solar and wind
PV solar has a very poor EROI, the next worst after ethanol. This is due to the energy needed to refine the silicon to ultra-pure standards. The ultra-clean, air-conditioned factory that makes those panels has to be built first (with fossil fuels) and it will operate for maybe 30 years if you're lucky. So some of the Energy Input must wait for 30+ years to be repaid with Energy Output.
Wind turbines in windy places do much better, but even then they have limited lives. A wind farm on Hawaii ran for 20 years and then needed a major refit, but they scrapped it and built another one instead. This sort of thing is only possible in an energy-plentiful economy. When we change to an energy-scarce economy, these projects will be un-doable.
