EE535: Renewable Energy:
Systems, Technology &
Economics
Energy Storage
Introduction to Storage
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Energy storage is the conversion of energy to other forms so that it may
be used at a later stage when required.
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Industrial scale energy storage technologies are measured in terms of
their power output (MW) and their capacity (MWh).
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The most common type of energy storage is ‘Spinning Reserve’ where
power plants operate at a level slightly below the point where they begin
to produce power. This allows them to react quickly to large swings in
demand.
The spinning reserve is the extra generating capacity that is available by
increasing the power output of generators that are already connected to
the power system. For most generators, this increase in power output is
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achieved by increasing the torque applied to the turbine's rotor.
Comparing Storage Technologies
• Most Important factors
– Cost & Return on Investment
– Efficiency
– Cycles
• Influential Factors
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Response Time (milliseconds to hours/days)
Energy Density (Size and Space Requirements)
Environmental Impact
Charge Time
Intermittent Wind Supply
Yearly, Bi-Monthly & Monthly Wind Output (MW)
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www.inference.phy.cam.ac.uk/withouthotair/c26/page_187.shtml
Wind is not a controllable energy
source, it is either blowing or it
isn’t and this power is distributed
to the electricity grid regardless of
whether it is needed or not.
The grid cannot store energy so
power plants have to match the
demand in real time.
Often several power plants will
operate in ‘Spinning Reserve’
mode during periods of high wind
output to cater for the inevitable
drop in wind.
Source: Eirgrid
•Typical 7 day Winter Demand profile (Saturday/Sunday generally lowest)
•Wind Output for same time period has no correlation to demand
•Issues with excess wind generation between 12.30 AM – 6AM
Storage Technologies – Flywheel Energy Storage
http://www.upei.ca/~physics/p261/projects/flywheel2/flywheel2.htm
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Flywheels are mechanical devices
which accelerate a rotor to a very
high speed, storing the energy as
rotational energy.
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These devices are most suitable
for high utilization applications as
their efficiency decreases in
proportion to their use.
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Suitable for short term lulls in wind
generation output (1-5min)
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Rated for > 100,000 cycles
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Used in F1 racing cars (0.1kWh,
4.6Wh/kg)
Storage Technologies – Flow Batteries
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http://www.netpowertech.com/e_menu_page.aspx?bid=4&id=10
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Flow Batteries allow energy to be
stored in two solutions existing at
different electrochemical
potentials.
All the energy is stored in the
electrolyte solutions so that the
capacity is determined by the size
of the electrolyte tanks
The system power is determined
by the size of the fuel cell stacks
which can be arranged in
series/parallel as required.
Suitable for daily lulls in wind
generation output (< 12hours)
Rated for > 10,000 Cycles
Storage Technologies – Pumped Storage
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www.hk-phy.org/energy/alternate/hydro_phy/images/pump_storage.gif
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Pumped storage is one of the
oldest energy storage methods.
During the night cheap electricity
is used to pump water into the
upper reservoir.
It is then released during the day
and peak times when it is more
valuable.
Additional features such as
frequency regulation and blackstart capabilities.
Pumped storage installations are
capable of providing power for
extended periods, from hours to
several days.
Rated for > 30 years
Establishing Volume of Water Required
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When designing a pumped
storage facility for a given capacity
the two most important factors are
water volume and height.
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If the height is doubled, only half
the water volume is needed for the
same power capacity.
Top Reservoir
Volume &
Density (p)
Elevation/Head (h)
Power
• Volume = gh / sec
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Pumps/Turbines
Efficiency (ɛ)
Lower Reservoir
Where ‘p’ is the density of water, ‘g’ is
acceleration due to gravity, ‘h’ is the
height drop and ‘ɛ’ is the efficiency of
the turbines/pumps.
Example
• A pumped storage facility is proposed for construction in the
west of Ireland to store excess power generated from wind
farms in the area. A potential location has been identified with a
head height of 250m between the upper and lower lakes. The
latest Siemens turbines with an efficiency of 90% will be utilised
in the construction.
What volume of water is required to store 12GWh ?
Solution
Power
Volum e/ sec 
gh
12x109
Volum e/ sec 
(1000)(9.81)(250)(0.9)
12x109
Volum e/ sec 
(2207250)
Volum e/ sec  5437m3
Volum e(total)  (5437m3 )(3600)
Volum e(total)  19.6 x106 m3
Proposed Storage Techniques
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Electric Car Grid Tie
– Current generation electric vehicles are capable of storing from 5kWh
up to 30kWh of capacity. Suggestions have been made that this
capacity could be made available to the grid in conjunction with smart
metering.
Hydrogen Production
– The process of extracting hydrogen from water is currently very
inefficient . Energy from renewable energy sources could be used to
create hydrogen and store it until it is needed for power generation or
for vehicle fuel.
Super Capacitors
– These devices can be used for storing small amounts of electrical
energy in applications where hundreds of thousands of cycles would
be common. Area’s such as regenerative braking in vehicles with
many stops/starts are suitable for these devices.
Turlough Hill (Wicklow Mountains)
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http://www.esb.ie/img/2.3.1_Turlough-Hill_800x600.jpg e
4x73MW Turbines
292MW for 5 hours
Head ~280m
Provides:
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Load levelling
Peak Shaving
Frequency Regulation
Black Start capability
• Running since 1974