The main driver right now in the US is that electricity produced from wind receives a federal production tax credit for each megawatt-hour delivered during the first 10 years of a wind farm's life. During low-demand times a wind farm can pay e.g. $10/MWh for customers to take the surplus power and still earn $14/MWh net from the tax credit (presently $24/MWh). Some states also offer per-MWh credits that can drive negative prices similarly to the federal PTC.
Solar PV power does not directly drive negative prices via the federal PTC in the same way as wind. Solar presently gets a 30% federal investment tax credit up front on the costs of constructing a solar farm. This tax incentive structure doesn't encourage generation during unprofitable times like the PTC. But a solar farm can curtail its output nearly instantaneously and at no risk to plant equipment. The same is not true of e.g. coal and nuclear generators; those facilities ramp output up and down significantly slower, and cannot generate at arbitrarily low fractions of nominal power. A cold start can take 15 hours for a legacy coal plant, longer for nuclear. Full shutdowns also impose extra costs due to equipment thermal stresses during restart. Slowly-ramping coal and nuclear plants may continue to generate negative-profitability electricity at some times of day because they can't adjust fast enough to be back at full output during profitable hours otherwise and because cycling reduces equipment lifetime.
"Impact of Load Following on Power Plant Cost and Performance: Literature Review and Industry Interviews" has a lot of good details on the impact on fossil plants from making additional adjustments:
One thing that really stands out, reading this 5 years later, is that measures to make coal more efficient (like integrated gasification combined cycle plants, IGCC, or going from subcritical to supercritical steam) also make it even less flexible. Plant operators will continue to extract what value they can from coal plants that are already constructed. But there's no plausible combination of factors on the horizon to encourage the building of advanced/new coal plants, even if Trump remains president for 8 years.
Solar PV power does not directly drive negative prices via the federal PTC in the same way as wind. Solar presently gets a 30% federal investment tax credit up front on the costs of constructing a solar farm. This tax incentive structure doesn't encourage generation during unprofitable times like the PTC. But a solar farm can curtail its output nearly instantaneously and at no risk to plant equipment. The same is not true of e.g. coal and nuclear generators; those facilities ramp output up and down significantly slower, and cannot generate at arbitrarily low fractions of nominal power. A cold start can take 15 hours for a legacy coal plant, longer for nuclear. Full shutdowns also impose extra costs due to equipment thermal stresses during restart. Slowly-ramping coal and nuclear plants may continue to generate negative-profitability electricity at some times of day because they can't adjust fast enough to be back at full output during profitable hours otherwise and because cycling reduces equipment lifetime.
"Impact of Load Following on Power Plant Cost and Performance: Literature Review and Industry Interviews" has a lot of good details on the impact on fossil plants from making additional adjustments:
https://www.netl.doe.gov/File%20Library/Research/Energy%20An...
One thing that really stands out, reading this 5 years later, is that measures to make coal more efficient (like integrated gasification combined cycle plants, IGCC, or going from subcritical to supercritical steam) also make it even less flexible. Plant operators will continue to extract what value they can from coal plants that are already constructed. But there's no plausible combination of factors on the horizon to encourage the building of advanced/new coal plants, even if Trump remains president for 8 years.