How can inverter be more efficient?

Posted by grattou on August 20, 2008, 10:09 pm

Hi,

I fail to see how inverter heat-pumps can be more efficient than fixed-
speed ones.

I guess this boils down to another question: a heat-pump probably has
a speed where its efficiency is maximum. Hopefully this is where fixed-
speed heat-pumps are set to operate. Is that right?

If so, when the inverter varies the speed, it moves away from the
optimum, and efficiency goes down.

It would seem to me, from a pure COP view point, and if my assumptions
are correct, that a fixed-speed heat-pump would have the maximum
efficiency. Now, comfort is also important, and off-on maximum heating
a not as nice as sustained, adapted, heating.

But this is not what is advertised. All vendors claim that COP is
better with an inverter, even though i assume there are lossses in the
current-inverting circuitry. How can that be? The only way I can see
that this would be true is if the COP increases as compressor speed
decreases, but I was never able to find any information about that.

And also -this is presumably linked- why do smaller heap-pumps have a
better COP than larger capacity ones?

Does anyone know the answer to this mystery?

Thanks you for your lights, Chris

Posted by danny burstein on August 20, 2008, 10:42 pm

>But this is not what is advertised. All vendors claim that COP is
>better with an inverter, even though i assume there are lossses in the
>current-inverting circuitry. How can that be? The only way I can see
>that this would be true is if the COP increases as compressor speed
>decreases, but I was never able to find any information about that.

I'd love to hear from the experts as well, but until they
get back to us here's my guess:

I believe a heat pump (whether operating as an a/c or as a heater...)
will have better efficiency when there's a lower temperature
difference across it. In other words, if you've got an inlet
temperature of 80 and an outlet of 70, the system is working
more effeciently than if it's pumping out 50 degree air.

Now it'll have to work 3X as long (per cycle time) with
the lower difference, but it'll be more than 3X as
efficient, so you'll come out ahead.

A "regular", or fixed speed, compressor (and the
rest of the system), will always be trying to give
that 30 or so degree temperrature change to the air
flowing through. (It'll cycle on and off, of course,
to match the thermostat setting).

So that inverter system will just gently give you
that five degree drop if that's all that's called for,
and will be much more efficient for it.

As an analogy, think of the MPG you get in a car
running at 90 mph versus 40 mph... While the rolling
resistance increases on a pretty linear slope,
the air resistance is exponential.

>And also -this is presumably linked- why do smaller heap-pumps have a
>better COP than larger capacity ones?

There you've got me... In fact, I'm in shock over
the latest offering from Mitsubishi, where they
have a new 9,000 BTU mini split (part of their
"Mr. Slim" line) with a claimed...

23 SEER.

--
_____________________________________________________
Knowledge may be power, but communications is the key
dannyb@panix.com
[to foil spammers, my address has been double rot-13 encoded]

Posted by on August 20, 2008, 11:07 pm

wrote:

Nope.

>If so, when the inverter varies the speed, it moves away from the
>optimum, and efficiency goes down.
>It would seem to me, from a pure COP view point, and if my assumptions
>are correct, that a fixed-speed heat-pump would have the maximum
>efficiency. Now, comfort is also important, and off-on maximum heating
>a not as nice as sustained, adapted, heating.
>But this is not what is advertised. All vendors claim that COP is
>better with an inverter, even though i assume there are lossses in the
>current-inverting circuitry. How can that be? The only way I can see
>that this would be true is if the COP increases as compressor speed
>decreases, but I was never able to find any information about that.
>And also -this is presumably linked- why do smaller heap-pumps have a
>better COP than larger capacity ones?

They don't.

>Does anyone know the answer to this mystery?
>Thanks you for your lights, Chris

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Posted by Thermo on August 22, 2008, 2:31 pm

The optimum operating point is not fixed - it's dependant on the
conditions at which the heat pump is operating at that particular
time.

The advantage of the inverter is that it allows the controller to
measure those operating conditions and then match the speed of the
compressor and fans to that load.

Posted by Andy Energy on August 24, 2008, 1:03 am

> Hi,
> I fail to see how inverter heat-pumps can be more efficient than fixed-
> speed ones.
> I guess this boils down to another question: a heat-pump probably has
> a speed where its efficiency is maximum. Hopefully this is where fixed-
> speed heat-pumps are set to operate. Is that right?
> If so, when the inverter varies the speed, it moves away from the
> optimum, and efficiency goes down.
> It would seem to me, from a pure COP view point, and if my assumptions
> are correct, that a fixed-speed heat-pump would have the maximum
> efficiency. Now, comfort is also important, and off-on maximum heating
> a not as nice as sustained, adapted, heating.
> But this is not what is advertised. All vendors claim that COP is
> better with an inverter, even though i assume there are lossses in the
> current-inverting circuitry. How can that be? The only way I can see
> that this would be true is if the COP increases as compressor speed
> decreases, but I was never able to find any information about that.
> And also -this is presumably linked- why do smaller heap-pumps have a
> better COP than larger capacity ones?
> Does anyone know the answer to this mystery?
> Thanks you for your lights, Chris

When a system cycles on and off the average EER is not the rated EER.
So a unit that may have a lower EER and does less cycling can actually
have a higher average EER. Than the unit that cycles more often.

Andy

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