Thermoelectrics in cryogenic cooling

Abstract

A use case for thermoelectric coolers (TEC) in cryogenic cooling application is presented where the heat load is small and the required temperature difference between the hot and cold sides ($\Delta \mathrm{T}$) is very high. The application shown here is the cooling of image sensors used in telescopes. These sensors need to be cooled to below −90C (to improve signal to noise ratio) with a heat load of less than 1W at an ambient temperature of $20\mathrm{C}$. This has been traditionally done using liquid nitrogen, however, this method requires high maintenance in having to periodically refill nitrogen. A TEC-based solution can offer a significant benefit in terms of lower cost and maintenance. A multi-stage TEC is generally well suited for applications which require a large $\Delta \mathrm{T}$ and low heat load. In order to determine if a commercially available multistage TEC is enough to meet the $\Delta \mathrm{T}$ requirement, an experimental setup was created using a vacuum chamber containing TECs inside and a liquid cooling plate on the bottom to remove the heat load from the hot side. Different types of TEC stacks were tested under an applied heat load of 0.6W on the cold side. It was found that although there are many commercially available multi-stage TECs for cryogenic applications, it alone is not enough to achieve the required $\Delta \mathrm{T}$ of 110C. Also, adding more than 4 or 5 stages to the multistage TEC is not very effective due to the very low COP and the exponential rise of heat dissipation in the lower stages. The maximum $\Delta \mathrm{T}$ that can be achieved by one multi-stage TEC was found to be only around 90C at a heat load of 0.6W. It is shown here that in order to overcome the $\Delta \mathrm{T}$ shortfall, an additional layer of 3 single-stage TECs (in parallel) needs to be added at the bottom of the multi-stage TEC in order to increase the total $\Delta \mathrm{T}$ closer to the required value (110C).