> The world needs a way to convert renewable energy into constant energy.

Not really or at least that represents a gross simplification of the situation.

Electricity demand is anything but constant - peak to trough intraday consumption can vary by up to a factor of 5. Grid engineering is all about matching lots of different sources with different generation characteristics with a varying (but quite predictable) demand curve.

All generation sources are intermittent - they are just intermittent in different ways. Solar and wind have well known limitations in this regard but at least the variation in output is predictable - particular solar, wind is typically accurately predictable up to a few weeks horizon.

The intermittency associated with thermal plants may be less in some ways but it has the disadvantage is that it is largely not predictable. For example, the average US coal plant will be unavailable for generation 15% of the time - so roughly 1 hour down for every 6 hours generating. Most of this 1 hour downtime is unscheduled/forced which is unpredictable. It's a myth that having a 1GW of fossil fuel generation capacity means you can reliability meet a peak demand of 1GW.

Also nuclear and coal are NOT good at load following - they operate most efficiently when producing the constant design output. Ramping up/down coal or nuclear output quickly is often not at all possible or is possible - depending on plant design - but with a large loss in efficiency and increased plant stresses and wear and tear.

Grid engineers have been maintaining this balancing act between unreliable generation and fluctuating demand for ever. In the past the focus was on coping with the intermittency caused by the failure modes of thermal plants. Increasingly now they are coping with the variable output of solar and wind generation but seem to be managing this - a bunch of European countries source more than 40% of their electricity from solar or wind and none required utility scale li-ion or have experienced increased grid instability.

The same tools are used to handle wind and solar intermittency as are used to handle thermal plant failure or inability to ramp up/down quickly - some hydro storage, backup idling natural gas plants, grid interconnections, etc.

Fundamentally the renewables revolution is happening quietly in the background is driven by simple economics. Coal and nuclear are just too expensive by a factor of 2 or 3 and natural gas, on-shore wind and utility scale solar are just so cheap in comparison. It's cheaper now to build a load of wind (or solar) and some backup natural gas generation - typically with a capacity factor of only 10% or so - than it is to meet demand with thermal fossil fuels. This is because wind and solar are capital intensive while NG plants are cheap to build but expensive to operate due to fuel costs. This combo (idling natural gas and wind and/or solar) is in the process of displacing everything else. 90% of the new generation capacity added in the US last year was of this nature. And a similar proportion is observed globally.

Meanwhile you have endless arguments about why wind and solar "cannot work" in fora, while all around the world it clearly IS working and analysis suggests it requires no technology breakthrough to get to 60%-70% carbon-free generation - many grids are well along this journey (40% to 50%) and none of the doomsday scenarios of massive load shedding, black-outs, etc. have occurred.