Thermodynamics

Heat and Temperature

First Law of Thermodynamics and Energy• Efficiency

Second Law of Thermodynamics and time (!)• Even MORE about Efficiency (temperature’s effect)

Unit 03 – Thermodynamics – Slide 1

Temperature Scales

• Heat – technically different

than internal energy

Unit 03 – Thermodynamics - Slide 2

• Temperature – how fast

molecules are moving

- Celsius (oC)

- Fahrenheit (oF)

- Kelvin (K)

Ex 3.1: Temperature

It is 25oC outside on a spring day.

What is the temperature in Kelvin?

What is the temperature in

Fahrenheit?

Unit 03 – Thermodynamics - Slide 3

Temperature and Heat

Heat (Energy) in Temp Up

Unit 03 – Thermodynamics - Slide 4

But how much?

Specific heat (c)

QIN = m*c*DT

Ex 3.2: Specific Heat

How many Joules of energy does it

take to raise the temperature of

5kg of aluminum 30oC?

Unit 03 – Thermodynamics - Slide 5

How about 5kg of

water?

Melting and Boiling

Takes energy to change state

Unit 03 – Thermodynamics - Slide 6

Solid Liquid (fusion)

Latent heat (L)

QIN = m*L

Liquid Gas (evaporation)

Ex 3.3: Latent Heat

How many Joules of energy does it

take to boil our 5kg water?

Unit 03 – Thermodynamics - Slide 7

(Note no change

in temperature.)

First Law of Thermodynamics

Energy is conserved

Unit 03 – Thermodynamics - Slide 8

Heatin – Heatout =

Work Done + Change in

Internal Energy

Qin – Qout = W + DEint

Thermodynamics Engines

W = 30 Joules

QH=100 Joules

QC=70 Joules

Power Plant

Low Temperature

High Temperature30 Joules

100 Joules e = 0.3 =

|All Arrows OUT| = |All arrows IN|

Unit 03 – Thermodynamics - Slide 9

Example 3.4: Efficiency

Unit 03 – Thermodynamics - Slide 10

Your car is around 25% efficient. If a

gallon of gas contains 1.3 X 108 Joules of

energy, how much heat is exhausted into

the air for every gallon of gas burned?

Example 3.5: Efficiency

Unit 03 – Thermodynamics - Slide 11

An coal fired power plant produces 2.5

Gigawatts of electrical power (2.5 GW(e))

and exhausts 4GW of excess heat into the

lake.

What is the total how many Joules/second

from coal must be put into the plant?

What is the efficiency

of the plant?

Question

Unit 03 – Thermodynamics - Slide 12

An coal fired power plant produces 2.5

Gigawatts of electrical power (2.5 GW(e)) and

exhausts 4GW of excess heat into the lake.

What is the efficiency of the plant?

A) 25%

B) 38%

C) 45%

D) 62.5%

E) 80%

Thermodynamics Engines

W = 2.5 Gigawatts

QH= ?? Gigawatts

QC= 4 Gigawatts

Power Plant

Low Temperature

High Temperature

2.5 Gigawatts

6.5 Gigawatts e = 0.38 =

Unit 03 – Thermodynamics - Slide 13

Question

A. 0.3 Gigawatts

B. 0.7 Gigawatts

C. 0.9 Gigawatts

D. 2.1 Gigawatts

E. 7.0 Gigawatts

An coal fired power plant produces

three (3) Gigawatts of electrical power

and has an efficiency of 0.30 and is

cooled using lake water. How much

thermal energy is put into the lake every

second?

Unit 03– Thermodynamics – Slide 14

Coal fired power plant

Is all about how you make steam to

turn the crank of the generator!

To

your

home

Unit 03 – Thermodynamics - Slide 15

You place a 80oC mug of coffee

inside an insulated container that

has an original air temperature of

24oC.

Unit 06 Entropy, Slide 16

80

24 0C 30 0C

30

10 0C

85

?

or

Second Law of Thermodynamics

No engine can

be 100%

efficient

Unit 03 – Thermodynamics - Slide 17

Heat goes from hot to cold

(unless you do work)

Low Temperature

Thermodynamics Engines

W

QH

TC

Power Plant

Low Temperature

High Temperature

TC

TH

emax = 1 -

Unit 03 – Thermodynamics - Slide 18

TH

QCThis is emax

Ex 3.6: Maximum Efficiency

An coal fired power plant

brings in its steam at

200oC and exhausts its

excess heat into the lake

water at 35oC. What is

the maximum efficiency

of the power plants?

Unit 03 – Thermodynamics - Slide 19

Question

A. Make the input to your engine hotter

B. Make the input to your engine colder

C. Make the exhaust from your engine colder

D. Make the exhaust from your engine hotter

You’re builder of an Indy

500 race car. You want

your car to get the most

power it can from the

gasoline it burns. What are

you most likely to do?

Unit 03– Thermodynamics – Slide 20

“Combined Cycle” (two stages)

Unit 03 – Thermodynamics - Slide 21

An modern combined

cycle turbine can take

steam in at 427oC. What

is it’s maximum

efficiency if it exhausts

the steam at 35oC?

TC

TH

emax = 1 -

427oC

35oC

Thermodynamics Engines

W

QH

TC

Power Plant

Low Temperature

High Temperature

TC

TH

emax = 1 -

Unit 03 – Thermodynamics - Slide 22

TH

QC

Why do it???

You could run your

factories by damming

up a stream and using

water power.?

Unit 03 – Thermodynamics - Slide 23

If generating electricity is so incredibly

inefficient, why do we do it? Why don’t see

just use water power like we used to?