Outline. Energy consumption Silicon solar cells Photovoltaic effect Thin film solar cells Dye sensitized solar cells Conclusions

Transcription

1

2 Outline Energy consumption Silicon solar cells Photovoltaic effect Thin film solar cells Dye sensitized solar cells Conclusions

3 What is energy? Basic quantitative property describing a physical system or object's state. A capacity of a system to preform work. ard.jpg rev1104/clairev / cartoonsmiling-desktop-computer--vector-illustration.jpg _Cartoon_of_a_Surprised_Woman_Cooking_clipart_image.jpg

4 Introduction World population growth

5 Introduction Energy consumption per capita Energy consumption per capita

6 Introduction Total primary world energy consumption EJ

7 Total world energy production by source in 2010 Introduction

8 Introduction Burning fossil fuels produce greenhouse gases

9 Introduction Earth a solar powered system 240 J of solar power every second per square meter.

10 Introduction What is the theoretical potential of solar energy? Incoming solar radiation 1370 W/m 2 The amount of the solar radiation that actually ~ 170 W/m 2 reaches the surface depends on: weather conditions, the amount of particulate matter and water vapor in the air, the time of day and season of the year, the angle of incidence of the radiation, the earth s distance from the sun, the amount of solar activity occurring.

11 Introduction Energy from the sun The total solar energy absorbed by Earth's oceans and land masses is app exajoules (EJ) per year. Total primary energy used 550 EJ (~ 0.02 %) Electricity 76 EJ In about every two minutes there is enough energy from the sun to produce energy to meet our demands for an entire year - if we could harvest it properly.

12 Introduction Harvesting energy from the sun PHOTOVOLTAICS (PV) CONCENTRATED SOLAR POWER (CSP) Convertion of light into electricity Reflection of sunlight / Concetration

13 Types of PV cells on the market (2010) Photovoltaics a-si 5% CdTe 4% CIGS 3% other 2% sc-si 38% mc-si 48% First Generation of Solar Cells Single Junction Silicon Cells Single Crystal Si (sc-si), Multycrystal Si (mc-si) Second Generation of Solar Cells Thin Film Cells Cadmium Telluride (CdTe), Copper indium gallium selenide (CIGS), Amorphous silicon (a-si) Third Generation of Solar Cells Multi-junction Cells Dye-sensitized solar cells Gratzel cells (DSSC)

14 Single Junction Silicon Cells First generation of solar cells - Silicon solar cells Photovoltaic effect (Edmond Becquerel, 1839). The first practical photovoltaic cell was developed in 1954 at Bell Laboratories. Single Crystal Si (sc-si) Multycrystal Si (mc-si) le_georg_slickers_wikipedia.gif

15 First generation of solar cells - Silicon solar cells Why silicon? It is a semiconductor. Intrinsic carriers are the electrons and holes that participate in conduction. Semiconductor material which has not had impurities added to it in order to change the carrier concentrations is called intrinsic material.

16 First generation of solar cells - Silicon solar cells Absorption of light When a photon hits a piece of silicon, one of three things can happen: E ph < E G Photons with energy E ph less than the band gap energy E G interact only weakly with the semiconductor, passing through it as if it were transparent. E ph = E G have just enough energy to create an electron hole pair and are efficiently absorbed. E ph > E G Photons with energy much greater than the band gap are strongly absorbed. However, for photovoltaic applications, the photon energy greater than the band gap is wasted as electrons quickly thermalize back down to the conduction band edges.

17 First generation of solar cells - Silicon solar cells Doping of silicon Doping is a technique used to vary the number of electrons and holes in semiconductors. n-doped silicon p-doped silicon

18 First generation of solar cells - Silicon solar cells p-n junction Doping can have a significant effect on the charge carrier density. Direction of the filed With the p- and n- materials separated carriers diffuse around randomly. Carriers cross into other region. The fixed ions cores are left behind set up an electric field. Electric field tends to keep holes in p-material and the electrons in the n-material.

19 Photovoltaic effect First generation of solar cells - Silicon solar cells Photovoltaic process is based on three important principles: 1. excitation of free mobile charge carriers due to light absorption, 2. the separation of the charge carriers, 3. collection of the charge carriers at the contacts.

20 First generation of solar cells - Silicon solar cells From cell to array P = U I PV cell circuits sealed in a protective laminate one or more PV modules assembled in a unit power-generating unit

21 Life cycle of silicon solar cell First generation of solar cells - Silicon solar cells Qurartz mining Arc furnance 2000 C Metallurgical grade Si HCl 300 C SiHCl 3 H C h Solar grade Si Impurities FeCl 3, AlCl 3 Monocrystal Si Multycristal ingot Si dust sawing Dopping gases AsH 3,PH 3 Cleaning SF 6, NF china.com/2f0j00obcewzrnglqf/monocrystalline-silicon-rods-p- TYPE-.jpg IKgeNrhnzf wafer PV Cell

22 First generation of solar cells - Silicon solar cells Energy pay back time the recovery time required for generating the energy spent for manufacturing a photovoltaic module. from 2 to 6 years for Si-cells depending on the module type and location. For silicon solar cells determined by the thickness of the silicon layer. t-is-the-payback-period-for-a-solar-pv-training-course%2f&ei=hba-u_cogspztqadu4g4aq&bvm=bv ,d.bgq&psig=afqjcnfttk0dkbzrb0gppsxnicroiycyaw&ust=

23 First generation of solar cells - Silicon solar cells Greenhouse gas emission Source g CO 2 e/kwh Coal 888 Oil 733 Gas 500 Solar Biomass 45 Nuclear 30 Hydro 26 Wind 26

24 Average price of PV cells and modules First generation of solar cells - Silicon solar cells

25 First generation of solar cells - Silicon solar cells Solar cell efficiency (measured at standard test conditions*) = P m E A c P m = cell's power output (in watts) at its maximum power point E = input light (in W/m 2 )=1000 W/m 2 A c = the surface area of the solar cell (in m 2 ) * STC specifies a temperature of 25 C and an irradiance of 1000 W/m 2 with an air mass 1.5 (AM1.5) spectrum. These conditions correspond to a clear day with sunlight incident upon a sun-facing 37 -tilted surface with the sun at an angle of above the horizon.

26 Equivalent sun hours (ESH) the hours that sun shines within irradiance of 1000 W/m 2. First generation of solar cells - Silicon solar cells Unit of sun = 1000 W/m 2 : the light power that is received by one square meter under optimal conditions (STC).

27 Calculation: The number of solar panels per person in Malta The annual energy consumption for Malta is 10 TWh, for electricity supply 2 TWh. The equivalent sun hours (ESH) for this region are 5 hours. The efficiency of the solar panel is 15 %, with the surface area 1.8 m 2. Calculate the number of panels per person in Malta to meet the electricity demand. The number of inhabitants in Malta is (2012), land area 318 m Calculate the peak output power of the panel. 2. Calculate the output energy per panel per year. 3. Calculate the total number of panels. 4. Calculate the number of panels per person.

28 First generation of solar cells - Silicon solar cells

29 First generation of solar cells - Silicon solar cells

30 First generation of solar cells - Silicon solar cells Silicon based cells Single crystallyne Si wafers the most dominant PV technology conversion efficiency of 25 %. Polycrstalline Silicon Cells conversion efficiency of 18 %.

31 First generation of solar cells - Silicon solar cells The Costs of Energy French energy costs for different generation technologies in Euros per megawatt hour (2011) Technology Solar farms 293 Onshore wind 69 Nuclear 50 Natural gas turbines without CO 2 capture 61 Hydro power 20 Cost ( /MWh)

32 First generation of solar cells - Silicon solar cells Advantages of Si-Cells Less defect in the structure, high efficiency Longevity Disadvantages of Si-Cells Expensive production process High initial costs Fragile

33 Second generation of solar cells Thin film solar cells Absorption coefficient of different semiconductors Materials with higher absorption coefficients more readily absorb photons, which excite electrons into the conduction band. Knowing the absorption coefficients of materials aids engineers in determining which material to use in their solar cell designs.

34 Thin Film Cells Second generation of solar cells Thin film solar cells Cadmium Telluride (CdTe) Copper Indium Gallium Selenide (CIGS) Amorphous silicon a-si Reduced cost of manufacturing, but lower efficiency (7-11%) Payback time < 2 years in Europe

35 Second generation of solar cells Thin film solar cells Heterojunction system junction is formed between semiconductors having different band gaps. Cadmium telluride sollar cell (CdTe) n-type CdS window layer p-type CdTe absorber layer Efficiency ~ 19 % Cots: less than 1$ per W

36 Second generation of solar cells Thin film solar cells CdTe energy band diagram Band gap of CdTe is 1.44 ev optimal range. High absorption coefficient only a few micron of the material is required for absorption.

37 Advantages of thin solar cells Second generation of solar cells Thin film solar cells Technology is relatively simple and inexpensive (for production of a-si, and CdTe). Less material required. Can be deposited on a wide range of substrates, including flexible, curved, and rollaway types. Absorption of sunlight at shorter wavelengths (for CdTe and CIGS). Disadvantages of thin solar cells Overall efficiency of around 10% for a-si cells and CdTe expected lifetime is shorter than the lifetime of crystalline silicon cells. Tellurium supply (tellurium is an extremely rare element). Toxicity of Cadmium. The disposal and long term safety of cadmium telluride is questionable. Production technologically demanding (for CIGS). Ultra-high-vaccum evaporator/cigs deposition tool, which is attached to the Copper Indium Gallium Diselenide cluster tool.

38 Dye-sensitized solar cell (DSSC) Third generation of solar cells Grätzel cells are based on a concept invented in 1988 by Brian O'Regan and Michael Grätzel. Different concept- a photo-electrochemical system.

39 Third generation of solar cells How does it work? 1. The incident photon is absorbed by Ru complex photosensitizers adsorbed on the TiO 2 surface. 2. The dye complex molecule is excided (S*), electrons are transferred into the conduction band of the TiO 2 electrode. 3. The electrons are transported with diffusion to the back contact (TCO). 4. The oxidized dye (S + ) accepts electrons from I - and regenerates in the ground state. 2S + + 3I - 2S + I 3-5. The oxidized I 3 - is reduced on Pt catalyst to I - I e - 3I -

40 Third generation of solar cells Advantages of DSSC cells low cost price, flexibility, light works under cloudy skies and non-direct sunlight. Disadvantages of DSSC cells: low efficiency (5 %), temperature stability of the electrolyte, high price of platinum and ruthenium dye.

41 Preparation of platinum catalyst at low temperature (up to 150 C) Reduction of platinum precursor (H 2 PtCl 6 ) with gaseous formic acid or ethylene glycol. Research work 10 nm SEM images of platinum deposited using spin coating method at 100 C (C) and 450 C (D). TEM image of prepared surface. The minimum amount of platinum catalyst needed for electrolyte regeneration is 10 mg/m 2!!! MATOH, Lev, KOZJEK-ŠKOFIC, Irena, ČEH, Miran, BUKOVEC, Nataša. A novel method for preparation of a platinum catalyst at low temperatures. Journal of materials chemistry. A, Materials for energy and sustainability, 2013, vol. 1,

42 Research work Preparation of conductive thin layer of platinum SEM images of platinum on plastic substrate at 100 C. TEM image of prepared surface. The amount of platinum needed for replacement of ITO is 50 mg/m 2. MATOH, Lev, KOZJEK-ŠKOFIC, Irena, BUKOVEC, Nataša, BUKOVEC, Peter. A method of deposition of Pt or Pd catalysts using gaseous reducing agents : publication number: EP (A1) München: European Patent Office, Str. 1-22

43 Third generation of solar cells Multi-junction Cells Multiple p-n junctions made of different semiconductor materials. Each material's p-n junction will produce electric current in response to a different wavelength of light. Increasing the conversion efficiency.

44 National Renewable Energy Laboratory (NREL) Conclusion

45 Solar energy - yes or no? Conclusion Advantages Widespread occurrence Free No maintenance Sustainable (in the future) Disadvantages Relability of supply (night) Environmental issues Recycling Expensive (at the moment)

46 Energy saved is energy generated. We do not inherit the earth from our parents, we borrow it from our children Native American saying