The
electrical energy is used to
produce hydrogen from water; carbon dioxide can be obtained from biomass gasification, cement
plants, or carbon capture from coal - fired
power plants.
So, the fact that the Sun is spatially «external» to the Earth greenhouse blanket is just as irrelevant to the location of the effective heat source (where actual heat is being
produced) as is the fact that the
power plant that generates the
power for your
electrical house heating system is «external» to your house.