Posted by Jeff on January 26, 2007, 6:03 pm
Robert Scott wrote:
> On 25 Jan 2007 19:23:01 -0800, "ghostwriter"
>>Question for the group, I am getting a quote on a geothermal heat pump
>>and was wondering what benifits might be had from coupling the closed
>>loop with a evacuated tube system. Ideally I would place the tubes in
>>front of the heat pump so that the heat pump only inputs the difference
>>between the solar produced temp and the temp necessary for heat
>>production. Anyone know of any reason why this wouldnt be possible.
>>Differences in necessary flows are the only reason I can imagine.
> Aren't evacuated tubes mostly for high-grade heat? A geothermal heat
> get by on low-grade heat, such as you might get from a simple
> plate collector. I'm not sure that the heat pump wouldn't actually
> by high-grade heat from an evacuated tube collector.
I think that is a pretty good point.
Also to be considered is that you wouldn't connect the collector
directly to the heat pump as you'd have to disconnect when the sun goes
down. Most solar water systems rely on a water tank heat store.
Is your heater, forced air or radiant?
You are going to want to crunch some numbers and the main variables
will be. What ambient temperatures are you dealing with? How much south
facing sunshine? And what is the COP of the heat pump at various input
> Robert Scott
> Ypsilanti, Michigan
Posted by ghostwriter on January 26, 2007, 7:59 pm
Actually that is the reason for going with evaculated tubes since they
dont have the conduction heat loses of flat plate systems. They
shouldnt lose any real amount of heat overnight, even when it is below
In place of the the water tank I intend to use the ground loop to store
as much as possible. The remainder will disipate into the ground, not
paticularly efficient but it allows the ground loop to be
Ground temp is usally 54F, a single set of tubes is rated by the
manufacurer as giving 40,000 BTU per day, normal geothermal heat pump
performance is about 3, higher tempatures will reduce that COP but
will reduce the absolute energy input needed by a comparable amount.
The major issue I will have will be that I will absolutly need to shade
the tubes during the summer or the COP of the AC side of the heat pump
will go straight down the tubes.
Posted by Simon on January 26, 2007, 8:41 pm
The upside of this idea is that the evacuated tube panel will transfer
heat much more efficiently into the relatively cold feed from the
ground loop (than into already warm DHW etc).
The amount of heat provided by a modest collector array will not raise
the temperature to the point that it will damage the heat pump, at
least when the ground loop circulation is in continuous operation (if
this is becomes a danger, then you have enough heat to switch out the
heat pump and just use the solar).
Posted by rogge on January 27, 2007, 11:18 am
If I understand you correct see this http://www.eviheat.se/ and click at the
picturs in swedish but I can explain if there are any questions. The company
Eviheat has constructed this concept and it works pretty nice. They claim it
has a COP of 7.
Posted by Gary on January 27, 2007, 6:36 pm
Maybe you don't have a way to do this, but it seems to me that if you are going
to add solar heating collectors you would be better off (more efficient) to use
the thermal output from the collectors to directly heat the house. Solar
thermal panels used for direct space heating would have a COP over a hundred,
and the heat would go into the house instead of into the earth where some
(most?) of it will be lost??
If you do just want to heat the ground, it seems like a very low tech
(inexpensive) collector could do this efficiently. If (say) the ground
temperature is 50F and the average day time air temperature is (say) 40F, then
the collector is only seeing a 10F delta temperature for losses -- it seems like
even pool collectors might work pretty well under these circumstances??
But, maybe I'm missing something?
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