Making the Most of Residential Batteries for Households and the Distribution Network
By Andreas T. Procopiou, Kyriacos Petrou, Luis (Nando) Ochoa
As residential battery energy storage (BES) systems become more affordable, more and more households will be able to store excess solar photovoltaic (PV) generation during the day and use it later at night, reducing electricity bills even further. This, however, can mistakenly create the belief or even the expectation that the widespread adoption of BES systems will inherently help reducing the reverse power flows and, hence, mitigate the associated impacts (such as over-voltages and asset congestion) on the electricity distribution network. However, existing controllers embedded in commercially available BES systems are ineffective in this matter, providing little to no benefits to the electricity network. Nonetheless, given the flexible controllability of BES systems, there is an opportunity to adopt advanced battery management strategies that not only provide benefits to their owners (lowering electricity bills) but also to electricity distribution companies, reducing power exports from households with solar PV and, thus, mitigating network impacts. These new BES management strategies could become an alternative to otherwise required costly network reinforcements, saving billions of dollars in investments.
Residential PV Systems – The Impacts on the Electricity Distribution Network
High levels of solar PV generation in residential areas is generally coincidental with low demand, when most people are at not at home (e.g., due to work, school, etc.). Because of this, most of the household PV generation is not consumed locally and is, therefore, fed back to the electricity distribution network.
This, however, occurs on an infrastructure that was not built considering PV installations, i.e., bi-directional power flows, from the substation to households and vice versa, were not part of the design process. Hence, when a large number of households export back to the network, the aggregated effect of these exports (reverse power flows) can potentially lead to technical impacts, such as over-voltage and asset congestion, that can compromise the integrity of the very infrastructure these households are connected to.
Off-the-Shelf Residential Battery Energy Storage Systems – The Limitations
Recently, with the gradually falling prices of the BES systems and the reduction of PV generation feed-in-tariffs in many countries across the world, more and more households are installing BES systems to store the excess of PV generation during the day and use it later at night, reducing electricity bills even further. This, however, can mistakenly create the belief or even the expectation that the widespread adoption of BES systems will inherently help reducing the reverse power flows and, hence, mitigate the associated impacts on the electricity distribution network.
Commercially available (off-the-shelf) BES systems are sized and operated for the sole benefit of the households. To drive costs down, their energy capacity is determined to accommodate for the average household demand and PV generation. Furthermore, their operation is based on storing of excess PV generation whenever available and using this stored energy only to reduce household energy imports. While this paradigm in sizing and operation is beneficial to the households, it provides limited benefits in terms of mitigating the impacts from excess PV generation. For example, on a sunny day with no one at home, the BES system can start charging from the unused PV generation relatively early in the morning, quickly reaching full charge before even high levels of PV generation period occur (i.e., around midday). Consequently, given that the BES system, after that time, cannot store more PV generation, it will be fed back to the distribution network. In other words, off-the-shelf BES systems today will not necessarily help mitigating solar PV impacts. A recent study performed on a real Australian distribution network with more than 4,000 households demonstrates and quantifies these limitations.
Consequently, the impacts due to large volumes of solar PV in residential networks are largely going to continue even if households install off-the-shelf BES systems. For electricity distribution companies, this means that network solutions such as replacing conductors and/or transformers for larger sizes or making the most of voltage regulation devices (e.g., on-load and off-load tap changers) will still be needed to mitigate voltage rise and asset congestions issues.
The Untapped Opportunity
Nonetheless, commercially available BES systems feature such technical capabilities and controllability that allow the adoption of control strategies that go beyond their built-in, off-the-shelf operation. As a matter of fact, this can be proven to be a unique – yet untapped – opportunity to adopt advanced battery management strategies that not only provide benefits to their owners (lowering electricity bills) but also to electricity distribution companies, reducing power exports from households with solar PV and, thus, mitigating network impacts. Such an approach could become a cost-effective alternative to otherwise required costly and time-consuming network reinforcements, saving billions of dollars in investments.
Advanced Battery Management Strategies – Benefiting Households and the Network
Any new battery management strategy that helps mitigating solar PV impacts by reducing excess PV generation must ensure that this does not come at the expense of the owners of the BES systems. In other words, households should still see as much reductions on their electricity bills as with the off-the-shelf operation. Furthermore, from a practicality and scalability perspective, it is desirable for new strategies to use as little information as possible and utilize the existing infrastructure. Otherwise, strategies that require, to different extents, communications, information and/or complex computations might prove too expensive and difficult to implement given the sheer number of BES systems that might be adopted in the years to come.
An example of an advanced battery management strategy that follows the aforementioned fundamental principles is presented in a recent article with quantifications on a real Australian electricity distribution network (IEEE Trans. on Power Systems, "Adaptive decentralized control of residential storage in PV-rich MV-LV networks"). The article demonstrates that a local BESS controller with considerably low implementation complexity and without the need of forecasting techniques, communications, or network information, can significantly mitigate PV impacts whilst still making the most of PV generation for households. This example highlights that it is indeed possible for practical and scalable approaches to be developed by researchers and industry alike.
Getting Ahead of the Game – The Need for Incentives and Standards
The adoption rate of residential BES systems worldwide is rapidly increasing from year to year. Taking Australia for example, which is on track to become the biggest home battery market in the world in 2019, the number of residential BES installations increases by almost a three-fold every year starting from just a couple of thousand installations back in 2015.
Given this rapid adoption of BES systems by households with solar PV, it is crucial for electricity distribution companies to understand as early as possible that the off-the-shelf operation of BES systems will not help mitigating solar PV impacts. However, there is a clear opportunity where these emerging technologies can provide an alternative to expensive and time-consuming network reinforcements. Nonetheless, for this to happen, there are two potential pathways: 1) households can be incentivized to adopt advanced battery management strategies, or 2) such strategies become mandatory.
The first pathway acknowledges the fact that households are helping electricity distribution companies deferring network investments. Hence, incentives such as direct payments or subsidized/discounted BES systems with those strategies can stimulate and accelerate the corresponding adoption. The second pathway, on the other hand, follows the philosophy of providing network support through regulation (e.g., standards, technical requirements). This is similar to what has already been seen with PV inverter functions such as Volt-Watt and Volt-Var which are required to be enabled (and with specific settings) in some parts of the world.
Andreas T. Procopiou is a Research Fellow in Smart Grids at The University of Melbourne, Australia. He has significant experience in network integration and control of distributed energy resources and future distribution networks from both the academic and industry perspectives. His research led to a number of academic publications, technical reports and one patent (filed by The University of Melbourne). Prior to his current role he worked as a Research Engineer at the largest R&D centre in Europe, Électricité de France R&D (France). Currently, he is one the lead researchers of industrial projects run in collaboration with EPRI, USA and AusNet Services, Australia. He holds a BEng (Hons) degree from Brunel University London (UK) and an MSc and PhD degree from the University of Manchester (UK).
Kyriacos Petrou is currently a PhD student of Smart Grids and Power Systems at the University of Melbourne, Australia. He has significant academic expertise in distribution network modelling, as well as integration and control of distributed resources, which has led to several publications, and one patent filed by The University of Melbourne. In addition, he has been part of two industry projects in relation to distribution networks; one in the UK and one in Australia. He holds an Meng degree from The University of Manchester (UK).
Luis (Nando) Ochoa is Professor of Smart Grids and Power Systems at The University of Melbourne, Australia and part-time Professor of Smart Grids at The University of Manchester, UK. His expertise in network integration of low carbon technologies and his extensive portfolio of industrial and academic projects have led to 150+ publications, 60+ technical reports, and two patents, one filed by Psymetrix Ltd (now part of GE) and one filed by The University of Melbourne. Prof Ochoa is an IEEE PES Distinguished Lecturer and is also Editorial Board Member of the IEEE Power and Energy Magazine. Prof Ochoa is an IEEE Senior Member since 2012. He holds a Bachelor's degree in Mechanical and Electrical Engineering from UNI (Peru), and a Research MSc and a PhD in Electrical Power Engineering, both from UNESP Ilha Solteira (Brazil).