How to store PV power with hybridization of lithium-ion batteries, supercapacitors

Researchers in Denmark have developed a new sizing strategy to combine PV system operation with lithium-ion batteries and supercapacitors. The proposed approach is claimed to reduce annual battery cycle by 13%.

A group of scientists at Aalborg University in Denmark has conceived a new sizing approach for combining PV power generation with hybrid energy storage from lithium-ion batteries and supercapacitors in an effort to improve storage operations and reduce operational costs.

“By intelligently combining lithium-ion batteries with supercapacitors, we're leveraging the strengths of each technology,” said the research team. “Supercapacitors handle the rapid power fluctuations that typically degrade batteries, while the batteries manage longer-term energy storage needs.”

In the research paper “Dual-level design for cost-effective sizing and power management of hybrid energy storage in photovoltaic systems,” published in Green Energy and Intelligent Transportation, the academics explained that their “dual-level” strategy adopts an adaptive low-pass filter (LPF) to distribute power among the grid, the battery, and the supercapacitor, which reportedly ensures that each component operates within its optimal parameters. 

In the proposed system configuration, the battery and supercapacitor are parallel to each other in an active connection. The PV system is connected to the same DC bus through a separate DC–DC converter as the battery and supercapacitor. For their modeling, they assumed a PV capacity of 6 kW and a battery capacity of 4 kWh.

Through a series of simulations, the scientists ascertained that the slope of the overall system's self-sufficiency decreases when the PV and battery capacity increase. “When it is over 60%, the slope of self-sufficiency becomes much smaller, which means the knee point (KP) is around 60%, and a lot of extra investment can gain only a little improvement in energy autonomy after this point,” they further explained. “Therefore, it is reasonable to set the self-sufficiency to 60% to balance the cost and the degree of self-sufficiency.”

They also found that, when the power ramp exceeds 10%, the time constant (TC) of the lithium-ion battery increases, and vice versa, the TC decreases. Moreover, they found that, on sunny days and overcast days, the proposed approach has minimal effect on the battery cycle reduction. “This is due to the few power fluctuations on these kinds of days, and the battery state-of-charge (SOC) has been in the optimal state without adjusting the TC,” they emphasized.

The analysis also showed that, compared to the fixed TC strategies, the new method reduces the battery cycles by 13.2%, from 1.06 to 0.91, with the SC cycles increasing “slightly” from 14.24 to 14.38, which the scientists said is acceptable due to the long lifespan of supercapacitors.

The new strategy is also said to improve SC utilization without affecting self-sufficiency and to avoid oversizing and overusing of the battery. “It is widely applicable to different areas and weather conditions, especially weather with fast-changing irradiance,” the scientists concluded.

Looking ahead, they want to include additional battery aging factors and validate their findings with real battery cells in field conditions. “Future research will also quantify economic benefits more precisely, providing a comprehensive techno-economic analysis,” they added.