The Physics of Energy sources Renewable sources of energy. Solar Energy

Transcription

1 The Physics of Energy sources Renewable sources of energy Solar Energy B. Maffei Renewable sources 1

2 Solar power! There are basically two ways of using directly the radiative energy emitted by the Sun! Convert radiation into heat through absorption Basic water heaters Solar concentrator power towers! Convert radiation directly into electricity through photoelectric effect Solar cells (or photovoltaic cells)! Both will be dependant on the Sun radiation intensity than can be collected Renewable sources 2

3 Solar irradiance IR Emission from Sun Collected power depends on:! The collecting area! The intensity of the radiation I(ν,T)! For the solar cells, the frequency of the radiation will also be important (E=hν) Visible UV Frequency (Hz) Photon energy hν Previous lecture: average maximum integrated (over the whole spectrum) irradiance ~ 1kW/m 2, assuming an atmospheric transmission of 70%. We need to take into account other effects Renewable sources 3

4 Geometrical effects θ zenith Collecting surface A Effective collecting surface area Ae Ae=Acos(θ) A Need for tracking system + atmospheric attenuation depends also on θ. So, the irradiance on the collecting surface will vary:! During the day! During the year (Sun lower on the horizon) + fewer hours of day light! Depending on the altitude Renewable sources 4

5 More generally Solid angle Ω Normal to surface S θ 1 Normal to surface D θ 2 D S Source with surface S Distance R Detector with surface D P " = I # S e # $ = I # Scos(% 1 ) Dcos(% 2 ) R 2 source spectral radiance I (in W m -2 Hz -1 sr -1 ) see lecture 1 For! more calculation on photometry see example 10 Renewable sources 5

6 Solar water heating! From the most basic system based on solar energy Usual temperature range will be C Useful for buildings / hot water or heating Renewable sources 6

7 Solar water heating Figure from wikipedia Evacuated space limit the heat exchange with outside lower heat loss Advantages: Fairly cheap and relatively easy to install Disadvantages: Needs sunny locations No tracking loss of efficiency Widely use in Mediterranean countries For the UK: a 4m 2 collector gives 100% supply to 2-4 people April to September Renewable sources 7

8 Improvement! Reaching higher fluid temperature (~ 500C) will drive a conventional heat engine! This can be achieve by the use of a solar concentrator! A tracking system will have to be adapted to follow the Sun x many Even with imperfections, temperature of up to 3000K can be achieved in practice. Renewable sources 8

9 Solar power tower CESA facility in Spain: 330 mirrors of ~ 40m 2 each irradiance of 3.3MW/m 2 on a surface of 4m diameter Total intensity ~ 40MW leading to an electricity production of ~ 7MW Renewable sources 9

10 Photo-electric effect! A photon of energy E=hν can interact and exchange energy with electrons of an atom! Example: photoelectric effect In order to ionise an atom we have to provide an energy at least equal to the ionisation energy of a given electron E i If the incoming photon has a energy (depending on its frequency) then we have ionisation We then have a cut-on frequency under which ionisation cannot occur hν E i Example: hydrogen E i =13.6eV A similar effect will be used in semiconductor technology in order to create a current leading to electrical energy: photovoltaic effect Renewable sources 10

11 Conductors, insulators and semiconductors! Silicon is the basis of semiconductor technology and is widely used in electronics (but also Germanium, GaAs.)! Here we are using its interaction with radiation properties to suit our purpose Energy diagram (band structure) of a semiconductor Conduction band When atoms are combined in molecules we have creation of covalent bonds through delocalised electrons Band gap E g When solids are formed we have then creation of energy bands leading to Conduction and Valence bands. Valence band Density of charge carrier electrons in conduction band varies as exp (! E ) g 2kT Conductor: electrons in the conduction band free to move Insulator: electrons trapped in the valence band Semiconductor: If an electron is promoted to the conduction band, it will be free to move within the semiconductor + creation of a hole The energy E g corresponding to the band gap between the conduction and valence bands depends on the material (1.1eV for intrinsic Si). Renewable sources 11

12 Solar radiation absorption When the Silicon is irradiated, each photon with an energy at least equal to Eg will be able to promote one electron from the valence to the conduction band Creation of an electron-hole pair We need Solar spectrum top of atm. hν E g! For intrinsic silicon Eg=1.1eV λmax=1.1µm will be dissipated in the crystal and heat the silicon A no photovoltaic effect possible B effect possible C Excess energy heating Si Section 7.4 ref 4 E = hν E excess g Efficiency of solar cell for various semiconductors and as a function of T Renewable sources 12

13 Doping Intrinsic Silicon Extrinsic Silicon: introduction of impurities Introduction of Boron (acceptor) Renewable sources Introduction of Phosphorus (donor) 13

14 Silicon band gap! For an intrinsic semiconductor (low level of impurities): E f =E g /2! When we introduce impurities in the semiconductor: Fermi level is changed Intrinsic semiconductor Extrinsic semiconductor n-type Extrinsic semiconductor p-type Conduction band Conduction band Conduction band Band gap E f Ef E g Ef Valence band E f (Fermi level): apparent energy level from which majority carriers are excited to become charge carriers Valence band Addition of electron donor impurities ions of higher valency Larger electron concentration as free carrier Valence band Addition of electron acceptor impurities ions of less valency Larger hole concentration as free carrier Semiconductor Donors Acceptors Silicon Germanium Phosphorus, Arsenic Boron, Aluminium Renewable sources 14

15 p-n junction! When an electron-hole pair has been produced we need to have them moving in opposite directions in order to avoid recombination and propagate a current through the semiconductor! Si can be doped with impurities! With electron acceptor impurity ions p doping! With electron donors impurity ions n doping! With a p-n junction we can create a diode for which electrons and holes can travel only one way once in the conduction band Figure from wikipedia! The p-n junction will need to have a bias voltage to create the current Renewable sources 15

16 p-n junction (diode) V-I characteristic Dark characteristic (no illumination) With illumination Renewable sources 16

17 Improvement! There are several causes for loss of efficiency! Some are intrinsic to the system nothing can be done! Some are avoidable and losses can be limited Reflection losses If n 0, n 1 and n 2 are the refractive index of the various materials, it can be shown that in order to reduce the reflections to a minimum we need: n = 1 n0n2 e = λ 4n 1 e n 0 n 1 n 2 A/R coating semiconductor Example: for silicon in air (or vacuum n=1) we can use a layer of SiO 2 as A/R coating Other way: shaping the surface to increase the number of reflections inside the material Renewable sources 17

18 Figure from wikipedia Cell design Renewable sources 18

19 State of the art in solar cells Renewable sources Figure from National renewable energy laboratory US department of energy 19

20 Applications! Commercial applications: solar cell efficiency is ~ 15-20%! A standard photocell current in full Sun light will produce a current of ~ 300A/m 2. Assuming a voltage of 0.5V 150W/m 2 Example: Turing building has been fitted with 100 solar panels Cost: k 250 Power: 10-30kW Disadvantages: Even if the price has dropped by a factor 10 since the 80s, it still is expensive Needs a sunny location Needs large surfaces to be covered / low efficiency Advantages: No pollution Free to run (excepted for replacement) Totally renewable energy source Renewable sources 20

21 References! Most of the material of this lecture is coming from! Ref4: Renewable energy resources, J. Twidell and T. Weir, second edition, 2006 Renewable sources 21