\documentclass{article}
\usepackage{multirow}
\usepackage{rotating}
%\multirow{nrows}[bigstruts]{width}[fixup]{text}
\begin{document}
\begin{tabular}{|c|c|c|c|}         %{|*5{c|}
\hline
wind & on shore & & $a_{1}$ \\
\hline
\multirow{5}{*}{Solar} & \multirow{2}{*}{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}
&  & Central receivers (towers)& $a_{5}$ \\
\cline{3-4}
&  & Stirling dish (parabolic dishes) & $a_{6}$ \\
\hline
\multicolumn{3}{|c|}{\rotatebox{19}{Gethermal}}&$a_{7} $ \\
 %\usepackage{rotating}       %\rotatebox{90}{Gethermal}
\hline
\multirow{6}{*}{Biomass } &&landfill gas&$a_{8}$\\%\multicolumn{2}{•}{•}
\cline{3-4}
&&Municipal Solid Waste (MSW)-incinerator&$a_{9}$\\\cline{3-4}
&&Municipal Solid Waste (MSW)-digestion&$a_{11}$\\\cline{3-4}
&&Sewage (biosolids)&$a_{12}$\\\cline{3-4}
&&Animal waste (manures)&$a_{13}$\\\hline
\multicolumn{3}{|c|}{Fuel cell}&$a_{14}$ \\
\hline
\end{tabular}
\end{document}