There are many places one can go to get an outline on the pros and cons of different methods for enclosure cooling. Although we will briefly touch on them here, this is really an in-depth article on how to choose a Peltier (thermoelectric) air conditioner, once you have committed to the technology.
Opening an enclosure and blowing a fan on heat sensitive components may be a cheap and easy way to cool things down, but it is dirty and dangerous. Exposing components to ambient dirt and debris can damage said components. It also increases the risk of shock to those working around the area. It is best to have a 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 but are a maintenance challenge when placed in hard-to-access, remote areas. Whether it is difficult installation, refrigerant leaks due to nearby vibrations, expensive compressor replacements or all of the above, endusers can find refrigerant-based cooling to be inconvenient and expensive to maintain in certain applications.
Heat exchangers and heat pipes are limited in performance due to ambient conditions. Certain applications will require more cooling than these technologies can provide. Finally, compressed air products will require the expense of maintaining a compressed air supply.
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.
Thermoelectric air conditioners create a closed system. When installed properly there is no exchange of ambient air into the enclosure. The cooling is created via the Peltier effect, which is 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 are free of refrigerants or other chemicals. These are solid-state cooling products. There are no filters to change, no compressor, and the only moving part is the fan. This means there is virtually no maintenance required and a long life expectancy.
Thermoelectric air conditioners excel in high ambient temperatures. Performance ratings are higher than what was available in the past. It is now possible to get up to a half ton of cooling from a thermoelectric cooler. This allows for thermoelectric cooling to be used in larger enclosures. Applications vary, of course, and small enclosures such as camera housings can be cooled with as little as a 100 BTU/hr thermoelectric air conditioner.
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.
Recent Changes to Thermoelectric Technology
In recent years, there have been innovations and improved products in the thermoelectric market. 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.
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 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.
Modern thermoelectric modules are able to 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.
Choosing a System
Factors to consider:
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 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 110°F or so. Therefore, cooling an enclosure to 95°F might strike you as warm, but it actually is more than cool enough to keep electronics happy and running. Using an air conditioner that is too large can mean less efficiency. Cooling significantly below ambient beyond 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. Shipboard or desert environments call for fans that can withstand corrosives such as salt spray or blowing sand. Rugged mobile applications require shock and vibration protection.
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 your enclosure, the more rugged a fan you will require in your air conditioner in order to maintain the integrity of your enclosure.
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 more likely 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.
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-mounted 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.
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. Be aware of what type of curve is being used and be sure you are comparing “apples to apples” when reviewing specs from different manufacturers. Ensure you are comparing air conditioners using the same rating method.
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.
This article was written by Emily Hutensky, Sales & Marketing Coordinator TECA Corp. (Chicago, IL). For more information, Click Here .