Solar and Renewable Energies€¦ · Lecture 2: Announcements • Reading: Wolfson, Chapter 2 •...

Physics 162:

Solar and Renewable Energies

Prof. Raghuveer Parthasarathy

[email protected]

Winter 2009

Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2009

January 8, 2009

Lecture 2: Announcements

• Reading: Wolfson, Chapter 2

• Homework Due: Problem Set 1 – due by 5pm.

• Homework: Problem Set 2. Due Thursday Jan. 15 (by 5pm)


posted on the web page

Last lecture

• Course Overview

• How much energy do we use? Where does it come from?

• Why do we use energy?


Last lecture

• Power = the rate of energy usageWatts (W) are a unit of power

• Typical “human” power 100 W

• Average US rate of energy consumption: ≈10 kW• Syllabus (read). Note clickers: participation grade, starting next week.


Units

• Quantifying physical phenomena requires a clear understanding of units

• Let’s discuss these...


Measurement

• In the beginning... counting. (1,2,3,...)

• Measurement: – Counting quantities relative to some standard.

– E.g. measuring a length of 3 feet: 3 × the length of some standard defined (by convention) as one foot.


Units

• Different standards can be used for the samephysical quantity

• E.g. feet, centimeters, parsecs for length

• These are “units”

• We can convert from one unit to another


Unit conversion


• An example:– There are 1.6 kilometers per mile.

– Convert the distance “2 miles” to kilometers

[Ask]


Unit conversion


• An example:– There are 1.6 kilometers per mile.

– Convert 2 miles to kilometers

2 miles × 1.6 km / mile = 3.2 km

Like writing 2 ×“1” = 2, since 1.6 km = 1 mileThe “unwanted” units cancel


Unit conversion

• Units can be treated algebraically, and only the correct calculation gives the correct units

• E.g. suppose we tried dividing instead of multiplying:Convert 2 miles to kilometers

2 miles ÷ (1.6 km / mile) =

2 miles × (1 mile / 1.6 km) = (2/1.6) mile2/ km

Nonsensical units – this can’t be right!


Unit conversion

• Another example:– “horsepower” (hp) is a unit of power.

1 hp = 750 W (actually 746, but we’ll be rough)

– Consider a car with a 200 hp engine

– What is the car’s power consumption, in Watts?

work on this (without a calculator)...

Answer: 200 hp × (750 W / hp) = 150,000 W

(Note 2 × 75 = 150, and there are 3 zeros.)


Units

• “SI units” – meters (length), kilograms (mass), seconds (time), Joules,... – a convenient set

• SI unit of energy: Joules (J)

• Other Energy units: Btu (British thermal units), calories, toe (tons of oil equivalent)

• (People use lots of different units for energy, so we often have to convert)

• (Don’t bother memorizing conversion factors; I certainly don’t remember how many Joules are in a Btu, e.g.)


Unit conversion

• You consume about 2000 Food calories of energy per day.

• There are 1000 calories per food calorie. (1 calorie = The amount of energy needed to raise the temperature of one gram of water 1° Celsius.)

• There are 0.239 calories per Joule.

• How much energy do you consume per day, in Joules?


Unit conversionWhich is correct?

2000 Food calories × 1000 calories/ Food calorie× 1 Joules/ 0.239 calorie

= 8,374,000 Joules

2000 Food calories × 1000 calories/ Food calorie× 0.239 calories / Joule

= 478,000 Joules

A.

B.


Unit conversion

2000 Food calories × 1000 calories/ Food calorie× 1 Joules/ 0.239 calorie

= 8,374,000 Joules

How much energy do you consume per day, in Joules?

A

Units can be manipulated algebraically.


Using scientific notation

(6.8 × 105) ÷ (3.4 × 103) = A. 2

B. 200

C. 2,000

D. 0.02

E. 2 × 108


Using scientific notation

(6.8 × 105) ÷ (3.4 × 103) = A. 2

B. 200

C. 2,000

D. 0.02

E. 2 × 108

This sort of math should be familiar.


Average power usage

• Last time we discussed the rate at which your body can produce energy – i.e. your power – and got a very rough intuitive feel for “Watts”

• How can we estimate your body’s average power consumption, in W, based on your ≈2000 Food calorie input? [Ask]


Power, Energy, and Time

How are power, energy, and time related?

A. Power = Energy / Time

B. Energy = Power / Time

C. Power = Energy + Time

D. Time = Power × Energy


Average power usage• Power = Energy / Time.

• SI Units: 1 Watt = 1 Joule / Second. – i.e. if you use 1 Joule of energy per second, your power consumption is 1 Watt.

• Energy intake = 8,374,000 Joules (calc. earlier)

• Time = 1 day – convert to seconds

– Do I remember how many seconds in a day? Certainly not!


Average power usage

• Time: 1 day × 24 hours/day × 60 minutes/hour × 60

seconds / minute = 86,400 seconds


Average power usage

• Power = Energy / Time

• SI Units: 1 Watt = 1 Joule / Second

• Energy = 8,374,000 Joules (calc. earlier)

• Time = 1 day = 86,400 seconds

• Avg. Power = 8,374,000 J / 86,400 s = 8.374 × 106 J / 8.64 × 104 s = 96.9 Watts

• Avg. Power ≈ 1 × 102 W = 100 W. (estimate!)


Average power usage

We just calculated that your body’s average power input (from food) is 100 W.

This is the same as your average power output. (Assuming steady weight):

Your body consumes as much energy as it expends – you’re not accumulating energy until you start to glow, or losing it until you disappear!


Jumping exercise

Recall that when jumping / doing knee bends, I said we were expending ~100 W.

You were actually expending ~200‐300 W, i.e. more than your average power

But as I said, 100 W is an order of magnitude value, designed to give a sense of the scale involved.


Review

By what factor is your average total power usage (as a 21st century US citizen) greater than the average power output of a human body?

A. 1 – they are the same

B. 10

C. 100

D. 1000


Avg. total power usage = 10 kW = 100 x 100 W

Important concepts

So far...

• Unit conversion!

• SI units for power (W), energy (J)1 Watt = 1 Joule / second

From earlier...

• Power and energy are not the same thing!Power = Energy / Time

These are often confused by the public


Energy

• We still haven’t defined what energy is – we’ll get to this soon. For now: consider energy a measure of the ability to “do things.”


Energy and Power• Suppose someone says: Lightning strikes carry lots of power – up to 1014 W. [True.] Therefore they can supply lots of energy; we should use them as an alternative energy source. What’s unconvincing about this “logic?”


WikimediacommonsA. Nothing; it’s fine

B. If the time span of the strike is short, the energy conveyed may be small

C. If the time span of the strike is long, the energy conveyed may be small

This is in fact the case (homework) – lightning can’t provide a “useful” amount of energy. (This doesn’t stop nonsense people post on the internet.)

Energy

• We still haven’t defined what energy is – we’ll get to this soon. For now: consider energy a measure of the ability to “do things.”

• First, let’s look at fossil fuels – Goals:– Seeing “where we are”

– Motivating the development of alternative energy

– Understanding the challenges that alternative energy faces


Fossil Fuels


natural gas, petroleum (oil), coal

Fossil Fuels

• Most (≈ 85%) of our energy comes from fossil fuel sources.

• What are these?

• What’s so great about them?

• What problems are associated with them?


Fossil Fuels

• What are they?• Hydrocarbons (molecules composed of carbon and hydrogen) formed from the decomposition of organisms long ago• Long ago = hundreds of millions of years – i.e. long accumulation of energy stored in the chemical structure of hydrocarbons


Fossil Fuels• Fossil fuels

– Coal

– Oil

– Natural Gas

• Formed from different geological processes


Natural Gas

Natural Gas• Methane (CH4)

carbon

hydrogen

from “World of Molecules”

Size 4 × 10‐10 m


OilLiquid Petroleum (i.e. Oil)• Petroleum includes gaseous and liquid components. “Oil” refers to the liquid (majority)

• A mixture of hydrocarbons

• (and impurities: sulfur, oxygen)

• for example, octane (C8H18)


carbon hydrogen

gasoline, e.g. contains many different hydrocarbons –octane, heptane, etc. ...

Coal• Coal

• Large molecules, mostly carbon

• Many varieties, grades– e.g. anthracite, ≈ 80% carbon

– e.g. bituminous, ≈ 50% carbon– the rest is oxygen, hydrogen, sulfur, ...

(an example of a chemical structure of coal. Each vertex is a Carbon atom. [Wikipedia: Karol Glab])


Fossil Fuels• What’s so great about fossil fuels?

• To answer this, let’s see what we do with them:

• We burn them. (Combustion) What does this mean?


Burning• This is combustion (burning) – a chemical reaction:hydrocarbon + oxygen

→ carbon dioxide + water + Energy

• CO2, H2O, and energy are the products!

• Energy Released: What we want. Energy from breaking C‐C and C‐H chemical bonds in the hydrocarbon, and forming C‐O and H‐O bonds.


Start with theseEnd up with these

Energy• Note:

• Total energy is conserved – same on both “sides.” We’ll discuss this more later.

• Left side (before burning): more “chemical energy” associated with the chemical bonds.

• Right side: Chemical energy is “released” and can be converted into other forms (typically thermal)


Combustion and energyhydrocarbon + oxygen

→ carbon dioxide + water + Energy

• How much energy is released?

• About 50 MJ/kg , i.e. 50 × 106 J per kg of fossil fuel. (Roughly the same for all fossil fuels) Remember this number.

• “Energy content” (Energy per unit mass)

• Is 50 MJ/kg a high energy content?...


Energy content• Fossil fuel energy content 50 MJ/kg

• How much gasoline per second would we need to burn to generate 100 W of power? (i.e. 1 100W light bulb, or 1 “human scale”).

• [Work on this... 2min] (Recall 1 W = 1 J/s)

• Do you have an answer?


A. Yes

B. No


• How much gasoline per second would we need to burn to generate 100 W of power?

• (Recall 1 W = 1 J/s)

• Q: Suppose we burned 1 kg of gasoline per second. What power would we generate? [Ask]

• Q: Suppose we burned 0.002 kg of gasoline per second. What power would we generate?...

• 0.002 kg/s × (50 × 106) J/kg = 100 × 10‐3 × 106 = 105 J/s = 105 W.



• How much gasoline per second would we need to burn to generate 100 W of power?

• (50 × 106 J/kg ) × X kg/s = 100 J/s– We want to determine X

• Note that the units cancel properly.

• X = 102 / 106 / (½ × 102) = 2 ×10‐6 kg/s, i.e. 2 milligrams* per second. Not much gasoline is needed – a little bit has a lot of energy!


* roughly the volume of an average grain of sand

Energy content• How does the energy content of gasoline compare to the energy content of a battery?

• We’ll learn how to calculate the energy content of batteries later. For now, I’ll just state it.

• “D” battery a bit less than 1 MJ/kg.


istockphoto.com

• Gasoline: > 50× greater energy density than batteries.• (This poses a challenge to any alternative energy sources that require battery storage, esp. solar)

Fossil Fuels

• What’s so great about fossil fuels?

• The main thing: very high energy content.

• Also: portable, “abundant” (in the past)


Solar and Renewable Energies€¦ · Lecture 2: Announcements • Reading: Wolfson, Chapter 2 •...

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