Thermionic cooling   
Thermionic cooling devices consist of two electrides separated by a vacuum. Cooling occurs when high energy electrons cross the vacuum from the negative side to the positive side of the device, reducing the temperature of the negative side. Borealis Technical Limited, one of the companies who are developing this technology, claim that the devices could be manufactured using semiconductor technology which could make them very cheap – predicted costs are in the region of £0.06 to £0.16 /W which is competitive with current vapour compression systems. A 100 mm square device could theoretically provide up to 300 W of cooling with a higher COP than vapour compression systems. Higher cooling duties could be provided by linking several devices together. Several companies are developing thermionic cooling devices but because of patent applications little further information has been published. 

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Thermo-electric (Peltier) cooling 

Thermo-electric cooling, known about since the 1830s, occurs when a current is passed across the junction of two dissimilar metals. One side of the device becomes hot and the other cold. Thermo-electric cooling systems are available commercially up to about 400 Watts for cooling electronic equipment at a cost of around £2.50 / Watt. Although single thermo-electric devices have low cooling capacities they may be connected together electrically to produce more cooling. Because they do not have a circulating fluid heat transfer is more difficult than for vapour compression systems. For large cooling duties additional heat transfer systems, possibly based on heat pipes would be required. 

When voltage Vin is applied to terminals T1 & T2 an electrical current (I) will flow in the circuit. 
As a result of the current flow, a slightly cooling effect (Qc) will occur at point (A) and a heating effect will be experienced at point (B) where it is expelled. 
A typical design of a thermoelectric cooler from Ferrotec Corporation  
 
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