The plunging price of solar panels and wind generators has made new clean energy strategies possible, strategies that were unimaginable even ten years ago. Solar panels and wind generators are close to miraculous. Their energy is clean, and once they are up and running, the cost of their electricity is almost zero. They are ideal candidates to fight climate change.

But they cannot do it alone. We all know that the sun shines barely half the time and the wind can die down over wide areas for days at a time. When that happens, of course, there is a gap in the supply of electricity from solar and wind farms. Since an economy needs power 24/7 regardless of the weather, there needs to be some easy and economical way to supply enough clean power to fill that gap.

Without cutting demand, there are two ways the gap can be filled: backup power generation and/or backup storage (e.g., batteries). To ensure no interruptions, those backup systems must be exceedingly robust and have the capacity to supply full peak power when peak demand occurs during gap times, especially those gap-times when solar and wind production falls off completely. And because, particularly in winter, the gap can last for many days, the backup systems must be capable of supplying uninterrupted power for at least several days and sometimes a week or more.In other words, the combined backup systems must be able to supply the full electrical load for the duration of the gap.

There are a number of technologies that might serve to help fill the gap. Each has its own advantages and disadvantages:

Presently, the gap here and around the world is being filled with power primarily from fossil-fueled plants and, sometimes, nuclear plants. Although nuclear is clean and can be economic (as it is in Asia), traditional nuclear power has its own set of problems, including political. As a result, countries undergoing extensive solar and wind build-outs are retaining, or even building more, fossil-fueled plants to fill the power gap. Farthest along this path is Germany. Germany has spent almost a half-trillion dollars on renewable energy. Yet because they are also closing their nuclear plants, they’ve needed to build more coal and gas plants to ensure 24/7 power for their industrial economy. With an increasing electrical demand for charging cars, the gap will grow even larger, and Germany will probably need to build still more fossil plants.

Long Transmission Lines. Because it can be windy when solar doesn’t produce at night or on days with heavy clouds, wind might theoretically be a gap-filling candidate. On the belief that there is always wind somewhere, more high-tension powerlines could conceivably bring power from a windy somewhere to locations where the wind has become calm. Those powerlines would have to span the continent because wind patterns and wind droughts can be very widespread. As the data (and California’s 2020 blackouts) show, the closest “windy somewhere” can be thousands of miles away. Moreover, that “somewhere” would have to have enough extra wind capacity to meet not only its own needs, but also the capacity to power regions where there might be little or no wind. For example, Texas and Iowa would have to build enough extra wind farms to supply themselves and power the eastern half of the country when the East is becalmed. Moreover, the East Coast would have to reciprocate by building enough extra wind generators to supply Texas and the Midwest when a wind drought strikes there. And even if that much wind farming were possible, the cost in money and land required for such a strategy would be prohibitive. So even very long transmission lines, although they might help somewhat, would not eliminate the gap.

Both Wind and Solar. It might seem that combining wind and solar would greatly reduce the size of the gap. When one isn’t producing, the other might be. But here again, the data show that sun and wind droughts can be simultaneous, cover thousands of square miles, and last for a week or more. Even in the daytime, when clouds cover the sun, solar production can drop by more than 90 percent. All of which means, of course, that whereas the two combined might sometimes help with the gap, we still need those sources of firm and/or stored, 24/7, gap-filling power.

Demand Response. One suggestion sometimes heard to help with the gap issue is something called demand response. That basically means cutting back or shutting down power use during the gap when solar and wind are not producing. From a practical standpoint, it’s hard to imagine an industrial economy throttling back for days at a time—or homeowners turning off air conditioners on a hot humid night. Were an economy to adopt such draconian measures, it would quickly be outcompeted by economies that used other methods to fill the gap so they could keep running.

Batteries. Large solar and wind farms often produce much more electricity than the system can use at the time. Batteries can be charged with that excess energy and supply that energy later, when needed, to help fill the gap. Unfortunately, the problem of scale intrudes here too. The gap is orders of magnitude larger than our best battery technology. For example, the largest battery in the world is Tesla’s $66 million, 100MW battery in Australia. If that battery were in California, it would only power PG&E’s 10,000 MW-per-hour customer base for less than one minute! Unfortunately, it will likely take decades before batteries will be able to store enough power to last more than a few hours, much less a week. In fact, as of this writing, the needed technology is not even on the horizon. When it comes to grid-scale power, it will be a long time before batteries can be anything more than a small, minor contributor to filling the gap.

Hydrogen. Hydrogen gas, on the other hand, is a much more scalable form of potential gap-filling power. Like batteries, hydrogen is a form of energy storage.It burns clean and pollution-free. It can often be substituted for natural gas and used to power electric generating plants. It might also power cars, trucks, and planes. But the problem is that unlike natural gas, hydrogen does not occur naturally.It must be manufactured. Most hydrogen today is produced by stripping it from hydrocarbons—a process that generates copious amounts of greenhouse gas. Alternatively, hydrogen can be made by the electrolysis of water. Often called green hydrogen, it is a way to store and later recover the energy it took to make it. But electrolysis takes electricity, lots of it. In fact, it takes substantially more electricity to make hydrogen by electrolysis than the electricity it can produce when burned. Moreover, if wind and solar electricity is used to make hydrogen, their uneven power output means the hydrogen plant will operate only part time. Those two factors make the process extremely expensive.So much so that making hydrogen with wind and solar power and then burning it in electric generating plants is far and above the most expensive way to make electricity. So although green hydrogen can fill most of the gap with clean energy, there is much that needs to be done to bring the cost into reasonable territory. On the other hand, if nuclear power is used to produce hydrogen, even with current technology, the cost of hydrogen could be cheaper than gas.

It is clear that, with the possible exception of hydrogen, these technologies, even taken together, would still fall far short of ensuring the reliability needed for a modern economy.

All this means that in order to meet our zero-carbon commitments on time, we will need new technology, new sources of clean, gap-filling energy that are available 24/7 on demand. To ensure the necessary reliability, these sources, plus any available storage, must be robust and have the capacity to meet peak electricity demand by themselves—without any help from the wind and solar farms that might be idle.

Happily, there are a number of such sources under development, some of which are scheduled for deployment in the next couple of years. The two most promising are advanced nuclear and Allam cycle gas. A short summary of each:

Advanced Nuclear. Most conventional nuclear power plants are water-cooled and operate under very high pressure. The advanced technologies are completely different. Those most likely to hit the market by the middle of this decade use no water, operate at atmospheric pressure, and require no operator or mechanical intervention for safety. The physics of their fuel prevents runaway overheats. Most make less waste than traditional reactors and some of the later versions can use our existing stockpile of waste as fuel. Best of all, costs are dramatically less because they can be built in factories and assembled on site.In fact, once we begin building them, their cost and build times should be comparable to that of a new coal plant. Optimists believe that in time their costs will experience the same reductions that solar and wind have seen. They will also represent the safest and most environmentally friendly 24/7 power source on the planet.

Allam Cycle Gas (ACG). Nearing completion in LaPorte, Texas is the first of what may be a solution to both declining jobs in areas dependent on fossil fuels and another way to cleanly and reliably fill the gap. Net Power’s ACG process produces only water and pure CO2 from the combustion of natural gas.The CO2 it produces from that combustion is not contaminated by other exhaust gases and is therefore ready for sale or sequestering. No expensive carbon capture is required. And because it uses CO2 to turn the generators, the ACG process also enables a much more efficient method of turning combustion heat into electricity.

The good news is that these two frontrunners can fix the gap problem and, combined with wind and solar, completely power the nation with clean electricity, especially as electrification of transportation and heating double electricity demand. If we’re serious about fighting climate change, we must greatly expand and accelerate the development, public acceptance, public and private financing, and rapid deployment of these clean, gap-filling energy sources.