Refrigeration systems that have compressors and conventional refrigerants are widely used. But issues over what refrigerants of the future might look like are causing some in the industry to look at ways to create cooling with an alternative to compressor-type designs.

Bill Wooden, a partner of BEPA LLC, Plano, Texas, has been working with Russian scientists on developing a system that doesn’t rely on conventional refrigerants.

“The approach and design is of a thermo-electric refrigerating system based on semi-conductor thermoelements as an electric energy source and their use for sources of refrigeration,” he said.

Wooden said the approach was tried in the 1950s but failed to compete with compressor-type units as some technological problems were not completely solved. Now he said he believes those problems have been overcome with the development of a Thermo-Electric (TE) module coupled with use of special heat conducting pipes.

“TE module-based refrigerators are used as miniature and autonomous sources of refrigeration,” said Wooden. “The hard-body TE freezer/refrigerator is radically different from a mechanical one. Specifically, in a hard-body TE freezer/refrigerator an electric current spontaneously creates a thermal differential without moving liquid or gas. Therefore such a freezer/refrigerator does not need constant maintenance and has a huge cooling potential.”

According to Wooden, other features of TE freezer/refrigerators are:

• Compactness and small mass with refrigeration capacities of up to 500 Watts per module (with larger wattage versions that can be created);

• Ability to provide a short blast of cold air in any minimum quantity;

• Working parameters of the mechanisms independent from their orientation in a gravitational field and are able to operate efficiently under extreme loads and in weightless conditions;

• High reliability;

• Ability to operate in a thermostable mode at any temperature, within the operating range, for as long as is necessary.

“The TE mechanism is ideal to control temperature because it takes only a simple switch of DC current within a TE to convert it from a refrigerator to a heater which is more economical than is a conventional heater,” noted Wooden. “The extrusion technology we developed to obtain thermoelectric materials provided material equal to the zone grown thermoelements in thermo-electric characteristics. Our high-pressure technology, extrusion method, changed the concept of alloy admixtures influence on n-type and p-type conductivity in connection with the changes in crystal structure.”

A REAL INNOVATION

A cycling mode of TE module operation in freezers/refrigerators was created with a special type of heat exchanger, heating pipes, and was “a real innovation in our scientific project,” he said. “Today, the main trend in the heat pipe development is in creation and improvement of variable conductivity pipes as the most effective way to control the temperature of an object. The ability of the heat pipes to act as heat switches and diodes theoretically opens up new technical possibilities. Thus, special attention should be paid to heat pipes of this type. The characteristics of a heat diode in heat pipes are used in this project.”

“Heat pipes belong to the class of heat transmission devices. Their common feature is a closed vapor-condenser cycle. A heat pipe is defined as a vapor-condenser hermetic device with capillary, gravitation, osmotic, centrifugal, and other forces serving for heat exchange and working in a closed cycle,” said Wooden.

“Heat pipes help in the separation of the thermal supply and discharge, i.e., transporting of heat from the most distant and random oriented parts in relation to each other. They also provide high heat conductivity of heat-transmitting systems; transformation of the heat flow density, i.e., concentration or dilution with a wide range of heat flows from the site of heat supply to the site of discharge; equalizing the temperature field and smoothing out temperature peaks; and control and stabilization of the object temperature.

“The technological potentials and qualities of heat pipes have opened up a new perspective of practical applications,” said Wooden. “Currently they are being aggressively introduced into different industries.”

After a study of the features of different heat pipes, those involved in the BEPA project have come to the conclusion that a two-phase contour thermosiphon should be used. “The reason behind it - it fully meets the need and its relatively simple construction makes it cost-efficient,” said Wooden.

With this, the goal was to design and create a household freezer/refrigerator with the cooling capacity of 400 Watts.

Several properties of heat pipes made it possible to solve such tasks as making the heat pipes transform and transmit cold from the cold brazing TE module to a low-temperature department such as a freezer chamber; transform and transmit heat from the hot brazing TE module to the ambient air creating a thermally stable condition between the ambient air and the freezing/refrigeration chamber; and provide the necessary cycle mode of the TE module-based freezer/refrigerator without significantly increasing the refrigeration/freezing chamber temperature by using the diode properties of the heat pipe.

The new type of refrigerator/freezer allows for:

• The TE module placed outside the cooling chamber;

• The volume of the cooling chamber to be increased;

• The TE module to work in a cycle mode;

• The freezer/refrigerator acting autonomous;

• Operating without moving parts;

• Not being affected by the orientation within the field of gravity;

• Noiseless operation;

• Low mass and small size.

The device operates as follows: An electric current is applied to the TE module creating a temperature differential between the cold and hot brazing. As a result, a temperature difference occurs between the vaporizing and condensation zone of the thermosiphon. This, in turn, transmits cold from the TE module to the freezing/refrigeration chambers. Simultaneously, a second thermosiphon removes heat.

REAL WORLD POSSIBILITY

“We have shown convincingly a TE module could be brought close to commercial status while at the same time be a competitive and functional alternative to conventional refrigeration systems,” said Wooden. “Every step has been practically substantiated and multiple experiments have been carried out both in the lab and elsewhere.”

Currently a refrigerator using this technology has been operating for well over a year. BEPA is now looking for what it calls a “commercialization partner to fully utilize this novel technology.”

For more information, contact Bill Wooden at wow@zowi.to or 214-293-2848.

Publication date: 05/07/2007