Storing the Sun

We are awash in solar energy - how can we store it for use in the dark?

Dec 02, 2024

Available and usable solar energy is about 300 times total global energy demand, but the inconsistent availability of that resource is a major barrier to its use. Batteries may come to mind first as a technology for storing electrons, and a lot of progress has been made in building systems big enough to address the challenge, but this essay describes several other interesting possibilities. It also offers a kids bookshelf analogy to introduce the concept of entropy and the ultimate fate of all energy!

Availability of Solar Energy Is 300 Times Total Global Energy Demand

The first essay in this set showed that usable solar energy globally 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 proposed by Bill Gates in 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. An earlier essay on Babcock Ranch in Florida described a community that is both energy self-sufficient, with a large community battery system, and highly hurricane resistant.

But let's take a wider view of storage options and include both well-known and experimental methods.

An Overview of Ways to Store Electrical Energy

Here is a diagram that organizes possibilities for storage in three categories – chemical, kinetic, and as heat (for use with heat pumps).

We start at the top with high-quality, concentrated 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. If you like the word, this is entropy in action. The 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).

Entropy – A Playroom Analogy

Here is an analogy I like to use for entropy and the energy required to reverse it. A parent, teacher or day care provider starts the day with all the toys and books 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 and books 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, or heat pumps) before letting it drop to heat. However you use it, it will end up as heat eventually. Entropy wins in the end unless energy is applied to reverse it (lift those toys!).

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.

Electron Pathways

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.

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.

Chemical Storage: Batteries and Hydrogen

Leading in familiarity are rechargeable batteries (Step 3) where solar-electric energy is used to reverse the chemical reactions in the batteries that store and release energy. 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.

Batteries are very likely the way to the future for storing renewable energy (see again sources for some large-scale projects and the first essay in this set on the potential for batteries in electric vehicles to serve this purpose), but let’s look at some other, innovative approaches.

Hydrogen (step 9 in the diagram) was the darling of the alternative fuels world two decades ago. When combined with oxygen the reaction releases huge amounts of energy 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 to create hydrogen gas.

Now if you could split water directly with sunlight, or use renewable forms of energy to drive the formation of hydrogen gas, 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.

A later essay in this set will have more to say about the potential value of hydrogen in a low-carbon energy system.

Kinetic Potential Storage: Pumped Storage and Mine Shafts

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, sighting 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 slowly, turning a turbine as it 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.

Storing and Reusing Heat

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 raise 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 is equal to the average annual temperature in that locale rather than current or recent temperatures. Deep soils store summer warmth for us to tap with heat pumps in the winter.

Other forms of geothermal energy as alternative sources, rather than for storage, will be presented in a later essay in this series.

Lots of Ways to Store Solar Energy – and a Thought on Efficiency

A final thought. There are many ways to store the huge excess of solar energy available to us: Chemical reactions (batteries and hydrogen), kinetic potential (pumped storage and mine shafts) or heat storage (water tanks and geothermal linked to heat pumps). Sunlight is a distributed energy source, perhaps best suited for local use near the point of capture. 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. Sounds like a good topic for another essay (stay tuned!).