glossaries: How to customize list of symbols with additional column for units?

I suggest to use a separate unit key for the symbol unit and refer to it with \glsunit. (Alternative: Use the user1...user6 keys for this feature)

In addition, this key must be used in the glossarystyle. I've defined a new style based on the already provided long3col style which displays the symbols in the requested order.

\usepackage[acronym,toc]{glossaries}              % use glossaries-package


\newglossary[slg]{symbolslist}{syi}{syg}{Symbolslist} % create add. symbolslist


\makeglossaries                                   % activate glossaries-package

% ==== EXEMPLARY ENTRY FOR SYMBOLS LIST =========================================
        description={Geometrical value},

        description={Height of tower},

        description={Energy consumption},

% ==== EXEMPLARY ENTRY FOR ACRONYMS LIST ========================================

% ==== EXEMPLARY ENTRY FOR MAIN GLOSSARY ========================================
    \newglossaryentry{Biofouling}{name=Biofouling,description={Some description}}

\setglossarystyle{long3col}% base this style on the list style
\renewenvironment{theglossary}{% Change the table type --> 3 columns
\renewcommand*{\glossaryheader}{%  Change the table header
  \bfseries Sign & \bfseries Description & \bfseries Unit \\
\renewcommand*{\glossentry}[2]{%  Change the displayed items
\glstarget{##1}{\glossentryname{##1}} %
& \glossentrydesc{##1}% Description
& \glsunit{##1}  \tabularnewline



    \printglossary[type=\acronymtype,style=long]  % list of acronyms
    \printglossary[type=symbolslist,style=symbunitlong]   % list of symbols
    \printglossary[type=main]                     % main glossary


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Just adding to Christian's answer: to use the unit macros inside the \si{unit} command defined by siunitx such as \metre and \kilo\watt used in his MWE, one should add the command \glssetnoexpandfield{unit} before setting any entries.

It is better explained in This Answer.