You hit the nail on the head there, and yes the details are important in assessing overall long-term feasibility and sustainability, However i believe this is a very viable opportunity that is staring Africa in the face, especially when you consider the limitations that energy poverty impose on economic growth. Plus the grid operators don't necessarily need to buy all the rigs upfront, they could entice a bitcoin miner to come and purchase power at the source, once consumers are connected the miner can turn off his machines when required to balance the power consumption between consumers and the miner. in addition, the point you brought up about heat energy being recycled for agricultural use is another benefit, and this is exactly what i am talking about that this is definitely a route that if planned and executed well, there is little to lose and much to gain, especially when new projects in rural areas are on the cards where immediate industrialization may not be likely, but a vibrant agricultural community already exists.