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.

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).

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.

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.