This particular rocket stove is built under a greenhouse, and the pipe is laid beneath what will be a raised bed within the greenhouse, which will provide the thermal mass to transfer the heat. The bed will stay nice and warm; an ideal place to start plants or grow tropical plants.
A rocket stove is much more efficient, but one of the best parts about it is that the smoke is literally burnt up inside the pipe, so the resulting exhaust is steam and carbon dioxide, instead of carcinogenic wood smoke.
Some people report that they can heat their home using just the sticks that fall from the trees in their yard – that’s how efficient a rocket stove is. It makes perfect sense in a greenhouse, because it is very inexpensive to build and maintain, and does not pose the fire risk a wood stove left alone might.
You can use scrap wood that would be wasted in larger wood stoves, such as blow down from storms, and all the dead sticks and twigs a tree normally sheds, which means you do not have to harvest anything living to provide fuel for your rocket stove.
If you are looking for alternative heat sources with renewable energy, look into the rocket mass stove for both your greenhouse and your home.