Renewable Energy is indeed a macro trend and hence be a good term long term passive investment. However the idiosyncratic risk embedded calls for broad diversification.
ETFs can indeed help to build exposure to a broad range of names.
Solar energy is maybe the first thing that comes to mind when mentioning renewable energies. It has indeed been understood by ETFs providers that developed attractive range.
We do use namely the Guggenheim Solar ETF, easily recognizable by his TAN US ticker. Details can be found here. We selected this ETF for the geographic diversification (many solar ETF are US only) and for the fact that no position exceeds 9% of the portfolio, Note that the fees are not cheap with a 0.88% TER.