Hydropower - MITweb.mit.edu/perron/Public/Hydro/101115-Hydropower-Perron.pdf · Head transformed to high velocity ... Hydropower–&T.Perron& & 8& a. Ex) fish that spawn in ... John

12.021: Earth Science, Energy & the Environment – Fall 2010 Nov. 15: Hydropower – T. Perron

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Hydropower I. Intro

A. Anecdote: History of hydropower in the eastern US a. Robert Walter & Dorothy Merritts, Franklin & Marshall College. Cover

story in Science, 2008 [PPT: Walter & Merritts in the field] b. While studying formation of floodplains by rivers in northeast US,

i. Discovered old surfaces, complete with tree stumps and even rough-hewn timbers, beneath meters of floodplain sediment. [PPT: field photos, cross-section]

ii. 14C dates for old surfaces as young as 300 yr!

iii. 210Pb dates (product of radon, t1/2 = 22.3 yr) for uppermost floodplain sediments ~1850 AD.

iv. These “floodplain” sediments were deposited rapidly! c. Conclusion, based on further fieldwork & archival research:

i. These sediments were deposited in mill ponds created by mill dams [PPT: example of mill in Hudson Valley, originally built late 1600s]

ii. Mill ponds were ubiquitous from colonial times through 19th century [PPT: mill pond locations in SE Pennsylvania]

iii. Densities in some locations approached 1 mill per km2 [PPT: mill density in early-mid 19th century in eastern US]

B. Lessons from this anecdote [PPT: Lessons]

a. Hydropower is one of the oldest means of communal power generation (mill designs used by N. American settlers date to classical times)

b. In pre-industrial times, having a local economy meant having hydropower c. Even today, hydropower remains a major power source worldwide, and is

the dominant renewable source d. The mill pond story also reminds us that hydropower has side effects

i. Mill ponds, and the sediment that accumulated in them, changed the face of the landscape

1. “Floodplains” where once there were marshy valley bottoms

2. Deeply entrenched rivers ii. As dams are removed or decay, accumulated sediment is released.

Implications for water supply, ecosystems, coasts? C. Lecture outline


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a. Hydropower statistics b. Practicalities: design and efficiency

c. Benefits and costs, with an example from river restoration

II. Statistics D. World

a. Total installed capacity 980 GW (~1TW) b. Total production ~3000 TW h / yr (makes sense: ~9000 hr/yr)

i. ~ 20% of world’s electricity ii. ~85% of renewable energy

iii. Some nations supply nearly all power with hydro (Norway >98%) iv. Canada, Brazil, Switzerland supply >50% with hydro

c. Theoretical global exploitable capacity: ~15,000 TW h / yr d. Annual global revenue from production ~$50 billion

E. US a. Total installed capacity 80 GW

b. Total production ~250 TW h / yr c. ~10% of total electricity (was 50% in early 20th c.)

d. ~50% of renewable energy e. Regionally, can be even more significant

i. Idaho generates nearly all of its electricity from hydro ii. Hydro-Québec [PPT]: major economic factor, regional energy

supplier F. Trends

a. Declining production in some developed countries (including US) i. All the good sites are taken [PPT: Reservoir construction in US

over time] ii. De-commissioning of large dams, non-renewal of permits

iii. New large facilities too costly or unpopular iv. Focus on other renewables

v. Ex) Massachusetts hydro production declined almost 6% from 2009 to 2010, and New England as a whole declined almost 9%

b. Growing production in other countries, including the developing world


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i. China is building new hydro almost as fast as they build new coal-fired power plants

ii. India, Asia, S. America, Africa currently exploiting a small fraction of potential capacity

G. Sources a. 2010 Renewables global status report (ren21.org)

b. Tester et al. (2005) c. USGS (http://ga.water.usgs.gov/edu/wuhy.html)

III. Practical considerations

A. Among renewables, hydropower is least abundant theoretically, but most exploited in practice.

a. Dwarfed by solar and wind [PPT: theoretical abundance of “exergy” sources]

b. Main reason: river networks concentrate energy, making it easier and cheaper to harvest

B. Energy budget a. [PPT: Schematic of hydropower station with dam] (more on types of

facilities shortly)

b. Basic equation: Power = (ΔPE/ + Δ KE) / time * efficiency:

P = !PE +!KE( )!t

=!PEV

+!KEV

"

#$

%

&'Q!

= "g!z+ 12"! u2( )"

#$

%

&'Q!

where t is time, Q is the flow rate [Volume/T], ρ is water density, z is elevation, u is mean flow velocity, and ε is efficiency.

c. For an impoundment system like the one we sketched, most KE is lost, and this becomes

P = !g!zQ"

d. Differences between different hydro facilities comes mainly from differences in !z and Q , in both the original, unmodified site and the engineered version

C. Efficiency


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a. Generally very high: mechanical efficiency (= work out / work in) > 90% for modern turbines

b. Can use different turbine types based on flow characteristics i. Impulse turbines (e.g., Pelton wheel)

1. Head transformed to high velocity (often with nozzle), which impinges on buckets or blades on turbine

2. For large head or variable flows ii. Reaction turbines (e.g., Francis or Kaplan turbines)

1. Driven by pressure drop in water 2. Must be sealed or submerged

3. For low head, wide range of flows D. Types of hydropower systems

a. Impoundment, usually involving dams i. Schematic is what we sketched earlier

ii. Components 1. Water is collected in a reservoir behind a dam, creating a

steeper head gradient than the original river. 2. Water is diverted into a “penstock”, a pipe that carries it to

turbines 3. Turbines turn a generator

4. Water is returned to the river downstream of the dam iii. Advantages

1. Produce lots of power in one spot à efficient 2. Can store water and produce in proportion to demand

3. Can double as irrigation, drinking water, recreation iv. Disadvantages

1. Impact of reservoir (more later) 2. More costly to build

3. Reservoir fills with sediment v. Examples

1. 3 Gorges Dam, Yangtze R., China [PPT] 2. Grand Coulee Dam, Columbia R., Washington [PPT]

a. Largest power-producing facility in US


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b. 5th largest hydropower installation worldwide c. Largest concrete structure in US

3. Glen Canyon Dam, Colorado R., Arizona [PPT] 4. Marmot Dam, Sandy R., Oregon [PPT]

b. Pumped storage i. 2-way flow: Pumped up to higher reservoir, then released down to

lower reservoir to generate power generation ii. Sometimes pumping is done with turbines that run in reverse

iii. Major advantage: generation can be tailored to demand 1. During low demand, excess power used to pump water to

higher reservoirs 2. During high demand, water released from higher reservoirs

3. This is a big issue with renewable sources, especially wind: a. How to create storage and redistribution

mechanisms to deal with variable production. b. I’ve heard compressed air being suggested for wind.

c. Pumped storage is currently the largest grid-storage method in the US

iv. Disadvantages 1. Requires multiple impoundments in close proximity

2. Pumping takes energy à lower overall efficiency v. Examples

1. Pyramid Lake (upper) and Castaic Lake (lower), CA. [PPT] a. Also a major water supply for LA

b. There’s a small impoundment power station at the outlet, which runs when the water is routed to LA.

c. Diversion, a.k.a. “Run of River” systems i. Water diverted from river, through penstock and turbines, then

back into river, with minimal or no storage reservoir ii. Typically built on rivers with steady, predictable Q

iii. Water may be run for a distance at small gradient to build up elevation head before being run through penstock

iv. Common to install a series of generation stations in a river reach v. Advantages


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1. Less impact on river (more details in a minute) 2. Don’t sacrifice velocity head

3. No reservoir to fill with sediment vi. Disadvantages

1. “Soft” power supply: no reservoir means production can’t be tailored to demand

2. Still requires a dam, with many of the impacts of impoundment syst.

vii. Examples 1. Niagara Falls

2. Bonneville Dam (one of several upstream of Portland), Columbia R., OR/WA [PPT]

3. John Day Dam, Columbia R., OR/WA [PPT] 4. Chief Joseph Dam, Columbia R., WA [PPT]

d. “Micro-hydro” i. Operational definition: < 100 kW

ii. Usually a small impoundment with penstock and power station iii. Advantages

1. Size of installation can be scaled to need 2. Clean, “fuel” (water) is already there

3. Can be installed off-grid iv. Disadvantages

1. Local environmental impact 2. Repair

v. Examples 1. Tungu R., Kenya [PPT]

2. Alternatives to impoundment systems: MIT D-Space class’s design for raft micro-hydro station in Lesotho [PPT]

IV. Benefits of Hydropower

A. No fuel cost B. Renewable and abundant: hydrologic cycle happens anyway

C. Relatively clean: no direct GHG emissions beyond construction


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D. High efficiency: > 90% mechanical E. Fast ramp-up

a. Can go from no generation to high capacity rapidly compared to other generation methods

b. Useful for emergency, unpredictable load, flow variations F. Scalability

a. Size of facility can be chosen based on demand b. Low-cost, low-capacity micro-hydro projects in remote areas and

developing nations G. Side benefits

a. Water supply b. Irrigation

c. Flood control d. Recreation

V. Costs and Impacts of Hydropower

A. Hydrological cycle may be free, but a. It can be unpredictable

b. Supply can change over time (e.g. melt from glaciers in N. Cascades) B. Reservoirs have finite lifetime before they fill with sediment.

a. Sediment would be transported downstream, but settles out in reservoir b. Reservoir lifetime can be << dam lifetime.

c. Can’t store water to adjust production to demand C. Hazards

a. Dam-burst floods (Ex: Johnstown, PA flood of 1889) [PPT: aftermath] b. Reservoir-induced seismicity

D. Social & cultural consequences, esp. of reservoirs a. Population relocation (e.g., 3 Gorges) [PPT: Inundation upstream of 3

Gorges] b. Archaeological site destruction (again, an issue at 3 Gorges)

c. Water resources disputes (upstream vs. downstream) d. Changes land use by indigenous populations. Ex) Cree and Hydro Québec

E. Ecological consequences: rivers are not just conduits for water!


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a. Ex) fish that spawn in revers, esp. salmon [PPT: Coho] b. Dams physically block river channel

c. Turbines kill fish d. Dams cause changes in bed grain size

i. Too coarse à fish can’t dig, eggs swept away ii. Too fine à not enough flow to supply oxygen to eggs

e. Reservoirs increase stream temperature. Fish like it cold & fast. f. Flow extremes can be important

i. Transport coarse sediment, reconfigure channel ii. Ex) lack of big floods in Colorado R. allowed invasive plants to

colonize immobile gravel bars F. Mitigation efforts

a. Run-of-river facilities b. Sediment removal & trucking

c. Controlled releases of larger flows (e.g. Glen Canyon) d. Fish ladders, fish trucking (no joke) [PPT: John Day Dam fish ladder]

e. Fish-friendly turbines (~5% mortality considered good) f. Dam removal

g. Case study: Marmot Dam Removal [PPT: Movie] i. Formerly a 13 MW impoundment facility

ii. Reservoir almost completely filled with ~106 m3 of sand & silt iii. Blocked passage of salmon

iv. Viewed as mid-scale dam removal experiment: 1. How long would river (& fish) take to recover?

2. Fate of sediment? 3. What was original state of river?

v. Estimated that sediment would move downstream over months to years

vi. Results: 1. Nearly all sediment gone from reservoir overnight

2. Fish return in days 3. But unclear how to define a full recovery – long-term

effects unclear


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VI. Summary

Hydropower is: A. Major energy source for many nations

B. Most widely exploited of renewable energy sources C. Not without undesirable side effects, but promising mitigation strategies are being

developed

VII. Resources A. Regional interest

a. Hydro-Québec: http://www.hydroquebec.com/en B. Engineering considerations

a. Tester, J. W., et al. Sustainable Energy: Choosing Among Options. Cambridge MA: MIT Press, 2005.

C. Impacts a. Clearinghouse for dam removal info @ UC Berkeley:

http://www.lib.berkeley.edu/WRCA/CDRI b. International Commission on Large Dams: http://www.icold-

cigb.net/listepublications.aspx c. King of Fish: The Thousand-Year Run of Salmon, by Dave Montgomery,

U. Washington d. USGS Study of Marmot Dam removal:

http://or.water.usgs.gov/projs_dir/marmot

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Hydropower - MITweb.mit.edu/perron/Public/Hydro/101115-Hydropower-Perron.pdf · Head transformed to high velocity ... Hydropower–&T.Perron& & 8& a. Ex) fish that spawn in ... John