Geothermal Buffer Tanks

MechTech_2

I have a client who wants to bury 2 geothermal buffer tanks underground (1 hot & 1 chilled), and also wants to optimize the time of use meter by charging the buffer tanks at the lower KW rate between 11pm & 7AM. Does anyone know of a resource to find the calculations for sizing tanks & heat pumps for such an application. Or maybe the juice isn't worth the squeeze. Any thoughts?

NRT_Rob

tricky

the "easy" thing to do is to calculate the maximum amount of heat/cooling energy you'll need for a 16 hour period and size your tanks to store that much energy at a temperature differential that works for your system.



that will result in darn big tanks though, sized for the coldest and hottest days of the year typically.



You could dramatically shrink the tanks by optimizing the size to store the full load for MOST, but not ALL the days of the year. But then you'd have to compare tank sizing and costs to the energy saved using bin data... probably easiest to price 3 likely tank sizes and pick a couple of sizing thresholds... say, 100%, 90%, 75% of "max charge" and compare the remaining yearly energy requirements to the difference in tank prices and see what makes sense. then if you see any trends, you can zero in on something that works for this project.



not easy. but doable, perhaps, if you have access to seasonal bin data.



if anyone has has a better idea I'll love to see it though

Mark Eatherton

I have a challenge....

To ANY heating contractor, forced error or hydronic.



On a system, that you know for a fact, that it was properly sized and not oversized, when you show up at "design" conditions, is the boiler doing a 100% duty cycle?



My personal experience, I have yet to see any system doing more than a 50% duty cycle, meaning the system is technically 2 times bigger than it really needs to be.



Just sayin'....



Anyone else?



ME

Slimpickins

LOL @ forced error

I love it! I will borrow that one if I may.

NRT_Rob

?

what does this have to do with the poster's question?



boilers are only available in certain sizes. hitting one 100% would be pretty rare even if your calculations were lean and mean.

Mark Eatherton

Borrow away Slim...

Rob, it goes to sizing. I know that you, as a diligent designer would never intentionally oversize any components, but when I show up in the field, and see one of your systems doing a 50% duty cycle, it tells me, that even though you are practicing due diligence, that there are many things that have a LOT of influence on real time heat loss that we don't/can't take into consideration that ends up making the physical plant 2 times larger than it needs to be.



I also know that you are not crazy enough to cut your physical plant in half. No one (other than maybe myself, on my own homes) is crazy enough to do that...



I guess, in a round about way, we agree that sizing a large thermal storage tank based on potential design conditions can be quite expensive, especially based on the potential of non use. If it were me, I would take the approach of a 50% sizing factor with the possibility of increasing storage if necessary. Build the underground vault large enough to double storage if need be, but install 1/2 of what is theoretically necessary, with all parties concerned being aware of the sizing option.



Will be interesting to see how many people have actually seen their physical plant doing 100 MPH at design conditions ;-)



ME

NRT_Rob

well

again, to be fair, boilers are always oversized. it's pretty hard to size a boiler even within 30% in most cases, especially since there are only about 3 boilers in existence sized for modern home heat loads (below 60kBTUs). If I have a 50k load, there is only one boiler that could hit that at near perfection. The vast majority of boilers are going to be 50% oversize on the top end right off the batt, at least, even if the load calculation were perfect.



While we agree that even designers as aggressive as I am are still oversized because the heat load models we are using are simplistic (and I've NEVER come up short, so I must be oversizing still, since I'm not yet perfect...), it's nothing like 50% on the calculation. Don't confuse boiler sizing resolution (especially given "product line lock in") with oversized load calculations.



If you had a 50% duty cycle on most systems, blind guess I would say maybe 30% of that is probably boiler sizing resolution issue and maybe 20% load overestimation related to solar gain and heat gain from other sources not taken into account. and it may be more like 40% or 10%.



but that assumes good calcs. if I had to throw a dart at most systems out there that even had load calculations done, knowing what I know of most supply house tactics in my area, I would say nearly all of them are in fact at least twice as big as they need to be ON THE CALC, plus the resolution problem.



but if someone is doing this kind of mass storage calculation and running real numbers, I definitely would not cut the calc in half. I'd just run the numbers and if you were near some tipping point for some reason, fudge down instead of up.

MechTech_2

Sounds like

alot of unknowns, maybe the juice isn't worth the squeeze. Perhaps I try something like this on my own house first, before I forfeit all my money on customers lofty ideas. Thanks for the debate.

zacmobile

peak period buffer sizing

here is a page out of viessmann's geothermal design guide, they give a formula for buffer tank (calorifier) sizing to utilize off peak times. that dot in the formula is a multiplication symbol (those wacky germans, you gotta love em!)



so as an example by their formula (which is kind of rule-of-thumby) if your buildings heatloss was say 15KW you would need a buffer tank around 1000 litres (260 gal)



15KW X 70 litres per KW = 1000 litres