Emissions-free energy system saves heat from the summer sun for winter

I'm confused, does this work by sunlight or heat?

"has the unique property that when it is hit by sunlight, it is transformed into an energy-rich isomer"

And

"The solar thermal collector is a ...."

Also what is the energy density? I assume the fact it isn't mentioned means that its pretty poor.

Does seem interesting though.

Edit: I'll answer my own questions:

They're getting referred to as photoswitches in the references, so I guess it's sunlight

And one reference quotes 155wh/kg, which seems not great.