An AlgaeRadiator is a flat container with channels running through it. In effect, it's like a heat radiator. The key difference is that it needs to be transparent. Through this you then run water, and grow Algae in it. At the other end, you get a Algae soup, which you can dry into AlgaeCake or use more directly.
The main use for an AlgaeRadiator is in providing BioFuel for a GeneratingSewageSystem or for augmenting a SolarTower.