We chose to use a water heater for this application because it contains many of the components needed in a single compact package.
Small capacity tank - The original system had a 64 oz (1.89 liter) HDPE bottle. The water heater selected has a 7 gallon (26.5) liter tank. This additional tank capacity should help limit temperature fluxations, which will be caused by adding heat to or removing heat from the alphatross assembly.
Heating elements - The original system contained a fluid-to-air heat exchanger with a fan, which was only capable of removing heat from the system. We believe that this causes the alphatross baffles to be colder than the melting point of Rb, causing depletion of Rb from the oven, and leading to maintenance. Our test setup included a 900W inline heating element scavenged from a failed coffee maker. We used a 2:1 step-down transformer to limit the maximum heat added to the system. However, this setup was cumbersome, and prone to failure, particularly in fluid connections to the heater.
Temperature control - The original system had NO feedback for temperature control. The heat exchanger fan was powered any time the pump was running. We replaced this with a cheap off-the-shelf PID temperature controller and SSR. Temperature feedback took some experimentation. Placing the thermocouple on the heater lead to fluid temperatures significantly below the target. The largest factor here was probably heat transfer into the fluid, which was poor due to poor circulation rates (more on this in the pump page).
Links about the Bosch water heater:
Product page: https://www.bosch-thermotechnology.us/us/en/ocs/residential/tronic-3000t-1098972-p/
Installation manual: https://www.bosch-thermotechnology.us/us/media/country_pool/documents/point-of-use-mini-tanks-manuals/6720801072_tronic3000t_es2.5-4-8_installation_instructions_en_es_usa_02.2019_us.pdf
Engineering Submittal Sheet: https://www.bosch-thermotechnology.us/us/media/country_pool/documents/downloads-for-bosch-products/engineering-submittal-sheets/final_ess_tronic3000t_es2_5-4-8_ess_06_2015_us_1.pdf