# A Machine Learning Approach for Solar Power Technology Review and Patent Evolution Analysis
## 4 Solar Technology Patent Review and Analysis
### Literature Analysis by Text Mining  
Clustering is also applied to academic literature. A total of 5610 documents are distributed into seven clusters. The academic publishing trend for each cluster is shown in the Figure 11. The seven clusters have similar increasing trends with Clusters 3 and 2 showing similar increases in research activity. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cluster** & **Topic Content** & **KEYWORDS/Key Phases** & **Patent Numbers (Total, CN, US, WO, EP)** \\ \hline
**1** & Grid-connected energy storage system, silicon based or lithium ion solar cell. & Battery, connect storage, inverter, charge, grid, wire, install, switch, conduction, AC, DC, conductive wire, lithium ion & 321, 317, 3, 1, 0 \\ \hline
**2** & Solar hydropower storage system, focusing on transferring waterpower into electricity especially for irrigation systems. & Water pump, storage tank, valve, pipeline, circulation, seawater, irrigation, condenser, pipe connect, heat collector & 619, 615, 3, 0, 1 \\ \hline
**3** & Solar battery module for both thermal hydro and PV solar systems. & Module, battery module, circuit, light, voltage, charge, sensor, inverter, board, array, signal & 156, 148, 5, 1, 2 \\ \hline
**4** & Material of thin film PV cells (focusing on silicon based and compound based). & Battery, thin film, crystal silicon, material layer, oxide, coat, electrode, insulation, metal, PV, Nano, glass layer & 197, 175, 20, 0, 2 \\ \hline
**5** & Solar hydropower system focusing on light absorbing materials of CSP. & Heat, thermal, heat exchanger, heat collector, light, material, surface, absorb, steam, evaporator, medium & 454, 402, 36, 8, 8 \\ \hline
**6** & Air processing system of CSP especially for fluid heat conduction. & Air conditioner, heat, storage, water, dust remove, indoor, channel, purification, sensor, greenhouse & 162, 155, 7, 0, 0 \\ \hline
**7** & Material of thin film battery and PV cells focusing on silicon based and organic materials. & Battery board, light, thin film, surface, component, material, organic, crystal silicon & 371, 342, 24, 1, 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of the patent clusters. Figure 10: Patent trend of all clusters. The topic contents for each cluster and the corresponding key words or phrases are listed in Table 3. Cluster 1 relates to CSP thermal collecting and storage systems with keywords of CSP plant, steam, efficiency, parabolic trough, thermal storage, heat and receiver. Clusters 2 and 5 are related to the simulation of grid-connected energy storage and supply systems, with Cluster 2 targeting on-grid systems and DC electricity supply (keywords: gGrid, load, voltage, PV module, simulation, DC, battery, capacity, network, electricity, demand and installation). Cluster 5 collects research on off-grid systems and AC electricity supply management (keywords: grid connect, PV, battery, off grid, simulation, management, converter, charge, network, AC, smart grid). Clusters 3 and 6 are related to integration of renewable energy generation systems (hybrid systems) such as the combination of wind power and solar power. Cluster 3 focuses on the economic impact of the major renewable systems and the integrated application of solar power and the other systems (keywords: renewable, electricity, battery, capacity, grid, electricity, wind, reduce, carbon, emission, PV, integration). Cluster 6 integrates hybrid systems of wind and solar or other renewable power generation systems combined with grid networks for electricity storage (keywords: hybrid, wind, PV, battery, diesel, generator, renewable, grid, resource, storage). Cluster 4 defines novel materials of PV cells (especially organic thin film), with high conversion and absorption efficiency (keywords: charge, cell, conversion efficiency, electron, organic, material, light, polymer, layer, acceptor, absorption, perovskite, PV).