There are a lot of different types of Arduinos out there. This page covers some of the more common Arduino boards and describes which ones are the best to use and which might cause you issues.
The Sun Harvester Shield has been tested with the Arduino Uno and the Arduino Mega 2560 (not the older Mega). I would recommend that you stick with one of these two boards. Note that the Mega has considerably more program space than the Uno. This extra space allows for a much more accurate sun position algorithm to fit on it. It also gives a lot more breathing room for future developments to the Sun Tracking / Heliostat Program.
If you are only interested in controlling a few machines, aren’t overly concerned about their accuracy, and don’t care if they have all of the current and future bells and whistles, then the Uno should be fine for you. If these things appeal to you though, then you should use the Arduino Mega.
Also note that there is a bug with some of the older Arduino Unos that keeps them from uploading programs over 30kb. This is a pain since the Uno version of the Sun Tracking / Heliostat Program is larger than this. The problem can be fixed by using the method described on this post on the Arduino forum, but you will need another Arduino to do it. This is perhaps another reason to use the Arduino Mega 2560.
Other Arduinos: (This is not exhaustive list by any means)
The Arduino Due does not work with this system. I have one, but haven’t had the chance to work out the bugs. Suffice it to say that you are probably better off avoiding it for now.
The Arduino Leonardo has 32KB of flash memory but 4KB of it is used by the boot loader. Basically, this means that the default Arduino Sun Tracking / Heliostat Program will not fit on it unless you are able to comment out some of the code that you might not need for your project.
The older Arduino Duemilanova should work; however, if you are using the Sun Harvester Shield you will have to run a wire from the extra SDA output in the upper left hand corner of the shield to analog pin output A4 on the shield. You will also have to run a wire from SCL to A5.
Similarly, the older Arduino Mega should work; however, if you are using the Sun Harvester Shield you will have to run a wire from the extra SDA output in the upper left hand corner of the shield to pin 20 on the Arduino. You will also have to run a wire from SCL to pin 21.
The older Arduino’s do not have the extra SDA and SCL outputs in the upper left hand corner, thus you have to make the connections yourself.