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Renewable Energy Resources

The big picture: Energy (power) reaching the Earth’s surface

Solar: Radiated energy at land surface

84,000 TW (TW = Terawatt = 1012 W)

Tides: Gravitational interaction

of Earth, Moon, and Sun

3 TW

Geothermal: Heat from radioactive decay

of long-lived isotopes within the Earth

44 TW

~99.95%

~0.05%

~0.003%

This power is ~ half that at the ‘top’ of the atmosphere

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Energy fluxes (Power per unit area)

Energy flux = Energy / (Area*Time)

Space- and time-averaged

Solar: 160 W/m2

Geothermal: 0.09 W/m2

Tide: 0.006 W/m2

We use these fluxes directly when

developing alternative / renewable

energy resources

Your text makes use of fluxes

The big picture: Energy use

2008 Global energy use ~ 500 exajoules

(5001018 J)

Equivalent to 15 TW power consumption

The breakdown:

13 TW Fossil fuels (85+%)

1 TW Nuclear

1 TW Renewable / Alternative

Mostly hydroelectric

Wee bits everything else

Po

we

r co

nsu

mp

tio

n (

TW

)

EIA 2008 data

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Review Problem: What was the per capita power consumption in 2008?

Given: Total power consumption of 15 TW in 2008

Global population ~ 7 billion

Globally, how many 100 W light bulbs are ‘you’ using at all times?

?

Time out

In kicking our ‘fossil sunshine’ habit, what

are the major challenges we face?

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46 coal-fired plants in Minnesota:

XCEL Energy (MSP):

30 plants @ 8961 MW total

ALLETE (DLH):

9 plants @ 1441 MW total

Great River Energy (Maple Grove):

3 plants @ 1400 MW total

Excelsior (Minnetonka):

4 plants @ 1035 MW total

Average MN fossil plant ~ 300 MW output

Typical fossil plant output ~ 200 – 500 MW

Typical nuclear plant output ~ 500 MW – 2 GW

Always keep in mind the ‘economies of scale’

Gut check: A useful measuring stick, perhaps?

Outline

• Geothermal

• Tides

• Solar

• Hydroelectric

• Wind

• Waves

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Sources of energy (heat) on Earth:

Solar input from above (~160 W / m2)

Geothermal heat from below (~90 mW / m2)

Wide range depending on geology / tectonics

~ 20 mW / m2 in subduction zones

~ 200 mW / m2 in volcanic provinces

Source for geothermal is radioactive decay of elements

(Uranium, Thorium, Potassium in crust and mantle)

Geothermal Energy

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Location, location, location…

Large-scale exploitation of

geothermal energy feasible in

regions of intense heat flow

In practice, this means volcanic

regions inboard of subduction zones

Example: “The Ring of Fire”

Geothermal Energy: Basics

General idea: ‘Mine’ heat from below

Note: In volcanic regions, geothermal

gradient can reach 80+ ºC/km

Approach:

• Drill series of deep wells

• Inject cold water from surface

• Recover hot water

• Run hot water through ‘heat

exchanger’ to concentrate heat

• Produce steam

• Steam turbine produces electricity

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Two basic approaches: Depends on geology

Vapor-dominated (“direct / dry steam”) Liquid-dominated (“flash / wet steam”)

Pressure at depth low enough

to allow vapor phase (steam)

High pressure maintains liquid

(water) phase at depth

CountryCapacity (MW)

2007

Capacity (MW)

2010

% national

production

USA 2687 3086 0.3%

Philippines 1970 1904 27%

Indonesia 992 1197 3.7%

Mexico 953 958 3%

Italy 811 843

New Zealand 472 628 10%

Iceland 421 575 30%

Japan 535 536 0.1%

El Salvador 204 204 14%

Kenya 129 167 11.2%

Costa Rica 162 166 14%

Top ten countries in terms of installed geothermal electricity capacity

Data: Bertani, R. (2007), "World Geothermal Generation in 2007", Geo-Heat Centre Quarterly Bulletin, 28 (3): 8–19

Holm, A. (2010), Geothermal Energy:International Market Update, Geothermal Energy Association

Recall: Average MN fossil plant output is 300 MW

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Wairakei Geothermal Field

Geothermal Case Study: New Zealand (North Island)

Situated on Pacific ‘ring of fire’; intense volcanic activity

Water-saturated geothermal reservoirs (“flash / wet steam” approach)

Output ~ 180 MW (How does this compare to a MN fossil plant?)

Produces ~7% of country’s energy needs; future goal is 20% (~2020)

Geothermal Energy for the home:

Residential Geothermal

• Same basic idea: Mine heat from

below surface

• ‘Produce’ heat for house /

business—not electricity

• Depths are shallow, i.e. a few

meters in your back yard

• Temperature difference is small

(~10 – 20 ºC)

• Requires efficient heat pump

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Geothermal Energy for the home: Residential Geothermal

Heat pumps: The basics

Requires special ‘refrigerant’

Captures latent heat of vaporization

in evaporator

Energy input compresses vapor,

thereby elevating its temperature

Releases heat of vaporization in

condenser

Liquid drops pressure and cools

Heat pump is part of overall system

On the ‘cool’ side, circulating fluid ‘mines’

heat from Earth (geothermal heat)

On the ‘hot’ side, circulating fluid moves

heat throughout structure

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latent heat of

vaporization

IN

latent heat of

vaporization

OUT

IN

External input

Geothermal Energy: Municipal applications

Is this feasible for Duluth?

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OTEC: Ocean Thermal Energy Conversion

a.k.a. the world’s largest heat pump…

Basic idea is simple: Can we somehow

exploit the temperature gradient in the

world’s oceans?

‘Evaporator’ uses warm surface water to

vaporize NH3

Vapor drives turbine = electricity

Deep cold water used in ‘condenser’

Theoretical efficiency ~ 6%

Modern systems ~2-3% efficient

However, oceans are HUGE heat reservoirs

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