Pumped storage hydro, PSH, is the largest-capacity form of grid energy storage available. As of 2017, the DOE Global Energy Storage Database reports, PSH accounts for over 96% of all tracked storage installations worldwide, with a total installed nameplate capacity of over 168 GW. The round-trip efficiency of PSH plants varies from 70% to 80%, with some plants claiming up to 87%.

 

The main disadvantage of PHS plants is the nature of the site, which needs 1) adequate geographical height and 2) adequate availability of water. Suitable sites likely are in hilly or mountainous regions, often in areas of outstanding natural beauty, which raises social and ecological issues. Many recently proposed projects, at least in the U.S., avoid highly sensitive and scenic areas, and some projects propose to take advantage of "brownfield" locations such as abandoned mines.

 

1) PSH Plants Perform Arbitrage Operations:  PSH plants have an upper and lower reservoir, and a building with several turbine-generators that can generate electricity when water flows from the upper reservoir, via water turbines, to the lower reservoir during hours of high demand and high wholesale prices, usually late afternoon/early evening.

 

The same turbine-generators can draw electricity from the grid to pump water from the lower reservoir to the upper reservoir, during hours of low demand and low wholesale prices, usually at nighttime.

 

The pumping losses causes the plant to be a net consumer of electricity, but the wholesale price differential is sufficient to provide an annual profit.

 

PSH plant reservoirs are quite small when compared to reservoirs of traditional hydro plants of similar power capacity, MW.

 

2) PSH Plants Provide Emergency Power on Demand: A grid operator, such as ISO-NE, to quickly obtain temporary power in case of an unscheduled outage of a major plant, often calls PHS plants to quickly provide filling-in electricity for several hours until other generators can be brought on line.

 

PSH plants usually can increase their outputs from 0% to rated output, MW, in less than 10 minutes, and maintain that output for up to 8 hours (longer at less than rated output) by using the maximum allowed water quantity from the upper reservoir.

 

The grid operator pays a high fee per MW and per MWh for that emergency service, in accordance with contract provisions.

 

3) PSH Plants Provide Balancing Services for Base-Loaded Plants: PHS plants can store the excess electricity from base-loaded plants (coal, gas, nuclear, bio) during times of lower demand (if the reservoir is kept at a low level in anticipation), and make it available to the grid during times of higher demand. This avoids base-loaded plants having to reduce their output, which would be less efficient, i.e., more Btu/kWh and more CO2/kWh. PHS plants would be paid for that service by the base-loaded plants, likely under buy/sell balancing agreements.

 

4) PSH Plants Provide Balancing Services for Wind and Solar, 24/7/365: If a PSH plant had four turbine-generator sets, then two of the T-G sets would be in generating mode to perform the balancing services, while the other two T-G sets would be in pumping mode, taking the outlet water from the generating T-G sets, and pumping it back into the water supply shaft. The water level in the upper reservoir would remain unchanged.

In that mode of operation, the generated electricity would be about 15% less than the pumping electricity, resulting in a significant electricity cost, which would have to be more than offset by balancing fees to maintain plant viability.

 

At present, the grid operator pays the fees to the balancing PSH plants; another “invisible” subsidy that makes wind and solar look less expensive. The fees are “socialized”, i.e., charged to ratepayers or taxpayers.

 

It would be more appropriate, if the owners of wind and solar systems (the disturbers) paid these fees.

 

These services would be at a high cost/kWh, because of the high capital cost of the PSH plant that likely would be operating at a low capacity factor. The main advantage is much less CO2 emissions than with turbine-generator and diesel-generator balancing plants.

NOTE: Europe, with much greater quantities of wind and solar on its grids, has some recently built PSH plants that have a T-G generator sets on the same shaft as the pumps, which enables very rapid changes in output for grid frequency control and balancing wind and solar.

https://www.eera-set.eu/wp-content/uploads/Technological-Developmen...

Balancing Wind and Solar in New England: In New England, wind (3.0%) and solar (2.2%) have become minuscule percentages of the NE energy mix after about 15 years of subsidies. Any peaking, filling-in and balancing services are performed by the large capacity of gas turbines on the NE grid. There would be a slight efficiency decrease due to performing those services, which at present is absorbed by gas turbine owners. NE has minor connections to nearby grids.

 

http://www.windtaskforce.org/profiles/blogs/the-true-cost-of-solar-...

http://www.windtaskforce.org/profiles/blogs/the-true-cost-of-wind-e...

http://www.windtaskforce.org/profiles/blogs/vermont-speed-renewable...

 

Balancing Wind and Solar in Ireland: However, when wind and solar become a major percentage of the NE energy mix, then the efficiency decrease of the gas turbines becomes highly noticeable, as it did in Ireland with only 17% wind in its energy mix. Due to the increase in CO2/kWh, the CO2 reduction due to wind was only 56% of what was claimed! Ireland does not have robust connections to nearby grids. See URL

http://www.windtaskforce.org/profiles/blogs/fuel-and-co2-reductions...

 

Balancing Wind and Solar in Germany: Germany has robust connections to nearby grids to spread the inefficiency over a larger number of generators during periods of higher wind and solar, which, due to increasing build-outs, are occurring more and more hours of the year.

 

During such hours with excess German generation, the wholesale prices frequently become near-zero, or negative, which is not so much of a problem for wind and solar, as they get reimbursed at (politically set) fixed feed-in tariffs, but it is a major problem for the traditional generators which have fuel costs and do not get reimbursed at fixed feed-in tariffs.

 

http://www.windtaskforce.org/profiles/blogs/german-renewable-energy...

http://www.windtaskforce.org/profiles/blogs/wind-and-solar-energy-l...

http://www.windtaskforce.org/profiles/blogs/germany-not-meeting-co2...

New England PSH Plants: NE has two major PSH plants. The Northfield plant is the major subject of this article.

 

Bear Swamp, rated storage/cycle 600 MW x 6 h = 3000 MWh

Northfield rated storage/cycle 1168 MW x 8 h = 9344 MWh

The actual values are less. See table 1B.

Production over about 8 hours is 12344 MWh, if both cycle capacities were fully available at the same time.

 

NOTE: Average NE consumption during a similar period on a high-demand day is 22000 MW, average x 7 h = 154,000 MWh

BEAR SWAMP HYDROELECTRIC POWER STATION

 

The plant is located in Deerfield River in Rowe and Florida, MA. It is used for:

 

- Time shifting, i.e., using night-time electricity (low wholesale price) to top off the upper reservoir and producing daytime electricity (high wholesale price).

- Ramping, i.e., provide filling-in electricity in case of an unscheduled outage of a plant on the NE grid, or to provide electricity during peak loads on the NE grid, usually late afternoon/early evening.

- The plant could be used for balancing variable wind and solar electricity, but that would require major modifications, because of the limited water quantity in the upper reservoir. Once that quantity is used up, the plant would have to stop balancing mode, and start pumping mode to top-off the reservoir. The same is true for the Northfield plant.

http://www.berkshireeagle.com/stories/bear-swamp-hydroelectric-gene...

https://www.energystorageexchange.org/projects/228

 

Table 1/Rated output

600 MW

Elevation drop

700 ft

Ramp up 0% to 100%

3 minutes

Duration

6 h

Production/cycle

3000 MWh

Pumping electricity

1.38 x 3000 MWh

Cycle electrical efficiency

1/1.38 = 72.5%, below average

Maximum annual production

365 x 3000 = 1,095,000 MWh

Actual average annual production

450,000 MWh

Plant capacity factor

41%

NORTHFIELD MOUNTAIN PUMPED STORAGE HYDRO PLANT

 

First-Light Power Resources, former owner of the Northfield Mountain Pumped Storage Plant, sold the plant to one of Canada’s largest pension investment managers, the Public Sector Pension Investment Board (PSP Investments), which is retaining properties from First-Light Power Resource, primarily located on the Connecticut River in Massachusetts and Connecticut, for $1.2 billion.

http://www.gazettenet.com/News/Local/FirstLight-Power-Resources-to-...

NOTE: The Canadian entity is collecting money from Vermonters to pay Canadian pensioners. The US debtor status was $8.3 trillion at end 2016, on which foreign entities collected at least $415 billion (5%) to pay pensioners, etc.

http://www.windtaskforce.org/profiles/blogs/cop21-flawed-trade-agre...

The Northfield plant, generating capacity 1168 MW after the recent replacement of the hydro turbine-generators, was built in 1976 as a companion to the former, base-loaded, Vermont Yankee nuclear power plant in Vernon, Vermont, which used to produce about 600 x 8766 x 0.9 = 4,733,640 MWh/y, of steady, near-CO2-free, electricity, at about 5 c/kWh. The VY production was almost as much as the 6,100,000 MWh/y supplied to Vermont utilities and was much more than generated by the Hoover Dam (4,200,000 MWh/y). See table 8.

 

PSH Plant Operation:

 

Arbitrage Mode: At present, the main use of the PSH plant is in arbitrage mode, i.e., generate electricity when daytime wholesale prices are high and use electricity to pump when nighttime wholesale prices are low. Even though the pumping electricity exceeds the generated electricity by about 20%, the difference in wholesale prices makes that mode a profitable operation. See table 1A.

 

Table 1A/Summary

NE Wholesale

MWh/y

$/MWh

$million/y

Generating

3404925

60

204.30

Pumping

4271600

40

170.86

Efficiency

0.797

Capacity factor

0.333

 

 

Net Revenue from arbitrage

33.43

 

The plant has an upper reservoir and a lower reservoir (Connecticut River) and 4 reversible, hydro turbine-generators, for a total rating of 1168 MW. The units can be electrically powered to pump water from the river into the upper reservoir. See table 1B for plant design parameters.

 

http://www.northfieldrelicensing.com/Pages/Northfield.aspx

http://www.northfieldrelicensing.com/Pages/features.aspx

 

Emergency Power Mode: The grid operator ISO-NE, to quickly obtain temporary power in case of an unscheduled outage of a major plant, often calls the Northfield plant to provide filling-in electricity for several hours until other generators can be brought on line.

 

The plant can increase its output from 0% to rated output, MW, in less than 10 minutes, and maintain that output for up to 8 hours (longer at less than rated output) by using the maximum allowed water quantity from the upper reservoir.

 

ISO-NE pays a high fee per MW and per MWh for that emergency service, in accordance with contract provisions.

 

Table 1B/Name

Northfield

Location

Massachusetts

Upper reservoir

Manmade

Lower reservoir

Connecticut River

Commissioned

1972

Nameplate rating, MW

1200

Average

Operating elevation range, ft

1000.5 - 938

988.75

Elevation differential, ft

62.5

Head range, ft

753 - 824.5

788.75

Head differential, ft

71.5

Reservoir capacity, gal

5600000000

m^3/gal

0.003785

Reservoir capacity, m^3

21196000

Useable storage, acre-ft

12318

m^2/acre

4047

m/ft

0.3048

Usable storage, m^3

15194568

Usable storage fraction

0.717

Conv. Factors

Q, Mass flow

Turbine-Generators, reversible to pump

4

m^3/ft^3

m^3/s

Tunnel design flow, ft^3/s

27000

0.02832

765

Flow, generating, ft^3/s

20000

0.02832

566

Flow, pumping, ft^3/s

15200

0.02832

430

T-G rating, new, MW

1168

T-G rating, old, MW

1080

Start-up, 0 to 1168 MW, min

10

Electricity supply duration, h

8

 

Electricity Generated Per Cycle: The plant generates about 9329 MWh per 8-hour cycle, based on the data in table 2. Some of the data were assumed, as they could not be found.

 

Table 2/Electricity supply/cycle

Generation

Head at reservoir surface, ft

838.75

Head loss*, ft

25.00

Head at turbine inlet, ft

813.75

Head at turbine inlet, m

H

248.03

Gravity acceleration, m/s^2

g

9.80664

Density, kg/m^3

d

1000

Mass flow, m^3/s

Q

566

Efficiency, turbine

n

0.920

Water power to generator, joules/s

P = ndQgH

1267468155

s/h

3600

Water power to generator, joules/h

4.56289E+12

kWh/million J

0.2777777

kWh/h = kW

1267468

Water power to generator, MW

1267

Efficiency, generator to grid

n

0.920

Electrical power to grid, MW

1166

Duration, h

8

Electricity to grid, MWh/8 h

9329

 

Electricity Used For Pumping Per Cycle: The plant uses about 11703 MWh per 10.53-hour cycle, based on the data in table 3. Some of the data were assumed, as they could not be found.

 

Table 3/Electricity used/cycle

Pumping

Head at reservoir surface, ft

838.75

Head adder*, ft

25.00

Head at turbine inlet, ft

863.75

Head at turbine inlet, m

H

263.27

Gravity acceleration, m/s^2

g

9.80664

Density, kg/m^3

d

1000

Mass flow, m^3/s

Q

430

Efficiency, turbine

n

0.920

Water power to turbine, joules/s

P = ndQgH

1022463251

s/h

3600

joules/h

3.68087E+12

kWh/million J

0.2777777

kWh/h = kW

1022463

Water power to turbine, MW

1022

Efficiency, generator to grid

n

0.920

Electrical power from grid, MW

1111

Duration, 566/430 x 8 h

10.53

Electricity consumed, MWh/10.53 h

11703

Electricity loss per cycle, MWh

2374

Cycles/y

365

Electricity loss/y^, MWh

866675

 

* Head adder = upstream of turbine + downstream of turbine

^ Electricity loss is similar to historic. See table 5.

 

Estimated Annual Net Revenue: The plant has revenues of electricity sales of about $204.30 million/y, and costs of electricity purchases of about $170.86 million/y, for net revenues of about $33.43 million/y. Other revenue is from ISO-NE-requested emergency services. See table 4.

 

Table 4/Electricity Trading Net Revenue

h/y

8766

s/h

3600

Flow, generating, 8 h, m^3

16312320

Fraction of useable storage used

1.074

107

%

Plant capacity factor, CF

0.333

Efficiency, Generation/Pumping

0.797

Electricity production, MWh/8 h

9329

8500

Cycles/y

365

Electricity production, MWh/y

3404925

Daytime average wholesale, c/kWh

6.00

Revenue, $/y

204295530

Revenue, $million/y

204.30

Electricity, pumping, MWh/10.53 h

11703

Cycles/d

365

Electricity consumption, MWh/y

4271600

Nighttime average wholesale, c/kWh

4.00

Pumping cost, $/y

170864003

Pumping cost, million$/y

170.86

Net Revenue*, $million/y

33.43

 

* Actual net revenue is less due to fewer cycles per year and partial cycles.

 

Historic Annual Net Generation: Below are the historic values for annual net generation, GWh/y. Those values date from before the replacement of the original turbine generators with new units of slightly greater capacity, MW. See table 5.

http://globalenergyobservatory.org/geoid/1239

 

Table 5

Net generation, GWh

2000

-614.760

2001

-552.245

2002

-512.425

2003

-392.991

2004

-386.698

2005

-329.032

2006

-371.347

2007

-579.880

2008

2009

 

Comparison Calculation of Hydro Power; MKS and US Units: The below table shows two sets of values for calculating the power output of a hydro plant using MKS units, the international standard, and US units, the unfortunate standard the US borrowed from the English some time back. See Table 6.

 

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.302.8171&a...

https://en.wikipedia.org/wiki/Hydropower

 

Table 6/Sample Calculation

 

 

 

MKS units

 

US units

Efficiency

n

0.85

 

0.85

Density

d

1000

kg/m^3

Density as weight

w

 

 

62.43

lb-force/ft^3

Conversion factor

 

 

35.31

ft^3/m

Mass flow, m^3/s

Q

80

m^3/s

2825

ft^3/s

Gravity accel’n, m/s^2

g

9.80664

m/s^2

Conversion factor

 

 

3.2808

ft/m

Head

H

145

m

476

ft

Power = ndQgH

P

96693470

joules/s

71309555

lb-f/ft^3 x ft^3/s x ft = ft.lb-f/s

Power

 

96.7

MW

 

 

Conversion factor

f

 

1.356

(ft.lb-f/s)/kW

Power = nwQHf

P

 

96.7

MW

- In US units, “density as weight” is in lb-force/ft^3, i.e., acceleration due to gravity is inherent in the unit.

- P is the water power driving the electricity generator, which has an efficiency, followed by a transformer efficiency, before feeding to the high voltage grid.

Hoover Dam Annual Electricity Generation: The main reason the plant has such a low capacity factor is a lack of water.

 

Table 8

Units

Hoover Dam

Mass flow

Q

m^3/s

1000

Height

H

m

222

Gravity acceleration

g

m/s^2

9.81

Density

d

kg/m^3

1000

Turbine efficiency

n

0.955

Water power to generator

P = ndQgH

joules/s

2079818100

s/h

3600

Water power to generator

P

joules/h

7.48735E+12

kWh/MJ

0.2777777

Water power to generator

P

kWh/h = kW

2079818

Rated output

P

MW

2080

Generator efficiency

n

0.980

h/y

8766

Capacity factor

CF

0.235

Production

kWh/y

4198755987

Production

TWh/y

4.199

 

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Maine Center For Public Interest Reporting – Three Part Series: A CRITICAL LOOK AT MAINE’S WIND ACT (excerpts) From Part 1 – On Maine’s Wind Law “Once the committee passed the wind energy bill on to the full House and Senate, lawmakers there didn’t even debate it. They passed it unanimously and with no discussion. House Majority Leader Hannah Pingree, a Democrat from North Haven, says legislators probably didn’t know how many turbines would be constructed in Maine if the law’s goals were met." . – Maine Center for Public Interest Reporting, August 2010  http://www.pinetreewatchdog.org/wind-power-bandwagon-hits-bumps-in-the-road-3/From Part 2 – On Wind and Oil Yet using wind energy doesn’t lower dependence on imported foreign oil. That’s because the majority of imported oil in Maine is used for heating and transportation. And switching our dependence from foreign oil to Maine-produced electricity isn’t likely to happen very soon, says Bartlett. “Right now, people can’t switch to electric cars and heating – if they did, we’d be in trouble.” So was one of the fundamental premises of the task force false, or at least misleading?"  http://www.pinetreewatchdog.org/wind-swept-task-force-set-the-rules/From Part 3 – On Wind-Required New Transmission Lines Finally, the building of enormous, high-voltage transmission lines that the regional electricity system operator says are required to move substantial amounts of wind power to markets south of Maine was never even discussed by the task force – an omission that Mills said will come to haunt the state.“If you try to put 2,500 or 3,000 megawatts in northern or eastern Maine – oh, my god, try to build the transmission!” said Mills. “It’s not just the towers, it’s the lines – that’s when I begin to think that the goal is a little farfetched.” http://www.pinetreewatchdog.org/flaws-in-bill-like-skating-with-dull-skates/

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