Dan wrote in :

Richard Tanzer wrote:

One seasonal advantage of a submerged pump, relative to a pump that
sits outside the pond, is that all the electrical energy that goes
into the pump is dissipated as heat into the water. So in the
winter, if your pump is drawing 100 watts of power, you are adding
100 watts of heat to the pond.

Actually, that would only be true if you had the world's most
inefficient pump, i.e., one that moved no water. A certain
amount of energy is 'wasted' as heat, but hopefully only a
small fraction of that consumed by the pump.


Dan, I believe you are mistaken. Even if I had the world's most
efficient pump, all the energy that goes into moving the water around is
lost to friction, i.e. drag forces. Otherwise the water would keep
accelerating as you continued to add energy.

Because the water very quickly reaches a more-or-less constant velocity,
whatever kinetic energy is imparted to the water by the pump must be
converted to thermal energy in the pond.

Consider two 100 watt electric pumps submerged in a fish pond. The first
motor is highly efficient, 90 watts of energy go into pumping water and
10 watts are lost as heat in the motor. The second motor is far less
efficient, 10 watts of energy go into moving water and 90 watts are lost
as heat in the motor. From the perspective of heating the water, the
motors are equivalent. The only difference is whether the heat is
generated in the motor itself, or in the pond in turbulent friction
(vortices and eddies).

Ahh .. the wonderful world of thermodynamics.