Tesla chief technology officer JB Straubel, the brains behind Tesla’s original idea to use lithium-ion batteries to power a car, has invested in a startup that stores energy using supermarket refrigerators and frozen salt water.
This week, that startup, Axiom Exergy, announced that it had raised another $2.5 million from investors including Straubel and a handful of early stage investors like the MIT Angels, a group of Massachusetts Institute of Technology alumni that invest in early stage tech startups.
The young company sells technology that plugs into super market refrigeration units and uses tanks of frozen salt water as a way to store energy and lower supermarket energy bills.
During peak times of energy use, like hot summer afternoons, Axiom Exergy’s technology switches a supermarket’s refrigeration units over to use frozen salt water tanks to keep cool. When the refrigerators are being cooled by the ice tanks, they don’t need to use their own power-hungry cooling systems.
The tanks freeze the ice at night, when grid rates tend to be low because less electricity is being used. The company’s software and data system continuously detects when grid rates fluctuate and manages the process.
The company says its ice cooling units, which it calls a Refrigeration Battery, can provide about six hours of cooling and can lower a customer’s energy bill. The systems can also provide backup cooling if the power grid goes down, enabling supermarkets to keep perishable food cold during blackouts.
The idea of using heating and cooling to store energy isn’t new, but entrepreneurs are beginning to think of new ways to manage so-called “thermal energy” using new and cheap computing technologies like data algorithms, sensors, wireless networks, and cloud computing.
For a look inside Tesla’s Gigafactory watch our video.
A decade-old company called Ice Energy also uses ice tanks for energy storage, but uses them to manage industrial air conditioning units instead of super market refrigeration units. Data center operators have used thermal energy storage for years, and Google is using a thermal energy storage system for a data center in Taiwan.
When most people think about batteries, they think of a AA or lithium-ion batteries that use reactive metals attached to electronics that charge and discharge. But energy storage is a more general term, and many different mediums can store energy.
Some companies like LightSail Energy have developed systems that compress air into tanks, which are used for storing energy. It requires energy to compress the air into the tanks, and when the air is released, the energy can be used.
Quidnet Energy, another startup, is using pressurized water injected into ground wells as a way to store energy. Pressurizing water and pumping it into underground wells requires energy, and when the water is later expelled or used in a high-pressure water cleaning system, that stored energy can be accessed.
Get Data Sheet, Fortune’s technology newsletter.
For years, some utilities around the globe have used what’s called “pumped hydro” energy storage systems. These stations basically use energy to move water up a hill, and then generate electricity by moving it back downhill.
Other startups have tried to build on the idea of pumped hydro energy storage, as it mostly only works where there’s a steep grade like a valley. A startup called Energy Cache, which was backed by Bill Gates, was working on storing energy using buckets of gravel attached to a sort of ski lift, but they abandoned the effort.
Axiom Exergy is a young company with a bright future. The startup is just getting its first commercial systems installed. The new funds, which also included investment from Victory Capital, the Element 8 fund, Sierra Angels, and the Propel(x) investing network, will be used to build systems for signed contracts.
Updated 9:30AM PST on August 4 to correct that the company’s name is Axiom Exergy, not Axiom Energy.