For the demonstration the two remote nodes (RN1 and RN2) are set up with differing message priority levels, with RN1 having a message priority of MP2 and RN2
Figure 8.10 Energy-aware, energy-harvesting node components. (1: core node, 2: energy multiplexer subsystem, 3: supercapacitor energy store, 4: thermal energy harvester, 5: wind energy harvester, 6: solar energy harvester, 7: vibration energy harvester.)
having a level of MP4. The sink node is attached to a PC, with the PC configured to display the messages received by the sink node on a terminal window.
In its initial state the energy-aware, energy-harvesting node and the energy multiplexer subsystem are connected to the supercapacitor energy storage module, which is uncharged, and to the vibration and solar harvesting modules. The energyharvesting modules are initially inactive, with the vibration source switched off and the paper clouds placed over the active surface of the solar module. A typical demonstration then proceeds as follows:
• The vibration source is started and the clouds removed from the solar module. The system then starts to harvest energy from these sources and charge up the energy store.
• Once the energy store has charged up sufficiently to operate the microprocessor node, status messages start to be received and displayed by the sink node. Initially, the node reports that it has the vibration and solar harvesting modules connected and that its energy store is at 2%, giving a power priority level of PP1. As a result of this, none of the messages from the two remote nodes are relayed. The energy-aware node sends a set of reporting messages each cycle reporting the level of the energy store and the production level of the connected harvesting modules.
• The level of energy in the energy stores rises over time to 20%, at which point the node switches to power priority level PP2. This causes the energy-aware node to start to relay messages from remote node RN1 but not from RN2, the node’s sleep period decreases, and these changes are reflected in the update messages sent to the sink node.
• The node’s energy store continues to increase, and at 40% the node switches to PP3 and the sleep duration is further reduced. This change is again reported by the sink node.
• The thermal harvester is then connected to the energy multiplexer subsystem and its heat source switched on.
• The next node status messages report the addition of the thermal harvesting module, its physical location on the energy multiplexer’s ports, and its current rate of harvesting.
• The vibration source is switched off, but the module is not disconnected from the energy multiplexer.
• The subsequent node status messages displayed by the sink node show the rate of generation of the vibration harvester to have dropped to zero, but that the module is still present.
• The wind energy harvester module is connected to the energy multiplexer subsystem, but no wind is provided to the miniature turbine, and the vibration harvesting module is disconnected.
• The next node status message received by the sink node reports that the wind energy harvester module has been connected and that it currently not generating any power. The message also reports that the vibration harvesting module has been disconnected from a specified port on the energy multiplexer subsystem.
• The energy in the supercapacitor store continues to rise, and after a period of time the node energy status messages report that it has reached 60% and that the node has changes to power priority PP4. The results of this are that messages from remote node RN2 start to be relayed and the sleep period of the node is further reduced.
• The source of wind is started to blow onto the miniature turbine.
• Subsequent node energy status messages report the now-nonzero level of energy harvesting from the wind energy harvesting module, along with the production levels of the other connected harvesting modules.
• “Clouds” are placed over 75% of the solar module’s active surface.
• Subsequent energy status messages displayed by the sink node report the drop in output from the solar module and that the level in the energy store has started to fall.
• After the level in the energy store has fallen sufficiently, the energy status message reports that the energy-aware, energy-harvesting node has changed its power priority level to PP3. As a consequence of this, the node’s sleep period is increased and messages from remote node RN2 are dropped instead of being relayed.
• With the node at PP3, the reported energy level in the supercapacitor store starts to rise again.
• The node reaches sufficient energy reserves to switch its power priority level back to PP4, reducing the sleep duration and permitting the relaying of messages from remote node RN2.
• As the level of energy in the energy store varies above and below 60%, the node reports changes to its power priority level between PP4 and PP3. The energy store gains during the lower power priority level and then starts to lose during the higher level. These energy level changes are reflected in the information reported to the sink node and also by the relaying or dropping of messages from remote node RN2.
• The thermal harvesting module is disconnected from the energy multiplexer subsystem and the clouds are removed from the active surface of the solar module.
• The next energy update message received by the sink node reports the removal of the thermal module from a specified port on the energy-multiplexer subsystem and the increased energy production from the solar module.
• Following a reduction in the total harvester power, the node settles into power priority level PP3, which is reported by the sink node.
• The message priority level of messages from remote node RN2 is increased from MP4 to MP3.
• The energy-aware, energy-harvesting node starts to relay messages from remote node RN2 to the sink node as a result of the increased priority of the messages now being sufficient in relation to the node’s current energy status.
• The energy bleed is used to the discharge the energy store until the power priority level reported by the node in its energy status messages to the sink
node drops to PP1. The effects of this drop in level are increased sleep duration and messages from both remote nodes being dropped.
• The message priority level of messages from remote node RN2 is decreased from MP3 to MP4.
• The energy-aware, energy-harvesting node starts to replenish its energy store, reporting the increase in energy level through the energy status messages that it sends to the sink node.
• The energy level in the node reaches 20% and the node switches to power priority level PP2 and reduces its sleep time.
• Messages from remote node RN1 are again being relayed to the sink node by the energy-aware, energy-harvesting node.
• Once the energy-aware, energy-harvesting node reports that it has reached power priority level PP3, the vibration energy harvesting module is reconnected to the energy multiplexer subsystem and the vibration source is restarted.
• The next energy status message displayed by the sink node reports the addition of the vibration harvesting module, the physical location of it on the energy multiplexer subsystem, and the current rate of energy harvesting.
• After a period of harvesting, the reported level in the energy store reaches 60% and the energy-aware, energy-harvesting node goes to power priority level PP4. This causes the sleep duration to be further reduced and the message from remote node RN2 to be again relayed to the sink node.
• With the rates of energy harvesting and consumption balanced, the energy – aware, energy-harvesting node remains at power priority level PP4, reporting the state of the attached energy-harvesting modules and its sensor module’s reading while also relaying messages from both remote nodes.
This demonstration demonstrates the active management and reporting of multiple energy-harvesting sources, along with intelligent message prioritization for energy management purposes. This demonstration has been performed at the Data Information Fusion Defence Technology Centre (DIF DTC) Conference 2009 and at the WiSIG Wireless Sensing Showcase 2009 .
The energy-aware, energy-harvesting sensor node described in this chapter has been developed for use within a demonstrator system, simulating use in an urban surveillance scenario. A plug-and-play approach to the configuration of energy-harvesting and storage modules has been adopted. This approach yields several benefits including the ability of configuration by nonskilled operatives and easy customization of the energy-harvesting system to suit the ambient energy available in a given location. Custom hardware has been developed to efficiently harvest, store, and process energy. Modules have been developed that allow the harvesting of solar, thermal, vibrational, and wind energy. Through the use of the energy-efficient hardware and intelligent energy management techniques implemented in software on the node’s microprocessor, the node can operate in an energy-neutral manner and device operation has been demonstrated, which achieves this goal.