Physics 162: Solar and Renewable Energies February 16, 2010 Prof. Raghuveer Parthasarathy raghu@uoregon.edu Winter 2010
Lecture 12: Announcements Reading: Wolfson Chapter 4 Homework: Problem Set 6, due Thurs. Feb. 25 (next week) No, the midterm results aren t ready yet (Thursday). Solutions posted! Prof. Parthasarathy will be gone next Tuesday (Feb. 23) lecture from the GTFs. (Billy Scannell will also take RP s Tuesday office hour: 1:30 2:30).
Last Time... Thermal energy: high entropy Thermodynamics: a fundamental limit on the conversion of thermal energy to mechanical work or electricity (any low entropy form of energy): e max = T 1 C T H e = work out / thermal energy in Carnot efficiency, e max : the maximum possible value of e for a heat engine any heat engine! Involves a ratio of temperatures, so must use absolute temperature (i.e. K)
Last Time... Actual efficiency less than e max. Necessarily: waste heat. Some calculations: e.g. a for a typical engine with T H = 300 C (600 K), T C = 25 C (300K), e max = 0.5. At best, you could convert 50% of the thermal energy into (e.g.) electricity.
Electricity I should mention: Why do we care so much about generating electricity? About 40% of the overall energy usage in the U.S. goes toward making electricity. Why is electricity so great? Easy to efficiently convert into other forms (heat, light, kinetic energy, etc.) Easy to transport just simple conducting wires!
Microwave exercise Microwave Exercise: most of you probably found that the efficiency of your microwave was 30 50% (i.e. 30 50% of the electrical energy thermal energy of the water). What happened to the rest of the energy? electrical energy electromagnetic radiation (microwaves) thermal energy generating microwaves is not very efficient! Microwave v. tea kettle: Microwave is less efficient, but can be faster (if more power); what determines the tea kettle s rate of heat transfer?... First, a brief aside...
Converting Energy We could take any forms and ask how we can convert energy between them: Mass Energy Kinetic energy Electromagnetic Radiation Gravitational potential energy Electrical Energy Etc. Thermal Energy Chemical Energy
Converting Energy Some we ve seen (e.g. kinetic electrical energy) Some you can try right now (e.g. kinetic energy thermal energy; rub your hands together) Some are complex but familiar (e.g. electromagnetic energy chemical energy, which plants do by photosynthesis, using sunlight to create molecules whose chemical bonds store this energy) Some are unfamiliar... Source: istockphoto.com
Electromagnetic Radiation Kinetic Energy A more exotic example of energy conversion Optical traps focused laser beams can move microscopic objects. Glass beads in water, radius 1 10 6 m from Prof. Parthasarathy s research lab
Who is Prof. Parthasarathy? Research: (a few examples) microparticles coated with membrane molecules (to grow new, complex materials) examining the mechanics of cargo transport in cells developing new microscopes & other optical tools My lab: 3 undergrads, 5 graduate students
A chart Who is Prof. Parthasarathy? undergrad grad + research Piled Higher and Deeper comics, 8/25/2008 (end digression)
Thermal energy Previously: Thermal energy, and its conversion to other forms. We ll say more about this in other contexts also, e.g. discussing geothermal power. First, another aspect of thermal energy: heat and heat transfer.
More motivation Heat transfer: important in general to understanding / using thermal energy Also very important to energy conservation by which I don t mean the fundamental principle of conservation of energy, but rather the colloquial usage. Energy is always conserved, but it often ends up (waste heat, friction, etc.) as useless high entropy thermal energy. We use lots of energy (at a rate of 10 kw per person, in the U.S.). Where? How can we use less?
Buildings About half of our energy consumption takes place in residential and commercial buildings. from Energy for Sustainability, J. Randolph and G. M. Masters (Island Press, 2008)
Buildings What is the largest use of energy in residential buildings (guess)? A. Heating (space heating) B. Cooling C. Lighting D. Refrigeration E. Water heating
Buildings About 47% of the residential energy use in the U.S. is for space heating (i.e. heating buildings) from Energy for Sustainability, J. Randolph and G. M. Masters (Island Press, 2008)
Heat Heat: Energy that flows due to a temperature difference. How does heat flow? ( Heat transfer ) Three mechanisms Conduction Convection Radiation
Heat Heat: Energy that flows due to a temperature difference. How does heat flow? ( Heat transfer ) Three mechanisms Conduction Convection Radiation
Conduction of Heat Thermal energy: Everything is in motion! Conduction of heat: Collisions between atoms transfer kinetic energy, and hence thermal energy. Example: Hot stove burner Hot pot Hot water. Note: all in contact.
Conduction of Heat Conduction: collisions of particles What could heat conduction depend on? [Ask] Block istockphoto.com
Conduction of Heat Conduction: collisions of particles What does heat conduction depend on? Temperatures (T H, T C ) Size, shape (d, A) Material T C A T C Block H d T H istockphoto.com T H
Conduction of Heat An equation you shouldn t memorize... Heat(H)(flow of energy): H H = ka T d... but which you should see makes sense T ( ) C Increasing(T H T C ): more heat Increasing A: more heat Increasing d: less heat What s k? T C A H d T H
Conduction of Heat k? We need something to describe an intrinsic property of the material (e.g. stone vs. metal vs. styrofoam) We call this the thermal conductivity, k. Demo: blocks
Thermal conductivity Table (text) Note: Air: low k Metals: high k Glass: higher than typical building materials (wood, fiberglass) Demo: wax
In fairness, for a glass building it s very good. not surprising... Photo: Brian Libby
Conduction of Heat Heat conduction: H = ka T H d T ( ) C A, d depend on geometry k (thermal conductivity) is a characteristic of the material
Conduction of Heat Heat conduction: H = ka T H d T ( ) C What are the units? Heat = energy flow (i.e. energy / time), so same units as power Writing the pieces: k has units of W / m K A has units of m 2 d has units of m T H T C has units of K (or C? hang on) So H has units of W m 2 K / m m K = Watts, as it should
Thermal conductivity: example A calculation: Suppose I live in a 10m 10m 10m cube, made of concrete with 0.5 m thick walls. I like to keep it at 22 C (72 F). Outside, it s 2 C (35 F). How much power do I need to expend? ( ) Let s use our equation: H TC Why? Isn t this the heat loss? I want to know the power I need to supply to keep T = 22 C. H = ka T d
Thermal conductivity: example These are the same thing: Due to heat conduction we re losing thermal energy at the rate given by H, and therefore our temperature would drop. To stop this, and maintain T, we need to supply the same amount of power (e.g. from a heater).
Thermal conductivity: example Suppose I live in a 10m 10m 10m cube, made of concrete with 0.5 m thick walls. I like to keep it at 22 C (72 F). Outside, it s 2 C (35 F). How much power do I need to expend? Concrete, look at table: k 1 W / m / K d = 0.5 m A = 6 10 m 10 m [6 faces of the cube] = 600 m 2. T H T C =...? 22 2 C = 20 C, or do I need to convert to K? A. 20 C B. Convert to K C. It s the same thing
Thermal conductivity: example T H T C...? 22 2 C = 20 C, or do I need to convert to K? A. 20 C B. Convert to K C. It s the same thing This is a difference in temperatures, not a ratio. It s the same in C and K Recall that C and K have the same increments, but the zero is different Or note: (22 + 273) (2 + 273) = 22 2 Or note that the difference in height between me and my son doesn t depend on where I measure height from.
Thermal conductivity: example... How much power do I need to expend? k 1 W / m / K; d = 0.5 m; A = 600 m 2 ; T H T C = 20 K H = k A (T H T C )/d = 1 W/m/K 600 m 2 20 K / 0.5 m = 12000 / 0.5 = 24,000 W = 24 kw. Pretty large! Suppose my walls were half as thick (0.25 m) but made of fiberglass, whose k is 20 smaller than concrete. What power would I need? A. 24/10 = 2.4 kw B. 24/40 = 0.6 kw C. 24/20 = 1.2 kw d is ½as big, so 2 as much H k is 20x smaller, so 1/20 as much H Combining: 1/10 th as much heat flow
Thermal conductivity: layers As seen, we can reduce heat conduction by choosing low conductivity materials. We can also make layers that include low k materials. An important example: double pane windows Glass air glass. (a.k.a. double glazed) Recall air has a very low thermal conductivity. (Sometimes use Argon, Krypton gases w/ even lower k; adds cost, though.) Can also have triple pane, etc. Can also have vacuum inside (k 0), but technically challenging, expensive
Insulation and R H ka Δ = T d The properties of building insulation are often described by R R = d/k [definition], so large R low AΔT H = conductivity (good insulation) R Typically ft 2 F h / Btu annoying units R isn t an intrinsic property of the material depends on d as well as k. Annoying, but... Useful since the R of the composite is the sum of the R s of the layers.
Insulation and R Useful since the R of the composite is the sum of the R s of the layers. E.g. 1 cm (0.01 m) thick glass has R = 0.01 m / (0.8 W /m K) = 0.0125 m 2 K / W [SI units] A 2.6 mm thick air layer has R = 0.0026 m / (0.026 W /m K) = 0.1 m 2 K / W Together, this glass + air + glass has R = 0.0125 + 0.1 + 0.0125 = 0.125 m 2 K/W. Note that this is pretty similar to the air alone the air dominates the behavior of the composite. Generally true it s a good idea to fill with low k, high R materials.
Convection Convection: bulk motion of a fluid (liquid or gas) E.g. hot gas becomes less dense, rises, and conveys thermal energy to higher regions. [Video] Details often complicated (fluid dynamics) General behaviors: Bigger temperature difference more convection Smaller pores convective flow more difficult (wool sweater)
Radiation Radiation. The sun... a fireplace... a hot stove burner... We see the electromagnetic waves these objects emit. All objects emit electromagnetic radiation It need not be visible EM radiation. You, for example, mostly emit infrared radiation. Night vision, Infrared thermometers (demo)
Radiation All objects emit electromagnetic radiation just due to their having some temperature Note that thermal radiation isn t the only way to emit light. A fluorescent light bulb, for example, emits light unrelated to its temperature. Also, a laser pointer... many other things. We re considering heat, and so are concerned with thermal radiation
Radiation EM radiation can travel through vacuum doesn t need stuff. (E.g. sunlight in space) Recall: Any EM radiation has a particular wavelength. wavelength, λ Source: Leiden University Quantum physics: In some ways, light behaves like a particle ( photon ) that carries a particular amount of energy Shorter wavelength higher energy