Using the sun as a heat source is nothing new. Twenty-four hundred years ago, Socrates observed:

Now in houses with a south aspect, the sun’s rays penetrate into the porticos in winter, but in the summer the path of the sun is right over our heads and above the roof, so that there is shade. If, then, this is the best arrangement, we should build the south side loftier to get the winter sun and the north side lower to keep out the winter winds. (Xenophon, Memorabilia of Socrates,VIII.14)

Later, the Romans discovered that if a south-facing portico and windows were covered with glass, the energy of the sun would be trapped, causing the building to stay warm into the night.