Sunlight can be harnessed as a source of clean energy in a variety of ways. With photovoltaic cells, sunlight strikes a photovoltaic material, exciting electrons in the photovoltaic material and resulting in a potential difference between electrodes placed on the photovoltaic material. This potential difference may be used to power an electric circuit or to store electrical energy in a storage device such as a battery. With living organisms, sunlight causes photosynthesis in an organism such as algae, causing the organism to grow. The organism then may be burned, for example as a carbon-neutral fuel source, and either the organism itself, or the secretions it produces metabolically, may be used as sources of bio-derived molecules, including ethanol and numerous other compounds that otherwise would be derived from petroleum.