Ten years ago I happened upon a Sun Oven on the last day of a big community 3-day yard sale. I dickered and got it for $8. It is an older one and when I got home I did some research and found similar models on ebay for about $30-40 (not the $150 - $300 they are listed for right now!).
The long side of my house (where most of my yard is) faces south and this is where I put the solar cooker when in use. The major benefits are: free power from the sun, keeps the heat out of the kitchen in the summer, is a back-up or 2nd oven. One Thanksgiving I used it to cook cornbread when I realized I hadn't planned my oven rotations with enough time! In the summer, it makes a fantastic fruit cobbler without all that heat in the house. It does a really good job roasting butternut squash. There is no integral temperature gauge so I bring out the one used inside my electric oven. On a nice sunny day it gets up to 350 degrees F in about 30 minutes.
Limitations: the pan size - have to plan accordingly to check that the intended pan can fit on the swing-like shelf. (It has an interior swing because there is an extendable foot on the bottom to adjust angle, thus the item being cooked stays level.) I have a blue glass lidded casserole dish that is pretty much dedicated for use with the solar oven. And the major limitation: sun. If the weather turns cloudy, that cobbler is more of a pudding! If I am cooking something that takes longer I do need to go out and reposition the cooker so that it tracks the sun.
From the videos - I am interested in compost heating. I'd like to have a greenhouse or hoop house benefiting from compost heat. The bicycle power project was fantastic - I once saw a bicycle powered video game at a school! We often exercise indoors in the winter (bike) and I've imagined harnessing that power for something like charging batteries or ?