Solar and Renewable Energies

Transcription

1 Physics 162: Solar and Renewable Energies February 25, 2010 Prof. Raghuveer Parthasarathy Winter 2010

2 Lecture 15: Announcements Reading: Wolfson Chapter 9 Homework: Problem Set 6. Due today. GTF Matt Briel: Extended office hours today, pm. Homework: Problem Set 7. Will be posted this evening (I ll ). Due Thurs. March 2, 5pm.

3 Last time: Specific Heat How much energy is required to change the temperature of an object by some amount? The relation between Q (amount of thermal energy) and ΔT (change in temperature) depends on how much material there is: Mass, M material property: specific heat, c Q = McΔT c is the specific heat

4 Last time: Geothermal Power Geothermal power. Calculating its magnitude, given the geothermal gradient. There is a lot of geothermal power, but it s very diffuse. Energy flow: Less than 0.1 W per square meter. Total geothermal energy flow over the planet? W. About 3 humankind s current energy consumption rate. Not uniformly distributed: some regions have large geothermal gradients

5 Last time: Geothermal Power Two categories of geothermal resources are practical / useful: Natural steam reservoirs. (Very desirable. Very rare.) Hot water reservoirs. Fine for heating; poor for electricity (why?...) We use geothermal power for Geothermal heating (use geothermally heated water) Electricity generation (low Carnot efficiency) Renewable? Depends on how it s harnessed

6 Last time: Geothermal Power Good for direct heating and small amounts of electrical power generation in particularly geothermal rich regions It s not going to solve our energy problems.

7 Last time: Solar Energy We began discussing Solar Energy The Solar Constant (S) is the power per unit area, where the area is oriented perpendicular to the sunlight Area A, perpendicular to sunlight Power P = S A S = 1400 W/m 2 The total amount of solar power hitting the Earth is about 10,000 times greater than humanity s power consumption there s a lot of solar power!

8 Last time: Insolation The power incident on a square meter of ground in Oregon is less than the power incident on a square meter in Arizona. We characterize this by the insolation the actual incident power per square meter of the Earth s surface. Depends on the tilt of the sunlight with respect to the surface Greatest in the tropics, lowest at high latitudes Greatest at midday, lowest at sunrise, sunset

9 Solar power Midday on a clear day, sunlight carries: 1000 W/m 2. Average insolation is smaller, depends on latitude, cloud cover, etc. Roughly 200 W/m 2. Higher in some places, but not tremendously lower (even in Oregon!). Consider a 7m 7m (21 ft. 21 ft.) = 49 m 2 50 m 2 area. The solar power hitting it is 50 m W/m 2 = 10,000 Watts! (Can we capture it? How efficiently?)

10 Solar power What can we do with all this power? Make things warm Generate electricity Storing energy as chemical energy (fuels or food) We ll discuss electricity ( photovoltaics ) first; the other things are simpler First, the basics of electricity Then, photovoltaics (EM radiation electrical energy)

11 Solar power Can we convert electromagnetic energy (light) directly into electricity? (No thermal energy, no turbines,...) Yes!(demo) How?

12 Electrical potential energy There s something called charge (can be positive or negative). Denote: q. There s something called voltage (V) a.k.a. electrical potential Units(SI): Volts (V) Electrical potential energy is E elec = q V An analogy: Grav. potl. energy E grav = M gh stuff Electrical potential energy is E elec = q V potential energy per unit of stuff

13 Current and Power Recall that we figured out that the power associated with falling water is M P = gh t There s a flow rate associated with charge. It s called current (I): I = q/t. Units (SI): Amperes ( Amps, A) flow rate Our analogy: q M, and gh V, so expect... P = I V. This is, in fact, true! Power = current voltage.

14 Current and Power P = I V. Power = current voltage. [demo]: Convert gravitational potential energy kinetic energy electrical energy M P = gh = IV t It works, but not all the grav. potl. energy in this demo is transformed into electricity: Low efficiency due to friction in this setup energy lost to thermal energy.

15 Current and Power P = I V. Power = current voltage. [demo]: Convert gravitational potential energy kinetic energy electrical energy efficiency M = power out / power in P = gh = IV t or energy out / energy in How to calculate efficiency? You can figure it out: Bulb: 1 Wif fully lit; about 0.3 W (we measured current, voltage; P=IV) Mass 1 kg, time 10s, h 1 m, g = 10 m/s 2. So input power about 1 W. So efficiency of this setup is about 30%. in the form we want

16 Storing electrical energy Consider a positive and a negative charge. + Separating them takes energy the separated configuration has more electrical potential energy + Greater E elec... and so a greater voltage associated with it This is how batteries work: chemical reactions separate charges, creating a voltage

17 Batteries Batteries: chemical reactions separate charges, creating a voltage Battery: V Hooking up the battery to a circuit, the voltage causes a current to flow In this setup, we get voltage V across the bulb V

18 Series and parallel circuits Suppose we connect two batteries in parallel, as shown. Is the voltage across the bulb A. V B. 2V C. Something else The current is double, but the voltage is the same. Keep our hill analogy in mind! Here: two hills next to each other the height is the same, but the amount of water flowing is greater. V V

19 Series and parallel circuits Suppose we connect two batteries in series, as shown. Is the voltage across the bulb A. V B. 2V C. Something else The voltage is double the two hills are stacked on top of each other. All this applies to any voltage sources e.g. solar cells that produce a particular voltage V V

20 Summary: Current, Voltage, etc. Charge q. (Positive and Negative) Electrical potential energy: E elec = qv. Electrical potential (i.e. Voltage) V, Units (SI): Volts (V) Current (I): I = q/t. Units (SI): Amperes ( Amps, A) Power P = I V. (SI units: Watts! (if Volts, Amps)) Series circuits: voltages add Parallel circuits: voltage same; currents add

21 Photovoltaics Photovoltaics ( PV ): Light voltage 1839: Edmund Becquerel (19 yrs. old) discovers that a voltage arises when he illuminates a metal electrode in a weak salt solution. This& related phenomena: the photoelectric effect. Explanation: light acts like a particle ( photon ) that carries energy (1904, Albert Einstein). (Einstein s Nobel Prize was for this, not relativity.) Longer EM wavelength A. greater B. lesser photon energy

22 Photovoltaics: history 1950 s: First practical photovoltaic devices (space program). High cost, but low weight, high reliability. By late 1980 s: many applications in places where power lines weren t feasible (off shore buoys, rural water pumping, etc.) 2000 s: lower PV costs, better efficiency lots of growth in grid connected PV devices (i.e. connected to the power grid) about 40% per year annual growth in PV power generation!

23 Semiconductors Photovoltaics are based on semiconductors. (Usually silicon.) We ll explore the basics of semiconductor physics. Semiconductors are also the basis of all modern microelectronics (computer chips, etc.) A milestone In 2006, for the first time, solar cell production used more silicon than the entire microelectronics industry!

24 Atoms Every atom consists of a nucleus, composed of protons (positive charge) and neutrons (no charge) Orbiting around are electrons (negative charge) The nucleus is much more massive than the electrons ( 10,000 ) electrons nucleus: protons & neutrons Helium

25 Crystals Most simple solids, composed of one type of atom or molecule, are crystals ordered arrays of atoms or molecules. E.g. copper, a lattice of copper atoms ice, a lattice of H 2 O molecules diamond, a lattice of carbon atoms quartz, a lattice of SiO 2 molecules There are also amorphous, non crystalline solids e.g. glass, disordered SiO 2 molecules. Many mysteries remain regarding amorphous solids...

26 Insulators and Metals In some solids, all electrons are tightly bound to their atomic nucleus. These electrons are not free to flow in response to a voltage. These materials are insulators. E.g. diamond, ice,... In some solids, some electrons are weakly bound to their atomic nucleus and are free to make up an electrical current. These materials are metals. E.g. copper, aluminum,... Are there things in between? Meaning?

27 Semiconductors Wolfgang Pauli (Nobel Prize winner), 1931: one shouldn t work on semiconductors, that is a filthy mess; who knows whether any semiconductors exist. Yes, they exist. ( Semiconductors ) Since then: basic physics technology widespread use (every radio, TV, computer, ipod, cell phone,...)

28 Semiconductors What is a semiconductor? An insulator in which electrons, if they had a bit more energy, would be free to move. A bit more: an amount that can be provided by thermal energy or other sources (e.g. light). So at T = 0 Kelvin, a semiconductor is an insulator. Under other circumstances, it acts like a metal. Semiconductors enable a great deal of control over electrical properties, which is why they re useful.

29 Semiconductors How much more energy does an electron need to be free (mobile)? At least the band gap energy, a characteristic of the particular semiconductor. Bound electron + band gap energy Mobile electron Silicon: E bandgap = 1.1 ev. (Don t memorize) ev A common unit for atomic energies. 1 ev = J. Recall electrostatic potential energy E elec = qv. Electron charge q = Coulombs (SI unit) So 1 ev is the energy of 1 electron in a 1 Volt hill.

30 Silicon (Si) Silicon a semiconductor 2nd most abundant element in the Earth s crust Siatom: 14 protons, 14 neutrons, 14 electrons. 10 of the electrons are very tightly bound to the nucleus The 4 valence electrons in the outer orbit are those that can be freed these are the relevant electrons. So we ll think of Si as a +4 charge nucleus with 4 valence electrons

31 Silicon (Si) tetrahedral crystal For ease of drawing: Silicon What happens if light hits?

32 Silicon + Light What happens if light hits? If the wavelength of the light is large... Photon energy less than E bandgap, For Si, wavelength > m... nothing happens; light passes through. (No change to electronic properties; transparent)

33 Silicon + Light What happens if light hits? If the photon energy is greater than E bandgap,... For Si, wavelength < m... we free an electron. And we also create a hole a vacancy that looks like a mobile positive charge. Light

34 Electrons and holes Illustration: a free electron and a hole:

35 Electrons and holes Note that the vacancy looks like a mobile positive charge

36 Electrons and holes Note that the vacancy looks like a mobile positive charge

37 Electrons and holes Note that the vacancy looks like a mobile positive charge

38 Electrons and holes What if the free electron and the hole find each other? They recombine, and we no longer have mobile charges: We don t want this... We ll see what to do shortly