The long-term cost-efficiency of renewable energy coupled with storage is among the undeniable positive outcomes of recent advancements in new renewable energy and energy storage technologies. Renewables with storage also produce sources of efficient, clean, environmentally-friendly energy with dramatically less greenhouse gas emission (GHGs) than fossil fuels. Energy storage continues to make the transition away from fossil fuels and towards a renewable energy future; and to a clean and zero-emission energy future, more and more of a fluid evolution. 

Solar and wind are intermittent energy sources (as most solar doesn't produce energy at night, and both wind and solar produce less energy when subject to weather conditions that limit their potential to generate energy). As the common saying goes, "solar does not produce energy when the sun doesn't shine, and wind energy doesn't produce when the wind doesn't blow". Now this is actually not always true with the most recent breakthroughs in solar energy; such as recent advancements in solar PV that allow photovoltaic panels to be efficient in all kinds of sub-optimal weather conditions.

Generally, however, in order to optimally generate energy from renewables, excess energy from times of peak generation with renewables should be sent to energy storage; thereby addressing the intermittency problem.

Fossil fuels do not tend to have intermittency problems but do produce a huge number of negative externalities, including pollution and GHGs, that always are created with the combustion of coal, oil, and gas for energy generation. The burning of fossil fuels is driving global warming; and in order to avoid the worst consequences of anthropogenic climate change, the world needs to transition away from fossil fuels to renewable energy. The current state of global energy production and consumption is far from static, however, with recent developments in sustainable energy technologies. There continues to be many recent breakthroughs in sustainable energy technologies, with none more critical than in the field of energy storage.

For cities, states, and countries, to make a successful, accelerated transition to renewables, energy storage needs to be implemented on a global level. It must also be noted that the transition from fossil fuels to renewable energy would be expedited if renewable energy storage was simply more affordable. Global investment in research and development, and deployment, of energy storage technologies, will help determine the pace of the worldwide transition away from fossil fuels. What is needed now throughout the world, is a focus on R&D of energy storage and clean energy technologies, in order for the transition to be accelerated. Just as critical are policies to deploy and optimally implement these technologies.

Means of Energy Storage

Widespread future use of renewable energy sources such as solar and wind are dependent on the development of effective, affordable means to store excess energy. The primary means of energy storage depends on the energy sources used, and therefore vary greatly. A few methods of energy storage commonly used today include:

• Pumped hydro storage (PHS)

• Batteries - lithium-ion (li-ion), flow batteries, etc...

• Compressed air energy storage (CAES)
• Thermal storage (thermal storage, and applications for excess heat generated in the energy production process, are also discussed in the  Solar and Solar Thermal, Combined Heat & Power, and District Heating articles)
• Flywheels, and other types of mechanical storage

Please see our article on - The Future Generations of Batteries

Following are some of the most promising emerging technologies for energy storage (with some limited commercial availability today):

• Hydrogen fuel cells**
• Ice storage technologies
• Supercapacitors
• Vehicle-to-Grid (V2G)*** and Vehicle-to-Home (V2H)
• Power2Gas
• Hydrogen; Ammonia; chemical energy & storage in gas, liquid, and synthetic forms to store and distribute energy

** hydrogen fuel cells are discussed further in the Clean Hydrogen in European Cities (CHIC); H2BusEurope article

Pumped hydro storage (PHS) remains the most used means for storing clean energy worldwide (over 94% of global energy storage capacity is PHS). Pumped hydro is also often the most cost-effective and readily available means of storage for large-scale energy storage projects. To implement PHS in a suitable location, all that is needed is an area in which both a higher and a lower reservoir can be developed.

Also, an area of existing hydroelectric generation can efficiently be developed for PHS. An existing hydroelectric generating facility can be readily improved upon to create a PHS facility - as long as the needed topography is present, and the reservoir system(s) already exist (such as in an existing hydroelectric dam with a lower reservoir/ area that can be suitably developed).

The lower reservoir in a PHS system acts as the energy storage component. When energy is needed, the water from the lower reservoir is pumped up to the top reservoir, run through the turbines in the hydroelectric dam, and then the water flows back down to the lower reservoir where it is once again energy storage.

Compressed air energy storage (CAES) is dependent on having an underground chamber, mine, or similar geological area for storing compressed air. CAES is more location-dependent than pumped hydro, but is also a method of energy storage that is growing in popularity worldwide.  As the needed areas for pumped hydro are already developed for hydroelectric generation, and/or are relatively easy to discover, PHS is much more widely used than CAES.

Large-scale electricity storage in batteries is the future of renewable energy storage. Currently, li-ion batteries have a higher energy density, are the least toxic, and are the best battery alternative for utility-scale energy storage (compared to lead-acid, nickel-metal hydride batteries, nickel-cadmium, etc...). Li-ion battery packs, like Tesla's Megapack (illustrated above), can replace natural gas peaker plants to generate a constant source of energy for power plants relying on intermittent sources of renewable energy as their primary energy source.

R&D by scientists and engineers worldwide will continue on next-generation batteries; improvements in lithium-ion battery technology, as well as alternatives to li-ion. Advancements in, and alternatives to, li-ion batteries include: graphene-based battery technologies, sodium-ion, lithium-sulfur, lithium-air, vanadium redox flow, and other advanced batteries...). Fuel cell batteries and flow batteries, such as hydrogen fuel cells, and rechargeable flow batteries, are promising new emerging battery technologies. These new battery technologies both have a low environmental impact (water vapor is the only by-product from fuel cells), but both technologies need more development before they are cost-efficient, or energy efficient.

***A technology which also seems very promising, but also needs to be developed more to become cost-effective, is using the batteries in electric vehicles (EVs) for storage. The good news is that EVs are gaining in popularity worldwide. It remains important that the energy to charge the EV comes from a renewable energy source in order for this form of energy storage to be truly clean. EV-based energy storage is also known as vehicle-to-grid (V2G).

Québec’s public electricity utility, Hydro-Québec (which has been known primarily for supplying Québec with hydroelectricity and pumped hydro storage) has partnered with the US DOE's Berkeley Labs to create V2G and vehicle-to-home (V2H) systems. These partners are working on V2G/ V2H power, and next-generation batteries for California and Québec (to start with), as seen in this linked article by Utility Dive. V2G systems implemented for use in municipalities on a regional level will use electricity stored in the batteries of privately-owned EVs connected to municipal V2G systems as a backup energy supply for municipal electricity grids during peak periods of energy demand. V2H systems will also allow EV and plug-in vehicle owners to use the energy stored in their car's battery as a temporary home power source during outages. That’s an integrated vision of housing and mobility for the 21st century!

Please also see: Next-generation batteries

and:

Benefits of hybrids, plug-in hybrids, and EVs