Thermoelectric Cooling: A Closer Look

THERE ARE MANY places one can go to get an outline on the pros and cons of different methods for enclosure cooling. It seems one article at least per calendar year runs in the trade magazines. Although we will briefly touch on them here, this is really an in-depth discussion on how to choose a Peltier (thermoelectric) air conditioner, once you have committed to the technology. Thermoelectric technology has seen advances and improvements in recent years. An overview of the technology will outline benefits and drawbacks to the end user followed by discussion of the newest innovations and factors to consider when specifying a thermoelectric enclosure cooler. Thermoelectric technology is easy to overlook due to upfront cost or power requirements. Once educated about and committed to the technology, however, thermoelectric cooling can be the best choice for harsh or hard-to-reach areas. Thermoelectric Cooling : A Closer Look Why Thermoelectric? Protecting electronics equipment from both over-heating and from the ambient environment is, hopefully, “a truth universally acknowledged”. It is best to have an active cooling component in place as part of your system. Before going into a detailed look at thermoelectric cooling, a few other technologies are quickly outlined below. Refrigerant air conditioners have their place- usually in large cooling applications and easy-to-access areas. Heat exchangers, heat pipes and compressed air systems can offer low cost cooling for smaller applications. Thermoelectric cooling is the best choice for remote areas and harsh environments such as wash-down areas, process automation systems, etc. Thermoelectric technology might be chosen over the previous mentioned options where ease of maintenance and long term operation costs are of concern. Thermoelectric Cooling : A Closer Look Electrical Insulation Thermoelectric Technology in the Food & Beverage Industry Industries across the board have a need for thermoelectric cooling in certain applications. Within the food and beverage industry, thermoelectric air conditioners are often used as part of a plant’s automation system. Cooling sensitive instrumentation equipment, control computers, thin clients and monitors, thermoelectric products excel in high ambient temperatures and can maintain NEMA-4 and NEMA-4X enclosures in wash down and corrosive areas. The Peltier Effect: Thermoelectric cooling is created via the Peltier effect: a solid-state method of heat transfer through dissimilar semiconductor materials. Two dissimilar conductors replace refrigerant in both liquid and vapor form. A cold sink (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another. A DC power source pumps the electrons from one semiconductor to another and a heat sink (condenser) discharges the accumulated heat energy from the system. Thermoelectric air conditioners create a closed system. When installed properly there is no exchange of ambient air into the enclosure. Left: cooling process control electronics enclosure in a beverage plant. Right: cooling communications equipment in a poultry plant. Thermoelectric Cooling : A Closer Look Thermoelectric air conditioners are low maintenance and have a long life expectancy due to the solid-state nature of the technology. Free of refrigerants or other chemicals, filters and compressors, the only moving parts are the fan. Thermoelectric air conditioners are typically more expensive to purchase up front compared to a refrigerant based cooler or a simple heat exchanger. However, the reduced need for maintenance and replacement parts must be considered as part of the value added to using the technology. Thermoelectric air conditioners create a closed system. There is no exchange of ambient air into the enclosure. Recent Changes to Thermoelectric Technology In recent years, there have been innovations and improved products in the thermoelectric market. Performance has increased and operation costs have decreased. Efficiencies in heat/cool products and temperature control have improved. Energy Saving New design features allow for energy savings, such as including a passive cooling mode when demand is lower. This will draw significantly less current than the active cooling mode used in times of higher demand, thus reducing operating costs without sacrificing cooling. Passive versus Active cooling modes PASSIVE 300 BTU/hr thermoelectric air conditioner ACTIVE Thermoelectric Cooling : A Closer Look Performance In the past, thermoelectric air conditioners were limited in performance to about 2,000 BTU/hr. Now, increasingly efficient designs are allowing for air conditioners with up to a half ton of cooling. This allows for thermoelectric cooling to be used in larger enclosures such as high heat-generating control panels. This is partly design innovation but also due to higher quality, more robust materials that are now available to manufacturers, such as more efficient heat sinks and more reliable thermoelectric modules. Efficiencies Modern thermoelectric modules heat more efficiently as well as cool (via reversing the polarity of the module). This allows more thermoelectric products to offer both cooling and reliable heating as the demand exists. By using the thermoelectric modules for heating, efficiency is increased since electric heaters are not needed. For either cooling-only needs or heat-and-cool applications, there is a greater ability for tight temperature controls. Thermoelectric air conditioners’ performance range is larger than it used to be. Top: camera housing cooling using a 200 BTU/hr thermo electric unit. Right: high demand control equipment enclosures being cooled with 6,ooo BTU/hr thermoelectric units. Choosing a System Below is a guideline for specifying a thermoelectric air conditioner. Consider the following factors: 1. How much cooling is needed The total load is created by the heat your equipment is generating as well as ambient conditions, delta-T, enclosure size, etc. How much cooling is required will Thermoelectric Cooling : A Closer Look be determined by many factors including the details of the enclosure and surrounding environment. There are many sizing tools available online, free from thermoelectric air conditioning manufacturers and enclosure manufacturers. 2. Allowable Enclosure Temperatures Thermoelectric air conditioners are often used to cool equipment below ambient. Most electronics are rated to 110F or so. Therefore, cooling an enclosure to 95F might strike one as a warm rather than cool temperature, but is indeed more than cool enough to keep electronics happy and running. Using an air conditioner with more cooling capacity than what is needed can mean less efficiency. Also, cooling significantly below ambient than what is needed can cause excessive condensation. In other words, design for allowable conditions both inside and outside the enclosure. This includes basing maximum allowable internal temperature around the known operating limits of your equipment rather than an arbitrary temperature. Know what the minimum enclosure temperature should be. If heat is needed in winter months, an air conditioner with heat function can be considered. 3. Ambient Temperature Range Along with understanding the environment, know what the ambient temperature range is expected to be. In an outdoor application, this range might be much larger than anticipated. Planning for the worst-case scenario is advisable. 4. The Importance of Fan Type The IP (Ingress Protection) standard describes a uniform system for classifying the degrees of protection provided by the enclosures of electrical equipment. The first number refers to the level of protection against penetration of solid objects into the enclosure. The second number refers to the level of protection against penetration of liquids into the enclosure. Air conditioners should - and often do - have both internal and external IP ratings. The external rating is influenced most by the hot side (external to enclosure) fan selection. Some environments call for fans that can withstand corrosives such as salt spray or blowing sand. Mobile applications require shock and vibration protection. Thermoelectric Cooling : A Closer Look NEMA Rating IP Rating 1 10 2 11 3 54 4 56 4X 66 6 67 12 52 13 54 Note: the comparison chart above is an aproximation NEMA & IP Ratings Comparison Therefore, it is quite important to check with the manufacturer about what type of fan is being used in an air conditioner and what other choices may be available. There are industrial fans, fans sealed from water, fans protected from water AND corrosives (such as salt, sand) and lastly, MIL-spec fans. Any of these will offer some protection however the higher the IP rating of the enclosure, the more rugged a fan will be required in the air conditioner to maintain enclosure integrity. Thermoelectric air conditioners are meant to maintain the enclosure rating and operate in that environment. Check the product literature or discuss your environment with the air conditioner manufacturer to ensure the air conditioner you choose will be adequate. 5. Input Voltage Most thermoelectric air conditioners are available in a multitude of input voltages. Systems with more cooling capacity are likelier to be 120 or 240 VAC although this is not always so. Larger systems running off of 24 VDC will naturally have a higher current draw as there are more thermoelectric modules used to obtain the cooling capacity. If using VDC input, any power supply added to the enclosure interior will add to the total heat load. This is an important consideration when determining the cooling requirement. Thermoelectric Cooling : A Closer Look Mounting styles rendering 5. What to Notice Regarding Air Conditioner Specs Currently, more conventional performance curves referencing the DIN 3168 L35L35 and L35L50 ratings are shown on air conditioner specifications. Traditional curves referencing delta-T are still used. Ensure you are comparing air conditioners using the same rating method. Example: performance curve per DIN 3168 (Left) and traditional performance curve (Right) 6. Mounting style A thermoelectric air conditioner will be designed by the manufacturer to be mounted part way through the enclosure (“through mount”), external to the enclosure (“flush mount”) or inside the enclosure (“internal” or “recessed” mount). Operation in any orientation is possible. Typically, “through mount” systems are the least expensive and offer greater cooling capacity. Although the true driving factor when choosing ought to be the design of the enclosure and location of other equipment. The air conditioner will perform best where there is adequate air flow. Choose the mounting style which gives both the hot side and cold side of the air conditioner at least several inches of breathing room whenever possible. Summary Thermoelectric cooling is best suited to applications where low maintenance is required or to harsh outdoor environments. Improvements to the technology and the designs provide more reasons to consider it. Utilize online information and work closely with a manufacturer of thermoelectric enclosure coolers to choose the best solution.