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mattie

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Reply with quote  #1 

Hello all
Here is a great design for a passive design greenhouse with an air to water heat exchanger.Thanks to Gary and Russell for sharing this.
http://www.builditsolar.com/Projects/Sunspace/GreenhouseHX/GreenhouseHX.htm
 
Here is the link to the PDF
http://www.builditsolar.com/Projects/Sunspace/GreenhouseHX/HeatPumpEnergyStorage.pdf

Regards Mattie


JoeK

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Reply with quote  #2 
This a great idea, and looks rather well built, but I wouldn't call it passive by any means. Fans, pumps and electric backup heat?
Backup heat probably won't be necessary for long, and it could easily be 100% solar by next year
mattie

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Reply with quote  #3 
Hello Joe
The title was a reference to the greenhouse structure,its the same type of design and works on the same principles as the Missouri U and many others with thermal mass used in the form of water .
The systems inside are not passive, i agree here, perhaps i should change the title something like passive design greenhouse with heat exchanger.This kind of falls into a grey area for me,is a passive home not passive anymore because it uses a heat pump or solar or hrv etc?I do get what you are saying though, not sure is the answer.
Regards Mattie
Garage_Hermit

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Reply with quote  #4 
Interesting point !

Perhaps this is good place to "park" the definition ?

http://en.wikipedia.org/wiki/Passive_house


G_H

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mattie

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Hello all
Heres a few questions and ideas that apply to the design of this greenhouse.
How is the sizing done when it comes to the choosing the air to water heat exchanger? The volume of the greenhouse is a factor ,how do you figure out the fan size and appropriate flow rate across the fins?

What would be the optimal flow rate for the pump? Ive seen in the article a pump with a vsm(variable speed motor) that works off a temperature difference between greenhouse and tank which seems a great option and energy saving too.
How effective would such a system be at cooling? This is where the pdf stops.I imagine the cooling load here will be more significant than the heating even with the glazing angle optimised for winter.So how do you find the right balance in sizing the heat exchanger taking into account both heating and cooling loads?
You do have the option of venting the excess energy or simply opening a door in summer but wouldnt it make more sense to harvest it.Something G_H mentioned was the Sonnenhaus design,where a large seasonal thermal storage tank is built inside a passive house, the added bonus here is that any heat loss from the tank ends up in your home.Couldnt this be applied to a greenhouse.
In the document the tank was left external for structural reasons.If this route had to be taken wouldnt a less insulated joining wall aid the heat transfer of lost energy from the tank to the greenhouse.

Hopefully some food for thought anyway.

Regards Mattie

GaryBIS

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Reply with quote  #6 
Hi,

I tried a somewhat similar scheme to pick up excess heat in the peak of my GH.  Mounted a water to air heat exchanger (similar to Russell's) up in the peak of the GH with a shrouded fan blowing air through it.  The water side of the heat exchanger connected to 5 water barrels along the back of the GH, so when the peak air was hot, the fan and pump turned on adding heat to the line of water barrels (which are all plumbed together).

I did a little testing on this, and it appeared to work fine -- as Russell says, even on cold sunny days there is lots of excess heat in the peak.

The problem for me came during a time in mid winter when we were not using the GH and had a long spell of very cold and cloudy weather.  The heat exchanger froze and broke many of the tubes.   I thought I had arranged the heat exchanger so that it would drain back to the barrels, but had I looked at the heat exchanger carefully, I'd have seen that you can't really mount it in such a way that it all drains back.  This is something only people in bitter cold climate have to worry about, and there might be ways to protect the heat exchanger -- mounting it down where Russell mounted his would probably make it easier to protect.

But, my next iteration on this will probably be to use the fin tubes (like the ones in hydronic baseboard) instead of a car radiator type heat exchanger.  This way I can get them to drain back.

Any other ideas on how to recover excess heat in the GH peak?

Gary
 
JoeK

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Reply with quote  #7 
Historically the term "passive" for solar was used to refer to designs/structures that did not use any energy input for heating, cooling etc. Elements like direct gain, thermosiphon, and circulation/ventilation driven by natural convection.  Using fans/pumps etc is referred to as "active" systems or design elements. They require an additional energy input. Though it can often be very low power use, if the power is out (whatever the source) the system does not operate effectively.  Some active systems are designed to still function on passive principles if power is lost. Typically there is reduced efficiency, but at least some effect can be maintained during power outage.
In describing a home/structure where fundamentally passive design elements provide a substantial portion of the heat/cool load without additional energy input, I would say something like:

Approximately 75% of the Heating/cooling needs are met passively, with the remaining load met using electric heating/cooling.
Or perhaps:
75% of the Heating requirement is met with the passive gain-storage-radiation cycle of the water tank, while the active hot air collector supplies the remainder.

I do not mean to apply either of those statements to this greenhouse per se, just examples.  I don't think this greenhouse would work very well if the fan and pump weren't powering the heat exchanger, so to me the design has little passivity.

I think the recent European standard that bears the name does muddle the waters a bit when it comes to semantics. Still it is a great standard to hold, and I'm glad to see it gaining momentum.   The name is appropriate in a sense because the highest mark for the standard, 0 energy consumed  (after construction) for heating/cooling. Would be a truly Passivhaus.

In no way do I wish to take away from this excellent example of ingenuity. I just think it is mislabeled to say it is passive when it is so dependent on powered inputs.

JoeK

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Reply with quote  #8 
The real gray area in terminology for me is PV-direct designs that use a small panel to power a pump or fan that automatically moves air/water when the sun shines. There is electric power generated, and it runs a mechanical device that requires some maintenance and is subject to eventual failure in time. Yet the sun is still the source of the power, and if it can run autonomously for a year or two between maintenance checks...
mattie

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Reply with quote  #9 
Hello Gary,
Bitter cold sounds like where i live in winter,probably mild compared to most on here.Some type of fluid(water) loop with added glycol provide freeze protection maybe. How do you sit a UFH(underfloor heating loop) in the air at the top of a Greenhouse? And is a heat pump unit required in order for this to work effectively? Maybe some piping not prone to freeze damage could be used here ,pump the colder section of the tank water through the loop to gather energy?

Or perhaps if the temperature drops internally the tank pump is triggered and fluid flow could be used to prevent freezing of the heat exchanger?
There are some powered devices out there that provide freeze protection ,used on air conditioning units and also HRV systems for colder climates, i wonder if the answer lies here?

These are all just spur of the moment ideas.Its a key area that needs solving though your right to focus here.Another simpler idea is a backup heater that is temperature triggered for the greenhouse.
Regards Mattie
mattie

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Reply with quote  #10 
Cant argue with that Joe ,well said on both points ,the passive label and the ingenuity.
Regards Mattie
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