Compressed air pipes

Compressed air storage vs lead-acid batteries – pv magazine International

Researchers from 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 that the former has a significantly lower Capex and a payback time of only two years.

Scientists from the University of Sharjah in the United Arab Emirates compared the storage potential of Compressed Air Energy Storage Systems (CAES) and conventional lead-acid batteries in an experimental setup and found that CAES offers a series operational advantages over electrochemical systems. “Our CAES concept is applicable to all places as it just needs reservoirs buried underground,” said research corresponding author Abdul Hai Alami. pvmagazine. “but it would really shine in hot climates.”

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

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

The performance of the two storage technologies was measured in terms of energy stored per cubic meter, costs and payback period. “In order to evaluate the performance of the system, three loads are used, a 6 W fan, a 100 W lamp and a 250 W drill,” the scientists explained. “An off-load condition was also performed to determine the full capacity of the system and compare it to theoretical calculations in terms of voltage and discharge time.”

The researchers also installed electrical cabinets to receive the generator load cables in order to study the suitability of the CAES system for industrial applications. “These include standard outlets for single phase and three phase with two different colors, protect against accidental electric shock, provide protection against ground fault leakage and finally the coated panel provides protection against environmental elements,” they wrote. underline.

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

According to their calculations, the theoretical maximum power output of the CAES system, at 12 bar pressure, should be 0.048 kW and its theoretical round trip efficiency was estimated at 86.6%. The system’s experimental maximum power output, however, was 27% lower at 0.035 kW and its experimental round-trip efficiency was about 60%. The battery turned out to provide continuous operation for about 50 minutes, after which the power supplied to the inverter turned out to be insufficient and the loads were immediately de-energized.

“The main factors to improve round-trip efficiency are larger air storage volumes to ensure constant pressure input to the air motors, better air temperature thermal management, connection of multiple tanks to the system and their operation in series or in tandem and finally having a pneumatic motor/generator as a single unit to overcome mechanical losses,” Alami explained. “Simulating and controlling system operation to allow inlet pressure variation to follow demand is another important factor in improving operation and scaling.”

In their cost comparison, the researchers considered an 840 kWh/3.5 kW CAES configuration and a 1,400 kWh lead-acid battery connected to a 3.5 kW battery inverter. The cost of the second configuration was estimated at $130,307 and that of the CAES system at $23,780.

“As a rough estimate, the break-even point with a battery storage system can be reached in 3-5 years depending on the required charge-discharge cycles of the battery and not including the price of the battery cabinet. , air conditioning and cooling load costs,” they pointed out, noting that while CAES capacity is solely a function of storage tank capacity and the space available for them, the footprint outside system floor is minimal and comparable to a battery enclosure connected to an air conditioning split.“The payback period, in this case, would be around 1-2 years,” they said.

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

Researchers Rashid Alrashid. Abdul Hai Alami and Ahmad Yasin

Image: University of Sharjah

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