\documentclass{article}
\usepackage{multirow}
\begin{document}
\begin{tabular}{|c|c|c|c|}
\hline
Wind&On shore&&$a_1$\\
\hline
\multirow{5}{*}{Solar}&Photovoltaic&On grid&$a_2$\\
\cline{3-4}
&&off grid&$a_3$\\
\cline{2-4)
&\multirow{3}{*}{Solar thermal}&Linear parabolic collector&$a_4$\\
\cline{3-4}
&&Linear parabolic collector&$a_5$\\
\cline{3-4}
&&Stirling dish(parabolic dishes)&$a_6$\\
\hline
\multicolumn{3}{c}{Geothermal}&$a_7$\\
\hline
\multirow{6}{*}{Biomass}&&Landfill gas&$a_8$\\
\cline{3-4}
&&Municipal Solid Waste (MSW)-incinerator&$a_9$\\
\cline{3-4}
&&Municipal Solid Waste (MSW)-digestion&$a_10$\\
&&Sewage (biosolids)&$a_11$\\
&&Animal waste (manures)&$a_12$\\
&&Forest residues&$a_13$\\
\hline
Fuel cell&&&$a_14$\\
\hline

\end{tabular}
\end{document}