Calculating Power Requirements
In order to power systems using ambient energy harvesting, several factors must taken into consideration to calculate the power required to operate the system in various states:
1 - Identify the sources of ambient energy to be used and the type of Energy Harvesting transducer to be used
2 - Characterize the power output of the EH transducer over various ambient conditions
3 - Looking at all the system components, calculate the power required for all states operation (e.g. sleep, sensing, wireless)
4 - Identify the EH conversion and power management electronics to be used and add the power used to the overall total.
5 - Size the energy storage device (solid state battery) to cover all the system energy storage and power delivery requirements.
Reference schematics of these systems can be found in the Cymbet EVAL-09 datasheet and the EVAL-10 data sheet.
EH Transducers
EH Transducers exist for several forms of ambient energy harvesting. The following table shows the fundamental characteristics of each:
Energy Transactions
The definition of an energy transaction is "the amount of discrete energy required to perform a certain task or functional transaction". This concept of energy transaction is very useful in the design of energy harvesting-based systems. In order to calculate the power budget and power boundary conditions for an EH-based system, all operating and quiescent power states of the system must identified. Each of these states requires an energy transaction level to function. Identifying all the various energy transactions will determine the sizing of the Energy Harvesting transducer and the energy storage devices.
Maximum Peak Power Tracking
To optimize the performance of energy harvesting based systems, it is critical the high efficiency energy conversion technique of maximum peak power tracking (MPPT) be used. MPPT can adapt to either constant impedance or variable impedance EH transducers. MPPT is used to match the impedance between the energy harvesting transducer and the system load as seen in the diagrams below. The EnerChip EP Energy Processor CBC915 implements an optimized version of MPPT.
Design Tips
When building energy harvesting based systems the following 10 Tips and Techniques will help improve system performance:
1 - Optimize for low average power in the system.
2 - Firmware efficiency is key; no loops, etc.
3 -Use hardware timers and interrupts. Isolate loads; all loads should be switchable.
4 - Watch power leaks via back-feeding other devices.
5 - Net power is a trade-off between dynamic and static power. Quick processing with high power may be better than slow processing with low power.
6 - Usually better to process data and send result vs. sending data for processing elsewhere.
7 - Wireless protocols and topology must be minimized.
8 - Power up sequencing – must understand implications of when to power each device along with other devices.
9 - Every MCU vendor provides hints and tricks to minimize power.
10 - Use Energy Processing devices that provide status indications so informed power management choices can be made.
