Solar Energy Solar Energy • Perpetual/Continuous resource: remain available in the same measure for an indefinitely long time • Origin: Thermonuclear fusion reactions occurring in the sun • 30 days of sunshine = the energy equivalent of the total of all the planet’s fossil fuels Advantages • Radioactive byproducts remain behind on the sun • Running costs are low. • No CO2 emissions- Reduced Greenhouse Effect • No SO2 emissions to cause acid rain. • Only pure radiant energy reaches the Earth Disadvantages • Solar energy is intermittent: dependent on weather conditions and the time of day • Solar energy is a diffuse source: Must concentrate to harness. • High initial cost of solar cells Two Categories Solar Thermal Solar Photovoltaic (PV) For Water Heating For Electricity Production Solar Thermal Energy • Using solar thermal technologies for heating fluids which can be used as a heat source or to run turbines to generate electricity. Solar thermal : How does it work? Solar photovoltaics • Solar Electric Systems • Photovoltaic (PV) systems convert light energy directly into electricity. • Commonly known as “solar cells.” • Simplest Ex: calculators Semiconductors • Conductivity can be readily modulated (by doping or electrical potential) offering a pathway to control electronic circuits. • Elementary – Si, Ge. • Compound – GaAs, InP, CdTe. PV Cells – Basics Band Diagram Doping in Si n-type p-type e- rich dopant (P, As) e- deficient dopant (B, Ga) 5th valence e- - charge carrier The hole acts as charge carrier n-type doping P-type doping PV Cell • A PV cell is a light illuminated p-n junction diode • Converts solar energy into electricity via photovoltaic effect. • When sunlight strikes the surface of a PV cell, photons with energy above the semiconductor bandgap impart enough energy to create electron-hole pairs. • https://www.youtube.com/watch?v=L_q6LRgKpTw&fe ature=emb_logo • https://www.youtube.com/watch?v=xKxrkht7CpY&feat ure=emb_logo p-n junction Major carriers : holes Minor carriers : ens Major carriers : ens Minor carriers :holes 1. generation of electron-hole pair by absorption of photon 2. Combination of electron-hole pair 3. electron and hole separation 4. Electrons in external circuit 5. Recombination of electron-hole Criteria for Materials to be Used in Solar Cell • Must have band gap from 1ev to 1.8ev. • It must have high optical absorption. • It must have high electrical conductivity. • The raw material must be available in abundance and the cost of the material must be low. Advantages of Solar Cell • PV cells provide clean and green energy. No harmful greenhouse gas emissions. • Can be installed anywhere including rooftops • suitable for smart energy networks with distributed power generation • Low operating and maintenance costs • PV panels are totally silent Disadvantages of Solar Cell • Solar energy conversion is affected by availability of light (night, cloudy days..) • Need inverters to convert DC to AC • Need to be connected to storage batteries for continuous supply • PV panels are fragile and can be damaged relatively easily. • High cost of installation. • It has low efficiency. Solar module and Arrays • A solar cell has power output enough to operate small devices like toys, pocket calculators etc. • For energy intense applications many cells are combined together. • Solar cells when connected in series have improved voltage output while connected in parallel have better current output. • Photovoltaic modules are formed by connecting many PV cells. Many modules when connected together form a panel and panels are connected together to form an array. Dye Sensitized Solar Cells • Uses Photosensitive Dyes • Some dyes when adsorbed on semiconductors like TiO2, shifts spectral sensitivity towards visible light • Has 4 components i) Semiconductor electrode (TiO2) ii) Dye sensitizer iii) Mediator ( Electrolyte) : I- or I3- complexes iv) Counter electrode ( C or Pt based) “Black Dye", an anionic Ru-terpyridine complex Working principle Photoexcitation of dye when exposed to light. The excited electron injected in the CB of TiO2 The oxidized form of the dye produced in this process is subsequently reduced by the electrolyte. The electrolyte is then regenerated by the reduction at the counter-electrode. The chemical electrolyte thus closes the circuit so that the electrons are returned to the dye. It is the movement of these electrons that creates energy which can be harvested. Advantages of DSSC • Can perform well in a wide range of lighting conditions • Efficiency is not affected by high temperature. • Its manufacturing process involves less consumption of energy • Sustainable and environment friendly technology.
