6, Dec 2012 / rebound


Intro Note: We are passionate about energy storage, but think our country’s current development approach is too slow and the resulting innovations too costly. The content below discusses this path and suggests a solution already pursued by REbound and other companies.

The Status Quo

This country’s approach to energy storage development can be summed up in one phrase: one-size-fits-all. This phrase refers to stand-alone technologies that store energy, in any market and any geography. Technologies that follow this approach, like redox flow batteries, can store anywhere from 10kWh to 200MWh in either behind-the-meter or utility markets around the country. Unfortunately, the trade off is sub-70% efficiency rates, high material costs, significant HAZMAT concerns and 12+ year payback periods. An even bigger concern is that US energy developers, dreaming about capturing the entire energy storage market worth hundreds of billions of dollars, keep throwing money at these general-purpose solutions.


REbound is one of several companies who recognize a better path. Tailored energy storage systems offer the same load shifting services with higher efficiencies, safer materials, and faster payback periods because they rely on storage systems embedded into larger infrastructure. REbound, for example, provides commercial freezer loads using thermal storage that leverages time-of-use (TOU) pricing. Other organizations like Ice Energy (air conditioning) and TAS (gas turbine inlet cooling) take similar approaches. We like that! Multiple companies creating tailored energy storage technologies is the best way to deliver solutions quickly, efficiently and economically.  After all, no singular tailored system can provide all our country’s energy storage requirements, but multiple solutions together can provide unmatched performance to specific market segments at significantly reduced costs. REbound’s solution pays for itself in roughly 80% faster than a redox flow battery with no incentives and today’s TOU pricing differentials. Similarly, ice storage systems embedded in air conditioning equipment like Ice Energy’s Ice Bear and Vogt’s Turbo Ice are already being installed across the country. Together, tailored solutions can be disruptive.

Cost, Performance and Purity

So why are tailored systems significantly cheaper than the status quo? The answer is storage material cost, performance and purity.

The one-size-fits-all approach places tremendous pressure on the storage material because it must absorb electricity during charging while efficiently producing electricity during discharge. This is a difficult task requiring extremely pure quantities of materials like VanadiumBromine, or Antimony that can form high power, reversible redox reactions. Unfortunately raw, unprocessed material costs alone can reach 60-100$/kWh, on a good day.

One way to think about storage material cost is by abundance. Using the USGS data above, which material do you think will be cheaper?
Another way is theoretical minimum cost, defined as the cost of the system with 100% round-trip-efficiency and no balance of system cost. This is basically the cheapest the system could every be.

The tailored approach places considerably less pressure on the storage material by offsetting electrical load instead of generating it. Tailored systems produce the same results as the status quo using common materials like water, which is used by REbound, TAS, and Ice Energy.

The advantages of tailored vs. one-size-fits-all storage only get clearer from here. The latter constantly fights nature to achieve high storage efficiencies because those expensive storage materials inherently don’t want to perform as electrolytes. Instead, they want to create bi-products via side reactions, degrade to other isotopes, stop moving across membranes, and interact with the other system materials. This is why A) practical vanadium flow battery storage capacity is 600kJ/kg while the theoretical limit is well over 1000kJ/kg, B) facilities that implement Polysulfide-Bromine flow batteries schedule solid sodium sulfate removal every 2 weeks and C) Sodium Sulfur (NaS) batteries must be vacuum insulated to remain above 300C. In simpler, personified terms, these systems rely on expensive rare materials that are unhappy performing a task forced upon them by engineers. The result: round-trip efficiencies rarely in excess of 70%.

Instead of fighting nature, tailored storage systems take advantage of natural cycles to boost efficiencies. Take REbound: at night, we generate ice using conventional vapor compression technology and, during the day, our customers use that ice, instead of electrically driven equipment, to provide freezing loads. The natural temperature swing between night and day helps REbound achieve round-trip efficiencies greater than 100%. This is possible because outdoor vapor compression systems generate ice more efficiently via colder, nighttime temperatures. To apply some numbers, a REbound system consuming 10kWh at night will avoid 12kWh of peak electrical purchases during the day. That’s what happens when your system works with nature and not against it, pretty nifty!

Finally, lets talk about purity. One-size-fits-all systems must use ultra-pure storage materials to minimize side reactions. This is why 20% of vanadium flow battery cost is devoted to electrolyte processing and why HAZMAT handles the sodium in NaS batteries during decommissioning.

Tailored systems don’t relying on a second order process that requires ppb levels of purity. REbound, for example, is simply freezing fresh water and melting it. Not the most exciting scientific reaction out there, but effective! However the system is implemented, the material purity requirements are orders of magnitude less than the general-purpose alternative.

Enough Money to Go Around

The global energy storage market is estimated somewhere in the hundreds of billions of dollars making the one-size-fits-all appeal crystal clear. However, while tailored technologies can, individually, only capture a portion of that market, the marginal opportunities remain fantastically huge. REbound can focus on the $32 billion global commercial refrigeration market, Ice Energy can capture air conditioning market, and TAS can secure turbine inlet cooling.  Together, tailored energy storage solutions can rightfully knock inefficient, expensive batteries from their current pedestal. After all, no supermarket will buy a Zn-Br flow battery with a 15-year payback to offset their freezer or air conditioning loads when a tailored energy storage system pays for itself in just a few years.

So what needs to change? A lot! Look around and tell us how many one-size-fits-all approaches you see? Do we use the same light bulb in our houses as we use in streetlights? No. Do we only drive one type of car? No. Do you only own one pan for cooking? I hope not. No general-purpose solution ever works well so why would energy storage suddenly be different? It isn’t. Recent federal funding solicitations resulted in $0 for tailored systems while batteries gobbled up $24,000,000. These funded technologies can’t economically solve our country’s energy storage problems. Tailored technologies, however, can deliver products faster, at a lower cost, and with higher performance. They must be supported.


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