Part 1
Geothermal Power

Why?
Growing demand for energy
Concern about CO2 from fossil-fuel burning
Recent Report, The Future of Geothermal
Energy, gives favorable assessment

Indian Point Power Plants 1 and 2, big energy producers in the NY City area

Electric Power Primer
Typical Big Power Plant generates 1GW
One billion watts … 109 J/s of energy
In a year it generates
109 J/s x 3.1x107 s/year = 3.1x1016 J/year
A nice round number is 1018 J
That’s the amount of power a 1GW plant
Generates in its nominal 30-year lifetime

US Electrical Power Production
Capacity
906 GW in 2006
rate of increase 1% per year
about 1000 big power plants
need 10 new ones each year

An aside …
controversial Cape Wind project
offshore Cape Cod would generated
0.4 GW by 120 turbines
Offsets growth
of New
England
power
demand for
about a year
or two …

Geothermal lumped into
“Other Renewable” … not much!

Where is the Heat?
Typical Geothermal Gradient:
typical region: 20 K/km
volcanic region: 100 K/km
Power production needs temperatures well
above 100 deg-C
So drilling needed to access heat

3.5 km – easy to drill, but not very hot
What’s that hot spot?

Yellowstone Caldera
Biggest Volcano in US
here I am
standing by Old Faithful
above 10,000 cubic km
of magma

6.5 km – expensive but routine, areas
of western US are hot

10 km – very hot, but pushing limits of
technology

Heat in Rock: Q = r Cp V DT
Heat = density * heat capacity * Volume * change
in Temperature
Density = 2500 kg/m3
Heat Capacity = 1000 J/kgK
Volume = 1 cubic km = 109 m3
DT = 100 K
So Q = 2.5 x 1017 J
A 1 GW power plant generates 3.1x1016 J/year,
so this is about tens years of a 1GW power plant

Remember 1018 J is roughly the amount of
energy produced by a power plant in its
nominal 30 year lifetime, so these estimates
indicate a huge supply of heat energy

How to access heat?
Drill 2 holes, one to inject cold water,
another to extract hot water
Circulate fluid
Use hot water to generate steam that turns
turbine of more-or-less standard design

Issues
Drill 2 holes … expense of drilling
Circulate fluid … low permeability of rock
Generate steam … dissolved minerals in water

Money Counts!
Any sort of mining or extraction is an
Economic Activity
that competes by price against alternatives
If the economics are not right
It will not be done
even if it is in theoretically possible to do

Solution to low permeabiliy
Artificially increase permeability by creating
fractures
“Hydrofracture” … pressurize well until
you crack the surrounding rock, routinely
used in oil extraction, at least for small
volumes of rock

60 MW Krafla power plant, Iceland: heat from 33 wells drilled into volcano
Tiny by US standards Lots of wells

Magma
chamber
Power plant

Part 2
Fresh Water
Possibly the most
Limiting Resource

How much water do you use in a day?

US Water Usage, %
Irrigation
Domestic Supply
Public Supply
Livestock & Aquaculture
Industrial
Mining
Thermoelectric Power
34
1
11
2
5
1
48

US Water Usage, billion gallons / day
Irrigation
Domestic Supply
Public Supply
Livestock & Aquaculture
Industrial
Mining
Thermoelectric Power
80
0.6
27.3
3.4
14.9
1.2
135
Total 262

Ogallala Aquifer

US Water Usage, billion gallons / day
Irrigation
Domestic Supply
Public Supply
Livestock & Aquaculture
Industrial
Mining
Thermoelectric Power
80
0.6
27.3
3.4
14.9
1.2
135
Total 262

Total 262 billion gallons/day
362 cubic kilometers per year
7 km
H20

Public & Domestic Supply 27.9 billion
gallons/day
266 gallons per person per day
drinking
cooking & washing dishes
washing clothes
flushing toilet

Cooling water for power
plants
135 billion
gallons/day
450 gallons per person per day
40 kWh average daily electrical
consumption per person in US
So 0.08 kWh per gallon
a gallon lights
the bulb for an
hour

Irrigation
80 billion
gallons/day
266 gallons per person per day
2750 calories average daily food
consumption per person in US
So 10.3 calories per gallon
2.7 calories per liter a gallon gets you a chip

Wheat: 3500 calories/kg
About 750 liters of water to grow a kilogram
Wheat: 4.6
calories/liter

Rice: 3700 calories/kg
About 1550 liters of water to grow a kilogram
Rice: 2.4
calories/liter

How much irrigation water does the world
need?
2000 calories/day minimum
At 3 cal/liter
670 liters/day
 6 billion people  365 days/year
= 1.46  1015 liters/year
= 14700 cubic kilometers per year
So how much is available ?

The
Hydrologic
Cycle
46,000 km3/year
transported on shore
108,000 km3/year
precipitated on land
62,000 km3/year
evaporated from
continental reservoirs
46,000 km3/year
runoff to oceans

Need 14,700 km3
Available 46,000 km3
So superficially about three times as much
water is available than is needed.
But consider …

Some runoff is in uninhabited regions
Runoff is uneven during the year and may
be lost to sea before it can be used
The rest of the biosphere uses water, too
Human populations are growing

Runoff is uneven during the year and may
be lost to sea before it can be used
Solution – Reservoirs (“Impoundments”)
created by damming rivers

Global
impoundments of
water
8400 km3
Not much growth
in last decade,
except in Asia-
Australia

Regional distribution of large dams

Dams in the US. Note that the
red symbols indicate high
hazard potential. Dam
maintenance has not been a
high priority for many
municipalities and other dam
owners.