Medical Electronics Manufacturing Fall 1999

THERMAL MANAGEMENT

Thermoelectric heat pumps offer flexibility when designing devices that need accurate and reliable thermal management.

Robert Smythe

Equipment engineers seeking to develop fast, accurate, and reliable medical and laboratory products are recognizing the importance of proper thermal management. As medical electronics evolve, the challenges inherent in designing products are expressed as increased throughput and accuracy while reducing the end cost. Determining the thermal requirements early in the design is essential to meeting those challenges and securing the best thermal solution. Medical equipment requires high reliability, and the thermal components play an important role in increasing the lifetime of the product. As such, the thermal components themselves must be even more reliable. Thermoelectric heat pumps are the best solution for a variety of medical applications including lasers, sample well plates, dispensing systems, imaging, refrigeration, centrifuges, electronics enclosures, microprocessors, microscopy, printer heads, thermoplastics and adhesives. They are successful in a wide range of products because of the design flexibility thermoelectrics offer.

Thermoelectric heat pumps are small, modular devices that cool or heat depending upon the direction of current flow. Constructed of solid-state materials, they are not limited by orientation against gravity, which is common with typical vapor compression cooling. The rate of heat pumping is proportional to the amount of dc power applied, so that precise temperature control is possible by regulating the input power. Thermoelectrics can be used to cool to –100°C or heat to above 200°C, making them the most versatile technology available for temperature-cycling regimens in medical products. In addition, their compact design enables temperature regulation of individual samples and components in a system. The modular design allows designers to optimize the amount of heat pumping for the application.

How Thermoelectrics Work

The principles of thermoelectrics were discovered in 1821 by a German physicist, Thomas Seebeck, who observed that if a closed circuit were made from two dissimilar metals, an electric current (dc) would flow when the junctions were maintained at different temperatures. The Seebeck effect is the mechanism of the thermocouple used in temperature-measuring instruments. In 1834, a French watchmaker, J.C.A. Peltier, found that by injecting current, a temperature differential was maintained between the two junctions. This is the Peltier effect. Considered only laboratory curiosities with no practical application, these discoveries went mostly unnoticed for a hundred years. Because all thermocouples were metals, significant amounts of energy conversion could not occur because the useful flow of electricity is inextricably linked to the wasteful flow of heat through the junctions. Only with the advent of the semiconductor was the development of practical thermoelectrics possible.

Thermoelectrics are solid-state heat pumps made from semiconductor materials. They have no moving parts. Thermoelectrics are manufactured in modular form, where a series of p-type and n-type semiconductor element junctions are sandwiched between ceramic plates. At the cold junction, heat is absorbed by electrons as they pass from a low-energy level in the p-type element to a higher energy level in the n-type element. A dc power supply provides the energy to move the electrons through the system. At the hot junction, energy is expelled to a heat sink as the electrons move from a high-energy element (n-type) to a lower level element (p-type). A typical thermoelectric cooling module contains as many as 127 junctions and can pump as much as 120 W of heat. The amount of heat pumped is proportional to the amount of current flowing through the thermoelectric. Therefore, tight temperature control (<0.01°C) is possible. By reversing the current, a thermoelectric can be used to heat. This is valuable when controlling an object in changing ambient environments or cycling at different temperatures. The sizes range from 2 to 62 mm, and multiple thermoelectrics can be used for greater cooling. Because of the relatively large amount of heat pumping over a small area, thermoelectrics require a heat sink to dissipate the heat into the ambient environment.

Designing-In a Thermoelectric

The physical design of a thermoelectric cooling or heating system is relatively simple. A thermoelectric module is positioned between the object to be cooled or heated (control side) and an object to which the heat is rejected (hot side). Design software is available to help select and model thermoelectrics in specific applications. Such software enables users to input the design conditions such as ambient temperatures, control temperatures, amount of cooling, types of heat sinks, and available power. The program selects appropriate thermoelectrics and demonstrates the performance characteristics. Technical assistance is available from major manufacturers.

Environmental Chambers and Electronic Enclosures. Thermoelectrics can be ideal for maintaining the temperature of small enclosures and chambers. Because there are no working fluids, thermoelectric air conditioners can be attached in any orientation. Typically, a thermoelectric is sandwiched between two heat sinks. One heat sink is placed into the enclosure, while the other remains outside. As current flows through the thermoelectric, the inside heat sink cools, allowing it to absorb heat from the enclosed air. Using a fan to circulate the air reduces temperature gradients within the enclosure and increases the efficiency of the thermoelectric. The hot-side heat sink increases in temperature as the heat is absorbed from the enclosure as well as from the joule heat pumped into it. The ambient air absorbs the heat from the hot-side heat sink. As with the cold side, a fan on the hot side will greatly increase the performance and efficiency of the thermoelectric. The temperature of the enclosure can be controlled through simple on-off thermostats or more precise controllers that adjust the input power to the thermoelectric depending upon temperature. Condensation removal can be accomplished by using drainage ports or incorporating absorptive materials and wick structures.

Sample Wells. One of the most common medical device applications for thermoelectrics is cooling and heating plates for biological samples and reagents. The advantage of using thermoelectrics in this application is its small size, pinpoint temperature accuracy, and ability to heat or cool by simply reversing the direction of current. A thermoelectric keeps the samples at an ideal storage temperature, typically 4°C, then quickly ramps to the exact temperature required for the reaction or test conditions.

A thermoelectric heat pump.

One of the best examples of thermoelectric effectiveness in sample well plates is polymerase chain reaction (PCR) cycling. The objective is to amplify the DNA strands for analysis. This occurs by cycling the samples between set points. The more rapid the cycling, the shorter the operation time is. Temperature uniformity and accuracy are critical to yields. Because a thermoelectric pumps heat in proportion to the amount and direction of the supply current, accuracy and reversibility are easily attained. Reliability is also critical. Until recently, this type of constant temperature cycling was normally detrimental to the life of the thermoelectric because of fatigue due to constant thermal expansion and contraction. A special thermoelectric that uses patented technology has been developed to survive these cycling conditions without degradation.

Liquid Chillers. Chilled liquid is often the most effective method of cooling a device or an object, especially those with larger cooling areas or densities. Running tap water through a plate can provide cooling, but this requires access to plumbing and does not afford temperature control. A closed-loop system addresses the plumbing problem, while active cooling by thermoelectrics or compressors allows for temperature control and cooling to below ambient. A thermoelectric system eliminates the maintenance and reliability issues associated with compressors. Digital temperature control offers temperature accuracy and set point flexibility. A thermoelectric chiller consists of a series of thermoelectric modules sandwiched between a conductive plate (through which liquid channels are drilled or molded) and a finned heat sink to dissipate the heat to the ambient air. A pump circulates the liquid through the plate where it is cooled to the desired temperature before absorbing heat from the object. Medical applications for a thermoelectric liquid chiller include therapeutic hot/cold pads, surgical lasers, and recirculating baths.

Centrifuges. Because thermoelectrics are solid-state devices with no working fluids, they are not affected by forces of gravity. The G forces created in a centrifuge would render a vapor compression cooling system useless. Thermoelectrics can control the temperature of a spinning centrifuge with no performance loss. Supplying power to the thermoelectrics is accomplished with brushes contacting a trace circuit. Additionally, the centrifuge's internal volume can be cooled by forcing cold air around the chamber, as in an environmental chamber.

Detectors. Optics are being employed in almost every industry. Light-wave detection is used in diagnostic and research instruments such as x-ray detectors, spectrophotometers, infrared, and CCDs. Using a thermoelectric to cool a detector enhances its resolution by reducing the dark current associated with thermal energy that limits the exposure time of the detector. Multistage thermoelectrics can be designed to cool as low as –95°C, allowing for maximum image resolution without the need for costly and bulky cryogenic systems.

Microprocessors and High-Density Electronics

Sometimes cooling the air around a board is not enough. Direct cooling of the heat-sensitive or heat-producing device can be more effective than cooling an entire space. Heat sinks are readily used to keep high-power components cool and are adequate in most cases where the ambient temperature is reasonable. When temperature control at or below ambient is required, thermoelectrics are sometimes the only solution. A thermoelectric can be mounted directly to a component, ensuring the correct operating temperature while dissipating the waste heat into the ambient air. When cooling below the dew point, insulating all of the cooled surfaces is important to prevent condensation from damaging the circuits.

Portable Specimen and Medicine Containers. Because thermoelectrics are run on dc, they are ideal for the portable storage containers used in transporting organs, fluids, and medical supplies. Using ice in a picnic cooler offers limited temperature control and storage time, in addition to possible contamination. Thermoelectrics run continuously on available dc power and provide any level of temperature control required. A thermoelectric cools the container without contacting the contents or inducing movement often caused by shifting fluids. An ac/dc converter allows such units to operate upon arrival at a medical facility.

Comparison of Cooling Methods. There are many different tools and methods for transferring heat. The optimal method depends upon the temperatures and tolerances of the application. Thermoelectrics are one of the most versatile methods of temperature control, but other methods may be more suitable. The following is an overview of other cooling techniques. Table I lists their advantages and disadvantages.

Table I. Comparison of cooling methods.

Fans and Blowers. By moving air past a hot component, heat will be absorbed by the air. The overall cooling effectiveness is determined by the air's flow and temperature, along with the component's size and power. Typically, fans and fan trays are used in cabinets for bulk cooling of electronics.

Heat Sinks. Generally, heat sinks are made from aluminum because of the relatively high thermal conductivity and low cost. They are either extruded, stamped, bonded, cast or machined to achieve a shape that will increase the surface area, so that heat is more readily absorbed by the surrounding cooler air. Most have a fin or pin design. Heat sinks, when used with fans (forced convection), can dissipate large amounts of heat while keeping the component at 10–15°C above the ambient. Heat sinks without fans (free convection) result in a higher component temperature because of the decreased impingement of air.

Liquid Cold Plates (Passive). These are typically made from copper, aluminum, or aluminum-clad copper tubing. The cold plate is attached directly to the object being cooled. Heat is absorbed by a liquid pumped through the plate. In an open-loop system, the cold plate is connected to a liquid source (tap water) that runs through the plate and is then released into a drain. A closed-loop system recirculates the liquid through a heat exchanger (radiator) via a pump.

Heat Pipes. A heat pipe is a sealed vessel containing a working fluid, typically ammonia, water, acetone, or methanol, although special fluids are used for cryogenic and high temperature applications. The heat pipe transfers heat by evaporating and condensing the working fluid. As heat is absorbed at the evaporator, the working fluid is vaporized, creating a pressure gradient within the heat pipe. The vapor is forced to flow to the cooler end of the pipe where it condenses, giving up its latent heat of vaporization to the ambient environment. The condensed working fluid returns to the evaporator via gravity or capillary action within the wick structure. Because heat pipes exploit the latent heat effects of the working fluid, they can be designed to keep a component very near ambient conditions. Although they are most effective when the condensed fluid is working with gravity, heat pipes can work in any orientation. Using forced air at the condenser allows for larger amounts of heat dissipation.

Compressor-Based Cooling. This conventional cooling system, found in commercial refrigerators and air conditioners, contains three fundamental parts—the evaporator, compressor, and condenser. The evaporator is where the pressurized refrigerant is allowed to expand, boil, and evaporate. During this change of state from liquid to gas, heat is absorbed. The compressor acts as the refrigerant pump and recompresses the gas to a liquid. The condenser expels the absorbed heat, along with the heat produced during compression, into the ambient environment. Compressor-based refrigeration is effective for large heat loads (1 kW or more), with the ability to cool far below ambient. These refrigerators must be used in their designed orientation, resulting in limited installation flexibility.

Summary

Given the diverse applications within the medical and laboratory industries, the need for reliable thermal solutions is apparent. Thermoelectrics have provided an effective method to control temperature in a variety of applications where reliability, accuracy, and size are critical. Thermoelectrics can be employed to cool solids, liquids, and gases as well as to stabilize electronics. The solid-state construction offers the highest possible reliability. Because they are regulated by the dc power applied, infinite temperature control is possible. Designing a thermoelectric system is simple. The critical thermal and mechanical requirements of medical equipment are ideal for implementing this advanced cooling technology.

Robert Smythe is vice president of sales and marketing for Melcor (Trenton, NJ).

Back to the Fall 99 Table of Contents