Greenhouse chiller system
This is a bit of a lengthy post because I want to try to anticipate questions and provide answers. I am not an HVAC professional, but I have read about refrigeration with an intent of designing my own systems. I am handy, having built a house and am building a greenhouse/pool structure now. I have a strong technical background in science and have worked a number of years doing laboratory research.
My current interest is to build a chiller system for my greenhouse (approximately 30x90 or around 2,700 sqft) where the waste heat goes into the pool (around 35K gallons). The pool (which has a conventional roof with an R-value of around 30 and walls with an R-value of around 20) and greenhouse occupy the same air space and my intention is to try and do some daily ‘load leveling’ where night-time heat is provided by radiation from the pool. The greenhouse covering is a 6 layer polycarbonate with a stated R-value of 3.7 and a transparency of 58%. According to my research the summer sun peaks at around 277 BTU/sqft/hr and with 58% transmission, that means 160 BTU/sqft/hr enters the greenhouse. I understand that essentially all of that is converted to heat, so the peak load would be on the order of 430K BTU/hr or around 36 tons. If it factors in, the zip code where the construction is going on is 22847 in the Shenandoah Valley of Virginia. The house is located in a bit of a frost pocket and it is fairly common for night-time temps to drop into the mid to lower 60’s even in July and August so there exists the potential for nighttime heating even during the summer. My plan is to experiment with aquaponics in the greenhouse and grow tilapia which also need water temps in the 80s, so there is another at least 6K gallons of water that can act as a heat reservoir if needed.
My goal is to keep the greenhouse temperature at 90F or lower (though I think not lower than 80F) and the pool at 80F or a bit higher (the pool is partially in-ground, though the primary heat loss would be through the surface, which is approximately 14x75 or around 1K sqft). I am also interested in piggy backing off this system and cooling my house (around 2,700 sqft, very well insulated) and another space (about 700 sqft, not as well insulated, but not intended for continuous use). I believe I need around 5 tons to cool the house and 1 ton for the other space during peak. I was considering also the idea of using a few tons of ice to act as a reservoir to further ameliorate peaks in cooling.
I only have access to single phase, 240 volt electricity, so it seems an electric powered compressor is not economically feasible. However, I have access to plenty (700+ gal) of propane. I have done some research on absorption systems as well as powering a compressor via a propane powered engine. It seems to me that the absorption system would be the most efficient, but I am not sure I grok absorption and what I have read indicates that the cost/benefit line is well north of 40 tons, so a propane-powered engine for a belt-driven compressor might be the best way to go.
What I am hoping to get is some advice on what sort of system to build, prices and where to get the parts. I am fine with assembling a system from components (I have a high vacuum pump (10-4 torr) and am comfortable with charging systems) and am working on a relative shoestring budget (there is no way I can go over $10K and $5-7K is probably the realistic max). I know this is challenging, but I also know that there is a plethora of used equipment available and I am well experienced in trading money for my time.
Thank you all for your time!
welcome to the site! you are correct, this place has a wealth of knowledgable people that could build a company that would put all manufacturers out of business!
you will probably spend $5-7k on mistakes and to help you design such a system over the internet would be quite difficult. it would probably cost me over $10k to build one myself (used) and i have over 20 years in design, engineering and repair.
Originally Posted by mitakeet
it sounds as if you are handy and have a handle on the beginnings of what you need to know.
i would recommend that you contact (become very good friends with) someone locally.
Don't fear the dream....or the reaper.
Originally Posted by jayguy
I concur Dr. Guy!
“If your work speaks for itself, don't interrupt.” ~Henry J. Kaiser
Thanks for your feedback! After posting this (nothing like having to write things down to induce focus, eh?) I started to think along the lines of instead of building a system that is capable of handling the peak load (very expensive indeed), that I instead build a system capable of handling the average load instead and use frozen water as the load leveling device. I calculated I would need a 24 hour average of 13 tons on the worst-case day (presuming the above numbers are meaningful) and if I built an insulated trough that contained 40 or so tons of water (a rather small volume I was surprised to learn, only about 1,300 cuft) and then froze it overnight I should be able to manage my worst peaks easily. Further, by going with several smaller compressors (I was thinking 3 5-ton, something I found on-line for around $1K a pop (single phase and 240 volt)) I could produce a system even more amenable to optimization and vastly more likely to fit within my budget AND be something I could build at my level of experience.
What do you think of this approach? Better, worse, just as dumb?
With 240 single phase being your voltage, and the water availibility, consider buying a used ice machine. Make ice and store it, could possibly do that within your budget if the electricity costs are in line. Use the ice into the pool as needed. As for the house, experiments don't heat when it's cold outside. Go with a tried and true system.
There is no way your budget will meet your needs even with free labor.
I see nothing about piping and a pump to feed all these buildings, then you have air handlers, the chiller itself, and now you are talking about ice banks, for under $10k?
Not to be rude but you are dreaming.
@CCSPIERCE, we heat with electric baseboard and since we live in a frost pocket, simply opening the windows at night and closing them in the morning keeps us cool almost all the time. The problem comes on those few hot nights and when we have parties and currently we are using portable window AC units for supplemental cooling, so I don't think it is much of a risk (other than lots of my time and possibly some money).
@zw17, I have often been accused of dreaming but one thing I have learned for certain from my efforts is that such 'impossible' things are often quite achievable if you are too stubborn (or stupid) to realize/accept 'reality'.
It would be a very fun project for sure. I hope you can pull it off.
Originally Posted by mitakeet
I will try to remember to post back with my experiences, then.