1
Physics 100
Lecture 23
Geothermal Energy
April 24, 2017
2
The Interior of the Earth
3
Plate Tectonics

Earth’s crust is divided into moving tectonic
plates. The intersections of these plates are
the focus of geologic activity.
4
Subduction Zone

Friction between the
plates brings hot material
near the surface:
• Volcanoes
• Geysers
• Hot springs
5
Hot Water
Geothermal
System
Heat from the Earth
produces steam that
can be used to
generate electricity.
One of 21 geothermal power plants called The Geysers in northern California
6
Types of Geothermal Resources

Hydrothermal Systems (shallow, small,
localized)
• Wet steam – superheated water flashes into
•

one part steam and four parts hot water.
Dry steam – the water boils far underground
and the steam is brought to the surface to
drive a generator.
Hot Dry Rock Systems – water from
surface is circulated through cracks in
hot subsurface rocks
7
Types of Geothermal Resources


Geopressurized reservoirs – hot, liquid
brines in large, very deep (3 km to 6 km
below the surface) areas and are under
very high pressure. We currently lack the
technology to fully exploit this resource.
Low-Temperature Geothermal
Resources – hot water can be brought to
the surface and used for space heating,
greenhouses, fish farming, etc.
8
Geothermal Energy in the US
Temperature <90C
o
Temperature >90C
Geopressured resources
9
Which state has the most
geothermal energy?
93%
A.
B.
C.
D.
Florida
Wisconsin
Utah
Vermont
4%
A.
4%
B.
0%
C.
D.
10
Which state has the most
geothermal energy?
A.
B.
C.
D.
Florida
Wisconsin
Utah
Vermont
Temperature <90C
o
Temperature >90C
Geopressured
resources
11
What makes geothermal energy
unique from other renewable
energies (solar, wind, biomass, etc.)?
44%
A. It produces no gaseous37%
emissions
at all.
B. It works both day and night.
19% the Sun.
C. It is not derived from
D. It can be harnessed anywhere in
the U.S.
0%
A.
B.
C.
D.
12
At what rate is energy conducted
through each square meter of
300-m-thick rock (R = 275 m2·°C/W) if
100%
DT is 205°C?
A.
B.
C.
D.
0.275 W
0.745 W
2.24 W
275 W
0%
A.
0%
B.
C.
0%
D.
13
At what rate is energy conducted
through each square meter of
300-m-thick rock (R = 275 m2·°C/W) if
DT is 205°C?
A. 0.275 W
B. 0.745 W
C. 2.24 W
D. 275 W
Qc A DT

t
R
2
1 m   205°C 


2
275 m  °C W
 0.745 W
14
Suppose a geothermal system
generates steam at 227°C when the
outside temperature is 27°C. What is
the maximum efficiency of the power
78%
plant?
A.
B.
C.
D.
20%
40 %
50%
60%
19%
4%
0%
A.
B.
C.
D.
15
Suppose a geothermal system
generates steam at 227°C when the
outside temperature is 27°C. What is
the maximum efficiency of the power
plant?
A. 20%
B. 40 %
C. 50%
D. 60%
TH  227  273  500 K
TC  27  273  300 K
emax
TC
300 K
1
1
 40%
TH
500 K
The steam temperature in a coal-fired plant is often 825 K,
for a max efficiency of 64%. In either case we can achieve
about half of the maximum efficiency, and geothermal
plants are less efficient than fossil fuel plants.
16
Problems with Geothermal
Energy



Many geothermal facilities vent smelly H2S
from the ground. High concentrations of
H2S are hazardous to human health.
Many minerals are contained in the
wastewater, affecting stream or lake
chemistry, corroding equipment, clogging
pipes.
Removing lots of water can cause the land
above to settle.
17
Residential Geothermal Energy:
Earth-Sheltered Homes


Subsurface
temperature is a
steady 52°F (11°C)
Underground areas of
the home are cooled
in the summer and
warmed in the winter
relative to
surrounding air
18
Earth-sheltered homes
De Soto, WI
Umbricht Haus,
Schweiz, Germany
Honingham, UK
Cumbria, UK
19
Geothermal heat pumps
(Ch. 5 pp. 142-146)

A heat pump draws energy from outside
a home to heat it, or from inside a home
Heat engine
Heat pump
to cool it.

It is a heat engine
operated in reverse.

See Chapter 5,
pages 142-146
20
Geothermal heat pumps
COLD
Figure 5.18 p. 143
HOT
HOT
COLD
21
Coefficient of Performance
heat transferred QH
COP 

electricity input W
22
How much heat can a 2.0-kW heat
pump add to a house in one day of
continuous operation if it has a COP
of 3.6?
A.
B.
C.
D.
590,000 BTU
360,000 BTU
200,000 BTU
184,000 BTU
Response
Counter
0%
A.
0%
B.
0%
C.
0%
D.
210
23
How much heat can a 2.0-kW heat
pump add to a house in one day of
continuous operation if it has a COP
QH
of 3.6?
COP 
 Q  COP  W
A.
B.
C.
D.
590,000 BTU
360,000 BTU
200,000 BTU
180,000 BTU
W
QH  COP  Pt 
H
 3.6  2.0 kW  24 h 
3.6  106 J
 172.8 kWh 
kWh
1 BTU
8
 6.22  10 J 
1055 J
QH  590,000 BTU
24
Geothermal heat pump
economics
Average
2017 rates
Electric
9.97¢
Nat. gas
51.17¢
X
X
Fig. 5.20
p.145