Solar PV CellsFree Electricity from the Sun?An Overview of Solar Photovoltaic ElectricityCarl Almgren and George Collins( editor)Terrestrial Energy from the Sun5 4 3 2 1 0.5Electron-Volts per Photonhttp://en.wikipedia.org/wiki/Image:Solar_Spectrum.pngRelative Solar radiation on Earth24 hour/365 day mean solar radiation received at the surface, in W/m2. It oscillates between a maximum of 275 M/m2in the deserts of the Middle East, to a low of 75 W/m2for misty isles in the Arctic.Origin of Photovoltaic cellsThe term "photovoltaic" comes from the Greek :phos meaning "light", and "voltaic", from the name of the Italian physicist Volta, after whom the unit Volts is named..The modern age of solar power technology began in 1954 when Bell Laboratories, discovered that silicon doped with certain impurities was able to generate electricity for satellites.What happens in the cell?Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon—creating a dc V-I source to extract energy from.An array of solar panels converts solar energy into a usable amount of DC electricity.Power Electronics Inverters convert the DC to mains AC to feed the gridQuantum characteristics of solar cellsA photon need only have greater energy than that of the semiconductor band gap in order to create electron-hole pairs but to penetrate deeply into the semiconductor the energy must be not far away from the band gap which for silicon is 1.1 eV and a wavelength of about 1 micronHowever, the solar frequency spectrum is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into undesired heat (via lattice vibrations called phonons) rather than into usable electrical energy.Photon absorptionWhen a photon is absorbed, its energy is given to an electron-hole pair in the crystal lattice. The electrons and holes have to move to the collection electrodes of the solar cell to create a V-I source. To keep from shadowing sunlight to the active solar area the top collections electrodes are made from a transparent conducting oxide like ITOMaximizing efficiencyA one-layer solar cell is limited to 20 percent efficiency in converting light to power, but materials with different bandgaps can be stacked in multijunction cells. Each layer responds to a different photon energy of sunlight to achieve 40 % efficiency.http://www.lbl.gov/Science-Articles/Archive/MSD-perfect-solar-cell.htmlSolar cell efficienciesCourtesy of L.L. Kazmerski, NRELMaximum Power PointA solar cell has a maximum-power point where the product of VI is maximum.The maximum power point of a PV cell varies dynamically with incident solar illumination. A maximum power point tracker tracks instantaneous power and uses this information to dynamically adjust the load so the maximum power is alwaystransferred, regardless of the variation in lighting.Photogeneration of charge carriersWhen a photon hits a piece of silicon, one of three things can happen:The photon can pass straight through the siliconThis generally happens for lower energy photons.The photon can reflect off the surfaceThe photon can be absorbed by the silicon which either:Generates heatGenerates electron-hole pairs, if the photon energy is higher than the silicon band gap value.If a photon has an integer multiple of band gap energy, it can create more than one electron-hole pair. However, this effect is usually not significant in solar cells.Sample of I-V curves at various levels of irradianceUniSolar PVL 136 Thin Film laminate cellsAir Mass 1.5 and 25C cell temperatureSTC - 1000 W/m2800 W/m2600 W/m2400 W/m2200 W/m2Anatomy of a PV Solar CellOther parts of a cell are needed to enable production of electricityBacking materialTCO/barrierCommonly ZnOor ITOConductive fingersSolar cell layers and how they are fabricated with plasma deposition processesTypes of PV CellsMonocrystalline vs. thin filmThe Energy ProblemWorldwide, an additional 10 TW electric energy will be needed by the year 2050. This is about 4 GW of capacity per week.How to supply this additional needed power Fossil fuel production is stretched - and releases CO2Would need one new 1 GigaWatt nuke every other dayBiofuels would consume massive amounts of agricultural resources like water,energy and fertilizer(NO gas from fertilizer is 300 times worse greenhouse gas)Nuclear Plants with no fossil fuel pollution at all http://www.greenandgoldenergy.com.au/The role of Solar energyA part of the solution as followsOn an average day, the earth at sea level is absorbing solar energy at rate of 120,000 TW but 170,000TW are entering the outer atmosphere.In space massive solar arrays could both cool the earth by shadowing and transmit power by microwaves10% efficient terrestrial cells are commonAnd 40% cells are achieved in researchBoth heat and electricity can be harvested on solar farms. In Japan and in America’s SW solar water heaters are commonHow Solar Photovoltaic Cells are usedUse is divided by size and purposeProvide low, independent power with no grid connection: i.e. calculators and garden lights.Power in remote areas difficult or costly to connect to the power gridHome-sized arrays to reduce grid-based electricity consumptionLarge industrial arrays on large scale buildings for reducing both peak and total grid power consumption( Walmart is a great example)PV Cell Power generationEfficiencies and CostsThin films cheap, but low conversion efficiency (10%) while crystaline cells give 40% they are expensiveAs of 2005, photovoltaic electricity generation costs ranged from ~60 cents/kWh down to ~30 cents/kWh in regions of high solar irradiation.Solar electric systems cost between 5 and 9 US$ per peak Watt, installedPeak watts x 20% x 24 hrs = AVERAGE Watt-hrs/dayexample: 1 kW peak = 4.8 kW-hrs/dayPayback is not there without government assistanceWhat do we do with this electricity?Mostly converted to AC using power electronics invertersThe inverter turns the DC electricity into AC electricity of the correct voltage and frequency needed for the grid.The electricity is then distributed to be used, either on-site or back into the distribution grid.Commercial 333 kW Inverter -Advanced Energy SolaronTMAlternative