GO FASTER

Luxury refrigeration begins with a lettuce for a llama

In a windowless lab in Madison, Wisconsin, Sub-Zero Group, Inc. grows 105 heads of Nevada lettuce every three weeks. It harvests the mild, sweet variety to test different models of its namesake refrigerators. Yet it’s simulations using Simcenter Amesim, constant adjustments to functionality, and collaboration with designers that advance a Sub-Zero refrigerator beyond a cold box and into a luxury appliance.

With over seven decades of history behind its name, Sub-Zero knows the value of distinguishing its brand as a monument to food preservation and a means to beautify the kitchen. The company is internationally recognized for its commitment to performance and aesthetics.

Anderson Bortoletto, Principal Engineer of Advanced Product Development for Sub-Zero, says the company’s goals include building appliances that use less energy, with as high a percentage of recyclable materials as possible. The specifications must be tailored to refrigerator models that offer proper humidity, precise temperature control, and purified air, and give customers a choice of elegant, customized designs.

Sub-Zero’s decades of work have resulted in spectacularly evolved machines: refrigerator models with a wide and spacious interior, clear compartments, and neatly fitting shelves and drawers. The exteriors are smooth and brushed with hand-finished surfaces. The models’ hinges, handles, and doors close gently and easily.

“Since our products are top of the line, we need to justify the cost to the customer. We can’t afford design that will deliver the most freshness but doesn’t look good. We’re going for a clean look that promotes and preserves the aesthetic. That ensures the longevity of the brand,” says Bortoletto.

A decade down the road, a Sub-Zero refrigerator still needs to be identifiable as a model made by the company. It has to showcase high-end refrigeration and sustainability. It must also communicate, to owners and guests, that it is a premium product.

It’s fitting that Sub-Zero has been meeting this challenge since 1945. That year, founder Westye F. Bakke built his first freezer prototype, from salvaged scrap metal, to store his son’s insulin at a consistent temperature. Bakke later asked architect Frank Lloyd Wright, for whom he had done work in the 1930s, to customize the first Sub-Zero models.

As today’s Sub-Zero’s product engineers focus on components, parts, and air flow, the company’s designers are equally hard at work. They’re busy crafting numerous desirable features, including flush installation, a wide variety of sizes and configurations, and a choice between the classic stainless steel Sub-Zero look or a panel-ready model with a custom finish.

They’re also making it possible for refrigerator models to be customized. This means the appliances can be flush with cabinets and drawers around them. They lack visible hinges and grilles. The refrigerator designs give a kitchen a seamless look. They also make it easy for an owner to move about in the kitchen.

Dynamic response of the appliance to perturbations

After weeks of tests, when the lettuce finally wilts, a Sub-Zero employee takes the heads home to feed to their pet llama. It’s the data that the product engineering and design teams gather, from a head’s first day to its last, that become the focal point.

One of the difficulties in finding the balance between aesthetics and performance is that most Insulating materials are opaque. That means they’re unattractive, says Terry Hardesty, Corporate Manager of Advanced Development at Sub-Zero.

“Customers expect crystal clear drawers so they can see the food in the fridge. They don’t want to have to open two to three hatches to get at items. At the same time, they’re looking for high food preservation. We work to ensure that the drawers and open space are beautiful, easy to access, and perform well,” says Hardesty.

The temperature of the whole refrigerator is not always as important as the temperature across a compartment. Sub-Zero’s team uses simulations to determine how a particular design and adjustments to various parts create a microclimate without a large temperature gradient front to back. The simulations allow Sub-Zero to shorten development time and reduce the amount of capital, expense, and manpower spent on product development.

Data collection and refinement assist engineers with developing a smarter control algorithm that will allow the refrigerator to dynamically respond to perturbations. Examples include opening the main door and adding new items.

“How will the refrigerator react if I put two six-packs of beer into it? How will it react if I load it up with 7 six-packs? The machine may have a different dynamic response for each action. Those are insights we want to have before we build an appliance. We want to get this information early on in the design process,” says Bortoletto.

How Simulations Improve Closed-Loop Systems

A refrigerator is a heat pump, continually transferring heat from inside the “box” to the condenser coils outside. It uses many components to accomplish this task, including insulation, an expansion device, heat exchange in the evaporator and condenser, and fans. The compressor is particularly important. Its job is to circulate coolant in the fridge. The electric motor that powers the compressor accomplishes its task by compressing refrigerant gas. The action generates heat, which is eventually released by the condenser.

Since energy efficiency is important to both regulatory agencies and their clients, Sub-Zero looks to minimize the amount of electricity that a refrigerator uses. They work to get every Sub-Zero fridge to use less than the U.S. minimum standard for refrigerators. A U.S. Energy Star certification reduces a refrigerator’s carbon footprint, lowers a customer’s utility bill, and in some jurisdictions, qualifies for rebates.

Simulations by Simcenter Amesim give Sub-Zero license to explore what parts like a compressor can do, at minimal cost.

“It’s a lot easier to test out an idea digitally. When we build a product to try it out, that takes more time and money. Right now we’re developing good correlations between the simulations and reality. The relief we get from that has been psychologically transformative. We evaluated a few other tools before using Amesim. They just didn’t work for us,” says Hardesty.

Variables that Sub-Zero is able to test with Simcenter Amesim include the gasket, or the air-tight seal along the edges of the refrigerator and freezer doors, the speed of fans inside and under the refrigerator, the capacity and efficiency of the compressor, and the suction line heat exchanger, a component that regulates the refrigerant flow and pressure while maximizing the cooling capacity.

The closed loop refrigerating system model has been developed to also decrease the total testing time spent on defining the optimum refrigerant charge and capillary tube restriction leading to maximum energy efficiency. The goal is to decrease design exploration time from two months to one week.

“With simulations, we can test anything from the length of the compressor run time to how long the operator fan is on. We don’t have to build prototypes. The various factors affect how much water an exposed head of lettuce will lose over time. Then we can evaluate what the refrigerator is doing and associate the impact on preservation,” says Bortoletto.

Simulation takes over trial and error testing. Bring down design exploration time from 2 month to a week

The Role of Maya HTT

Sub-Zero is able to accomplish a great deal with Simcenter Amesim partly because it works closely with Maya HTT, a long-time partner of Siemens. Its staff frequently collaborates with Siemens to offer clients software, testing, AI, and engineering services. Maya HTT was integral in Sub-Zero deploying Simcenter and leveraging it with their refrigerator development.

The first phase involved building a library of components in order to model the entire fridge, including the heat exchanger within the fridge, optimizing the whole system, and making it run better in the simulation. In the second phase, Maya HTT made a library of components for the refrigerant loop, in which coolant circulates to cool the fridge. The third phase saw Maya HTT finishing the fridge component loops, adjusting the heat exchanger and condenser, and optimizing the overall efficiency of the fridge.

The library of custom Simcenter Amesim components was particularly helpful.

“This extensively customized closed refrigerant loop library has allowed Sub-Zero to maximize efficiency, with components tailored precisely to their products,” says Garrett Keenan, Maya HTT.

Bortoletto says the beauty of the relationship is that Maya HTT has an in-depth understanding of Sub-Zero’s people and appliances.

“Since they’re our long-time partner, they’ve gotten to understand what we’ve been trying to accomplish over time. Without Maya HTT, we couldn’t have gotten closer to the ideal end state for the refrigerators,” says Bortoletto.

The 1D simulations that Sub-Zero accomplishes with Simcenter Amesim provide many advantages. These simulations have a low computation cost, offer a system-level/parametric study with a quick turnaround, and provide fast and easy model preparation.

Sub-Zero is transitioning to using simulations as naturally as any physical product development tool, says Scott Wareing, senior vice president of operations and product design for Sub-Zero.

“Engineers will model and simulate components, sub-systems, and entire products prior to being physically built. Simulation becomes a core competency,” says Wareing.

Sub-Zero’s next move with Simcenter Amesim will involve using the software to refine Cove, its line of dishwashers.

Bortoletto wishes Sub-Zero could have started with dishwashers.

“They’re an easier product to model with simulations. After using Simcenter Amesim to improve refrigerators, we know what we’ll learn,” says Bortoletto.

Simcenter Amesim will help with modeling and predicting the behavior of a dishwasher, such as different types of cycles. It will also allow engineers to reduce cycle time and attain minimal water usage.

“These things impact the environment in a positive way and improve the performance of the product,” says Bortoletto.

For now, Bortoletto says Simcenter Amesim will remain a “big enabler.” The tool boosts Sub-Zero’s capacity to adapt to ever-evolving consumer expectations.

Challenges / Next Steps

“We always want to find the sweet spot between food preservation, great design, and energy consumption. We try to evaluate those attributes simultaneously. Finding a way to decrease the time and cost to get those insights hasn’t led to an enormous design breakthrough. Sometimes that takes years,” says Bortoletto.

Bortoletto says Simcenter Amesim currently gives Sub-Zero the chance to clearly define objectives and validate models before implementing changes. Later simulations may help Sub-Zero write better algorithms for software to power smart refrigerators.

Bortoletto foresees refrigerators becoming easier for owners to adjust.

Using an app on a smartphone, a customer may be able to change the machine’s settings to preserve a particular type of cheese or beverage in a certain spot. They’ll also be able to ensure that a machine is energy efficient overall.

“Currently, customers want a Sub-Zero refrigerator because the machine offers the ability to maintain optimal conditions in refrigerator microclimates, coupled with reduced energy use, green materials, and luxury design. These reasons will remain why a customer will want a Sub-Zero refrigerator for years to come,” says Bortoletto.