Vapor compression is used to cool buildings and data centers because it is the most efficient cooling technology available. A typical vapor compression cooling system will produce twice as much cooling power as it consumes in power depending on the operating conditions. No other cooling technology comes close to this level of efficiency. The most common competing technology is thermoelectric-based Peltier devices, which draw at least 2 times more power than it provides in cooling, or at least 4 times more power than a vapor compression system under the same operating conditions.
By comparison, Vapor compression has a maximum Carnot efficiency of 80% and a commercially attained efficiency of 60%. Thermoelectric systems, on the other hand, have a maximum Carnot efficiency of 35% and a maximum commercially attained efficiency of ~15%. The attained efficiency of thermoelectric systems is one-fourth of that attained by vapor compression systems.
This means that a vapor compression system providing 1000 watts of cooling with a Coefficient of Performance of two (COP= Cooling Watts/power draw Watts) will draw 500 watts of power to achieve that cooling capacity. A thermoelectric system providing 1000 watts of cooling at the same temperatures with a COP of 0.5 (Cooling Watts/Power Draw) will draw 2 KW of power to achieve the same cooling load.
This COP comparison holds true whether the system is cooling air, a pumped liquid chiller, or a direct cooling approach is used. The vapor compression cycle will always be more efficient than a concomitant Thermoelectric based cooler due to the innate limitation of the technology. No amount of packaging can overcome the efficiency limitation. As we shall see in additional posts, the efficiency limitation of TECs causes a number of additional limitations in their use in a wide range of cooling applications.
The bar graph shown here was generated by running tests on Aspen’s ECU-550, a vapor compression-based ECU for electronics enclosures, and a competing thermoelectric device with the same nominal capacity. The data was gathered over a wide range of ambient temperatures and case temperatures. The data was only limited by the failure of the thermoelectric system to provide adequate cooling under high ambient, low case temperature, and high delta T conditions. As can be seen from the graph the vapor compression system is 65% smaller, used 77% less power, and weighs 82% less than its thermoelectric counterpart.
