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Transcript of Pumped Storage Speaker 9655 Session 664 1
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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.