The people in northwestern Argentina live an austere life. Their villages near the Bolivian border lie up to 4000 meters above sea level. The sunlight here is intense, the air thin, and the nights very cold. To heat their homes, they have cut much of the nearby vegetation for fuel. The region is in danger of desertification.   Simple solar collectors are being used to head off this danger. German mechanical engineer Christoph Müller lives in the area and for years has been helping villages install solar energy equipment. He works for the Argentinean environmental group Ecoandina.