Mirrors used to be difficult and expensive. There's no practical way (pre-semiconductor-era) to make thermodynamic engines on sunlight without a method to concentrate it, to achieve high temperatures.
At any rate, the areal power density is really low and wouldn't have been a good engineering choice, generally, even if it were available. (Agricultural pack animals were solar-powered machines all along. But, it takes no human work to build or maintain the fields of grass that they graze off).
There's an essay and a large HN thread about a related idea—why ancient Romans did not develop (combustion-powered) steam engines, what technological barriers prevented that,
Iron-age solar engines may have been impossible twice-over: impossible because of the lack of mirrors, and impossible again because of a lack of metallurgy for building high-pressure steam vessels.
At any rate, the areal power density is really low and wouldn't have been a good engineering choice, generally, even if it were available. (Agricultural pack animals were solar-powered machines all along. But, it takes no human work to build or maintain the fields of grass that they graze off).
There's an essay and a large HN thread about a related idea—why ancient Romans did not develop (combustion-powered) steam engines, what technological barriers prevented that,
https://news.ycombinator.com/item?id=32607187 (Why no Roman industrial revolution? (acoup.blog)", 519 comments)
Iron-age solar engines may have been impossible twice-over: impossible because of the lack of mirrors, and impossible again because of a lack of metallurgy for building high-pressure steam vessels.