Does Renekton Legend ability work on Accelerate ability?

Yes, Renekton's legend ability can be used to pay the Energy portion of the Accelerate cost.

Renekton, Butcher of the Sands adds [2] Energy to your rune pool with the restriction "Spend this Energy only to play units or activated abilities of units." Accelerate (Rule 805.1.a) is an optional additional cost of [1] Energy and 1 Power that you pay as part of playing a unit — during the "Pay Costs" step (Rule 357 / 805.2). Since that Energy is being spent while playing a unit, it fits Renekton's spending restriction.

Note two things: (1) Renekton only adds Energy, not Power — you'll still need to provide the 1 Power from elsewhere. (2) Renekton's own ability requires exhausting him and paying [A][A], so you need to activate it before or during the Pay Costs step (it has Reaction, so you can activate it as an Add reaction during cost payment per Rule 429.3).