What is the most efficient type of energy storage?

Helen

Mar. 07, 2024

SEE INFOGRAPHIC: Energy storage technologies [PDF]

Currently, the following list shows the ways to accumulate energy and the main technologies that allow efficient energy transformation and storage:

Hydroelectric pumping

The most efficient large-scale storage system in operation. This is a cost-effective and proven technology that provides stability to the electrical system and can generate significant levels of clean energy with rapid response times.

These facilities have a reversible motor that, during periods of excess energy, ambient air is stored at high pressures in underground cubicles. It is a mechanical storage system comparable in capacity to hydroelectric pumping.

Thermal storage

It consists in accumulating energy in materials that allow it to be retained and released in a controlled manner, through methods ranging from cooling through ice accumulation to exposure to extremely high temperatures.

This is a device capable of storing large amounts of electrical energy in the form of electrostatic charges, meaning there are no chemical reactions. Supercapacitors can be charged or discharged in a matter of seconds, thus being ideal for responding to energy peaks or brief supply interruptions.

Flywheels

This is a mechanical storage system consisting of a metal disc that starts to spin when a torque is applied to it, then, with the action of braking the wheel applying a firm braking torque, electrical energy is conserved in kinetic form.

This is a device that stores energy in chemical compounds capable of generating electrical charge. There are many types, such as lead-acid batteries, lithium ion or nickel cadmium batteries. The main advantages of batteries are their rapid response (milliseconds), their ease of installation and scalability and, finally, the multiple benefits they can bring to renewable assets to which they connect.

This is a type of continuous chemical storage. It differs from batteries in that it is supplied continuously with hydrogen from the outside allowing its constant use. There are other types of fuel cells, but hydrogen is the most commonly-used fuel.

Lithium batteries: the future of storage

In recent years, the renewable energy sector has seen in lithium-ion batteries the solution to its main problem: the storage of generated energy. Being one of the smallest elements in the periodic table, lithium has a high electrochemical potential and can accumulate large amounts of energy. With the desirable low weight and high efficiency, only one obstacle has so far prevented lithium batteries from becoming the standard storage technology for renewable energy: their high cost.

This situation, however, seems to be changing. According to a recent study by Bloomberg NEF (BNEF), the cost of lithium-ion batteries will be significantly reduced in the coming years — beyond even the 85% reduction that occurred from 2010 to 2018. Specifically, BNEF predicts a 50% reduction in the costs of lithium-ion batteries per kW/h by 2030, as demand takes off in two different markets: stationary storage and electric vehicles.

This will lead to worldwide energy storage facilities multiplying exponentially, from modest 9GW/17GWh implemented from 2018 to 1,095 GW/2,850 GWh by 2040. This dramatic increase will require an estimated investment of US$662 billion.

5 ways of energy storage and how effective they are with regard to energy losses.

Energy sector’s future has always been associated with the fact that humanity will learn to accumulate energy effectively and to use it as necessary. The future is coming more confidently now and with different efficiency.

The efficiency of accumulating technologies is determined by the ratio of useful energy used to the total amount of energy received by the system. In other words, the efficiency of the batteries is evaluated in terms of energy losses during storage compared to the total amount of energy that was “poured into the system”.

Interestingly, that the assessment of batteries efficiency is often carried out in comparison with an external combustion engine, which has an efficiency of 23%, according to common data. In most cases, according to official information, of course, all energy storage systems show efficiency factors significantly higher than those of external combustion engines.

5 ways to store energy and how effective they are through the lens of “leaks”:

Lithium-ion batteries. “Fresh”, agile, expensive (it is becoming cheaper)

The most hyped type of energy storage. Thank you, Elon Musk. Energy storage in lithium-ion batteries is considered one of the most efficient. But for the time being, until the battery begins to degrade.

Pros: fast construction (Musk built the object in 100 days in Australia), almost instantaneous output of the stored energy to the network (tenths of a second).

Cons: price, degradation, disposal (its absence)

Tesla produces both domestic and industrial lithium-ion batteries (not to mention batteries for electric cars). And their efficiency under ideal conditions is approximately the same. But ideal conditions are rare.

The efficiency of the Tesla Powerwall inverter, a household battery, is 90%. This figure is indicated for the “complete cycle” of charging/discharging the battery: “alternating current to the battery and back to alternating current”, the specification for the battery says.

The performance measures of an industrial battery system, Tesla Powerpack, are about 88 - 89%.

If the capacity of one Powerpack is 210 kWh, then after one storage cycle, 186–187 kWh will be available for use back.

That’s if the battery is new and works at the optimum temperature. Storage system performance measures are indicated for operation at a temperature of 25 °C – too high or low temperatures damage the efficiency of a lithium-ion battery, although the specification states that they can operate at temperatures from -25 °C to + 50 °C.

Studies show that if the battery is fully charged for some (short) time at an ambient temperature of 40˚C, its capacity (the ability to store energy) will decrease by 35% in a year, even if it is used by small scale.

One of the main problems with lithium-ion systems is that they degrade over time. The degree of degradation depends on the intensity of the battery use – how often it worked during peak loads, how often it was discharged to zero. But on average, after ten years, the battery capacity decreases to “economically disadvantageous” levels (it applies to industrial facilities) – 50 - 70% of the declared for new batteries. Less capacity is less efficiency.

Degradation results another problem with lithium-ion batteries – their disposal.

Besides lithium-ion, there are batteries based on other metals. But their efficiency is slightly lower. As well as the cost. Below are data of researchers from China. The number of charging– discharging cycles, safety of use, energy density, cost and efficiency are described.

The cost of lithium-ion batteries (as of January 2018) is estimated at $213 - $640 per kWh, lead-carbon – $142 - $213 per kWh, vanadium flow batteries – $425 per kWh.

Power-to-gas. Gas storages: innovative, “green”, less efficient

Pros: it is possible to supply methane directly to gas networks, container transportation

Cons: low efficiency, expensive electrolysis process

The logic is simple: excess electricity is used to produce hydrogen in the electrolysis process. Further, hydrogen can be converted to methane if there is a carbon source “nearby”. Gas is stored in special containers and output as needed. The efficiency of energy use is clearly shown below when renewable electricity is converted to hydrogen and then to methane.

The standard efficiency factor for such systems is about 50%.

Although scientists conduct researches in the field of high-temperature electrolysis and methanation, their efficiency exceeds 75%. Electrolysis at especially high temperatures around 800 °C is a significant advantage for increasing the efficiency of power-to-gas technology. We are waiting (or investing money in the technology).

Power-to-gas technology is the most efficient for long-term energy storage. With intraday charging-discharging losses are too large.

For example, the French company HDF Energy will build a solar park in French Guinea, within the framework of which there will be created hydrogen energy storage for long-term periods, but for short-term – lithium-ion batteries. The capacity of the solar park is going to be 55 MW, the capacity of a hydrogen storage battery –130 MWh, and the lithium-ion capacity – 10 MWh.

PSP. Verified, large-scale, but not agile.

Pumped storage power plants are not as fresh as lithium-ion batteries or hydrogen storages. It has stood the test of decades, and it is the only accumulation technology used in Ukraine. Recently, it is being used too active, because after the launch of the market it is necessary to balance much more often.

Pros: inexpensive storage, high capacities

Cons: expensive and lengthy construction, the need for a suitable landscape

The data on the efficiency of the Tashlyk PSP

In the generator mode, the maximum efficiency is 87% (energy conversion efficiency while producing electricity in the process of draining water); in pumping mode (pumping water into the upper tank) – 90.8%. We emphasize – the maximum efficiency.

Commonly, the efficiency of the PSP is about 0.75, which means that out of every 100 kWh taken by the PSP from the system, 75 kWh is returned. And it applies to modern, more or less, technologies. If we are talking about PSPs built in the 60s, then the efficiency can be at the level of 40%.

Of course, it also depends on the size of the impoundment, its climatic location and other factors that affect the amount of water in the reservoir, which, among other things, affects the efficiency of the PSP.

Gravity energy storage systems. Unusual, cheaper, but ineffective

The principle of work is based on gravity and friction, basically, it is similar to the PSP. But without water.

Pros: fast construction and comparative low cost

Cons: new technology with uncertain efficiency

The principle of work is based on gravity and friction, basically, it is similar to the PSP. But without water.

An interesting example of gravity storage was created by the American company (Advanced Rail Energy Storage North America) using the mini-railway in Nevada. Instead of pumping water into the upper tank when storing energy, the ARES project lifts carriages with ballast closer to the top of the hill; when energy is output, the carriages are lowered from the hill.

Each of the carriages is equipped with a 2 MW generator, when lifting it works like an electric motor, and on the descent it gives energy to the network. The efficiency of the entire system is estimated at 80 - 86%.

The difference in height between the points is 900 m (and this is a very large height difference, most PSPs do not have this), given that the length of the tracks along which the carriages move is 8 km. At its peak, the system output up to 50 MW of capacity. This is about 1.5 MW from each of 32 carriages (including all losses).

It is said that the launch of carriages on the move is very fast – from 5 to 10 seconds. This is not as good as with lithium-ion batteries, but in the area where the project was created, the inconsistencies of renewable energy sources are balanced mainly by gas thermal power plants, which are engaging for half an hour.

The ARES pilot experience was limited to tests at a distance of 240 m. In addition, critics insist that 80–86% efficiency is possible only with a short carriage path and the longer the descent and lift sections, the greater the loss.

Another no less interesting project works on the same gravitational principle – energy storage in a tower. The principle of operation – a six-star crane stands in the center, and concrete cylinders weighing 35 metric tons lie at a distance from it. When an excess of solar or wind electricity occurs, the electric engine is activated, and the crane, controlled by an automatic algorithm, lifts the concrete blocks one by one, folding them into a tower around its axis. The storage system is “fully charged” when the crane created a tower of concrete blocks around itself. The tower can store 20 MWh of energy. When there is a shortage of electricity in the network, the system starts in the opposite direction, and electricity is supplied to the network due to gravitational energy.

When the network runs at a low level, the engines come back into action – except that instead of consuming electricity, the engine is driven in reverse by gravitational energy and thus produce electricity.

This energy storage system was developed by the Swiss startup Energy Vault. The demonstration facility is located in Italy, near Milan.

Whereas concrete is much denser than water, lifting a concrete block requires (and therefore can store) much more energy than a reservoir of water of the same size, the developers note. The efficiency of such a system is about 85%.

Thermal energy storage. Or a heated “philosophical” stone by Siemens

Pros: inexpensive
Cons: Not very efficient, only suitable for seasonal energy storage

A couple of years ago, Siemens, which is actively developing wind power, proposed a solution for energy storage in Northern Germany. The excess energy produced by the wind farm is converted into heat, heats stones (up to 600 C), protected by an insulated cover. When there is a need for additional electricity, the steam turbine converts thermal energy back into electricity. This is the basic scheme of work of the energy storage “on hot stones”.

The project itself is inexpensive in the organization, but its effectiveness is still low. It is planned that a full-scale storage facility will be able to hold about 36 MWh of energy in a container with about 2000 cubic meters of the rock. Using a boiler, the accumulated heat can generate so much steam that the Siemens compact steam turbine can produce up to 1.5 MW of electricity up to 24 hours a day. In the early stages of development, the efficiency of the stone storage will be about 25%. In the future, the concept has an efficiency potential of about 50%. In other words, of the 36 MWh enter the storage, 18 MWh remains at the output.

Accumulation ways

Efficiency

Reaction

Expensive (+)/ inexpensive (-)

Durability

Mobility

Li-ion

88 – 90%

Power-to-gas

50%

+/-

PSP(new)