Energy for the 21st Century

Fall 2009, Lecture 3

Hydropower, Geothermal, and OceanEnergy

HYDRO

Geothermal

Hydro and Geothermal Energy’smarket share in the United States

Power from water: tides, waves, rivers, andwater falls

Hydropower is the most important and widely-used renewablesource of energy.

Hydropower represents ~20% of total electricity production.

Canada is the largest producer of hydroelectricity, followed bythe United States (about 10%, down from ~40% around 1950)and Brazil.

Approximately two-thirds of the economically feasible potentialremains to be developed. Untapped hydro resources are stillabundant in Latin America, Central Africa, India and China.

Hydroelectricity

Clean but no globally abundant and with somenegative environmental impact

Instead of releasingwater into the river, itcan be stored in alower reservoir. Waterthen can be pumpedback into the higherreservoir at off-peakhours

Turbine can weigh up to170 tons and turn at 90rpm

Shaft connectingturbine to generator

The Hoover dam has17 generators rated at133 MW

Advantages to hydroelectric power:

Fuel is not burned so there is minimal pollution Water to run the power plant is provided free by nature Hydropower produce little greenhouse gas emission Relatively low operations and maintenance costs The technology is reliable and proven over time It is reliable - the “fuel” is almost always there

Disadvantages of hydroelectric power:

High investment costs Susceptible to long-term climate changes Limited to some regions of the world In some cases: flooding of land and wildlife habitat

loss or modification of fish habitatdisplacement of local populations

Possible impact on regional climate (?)

Ocean Energy

Energy exists in the oceans in several different forms, whichhave different origins. Two of the most significant forms are

* marine currents, caused by tidal effects and thermal andsalinity differences

* ocean waves, generated by the action of winds blowingover the ocean’s surface.

Tidal streams are mainly caused by the movements ofoceans and are driven by the interaction of thegravitational fields of the earth, sun and moon.

The potential resource for tidal stream is estimated as 5TW, which is comparable to today’s global electricityconsumption.

However, energy extraction is practical only in a fewareas where the currents are concentrated near theperiphery of the oceans, or through straits and narrowpassages between islands and other landforms.

A current velocity of 1 m/s is needed before the site isdeemed economic

The tides are generated by the rotation of the earth within thegravitational fields of the moon and sun. The relative motions ofthese bodies cause the surface of the oceans to be raised andlowered periodically, according to a number of interacting cycles

• A half-day (semi-diurnal) cycle, due to the rotation of the earthwithin the gravitational field of the moon. This results in a period of 12hours 25 minutes between successive high waters.

• Daily (diurnal) tides occur in some regions such as the Gulf ofMexico. These have only one high tide and one low tide in a 24-hourperiod.

• A 14-day cycle, resulting from the superposition of the gravitationalfields of the moon and sun. At new moon and full moon, the sun’sgravitational field reinforces that of the moon, resulting in maximumtides or spring tides. At quarter phases of the moon, there is noreinforcement, resulting in minimum or neap tides.

Floating (≥ 50 m deep) orfixed to seabed (~ 20 m deep)

Similar to wind turbines with theadvantage of predictability

Power of a current ∝ v3

Assume 30% efficiency

The power that can be generated increases with rotordiameter and flow speed

Very little is commercial to date. One British company, MarineCurrent Technology, has installed a 1.2 MW dual turbine

A 10.5 MW system is be commissioned in 2011 in the UK

Three 1.2 MW turbines are scheduled for Canada in 2009

16 m

14 rpm to avoid marine life running into the blades

La Rance Power Station,which became operational in 1966,uses tidal energy and produces 240 MW (24 10-MW turbines)

A dam 775 meters long (330 m for power plant) was built infront of a 22 square kilometer basin. The tidal range (thedifference between high and low tides) averages 8 metersand reach up to 13.5 meters.

Ocean Energy: what is it and where is itavailable in large quantity?

Tides (in especially in estuaries) and Waves (2% of800,000 km of coastline where power flux> 30 kW/m)

Wave-Energy Conversion Techniques

They are more than 200 years old!

Any device that can create waves can also convert it back

Heaving and pitching devices

Cavity resonance (“piston”)

Because it may be impractical to have large sections ofcoastal water occupied by wave-energy conversiondevices, wave focusing techniques may be required

Linear Waves in Deep Water

hH

λ

Linear Wave Theory in Deep Water

Swell = wave of long wavelength and small height.

For H/λ < 1/50, linear wave theory works.

Deep water conditions are satisfied if h > λ/2

(Note that there is more energy in deep water than inshallow water)

Then, λ ≈ gT2/2π and c ≈ gT/2π

Total energy (kinetic + potential) of a wave E = ρgH2 λb/8and the wave power (i.e., the transfer of wave energy in thedirection of the wave) P = ρgH2cgb/8, where b is the width ofthe crest (or wave) and cg the group velocity (≈c/2)

A wave with H=1 m and a 10 sec period deliver 165 kJ/m ofenergy per width or 8.3 kW/m of power per width

There are many types of waves…

Swell = wave of long wavelength and small height. For H/λ < 1/50, linear wave theory works. For H/ λ > 1/50, itdoes not.

Nonlinear waves are formed in shallow water; one specialcase is a solitary wave.

In random sea, there is a superposition of multiple linearwaves.

Most wave energy conversion devices are resonantdevices, thus are optimized to work at one wavelength.

References

Ocean wave energy conversion, by M.E. McCormick(Dover, 2007)

Wave and marine current energy, by R. Boud, InternationalEnergy Agency (2002)

Future Marine Energy, by J. Callaghan and R. Boud, TheCarbon Trust (2006)

Geothermal Energy

First electric power generation from geothermal energy startedin 1904 in the Italian village of Larderello.

The average increase of temperature with depthunderground is 3ºC/100 m. At Larderello, it is 10X larger.

With today’s drilling technology, reaching >1 km is easy.

If fluids exist (or can be inserted) in reservoirs at elevatedtemperatures, they can be brought up and become asource of power.

These fluids are complex mixtures of water with multipleminerals and hence can be corrosive

To be economical, a geothermal power plant must functionfor 25 years

Growth of electric power produced from geothermal plants

Geothermal power plant in IcelandAn assessment of the total potential for electricity production from the high-temperature geothermal fields in Iceland gives a value of about 1500 TWh (total) or15 TWh per year over a 100 year period. The electricity production capacity fromgeothermal fields is now only 1.3 TWh per year

Advantages of Geothermal Energy

Geothermal heat requires no purchase of fuel.

Emission of undesirable substances is small.

Geothermal power plants are unaffected by changing weatherconditions and work continuously, day and night.

Geothermal energy is extremely price competitive in someareas.

It also offers a degree of scalability: a large geothermal plantcan power entire cities while smaller power plants can supplymore remote sites such as rural villages

Impact of geothermal energy in terms of land usage

Geothermal ~ 1,200 m2/MW

Coal ~ 40,000 m2/MW (power station + area to be strip-mined for 30years

Photovoltaics ~ 60,000 m2/MW

Impact of geothermal energy in terms of CO2 emission

Geothermal ~ 0.06 kg/kWh

Natural gas ~ 0.59 kg/kWh

Coal ~ 1.13 kg/kWh

Potential Problems with Geothermal Energy

The geothermal fluid is corrosive and at a relatively low temperature (limitingthe efficiency of heat engines)

Construction of the power plants can adversely affect land stability in thesurrounding region. This is mainly a concern with Enhanced GeothermalSystems, where water is injected into hot dry rock where no water was before.

Hot water from geothermal sources will contain trace amounts of dangerouselements such as mercury, arsenic, antimony, etc. which if disposed of intorivers can render their water unsafe to drink. For example, H2S, a non-condensable gas, needs to be treated before release. Re-injection of brine intothe earth minimizes pollution.

Eventually, specific locations may cool down. For example, the world's second-oldest geothermal generator at Wairakei has reduced production. If left alone,however, these places will recover some of their lost heat, as the mantle hasvast heat reserves.

Core temperature is ~6,650°C

Mantle temperature is ~2,200°C

Where is geothermal energy coming from?

Why is geothermal energy produced in some countriesand not in others?

Where is geothermal electricity produced?

Note that somecountries areincreasing theirproduction veryrapidly (e.g.,Turkey)

Geothermal powerproducing plantsin California andNevada (2005)

Largest in theWorld

Electrical and non-electrical geothermal capacity in 2000

Different uses of geothermal energy

Five components for a commercially-viablehydro thermal geothermal plant producing

electricity

Large heat source

Permeable reservoir

Supply of water

Overlying layer of impervious rock

Reliable recharge mechanism

springrain

Permeable rock

200 to 300 ºC

faults

magma

heat

Onset of boiling

~ 2k

m

What can a geothermal heat pump do for you?

A typical home in Rochester uses about 1,000 gallons of fuel oilin winter. If fuel oil is at $4.55 a gallon, it will cost you about$4,500.

With a geothermal heat pump, it will cost about $ 1,200.

The pump can also be used to cool the home in the summer,because the temperature a few meters below ground remainsconstant.

Heat pumps reduce the required peak capacity. Totalinstallations in US is > 5GW (and >7 GW worldwide) for >350,000 units

Rochester data source: http://www.gleasonheating.com/Geothermal.aspx

References

Geothermal power plants: Principles, applications, and case studies, by R.DiPippo (Elsevier, 2005)

Geothermal Energy—Clean Power From the Earth’s Heat, by W. A. Duffieldand J. H. Sass, USGS circular 1249 (2003)

Geothermal Energy- An alternative resource for the 21st century, by H.Gupta and S. Roy (Elsevier, 2007)