"Energy cannot be created or destroyed, it can only be changed from one form to another."
From home-scale batteries like the 14kWh Tesla Powerwall to larger systems like an advanced 1.6MWh battery now used in large solar farms internationally, when it comes to energy storage, most people will think of batteries.
However there are other storage systems being used worldwide for renewable energy projects small and large.
Here are some older and newer technologies being utilised today.
1. Pumped Hydro
An ambitious new plan is underway in Australia to revitalise a large hydro electric system with more pumped hydro storage. At first glance using electricity to power giant pumps seems counter-productive but it makes a lot more sense when you see pumped hydro for what it really is - a giant battery.
Pumped hydro is far and away the most prevalent form of energy storage worldwide, making up a huge 99 per cent of it.
It operates on an incredibly simple premise - energy can be stored for as long as needed in the form of gravitational potential energy.
First, excess electricity is used to pump water from a low reservoir into a higher reservoir. Then, in times of high demand for electricity, the water is released back down the slope and through a hydroelectric turbine.
Water can be stored in the high reservoirs for as long as needed, without the cycle limitations of chemical batteries. The only potential energy loss happens through evaporation, though this too can be minimised by enclosing the reservoirs.
Compared to alternatives like battery farms, the infrastructure takes a long time to deploy and without careful placement can risk disrupting sensitive environments. As with any kind of hydroelectric installation, drought can severely effect operation.
2. Compressed Air
The second most popular form of energy storage is compressed air energy storage, or CAES. This operates on a similar principle to pumped hydro, using air instead of water. Instead of powering a pump, excess electricity is used to compress air, which is then stored under pressure in an underground reservoir. When electricity is needed again, the air is heated and driven through an expansion turbine.
Compressed air energy storage was used to deliver power to households as far back as the 1870s, around the same time as the invention of the light bulb.
Compared to more modern techniques its efficiency can be a problem however, ranging from only 40 percent to over 70 per cent, depending on how it is deployed.
Because of this, the technology has rarely been used on a large scale. Only two large CAES plants are in operation at this time.
A more experimental technique is being explored, where the heat generated from compressing the air is captured and used again later to reverse the process. These systems have the potential to be far more efficient.
3. Molten Salt Solar
Molten salt, or liquid sodium, batteries offer both high energy density and high power density. As a sodium-sulphur rechargeable battery, it also employs cheap and abundant materials. Operating temperatures of 400-700oC, however, mean there are management and safety issues plus stress on the battery components. In 2010, Italy opened a 5-MW solar farm, the first in the world to use molten salt technology. Newer designs have brought temperatures down to 245 oC. A liquid-metal battery using molten sat as a separator are being developed in the US with funding from the Bill Gates Foundation.
While both pumped hydro and compressed air operate on an electricity in/electricity out basis, molten salt solar storage captures and keeps renewable energy in its original form of heat. Without the energy lost in multiple state transfers, molten salt storage is one of the cheapest and most efficient forms of large-scale energy storage, even beating out pumped hydro by some estimates. Molten salt storage can be deployed at as little as 10 per cent of the cost of large-scale batteries.
Coupled with molten salt, solar thermal technology becomes far more valuable for its potential to provide reliable, renewable baseload power.
The Ouarzazate Solar Power Station in Morocco will eventually be able to produce 580 MW in peak times. The completed first phase of the project, Noor 1, has a molten salt storage capacity of three hours, but when the plant is completed it will run for 20 hours a day.
4. Hydrogen
Hydrogen energy storage is the least like a battery and the most like a resource. it's a way theoretical excess renewables could be bottled up and shipped across the world as already occurs with coal, nuclear fuel and LNG.
Hydrogen is created from water in a process called electrolysis, which uses electricity to split H2O's hydrogen from its oxygen. From there, hydrogen can be used as a fuel for vehicles that's almost on par with petrol or diesel. It can also be run through a fuel cell with oxygen, essentially reversing the process of electrolysis to produce electricity, heat and water.
Although hydrogen energy storage has seen renewed interest in recent years, it's still far from a perfect solution. Hydrogen suffers from relatively low round trip efficiency at around 30 to 40%, but it does have one main advantage over other energy storage technologies: its ability to be sold and exported across the world.
One roadblock still exists for hydrogen as an export, however. The gas must be compressed into a liquid for transportation overseas, and the process required to do this is currently too energy intensive to be viable. An Australian pilot program is aiming to investigate the feasibility of converting hydrogen into ammonia for ease of transportation.
Sources: Gizmodo, MIT Technology Review