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Lecture 21
Non-solar energy
sources
Tidal
energy
Geothermal
energy
Small
energy sources compared to solar flux (Fig. 1)
Tidal power
Energy
related to the movement between earth, moon and sun
Power
generation similar to hydropower
Approaches
Impoundment
In-stream
systems
Assessment Tidal
power
Clean
energy production
Impoundment
systems
Reliable
flow systems
Large
structures needed
Very few
useful areas
Potential
for severe interference with ecology
In-stream
systems
Little
environmental impact
Small
systems
Geothermal Energy
Use of energy related to heat produced in the
inner of the earth ΰ geothermal gradient
Approaches
Unusual
high gradient with natural water flowing ΰ electricity production
Natural
Regular
gradient ΰ heat pump; heating-cooling systems
A few data
Average
continental heat flow ~ 0.055 W/m2 = 55 milliWatts/m2
Average
oceanic heat flow ~ 0.06 W/m2
Heat
flow equation:
Q = - W
dT/dx
Q heat flow; W
thermal conductivity
Temperature
gradients:
(10 < dT/dx
< 50) oC/km
Example:
Conventional
Geothermal Systems
High
Temperature systems: T > 200oC
Two
types (Fig. 3):
Vapor
dominated system ΰ no cap rock present
Hot
water dominated system ΰ cap rock
Essential
ingredients for geothermal system:
Heat
source ΰ high T gradient
Permeable
rock
Water
Systems
used to produce electricity using Carnot cycle: h=(Tin-Tout)/Tout ~ 40%
Environmental
concerns with geothermal energy
Considerations
(ΰ hydrothermal ore deposits)
Temperatures
are high:
150oC
< T < 400oC
Pressures
are elevated
10 bar
< p < 2000 bar
Solubility
of minerals in water depends strongly on p and T; in most cases solubility
increases with increasing T and p
ΰ Increase in T (and/or p) causes elements to
go into solution
ΰ
Decrease
in T (and/or p) causes deposition of ore minerals
ΰ
Changes
in T and p are dominant for formation of ore and other minerals
Consequences:
Release
of S ΰ formation of SO2 etc.
As water
cools, minerals come out of solution ΰ clogging of pipes etc.
Presence
of heavy metals ΰ toxic
Mineral
load of geothermal brines ΰ re-injection
Future uses of
geothermal energy
Hot dry
rock systems
Space heating
and green houses
Heat
pumps
Enhanced
Geothermal systems
Assessment of
geothermal energy
Current
use:
Production
of electricity from hot water or vapor systems with very high temperatures
Relatively
low impact on environment (release of SO2; metals)
Few
locations fulfill conditions for such a system ΰ little possibility for expansion
Future
use:
Hot dry
rock systems could expand electricity production (not very successful so far)
Low
level heat can be used for heating/cooling in many locations ΰ example
Enhanced
geothermal systems hold very large, but questionable potential
The grand picture
Who is
using materials?
Current
Population
Growth
rate ΰ 2050
What are
we using it for?
Domestic
use
Transportation
Industrial
use
Agricultural
use
At what
rate are we using materials?
Minerals
Water
Energy
Energy
options
Direct
use
Conversion
Electricity
Fuel
cells
Hydrogen
Energy
Sources - Estimates
Exhaustible
Renewable
Sustainable
Energy carriers
Energy
carriers are not new sources of energy, but provide new methods of providing clean
energy
Electricity
Fuel
cells
Hydrogen
Fuel Cells
Fuel
cells use the reaction between hydrogen and oxygen to produce electricity
Basic
steps:
Ionization
of Hydrogen ΰ production of protons and electrons
Membranes
provide separation of electrons and protons ΰ charge separation
Separate
pathways for electrons and protons ΰ flow of current
Combination
of H and O ΰ Water
Starting
ingredients: H and O
Final
product: Water
Systematics of fuels
cells (Fig. 4)
Assessment of Fuel
Cells
Basic
process uses H and O to produce H2O ΰ no direct pollution from process
Problem:
Production of H
Currently,
most H is from CH4 ΰ part of hydrocarbon cycle
Future production
might be from water ΰ needs electricity to split H2O
(coal; nuclear; wind; hydro)
Current
efficiency not competitive, but research is underway to improve fuel cell
performance
Hydrogen
Hydrogen
and Oxygen combine to form water: the reaction is exogenic
Problem:
There is very little free hydrogen present on earth ΰ hydrogen has to be separated first ΰ energy is needed
There is
always energy lost in the production of hydrogen, the specific amount depends
on the process (comparable to electricity production)
Advantage:
no pollution at point of use
The hydrogen
approach (Fig. 5)
How much hydrogen
do we need?
Estimate:
40x106 tons/yr
to run
100x106 fuel cell powered cars
or to provide
electricity to 25x106 homes
Options:
Distributed
Generation
1x106
neighborhood systems
Small
reformers: 67,000 refueling stations
Centralized
Production
Coal/biomass
gasification plantsΰ 140 stations with size similar to todays
coal plants
Nuclear
water splitting: 100 nuclear plants only
for H
Oil and
gas refinery: 20 plants, similar to refinery stations using oil and gas
Near and long term
scenarios for the hydrogen system (Fig. 6)
Assessment
Hydrogen
is an energy-intensive system ΰ conversion losses
Use of
hydrogen produces only water ΰ pollution moves to hydrogen production
facilities
Production,
transport and storage questions need to be addressed
Large
investment needed to change from current hydrocarbon based system to hydrogen
based system
Energy overview
Energy consumption
in the G8 countries (Fig. 7)
Energy situations
Today:
6.5x109 people x 2 kW/person ΰ 13 TW
EU,
ΰ Rest of World ~ 0.85 kW/person
2050:
9.4x109 people
Scenarios:
Rest of
world use 0.85 kW/person ΰ15.2 TW
Rest of
world use 2 kW/person ΰ 24.4 TW
Rest of
world use 3 kW/person ΰ 32.4 TW
Energy estimates (Fig. 8)
Observations
Our
economy currently is run on fossil fuels
Resources
sufficient for more than 30 years
Severe
environmental consequences
Current
nuclear power (thermal reactors) have limited potential; nuclear cycle is not
complete yet; breeder reactor and/or thorium reactors have large potential, but
are controversial and not available in the near future
Current
alternatives (hydro, wind, biomass etc.) can contribute, but have limited
potential
Two
energy sources have very large potential:
Fusion
power ΰ not developed yet, will need large, centralized stations
Direct
conversion of solar energy ΰ can be used on many different scales, but
needs considerably more development
Suggestions
Conservation:
Reduce energy use by switching to more efficient cars, houses etc., less energy
intensive food
Change
from centralized to decentralized energy sources
Diversification
of power generation