I'm sure if one selected Economy mode in WIndows Power plan, one might be limited to mostly 800-1200 MHz and easily achieve the 15W TDP envelope...
(you can play with HWMonitor and CPU-Z/bench/stress CPU to figure out what sort of clock speeds will keep you at a true 15 W level of consumption
If that is too slow, perhaps Intel's XTU might allow more flexibility with clock speeds under 1,2,3 or more cores loaded....