While it can be difficult to scavenge enough heat from ambient outside air, using underground "line sets" can be a good alternative. In new construction this is fairly straight forward; you are already excavating on site. For retrofits there are already compact and drivable drilling rigs in some cities that can drill the necessary depth well bores for the necessary underground heat exchange. These of course can run in reverse in cooling season...I'm hoping to see the retrofit industry gain significance in the next decade.