We caught up with Kipp Bradford - CTO of Heat pump company Gradient and former senior Research Scientist at the MIT Media Lab. We shared the outcome of the one-week design sprint and subsequent prototyping we covered in part one of this series. After a few working sessions, we evolved the design to consider a couple of things:

1. To freeze ice we have to move that heat somewhere - in a home this heat is usually wasted, or even worse, causes an A/C unit to use more power to remove the heat from the home. What if we could store this heat energy alongside the cold?

2. A conventional refrigerator/freezer combo cools the freezer, and then meters the cold freezer air into the fridge when needed. We have to make this work for the freezer as well as the fridge. This means we can't use regular ice as the phase-change material - we have to lower the freezing point.

The New Ice Box

It stores cold in a “cold battery” powered with a mixture of water & glycol

It stores heat in the form of a water heater - where the water is used in the kitchen for the dishwasher and the hot water tap:

Why store hot water? Refrigerators currently are a type of heat pump. We’ll get into the details later in this article, but if you look at the way current refrigerators work, they cool the inside by pumping heat to the outside of the unit. That heat then dissipates into the air. We are imagining a system where we capture and use that “waste heat” to make our kitchen more efficient. 

When you add storage capacity to the cold and hot sides of the heat pump, things get really interesting. Imagine a couple of scenarios:

You run your dishwasher, the hot water for the dishwasher is connected to your refrigerator - as the dishwasher is running, the heat pump heats water to replace the hot water the dishwasher used. To heat the water in the water heater, the cold has to go somewhere, so the otherwise wasted cold charges up the cold battery in the fridge.

Remember the "duck curve" problem from part one of this series? As we add more solar energy generation to our electrical grid, we will need a way to store the energy generated in the middle of the day when the sun is out.

This refrigerator's new thermal storage capability enables its heat pump to run on solar energy during the day, and deploy the cold through the day through its “cold battery.” instead of resorting to fossil-fuel based energy.

The Benefits

The reason we are excited about this concept is that with a few tweaks to the design of a refrigerator we can achieve 3 beneficial outcomes:

1. Less Energy

2. Clean Energy

3. Better Food, Less Waste

We’ll look at each of these in detail.

Less Energy

The way Energy Star measures the efficiency of a refrigerator is how much electricity it uses in a year (kWh/yr) for a given capacity of the fridge.  There are 2 primary ways to affect this efficiency - better insulation so we can use less power to keep it cool, or a more efficient way of turning electricity into cooling energy. This second method is defined as the system’s Coefficient of Performance (CoP) and is defined as the amount of energy you can add or remove from a system over the amount of work it takes to do so: COP=|Q|/W

A standard household refrigerator will have a CoP in the range of 2-4, meaning for every watt of energy put into the system, it can move 2 to 4 watts of energy out of the fridge into the air around it. A heat pump water heater has a similar CoP, in the range of 2.5 to 4.5.

What is exciting about this coupling of the cold battery and water heater is that if we look at a conventional system with the water heater and refrigerator separately, the CoP is averaged based on how much power is used for each. The system will have a combined CoP of 2-4.5. If we look at this new design - the COP is additive, meaning it will be in the range of 4.5 to 8.5!

The reason that the efficiency of this system can double is because a water heater is wasting the cool air it produces, and a refrigerator is wasting the warm air it produces. If we link them together they can work using each other’s waste.

The key to this system working is thermal storage. You wouldn’t want to cool down your food just because you wash your hands with warm water! The ability to store cold and hot means that these systems can generate the hot and cold at different times than using the hot and cold. This brings us to -

Clean Energy

The “load balancing” capability of this refrigerator was where this concept was born, and remains one of the core aspects that makes this fridge so compelling. This is covered in detail in part one of this series.

Better Food, Less Waste

For a solution like this to make a market impact, we need to consider how it can be differentiated from the competition. Paying more money solely for more efficiency can be challenging to reach a mass market if efficiency is the sole value proposition. By adding thermal storage to a refrigerator, it unlocks a few capabilities that are not available on currently available refrigerators.

True Multi-Zone Capability: Current refrigerators have crisper drawers that allow you to control the humidity, and some have a single “fresh drawer” that allows for certain items to be kept just above freezing to keep them fresher longer.  These features are great, but they don’t work as well as they could, and are quite limited in their ability to dial in specific temperatures for each zone.

Because this refrigerator has a “cold battery” - it greatly simplifies a system to control multiple zones. All that is needed to have multiple controllable zones is a fan to move heat to the battery from each compartment, and a temperature sensor to tell the fan when to activate - these are fairly low-cost features that can help people keep their food fresher longer, allowing each compartment to adapt to its contents.

In the next article, we'll look at market pressures that keep our refrigerators from evolving, and how we might take a smaller step in the right direction.