Expert Talk: Battery storage in the energy transition

Power system and energy transition: new challenges

You want electricity to be always available at your electric outlets. In other words, you want a reliable supply of power. A careful balance between generation, by power plants, and consumption, at demand side, is essential to secure the quality and reliability of a power system. Such a balance is challenged by the turbulent and uncertain nature of the electricity demand. In simple words, the power consumption might vary significantly during a single day and all over the year. For instance, a concentrated consumption in morning and evening at the start and end of working hours (i.e. peak hours), respectively, is in clear contrast with quite low demand in between.

The ongoing energy transition brings additional challenges to manage the equilibrium of a power system. Significant penetration of renewable wind and solar electricity into the energy mix together with the increasing electrification because of increased use of ICT systems, electric vehicles and heat pumps, modifies the circumstances. The potential imbalance of the system might grow both in time dispersion and intensity due to the intermittent nature of the generation and the extra electricity consumption.

A discussion of power system reliability and quality in view of the energy transition deserves a comprehensive techno-economic analysis and is very case dependent. For instance, many scenarios could be investigated depending on the share of centralized vs. distributed generation of wind and solar electricity which is beyond the scope of this note. What follows is mostly relevant to a simple case where the traditional existing electricity grid is complemented by a distributed generation of a specific renewable source of power: residential generation of electricity by photovoltaic cells (PV).

The role of battery storage

The typical significant misalignment between the PV generation and electricity demand in a residential building is schematized in Fig1. The generation surplus from the PV panel (green dashed area, Fig.1) could be injected to the grid in return for a financial incentive. But, what happens to this energy surplus if the grid refuses to accept it, e.g. due to insufficient grid capacity? Should this green power be curtailed then? Of course not.

Battery energy-storage systems (BESS) are good candidates to increase the flexibility of a power system by using the available grid capacity in a more efficient way. They save the PV generation surplus for later self-consumption during peak hours or injection to the grid when possible. BESS are based on a rechargeable electrochemical cell (EC) that can store and release electricity during charge and discharge, respectively. Many different EC technologies are available, at different levels of maturity, each with a unique combination of materials for the main components of the cell, i.e. anode, cathode, and electrolyte. The energy-storage performance of each EC technology is mostly dictated by the chemical nature of the cell components and is assessed by the following important parameters:

1. Specific energy per unit of weight (Wh/kg) and volume (Wh/l): the minimum storage size in Wh (watt-hour) to avoid curtailment[1] is equal to the ensemble of generation surplus (green dashed area, Fig.1). More specifically, the area confined below the generation (green line, Fig.1) and above consumption (red line, Fig.1) profiles in a power-time plot and amounts to 4 kWh for the example in Fig.1. This storage size would be currently offered by a [100 kg, 70 l] lead-acid, a [200 kg, 250 l] redox-flow, or a [35 kg, 38 l] lithium-ion EC technologies. In other words, lightest weight and smallest size BESS systems are possible with lithium-ion technology thanks to their high specific energies. Although size and weight limitations for residential storage are less strict than in electric vehicles, smaller BESS units are preferred in residential areas for evident reasons such as ease of integration.
2. Specific power (W/kg or W/l): electrochemical cells during discharge and charge have to respect a specific voltage window defined by low and high cut-off voltages. The cell activity outside this zone is known as the over-discharge or over-charge and can lead to accelerated aging and safety hazards. The cell voltage should be continuously controlled by the battery management system (BMS), as the energy drain from and injection to the cell is concurrent with a continuous drop and rise in voltage, respectively. The rate of such a voltage change is correlated to the power capability. In another words, a cell subject to a higher power drain or injection reaches the cut-off voltages sooner. Therefore, a cell might refuse to charge or discharge even if it possesses enough storage room or enough energy, respectively. Accordingly, besides having enough room for generation surpluses, BESS systems have to be compatible with both the power rating of PV generation (green line, Fig.1) and residential load (red line, Fig.1). A careful analysis of the load and generation profile together with the intrinsic behavior of an electrochemical cell technology is required to optimally size a BESS system. Similar to the specific energy, the specific power capability (W/kg or W/l) of lithium-ion batteries is significantly higher than the power rating in other EC technologies such as lead-acid, Ni-MH, Ni-Cd, and redox-flow.
3. Efficiency: Efficiency or better said ‘cycling efficiency’ of an EC is usually defined as the ratio between the energy output (E_o) and input (E_i) during a charge-discharge cycle. For instance, an  efficiency of 100% would mean that a battery only charged using the PV surplus generation (E_i, green dashed area, Fig.1) could be sufficient to compensate, upon discharge, for the PV generation shortage (E_o, sum of red-dashed areas, Fig.1). In practice, however, the efficiency is always smaller than 100% because a fraction of energy input to the cell during charge will be lost as heat. Cycling efficiency is a complex function of the power, temperature, and age of the BESS system. Pristine lithium-ion batteries are characterized by a typical efficiency of 90% whereas other technologies such as lead-acid and redox-flow batteries are less efficient (~80%).
4. Lifetime:The battery reaches its end-of-life (EOL) when its energy and power characteristics fall below a certain threshold. Such a threshold is usually set to a fraction of battery energy (Wh) or capacity (Ah) at the beginning of service life, with a typical value of 20%. Therefore, the useful age of a battery depends on how fast it experiences a 20% loss in capacity and power. This mainly depends on the operating conditions (i.e. charge, discharge, rest periods, and temperature) and the rate of chemical and mechanical degradation inside the cell. The current generation of batteries offered for the residential storage (lithium-ion, lead-acid, and redox-flow) are usually guaranteed for a period of 10 years or a few thousands of deep cycles whichever comes first. In a residential storage application and in the view of the current mature battery technologies, the need for battery replacement is inevitable after 10-15 years.

Current status of battery technologies: how far & how expensive

Unlike the portable electronics and transport applications (e.g. electric vehicles, start-lightning-ignition), batteries are much less exploited, so far, in residential storage. Although there is a good prospect for many battery technologies to penetrate into the market of residential storage, the mature options are currently quite limited.

We usually report the price of a battery technology as a specific price normalized to the storage size ($/kWh). This specific price is a function of technology (e.g., lead-acid, lithium-ion, etc.), maturity, and the storage size. The chemical nature of the components inside a cell has a significant impact on the final price of a given technology. The material cost of a battery is sensitive to: 1) raw material abundance in the nature and geographical distribution, 2) synthesis and processing route, 3) possibility of a feasible recycling technology for material recovery at the end of battery life. The maturity of a battery technology can be characterized by its global cumulative production (CP) in Wh. This parameter can also be used in the so called experience-curve analysis to predict the future decline in the production costs driven by the economy of scale. The specific price is a function of the battery size: for instance, a single (~10Wh) 18650 cylindrical lithium-ion cell could cost 20% more per kWh compared to a small lithium-ion battery pack (100Wh) in a laptop.

1. Mature (CP >1 TWh): the current cumulative production of lead-acid and Lithium-ion batteries are beyond 10TWh and 1TWh, respectively. The majority of these battery productions are in the form of small capacity and end up in applications such as start-lightning-ignition (lead-acid, <1kWh) and portable electronics like smartphones (lithium-ion, <0.1 kWh). Lead-acid batteries have a price tag of well below 500 $/kWh whereas the price for lithium-ion batteries vary in a broader range of ~ 400 to 1500 $/kWh.
2. Maturing (CP <100 GWh): lithium-ion batteries for electrification of road transport are a notable example for a steadily maturing battery technology. Here, large battery packs in the range of 10 to 100 kWh are assembled to power the hybrid, plug-in hybrid and full- electric vehicles. Such battery packs have currently a specific price of ~ 250 to 600 $/kWh.
3. Emerging (CP <1GWh): application of medium size Li-ion and lead-acid batteries (1-20 kWh) for residential storage (Fig.2a) is emerging with a global installation of ~ 1GWh. In recent years, redox-flow batteries (Fig.2b) are getting more attention and besides to the utility applications (<0.5 GWh), some compact versions are currently offered for the residential storage. The current cost of these residential batteries varies between 500 and 2000 $/kWh.
4. Non-commercial (R&D): Many promising cell technologies are under development for the next generation batteries in the scientific community. The goal is to go beyond the state-of-the-art lithium-ion batteries with respect to the performance, sustainability, and safety:
   1. electrolytes with higher electrochemical and thermal stability: e.g. composite and solid
   2. electrodes based on high energy and/or sustainable active-materials: e.g. Na, Si, S, O₂

Feasibility & future trends

What is the time lag between the investment and payback for residential storage? The payback time is a complex function of electricity price, governmental incentives, and battery price. It is currently estimated to be 8 to 15 years subject to the current trends and according to the existing literature. A payback time as early as 5-6 years could be envisaged in near future given the rising price of electricity and falling price of batteries. The experience-curve analyses available in the literature suggest that the price of the residential-storage batteries will scale ~ 12-15% down for every doubling in the cumulative production. This would correspond to an approximate price of 250 to 400 $/kWh for the residential batteries by the time of maturity (i.e. CP >1 TWh).

Conculsions

References

1. M. Safari. ‘Battery electric vehicles: looking behind to move forward,’ Energy Policy 2018 (115): 54-65.
2. B. Joos, T. Vranken, W. Marchal, M. Safari, MK. Van Bael, A. Hardy. ‘Eutectogels: a new class of solid composite electrolytes for Li/Li-ion batteries,’ Chemistry of Materials, 2018 (30): 655-66
3. O. Schmidt, A. Hawkes, A. Gambhir, I. Staffell. ‘The future cost of electrical energy storage based on experience rates,’ Nature energy, 2017(2): 17110.
4. V. Muenzel, I. Mareels, J. de Hoog, A. Vishwanath, S. Kalyanaraman, A. Gort. ‘PV generation and demand mismatch: evaluating the potential of residential storage,’ IEEE Power & Energy Society Innovative Smart Grid Technologies Conference (ISGT), Washington 2015: 1-5.

The EnergyVille battery research

Written by Momo Safari and An Hardy, both professors at the Institute for Materials Research at UHasselt and researchers at EnergyVille. Momo is a battery expert and An is specialised in chemical synthesis methods for inorganic nanomaterials. In this expert talk they give an overview of what the current status of battery technologies is: how far developed & how expensive are the battery material trends? What are future trends?