This one intrigued me, using the words low cost and passive energy. The only thing I didn’t see mentioned in the article is scalable, but hopefully someone has thought that part through.

https://www.fastcompany.com/90621594/this-low-cost-water-filter-runs-on-just-sunlight?partner=feedburner&utm_source=feedly&utm_medium=webfeeds&utm_campaign=Feed%3A+fastcompany%2Fheadlines+%28Fast+Company%29