Energy Systems - Energy Storage

pumped hydro storage (PHS) [mechanical]

1. Dominates all forms of energy storage, with over 150 GW representing >95% of storage capacity 2. Surplus generation capacity used to pump water from a lower reservoir to a higher reservoir. When demand grows, the water is released back to the lower reservoir through a turbine to generate electricity. There are reversible turbines that can also act as pumps that serve well for pumped storage. Round-trip efficiency (RTE) is on the order of 75-80%.

Non-Dispatchable Power

A power system that cannot be scheduled to produce power when needed Many renewable energy sources like solar PV, wind, tidal power, and wave power fall into this category of non-dispatchable. You get the energy when it is available

fuel cell

An electrical-chemical device that converts fuel, such as hydrogen, into an electrical current.

Rechargable batteries [Electrochemical]

Batteries consist of 3 key components: the anode, cathode, and electrolyte. At the anode (the negative terminal), oxidation reactions occur that release free electrons. These electrons flow through an external wire (where they can be supplied to the grid or used to run devices) to the cathode (positive terminal) where reduction reactions occur. The electrolyte allows a flow of ions to balance the buildup of positive charge in the battery at the anode and negative charge at the cathode that would otherwise degrade performance. During recharging, these reactions are reversed to return the battery to its original state and the ions in the electrolyte return to the anode. There are many different battery technologies, the best known of which are Lead-acid, Nickel Metal Hydride, and Lithium-ion. Lithium-ion batteries have the advantages of high storage capacity to weight ratio and high power delivery, both of which make them well suited to automotive use. However, they have a risk of fire if they are overcharged or overheated (especially if damaged), contain toxic materials, and are hard to recycle

Interconnection

Electricity storage can be connected to: 1) the high voltage transmission network to help integrate large scale wind and solar PV farms and to provide frequency and voltage support; 2) the medium-voltage distribution network for frequency and voltage support, peak shaving, and emergency backup power at sites of large electricity consumption; and 3) the low voltage network to integrate rooftop renewables and shift loads to lower peak demand and save money

Frequency and Voltage support

Fast-responding energy storage like batteries can even be used to help balance very short time scale fluctuations in the balance between generation and demand which could otherwise destabilize the electrical grid.

Flow Batteries / regenerative fuel cells

Instead of having an anode, cathode, and electrolyte like a rechargeable battery, a flow battery has a liquid anolyte (in one tank), a liquid catholyte (in a second tank), and an ion-selective membrane held between two electrodes. The anolyte and catholyte are pumped across their respective side of the ionselective membrane (which consumes a little of the electricity of the battery, one disadvantage of this kind of battery). Electrons flow from the anolyte side of the flow battery out to the grid and back to the catholyte side and ions pass through the membrane to balance the charge. Energy storage capacity is a function of the electrolyte (anolyte and catholyte) volume, while power is a function of the electrode surface area. This "decoupling" of storage capacity and power delivery is an advantage for flow batteries over rechargeable batteries. In rechargeable batteries, you need more or bigger batteries to scale up, which is expensive. If you want more storage capacity for a flow battery, you just need bigger electrolyte tanks, which is not so expensive. Also, flow batteries can be recharged indefinitely (either using electricity or by replacing the electrolytes) unlike rechargeable batteries which only last a few thousand cycles. Two types of flow batteries you may hear about are Vanadium Redox and Zinc Bromide. The chemistry is too detailed to go into here. There are many different chemistries under development. Costs are coming down and flow batteries may be a good commercial option for bulk storage of electricity versus lithium-ion, especially for longer duration, stationary (not in cars) applications.

cryogenic storage [thermal]

Large amounts of energy can be stored by using surplus electricity to cool air (after removing water vapor and CO2) to negative 195 Celsius, at which point it becomes a liquid, reducing in volume by a factor of roughly 1000. Then when the electricity is needed, the liquid air is expanded back into a gas. The resulting high pressure air is used to run a turbine-generator. While the liquefaction and expansion to generate electricity on its own is not very efficient, efficiency is increased by recycling the heat taken out during liquefaction and the cold produced by expansion. Efficiency is further improved when waste heat or waste cold are available to supplement the process. The advantages of cryogenic storage is that it uses long-lived and well-understood components and can in principle be located anywhere (unlike pumped hydro or compressed air energy storage).

Load shifting

One of the most obvious applications for energy storage, even in the absence of renewable energy, is to charge storage during offpeak hours when electricity is cheap and to discharge the storage during peak hours in place of running peaker plants. This strategy flattens out the net load curve, making it easier to manage

how can you make intermittent renewable energy dispatchable?

Storing intermittent renewable energy makes it dispatchable and a potential candidate for contributing to base load and providing load-following or peak load power.

peak load power

Supplied during times of high demand by smaller units that are used infrequently (and therefore supply more expensive electricity) and are generally less efficient simple turbines (not combined cycle). Peak load today is supplied by fast-starting generators (called "peaker plants") like gas turbines or combustion engines and pumped storage.

base load power

The minimum level of electricity demand over time span supplied by large power plants with high capacity factors that supply inexpensive electricity, like coal, nuclear, geothermal, biomass, combustion, and hydroelectric

intermittent renewable energy source are mostly used as ____today?

Today, intermittent renewable energy sources are mostly used to displace fuel that would otherwise be consumed by non-renewable power stations.

renewable integration or renewable firming

Using storage to manage intermittent generation is called renewable integration or renewable firming, where "firming" means making intermittent renewables reliable and dispatchable.

Flywheels [mechanical]

a motor-generator is used to spin a mass at very high speed (tens of thousands of rpm), storing excess electricity as rotational (kinetic) energy. Friction can be reduced by magnetic (frictionless) bearings and enclosing the flywheel in a vacuum to eliminate air resistance. They have the advantage that they have very long lifetimes (unlike batteries, their life is not shortened by discharge/recharge cycles). However, care must be taken to reinforce the containing structure or locate it underground since failure can be catastrophic. They are typically 4 used for very short term balancing of electricity supply and demand (called "frequency regulation"). Not widely used for grid energy storage

load following power plant

a power plant that runs intermittently to meet increases in demand use generators like gas turbines, combined cycle gas turbines, and engines running on diesel or gas to ramp generation up or down

Hydrogen [electrochemical]

always bonded to something else, requiring energy to separate it and compress it for storage to improve energy density. To store surplus renewable energy without emitting carbon, a good approach is electrolysis (splitting of water using electricity). This splitting is done using one of two approaches: alkaline electrolysis and Proton Exchange Membrane (PEM) electrolysis. Efficiencies are now upwards of 70% but producing Hydrogen this way is still relatively expensive (especially PEM electrolysis which relies on expensive platinum as a catalyst). Remember, a catalyst is a material that makes a reaction go faster but is not consumed. There is a great search underway for substitutes for platinum that perform as well but cost much less. PEM electrolysis in principle is like running a fuel cell backwards, using electricity to split water into oxygen and hydrogen rather than generating a flow of electricity and joining the hydrogen and oxygen together to make water and heat

Chilled water/ice storage [thermal]

an effective, low cost way of shedding air conditioning load that stresses the grid during heat waves. Ice is made at night when electricity is cheap and temperatures are cooler. Then this ice is used to cool buildings during the hot hours rather than using electricity to run air conditioners. The electrical load for cooling is effectively shifted from mid-day peak hours to nighttime off-peak hours

intermittent energy sources

any source of energy that is not continuously available due to some uncontrollable external factor

Molten Salt [thermal]

best known for storing heat captured in concentrating solar power (CSP) generating plants. The salts (a mixture of sodium-, potassium-, and calcium-nitrate), are heated to between 500 and 600 degrees Celsius and stored in superinsulated tanks to avoid losses and can be used to create superheated steam to drive steam turbines. Typically, CSP plants with storage are designed to have enough molten salt to continue generating power for 4- 6 hours after the sun has gone down

why can't intermittent renewable energy be added to today's grid in large quantities without storage?

it will make the grid much harder to manage because supply and demand of electricity on the grid must always be balanced throughout the entire grid. Right now, large sudden short-term changes in wind or solar PV power must be handled by ramping gas turbines or other flexible generators up and down. Intermittent renewable energy could cause power to flow backward or cause voltages that are too high in the distribution grid (the low voltage grid that delivers power to end consumers). It is also possible that intermittent renewables could produce so much power that they lead to overgeneration: a situation where it would make sense to ramp coal and nuclear plants down, but it can't be done because it is technically not possible. In those situations, electricity prices can go negative (the utility pays for people to consume power) and the renewable power might need to be curtailed (turned down or off, meaning it isn't available to lower carbon dioxide emissions). Energy Storage can prevent this curtailment by absorbing excess generation and allowing it to be dispatched during shortfalls.

Gravitational Potential Energy Devices (non-PHS) [mechanical]

raising train cars up a hill using electric motors and then generating electricity through regenerative braking when the trains descend by using the electric motors as generators. This approach has been named Advanced Rail Energy Storage (ARES). We also saw an example with cranes stacking concrete blocks - the electric motors used to lift the blocks can also be used as generators when the blocks are lowered to the ground.

Electrolysis

splitting of water using electricity

Water heater thermal storage

the amount of energy stored depends on the temperature and volume of the water in the tank. The amount of insulation determines the loss of thermal energy over time. The stored heat can be used for hot water or space heating. Water is a good medium in which to store heat because it has a very high heat capacity, although the temperature is obviously limited because the water will boil at 100C