Four emerging solutions to the need for cheap storage for renewable electricity

Westmill Solar Park with part of Westmill Wind Farm in the background, on the Wiltshire/Oxfordshire border in England. [photo: Neil Maw, Westmill Solar Co-operative]

We can’t control when the wind blows, and the sun doesn't shine at night, so with an increased proportion of these renewable energy resources supplying electricity to the grid, it means that countries will more and more have to balance overall electricity supply and use better in order to meet demand the most efficiently and economically.

Sometimes too much electricity is being generated, leading to the possibility of wastage, and sometimes there is not enough to satisfy demand, in which case either more polluting coal power stations must be brought online, prices put up, or, at worst, power cuts implemented.

The much sought-after solutions for this problem are the missing link in the widespread adoption of renewable electricity. They will either be cheap, easy-to-use energy storage or better, dynamic, management of electricity demand.

Many technologies are vying to be the go-to solution for filling this gap.

While different storage technologies such as lithium ion batteries or compressed air storage are becoming interesting for their practicality and price, more imaginative, novel solutions are also emerging. Here is a quick overview of four of them, including a new battery which could not only fit the bill best but also desalinate saltwater while it is being charged.

1. Get paid to use energy

For some time, several countries' grids have been running systems which reward industrial users who have their own spare generating capacity for selling it to the grid when there’s not enough coming from traditional power stations. In the UK, this has been achieved under a scheme called Short-Term Operating Reserve (STOR) that has been run by Flexitricity with the National Grid.

The same company is now operating 'Demand turn-up'. This is a demand management solution that will actually pay customers to use electricity when there’s too much wind, rather than it being wasted. Participating users are rewarded for increasing consumption or reducing generation at their industrial sites, so that the National Grid doesn’t have to pay wind farms to shut down, as it does now.

Small generators can be paid to reduce generation for the toughest half-hours in a high wind times. The more a generator is used in normal business activities, the more it can earn. These include operators of Combined Heat and Power (CHP), anaerobic digestion, including sewage and landfill gas, and small hydro generators with reservoirs.

Business energy users with flexible needs are the ones who can benefit from this scheme. They can earn by consuming more when wind output is high. Such needs include water pumping, heat and cold storage, space cooling and any non time-dependent manufacturing purposes.

Flexitricity’s chief strategy officer, Dr Alastair Martin, says this idea marks a significant milestone in the evolution of how businesses consume electricity. “It opens up a world of possibilities for business and for renewables developers. Currently, when the wind is at its strongest, the Grid turns large power stations down or off. But it can’t turn down all of them, so sometimes it has to turn off some of the wind farms. This wastes a free resource.

“Now, businesses can boost productivity for minimal extra cost and are incentivised to do so. In turn, the Grid can increase the amount of electricity distributed to homes from clean, renewable energy sources.”

This is in essence an evolution of Economy 7 – brought in to encourage use of nuclear power since those generators can't be turned off at night – made possible with smart tech. But there's another old idea out there that's being given a new, more reevant lease of life with smart tech.

2. Community storage

In America, electric cooperatives are set to roll out the use of electric water heaters as a demand response (DR) and energy storage tool in their service territories, following the launch this month of The National Community Storage Initiative. This is designed to promote growth in a novel, community-based approach to energy storage dubbed “community storage”, and is supported by legislation passed last year, the Energy Efficiency Improvement Act of 2015.

Energy co-ops have been in existence in the USA since the 1930s, to bring supply, price and environmental benefit especially rural members. They are a much more recent phenomenon in Europe.

The Energy Cooperative of America already offers its New York State customers a Demand Response Program in conjunction with the New York Independent Systems Operator (NYISO). In this scheme, businesses that participate are rewarded for temporarily reducing their electricity load when demand needs could outpace supply. Customers contribute to energy load reduction (decreasing usage or using onsite generators) during times of peak demand between the hours of 11AM to 6PM.

But, as with the Flextricity scheme in the UK, this new proposal would store surplus energy in residential electric water heaters to avoid it being wasted. There are 50 million such tanks the USA so this would create a significant storage resource with substantial environmental and cost benefits, according to a new report, The Hidden Battery: Opportunities in Electric Water Heating

commissioned from the Brattle Group by the National Rural Electric Cooperative Association (NRECA), the environmental organization Natural Resources Defense Council (NRDC), the Peak Load Management Alliance (PLMA), and Great River Energy (GRE).

The storage is thermal (so can’t feed electricity back to the grid), and, unlike community renewable energy, where multiple customers buy in to a project at a single site, the storage tanks are distributed throughout the co-op's territory. Customers would use the hot water later, when they need it, and be rewarded with reduced electricity bills. This 'fast response' solution as it is called will save far more carbon and money than other related techniques, as shown in this graph:

This is already being done in Minnesota, where Great River Energy has been able to store a gigawatt-hour of thermal energy each night by charging the electric resistance water heaters of 65,000 end-use members.

Currently it only works with water heaters from three specific manufacturers that offer high efficiency. They are grid-connected from 11 p.m. to 7 a.m., Connett said, and then when the DR program is in effect, they are disconnected until the next charging period. 

“When the wind is blowing or the sun is shining, large capacity water heaters can be enabled to make immediate use of that energy to heat water to high temperatures. The water heaters can be shut down when renewables are scarce and wholesale costs are high.” The water can then be used whenever it is needed.

Keith Dennis, NRECA’s senior principal for end-use solutions and standards, said: “Co-ops have been controlling large water heaters for decades in order to reduce demand at peak times, which also reduces members’ electric bills. A community storage program using advanced water heaters allows us to do even more: we can store energy, we can optimize the power grid by shaping demand and we can integrate more renewable resources”.

3. Lithium Ion: the current favourite

The current favourite for battery storage is lithium ion – the type found in many computing devices and electric cars.

One backer of this technology is John Hingley, who runs Renovagen, a company making off-grid roll-outable photovoltaic (solar) power generators for emergency relief and military users. His system is driven out to a location and simply rolled over the ground. Within minutes power is being generated and stored. "We can fit 18kWp in the small trailer/air-pallet container in the video and up to 300kWp or more could fit in a 40ft shipping container - all deployable by vehicle tow in a few minutes. Energy storage and inverter systems are built-in – it's a "micro-grid in a box" – also enabling multiple units to be run in parallel to build up larger micro-grids."

Rollable PV being used by the military.

Right now he uses lithium ion as it's cheapest. "We've already seen proposals for [lithium] storage systems come down from £1,500/kWh three years ago to £350/kWh today. Driven by the EV industry, like Tesla's Powerwall offer for solar-powered homes and its Powerpack offer for businesses and utilities, that trend will continue," he says.  "The solar + storage tipping point for 'off-grid parity' (same price as diesel) in most off-grid areas of the world I believe is broadly around £2/W and £250/kWh installed and we are getting very close to that point. Some options are already there, it's just a matter of how do you deploy?"

The container holds lithium ion batteries for storing the solar power.

Hingley is looking forward to the day perovskite PV is commercialised. "We're currently using CIGS PV, but the potential with perovskites (in particular, if printable under atmospheric conditions) is for significant cost reductions and efficiency improvements that could make this sort of technology the cheapest way of providing portable power almost anywhere in the world.

"In combination with leaps forward in the cost effectiveness of energy storage technology this can become hugely competitive and displace diesel burn across a wide range of industries – disaster relief, humanitarian, military, mining, agriculture and other remote commercial and industrial applications."

4. On the cusp: the battery that runs on saltwater

But lithium is not cheap, found in remote areas of the world, and comparatively rare. What if reliable and safe batteries could be made from something really common – and solve another problem brought about by climate change and rising populations: the increasing scarcity of drinkable water?

Step forward sodium batteries, that run on saltwater – sodium makes a salt, is similar chemically to lithium, and makes up more than 2.6 percent of the Earth's crust.

Laurence Croguennec, a material scientist at the CNRS Institut de Chimie de la Matière Condensée de Bordeaux, says that "As the chemistry is very close to that of the lithium battery, from that point of view there are no major difficulties; the mechanisms are the same ones and all the industrial processes for their production are the same. One remaining problem is that sodium is less efficient as a charge carrier, however. A sodium battery loses 0.3 volts as to compared to a lithium battery. You have to develop materials that can function at higher voltages and that provide ample capacity."

But the technology is advancing fast. Last December Wattstor, a British company, announced it had made the first British installation of sodium ion batteries made by US company Aquion on the premises of Henbo Energy Storage, a newly launched storage installation company in Portadown, Northern Ireland.

Aquion Energy's M-Line Battery Module, 22kWh on a PV off-grid system in Hawaii, installed a year ago.

This battery is first one to receive certification for the maximum use of available recycled materials and an optimisation of the amount of the product that can be recycled. Wattstor Director Michael Danes insists the technology has a long (expected) lifespan. “These new batteries use a completely organic electrolyte in the form of saltwater and a potential lifespan of 15-20 years. In a market dominated by lead acid and lithium, it is encouraging to know that sustainable battery chemistries are being developed.”

The unspectacular looking Wattstor installation – the black box is the sodium batteries.

But your devices may soon be running off something like this – a small, rechargeable sodium battery.

James Dean, manager of rival company Circuitree, says the batteries will eventually be cheaper than lithium ion. “Due to the abundant nature of materials in their construction, this allows us to deliver much greater capacity at lower cost, giving our customers increased energy independence, affordably, from a unique and sustainable product”.

That's good news. But it gets better. Researchers at the University of Illinois have last month reported that they have come up with a refined design of the sodium battery that does all of this but also removes the salt ions from the water in the process to create drinkable water. In other words, it has the potential in the future to be used in a solar powered desalination plant that also stores the electricity generated in the daytime so that it can power itself when the sun goes down.

Here's how it works. In a regular sodium battery, one electrode receives desalinated water, but only for a while, as the sodium ions then flow with the current back the other way. The Illinois researchers have developed a filter to stop this occurring while letting the battery continue to work.

On of the researchers, Kyle Smith, explains that "In a conventional battery, the separator allows salt to diffuse from the positive electrode into the negative electrode. That limits how much salt depletion can occur. We put a membrane that blocks sodium between the two electrodes, so we could keep it out of the side that's desalinated."

Schematic of the battery design. Courtesy: Journal of The Electrochemical Society, 163 (3) A530-A539 (2016)

The researchers, (the other one is Rylan Dmello), have modelled how their device might perform with salt concentrations similar to seawater, estimating that it could recover around 80 percent of desalinated water. But seawater contains other contaminants so work is ongoing to test it using real seawater.

Most water desalination uses reverse osmosis, which pumps water under high pressure through a membrane to separate the salt. This is expensive and energy-intensive forcing reverse osmosis plants to be large in order to be cost-effective. The Illinois researchers believe that their design will use very little energy to extract the salt, meaning small-scale, desktop desalination could be possible.

Andy Glass, Executive VP, Chief Technology and Marketing Officer at JET Recycling America, Inc., comments that: "Massive scale energy storage and balancing is the biggest obstacle to proliferation of sustainable energy technologies like wind and especially solar. Shortage of potable water and arable soil are huge environmental threats. It's exciting to think that a combined solution like this could be scalable to global needs. This could also be a tremendous boon to remote areas where increased access to electrical power and potable water can change and even save lives.

"Water production from this method does not have to be compared to mass-scale desalinization needed for crop irrigation, for example. Production of safe drinking water is a different problem and more urgent in many areas of the world. This technology sounds like it may be implementable first and most practically as a local solution."

Peter Murphy, the Founding Director at UK Water Ltd., agrees, calling it "an excellent solution for water stressed areas," adding that: "If we can address the energy and waste salt issues then this has real potential."