Do we have batteries today that can smooth out the fluctuations of sustainable energy sources?
Unusual ways of storing energy in the grid. Is it time for flow-through batteries or do we need to wait?
Words by: Jan Strmiska
Sustainable energy sources are prone to large and unpredictable power fluctuations, which poses a huge challenge to the grid. Fluctuations aren’t usually resolved by reducing the output of energy sources at peak times, but rather by storing excess energy and sending it back to the grid when needed. In fact, severe fluctuations can even lead to a blackout, meaning the whole grid collapses. However, storage is not free and the question is whether it can be worthwhile given the current state of technology.
Why should the network collapse?
In most cases, there is a loss of network frequency, which makes connected generators stop synchronizing. This is followed by a cascading failure of the individual parts of the network, which then needs to be restarted — a process that is painfully slow. That’s why blackouts can last quite long. To prevent such a situation 2 types of balancing and backup resources are needed. Ones capable of responding within milliseconds and others with response time counted in minutes or even hours. To illustrate this, imagine that you would have a server room with UPS battery backup for sudden power outages. And on top of that, you would also use a diesel generator in the basement — the type that hospitals have to be able to operate in case of an outage lasting tens of hours.
So how to store energy?
For a quick response, there are flywheel (FES) and battery storages. And as an alternative to that basement diesel is pumped-storage hydroelectricity (PHS) and compressed-air energy storage (CAES). These are suitable for a more gradual ramp-up of slump absorption and energy release over a longer period of time.
Redox flow batteries (RFB)
Redox flow batteries (RFBs) are another alternative for short-term energy storage. Unlike the currently more advanced and certainly the most popular lithium batteries, this is essentially a flow-through chemical reactor where a vanadium solution serves as the electrolyte. They are more economical at large capacities than lithium batteries, which are made up of individual small cells — each with its own electronics — and so their cost increases essentially linearly. This is not the case with redox batteries, so at high capacities of tens of megawatts, they are a more cost-effective solution. Therefore, these batteries will be nothing for the new Tesla or iPhone, but quite something for the energy sector.
What are the other advantages of vanadium RFB batteries?
In the first place, longevity. In a simulation of twenty years of operation, no degradation is observable. Using traditional calculation, this means a lifetime of 100,000–200,000 cycles. In fact, you could say that these are essentially barrel batteries. Think of them as electrolyte tanks. But unlike conventional solutions, the chemical reaction doesn’t take place inside the electrode, which will gradually tear and wear out from the inside, but only on the surface. Therefore, the whole process is gentler and contributes to a longer lifetime. They can be charged and discharged several times a day for years. They are also easier to recycle because they are constructed from recycled plastics and aluminium. Moreover, their electrolyte can be reused or used in steel production.
Is there any progress in the field?
Yes, for example, the Czech chemical engineering start-up Pinflow Energy Storage is making progress in increasing the density of current. And that means more power packed in the same size. European development projects HIGREEW and HYFLOW are also looking into this. The chemists claim that their research will eventually make lithium batteries look obsolete due to their relatively low lifetime and lousy recyclability.
There are RFB batteries for houses too
RFB batteries don’t always have to be truck-sized. Australian zinc-bromide flow battery manufacturer RedFlow makes these flow batteries for homes, which is a great solution for increasingly popular island systems. They are competitively priced compared to the Tesla Powerwall. Bad tongues claim that there is a threat of toxic bromine leakage. Redflow marketing manager Sciobhan Leahy has a clear answer to such an objection, “Tesla has to be very, very, very sure that their lithium-ion battery won’t ever cause a house to burn down.” He’s referring to the fact that today’s most popular batteries are virtually unquenchable.
But the comparison isn’t fair anyway, because the main competitor to RedFlow’s technology is lead-acid batteries, which are widespread mainly in Australia and Africa.
RFB has problems too
The downside of RFB batteries is that it’s difficult to measure their state of charge. One option is to monitor changes in the colour of the electrolyte. They also experience parasitic energy loss because they need electric pumps to operate. Moreover, the chemical reaction is exothermic, so some energy is inevitably lost during operation.
There are not many good solutions
When you look at all the pros and cons, it is actually surprising that lithium batteries are still used in large industrial installations. Their production is energy-intensive and produces a lot of greenhouse gases (see Baumann, 2016). Renewable systems can reach emission levels of around 100 g CO2e/kWh when the lifetime of the system is taken into account. This is rather devastating for the environment, not to mention the costs.
This is because the conversion of gravity, carbon or fissile material into electricity is done in conventional systems by dispatching hydro, coal, gas and uranium. In comparison, the use of mostly intermittent sources to power the power grid looks like an unsolved problem for now because it requires storing electricity in huge quantities. At the moment, there is no mature, scalable technology that can perform this function in a way that could be considered economic.