Lesson 20: Solar Energy PDF

Lesson 20: Solar Energy Overview In addition to being the ultimate source of just about all the types of primary we use on 일사량 Earth, radiative energy from the Sun, or solar insolation, is also a valuable energy source in and of itself. It can be...

Lesson 20: Solar Energy Overview In addition to being the ultimate source of just about all the types of primary we use on 일사량 Earth, radiative energy from the Sun, or solar insolation, is also a valuable energy source in and of itself. It can be used as a passive space heating source, a water heating source, and even as the source of heat for thermoelectric power generation when concentrated using mirrors. Furthermore, solar energy can be used to generate electricity directly in a solar cell. How this is done is unlike any form of electricity generaton we have discussed thus far and requires delving into the chemistry of semiconductors, which are the heart of a solar cell. Solar cells make up solar panels, solar panels make up solar arrays, and solar arrays are the generators in PV systems, the workings of all of which, plus the economics of solar systems, are addressed in this lesson. Lecture Outline 1. 99.9% of the energy we receive at the Earth’s surface comes from the Sun 2. Just about all the different forms of primary energy we rely on are or were ultimately sourced by the Sun a. Not just renewable energies, but also conventional primary energies 3. Solar radiation can also be used directly as a source of energy for doing work 4. Four major forms of solar energy: passive solar, solar heating, concentrated solar, and solar photovoltaics or solar PV 5. A solar PV system is made up of solar PV panels, the panels are made up of solar cells, and solar cells are made up of semiconductors, which are the heart of PV systems 6. Semiconductors a. Lie along a spectrum that extends from conductors to insulators i. Conductors conduct electricity while insulators do not b. This difference has to do with how tightly conductors and insulators hold onto electrons in the outermost electron shell surrounding the atomic nuclei  i. The outermost electron shell of an atom is known as the valence shell 1. A good conductor, such as copper, has only one electron in its valence shell 2. A good insulator has a full valence shell and the electrons within this shell are tightly held onto by the protons in the atom’s nucleus c. Electrons with high enough energy can break out of the valence shell and begin to flow i. The energy level of the electrons in the valence shell is referred to as the valence band ii. The energy level of electrons that break out of this shell and can flow is known as the conductive band d. The difference in energy level between the valence band and the conductive band is known as the band gap i. This gap is the minimum amount of extra energy that an electron needs to acquire in order to jump from the valence band to the conductive band 1. In insulators, the band gap is very large 2. In conductors, on the other hand, the band gap is very small; so small in fact that the conductive band overlaps with the valence band e. Semi-conductors are materials with moderate to small band gaps, gaps that are less than those of insulators but greater than those of conductors i. Under normal circumstances, a semiconductor is a poor conductor of electricity ii. If, however, the electrons in the valence band of a semiconductor become sufficiently energized, they will jump to the conductive band, and the semiconductor will carry a current 7. The creation of semiconductors through doping a. Semi-conductors are made from poor conducting material that is “doped”  with impurities to alter the material’s electronic properties in a controllable way i. The most common material used to make semiconductors, particularly those found in solar PV panels is silicon ii. Its insulating properties are reduced by melting down the silicon and mixing into the melt one or more other elements that reduce the silicon’s insulating properties and turn it into a semiconductor b. Two basic types of semi-conductors are produced through this process: N- semiconductors and P-semiconductors i. N-semiconductors are created by doping the silicon with an element that has one or more extra electrons 1. The extra electrons lead to bonds with an extra electron that is relatively loosely attached ii. P-semiconductors, on the other hand, are created by doping the silicon with an element that has fewer electrons 1. The fewer electrons lead to bonds that are missing electrons, k.a. electron holes iii. Both electrons and electron holes can move about c. NOTE: while the N and P semiconductors have extra or fewer electrons, respectively, than pure silica, the number of electrons in each balances the number of protons in each, so both semiconductors are electrically neutral 8. P-N junction a. When a N-type semiconductor is joined with a P-type semiconductor, the relative excess of electrons in the N-type semiconductor are attracted by the relative excess of electron holes in the P-type semiconductor and vice versa b. And as these negative and positive charges move across the boundary between the N- and P-type semiconductors, they transform what were 인 & 붕소 neutral atoms of phosphorous and boron fixed in the silicon matrix on either side of the boundary into positive and negative ions respectively. c. The oppositely charged ions on either side of the P-N junction create an  electric field that opposes further electron and electron hole movement between the two semi-conductors i. The region of the electric field is k.a. the depletion zone 9. P- and N-semiconductor band gaps a. While the width of band gap may be the same between the two sides of the semiconductor, the absolute energy level at the bottom of the band gap is greater on the P-side than the N-side i. The maximum energy of the valence band on the P-side is higher than that on the N-side, so takes lesser energy to jump electrons up into the conductive band on the N-side than the P-side ii. Importantly, however, the maximum energy of the valence band on the P-side is still is still less than the minimum energy of the conductive band on the N-side iii. So, a P-N semiconductor only allows current to flow in one direction: from the P side to the N side 10. Solar Cell and the P- and N-band gaps a. Recall that for electrons to flow freely in a semiconductor, they must be energized enough to jump out of the valence band and into the conductive band b. P- and N-sides are designed to have different absolute energy levels at which an electron will jump from the valence to the conductive band i. Band gap width may be the same on the two sides of the semiconductor, but the absolute energy level at the bottom of the band gap is greater on the P-side than the N-side ii. Consequently, it takes lesser energy to jump electrons up into the conductive band on the N-side than the P-side iii. Electrons in the conductive band tend to concentrate on the N-side c. Electrons jumping into the conductive band on the N-side leave behind electron holes and positively charged ions i. The latter preferentially push the holes into the P-side  ii. Electron holes tend to concentrate on the P-side d. There are two sources of energy that can energize electrons out of the valence band and into the conductive band i. Heat ii. Sunlight 1. Energy in this case is carried by photons 2. Electrons that absorb the photon energy rise into the conductive band e. When heat is the source of energy, the higher the heat, the more the band gap is reduced across the entire P-N junction i. This reduces the energy needed to bump electrons into the conductive band across the entire solar cell ii. This reduces the electron gradient and thus the energy output of the solar cell f. When sunlight is the source of energy, the band gap remains unchanged g. Solar cell efficiency is highest when the temperature is cold and there’s lots of sunlight 11. Solar Cell Operation a. In a solar cell circuit, electrons leave the N-side and travel via a conductor across a load to the P-side b. If the solar cell is in complete darkness… i. The only energy source for bumping electrons into the conductive band is heat ii. Electrons that gain enough energy to pop into the conductive band and move about iii. The electrons leave behind an electron hole that also moves about, but within semiconductor material bonds iv. The ions on either side of the P-N junction in the depletion zone,  preferentially exert a push-pull on both the electrons in the conductive band and the electron holes 1. The electrons tend to congregate on the N-side 2. The electron holes on the P-side c. If in sunlight… i. The generation of free electrons in the conductive band increases, as does the generation of electron holes ii. This leads to even greater concentrations of electrons on the N-side and electron holes on the P-side iii. The net result is an increase in the voltage between the solar cell terminal on the N-side and the solar cell terminal on the P-side d. If in sunlight and the N- and P-side terminals are connected using a conductor that completes a circuit… i. The excess electrons on the N-side move across the conductor to the P-side where they connect with excess electron holes ii. The sunlight continues to energize electrons into the conductive band iii. This in turn maintains the excess concentration of free electrons on the N-side and electron holes on the P-side iv. Electrons continuously travel through the conductor from N- to P-side providing electric energy for powering a load 12. Solar cell construction a. N-side faces the sun i. Covered by thin terminal strips for collecting electrons ii. A non-reflective film to reduce solar energy loss due to reflection iii. A glass cover to protect the all of the above b. P-side lies below the N-side i. Solar energy has to penetrate the N-side to reach the P-side 1. Consequently, most solar energy absorbed on the N-side or in  the depletion zone ii. Far end of the P-side covered by a continuous metal plate that is the terminal that receives electrons iii. This in turn is covered by a protective back cover 13. Solar panel construction a. Panel is made up of multiple cells b. Cells are arranged in series, so total panel voltage is the sum of cell voltages 14. Types of PV a. Monocrystalline i. Made from single crystals of doped silicon ii. Take the longest time to grow and make and thus most expensive iii. However, also the highest efficiency (~17-22%) b. Polycrystalline i. Made silicon that is cooled faster and that forms multiple interlocking crystals ii. Shorter time to grow and thus cheaper to make iii. Lower efficiency (15-17%) c. Film PV i. Rapidly quenched doped silica ii. No crystals, i.e., amorphous, but can be made into thin flexible sheets that can cover surfaces, such as windows iii. Fastest and cheapest to make iv. Lowest efficiency (10-13%) 15. Solar PV System a. Solar panels arranged in series and parallel i. Panels arranged in series referred to as a string  ii. Strings of panels are arranged in parallel b. Inverter to convert DC current from panels into AC current for… i. Use in the home ii. Injection onto the grid 16. Solar Resource and solar panel output a. Although solar is intermittent due to cloud cover and no sunlight at night, the resource is predictable b. Furthermore, it tends to peak close to when daily demand for electricity peaks c. Solar insolation at different locations given in kWh/m2/day i. Highest nearest equator and least toward the poles ii. Varies seasonally, however, because of the tilt to the Earth’s rotation axis and the Earth’s orbit about the Sun 1. Insolation in N Hemisphere greater during our summer a. When Earth’s rotation axis is tilted toward the Sun d. Insolation resource is often given in terms of three different types of solar panel orientations and tracking i. Fixed panel that’s laid flat on the Earth’s surface 1. Equates to the least solar energy ii. Panel with fixed orientation but tilt that varies with the seasons 1. 1-axis tracking 2. Equates to more solar energy iii. Panel with orientation and tilt that tracks the Sun over the seasons 1. 2-axis tracking 2. Equates to the most solar energy 17. Solar economics a. Major cost components of solar i. PV modules ii. Inverter iii. Balance of system hardware costs 1. Wiring, switches, mounting system, etc. iv. Soft system costs 1. Installation labor, permitting, etc. b. Reduction in solar costs has been driven in large part by incentives i. These are currently set to sunset in the next couple of years for residential customers