I tried the setup with the ground radiator almost all the way into the ground. Then I placed the TEG on top of the ground radiator and then a PC CPU heat sink on top of the TEG. The test was done in the evening and I used a multimeter to measure voltage of the device. I did not produce any voltage. I will try the test again in the day time so that the ambient air will be hotter.
Also of note is that I did not use any thermal paste on the device. I think thermal paste will be highly important to get decent performance from my setup.