Solar energy is even indoors an alternative to thermal and vibrational energy harvesting. This indoor photovoltaics use both the natural light and artificial light.
On the Feasibility of Indoor Light Energy Harvesting for Wireless Sensor Networks (Carlos Carvalho and Nuno Paulino) discusses indoor light energy harvesting of the attenuated natural illumination. The authors describe the powering of a low power wireless sensor node with an average power level of 11.51 µW. This low average power level is achieved by a very low duty-cycle of the system making one transmission every hour only.