Engineering of Photo-Rechargeable Energy Storage

Solar photovoltaics (PV) is a very promising renewable energy technologies as it is abundant and pollution-free. However, the major drawback of PV power is its intermittency. Integration of batteries with solar modules can reduce overall PV system costs and increase the practicality of PV power. Integration of the photovoltaic cells with supercapacitor storage proved feasibility of combined photovoltaic energy generation and storage but the supercapacitors had low energy storage capacity. Photovoltaic cells with integrated Li-ion batteries as energy storage were demonstrated but had a complex structure due to multiple PV cells; low efficiency due to a mismatch between the PV cell and battery; and low storage capacity due to TiO2 nanotube structure as battery electrode material which possesses high power density but low energy density. There is a need for an efficient integrated photovoltaic rechargeable energy storage system that is cost effective. The objective of this research was to develop an integrated photovoltaic/energy storage device that uses a single solar cell to simply manufacturing (roll to roll); is more efficient by using MPPT with boost converter; and has higher energy storage using a suitable battery electrode material. The performance of DSCs, PSCs with a MPPT boost converter using discrete single solar cells to charge batteries were investigated to determine if the MPPT boost converter can provide the boost function with maximum PV power. Integrated PV/energy storage devices using DSCs, PSCs and Li-ion batteries were fabricated and characterized with MPPT. Li4Ti5O12 battery material had greater than 10% higher specific capacity and better cycling stability with 10% higher energy storage efficiency in both half and full cells than TiO2 tested. The efficiency of PSCs were 14.2% vs 7.8% for DSCs. MPPT led to 9% overall efficiency for PSC-charging vs 5% for the DSC. This was attributed to the higher efficiency of PSC. The PSC efficiency decreased by 2.2% while DSC was more stable with a decrease of 0.4% for 10 cycles studied. The overall efficiency was 4.2% for PSC integrated cell. The overall efficiency of the PSC integrated cell was lower than the discrete PSC charging of a Li-ion coin cell. This was attributed to the lower efficiency of the rear illuminated solar cell in the integrated cell. The storage efficiency of the integrated cell was comparable to that of discrete DSC charging of Li-ion coin batteries. A decrease in discharge capacity of the integrated cell was observed similar to that of the thin film battery studied separately. This integrated device had less complex fabrication, higher storage capacity and was more efficient in comparison to previous reports on third generation solar cells such as dye sensitized solar cells. A simple analysis demonstrating the use of the integrated cell for PV ramp rate application showed that the PSC/Li4Ti5O12-LiCoO2 integrated cell had storage time \u3e 150 secs to satisfy the desired 10%/min ramp rate and also could satisfy even lower ramp rate of 5%/min which required 424 secs. PSCs are at an early development stage and have achieved high efficiency, but stability is a major concern. If this problem is solved then, use of PSC for PV-battery integration will be promising