The Big Chill

Anyone who has ever pulled an all-nighter before an exam knows the importance of a steady consumption of caffeine-intensive beverages. In fact, when Igloo asked college students what they wanted most in a compact refrigerator, their unequivocal response was: "Forget the ice tray, we want more space for six-packs."

Igloo design engineers were already familiar with thermoelectric (TE) cooling devices used in small ice chest applications, and they had a hunch that the compact technology would free up just the kind of interior space that college kids were clamoring for. But they were skeptical of its ability to cool a 2.0-cubic-ft refrigerator. They found out it does, as evidenced by the recent introduction of the SpaceMate^TM, the first thermo electric-powered dorm-size unit.

Cool running. "In school, mechanical engineers learn that the most efficient heat transfer takes place at a constant temperature, since entropy is minimized," says Fred Schmidt, Igloo's director of engineering. "So my reaction to a thermoelectric device was, 'I know it will remove heat, but since it involves a temperature gradient, will it remove enough heat?' We felt we were asking an awful lot out of this little chip to pump about 50W, which includes the heat generated by the device itself."

Although many mechanical design engineers are only just now finding out about the technology, the concept of thermoelectricity has been around since the early 19^th century. That's when scientist Jean Peltier discovered that passing a dc current through two dissimilar electrical conductors causes heat to be transmitted or absorbed at their junction. In TE--also known as solid state--cooling, the circulating direct current is analogous to the refrigerant used in a mechanical compressor system to carry heat from the thermal load.

A typical TE cooling engine consists of a TE device (or module) and a pair of combination heat sink/fans (see diagram, next page). Each system has a unique capacity for pumping heat, which depends on a number of variables, including physical and electrical characteristics of the TE device, the efficiency of the heat exchangers, and the ambient temperature. "Many design engineers do not realize it, but TE devices can pump loads up to several hundred watts, although the most practical use today is in applications involving less than 200W. It really comes down to the trade-offs a design engineer is willing to make to get the performance he or she wants," says Chuck Cauchy. He's president of Tellurex, a manufacturer of TE devices that worked closely with Igloo to develop the new refrigerator.

The question, then, for design engineers at Igloo was, "How can we design a thermoelectric-powered refrigerator that matches the performance and price of a compressor-based unit?"

Delta T is key. When it comes to TE cooling systems, temperature management is everything (see diagram, next page). For design engineers at Igloo, what it basically came down to was figuring out how to minimize the thermal resistances across the cooling engine. This is particularly key across the outside (hot) sink, because as current passes through a thermoelectric device, it pumps heat from the inside sink to the outside sink, generating some additional heat in the process. Every interface in the stackup creates a resistance, which causes an increase in either the TE device's hot or cold junction temperature. And every increase means the temperature of the soda you're trying to keep cool goes up.

"The first thing we did was build a unit and put thermocouples everywhere. That way, we could figure out what was happening and what elements of the system were important and what ones were not," recalls Schmidt. "Our goal was to make the temperature difference between the ambient and load as big as possible."

It quickly became obvious that the design of the inside heat sink is not nearly as critical as the design of the outside heat sink. In fact, the inside heat sink/fan assembly is a relatively simple design. As air continually circulates through the inside of the chest, it is pulled up into a fan. The fan pushes it across the inside heat sink, helping to promote efficient transfer of heat to the cold shoe, which is a 1-inch-high solid piece of aluminum. "We tried to light-weight the cold shoe by extruding holes in it, but we lost efficiency," recalls Schmidt.

To fill in any voids between the heat sink and the cold shoe and the chip's ceramic substrate, design engineers applied a high-conductivity thermal grease "With the first unit we built we actually tried lapping the surface of the heat sink to get better surface contact," explains Schmidt. "But while the extra machining step improved the performance, it would have added a dollar or two to the cost of the heat sink," says Schmidt. "Using the as-extruded aluminum surface which has a flatness tolerance of 0.003 inch in conjunction with thermal grease at the interface gave us an acceptable performance at a much lower cost."

Designing the outside heat sink, which takes heat from the hot shoe and transfers it to the ambient, was trickier, because it also has to dissipate the additional heat generated by the TE device itself. "We tried pushing outside air across the heat sink, but we were not getting an even flow. One side of the sink would be more heavily loaded, which created hot spots," Schmidt explains. "Using the fan to pull air across the heat sink was much more effective."

To maximize thermal performance, design engineers applied another layer of thermal grease at the interface between the TE device's ceramic substrate and the heat sink. In designing the heat sink itself, says Schmidt, engineers really pushed the limits of the extrusion process. To wit, the fins on the extruded aluminum heat sink are 1.25 inches tall, with a mere 3/16-inch spacing between them. "Obviously, one big chunk of aluminum would have done the job, but we wanted to keep the size and weight of the overall unit to a minimum," explains Schmidt. "So we designed a heat sink with a very high-aspect ratio to maximize the available surface area. But it is a pain to make--imagine taking all these little metal fingers and shoving a billet between them."

Engineers even discovered that the quality of the mechanical clamp used to install the assembly impacts the overall thermal performance of the module. "You can kill the unit's cooling capability just by backing the screw up half a turn," says Schmidt.

Use of a new thermoelectric device, called the ZMAX^R power chip, from Tellurex was critical to the development effort. Thanks to a new, patented metallurgical manufacturing process, the delta T of the chip is 2 to 4 degrees C higher than comparative devices. For higher performance, engineers could have simply added another TE device, and many design teams do just that. But by working all the delta T's, Igloo was able to meet all performance goals at a competitive price.

By continuing to refine the design of the heat exchanger and taking advantage of advancements in thermoelectric technology, Schmidt believes there is room to push the envelope. "By streamlining the cooling engine, I think we can increase the size of the refrigerator to 2.5 or 3.0 cubic ft," says Schmidt.

Cool running. The result? Meet Igloo's SpaceMate^TM, the first compact fridge to use thermoelectric cooling. With no need for a bulky compressor, the 2.0-cubic-ft unit holds 82 12-oz cans--one-and-a-half six packs more than a standard 2.5-cubic-ft compressor-based unit. Taking advantage of the company's expertise in coolers, the unit's lightweight (28 lb) body's blow-molded polyethylene construction (with carrying handles) resembles an ice chest, but boasts the cooling performance of a refrigerator.

In fact if there is any doubt about the unit's ability to keep things cool, Tellurex once inadvertently froze the entire contents of the fridge--refreshments intended to show visiting customers the fridge's capabilities. The only noticeable trade-off is the absence of a freezer compartment. "We talked about making one, which would have been difficult to do using thermoelectrics, but the question is, 'Does anyone actually use the freezer compartment?' Look in most compact refrigerators and you'll probably see a miniature glacier flowing across the ice cube tray," says Schmidt. "So we stuck with our original goal, which was providing more space for drinks."

We'll drink to that.

Tellurex has an excellent primer on thermoelectric technology at www.tellurex.com/resource/txfaqc.htm  and also offers a thermoelectric sample kit for $150 that includes a thermoelectric module, fan, heat sink, and cold plate. More information on the Igloo SpaceMate^TM can be found at www.broco.com/MjjU/prince.html           

Thermoelectric technology involves passing current through two dissimilar semiconductor materials to release or absorb heat. The diagram at left shows a typical stackup of a thermoelectric cooling module, consisting of a TE device, conducting and insulating layers, and two heat sinks. The "x" notation refers to the distance each layer is from the thermal load.

As current passes through the thermoelectric device, it pumps heat from the inside (cold) sink to the outside (hot) sink. T_COLD is the temperature at the cold-side mounting surface of the device and T_HOT is the temperature at the hot-side mounting surface of the device. The Delta T between the two surfaces is a fixed number that is determined by the characteristics of the semiconductor material, thermal load, and to some extent the ambient temperature.

The accompanying graph describes the relative temperature at various locations. With no thermal resistances within the system, T _REFRIGERATOR would equal T_COLD and T_HOT would equal T_AMBIENT. But some thermal inefficiencies always exist in the real world, leading to increases in T_HOT and T _REFRIGERATOR .

Mechanical engineers try to maximize the temperature difference between the inner and outer heat sink, and while they cannot do anything to change the Delta T of the TE device itself, they can maximize its cooling performance by minimizing the temperature drops across the system. Reducing the thermal hit across T_HOT is particularly crucial, as it has the effect of moving the entire curve downward. Also, note the impact of T_AMBIENT, illustrating just why refrigerators of any sort do not operate as well in extreme temperatures.

Mechanical designers can reduce these various thermal resistances in a variety of ways. Strategies include usingmaterials with higher thermal conductivity; applying thermal grease to provide better contact between interfaces; increasing the surface areas of the heat sinks; and reducing the length of heat paths.

Big party tonight and the beer is at room temperature. How long until it's drinkable? Unable to resist getting an answer to this important question, we immediately rushed out and bought a couple of warm six-packs (of soda, since anything stronger would have prevented us from meeting deadlines). Under admittedly unscientific conditions, we intended to pit the Igloo SpaceMate^TM against the Danby Designer (a comparable, compressor-based dorm-size fridge) in a head-to-head competition.

To even out the playing field, we turned both temperature dials to "maximum" and stocked each unit the night before with an assortment of condiments. The soda--measuring in at a slightly cooler temperature (70F) than our stuffy office (74F)went in first thing the next morning. Then we sat back in anticipation. Every hour or so, we'd shove a kitchen thermometer into an open can, record the temperature, and take a sip.

Two hours later, we pronounced the soda in the Danby Designer drinkable at a temperature of 40F. And although the soda in the SpaceMate^TM Igloo was only at 48F, we deemed it within acceptable limits for drinking, too. Over time, the temperature difference became almost negligible. Our conclusion? The compressor must be more efficient, since it cooled the soda down somewhat quicker . But it was sort of a wash, since we didn't detect a really noticeable difference in temperature. Plus, the Igloo can actually hold nine more 12-oz cans than the Danby.

We shared our data with engineers at Igloo and Tellurex who designed the SpaceMate^TM. And although they chided us for our loose experimental controls, they did agree at least in principle with our conclusions. "In general, less work is consumed whenever you use a phase change to move heat, so a compressor is going to move the heat a little bit faster," says Chuck Cauchy, president of Tellurex. Yet, he notes, the cooling rate of any thermoelectric system depends on a number of variables including how much cooling capacity is designed into the system and the efficiency of the heat exchanger. "For enough money, you can always boost performance. The challenge is deciding how good is good enough to sell a product at an acceptable price."