A cycle, Not a Thermal Battery

Author: Kevin Davis

Very often Rebound is mistaken for a thermal battery, which I suppose is a reasonable misunderstanding seeing as we A) leverage ice generated during off-peak hours and B) can mitigate peak demand charges.

However, what we’ve actually developed is an entirely new high capacity cooling cycle, something that hasn't been done since the early 20th century. [1] And its important to point out the distinctions. First though, some context.

In a sector with fairly trivial innovation, thermal battery development is one area that has gained a lot of attention over the last 10 years. With that attention has come improvements. What arguably started with Calmac almost four decades ago led to Ice Energy in 2003 and now Axiom Exergy in 2014. Companies like Buco, Baltimore Air Coil and Viking Thermal Solutions also have thermal battery products.

However, while these batteries add value mitigate peak energy expenses, they still leverage legacy, compression-based refrigeration equipment to produce the stored cooling capacity. And this is where Rebound and thermal batteries diverge.

Our team thrives on solving complex thermofluid challenges. One of the biggest, if not the biggest, challenges facing the refrigeration industry is how to develop a reliable, economically practical alternative to legacy, compression-based cooling. In developing IcePoint®, we indeed wanted to leverage ice storage, but our focus wasn’t isolated to shifting electricity purchases. Instead we set out to invent an entirely new cooling cycle, which is exactly what we are commercializing.

Rebound is currently operating the first commercially-deployed, closed loop, freeze point suppression cycle in the world. In fact, in the few days prior to publishing this article, 4200 lbs. of strawberries were frozen by IcePoint’s pilot system. So while words about IcePoint’s novelty might not mean much to the majority of readers, the primary take away is that this is the first step to reshaping how the world produces cooling.

The short, general comparison:

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The longer, detailed comparison:


Thermal batteries use a single solution that goes through a freeze-thaw cycle, repetitively, to first store thermal energy and then utilize it. This single solution has a freeze point between -10°F and 10°F.

IcePoint® uses two different materials, which are mixed and then separated. The only material that is ever frozen is pure water, which has a freeze point of 32°F.


Thermal batteries use legacy, compression-based equipment to freeze their mixture at temperatures as low as -10°F. This low temperature means the legacy equipment must work harder at an efficiency about 1.5-2X lower than if they were freezing their mixtures at 32°F. This, in combination with heat gains during the storage process, lead to a round trip efficiency comparable to other batteries of about 80%.

IcePoint® also uses legacy equipment to freeze its water, but at much higher efficiency because freezing occurs at 32°F. Due to the mixing step that leverages freeze point suppression and the separation step that leverages high efficiency nanofiltration, IcePoint® provides cooling at 35% greater efficiency.

Heat Exchange Surface Area

Most thermal batteries use ice-on-coil configurations meaning that there is a heat transfer fluid flowing through their mixture in tubes that both freeze the mixture and extract the energy from it for deployment. As a result there is a limit to the amount of energy that is transferred to the heat transfer fluid at any given time.

IcePoint® has hundreds of times more  surface area available for heat transfer because the solid (ice) is stored in direct contact with it’s own heat transfer fluid (freeze point suppressant) that can be pumped at dynamic flow rates to the facility, allowing for the bursts of cooling that add such significant value. No other low temperature cooling technology in the world offers this functionality.

Cost & Economic Value

Using ice-on-coil requires either large copper heat exchangers or secondary glycol loops both of which increase the cost per kWh. Additionally, thermal batteries only mitigate peak energy expenses limiting their value generation.

On a per kWh provided, IcePoint® is 45% less expensive than a thermal battery. Additionally, IcePoint’s ability to supply bursts of cooling and subsequent acceleration of blast freezing cycles boosts revenues for facilities, providing an enormous value add unattainable by batteries.


[1] Lower capacity cycles like magnetic refrigeration, thermos-acoustic, hot electron and thermoelectric have been invented with only the latter of those seeing any traction. 

We're Back, And We've Brought a Commercial Pilot

Author: Kevin Davis

Well, that was an embarrassingly long break from blogging. Lets chalk it up to being bandwidth limited in 2017 and we'll do better in 2018. Hopefully folks are keeping up with our funding success via the news feed, but what’s missing are some insights into how the technology is advancing. So lets catch you all up by diving into the biggest news, our proof of concept pilot.

A Reminder

Rebound manufactures IcePoint®, the most flexible, efficient and economically optimized cooling equipment ever developed. The cooling product integrates with existing freezer systems at cold storage facilities and provides the control to deploy bursts of high capacity cooling, ideal for accelerating blast freezing (ie: freezing food faster) and mitigating peak energy expenses. No other cooling technology in the world offers this control and Rebound accomplishes it with a 35 percent efficiency gain. Facilities not only realize a boost in revenue from increased product throughput, but they also benefit from decreased energy expenses.

The Deployment

On January 15th the long-awaited IcePoint® system arrived on site in Oxnard, CA. Its a proof of concept pilot that provides 2.8 - 14 tons of refrigeration (TR) at -22F. It went live on February 6th and by March 31st commissioning was completed. Rebound is currently collecting data and working with Lineage to ensure the pilot meets the appropriate figures of merit.


The Primary Highlight

Rebound is currently operating the world’s first, closed loop, freeze point suppression cycle in a commercial environment. That’s a milestone six years in the making and one that should be celebrated! Think about it, there’s A) vapor compression, the dominant technology developed in the mid 19th century, B) absorption cycles, really only economically practical for air conditioning, invented around the same time period, and then C) IcePoint®, invented in 2012 and now currently being commercially validated by Rebound alongside our pilot partner, Lineage Logistics. So, I’ll just throw out some much deserved public acknowledgment of the hard work our technical team put in to make this a possibility. Well done and thanks for those long hours over the 2017 holidays!

The Primary Challenge

My hunch is Russell and his team will dive a little deeper into what kept them up at night in a follow up blog, but meanwhile, I’ll give you a higher level snapshot.

The primary challenge we’ve experienced is waste heat availability. As a reminder, our system provides bursts of cooling to freeze food faster. We then utilize the heat pulled out of the food products to drive our cycle. So, as you might imagine, when there aren’t many strawberries being frozen in Oxnard, we neither have the opportunity to provide much supplemental cooling nor have the heat to separate our refrigerant. Its tough to power a thermal separation process without a thermal input :-) The upside is that A) we have a kick ass engineering team that was able to re-design our controller to enable a more effective separation process under tough conditions, albeit at a slow pace and B) we learned a lot about the design limitations and subsequent complications that waste heat availability creates. In fact, due to this experience, we’re transitioning to a membrane-based separation design that provides similar efficiencies without a heat input. Had there been a constant flow of berries, we may not have been forced to revisit the design, which turns out to be a great business decision as it makes our technology easier to install and scale.

As for waste heat, its just about Mother’s Day, so according to frozen strawberry experts,  loads will begin to arrive at the facility with tremendous frequency. By early June the facility will be overloaded, waste heat won’t be a topic of conversation and we’ll be hitting those figures of merit the Rebound and Lineage teams are anticipating.

What’s Next?

While our team waits patiently for strawberries to see IcePoint® operating at 100% utilization, we are also designing the full-scale, production-ready system. We recently leased a 10,000 ft2 facility in Denver and hired a Director of Manufacturing in anticipation the next big step, the transition from R&D to production. We’ve also begun assessing IcePoint value at other facilities, freezing different products, by gathering data and producing techno-economic models. We’ve got big plans for 2019, and the results of this pilot unlock our ability to raise additional capital, deploy full-size systems and continue ramping up manufacturing and sales.

See you in LA, IcePoint™

It’s been an intense summer and fall here are Rebound, but today we hit a major milestone. The demonstration unit that the team has been developing for the past 10 months left Rebound’s lab for third party testing by SCE. This follows a strenuous internal commissioning process that pushed the system to its limits and taught us a myriad of invaluable lessons about building and operating IcePoint™. 

Things You Won't Learn In School

This summer, I was lucky enough to spend 10 weeks interning in Rebound’s Denver office, soaking up as much wisdom as I could.  As a tree-hugging, Mechanical Engineering Masters student looking for a challenging internship, everything about Rebound seemed to fit.   I was drawn in by their focus on environmental impact, the innovative thermodynamic problem they were solving, and a company structure (read: startup) that entrusts interns with real projects.  As anticipated, the experience checked all of these boxes.  However, as I look back on my summer, it is the experiences I had not anticipated that feel most valuable, all of which I would have never been exposed to in school.  I have attempted to package these lessons in four, bite-sized takeaways for quick and easy digestion. 

Attention Millennial Energy Engineers: Refrigeration = Impact

At Rebound, we recently had the honor of presenting at ATMOsphere America, the largest natural refrigerant conference in the United States. This was a great opportunity to introduce ourselves to some of the biggest names in refrigeration: Danfoss, Hillpheonix, Emerson, and Zero Zone were just a few of the companies we talked to about IcePoint™. Overall, it was a great experience!

One aspect of the conference really caught our attention: the lack of young people. To illustrate this point, I want to take a slight detour. About a month ago Kevin and I were on the phone with a Director from one of the aforementioned large refrigeration companies. That company had recently visited our lab and this individual wanted to know more about what we were doing. One comment he made really stood out: “What is surprising to me is that you all are doing this and your are not even 50!” What he was getting at was not something neither Kevin nor I had given a lot of thought to, as we tend to surround ourselves with startup entrepreneurs working in other industries: Refrigeration is an industry dominated by engineers with 30+ years in the sector. 

MVP Wrap Up: Success

Back in August, I wrote about testing some customer hypotheses while waiting for Phase II of the NSF project to begin. Since then, we completed the MVP work and Phase II has begun.

After a few months getting the MVP reliably chilling a chest freezer in my garage, we deployed it at Whole Foods. As you’ll recall from the previous post, we wanted to use this MVP to validate our assumptions about our customers needs.