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
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 SpaceMateTM, the first thermo electric-powered
To move heat from the inside to the outside of the SpaceMate,
engineers designed a novel cooling engine consisting of a thermoelectric
device and two combination heat sink/fans. As air continually circulates
through the inside of the chest, the inside heat sink picks up the heat.
(Arrows indicate the direction of air flow through the system.) As current
passes through the thermoelectric device, it pumps heat from the inside
(cold) sink to the outside (hot) sink, which must also dissipate the
additional heat generated by the thermoelectric device. To achieve uniform
flow and better heat transfer, the outside fan pulls—rather than
pushes—ambient air across the hot sink.
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
19th 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
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
Igloo SpaceMateTM specs:
Interior storage capacity
2.0 cubic ft (82 12-oz cans)
Power delivered to the
Approximately 46F below ambient
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,"
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 ZMAXR 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
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
SpaceMateTM, 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 SpaceMateTM 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. TCOLD is the
temperature at the cold-side mounting surface of the device and THOT
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 TCOLD and THOT would equal
TAMBIENT. But some thermal inefficiencies always exist in the real
world, leading to increases in THOT 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 THOT is particularly crucial, as it has the effect of moving
the entire curve downward. Also, note the impact of TAMBIENT,
illustrating just why refrigerators of any sort do not operate as well in
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
SpaceMateTM 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 SpaceMateTM 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
SpaceMateTM. 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."