Compressed air storage vs. lead-acid batteries

From pv magazine Global

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.”

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