Heres the items you will need.
1) Computer cooling fan
4) General purpose diodes
Resistors + potentiometer (I used a 10k pot and ~2.8k resistor)
200-300 micro ferrads worth of capacitance
small perf board
small project enclosure (An Altoids tin would work fine)
some sort of connector (I used an rj45 and a keystone jack that I had disassembled)Tools
Needle nose pliers
Total cost to build 2 units ran me around $40 US with parts to spare.
I bought the resistors, dioeds and capacitors in a "variety pack" and saved a couple of bucks
The fan that you use is the biggest factor, you want one that turns very easily preferably with large surface area across the blades. For my purposes I needed a fan capable of generating 0-10 volts DC
After quite a bit of experimentation I settled on a Radio Shack "Brushless 12VDC Cooling Fan" #273-238
The equipment that all this will plug into is a Veris Industries H8820 Acquisuite
which in a nutshell is an embedded linux device for doing power and building monitoring. It has several a 0-10VDC pins to use. These are not cheap but we already have one so why not use it to its full capacity.
Another (cheaper) option might be a Z2TEN
one-wire device and associated bus master like a HA7Net.
Because I'm using the H8820 as input I measured the impedance of the inputs and determined that its ~12.7K so my rectifier circuit will closely match that value (10k pot +~2.8k resistor)