Here’s an excerpt about them from The Book of Knowledge, pub. 1919, scanned by Google Books.
The Arizona Republic on this early attempt at solar power
The company’s steam engine worked by concentrating sunlight on curved reflectors and into a specially designed high-pressure boiler. It could be used on any engine equipped with the boiler.
The book said that, with the company’s equipment, a square yard of sunlight could melt silver, gold, glass or wrought iron, and 2 square yards of sunlight could create a temperature of more than 25,000 degrees.