Improvement of converter efficiency in partial load using temporary storage with lithium-ion capacitor

Electrical output efficiency from renewable energy sources tends to be lower because the converters, such as power conditioning subsystems (PCSs), are less efficient in partial load. Operation in partial load is common because of the fluctuations are typical in natural energy. We proposed and developed a high-efficiency converter system that uses a renewable energy source such as photovoltaics (PV), lithium-ion capacitors (LiCs) for temporary energy storage, a converter for external output, a pyranometer, and a controller. PV and LiCs are directly connected without a converter, and LiCs store electrical energy with ultra-high efficiency (approximately 99.4%) of PV output. The converter for external output can convert with high efficiency at the rating at one stroke from the stored energy in LiCs, while continuing to generate PV power. In general, a PCS for PV is operated at low efficiency at low solar radiation intensity of approximately 350 W/m² or below, and the operation stops when solar radiation intensity is less than approximately 200 W/m². In the proposed system, LiCs temporarily store energy when solar radiation intensity is less than 350 W/m² or when the conversion efficiency of PCS drops below 80%. After solar radiation intensity increases or LiCs are recharged, PCS can output at over 92% efficiency by converting stored energy in LiCs at one stroke. As a result, the proposed system increased annual gross power generation in Japan by more than 12%.