Been trying to remember something I meant to try in a
greenhouse once (but never did). The idea was that you
filled black tubes with a compound that has a high latent
heat (higher than water). During the day they heated up
and then released the heat during the night to stop frosts.
I thought it was a super-saturated solution, but these guys
are using the solid

http://www.kellysearch-extension.co.uk/Science0704.htm

They've got slightly different goals than just avoiding
condensation, but it should work to some degree in a
simple form

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