant element on earth but it is not purely available in nature. It is very high in energy and can be used in fuel cells to produce electricity by combining with oxygen with only

water as a by-product.

Energy storageChemical: Hydrogen generation

Hydrogen can be produced or extracted from a variety of feedstocks

Energy storageChemical: Hydrogen generation

Hydrogen generation is becoming increasingly important because it can be stored easily and used later to produce clean energy. Storing solar energy by splitting water into hydrogen and oxygen has long been a

promising idea because both of these sources (water, sun) are so abundant.

Energy storageChemical: Hydrogen generation by water splitting


• Water splitting involves the absorption ofabundant sunlight with a semiconductor electrodeto produce electron-hole pairs, followed byoxydation and reduction of water to generateoxygen and hydrogen fuel.

• The free energy charge for the conversion of one

molecule of H2O to H2 and1

2O2 under standard

conditions is 237.2 kJ/mol which corresponds to ∆E= 1.23V per electron transferred (Nernst equation).

• To drive this reaction with light, the semiconductormust be able to absorb radiant light with photonenergies >1.23eV.

• This process must generate two electron-hole pairsper molecule of H2 or four electron-hole pairs permolecule of O2

• The efficiency of the water splitting process can be evaluated by calculating the external quantum yield (QY)

• QY =𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠=

𝐼×ℎ𝑣

𝑒×𝑃• I: electrical current• e: fundamental charge of the electron• P: total power of photons• h: Planks’s constant• v: frequency of incident light


Energy storageChemical: Methanol generation

Energy storageElectrochemical: Batteries

Documents

A SHORT COURSE ON SUSTAINABLE ENERGY -- Part IImatecss.org/sites/default/files/Part_II_Short course new materials.pdf · Energy Conversion: Photovoltaics Overcome the Shockley-Queisser