The Success of Battery Energy Storage Systems

According to the International Energy Agency (IEA), the global installed capacity of battery storage projects has soared from 0.2GW to 3.1GW in the last five years. This is over fifteen times increase, and the trend does not show any signs of reaching a plateau. There are two reasons behind it: batteries keep on getting cheaper and modern power electronics coupled to batteries provide unrivalled versatility to satisfy the needs of changing power systems.

The marriage of batteries and power electronics is called a Battery Energy Storage System (BESS).

BESS technology is the best available vehicle to convey the transition from conventional power systems to less nuclear-intensive decarbonised, decentralised and digitalised energy infrastructures.

As prices of solar and wind continue to drop, the transition is speeding up. This is displacing fossil fuel-based generation plants and will continue to do so in the coming decades. The issue is current power systems were conceived to run on conventional (fuel-based) generators and the implications of this are deeper than meets the eye: BESS are needed not only because they will allow owners trading in energy markets or going off-grid; they are also needed because without them a continuing increase in the share of renewables in the mix would be nearly impossible to take in.

Technical reality is that for every new installed watt of wind or solar energy, our power systems become a bit weaker to the point that a poor management of this situation will result in the collapse of the backbone that ensures power is delivered reliably to everyone.

The management of this situation is a growing market called ‘Ancillary Services’. Wealthy countries with less resilient power systems feature more development than the average in this kind of business and are fostering the onset of solutions where unprecedentedly large batteries are employed to guarantee network stability. Those ones arriving first are cashing up very well and already displacing fuel-based technologies that have so far provided their services in a monopolistic way, charging large sums for a quality of service well below the standards achievable by a BESS.

The flagship of these projects is the 100MW / 129MWh Hornsdale Power Reserve (HPR), owned and operated by NEOEN in South Australia: in the first year of operation it recovered nearly a fifth of the total construction plus operational costs (it will pay itself in a few more years), eat up more than 50% of the frequency regulation market and provided 40M AUD savings to consumers. How this pricing strategy may develop in the future is hard to guess given that this market may soon be collapsed by newcomers seeking similar profits.

Figure 1 – The arrival of the HPR BESS caused a dramatic reduction in payments made for Frequency Control Ancillary Services (FCAS), resulting in lower prices for electricity to customers.

It is only a matter of time that countries with stronger power systems will experience a similar urge for network support as the share of renewables in the energy mix increases even further. In Europe we will see this happening first in the UK and around the North Sea.

Batteries begin to make economic sense and they can ease the transition we want to have, but how does a power system lose its feet when wind and solar are introduced? Weren’t these technologies supposed to be better? Short answer is wind and solar are good but to gain a deeper insight as to why they are disrupting the established order we need to get a bit more technical.

NETWORK FREQUENCY STABILITY

A power system is the infrastructure and operational intelligence that produces, stores and delivers energy within areas that range from small islands to entire continents. They are prone to instability in many ways. Wind and solar are good… It is just our power systems were not conceived to accommodate a lot of them since they were designed with synchronous generation in mind.

Moved by explosions, steam or water, large synchronous machines create the electric and magnetic forces that glue the network together across entire nations as they rotate synchronously, at the same speed. Try to make one of them spin at a slightly different speed and that glue will counteract your efforts with the strength of a continent to the point that the opposing forces will make something break before you can move it any further. This cooperative effort is what makes conventional power systems robust and its design is very clever.

Frequency is ubiquitous, but it is not fixed. Its value depends on the balance between generation and demand through the system inertia and it is controlled within tight margins.  The moment it diverges more than 0.15 Hz from 50Hz, network operators start adjusting knobs to bring it back to centre. Inertia is opposition to change. A power system with more and bigger synchronous generators is more inertial and more difficult to be brought out of equilibrium than a lighter one.

Unfortunately, wind and solar plants do not add much to the glue. It is easy to see why solar does not: there is nothing spinning there. Some inertia can be obtained by oversizing the inverters and running them constrained, which creates room for delivering additional power when needed but not to absorb it.

Wind turbines can potentially do the job but despite many efforts there is not to date an established solution that allows harnessing their inertia in full, so in practice they are both insensitive to changes in network frequency and they do not provide any opposition to changes in frequency. The inertial behaviour of wind turbines nowadays is at its best, very lightly harnessed and short-lived, but we may be seeing effective developments in the field as the necessity for truer inertia becomes more urgent.

There is an analogy that describes the dynamics of frequency very well. Imagine a double-platform weighing scale. The beam that joins the two platforms together has a certain mass and swings left or right according to the weight placed on each platform. The level of unbalance is measured by a gauge.

In this analogy, the following magnitudes are related:

• Weight on one platform = power generated by power plants
• Weight on the other platform = power consumed by industries and cities
• Gauge = measurement of power system frequency
• Beam mass = amount of synchronous generation in the power system (inertia)

Power system operators perform complex dynamic simulations to assess the robustness of their networks. The analogy of the balance allows performing a simplified simulation in our own minds when we understand that for a given weight difference a heavier beam will move less and more slowly than a lighter one because it has a greater moment of inertia.

What all this boils down to is that when renewables displace synchronous, the power system losses its glue. There is less spinning mass holding everything together and unbalances in generation and demand result in bigger and faster swings in frequency (the beam mass is reduced), which means there is less time for the network operator to restore the system balance. Loss of control is a possibility for the first time so on the way to become cleaner we are thinning the foundations of our power systems and risking the transition itself.

BESS systems can provide relief for this situation better and are flexible to deliver a broader range of services than any other technology at competitive costs, which explains their success beyond simply storing energy to trade it when prices are favourable.

In a BESS the inertia is provided by the battery together with fast-acting control systems. To put things into perspective, primary response to frequency contingencies must be delivered within the first 2-5 seconds of an event. This is a huge effort for any fuel-based generator, but the BESS installed at HPR has been commissioned to deliver the service within 0.15 seconds: this is very close to true inertia.  We believe the response can even be instantaneous by using more advanced control systems based in ‘grid-forming’ algorithms.

Considering the relevance and prospects of BESS technology, at Norvento we are developing our own grid-forming BESS to supply the isolated network of our CIne headquarters. The extended capabilities of this system will allow joining two isolated networks, making the resulting network stronger and more resilient, in the exact same way several synchronous generators collaborate through electric and magnetic forces to create an entire power system.

Figure 2 – Engineering Test at Norvento Headquarters: CIne. The situation on the left is representative of a power system where synchronous generators coexist with renewables: a diesel generator creates a network alone and a Grid Follower BESS simply keeps its current constant. The right side depicts a power system which is reinforced with a Grid Former BESS (Grid Former BESS + Grid Former Diesel). Notice the reduced frequency swing compared to a conventional Grid Follower BESS (left) when the same disturbance is applied. The BESS is configured to show a large inertia and as a result it absorbs fast power transients resulting in smaller frequency swings. The Diesel generator delivers the steady state load increase, but the BESS is handling the transient.

A SHORT THOUGHT ON THE VERSATILY OF BESS

It has been mentioned previously that BESSs provide unrivalled versatility. The US Department of Energy (DOE) offers access to a database where electro-chemical energy storage figures as a prevailing technology that can deliver up to 30 different types of network services (‘use cases’), and it is surpassed only in two of them by pumped hydro storage, where a very large energy storage is required.

The reader may be inclined to consider that the benefits of BESS can only be harvested placing battery behemoths at the weak spots of transmission networks. We believe this is not true: the ubiquity of frequency means the network can be supported at any node regardless of its size and relevance. A possible and unplanned outcome is that the proliferation of smaller BESS will naturally reinforce our power systems in the same manner new synchronous machines did as our cities grew in the past. This will happen at the distribution level, but we can speculate even further:  electric vehicles are BESS on wheels. As of July 2019, there are in Spain above 20.000 100% electric EVs representing a total energy storage of 700 MWh (over five times HPR). Considering cars are parked most of the time, this number looks like a very decent amount of distributed stabilizing power and it will only grow bigger. Will societies be smart enough to use it?

REFERENCES

• Energy Storage, Tracking Clean Energy Progress
• DOE Global Energy Storage Database
• Energy Storage Projects: a global overview of trends and developments
• Definition and classification of power system stability
• Battery energy storage’s role in a sustainable energy future
• The role of Fast Frequency Response (FFR) in keeping the lights on