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Solving the Energy Storage Problem Will Put an End to Burning Fossil Fuels

 

Energy storage remains the chief inhibitor in our quest to move from fossil fuel-derived energy to that which we can glean from the sun, wind, and waves. These three energy sources suffer from one common problem, intermittency. If we can store the excess energy we can derive from them then intermittency goes away. But until now we have been trying to use chemical-based batteries the primary storage method which means we add a level of complexity to energy solution. What’s needed is simplicity.

Think of it this way. If the sun heats water in a tank next to an array of photovoltaic panels, then we have captured the energy of our star twice. When the sun goes down, the water with its trapped heat is, in fact, an energy storage sink, albeit not a terribly useful one except for taking a shower or bath. So find a medium other than water that is more efficient at transferring solar energy and converting it to heat and we have a potential storage technology that can be used to power our lights, heating, and air conditioning after the day is done.

So what are likely candidates to create adjunct storage for renewable energy technologies to create a continuous, reliable supply of electricity?

  • Compressed air – pressurizing air using the excess energy produced by renewable sources is not a new idea. Known in the industry as CAES (Compressed Air Energy Storage) the concept is very simple. Build an air-tight storage tank, or find a natural one such as an underground reservoir, cavern, or mine (salt mines work really well) and install compression equipment on the surface with pipes connected to your storage locale. When the sun sets or the wind dies, release the compressed stored air which then powers a turbine connected to a generator.
  • Carbon dioxide (CO2) – using captured CO2 is seen as a way to turbocharge existing thermal power plants. Whether as a supercritical fluid or just liquefied CO2, the potential to turn this greenhouse gas into a power generating technology that can be integrated into both existing thermal-based as well as renewable energy systems has led to some early turbine prototypes that can handle the liquid or supercritical state of the CO2.
  • Norbornadiene – a liquid hydrocarbon may prove to be the best thing that ever happened to solar energy. Combined with photovoltaic panels the norbornadiene chemically is altered when exposed to light turning it into quadricyclane, a chemical with energy densities similar to high-performance batteries. This photochemical reaction is being called “solar fuel” and in a closed system fuel cycle could generate power from photovoltaic cells continuously 24×7.
  • Molten salt – is increasingly being seen as the answer to large-scale energy storage. Using the excess energy produced by renewable sources to heat the salt it becomes a stable storage medium. Today salt is being used this way in solar thermal energy plants that focus the light of thousands of mirrors to convert the energy to heat which is then stored in the molten salt, and later released to produce steam to drive turbines connected to generators.
  • Molten silicon – is based on the same concept as molten salt, but the advantage silicon gives is its much easier to store than salt because it doesn’t corrode the storage medium in which it is placed. Another advantage is liquid silicon’s thermal gradient. Silicon can be heated to temperatures 2 to 4 times higher than molten salt which creates even greater latent energy. Heat-tolerant pumps developed by MIT engineers can handle the high temperatures of molten silicon and through heat exchangers capture the energy to drive steam turbines.
  • “Sun in a box” – also called TEGS-MPV (Thermal Energy Grid Storage-Multi-Junction Photovoltaics) uses the excess electricity produced by any renewable source by passing the current through a heating element. The energy is then absorbed by molten silicon which turns glowing white when it reaches temperatures of 2,370 Celsius (4,300 Fahrenheit). The light from the silicon is then turned into electricity by multijunction photovoltaics, specialized solar cells. The electricity generated by the solar cells can then be used for HVAC systems and lighting. The MIT engineers who have proposed the “Sun in a box” believe a single storage system combined with wind and solar systems could continuously power a small city of 100,000 homes. The system would cost half the amount of current pumped hydroelectric storage energy systems, which currently is the least expensive mass energy storage technology on the market.

The “Sun in a box” concept is pictured here, a system to store excess power from solar and wind energy by heating liquid silicon turning it white hot. The tanks needed for this technology would probably be built of graphite so that they would be hot inside but cool to those touching the wall from the outside. (Image credit: Duncan MacGruer)

Len Rosen's picture

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Discussions

Jarmo Mikkonen's picture
Jarmo Mikkonen on December 15, 2018

How is using CO2 as the working fluid of thermal power plants related to energy storage? It is more efficient but not a form of storage.

Bob Meinetz's picture
Bob Meinetz on December 17, 2018

Jarmo, the technique uses advanced marketing technology, borrowing established "sun in a box" electromagnetic principles to turn CO2 blue-hot and overcome the fundamental laws of skepticism. The best thing to happen to thermodynamics since Baked Bread, by 2050 it will make cars powered by their own exhaust possible. Think of it! (but not too hard, or you might realize it's written by someone who has been trained to sell things to people at whatever price they can be convinced to pay).

Bruce McFarling's picture
Bruce McFarling on December 15, 2018

That is a good question ~ the link included at that point does not address that point. One can imagine that if there is a phase change involved in going from compressed gas or liquid CO2 to supercritical CO2, then holding CO2 in the supercritical state would be a way to store energy, and that if one had a 300K+ heat source, the supercritical CO2 generator would allow a time gap between availability and generation ...

... but there is nothing like that even alluded to in the piece that is linked to. Doing a comparison between existing proven technologies and storage technologies that the reader is required to invent while they are reading the piece seems to be a bit of an unbalanced comparison, to say the least.

Stephane Bilodeau's picture
Stephane Bilodeau on December 15, 2018

Thermal Energy Storage (such as the molten salt, silicon and other phase change materials) is often underestimated, but it will become one of the key actors in that quest to move from fossil fuel-derived energy. 

Bruce McFarling's picture
Bruce McFarling on December 15, 2018

The framing of this piece is a bit silly. Different storage technologies that have different cost of capacity, different cost per cycle, store energy available in different forms are not competing to see which will be the exciting magic silver bullet ... the useful ones, which will surely include some form of battery technologies, will form a complement of storage capacities that each take up a share of the storage task that it is best suited for. 

John Miller's picture
John Miller on December 17, 2018

Historically and today the most efficient and cost-effective electric power energy storage technologies have been and are today: 1) hydropower pumped storage, (chemical based) batteries and more recently solar thermal (molten salt based) storage.  Yes, there are numerous other ‘feasible’ technologies to store power, such as more recent test applications of compressed air storage, flywheels, chemical conversion alternatives, etc.  However, application of these alternative/developing technologies is generally very inefficient (compared to pumped storage & chemical based batteries), and often have limited energy storage compacity(s).  These factors often make these experimental/developing power storage technologies comparably very expensive per KWhr.  In other words, the average (daily) capacity factor of solar PV is normally on the order of 25%, but utilizing relatively inefficient power storage such as compressed air will be at a cost that will significantly reduce this daily average capacity factor level of a given solar PV generation facility-capacity.

For future electric power storage technologies to be reasonably cost effective or affordable, they also need to be efficient, and, ‘not wear/lose capacity significantly during each year of operation’; another factor that reduces the economic attractiveness of several energy storage technologies listed in this post.

Bruce McFarling's picture
Bruce McFarling on December 22, 2018

Yes, battery storage has a User Cost, as several of these technologies do, while PHS does not. That is why a complement of both battery storage and PHS can be more economically efficient than either separately ... using the PHS first to clip the "base" of the net surplus curve and battery power to clip the "peak" of the net surplus curve improves the capital cost efficiency of the PHS and a multiple turbine PHS system can sell spinning reserve whether storing or generating.

So "cheaper than PHS" is not as cut and dry a concept as it would appear from simply reading a standard cost per KWh stored under some assumed number of cycles per year of free-standing storage.

All of the links in the piece are incestuous links to other blog posts from the same site, so I haven't read the actual Sun in a Box work ... but as described it relies on converting electricity into heat to store the heat and convert that into electricity via the light emitted by white hot liquid silicon. I'd want to see the conversion efficiencies to be convinced that is more efficient electricity-to-electricity storage than PHS, which in modern implementations is around the 80% range. "Cheaper per KWh storage capacity" is one thing in a short-run perspective when the marginal cost of solar or windpower otherwise curtailed is effectively $0 ... but in the long run perspective looking at the total system cost of building the wind and solar generating capacity, then storing the energy, then retreiving it, lower round trip energy efficiency implies greater solar and windpower investment costs for the same delivered dispatchable KWh.

Barry Doyle's picture
Barry Doyle on December 20, 2018

I believe the problems of heat loss( contributing to warming the globe) and terrible inefficiency for air storage and molten anything....make these solutions improbable from a basic Pyshics stand point.  

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