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SolarInterested

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Reply with quote  #11 
Quote:
Originally Posted by GaryBIS
... 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.

That seems like a better and less expensive design. Will you still use a fan either at the either the fin tubes or the drums?

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JoeK

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Reply with quote  #12 
Gary have you seen the thread on Subterranean Heating and Cooling in this forum?

Mattie I think a UFH and a peak heat collector could be connected with much success. Could work, from a thermal transfer standpoint, even without a storage tank. The heat would be picked up from the peak of the GH and then absorbed by the soil surrounding the UFH. it would then return to the peak cool and ready for another cycle. Essentially a heat pump using water as transfer fluid and the soil for storage mass. I wouldn't be surprised if you could even get away without an expansion tank, since the heat will not accumulate in the flowing pipes very much. but a good idea to have a small one anyway.  Freeze protection would be necessary in my climate and I can think of a few options.

Your idea of just having a temp switch force circulation to prevent freezing should work, and gets points for the simplicity. It would allow the heat collection loop and UFH to connect directly and simply in a closed loop. a very low power circulation pump would suffice if this is a closed loop.  The drawback would be circulating water at the top of the greenhouse would result in substantial heat loss since the top is near the exterior, and presumably the glazing. This forced circulation would in effect be cooling the storage mass and sending the heat out the top of the greenhouse.

Another option and perhaps the most efficient (both thermally and in terms of power consumption) would be a drainback design.  This requires a drain/expansion tank, but it only need be large enough to drain the collection loop overhead. Perhaps only a few gallons in a small system. the pump would have to be powerful enough to overcome the head pressure at startup, but then could circulate the rest of the day with very low power consumption. Might be possible to use PV direct but overcoming the head pressure will be a challenge.

Third option is perhaps the easiest of all, just put some heat tape on the back of the collection loop, or around the sides of a heat exchanger. This is quick and easy, but consumes the most power and also is heating the top of the greenhouse where loss is high.

mattie

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Reply with quote  #13 
Hello all.

I like the idea of a tank and peak loop.This gives the option to draw from the tank to meet demand whenever its required.

Another notion here is water filled pipes(glycol for freeze protection) that sit at the peak.Two heat exchanger coils(indirect) one in the pipes at the peak and another in the tank.The pump with the temp difference trigger between the peak of the greenhouse and tank could work well here i think.

Regarding the temp switch your right on the energy loss out of the peak area of the greenhouse ,this will push inefficiency in the system.Perhaps a smaller separate loop with a short run of pipe for freeze protection is an option here.
Another factor to think about here is that energy does sit in the peak area for harvesting potential ,so it would be down to r values and material choices within the structure along with external temperatures as to the rate at which the heat loss occurs.

I think the drain back system is a great idea Joe ,this could provide the freeze protection and suits the tank design, once like Gary mentioned it can drain completely.I'm not sure off hand what size pump would be needed to deal with the height issue, there is ofc the issue of more power more cost with a pump,then again its not a huge climb so might not be too bad.

Regards Mattie
Garage_Hermit

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Reply with quote  #14 
@ Mattie, for Item #5

Mattie, I guess your questions are directed at the GH designer, and seeing as he is unlikely to respond, I am taking them as rhetorical, and having a stab at them myself !

  • Q       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?

A     Faced with the same issue, I would go by one of three routes, namely:
1. Empirical-Iterative-Pragmatic-Heuristic (by using what I had available, lying around or beggable), such as a scrapyard automobile radiator of such and such a size, and a fan likewise, tank likewise; then just build it an run it and see if it worked, and adjust accordingly…;
2. Calculation by thinking it through and designing to the numbers (requires input data) (= dimensions & parameters); and output data (= requirements and values / magnitudes);
3. Off-the-Shelf: purchasing a heat pump or a heat exchanger of the required capacity  (latter determned by catalogue sources or specialist’s visit to site); presumabley, the required control package would also be included;

the actual route taken depending heavily on how I was feeling on the day, and the state of my finances…

=============

  • Q      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.

A      Your question in fact answers itself !  The optimal flowrate for the pump would be determined by a programmable logic controller, receiving inputs from various sensors such as date, time (= night-day-insolation), temperatures, pressures, dewpoints, enthalpy…). The PLC drives the pump speed controller (PWM etc.). The PLC also would have access to library files, containg reference climate data (insolation and max/min temperatures, pluviometry, degree-days etc.) over several years, and maintenance logs, containing the actual system settings for a given date (= day of year), for real time comparison…  The PLC could also run other drivers such as additional fans and/or heating, vent opening/closing, IR / fluo lighting on/off, alarms etc.

================

Q      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?

A       Irrespecitive of the basic solution retained (empirical, calculation, off-the-shelf), this aspect comes down to managing enthalpy. Some quantity of thermal inertia must be introduced into the system, to provide a « buffer » (analogy = a ‘’heat header tank‘’» or a ‘’coolth header tank‘’…) in order to cater for inversion of load between HEATING (energy) (required, for example October thru May…) and COOLING (exergy) (required the remaining months).  Boils down to saying, if you need a tank of hot water for warming in Winter, then you also need a tank of cold water for cooling in Summer…). Can you use the same tank ? If so, how do you manage the changeover period, when the tank contents have to change from HOT to COLD…). Etc.  Would no doubt require some severe modelling…

==================

Q    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.

A       I am sure that the SonnenHaus concept contains multiple gems that nobody has unlocked yet !  (going there, immediately after this !)

====================

Q       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.

A   This aspect would be taken care of, I suspect, in the design approach: ‘’whether you do or you don’t’’ insulate... It reminds me of Gary-BIS’s Low Inertia Sunspace »… Yes, of course U R dead right !  Insulate the outside walls = heated envelope, but *not* the inside partition…

(I know some folks that cannot use their ‘’Very Expensive Smart Verandah’’ for several months of the year, because yrs ago, they thought that putting 8 inch of styro under the slab was overkill.  Result : the place is a BLACK HOLE and *drains* their entire HOUSE… (whence the saying, ''You Gets Watts U Pays For''…)


@Joe

Hate to disagree, but in my book, Mattie’s reference surely qualifies as a ‘’passive greenhouse’’…

However, rather than starting a rant on here ( !) I will start another post in General Discussion.

Best,

G_H

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GaryBIS

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Reply with quote  #15 
Quote:
Originally Posted by SolarInterested
Quote:
Originally Posted by GaryBIS
... 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.

That seems like a better and less expensive design. Will you still use a fan either at the either the fin tubes or the drums?


I'm thinking that some type of fan to get a little air velocity over the fin tubes would improve performance quite a bit.  

I think this is one area where the compact car radiator type heat exchanger may have an advantage in that its easy to mount a fan that directs a lot of airflow through the heat exchanger.

An interesting thing about using the fin tube pipes is that if they are in the sun, they will also pick up some direct solar radiation -- kind of like a solar water heating collector or the Zomeworks Big Fin collectors.

Gary
JoeK

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Reply with quote  #16 
I'll check out that post in General Discussion, very curious to hear your reasoning. thanks as always for the perspective GH.

In response to the changeover period.

"Boils down to saying, if you need a tank of hot water for warming in Winter, then you also need a tank of cold water for cooling in Summer…). Can you use the same tank ? If so, how do you manage the changeover period, when the tank contents have to change from HOT to COLD…). Etc.  Would no doubt require some severe modelling…"

you may be overthinking it. The changeover period inherently occurs when there is neither much heating, nor cooling required. At these times one has little dependance on the storage tank and I it should be easy enough to make the transition.  Not to mention that a tank at say ~70*F will feel warm on a winter night and cool in the summer's day, at the same temperature. So the tank doesn't actually need to have a large swing in temp during the change. And the changeover period is typically a period of weeks, or even up to a couple months where weather is in the "goldilocks" zone.

As I see it:
In winter one is harvesting the heat from the greenhouse during the day, and transferring via the heat exchanger into the tank. Then at night the heat is exchanged back into the greenhouse. 
In summer time one can use the tank as cooling during the day by exchanging heat with the greenhouse, but at night would want to exchange that heat with the exterior cool air of nighttime. Essentially draining off the heat accumulated in the tank during the day's cooling cycle.
In the transition times the greenhouse will not need that much heating or cooling so the exchangers won't need to run as often.  The operator will notice the daily tank temp starting to climb, at the same time greenhouse heat requirement at night is dropping off.  During this time the system could likely sit idle and temps in the greenhouse would remain favorable.  One could simply vent the heat out during the day instead of storing it and the tank temps would begin to drop. For faster drop flip on the outdoor heat exchanger at night, but don't add any more during the day, rely on venting to maintain day temps in the GH.
GaryBIS

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Reply with quote  #17 
Quote:
Originally Posted by mattie
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


Hi Mattie,
All good things to think about, but I'm kind of liking the idea that the fin tubes could easily be arranged to drain back to the barrels reliably when the pump shuts off, and I like that.

I just need to find a source for the fin tubes that is not to expensive.

Gary
cwwilson721

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Reply with quote  #18 
They ain't cheap, that's for sure.

A possibility is a HVAC company that just replaced someone's baseboard heat...Or someone installing underfloor heat, and replacing it.

Both options around here are few and far between
SolarInterested

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Reply with quote  #19 
Quote:
Originally Posted by GaryBIS
...I just need to find a source for the fin tubes that is not to expensive.


Maybe this is another application for aluminum flashing? If you constructed tubes & fins similar to those in your hydronic collector wouldn't they work well is this application?

Mike

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Both temperature rise and airflow are integral to comparing hot air collectors
GaryBIS

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Reply with quote  #20 
Quote:
Originally Posted by SolarInterested
Quote:
Originally Posted by GaryBIS
...I just need to find a source for the fin tubes that is not to expensive.


Maybe this is another application for aluminum flashing? If you constructed tubes & fins similar to those in your hydronic collector wouldn't they work well is this application?

Mike



Hi,
I have thought about doing that -- it would certainly be easy and much cheaper than the fin tubes.  The aluminum area would not be as great, but maybe it would be good enough.  And, I already have a box of the 6 inch fins on hand.

Gary
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