By a paper, do you mean the laws of thermodynamics? The fans also eventually also dissipate their energy as heat when they push air around. There is an altern

By a paper, do you mean the laws of thermodynamics? The fans also eventually also dissipate their energy as heat when they push air around. There is an alternative heater design though for the same power, which is a heat pump (the same as an AC, run in reverse), which can be much more efficient at heating (down to -5F outside, or so, when they may freeze over and stop working entirely)