data centers do generate lots of heat.
I work in one, too.
But the heat is volume, and temperatures are not relatively high, compared to the needs of current Stirling engine designs that could produce power.
Green technology is still more a fad than a actual driving force. Just take my word for now, the cost to replace fossile fuels with renewable green energy has already become about even, even not factoring in climate warming concerns and global politcial difficulties. But, about you idea of data center energy.
Here in my world class data center, they run air conditioning to chill the data room, even in winter, with sub-zero temps outside, as they pull the air from inside the building, which is first heated. They do not figure it is worth the money to change it, too. Convince them of an exotic heat recovery system?
Often they cool during the day, and heat at night. A very simple system of thermal storage could save them money, using well accepted methods, but nobody I know does these things.
It will be a long and difficult road for our favorite engine type.