Les "fossil fuel", se sont les combustibles fossiles (pétrole, gaz, etc.)
Vu le (peu de) contexte, j'imagine que switch doit se comprendre comme un "changement" pour d'autres énergies, un abandons de celles là.
gabs2009: the "context" for "fossil fuel switch" is not a gloss in French, but the sentence in which the phrase is found, and more if possible. Otherwise it's a private conversation, not a Forum discussion. "Fuel" is used free-standing in USE as both a noun and a verb.
"In practice, the most likely sectors for CDM projects, with promising investment prospects, are renewable energy (hydropower and wind), biomass and biogas (residues from sugar, rice, wood production, agriculture), waste and waste water treatment (landfills, animal farms, tapioca starch), fossil fuel switch (food, beverage, iron, steel, pulp, paper, rubber, wood).
There you go. I just kept it concise to not bother other people too much, but ok.
Thanks, gabs2009. I think now that "fossil fuel switch" means "replacement of [a switch from] one fossil fuel by [to] another [presumably more efficient fossil fuel]". "remplacement par d'autres combustibles fossiles [plus efficaces]". The English sentence structure is odd: the elements change character in the series. 1. general class [of non-fossil energy] (specific types/sources), 2. General class  (specific sources), 3. Combination of source and application  (specific sources), 4. Alternative choice of fossil fuel (industrial users of fossil fuel energy).