# A Machine Learning Approach for Solar Power Technology Review and Patent Evolution Analysis
## 2 Literature Review
### Solar Power Cells and Energy Storage
During off-peak hours, the CSP's power generation can be adjusted according to electricity demand. The power generation can be shut down quickly and the accumulated heat can be stored by the molten salt [12]. Today's most advanced CSP systems are towers integrated with two-tank, molten-salt thermal energy storage, delivering thermal energy at 565 \({}^{\circ}\)C for integration with conventional steam as Rankine power cycles. The power towers trace their lineage to the 10-MWe pilot demonstration of Solar Two in the 1990s. The design lowered the cost of CSP electricity by approximately 50% over the prior generation of parabolic trough systems. However, the decrease in cost of CSP technologies has not kept pace with the falling cost of PV systems [13]. [PERSON] et al. [14] examined and compared two energy storage technologies, i.e., batteries and pumped hydro storage (PHS), for the renewable energy powered micro-grid power supply system on a remote island. It was found that the employment of conventional battery had higher life-cycle costs (LCC) than the advanced deep cycle battery, indicating that using deep cycle batteries is more suitable for a standalone renewable power supply system. The pumped storage combined with battery bank had almost half LCC as a conventional battery, making this combined option more cost-competitive than the sole battery option. Solar photovoltaic (PV) technologies may also be used to convert solar energy into long term storable forms by using electricity to cause chemical reactions, such as the conversion of water to hydrogen and oxygen. Solar PV systems produce no greenhouse gas emissions during operation, do not produce other pollutants such as oxides of sulfur and nitrogen, and limit the use of water for cooling [15]. Knowledge of solar radiation is important for the integration of energy systems using solar panels on buildings, greenhouses, or with grid networks. For the optimal management of energy, the development of forecasting tools is needed to anticipate the rates of energy consumption. Since global horizontal irradiation data are rarely measured, [PERSON] et al. [16] built an artificial neural network model to estimate the values. As solar collectors are often tilted to face the sun, a second ANN model was further developed to transform horizontal irradiation data into global tilted irradiation data. The most widely adapted solar cell is constructed with silicon wafers and accounts for about 90% of the total global output [17]. Due to the shortage of raw materials, the traditional silicon wafer solar cells are not meeting the demand and cost requirements of the fast-growing global market. Thin film conductors have become the technology focus of new generation solar cells since they do not require much silicon. There are many types of thin film solar cells, including germanium films (amorphous germanium a-Si, microcrystalline germanium c-Si, stacked a-Si/c-Si), compound semiconductors (copper indium gallium selenide CIS/CIGS, cadmium telluride CdTe) and dye sensitization solar cells (DSSC) [17]. Although thin film solar cells have low energy conversion efficiency, low mass production yield, and high costs, there are many advantages such as material savings since they can be fabricated using inexpensive glass or plastic substrates, can be customized and offer greater flexibility for structural applications. The tandem cell is a PV cell which uses two solar cells with different absorption characteristics enabling a wider range of the solar spectrum to be converted to energy. A transparent titanium oxide (TiOOx) layer separates and connects the two cells. The TiOx layer serves as the electron transporting and collection layer for the first cell, and is the foundation that enables the fabrication of the second cell to complete the tandem cell architecture [18]. The technical difficulty of the tandem battery is that the current generated must match and the currents generated by the two layers of the battery are not easy to synchronize.