The potential for capturing sunlight to generate usable energy is 300 times total global energy demand. That is not just for electricity, but for all energy needs. Add flexibility in location of collectors and options for local use and solar seems like the perfect approach for moving toward the all-electric economy (c.f. Bill Gates' How to Avoid a Climate Disaster) while minimizing impacts on already strained transmission grid systems.
But there is that one big problem - solar panels don't produce electricity in the dark!
So, while wind and solar are the fastest growing sources of electrical energy, for these to become part of baseline production not just the intermittent icing on the electricity cake, we need ways to store solar energy, lots of it.
Batteries! I hear you say. True, and that is the conventional approach, and steady progress is being made in battery technology, with some major, community-scale installations in place. Examples from Australia, Hawaii and California are included in the Sources section at the end of this essay.
But the fun here is in thinking about other possibilities, so let's take a wider view of storage options, including some experimental ones, and some wrinkles on ones we've tried before.
When thinking about or comparing related topics my first inclination is to try to put them together into a single diagram that captures how they relate to one another. Here is an attempt to do that for energy storage mechanisms organized into something like a metaphorical (meaning terribly imprecise - my apologies to any physicists reading this) energy pyramid.
We start at the top with high-quality radiant energy received from the sun that can be put to many uses, and end with low-value, low concentration energy in the form of heat at the lower right. If you like the word, this is entropy in action. The concentrated, high-quality energy gifted to us by the sun runs "downhill" through uses that increase entropy until it is the least concentrated form of energy, heat, reflected in the temperature of air or water (for example).
Let's add an analogy to that metaphor: A parent (or day care) starts the day with all the toys neatly arranged by category on shelves and up off the floor. Through the action of many children (I had a graduate student once who called them entropons), all the toys at the end of the day are spread evenly across the floor - lowest state of organization - maximum entropy. Energy is required (as every parent, caregiver or teacher knows) to get those toys back on the shelves in the right order for the next morning.
What this means for solar energy is that we should make the best use of the highly concentrated, highly organized energy in sunlight before losing it to just heat. Connecting your solar panel output directly to an electric resistance heater, sunlight to electricity to heat, is not the best and highest use of that precious resource. Better to use that energy for more demanding applications (like light bulbs or a refrigerator) before letting it drop to heat.
To continue the classroom analogy, you want the kids to do creative and wonderful things with those toys before they end up spread evenly across the floor (the toys that is). Sun-to-electricity-to-heat is like just dumping the toy shelf over directly onto the floor - so much more could have been done!
One way to assure availability of that high quality energy is to store it for later use.
Getting back to our diagram, let's follow some potential pathways those electrons received from your solar collector might travel, focusing on methods for storing the generated energy.
Starting at the top, high-quality, highly organized energy from the sun is converted to electricity by a photovoltaic solar panel (step 1 on the figure). All the direct uses of that energy that do not involve storage are summarized as step 2. This would include using the energy near the source, or transferring it out to the grid. In either case, as the energy is used (hopefully many times) it will be converted eventually to heat.
There are limitations on transfers to the grid. Beyond the engineering required to link many thousands of local sources to the grid, there is the need to balance supply and demand for electricity. The utilities can't accept your electrical energy if supply already exceeds demand.
If we can't use the power immediately and the grid can't accept it (or we would rather keep it anyway), we are back to storage, and the bottom lines in the figure. The categories there are storage as chemical energy, as potential for kinetic energy (like water running downhill), and, with caveats, even as heat. The numbering system in the diagram goes from the most familiar to the most experimental. Hopefully this organizational chart is useful for you and maybe includes a few terms that are new.
Leading in familiarity are rechargeable batteries (Step 3). From cell phones to cars, and all the way up to whole neighborhoods, the technology is available, and is being improved constantly to increase capacity, safety, longevity and efficiency of storage and reuse,. An earlier essay presented everything from community-scale battery mega-packs, to parking lot canopies that could feed electric cars directly (no grid required), to the intriguing idea that battery systems in electric vehicles, if they come to dominate transportation, could provide much of the local energy storage required.
Batteries are very likely the way to the future for storing renewable energy, but, for fun, let's look at some other methods.
Steps 4, 5 and 6 are ways to reverse entropy (if you like) by applying electrical energy. Early solar installations often ran water through the collectors and stored warmed water in a large bin in the basement, rather than converting the sun's energy to electricity - a simpler mechanical system that did not rely on the grid. But heat is that low-intensity, low-organizational form of energy that actually requires energy input to concentrate it for use.
We are talking here about heat pumps, which are becoming a first-choice method for heating and cooling buildings in many areas. Heat pumps are basically reversible air conditioners that can be used to make hot air out of cold or cold air out of hot. The process is many times more efficient than using electricity directly in resistance heaters (like electric baseboard units).
So generating warm water with solar collectors and then using a heat pump to lift that warm water to a usable temperature (for heating, hot water, etc.) captures steps 4 and 6. Think putting the toys back on the shelf - requires energy!
Step 5 in the figure, geothermal, is similar to 4 except that the heat source is not solar-warmed water, but the soil under your feet. Did you ever think of geothermal heating as drawing on solar energy? It is indirect, but the reason the soil a couple of meters down in colder climates is so warm relative to air temperatures in winter is that it reflects the average annual temperature in that locale rather than the current or recent temperatures. Deep soils store summer warmth for us to tap with heat pumps in the winter.
Pumped storage (step 7) uses excess electrical generation when consumer demand is low to pump water from a low elevation reservoir to a high one. When demand is higher than generation, the water is released and flows down through a turbine to generate additional power.
While the concept is simple, requirements for location are stringent, including access to or capacity for large amounts of water at both ends of the system and a big elevational gradient.
As a result, siting this kind of facility can be controversial! The most famous example, perhaps, is the Storm King project proposed by ConEd for the New York city power system in 1962. The project would have flooded a major forest reserve, carved out a large section of the mountain to support the power plant, run high tension wires across the river and otherwise forever altered the nature of this part of the Hudson Valley. The story of the 17-year battle that ended the project is well-told in a recent article from the Times-Union out of Albany, NY (see Sources).
According to the U.S. DOE, there are 43 working pumped storage operations that provide 93% of utility-scale energy storage, with the potential to double that total. Current total U.S. capacity of those plants is 29 gigawatts - a vanishingly small fraction of the roughly 1.1 billion gigawatt generation capacity of current U.S. power plants. So even though the efficiency of recapture of the energy used to pump the water uphill can be as high as 70-80%, this does not seem to be our answer.
A recent article captured my interest as it applied the same principle to deep mine shafts. Abandoned shafts can be fitted with very(!!) heavy weights that are lifted whenever power generation exceeds demand. When needed, the weight can be allowed to drop, turning a turbine as it slowly descends to the bottom of the shaft.
Several new companies are investing in methods for recycling abandoned mine shafts to provide stability and continuity in energy delivery from intermittent sources like solar and wind. Sounds like a win-win.
A final, chemical method for energy storage that is seeing a resurgence in interest is the production of hydrogen gas without the conventional intermediate step of requiring electricity to drive the separation of water into oxygen and hydrogen. That is a very energy intensive process.
Hydrogen was the darling of the alternative fuels world two decades ago. Yes, hydrogen releases huge amounts of energy when combined with oxygen and produces only water as a waste product. Often overlooked, somehow, in the hyped stories of the 2000s was the very high energy cost of splitting water molecules. Unless a renewable source of energy is used to drive that process, this is not a net gain for the planet.
Now if you could split water directly with sunlight, that could be a game changer.
And that is where the innovations are happening. Using different catalysts and techniques for keeping the oxygen and hydrogen from recombining, there have been some amazing increases in the efficiency of using the energy in sunlight directly to produce hydrogen gas. Stored gas can be converted to electricity using existing fuel cells as needed. Hydrogen also has a raft of uses in the chemical industry.
Of all the potential uses of hydrogen, I hope it never becomes the fuel of choice for cars and trucks. I have no intention of driving a car with a tank full of one of the most explosive gases known and hope those driving around me will feel the same way! I don't even want to think about self-driving cars with hydrogen in the tank! Remember the Hindenburg…
For now, we have lots of potential ways to store solar energy, summarized in the diagram above. We can drive chemical potential (batteries and hydrogen), kinetic potential (pumped storage and mine shafts) or heat storage (water tanks and geothermal, with the caveat that heat pumps will be needed). While these processes and others can be ranked as most (or least) likely to succeed given current technologies, we can never be sure where the next innovation will come from, and how it might alter those rankings.
A final thought. With the prospect of such huge amounts of accessible solar energy, maximizing efficiency in the usual sense may not have to be the first objective. Planning for generation and storage of renewable solar energy might focus as much on decentralization, redundancy and reliability as on initial cost. Maybe many smaller systems for local storage, with less reliance on regional grids, might offer the most secure way to use that vast flow of solar energy.
Sources
Renewables as fastest growing sector
https://www.eia.gov/outlooks/steo/report/electricity.php
A Mega-battery project in Australia
https://www.nytimes.com/2022/06/08/opinion/defense-production-act-solar-power-australia.html
Rapid growth in home solar and storage in Hawaii
https://www.nytimes.com/2022/05/30/business/hawaii-solar-energy.html
Mega-battery project in California
https://www.bbc.com/future/article/20201217-renewable-power-the-worlds-largest-battery
Improvements in battery systems
https://www.nytimes.com/2022/07/12/business/electric-vehicle-batteries.html
Sources on Pumped storage and Storm King
https://www.energy.gov/eere/water/pumped-storage-hydropower
https://en.wikipedia.org/wiki/Storm_King_Mountain_(New_York)
https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity
Gravity storage in mine shafts
https://www.bbc.com/future/article/20220511-can-gravity-batteries-solve-our-energy-storage-problems
Hydrogen