As an energy engineer you have been asked to work in the field grid based energy

Question

As an energy engineer you have been asked to work in the field grid based energy storage systems;

your brief is as follows:

i. Explain the outline principles of operation of such systems and why they are used i.e. advantages.

ii. Review the mechanical design features materials, construction and so on of two types of energy storage system.

iii. Give examples of energy storage schemes in operation.

iiii. Design; produce an outline a design for energy storage system based on existing the technology that will deliver 0.5 MVA on a short term basis (you can choose the short term period based on an evaluation of the constraints that would be in place) to a 50 Hz grid system.

v. Summary

(I need in depth explanation please).

Explanation / Answer

Energy storage is the capture of energy produced at one time for use at a later time. A device that stores energy is sometimes called an accumulator or battery. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped.

Some technologies provide short-term energy storage, while others can endure for much longer.

A wind-up clock stores potential energy (in this case mechanical, in the spring tension), a rechargeable battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. Food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.

Ice storage tanks store ice frozen by cheaper energy at night to meet peak daytime demand for cooling. The energy isn't stored directly, but the work-product of consuming energy (pumping away heat) is stored, having the equivalent effect on daytime consumption.

Grid energy storage (also called large-scale energy storage) is a collection of methods used to store electrical energy on a large scale within an electrical power grid. Electrical energy is stored during times when production (especially from intermittent power plants such as renewable electricity sources such as wind power, tidal power, solar power) exceeds consumption, and returned to the grid when production falls below consumption.

As of 2017, the largest form of grid energy storage is dammed hydroelectricity, with both conventional hydroelectric generation as well as pumped storage. Alternatives include rail potential energy storage, where rail cars carrying 300 ton weights are moved up or down a 8 mile section of inclined rail track, storing or releasing energy as a result or disused oil well potential energy storage, where 100 ton weights are raised or lowered in a 12,000 ft deep decommissioned oil well.

Benefits of storage and managing peak load

The stores are used – feeding power to the grids – at times when consumption that cannot be deferred or delayed exceeds production. In this way, electricity production need not be drastically scaled up and down to meet momentary consumption – instead, transmission from the combination of generators plus storage facilities is maintained at a more constant level.

An alternate and complementary approach to achieve the similar effect as grid energy storage is to use a smart grid communication infrastructure to enable Demand response. These technologies shift electricity consumption and electricity production from one time (when it's not useful) to another (when it's in demand).

Any electrical power grid must match electricity production to consumption, both of which vary drastically over time. Any combination of energy storage and demand response has these advantages:

fuel-based power plants (i.e. coal, oil, gas, nuclear) can be more efficiently and easily operated at constant production levels
electricity generated by intermittent sources can be stored and used later, whereas it would otherwise have to be transmitted for sale elsewhere, or shut down
peak generating or transmission capacity can be reduced by the total potential of all storage plus deferrable loads (see demand side management), saving the expense of this capacity
more stable pricing – the cost of the storage or demand management is included in pricing so there is less variation in power rates charged to customers, or alternatively (if rates are kept stable by law) less loss to the utility from expensive on-peak wholesale power rates when peak demand must be met by imported wholesale power
emergency preparedness – vital needs can be met reliably even with no transmission or generation going on while non-essential needs are deferred
Energy derived from solar, tidal and wind sources inherently varies – the amount of electricity produced varies with time of day, moon phase, season, and random factors such as the weather. Thus, renewables in the absence of storage present special challenges to electric utilities. While hooking up many separate wind sources can reduce the overall variability, solar is reliably not available at night, and tidal power shifts with the moon, so slack tides occur four times a day.

How much this affects any given utility varies significantly. In a summer peak utility, more solar can generally be absorbed and matched to demand. In winter peak utilities, to a lesser degree, wind correlates to heating demand and can be used to meet that demand. Depending on these factors, beyond about 20–40% of total generation, grid-connected intermittent sources such as solar power and wind turbines tend to require investment in grid interconnections, grid energy storage or demand side management.

In an electrical grid without energy storage, generation that relies on energy stored within fuels (coal, biomass, natural gas, nuclear) must be scaled up and down to match the rise and fall of electrical production from intermittent sources (see load following power plant). While hydroelectric and natural gas plants can be quickly scaled up or down to follow the wind, coal and nuclear plants take considerable time to respond to load. Utilities with less natural gas or hydroelectric generation are thus more reliant on demand management, grid interconnections or costly pumped storage.

Demand side management and grid storage

A sense of units and scale for electrical energy production and consumption
The demand side can also store electricity from the grid, for example charging a battery electric vehicle stores energy for a vehicle and storage heaters, district heating storage or ice storage provide thermal storage for buildings. At present this storage serves only to shift consumption to the off-peak time of day, no electricity is returned to the grid.

The need for grid storage to provide peak power is reduced by demand side time of use pricing, one of the benefits of smart meters. At the household level, consumers may choose less expensive off-peak times for clothes washer/dryers, dishwashers, showers and cooking. As well commercial and industrial users will take advantage of cost savings by deferring some processes to off-peak times.

Regional impacts from the unpredictable operation of wind power has created a new need for interactive demand response, where the utility communicates with the demand. Historically this was only done in cooperation with large industrial consumers, but now may be expanded to entire grids. For instance a few large scale projects in Europe link variations in wind power to change industrial food freezer loads, causing small variations in temperature. If communicated on a grid-wide scale, small changes to heating/cooling temperatures would instantly change consumption across the grid.

Energy storage – At a glance
Purpose of energy storage
Energy storage is the storage of some form of energy that can be drawn upon at a later time to
perform some useful operation. All forms of energy are either potential energy, chemical or
gravitational energy:
A wind up clock stores potential energy (in this case mechanical, in the spring tension);
A battery stores readily convertible chemical energy to keep a clock chip in a computer
running even when the computer is turned off; and
A hydroelectric dam stores power in a reservoir as gravitational potential energy.
Energy storage became a dominant factor in economic development with the widespread
introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural
gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal,
electricity must be used as it is generated.
Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored
as electrical energy. This meant that changes in demand could not be accommodated without either
cutting supplies (e.g., blackouts) or arranging for a storage technique.
Paving the way for renewable base-load energy
Many renewable energy technologies such as solar and wind energy cannot be used for baseload
power generation as their output is much more volatile and depends on the sun, water
currents or winds. Batteries and other energy storage technologies therefore become key
enablers for any shift to these technologies.
The power storage sector generally includes traditional batteries, but also covers hydrogen fuel cells
and mechanical technologies like flywheels that are straight potential replacements for batteries. More
and more research is also conducted in the field of nanotechnology as ultra-capacitors (high energy,
high power density electrochemical devices that are easy to charge and discharge) and nanomaterials
could significantly increase the capacity and lifetime of batteries.
For more detailed information on nanotechnology and nanomaterials, you may consult the forthcoming
research report by Mora Associates Ltd. “Nanotechnology in the cleantech sector”.
Grid energy storage
Grid energy storage lets electric energy producers send excess electricity over the electricity
transmission grid to temporary electricity storage sites that become energy producers when
electricity demand is greater, optimizing the production by storing off-peak power for use
during peak times. Also, photovoltaic and wind turbine users can avoid the necessity of having
battery storage by connecting to the grid, which effectively becomes a giant battery.
Photovoltaic operations can store electricity for the night’s use, and wind power can be stored for calm
times. It is particularly likely that pumped storage hydropower will become especially important as a
balance for very large-scale photovoltaic generation.

Potential technologies for grid storage
The use of underground reservoirs as lower dams has been investigated. Salt mines could be used,
although ongoing and unwanted dissolution of salt could be a problem. If they prove affordable,
underground systems could greatly expand the number of pumped storage sites. Saturated brine is
about 20% more dense than fresh water.
Storage methods
Chemical energy storage
Chemical fuels have become the dominant form of energy storage, both in electrical generation and
energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel fuel, natural
gas, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel and hydrogen. All of these
chemicals are readily converted to mechanical energy and then to electrical energy using heat engines
that used for electrical power generation.
Liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation.
Unfortunately, these produce greenhouse gases when used to power cars, trucks, trains, ships and
aircraft. Carbon-free energy carriers, such as hydrogen and some forms of ethanol or biodiesel, are
being sought in response to concerns about the consequences of greenhouse gas emissions.
Hydrogen
Hydrogen is a chemical energy carrier, just like gasoline, ethanol or natural gas. The unique
characteristic of hydrogen is that it is the only carbon-free or zero-emission chemical energy carrier.
Hydrogen is a widely used industrial chemical that can be produced from any primary energy source.
Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible. Such
production will require more energy than is currently being used, and require large capital investment
in hydrogen production plants. Because of the increased costs, hydrogen is not yet in widespread use.
If hydrogen production costs were to be reduced, hydrogen fuels may become more attractive
commercially, providing clean, efficient power for our homes, businesses and vehicles.
Biofuels
Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to
replace hydrocarbon fuels. Various chemical processes can convert the carbon and hydrogen in coal,
natural gas, plant and animal biomass, and organic wastes into short hydrocarbons suitable as
replacements for existing hydrocarbon fuels.
Electrochemical energy storage
An early solution to the problem of storing energy for electrical purposes was the development of the
battery, an electrochemical storage device. It has been of limited use in electric power systems due to
small capacity and high cost.
Batteries
A battery is a device that transforms chemical energy into electric energy. All
batteries have three basic components in each cell – an anode, a cathode, and
an electrolyte and their properties relate directly to their individual chemistries.
Batteries are broadly classified into primary and secondary.
Primary batteries are the most common and are designed as single use batteries, to be discarded or
recycled after they run out. They have very high impedance1 which translates into long life energy
storage for low current loads. The most frequently used batteries are carbon-zinc, alkaline, silver
oxide, zinc air, and some lithium metal batteries.Secondary batteries are designed to be recharged and can be recharged up to 1,000 times depending
on the usage and battery type. Very deep discharges result in a shorter cycle life, whereas shorter
discharges result in long cycle life for most of these batteries. The charge time varies from 1 to 12
hours, depending upon battery condition other factors. Commonly available secondary batteries are
nickel-cadmium (NiCad), lead-acid, nickel metal-hydride (NiMH) and lithium-ion (Li-Ion) batteries.
Some of the limitations posed by secondary batteries are limited life, limited power capability, low
energy-efficiency, and disposal concerns Fuel cells
Fuel cells were invented about the same time as the battery. However, fuel cells were not well
developed until the advent of spacecrafts when lightweight, non-thermal sources of electricity were
required. Fuel cell development has increased in recent years to an attempt to increase conversion
efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity.
Like a battery, a fuel cell uses stored chemical energy to generate power. Unlike batteries, its energy
storage system is separate from the power generator. It produces electricity from an external fuel
supply as opposed to the limited internal energy storage capacity of a battery. [2]
Electrical energy storage
Capacitor
Capacitors use physical charge separation between two electrodes to store charge. They store energy
on the surfaces of metalized plastic film or metal electrodes.
When compared to batteries and supercapacitors, the energy density of capacitors is
very low – less than 1% of a supercapacitor's, but the power density is very high, often
higher than that of a supercapacitor. This means that capacitors are able to deliver or
accept high currents, but only for extremely short periods, due to their relatively low
capacitance. [2]
Supercapacitor
Supercapacitors are very high surface area activated carbon capacitors that use a molecule-thin layer
of electrolyte, rather than a manufactured sheet of material, as the dielectric to separate charge. The
supercapacitor resembles a regular capacitor except that it offers very high capacitance in a small
package. Energy storage is by means of static charge rather than of an electrochemical process
inherent to the battery. Supercapacitors rely on the separation of charge at an electrified interface that
is measured in fractions of a nanometer, compared with micrometers for most polymer film capacitors.
The lifetime of supercapacitors is virtually indefinite and their energy efficiency rarely falls below 90%
when they are kept within their design limits. Their power density is higher than that of batteries while
their energy density is generally lower. However, unlike batteries, almost all of this energy is available
in a reversible process.
Superconducting magnetic energy storage (SMES)
In a SMES system, energy is stored within a magnet that is capable of releasing megawatts of power
within a fraction of a cycle to replace a sudden loss in line power. It stores energy in the magnetic field
created by the flow of direct current (DC) power in a coil of superconducting material that has been

The stored energy can be released back to the network by discharging the coil. The power
conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct
current or convert DC back to AC power. The inverter/rectifier accounts for about 2-3% energy loss in
each direction. SMES loses the least amount of electricity in the energy storage process compared to
other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater
than 95%. [14]
Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES is
currently used for short duration energy storage. These systems have been in use for several years to
improve industrial power quality and to provide a high-quality service for individual customers
vulnerable to voltage fluctuations.
SMES systems are typically installed on the exit of the power plants to stabilize output or on industrial
sites where they can be used to accommodate peaks in energy consumption (e.g. steel plants or rapid
transit railway) cryogenically cooled

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