Re: Need feedback on pond design

Richard Tanzer wrote:

Dan <NoSpam@xxxxxxxxx> wrote in news:wuudncUusup7UDPeRVn-pg@xxxxxxxxxxx:


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. 

Yup, you're correct. I was merely talking about the heat generated
by the motor itself, not the heat generated by the friction of the
water since that's pretty constant whether the motor is submerged
or not.  Say, as in your example, a highly efficient pump only wastes
10% to heat, a 100 watt submerged motor is only adding 10 watts of
heat to the water over an external pump of the same efficiency, right?
In any case, every bit helps (except in the summer that is :) ). BTW,
any idea how efficient 'high efficiency' pumps really are? I haven't
a clue.  Is 90% about right?
.

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