Its been a few years, but I spent a summer touring German technical schools. They had a lot of solar projects. I remember one that used alcohol in the heat exchanger and I think it might have been under a vacuum. The main reason was to lower the boiling point because you gain more latent heat transforming to steam. It also requires less btu to superheat steam vs warming liquid, usually half. So in short, consider a low temperature solar boiler for efficiency, I'm sure easier said than done.
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