An application note from Silicon Labs on energy harvesting reference design user’s guide(PDF!):
The purpose of this reference design is to demonstrate an ultra low power wireless sensor, powered from an energy harvesting source. This application is typical of systems which wake periodically to measure and transmit results. Since it is powered from an energy harvesting source, no batteries need to be replaced for the life of the system (life expectancy is greater than 15 years or 7000 mA-H) and the wireless node can be designed with a very thin profile (battery height is 0.17 mm). The system consists of two components: a wireless Sensor node and an EZRadioPRO® USB Dongle. The Sensor Node uses a Silicon Labs Si1012 wireless MCU. The Dongle uses a Silicon Labs C8051F342 MCU and a Silicon Labs Si4431 radio.