Solar and Renewable Energies - University of Oregon

Physics 162:

Solar and Renewable Energies

Prof. Raghuveer Parthasarathy

[email protected]

Winter 2010

Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2010

February 16, 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 Scannellwill 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):

1 Cmax

H

e TT

= −


• e = work out / thermal energy in• Carnot efficiency, emax: 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 emax.

• Necessarily: waste heat.

• Some calculations: e.g. a for a typical engine with TH = 300 °C (600 K), TC = 25 °C (300K), emax = 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

• 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...


generating microwaves is not very efficient!

Converting Energy• We could take any forms and ask how we can convert energy between them:


Kine

tic ene

rgy

Gravitational potential energyChemical EnergyEle

ctrica

l Ene

rgy

Electromagnetic Radiation

Mass Energy Ther

mal E

nerg

y

Etc.

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

– examining the mechanics of cargo transport in cells

– developing new microscopes & other optical tools

– My lab: 3 undergrads, 5 graduate students


(to “grow” new, complex materials)

Who is Prof. Parthasarathy?• A chart


grad + re

searchunde

rgrad

“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 atomstransfer 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]

istockphoto.com

Block


Conduction of Heat• Conduction: collisions of particles

• What does heat conduction depend on?– Temperatures (TH, TC)

– Size, shape (d, A)

– Material

istockphoto.com

Block

TH


TCd

TH

ATC

H

Conduction of Heat• An equation you shouldn’tmemorize...

• Heat (H) (flow of energy):


d

TH

ATC

H

( )H CHk A T

dT

=−

• ... but which you should see makes sense

• Increasing (TH‐TC): more heat

• Increasing A: more heat

• Increasing d: less heat

• What’s k?

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

not surprising...Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2010

In fairness, for a glass building it’s very good.

Photo: Brian Libby

Conduction of Heat• Heat conduction:


( )H CHk A T

dT

=−

• A, d depend on geometry

• k (thermal conductivity) is a characteristic of the material

Conduction of Heat• Heat conduction:


( )H CHk A T

dT

=−

• 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 m2

d has units of m

TH‐TC has units of K (or °C? – hang on)•So H has units of W m2 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:

• Why? Isn’t this the heat loss? I want to know the power I need to supply to keep T = 22 °C.


( )H CHk A T

dT

=−

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 m2.• TH‐TC = ... ? 22‐2 °C = 20 °C, or do I need to convert to K?


A. 20 °CB. Convert to KC. It’s the same thing

Thermal conductivity: example• TH‐TC ≈ ... ? 22‐2 °C = 20 °C, or do I need to convert to K?


A. 20 °CB. Convert to KC. 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 m2; TH‐TC = 20 K

• H = k A (TH‐TC)/d = 1 W/m/K × 600 m2 × 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 kWB. 24/40 = 0.6 kWC. 24/20 = 1.2 kW

d is ½ as big, so 2× as much Hk is 20x smaller, so 1/20 as much HCombining: 1/10th 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‐kmaterials.

• An important example: double pane windowsGlass – 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

• The properties of building insulation are often described by “R”

• R = d/k [definition], so large R ↔ low conductivity (good insulation)

• Typically ft2 °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 sumof the R’s of the layers.

H k A Td

=Δ


H A TR

=Δ

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 m2 K / W [SI units]

A 2.6 mm thick air layer has R = 0.0026 m / (0.026 W /m K) = 0.1 m2 K / W

Together, this glass + air + glass has R = 0.0125 + 0.1 + 0.0125 = 0.125 m2K/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 Rmaterials.


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.

Source: Leiden University

wavelength, λ


• Quantum physics: In some ways, light behaves like a particle (“photon”) that carries a particular amount of energy

• Shorter wavelength ↔ higher energy

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