Replacing MPPT with direct PV-battery coupling

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From pv magazine Global

Researchers at the Institute of Energy and Climate Research IEK-5 Photovoltaik, an entity under Germany’s Forschungszentrum Jülich GmbH, have proposed the use of direct PV-battery coupling as an alternative to MPPT, in order to optimize PV system performance.

“We address the direct coupling and integration between PV and batteries at a single PV-module scale, so potentially the PV-battery modules are applicable for various scales of PV installations,” researcher Oleksander Astakhov told pv magazine. “In general, a PV-battery module is applicable in different climates and utilization scenarios, at the same time to achieve optimal operation one would need to choose a specific PV-battery combination for one or another climate zone.”

The scientists described their findings in “Efficient Power Coupling in Directly Connected Photovoltaic-Battery Module,” which was recently published in RRL Solar. They noted the need to eliminate MPPT with the costs it adds to PV systems, power overhead due to self-consumption, lower reliability, and higher maintenance.

“In contrast, a direct PV-to-battery connection can be a simple and cost-effective alternative,” they said.

The German group compared the temperature and irradiance maps calculated for coupling factors to temperature and irradiance data from a hypothetical solar plant in Widderstall, Germany.

“The performance of direct connection was evaluated by comparing obtained coupling factor dependences to the power efficiency of power converters used in literature,” they said. “Finally, real charging measurements were used to validate the calculated coupling factor maps and discuss the effect of load.”

The academics tested the direct PV-battery connection on a shingled PV panel based on heterojunction solar cells and a 160 mAh lithium-ion battery. The measurements of I-V curves showed that the proposed combination could maintain a coupling factor of more than 90% in the wide range of irradiances and power outputs.

“Comparing these results to the power efficiency of MPPT devices revealed that direct coupling can provide superior performance at peak conditions and a wider dynamic range, i.e., wider range of high coupling factor on the irradiance scale,” the scientists said. “In the charging measurement, the battery was completely charged by the PV module at a constant 25 C temperature under 1 Sun irradiance resulting in 17.06% overall efficiency. The coupling factor remained above 95% in a major part of the measurement in a wide range of battery state of charge.”

Recently, researchers from the Institute of Energy and Climate Research IEK-5 Photovoltaik proposed the integration of battery storage in hydrogen production systems with PV-powered water splitting electrochemical (EC) cells. The researchers said that directly integrating batteries into such systems, without adding power management electronics, could help to smooth out PV power fluctuations and stabilize the performance of EC components.

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