JOHANNESBURG, July 16 (Reuters) - South African state-owned power utility Eskom aims to shift to mainly clean energy sources by 2040 from its current predominantly coal-based generation fleet, it said on Wednesday as it laid out its latest plans for the change.

Eskom aims to have 32 gigawatts (GW) of renewable energy capacity by 2040, compared to less than 1 GW now, while it wants its coal capacity to shrink from 39 GW to 18 GW over that time, a company presentation on its energy sources to lawmakers showed.

Eskom said it would roll out renewable energy projects through a combination of "repowering" older coal-fired power stations slated for closure and new projects.

Repowering involves replacing old power-generation equipment with newer technology. Eskom has identified several old coal plants where it will replace generating units with renewable energy or gas-fired plants.

Eskom will have an in-house renewable energy business unit to implement projects and partner with private companies.

Among obstacles to its clean-energy goals, Eskom cited its 400 billion rand ($22.31 billion) debt burden, which it said continued to slow investment in renewable energy.

It also mentioned growing debts it is owed by South African municipalities and uncertainty over regulated power tariffs, which it argues are below the level required to cover its costs.