Energy Storage

Image: RPI

Researchers from Rensselaer Polytechnic Institute in the United States have developed a special class of materials for bulky calcium ions, providing pathways for their facile insertion into battery electrodes.

From pv magazine

Against a backdrop of soaring prices and predicted shortfalls of lithium-ion battery materials, the search for inexpensive, abundant, safe, and sustainable battery chemistries has never been more critical. Calcium has been considered in batteries, but the larger size and higher charge density of its ions, relative to lithium, have posed challenges for their insertion into electrode materials.

Now, researchers from Rensselaer Polytechnic Institute in the United States have reported progress in addressing this issue and unlocking the potential of high-performing calcium-ion batteries.

“The calcium ion is divalent, and hence one ion insertion will deliver two electrons per ion during battery operation,” said Nikhil Koratkar, the John A. Clark and Edward T. Crossan Professor of Engineering at Rensselaer. “This allows for a highly efficient battery with reduced mass and volume of calcium ions.”

However, the larger size and higher charge density of calcium ions relative to lithium impairs diffusion kinetics and cyclic stability, he added. The team has overcome this problem by developing oxide structures containing big open spaces (heptagonal and hexagonal channels). In their work, an aqueous calcium-ion battery is demonstrated using orthorhombic and trigonal polymorphs of molybdenum vanadium oxide (MoVO) as a host for calcium ions.

The researchers have demonstrated that calcium ions can be rapidly inserted and extracted from the material, with these tunnels acting as “conduits” for reversible and fast ion transport. The findings indicate that MoVO provides one of the best performances reported to date for the storage of calcium ions.

Specifically, for trigonal MoVO, a specific capacity of ∼203 mAh g⁻¹ was obtained at 0.2C and at a 100 times faster rate of 20C, an ∼60 mAh g⁻¹ capacity was achieved. The open-tunnel trigonal and orthorhombic polymorphs also promoted cyclic stability and reversibility. These findings were recently published in Proceedings of the National Academy of Sciences (PNAS).

“Calcium-ion batteries might one day, in the not-so-distant future, replace lithium-ion technology as the battery chemistry of choice that powers our society,” says Koratkar. “This work can lead of a new class of high-performing calcium-based batteries that use earth abundant and safe materials and are therefore affordable and sustainable. Such batteries could find widespread use in portable and consumer electronics, electric vehicles, as well as grid and renewable energy storage.”

Author: MARIJA MAISCH

The experimental setup at the campus of the University of Sharjah. Image: University of Sharjah

Researchers in the United Arab Emirates have compared the performance of compressed air storage and lead-acid batteries in terms of energy stored per cubic meter, costs, and payback period. They found the former has a considerably lower Capex and a payback time of only two years.

From pv magazine

Scientists from the University of Sharjah in the United Arab Emirates have compared the storage potential of compressed air energy storage (CAES) systems and conventional lead-acid batteries in an experimental setup and have found that CAES offers a series of operational advantages over electrochemical systems. “Our CAES concept is applicable to all locations as it just needs tanks buried underground,” the research’s corresponding author, Abdul Hai Alami, told pv magazine. “but it would really shine in hot climates.”

In the study “Experimental evaluation of compressed air energy storage as a potential replacement of electrochemical batteries,” which was recently published in the Journal of Energy Storage, the UAE group described the experimental setup as a unit combining a CAES system operating as an AC generator that is connected to various loads through an electrical panel. Its performance was compared to that of a 12 V, 70 Ah battery provided by US-based Incoe Corporation, which was connected via a 600 W inverter through an inversion circuit to the same load.

The CAES system consists of an air motor connected to a 3-phase, permanent magnet AC generator supplying 380 V and 5 A. The connection was made either directly or through a 1:2 or 1:4 speed-up gearboxes to compare the discharge time and output energy quality. The system also had a rudimentary heat exchanger – a pipe loop in a water bath – to control air temperature, which affects air density significantly, resulting in better system efficiency. The scientists tested two different motor sizes of 5 hp and 9 hp, respectively.

The performance of both storage technologies was measured in terms of energy stored per cubic meter, costs, and payback period. “In order to assess system performance, three loads are operated, a 6 W fan, 100 W lamp, and a 250 W drill,” the scientists explained. “A No-load condition was also performed to determine the full capacity of the system and compare it with theoretical calculations in terms of voltage and time of discharge.”

The researchers also installed electrical cabinets to receive load cables from the generator in order to investigate the suitability of the CAES system for industrial applications. “These include Industry-standard sockets for single phase and three phases with two different colors, protect against inadvertent electric shocks, provide earth leakage protection and finally the coated panel provides protection against environmental elements,” they emphasized.

The academics explained that the quality of the energy generated by the generator’s air motor, which is in turn activated by the kinetic energy coming from the storage air cylinders of the CAES system, is provided by maintaining operation at the rated rotational speed of the generator in order to satisfy the minimum output voltage and operational frequency that will eventually be supplied to the end-users.

According to their calculations, the theoretical maximum output power of the CAES system, at 12 bar pressure, should be 0.048 kW and its theoretical roundtrip efficiency was estimated at 86.6 %. The experimental maximum output power of the system, however, was 27% lower at 0.035 kW and its experimental roundtrip efficiency was around 60%. The battery was found to ensure continuous operation of around 50 minutes, after which the power supplied to the inverter proved to be insufficient and the loads were immediately unpowered.

“The main factors to enhance roundtrip efficiency are the larger air storage volumes to ensure consistent pressure input to the air motors, better thermal management of air temperature, connecting more than one tank to the system and operating them in series or in tandem and finally having an air motor/generator as a single unit to overcome mechanical losses,” Alami further explained. “Simulation and control of system operation allowing input pressure variation to follow demand is another important factor in enhancing operation and scale up.”

In their cost comparison, the researchers considered an 840 kWh/3.5 kW CAES setup and a 1400 kWh lead Acid battery connected to a 3.5 kW battery inverter. The cost of the second setup was estimated at $130,307 and that of the CAES system at $23,780.

“As a rough estimate, breakeven point with a battery storage system can be achieved within 3–5 years depending on charge-discharge cycles required from the battery and no including the price for battery cabinet, air conditioning and the costs of the cooling load,” they highlighted, noting that if the CAES capacity is solely a function of the storage tanks capacity and space available for them, the above-ground footprint of the system is minimal and comparable with a battery enclosure connected to an air conditioning split unit. “The payback period, in this case, would be around 1–2 years,” they said.

The research team is currently considering how to bring the proposed storage technology closer to commercial production. “The system is intrinsically compatible with off-grid solar farms, charging the tanks with high-pressure 100 bar air compressors with a capacity of around 30 kW,” Alami said. “With tanks available and amenable to underground burial, the main technical issues of the system are clear and manageable for such installations. Also, the system can power EV chargers in off-grid locations.”

Author: EMILIANO BELLINI

Image: pv magazine, Eckhart Gouras

Greener says that battery storage could help large electricity consumers in Brazil to cope with sharp differences between peak tariffs and off-peak tariffs.

From pv magazine

Batteries are already competitive for consumer energy storage in behind-the-meter applications in several Brazilian states. Marcio Takata, the director of consulting company Greener, Marcio Takata, described this market opportunity during the Greener Business Summit earlier this month in Sao Paulo, Brazil.

He presented a case study involving an industrial consumer in the area of ​​Cosern, in the state of Rio Grande do Norte. The difference between the tariff at peak hours and the off-peak tariff is BRL 3.004 ($0.55)/MWh.

According to Greener, such consumers could reduce their annual bills from approximately BRL 273,600 to BRL 173,600. They would achieve 36% savings on the energy tariff by installing a lithium-ion battery with a capacity of 0.6 MW/2.5 MWh.

In the simulation, the consumer would have a reduction of BRL 123,800 in the peak tariff, from BRL 173,400 to BRL 49,600, and an increase of BRL 21,800 in the off-peak tariff, from BRL 103,200 to BRL 124,000. The net savings with storage would therefore be BRL 102,000.

The investment, for a useful life of 15 years, was estimated at BRL 11 million, with an operating cost of BRL 55,000 per year. There is a need for reinvestment in the tenth year of the asset, consisting of 15% of capex, or BRL 1.65 million. The tariff adjustment for the period was 6% per year.

The difference between peak and off-peak tariffs is crucial to the viability of battery storage to manage the load. According to the Greener survey, this difference can range from BRL 354/MWh in the CEA concession area, in the state of Amapá, to BRL 3,971/MWh, in the Equatorial concession area, in the state of Pará. In addition to the distributors in Pará and Rio Grande do Norte, other distributors such as Coelba (BA), Enel RJ and EMS (MS) have peak and off-peak rates with differences above BRL 3,000/MWh.

Battery storage is seen as a way to diversify for companies that already operate in the solar sector. Sao Paulo-based Brasol, for example, is expected to invest in its first storage projects this year.

Author: LIVIA NEVES