Until fairly recently, almost all refrigerated laboratory incubators used a compressor-based cooling system. While these systems are still very prevalent, some incubators now use a Peltier cooling system instead. Each of these types of systems comes with its benefits and drawbacks, including those related to cost, performance, and practicality.
In this post, we explain how the two types of cooling system work and their benefits and disadvantages.
Peltier-Based Cooling
A Peltier (or thermoelectric) cooling system is based on a DC voltage running through two junctions joined by thermocouples.
Thermocouples comprise two electrical conductors with different Seebeck coefficients, usually semiconductors. N- and p-doped bismuth chloride are often used. The ends of the conductors are linked together with copper. Multiple thermocouples connected by copper bridges make up a Peltier element.
The copper bridges are bonded by ceramic plates, usually made of aluminum oxide. When a current runs through the element, it removes heat from the “cold” plate causing it to cool and to remove heat from the environment. Heat is transferred along elements to the other “warm” plate.
Fans help to divert the flow of cold and warm air on either side of the module. Multiple coolers can be connected side by side to increase the cooling capacity of the system. That being said, it is difficult to maintain low temperatures (below 10°C) with a Peltier system.
The MyTemp Mini Digital Incubator (heat and cool version) and the SRI Drosophila Peltier Incubators use a Peltier system for cooling.
Peltier systems require more energy input than comparable compressor-based systems. However, while heat is emitted from Peltier systems, it is comparatively less than in compressor-based systems. This means that there may be some savings in terms of air conditioning load in the area where the unit is housed, which could help offset the energy efficiency differential.
Advantages of Peltier Systems
Here are the main advantages of thermoelectric cooling systems:
- Quiet and free of vibrations as they don’t contain moving parts
- Location isn’t an issue and they can be moved so are suitable for portable units
- Can be small and lightweight
- No flammable or ozone-depleting refrigerants are required.
- Economical to produce
- Require little maintenance
- Less complex and easier to replace than compressor-based cooling systems
- Reversing the polarity will switch to a heating system
Disadvantages of Peltier Systems
Peltier systems also come with their drawbacks:
- Cooling is generally slower than in compressor-cooling systems
- Complex, multistage systems are required for larger temperature differentials
- Can’t provide low temperatures (below 10°C)
- Not very energy-efficient compared to compressor-based systems (although control technology means cooling can be more accurately measured than with a compressor, so these systems can be energy-efficient for small temperature gradients)
Compressor-Based Cooling
Compressor-based cooling involves the use of vapor compression to cool a unit. This is the most commonly used type of cooling system found in both domestic and commercial refrigeration systems. Indeed, most of our refrigerators and freezers, as well as other refrigerated systems, including cooled lab incubators, use a compressor-based cooling system.
The MyTemp Digital Incubator and the SRI3 BOD Low Temperature Incubator both use a compressor-based cooling system.
Compressor-based cooling involves a complex system, but below is a simplified version of what happens. (Note that this is a cyclical system -- we’ll start with the refrigerant entering the compressor.)
- The refrigerant enters the compressor as a low-pressure gas. Pressure is applied to compress the refrigerant to a higher-pressure (and higher-temperature) gas.
- This gas then travels through the condenser where it is cooled to a high-pressure liquid. It’s at this point that heat is rejected from the system. Cooling is carried out by means of cooling water or cooling air.
- The liquid refrigerant then goes through the expansion valve, where it there is an abrupt drop in pressure, converting it to a low-pressure liquid/gas mixture. The pressure reduction lowers the temperature of the refrigerant.
- The refrigerant flows as a liquid/gas mixture through the evaporator. Here it is heated to a low-pressure gas. It is at this point that heat is removed from the area to be cooled.
Although this is a more complex type of system, it is tried and tested and is capable of providing very low temperatures.
Advantages of Compressor-Based Systems
- Energy-efficient compared to Peltier-based systems
- Can handle large temperature differentials
- Capable of providing low temperatures
- Cooling can occur more quickly, for example, after door openings
Disadvantages of Compressor-Based Systems
- Use non-environmentally-friendly flammable chemical materials
- Dependent on proper positioning (the system can fail if it’s tilted or rotated)
- Can be noisy and may vibrate
Conclusion
An increasing number of refrigeration systems use Peltier cooling technology. However, its main drawbacks, including low energy efficiency and relatively high cost for a given cooling capacity mean that it is far from enjoying widespread use. Until developers find a way to produce inexpensive and efficient Peltier systems, compressor-based models will likely continue to dominate.
That being said, Peltier systems can still be ideal in certain situations, particularly where only small temperature differentials and low cooling capacity are required. The many benefits of these systems, including compact size, simplicity, portability, and lack of noise and vibrations can make them suitable for various applications and environments.
