Renewable Long-Life Biocidal Hydrophilic Coating for Condensing Heat Exchangers

Future manned spacecraft and lunar or Mars outposts will need a condensing heat exchanger (CHX) to control humidity in the cabin atmosphere. Condensing surfaces must be hydrophilic to control the condensate flow and ensure efficient operation, and biocidal to prevent growth of microbes and formation of biofilms on condensing surfaces. Coatings must be extremely stable, adhere to the condensing surface, and maintain hydrophilic and biocidal properties for many years. We propose to develop a zeolite coating system that incorporates two key innovations: (1) modifications to the coating chemistry to enable much longer life than prior coatings, and (2) an in situ cleaning process that can decontaminate the surface and renew hydrophilic properties. In Phase I we will prove the feasibility of our approach by developing preliminary cleaning formulations, developing chemical analysis models to predict coating lifetime, producing trial coatings, and demonstrating coating performance and the effects of cleaning. In Phase II we will scale up and refine the coating process, produce sample coupons and heat exchanger cores, and measure the coating performance after long-term exposure to prototypical environments. We will also develop and demonstrate a coating regeneration process and associated hardware suitable for ground testing and eventual demonstration on the International Space Station (ISS).

The primary NASA application will be environmental control and life support (ECLS) systems on future manned exploration missions, including spacecraft and rovers designed for long missions that require active ECLS systems. The coating technology will be equally applicable to any active life support system, including potential upgrades to the ECLS system on the International Space Station. Implementation of our coating and renewal system on the ISS can potentially save large amounts of crew time and large costs associated with reapplying the existing hydrophilic coatings to the condensing heat exchangers.

The long-life, biocidal coating is potentially inexpensive and can be used to prevent microbe growth in condensing heat exchangers used for dehumidification in terrestrial HVAC systems. Benefits would be enhanced component performance and improved air quality. Another key market will be balance-of-plant equipment for terrestrial fuel cells, which will include water management components that will also require hydrophilic and antimicrobial coatings. The long-life coatings developed in the proposed program will enable these condensing heat exchangers to operate reliably with good performance for a long service life.

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