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Heat Pump and water - Page 7

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Posted by Robert Scott on November 2, 2006, 8:52 pm

There is no doubt that higher inlet water temperature to a heat pump results in
higher COP.  But you have yet to show that there is any significant increase in
temperature of the water delivered to the heat pump due to your suggested tank;
and I and everyone else who has commented on your design seems to think there
would be no such increase in temperature.

Have you considered the time-constant of the thermal mass involved?  Have you
considered the pattern of heat usage?  Your suggestion offers an advantage only
if the heat demand is very spikey, and if the time-constant of the field is
fairly short.  The "problem" you are trying to correct is excessive fluctuation
in the temperature of the underground field.  Because if there were no
temperature fluctuations, then the field already would be operating optimally.
Buffering an already-stable temperature source (as you suggest) adds no benefit
to the system.  But that is just what I see in the operation of my heat pump -
an essentially stable temperature (when viewed on a daily basis).  The changes I
see are slow and take weeks to mature.

In that sense, your tank buffer would be attempting to perform a seasonal heat
storage feat, which I think you will agree is not practical.  Now if the
temperature of the soil around the pipes cycled by several degrees every time my
heat pump cycled on and off, then I would have to agree with you.  In that case
a buffer filled by a constantly-flowing extra pump would harvest more heat from
the field than if the temperature were allowed to yo-yo.  But that just doesn’t
happen.  And if it did, that would mean I had a seriously undersized field.

When they size these fields, the installers look at the expected heating load of
the house.  The more money they put in the field, the more efficiently your heat
pump will run.  But the relationship is not linear.  As more pipe and earth is
added, the incremental improvement in efficiency becomes smaller and smaller.
At some point it just doesn’t pay to add any more pipe.  Your solution is
different inthat it costs money up-front and it costs money on a continuing
basis to operate the extra pump.

If you took all the money that you would spend on tanks and plumbing and valves
and controllers and simply ask the installers to add 100 more feet of trenching,
I believe you would reduce the thermal resistance to the earth and increase the
temperature of the water and the COP of  the heat pump more for less money.

Robert Scott
Ypsilanti, Michigan

Posted by News on November 3, 2006, 12:19 pm

It appears you still haven't got it. There will be no increase in
temperature in the tank - it will be at ground temperature.

The tank concentrates heat far more than earth. Convection of the water
inside the tank is far greater than earth.

What this means is that when the heat pump kicks in it works at a constant
COP extracting heat from the water. If the tank temperature drops because
the heat pump is returning very cool water the loop pump kicks in and starts
to dump warmer water from the earth.

Compare that to the heat pump that is only acting on a slinkie loop in the
earth. The earth may get cool quite quickly around the slinkie loop.  The
heat convection of the earth is poor so the earth does not warm up fast
enough leaving the temperature of the earth around the loop quite low. This
means the COP initially will be quite high and then sharply drop off as the
earth around the loop cools.  Some heat pumps in winter can't raise water to
DHW temperatures with say the earth initially at 5C. Yet!!  Heat extracted
from flowing water at 5C will result in DHW temperatures.

With running water it is always 5C as the cooled water runs away and
instantly replaced by fresh water at 5C. However with earth it will be
cooled way down, even below freezing.

The tank (thermal store) concentrates the earths heat into one efficient
means of extracting the heat that will remain constant for most of say the
run time operation to heat a DHW tank to DHW temperatures.

Get it? :-)

This is all open and specific to site and location of course.   A simple
pump running through a long earth loop ensuring a thermal store is all at
the earth temperature can operate when there is no demand for heat in the
house to store the heat in the thermal store - a buffer.

DHW is spiky by nature.  The thermal store tank also increases the COP of
the heat pump over the run time of the heat pump.  Makers say a Heat Pump
has say a COP of 3 - that is at peak and optimum conditions and the
"average" COP may be well below that. Many people have been disappointed
with the running bill of heat pumps.

In the UK there is the SEDBUK tests which test boilers. What they do is test
a boiler on a "typical" British CH system to average British climatic
conditions.  The best is 92%, and these figures are stated by the makers.  I
have "engineered" a system to get the boiler to more than SEDBUK, as I know
what I am doing - SEDBUK use an average CH system, with basic controls.
These boilers at "optimum" conditions can reach near to 100%, but the test
is to average, so representative. There is no such thing with Heat Pumps and
invariably the optimum COPs will be quoted.

Read above, re: the earth over cooling around the slinikie pipe lowering the
COP and unable to reach DHW temperatures.

That is one of the points. Your slinkie is not operating at a stable
temperature at all. The heat pumps cools around it.

That is exactly what it does, lowering the COP and in mid-winter the heat
pump is unable to raise to DHW temperatures meaning expensive COP 1 heating
elements have to supplement.

You could have a slinkie which is massive and hold a hell of lot of water in
itself. This will get around the problem as the water in the loop will have
stabilised at earth temperature and may be enough to supply DHW alone with
only two passed of the water through the loop.

That is extra expense of course,and you need the ground to do that.  Most do
not have a rear garden the size of a park.  Over a longer period of time a
smaller loop can store the earths temperature in a concentrated thermal
store tank.

As above.

The money up front is offset by not having a large loop, a smaller loop can
be used, and a higher average running COP, which means cheaper running

100 more feet of trenching?  Few have the luxury of that amount of space

Take this thermal store buffer tank and have it overground, then have solar
panel also feeding it too, and then it will really shine.

Posted by Robert Scott on November 3, 2006, 12:42 pm

OK, now I get it.  You are talking about DHW and I am talking about home space
heating.  If the heat pump is being used to supply DHW, then a small slinkie and
a thermal buffer tank do make sense.

But here in Michigan, in my home the DHW bill is about 15% of the home space
heating bill.  In fact my heat pump only serves space heating, which is not

I couldn't agree more.

Robert Scott
Ypsilanti, Michigan

Posted by SJC on November 3, 2006, 2:20 pm
 Seems like an expensive system for just DHW. Solar thermal collectors
and a heat exchanger would be cheaper than a heat pump. If you are using
a heat pump for home heating, then there are not enough BTUs with this

You seem determined to do this. So let us know how it goes.


Posted by News on November 3, 2006, 8:45 pm

Heat generated by a solar collector is best fed directly into a DHW tank,
when the tank is up to temperature the solar gain can be pumped into a
thermal store in which the heat extracts heat from.

There are read what I wrote and understand it.

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