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July 29, 2008

Achieving Zero Carbon – Self Powering Electronic Systems

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What if someone were to tell you that you could build a product that, following manufacturing, would use zero power and produce no carbon emissions? My first response would be, "yes, if you have abundant energy in the product’s environment that could be harvested efficiently".  So what "energy" is there to harvest?  Let’s take a look at the available sources and methods for creating a zero power system and what limitations exist today that limit the performance.

First, we need to have energy of some sort to power our device.  Energy resources will limit the performance of our system, so knowing our energy limits is the first step in determining what is possible.  Here’s a list of sources and the availability of each:

TYPE ENERGY DENSITY AVAILABLIBILITY IMPLEMENTATION COST
SOLAR (PV) HIGH HIGH MEDIUM - HIGH
WAVE HIGH MEDIUM HIGH
TIDAL / LUNAR HIGH MEDIUM HIGH
GEOTHERMAL HIGH LOW HIGH
WIND MEDIUM MEDIUM MEDIUM
VIBRATION LOW LOW LOW
THERMAL LOW LOW LOW
PRECIPITATION LOW LOW LOW
RADIO EMISSIONS LOW HIGH LOW
ZPF ? HIGH ?

OK, The ZPF (Zero Point Fluctuations) line item is a stretch - but I included it since at the nano-scale, it actually may be a source of energy and it does exist.  So, out of the above list solar is probably the best for most applications.  Depending on the season and where on earth you are located, the sun rains down energy on the surface anywhere from 2 kWhr/m^2 to over 7 kWhr/m^2. For applications such as buried high-way sensors, vibration could be a good source of power (and even solar).  Water running down a drain could also be tapped to supply power to small monitoring systems.  If the equipment is really frugal with its power, radio emissions from the many radio and TV stations could be harnessed to supply microwatts (possibly charge capacitors for use later).

To understand what it would take, let’s use an example of a system I saw many years ago that really needed to be completely independent from any power source - a golf course sprinkler head.  This was a neat idea that a major irrigation manufacturer dreamt up (and you know who you are...).  Here was the problem - a golf course must have water to keep "green" and the system must easily adapt to changes in the location of traps and greens.  Conventional zoned systems that use wires cannot easily be moved.  Each head needs to be controlled and require power.  Each head is used a few hours each week.

So the solution was to have each irrigation head be a self contained system that was totally independent and could be installed anywhere along a pipe without requiring any power.  Each head would communicate wirelessly to repeaters (or a mesh network) controlled from a central PC.  Moisture sensors could also be spread around the network to more accurately control the irrigation so not to waste water.  To solve the problem of power, at the top of each head a small solar panel was used to charge a battery (the weak link - batteries require replacement) capable of opening the valve and maintaining the electronics.  If the grass started to grow over the solar panel while the head was off or the battery required replacement, the head could signal a problem to the central computer. 

The overall function was to water the grass under control from a central system, require no external power source and rarely need service - a very nice application of a completely autonomous system harvesting energy to perform its function.  Could it be improved?  How about using the high pressure water during the watering cycle to run a small turbine and charge the battery - helps if there’s not enough sun.  Of course low power electronics makes this all possible (see http://national.com/powerwise for more information on reducing energy consumption in electronics).  If the electronics draw more power than the solar panel can supply (and the head is small), then the battery will go dead (along with the grass).  Additionally, the valve must use hydraulic pressure to help hold it open to minimize the energy required - a common feature of irrigation valves.

Let’s take a look at another system - this one hypothetical; an attic temperature sensor.  The idea here is to monitor the attic temperature for control of an exhaust fan that helps reduce the cooling cost of the home.  Now there’s a new problem - no direct sunlight and limited ambient light.  There are several sources of power in the attic, but most notably a large differential temperature (in summer or winter).  So the question is, how could this sensor be built? I’m going to suggest a thermal generator charging a battery or capacitor. During times of peak temperature differentials such as mid-day in summer, the generator charges the battery and monitors the temperature communicating wirelessly to the controller in the fan.  The actual device could be shaped like a spike that is thrust into the ceiling wall-board and protrudes through the insulation.  The cold side is inside the house (in the ceiling) and the hot side is in the attic... could it work? You tell me... till next time!

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