# A Machine Learning Approach for Solar Power Technology Review and Patent Evolution Analysis
## 2 Literature Review
### Solar Power Cells and Energy Storage
The model captures the characteristics of forecasting errors and revises the predictions by combining data clustering, variable selection and neural networks. The combined model approach uses the improved k-means clustering algorithm, the least angular regression algorithm and back propagation neural networks. PV storage systems can be divided into off-grid, on-grid and hybrid systems. The off-grid system, or stand-alone system, consists of battery packages, photovoltaic charge and discharge controllers, battery packs, off-grid inverters and AC/DC converters. The controller manages the charging and discharging of the battery and protects the battery from over charging and completely discharging. The function of the off-grid inverter is to convert the DC power into AC power and provide it to a system or a utility grid. The design of a stand-alone system must take into account the capacity of the battery to be used at night, knowing the power load, predicting cloudy days and determining the requirements of solar cell module boards. The design is more complicated and more expensive. The typical application is used in high mountain areas, outlying islands or undeveloped areas without power grids. Figure 2 shows the operation concept of an off-grid storage system [26]. The on-grid system consists of a PV array, a PV controller, battery packs, a battery management system, an inverter, an energy storage unit and a dispatch control system [27]. Solar panels convert light energy into electricity which charges the lithium battery pack. DC power is converted to AC power through the inverter. The controller continuously switches and adjusts the working state of the battery pack according to changes in sunshine intensity and the load status. The electricity is sent to the DC or AC converter for immediate use or the excess DC power is sent to the battery pack for storage. When power generation cannot meet the load demand, the controller uses power from batteries to ensure the continuity and stability of the system. The on-grid inverter system consists of several inverters, which convert the DC power from the battery into a standard voltage for the user-side low-voltage grid or for transmission to the high-voltage grids. The advantages are a safe and simple design, easy maintenance, with efficient solar energy generation that is higher than stand-alone systems. Figure 3 shows the operation concept of the on-grid storage system [26]. The hybrid solar photovoltaic system combines the on-grid system with more battery modules. The PV system generates power and supplies the load to charge the batteries simultaneously in the daytime, and then the power company supplies electricity at night. There is sufficient battery backup which makes the system suitable for public facilities. Hybrid systems are more complex to design and more expensive to set up. The system architecture is shown in Figure 4[28]. Figure 2: Schematic view of an off-grid system and key components. Utilizing battery storage systems can reduce the intermittent output of PV generation systems and store larger amounts of energy. [PERSON] et al. [PERSON] et al. (2018) designed an optimal charging and discharging schedule for battery storage systems such that the power loss from transmission systems interconnected with large PV generation systems is minimized. A mathematical model to simulate the charging procedures was proposed in this study, and the minimum line loss problem considering intermittent output was built into the operations support system. The optimal charging and discharging scheduling of battery storage systems was obtained using a genetic algorithm. [PERSON] (2018) also proposed a model for a combined solar PV with batteries and super-capacitors that helps reduce power injection losses to the grid during peak demand. Research and development of silicon heterojunction solar cells have seen a marked increase since the recent expiry of core patents [PERSON] (2018).