Supercapacitor-Based Energy Storage for Residential PV Applications: A Quantitative Comparative Study

Supercapacitors store energy electrostatically, have cycle lives of more than 10^6, and react to load transients in less than a second. All these properties have prompted several authors to suggest the use of supercapacitors as an alternative to electrochemical batteries in residential PV systems, which do not require maintenance. But the current literature is split, and methods that are proposed as fully battery-free for the supercapacitor have tended to be qualitative, without considering energy balance, conversion losses, or component lifetime, while methods that are proposed as simply using the supercapacitor as a transient buffer in a battery–supercapacitor hybrid have tended to be quantitative. The present work directly tackles this gap. A simulation of a 5 kW residential PV system, equipped
with a 40 kWh energy store, is created with one-second resolution, including capacitor self-discharge (leakage) and equivalent series resistance (ESR) and a voltage dependent buck–boost converter. The three storage architectures are analyzed: a capacitor-only store, a LiFePO4-only store, and a right-sized hybrid that integrates both. The capacitor-only architecture is demonstrated to be able to meet the load demand, but with a leakage of 3.4–3.9 kWh per day (17–20% of the load demand) and a round trip efficiency of only 68–86% versus 94% for LiFePO4; the bank also takes up 5–13 m^3. This is the limiting constraint in a levelisedcost analysis, where the maximum cycle life is constrained by calendar life, which gives the levelised cost of storage of 1.03– 4.12 USD/kWh – some 23 to 92 times that of LiFePO4. Either 68 equivalent full cycles per day or a cost of 109 USD/kWh of
the capacitors would be needed to break even with LiFePO4. In contrast, a small 0.3 kWh supercapacitor buffer (only 0.9% of the capacitor-only buffer size) embedded in the hybrid architecture is shown to reduce the current-ripple stress on the battery by 91%. These results suggest that the supercapacitor is not a viable bulk energy storage device, but could be applied in the power layer of a residential PV system where frequent shallow cycling could be economically justified.