There are a number of different types of energy storage each with their strengths and weaknesses but most widely discussed are batteries and pumped hydroelectric storage. In storing renewable energy and dispatching it when power demand exceeds supply or when the price is right, pumped hydro energy storage and batteries are likely to do much of the heavy lifting. Since applications such as Solar Energy, Wind Power and more along with conditions vary from one technique to another, a comparison is needed to assess what type of energy storage is needed.

• Technical aspects

Modern batteries, such as Li-Ion and Na-S, have an excellent energy density that opens up the possibility of storing energy closer to the consumer, which may be advantageous. Having a high energy density also means that smaller facilities can be used, having a positive impact on the price of energy as a finished product. Apart from things such as expensive components and chemicals, energy storage efficiency and lifespan also have an impact on the resulting price. With higher efficiency, less energy is lost during the storage period, resulting in a lower price. Likewise, long-lasting energy storage, such as PHES that can remain functional for up to a hundred years, can store energy at a significantly lower cost than storage that needs to be replaced more often, such as Na-S batteries that last only about 15 years.

• System Support

Pumped hydro is based on well-established synchronous generation, providing critical ancillary services to the grid through inertia, frequency and voltage support and sufficient fault level support. Battery inverter technology is still catching up on most of these fronts. The potential for batteries to provide 'synthetic inertia' or rapid frequency response is high, but this is balanced by their dependence on system strength to provide this support. They offer minimal support with fault levels but can still provide some support for system frequency and voltage regulation.

• Implementation Period

There is no doubt that the implementation period of battery storage is faster than the pumped storage of hydropower. Pumped hydro, by comparison, is a technology that takes a lot longer to implement. Typically, development activities (including optimizing the technical solution, environmental and social assessments, arranging financing and finalizing the design) take two or more years to complete and construction takes place another two to three years.

• Capital cost

Pumped hydro has a very low price per megawatt hour, ranging from about $200/MWh to $260/MWh and currently, battery costs range from $350/MWh to almost $1000/MWh. The capital cost of pumped storage projects around the world ranges from about $1.5 million to $2.5 million per MW installed while cost of installing a grid-scale battery solution ranges from about $3.5 million to $7.5 million.

• Efficiency and capacity

For both batteries and pumped hydro, some of the electricity is lost when the stored energy is charged and discharged. The round-trip efficiency of both technologies is usually between 75% and 80%. Degradation is a particular consideration for batteries. Batteries degrade as they age, which reduces the amount they can store. Expected life of the batteries to be around 15 years (depending on how the batteries are operated). By the end of that time, the battery capacity is expected to fall to less than 70% of its original capacity.

• Safety

No storage solution can be considered to be sustainable unless it is safe. Dam safety is the greatest risk associated with pumped storage. If this occurs, dam failure can affect downstream communities and the environment, with a potential impact that is likely to be far greater than a battery safety incident. However, pumped hydro technology is mature, dam risks are generally well understood and managed, and the frequency of dam safety events is low. The thermal leakage leading to explosions and fires is the main safety concern for batteries. The severity of this risk will depend on how the battery project is being implemented. In a modular arrangement, thermal leakage would be localized, not affecting the entire bank. However, due to the very rapid deployment of evolving battery technologies, safety standards may not be rigorously enforced.

• Environmental impact

No energy storage methods come without environmental impact, even if the method does not affect the environment while operating chemicals and components. It is always produced and recycled at some environmental cost, either as raw materials or as production of pollution. For example, PHES, the most commonly used energy storage facility, occupies large parts of the surrounding area, disturbing wildlife and associated eco-systems. Hydroelectric power plants have been constructed to the extent that the streams are suitable for PHES installations. The remaining large capacities are protected as nature reserves and as a result of social protests. Chemical disposal issues are an attribute shared between several types of battery energy storage, since they all rely on chemical components to react. The exceptions are mainly Na-S batteries, as they are made from non-toxic materials that are easy to recycle. Lead-acid batteries can be considered easily recycled to some extent, but lead in them poses a threat to leakage ecosystems and ultimate disposal.

• Social impact

In some way, all energy storage systems will be noticed by the public. The price of energy storage varies between technologies, affecting the price that the consumer has to pay for electricity or thermal energy. Although most can be hidden in industrial areas in the outskirts of cities, some attract more attention. The most notable is the PHES, which is hindered by social protests due to their large social and environmental impact on the local area. Some batteries, such as Li-Ion and Na-S, contain reactants that may act violently. Most modern Li-Ion cells keep lithium bound to keep it from ignition at all times. In Japan, a fire was reportedly caused by a Na-S battery in 2012.

DEDUCTION:

Pumped hydroelectric storage (PHES) continues to be the most solid contender for long-term storage and will long be the main choice due to the maturity of the technology. Li-Ion or Na-S batteries certainly are the best options for short-term energy storage. Batteries storage provide quick response times, however they have yet to establish their capability to provide the full range of ancillary facilities needed to support the grid. Pumped hydro remains a milestone, proven and reliable technology proficient at meeting grid needs and providing sustainable output for the coming centuries.

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