Pumped Storage Speaker 9655 Session 664 1

8/3/2019 Pumped Storage Speaker 9655 Session 664 1

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HYDROVISION 2011Evaluation of Pumped Storage Operations for Coordination with

Wind Resources and for Supplying Ancillary Services.

Authors

Diana Hurdowar-Castro, Ph.D., P.Eng, Director, Power and Water Optimization, Hatch,

Ontario, Canada

Dieter Matzner, MScE, Principal Power Consultant, Hatch Woodmead, South Africa

Francois Welt, Ph.D., P. Eng., Senior Optimization Specialist and Mechanical Engineer, Hatch,

Ontario, Canada

Abstract

Pumped storage has the capability to support renewable power projects by providing the

necessary generation required to firm such supplies. Pumped storage capacity can also be

used to trade between off and on-peak energy and to provide ancillary services especiallyfor systems where conventional hydro is small or nonexistent. Spinning and non-spinning

reserves can be produced in both the generating and pumping modes of operation,depending on the selected design of the pumping/generating units and control system.

This paper discusses pump/generation facilities and their interaction in the market by

describing specific experiences gained with pumped storage plants. Of particular interestis the growing role of wind power in the overall energy mix and the increased variability

that results from its large scale implementation. To demonstrate the economic and

operational viability of pumped storage in re-regulating wind energy, an optimizationmodel has been setup for a potential pumped storage plant and wind farm in Lesotho,

Africa. Results were generated over a full year of operation for various wind scenariosand an associated pumped storage plant.

With a growing demand for energy storage and rapid expansion of renewable energy, the

analysis discussed herein is applicable to the evaluation of any future large scale

renewable project.


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Introduction

Pumped storage stations are used worldwide as a means to increase on-peak powerdelivery capability by storing energy during low demand periods. This is particularly

useful in areas that do not have a high percentage of conventional storage hydro power in

their generation resource mix and which are heavily base loaded with nuclear or coal

fired plants (e.g. parts of North America, Asia and Europe). With the increasedpenetration of non firm renewable energy such as wind, the issue of energy storage is

becoming even more significant as wind generation is highly variable and can take placeat any time of the day or night.

In addition, the use of renewable energy requires a higher amount of regulation andspinning reserves. Pumped storage stations can significantly contribute to the ever

increasing reserve requirements. In some particular cases, pump storage stations serve a

dual purpose, i.e. not only do they firm up the on-peak capacity, but they also provide

water for irrigation purposes.

Design ConsiderationsPumped storage stations use an upper and one lower reservoir to re-circulate water in a

close loop cycle. A pumped storage station can have two artificial reservoirs; however,

many facilities have the lowerreservoir located on an existing

waterway. There are also

concepts and preliminary

designs where the lowerreservoir is an (abandoned)

underground mine or an

underground excavation madespecifically for the pumped

storage project. It is preferable

that the two storage areas arequite close to each other to minimize losses in the tunnels and conduits, and the cost of

long penstocks. A typical pump storage station configuration is shown in Figure 1.

Pumped storage stations operate on a daily, weekly or even seasonal basis, the differencebeing the size of the upper reservoir and inflows. When the upper reservoir is part of the

watershed and receives natural inflow from upstream, it is considered to be an in-stream

pump storage station and the operation may be affected by the changes in precipitation.

Typically, the elevation differential between the upper and lower reservoir is quite large

to minimize the amount of water that is required to store. Plant heads are usually greaterthan 100 m, and there are many instances where plant heads are in excess of 300 m (e.g.

Bleinheim Bilboa, NY, USA). Pumped storage stations with 700 or 800 m heads are

also increasingly common worldwide (e.g., Kazunogawa, Japan, LaCoche, France).


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For a given amount of energy storage, the volume of water in the system is inversely

proportional to the head. Therefore the physical size and cost of reservoirs, pump-turbineequipment and water conduits is reduced as head increases. For this reason higher head

pumped storage developments are generally more economic.

Large pumped storage stations are more cost effective and plant capacities tend to bequite large. Most plants have a capacity that is at least a few hundred MW, and a

significant number of pump storage stations have a capacity in excess of a 1000 MW(e.g., Ludington with 1800 MW, Michigan, USA) especially for the newer plants constructed

in the 1960s and beyond.

The (cycle) efficiency of a pumped storage station is the ratio of energy output to energy

input. Efficiency for plants constructed from the 1960s to mid 1980s is usually 68% to

74%. More modern plants have an overall cycle efficiency in the 72% to 78% range,

with some designs approaching 80%. Traditionally, very large motors and generators inthe power industry (and other industry) have been single speed synchronous types. This

means pump-turbines will operate at a single point of operation in the pump mode, e.g.,the flow and MW range reduces to a single point at any given head. However, over thepast 20 years, variable speed technology has advanced to large hydro units. This is an

increasingly popular design for pumped storage station as the variable speed permits a

wide range of operation in both pumping and generating modes. This allows foradditional reserve capability, especially in the pumping mode of operation, as well as

modest increases in efficiencies. However, the cost of such units can be as much as 30%

higher than the fixed speed type. Such units have been installed in Europe (e.g.,

Goldisthal, Germany) and Asia (e.g., Kazunogawa, Japan).

Plant Operation

In most cases, units are used in pumping mode during off-peak periods, typically at night,and in generating mode during the day. For the vast majority of units, there is a minimum

amount of time required to switch from the pumping to generating mode, or vice versa.

For newer units the minimum time for the changeover is fairly short (e.g., of the order of10 minutes), but it can be of the order of 1 hour and can be even two hours for certain

operations.

In older plants, it may also be required that no more than one pump cycle per day beused, and some restriction may also exist on the generating side. These rules of operation

are imposed to prevent excessive wear and tear on the units, but also to facilitate

scheduling of the operation in a smooth and predictable fashion. Newer plants have beendesigned for a much more flexible operation and may not exhibit such restrictions (e.g.,

Dinorwig, UK, with up to 35 mode changes per day).

Operation in a Coordinated Hydro-Thermal Environment

In this regulated environment, the use of available resources is well coordinated and

planned ahead of time according to the best available load forecast. The utilization of apump storage station is then heavily dictated by the difference between on-peak and off-

peak load demand.


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The cost savings comes from the ability to displace more expensive non hydro peaking

units required to meet load,such as diesel or natural gas

fired units, during on-peak

hours. The savings in

production costs has to begreater than the sum of lost

revenue from the 20%-30%loss in cycle efficiency,

amortized debt payment and

expected equity cost for theplant operation to be

profitable. As a result, pump

storage stations have usually

a fairly low utilization factor(0.08 to 0.18). A typical

schedule of operation isshown in Figure 2, where theplant is operated in pumping mode for a few hours at night, and similarly for a few hours

in generation mode during the day.

Operation in an Open Energy Market Environment

In this environment, the utilization of a pump storage station is heavily dictated by the

difference between on-peak and off-peak market prices. Similar to the hydro-thermal

environment, the price differential has to be greater than the sum of losses in pumpgenerating efficiency (e.g., 20-30%), amortized debt payment and expected equity cost

for the plant operation to be profitable. As a result, there is a strong incentive to pump at

low price hours and generate during high price hours. Because of price variability, theeconomic operation of a pump storage station may lead to a more dynamic schedule with

an increased number of starts and stops or mode changes, as compared to the operation

within a regulated environment such as described previously.

Operation to Provide Ancillary Services

Conventional hydro power plants are very effective at providing regulation and spinning

reserves, as compared to other non hydro resources. The same characteristics apply topump storage units when operated in generating mode. In general, pump storage units can

provide the following ancillary services:

Operating Reserve Load Following. Regulation Reserve (up and down regulation). Black Start. Supply or Absorb Reactive Power.Newer units have been designed to be able to start very quickly, which means that they

have excellent capability to respond to reserve requests.


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When operating in pumping mode, fixed speed pump storage units cannot provideancillary services. This is where variable speed units can be very useful, as they can

provide the same amount of operating, load following or regulation reserve as compared

to the generating mode.

Operation with Water Management

A few pump storage stations are part of a more complex hydro system and as suchrequire more elaborate water management. This is the case when there is significant

upstream inflow in one of the reservoirs. Plants with significant storage can also be used

for flood control. In other cases, pump cycles may be interrupted during the high flowperiods, as the water must be carried downstream while generating at all hours of the day.

Other plants may not always be free to pump and generate according to market

incentives, e.g., when they must follow a very well defined irrigation schedule withpumping required over extended periods of time.

Although not all pumped storage stations can be used to provide ancillary services,

especially when considering older plants, many other plants in various parts of the world

do contribute significantly to the ancillary service requirements (e.g., Ludington in theUSA, Dinorwig in the UK, etc.)

Operation with Wind EnergyStorage of wind energy (as is for the case study described below) has been the focus of

considerable attention over the last few years. A number of options have been considered

such as the use of batteries, compressed air energy storage technology, fly wheel inertia,

only to mention a few. Pumped storage is the most established technology, particularlywhen considering large amounts of energy storage. However, pumped storage may also

incur the highest initial capital costs and investment requirement.

The design of pump storage stations for storing wind energy requires some special

consideration due to the random nature of the wind energy resource, as follows:

Sufficient reservoir storage capacity to handle extended periods of high or lowwinds.

Flexible range of operation as wind can fluctuate rapidly over time, with the use ofmultiple units so that high efficiencies can be maintained over a wide range bychanging the unit allocation. Variable speed designs are generally considered

necessary to accommodate varying pumping power that is available from wind

energy generation.

Good ancillary service capability to contribute to the higher reserve requirementsthat the use of renewable energy imposes on the system. The ability to provide

ancillary services can be a key factor in the profitability of the plant operation.


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Case Study Lesotho HighlandsLesotho is a country land locked by South Africa as shown in Figure 3. The landscape is

characterized by hill (almost cliff-like) type terrain and characteristically high winds.

Apart from the logistics associated with installing wind in this geographic area, theseregional characteristics provide for an ideal opportunity for wind power production.

The opportunity to export wind energy to South Africa appeared economically viable

with the introduction

of the South AfricanFeed in Tariff

program in 2009

which provides for

attractive tariffs forrenewable resources

such as wind.However, exportingof large scale wind,

produced within

Lesotho, wouldrequire firming for

delivery to South

Africa.

The basis of the study

was to gain insight into the operations that a pumped storage station (PSS) could provide

to improve the delivery reliability and firming of renewable energy to the South Africanelectricity market. Specifically the study included a preliminary evaluation of operations

of a wind-PSS hybrid system and the wind power capacity required to support an on-peak

firm delivery of energy over a specified period of time each week. Various peakdeliveries were evaluated along with a staged PSS development of 1,000, 2,000, 3,000

and 4,000 MW.

Simulation of the wind-PSS system was carried out using the Vista DSSTM

model. Thismodel optimizes the hourly operations of all reservoirs, units, and spillways in view of

external energy sources like wind and other factors like market prices, load, inflow, and

operational constraints. For the study herein, the model was used to produce anoperations pattern of pumping and generating on an hourly basis over a year-long period.

The Model

The Vista model simulates the hourly operation of the hydraulic system using detailed

operational procedures analogous to those used in actual practice. The model uses

detailed physical system and operational constraints to schedule plant dispatch, by unit, in

a manner which optimizes revenue but which abides by defined system constraints:


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Reservoir physical and operational limits Unit characteristics and operational limits Historical inflow sequences Channel lag and route characteristics River flow constraints

Market price forecasts Transaction opportunities Firm contracts Transmission constraints tie lines Maintenance

The model uses a series of arcs, which link plants, power canals, spill channels and river

reaches as shown in Figure 4. Generating facilities are connected to buses and

interconnected to the system via transmission lines which are tied to load centers and

available markets as shown in Figure 5

PSS Setup

The features of the PSS system configuration aredescribed below and shown in Figure 6:

Two storage reservoirs were defined, with theupper reservoir located 800 m above the

lower reservoir to provide the desired head.

Four units (250 MW each) were specified atthe PSS capable of pumping and/or

generating up to a maximum capacity of

1,000 MW, depending on the net head andwater volume moving between the tworeservoirs.

A controlled spillway was placed at eachreservoir to handle extreme flow conditions. The plant tailrace was defined to provide a

water level that is equal to the downstream

reservoir elevation.

No natural inflows at the upper or lowerreservoirs were considered in the analysis.


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Characteristics of the PSS

In order to perform the analysis several assumptions on the characteristics of the PSSwere required. The pumping and generating capability of the plant was modeled as four

reversible units with a rated head of 800 m. The units were assumed to be able to operate

in a continuous manner in both pumping and generating modes, so that it can continue to

operate even under erratic wind conditions.

Typical generating and pumping characteristics were adopted. In the generation mode apeak efficiency of 89% is achieved at rated head and a flow of 32 m3 /s. In the pumping

mode, 92% efficiency is achieved at a flow of 30 m3/s. The variation of efficiency versus

flow for both generating and pumping modes are shown in Figure 7.

Pumps are typically available as single speed types, however variable speeds are

available but tend to be more expensive. For the purposes of this preliminary level study,

the continuous mode was employed to ensure maximum use of incoming wind power.

The system was modeled to include up to four pumped storage stations each comprising aset of two reservoirs and corresponding pumped storage facilities.

Storage Characteristics

The upper and lower reservoirs were assumed to have a storage capability of up to 13

Million Cubic Meters (MCM), which would permit accumulation of energy over asustained period even under a continuous pumping mode of operation. The elevation to

volume curve has been assumed to be linear, with both reservoirs sharing the same

characteristics.

Plant Tailwater Characteristics

The plant tailwater elevation was assumed to be the same as the downstream reservoirelevation. As a result, the plant head is simply,

Plant Head = upper reservoir elevation lower reservoir elevation.

Wind Data

Mesoscale data indicating the general spatial wind speed patterns over the terrain was

available. For modeling purposes however an hourly wind time series was required to


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analyse wind coordination with the proposed PSS. Procurement of general hourly time

series data (satellite derived) for each of three wind farm locations was obtained. In orderto perform the analysis the Goldwind 77 1.5MW wind turbine with an 85m hub height

was assumed. The power curve from the manufacturer was adjusted to the site conditions

and used to develop an hourly power delivery pattern over the designated yearlong period

for each site.

TransmissionThe wind energy was modeled as a set of external energy sources. Three buses,

representing the three different wind farms were defined to incorporate the wind spatial

diversity. A bus collecting generation from the various sources (wind and PSS) waslinked to the South African power market. The various buses were all connected via

transmission lines of unlimited capacity. The wind energy can be transmitted to the main

collecting bus where it can be sold directly to the South African power market under the

preferential tariffs (e.g., REFIT), or it can be used to power the pumping units.

Simulation MethodologyThe operations simulated were formulated on the basis that the Lesotho wind andpumped storage system would be isolated with only the ability to export energy i.e.,

South African off-peak energy was not available for pumping. Consequently pumping

for reservoir refill could occur using wind energy resources only during the off-peakhours.

In the off-peako wind energy is used to pump water to the upper reservoirs.o once the reservoirs are full, excess wind energy is sold as secondary

energy into the S.A. market.

In the on-peak

o wind energy is sold directly to the market as part of the firm delivery.o shortfalls in meeting the firm are supplied by the generation cycle of the

PSS.

o if the reservoirs are empty and firm can not be met the model records themagnitude of the shortfall.

Shortfalls in meeting the on-peak firm requirement were recorded and subsequently used

to determine system reliability for the level of wind and PSS installed.

A number of runs were performed which simulated operations for delivering 25, 30 or 35

hours of on-peak energy to a firm value of 1000, 2000, 3000 and 4000 MWcorresponding to a PSS staged development of equivalent scale. The reliability

associated with meeting the firm commitments were assessed as a function of the wind

capacity installed.


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Simulation Results

As the model simulateshourly operations over an

entire year the interaction

between the incoming wind,

pumping, generating,reservoir usage and energy

export delivery was closelymonitored and preliminary

conclusions formulated on

the reliability of supply.Simulations provided an

opportunity to specifically

determine the reliability of

meeting firm commitment,the average shortfall, the unit operations and usage of the reservoirs.

Figure 8 shows the reliability results for a 25 hour per week on-peak delivery of 1000,2000, 3000 and 4000 MW and the associated wind farm capacity required to achieve

various reliability levels. For example, results indicated that to achieve a 95% reliability

level, in terms of delivering 1000 MW for a 25-hour on-peak weekly commitment,approximately 600 MW of wind would be required.

The reliabilities curve asymptotically at higher reliability levels due to the magnitude of

wind required to meet firmdemand at that level. This

trend can be attributed to

specific weeks during theyear when the wind is

unusually low and the

reservoirs are completelydepleted. These specific

periods of time are seen to

require significant

additional wind capacity toincrease firm reliability.

The number of shortfalls

encountered during on-peakhours over the duration of

the yearlong simulation and their associated magnitude, were sorted and categorized into

a plot as presented in Figure 9 showing probability of occurrence and average shortfall

for the various wind farm capacities modeled. With increasing wind supply the average

shortfall and likelihood of occurrence decreases.


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For all simulations, the

operation of the pump

storage plant was

essentially optimized to

minimize the number of

shortfalls, given the windinput energy and the firm

delivery requirement. A

typical operation, in terms

of generation and pumping

operations over a week

long period for the 25-

hour 1000 MW firm

requirement is shown in

Figure 10.

The capability of the pumped storage plant to firm wind energy is further illustrated in

Figure 11, which shows the wind energy input over a week long period along with the net

energy output. The on-off shaping is visibly present on the output side, while being

totally absent in the raw wind input energy.

Upper and Lowerreservoir utilization is

recorded throughout the

yearlong simulationperiod. Figure 12 shows

the upper reservoir usage

for a 2-week period for the25-hour 1000 MW case.In the cases modeled

herein the storage of the

reservoirs was set and theoperations limited by the

storage capability defined.

Conclusions

Storage of wind energy has been the focus of considerable attention over the last few

years. A number of options have been considered such as the use of batteries, compressedair energy storage technology, fly wheel inertia etc. Storage can be an important

component of any generation system when export of the wind energy, as part of a firm

commitment, is required. Pumped storage is the most established technology,particularly when considering large amounts of energy storage. However, pumped

storage may also incur the highest initial capital costs and investment requirement.


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As part of a study to assess the coordination of operations, between proposed wind farms

and PSS facilities inLesotho, an analysis was

performed to determine the

likely operations, reservoir

usage, reliability of firmdelivery and average

shortfall for severalproposed systems. In the

analysis, a review of the

coordinated operations of alarge scale wind resource

combined with a pump

storage facility ranging

from 1000 to 4000 MW waspresented using an optimization model to project the level of coordination. Estimates on

the size of wind farm, for specific pumped storage facility sizes, was determined forseveral reliability levels.

This information was subsequently used to assess the economic viability of the proposed

works.

Dr. Hurdowar is Director of Power and Water Optimization, Hatch, Canada, with over 20 years of

engineering experience. Her most recent emphasis has been on hydro operations which includes

optimization analyses for power generation, power system planning, energy deregulation, relicensing, and

most notably coordination between renewable power sources and hydro-thermal coordination.

Mr. Matzner is principal power consultant, Hatch, South Africa. Dieter leads the renewable power team of

Hatch in Africa with its' main focus of developing wind, solar (PV, CPV and CSP) and hydro-electric

projects for clients.

Dr. Welt is a Mechanical Engineer specialized in the development of software solutions for the hydro

industry. He has held lead technical positions in the design and implementation of water management

systems for electricity producers.

Documents

Pumped Storage Speaker 9655 Session 664 1