Week 12 Assignment
Bryan Tiong
Renewable energy is often not available all the times. So, one must store energy for use
when desired. Describe briefly different energy storage technologies available with special
focus on efficient storage of wind and solar energy at the small and large scales.
Energy Storage
People still need electricity when the wind isn't blowing and the sun isn't shining, which is why
renewable energy developers are increasingly investing in energy storage systems. They
need to sop up excess juice and release it when needed.
However, storing large amounts of energy, whether it's in big batteries for electric cars or
water reservoirs for the electrical grid, is still a young field. It presents challenges, especially
with safety.
The most recent challenge first appeared in May, three weeks after a safety crash test on the
Chevrolet Volt, General Motors Co.'s plug-in hybrid. The wrecked vehicle caught fire on its
own in a storage facility, raising questions about its lithium-ion battery.
Motivation for energy storage
1.
2.
3.
4.
5.
6.
7.
Variation in energy demand
Variations in energy supply
Interruptions in energy supply
Transmission congestion
Demand for portable energy
Efficiency of energy system
Energy recovery
Energy storage of solar energy
Solar energy is not available at night, and energy storage is an important issue because
modern energy systems usually assume continuous availability of energy.
Thermal mass systems can store solar energy in the form of heat at domestically useful
temperatures for daily or seasonal durations. Thermal storage systems generally use readily
available materials with high specific heat capacities such as water, earth and stone. Welldesigned systems can lower peak demand, shift time-of-use to off-peak hours and reduce
overall heating and cooling requirements.
Phase change materials such as paraffin wax and Glauber's salt are another thermal storage
media. These materials are inexpensive, readily available, and can deliver domestically
useful temperatures (approximately 64 °C). The "Dover House" (in Dover, Massachusetts) was
the first to use a Glauber's salt heating system, in 1948.
Solar energy can be stored at high temperatures using molten salts. Salts are an effective
storage medium because they are low-cost, have a high specific heat capacity and can
deliver heat at temperatures compatible with conventional power systems. The Solar
Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m3 storage tank
with an annual storage efficiency of about 99%.
Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity.
With grid-tied systems, excess electricity can be sent to the transmission grid, while standard
grid electricity can be used to meet shortfalls. Net metering programs give household
systems a credit for any electricity they deliver to the grid. This is often legally handled by
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Bryan Tiong
'rolling back' the meter whenever the home produces more electricity than it consumes. If
the net electricity use is below zero, the utility is required to pay for the extra at the same rate
as they charge consumers.[103] Other legal approaches involve the use of two meters, to
measure electricity consumed vs. electricity produced. This is less common due to the
increased installation cost of the second meter.
1. Molten Salt Technology (Large Scale)
*Solar powered molten salt technology
Molten salt can be employed as a thermal energy storage method to retain thermal energy
collected by a solar tower or solar trough so that it can be used to generate electricity in
bad weather or at night. It was demonstrated in the Solar Two project from 1995-1999. The
system is predicted to have an annual efficiency of 99%, a reference to the energy lost by
storing heat before turning it into electricity, versus converting heat directly into
electricity. The molten salt mixtures vary. The most extended mixture contains sodium
nitrate, potassium nitrate and calcium nitrate. It is non-flammable and nontoxic, and has
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already been used in the chemical and metals industries as a heat-transport fluid, so
experience with such systems exists in non-solar applications.
The salt melts at 131 °C (268 °F). It is kept liquid at 288 °C (550 °F) in an insulated "cold"
storage tank. The liquid salt is pumped through panels in a solar collector where the focused
sun heats it to 566 °C (1,051 °F). It is then sent to a hot storage tank. This is so well insulated
that the thermal energy can be usefully stored for up to a week.
When electricity is needed, the hot salt is pumped to a conventional steam-generator to
produce superheated steam for a turbine/generator as used in any conventional coal, oil or
nuclear power plant. A 100-megawatt turbine would need a tank of about 30 feet (9.1 m)
tall and 80 feet (24 m) in diameter to drive it for four hours by this design.
2. Off grid photovoltaic (PV) system (Small Scale)
Solar cells produce direct current (DC) power which fluctuates with the sunlight's intensity. For
practical use this usually requires conversion to certain desired voltages or alternating current
(AC), through the use of inverters. Multiple solar cells are connected inside modules. Modules
are wired together to form arrays, then tied to an inverter, which produces power at the
desired voltage, and for AC, the desired frequency/phase.
Many residential systems are connected to the grid wherever available, especially in
developed countries with large markets. In these grid-connected PV systems, use of energy
storage is optional. In certain applications such as satellites, lighthouses, or in developing
countries, batteries or additional power generators are often added as back-ups.
Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity.
With grid-tied systems, excess electricity can be sent to the transmission grid. Net
metering programs give these systems a credit for the electricity they deliver to the grid. This
credit offsets electricity provided from the grid when the system cannot meet demand,
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Bryan Tiong
effectively using the grid as a storage mechanism. Credits are normally rolled over month to
month and any remaining surplus settled annually.
Different chemicals can be combined to make batteries. Some combinations are low cost
but low power. Also, others can store huge power at huge prices. Lead-acid batteries offer
the best balance of capacity per dollar and it's a common battery used in stand-alone
power systems.
The lead-acid battery cell consists of positive and negative lead plates of different
composition suspended in a sulphuric acid solution called electrolyte. When cells discharge,
sulphur molecules from the electrolyte bond with the lead plates and releases electrons.
When the cell recharges, excess electrons go back to the electrolyte. A battery develops
voltage from this chemical reaction. Electricity is the flow of electrons.
In a typical lead-acid battery, the voltage is approximately 2 volts per cell regardless of cell
size. Electricity flows from the battery as soon as there is a circuit between the positive and
negative terminals. This happens when any load (appliance) that needs electricity is
connected to the battery.
In 2012, solar panels available for consumers can have a yield of up to about 17%, while
commercially available panels can go as far as 27%. Single p-n junction crystalline silicon
devices are now approaching the theoretical limiting power efficiency of 33.7%,
Energy storage of wind energy
1. Flywheel energy storage method (Small scale)
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*Components of a flywheel motor
* An example of a flywheel generator
Flywheels are mechanical devices that use inertia to store and cycle energy. When
used with wind turbines, wind power is used to generate electricity used to spin the
flywheels. Stored energy capacity can be increased by increasing the speed of the
flywheel's rotation or by adding additional flywheels. This technology is still in the
developmental stage for wind energy storage. It is being used in some small-scale
operations. Flywheels may be used to store energy generated by wind turbines during
off-peak periods or during high wind speeds. The efficiency of the system depends on
the presence of high wind speeds.
2. Pumped-storage hydroelectricity (PSH)
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*An example of an ‘Energy Storing Wind Dam’
*An example of a pumped-storage power plant.
Pumped hydro storage uses wind turbines to power pumps that move water from the
lower to upper reservoirs during periods of low energy demand. Energy is then
generated by releasing water through hydro turbines into lower reservoirs during
periods of higher demand. This process requires at least 400 feet of geographic
elevation variance to create the needed higher and lower reservoirs. However,
technologies using underground reservoirs or reservoirs at equal elevations are being
explored. The potential energy storage capacity with this method is limited by the size
of both the upper and lower reservoirs. This method is often carry out in large scale
operations. PSH reported energy efficiency varies in practice between 70% and
80%, with some claiming up to 87%.
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3. Compressed air energy storage
*Example of CAES
Compressed air storage technologies use wind energy to compress and store air in
chambers beneath the ground. The air is stored until demand increases. Then it is
released into a gas turbine, where it is mixed with natural gas and burned to produce
electricity. For this system to work, turbines must be placed where geology offers
adequate underground chambers. The size of the chambers, generally caves or
abandoned mines, dictates the amount of energy that can be stored. Large scale
operations can increase the efficiency of CAES.
Energy storage systems often use large underground caverns. This is the preferred
system design, due to the very large volume, and thus the large quantity of energy
that can be stored with only a small pressure change. The cavern space can be
easily insulated, compressed adiabatically with little temperature change
(approaching a reversible isothermal system) and heat loss (approaching an
isentropic system). This advantage is in addition to the low cost of constructing the
gas storage system, using the underground walls to assist in containing the pressure.
There are three ways in which a CAES system can deal with the heat. Air storage can
be adiabatic, diabatic, or isothermal:
i.
Adiabatic storage retains the heat produced by compression and returns it to
the air when the air is expanded to generate power. The efficiency is around
70%
ii.
Diabatic storage dissipates much of the heat of compression with intercoolers
into the atmosphere as waste; essentially wasting, thereby, the renewable
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iii.
Bryan Tiong
energy used to perform the work of compression. Diabatic storage have a 54%
thermal efficiency
Isothermal compression and expansion approaches attempt to
maintain operating temperature by constant heat exchange to the
environment. They are only practical for low power levels, without very
effective heat exchangers. The theoretical efficiency of isothermal energy
storage approaches 100% for perfect heat transfer to the environment. In
practice neither of these perfect thermodynamic cycles are obtainable, as
some heat losses are unavoidable.
4. Flow Batteries
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*Examples of flow battery.
According to energy storage consultant Petra de Boer, flow batteries are a flexible
energy storage option--adaptable to high-capacity storage as well as high-voltage
applications. Flow batteries produce energy through a chemical reaction of
electrolytes pumped through a reactor cell from exterior storage chambers. The
process is similar to that within fuel cells. This type of storage is not yet viable for largescale operations, but may be in the future as technologies evolve.
References
1. http://en.wikipedia.org/wiki/Solar_energy#Energy_storage_methods
2. http://en.wikipedia.org/wiki/The_Solar_Project#Solar_Two
3. http://www.amgs.or.kr/Eng/property/board/sub_4_9/Lesson%2012%20Ener
gy%20Storage.pdf
4. http://www.solardirect.com/pv/batteries/batteries.htm
5. http://www.ehow.com/list_5984642_types-turbine-energy-storagetechnology.html
6. http://en.wikipedia.org/wiki/Flywheel_energy_storage#Wind_turbines
7. http://en.wikipedia.org/wiki/Flywheel_energy_storage
8. http://www.eenews.net/public/climatewire/2011/11/30/1
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