I know some of you are using passive solar heating in your greenhouses so I'm wondering if you can tell me how to go about figuring how much water storage I'll need in mine to keep the inside above freezing.

I just finished building a 12ft x 20ft pvc hoophouse. The top of the bows are about seven feet or thereabouts. It's covered in a single layer of 4mil builders poly. I have black landscape fabric over the entire floor. The ends are also covered with the same plastic and I have a door in each.

I've already put five 55gallon plastic drums of water inside, two are black and three are call it a medium gray. All are sealed. The black ones are in two corners, the gray ones I'll put a plant bench on top of for seed starting. None are directly touching the walls. The house gets direct sun for about all morning and the early afternoon then varying degrees of filtered sun from about one to two o'clock onwards. Night time interior humidity will be fairly high.

Typical low temperatures would be about twenty five degrees, occasionally down to twenty two, rarely down to twenty. I've got frost tender citrus such as lemons and limes inside so I want to keep the interior above freezing. To the extent possible I'd like to do this via passive solar. When that is not enough I have two 1,000 watt space heaters and a small fan already set up.

I know that putting an insulated cover such as one of the clear pool covers would help a lot, but for the moment I want to explore how far I can go with just the single sheet of poly. How much more water storage do I need to add? Or is it that I can't put enough out there to get the job done?

.....Alan.

I have plans that uses passive geothermal as heat source and sink. The geothermal is simply ordinary earth, not the hot lava rock beds that is usually associated with the term. The passive geothermal will provide excellent cooling in the summer and heating during winter. It is a combined approach where you also do a rainwater recapture system which can also serve as a heat buffer during the days of intense sunlight.

The major investments would the digging to bury the pipes to serve as heat exchangers, and the building of ferrocement tanks that line the walls of the greenhouse to serve as heat buffer and cistern for rainfall recapture. The thermal capacity of a moist soil is several orders of magnitude greater than that of air.

And contrary to the most common use of greenhouses, if I were able to build a unit one of these days, I intend to plant in the soil of the greenhouse. Most greenhouses of today are highly sanitized where the ground is paved and you plant in pots up on benches. That is very excessive waste of resources, if I had my own way.

Alan, in your greenhouse it will be difficult to add enough water to maintain a temperature in the upper 30F when the outside temperature falls to the low 20s. You have five 55-gallon drums = 275 gallons water = 2,200 pounds of water. It takes 1 BTU to raise 1 pound of water 1 degree F., or when 1 pound of water losses 1 degree F. it gives off 1 BTU. Therefore, for every 1 degree decrease in temperature of the water in your drums (2,200-lbs.) during the night, your greenhouse will receive 2,200 BTU's heat. Therefore if on a cold night the water temperature in your drums falls 4 degrees your greenhouse will receive 8,800 BTU's. Being covered with a single sheet of polyethylene, and having doors on both ends, 8,800 BTU's will only be a drop in the bucket of what your greenhouse will require to maintain a 15 to 18 degree elevation, when your area receives cold weather. Here is the formula to determine the amount of BTU's your greenhouse will require.

Use this formula to determine the approximate heating requirements of your greenhouse: A X D X 1.1 = Btu's. "A" is the total wall and roof surface area of your greenhouse. "D" is the difference between coldest outdoor winter temperature and the night temperature desired in your greenhouse. "Btu's" is the heat requirement. Subtract 30% if greenhouse is insulated using double glazing or polyethylene liner. Subtract another 30% if it is a lean-to greenhouse on heated wall.

Congratulations on you greenhouse. - Millet

According to Millet's formula, my lean-to green room requires 22,500 btu's per night to maintain 75 degrees inside when it's 25 degrees outside at night. Now durning the day, if the sun is shining, it gets to 85 degrees. I use a small electric heater the sits on the floor that is set at medium. It runs about one minute per cycle and off 4-5 minutes. This use to be an outside front porch that I inclosed. Walls and ceiling are insulated. The large windows are insulated sliding glass door replacement glass. Which were 75 bucks apiece. If I had used a custom sized glass, the cost would have been three times that. This was the porch before I inclosed it.
after

This may sound silly, but works for me. Put an aquarium heater in one or more of the buckets. They turn on automatically when the temp drops below a certain temp....

This works well for me, but my enclosures are smaller and covered with blankets (at night)

Frank

what it condenses down to is what is your annual cost of heating and cooling plus interest on investments and labor.

Over here, the current commercial greenhouses (30' wide x 48' long by 14' high) that are maintained for continuous rice breeding costs $1,800 per month on propane fuel during the winter and $1,200/month for cooling costs during the summer. It is possible to bring the fuel and electric costs to a hundred dollars or less per month using passive geothermal methods.

The best approximation for elliptical areas, whose radii or diameters are not equal length, would be

Area = pi x a x b
where a=major radius, b=minor radius

Similarly, the volume of an ellipsoid would simply be

V = 4/3 x pi x a x b x c
where a,b,c are corresponding radii from the three major axes.

Take note, that if the radii are equal, voila, you get the formulas for circles or spheres. So if you remember this series of formula, you are generally cool for approximating curved surface areas or curved object volumes, and they don't need to be perfect spheres or circles.

The above formulas were first written by my math idol, Archimedes. A few years ago, historians have uncovered many documents from the prayer pamphlets whose original pages originally belonged to Archimedes but were written over by the monks, and when they reconstructed those old notes behind the prayer writings, using X-rays and other lithography techniques, they found out that Archimedes has laid out the foundation for calculus, 100 years ahead of Newton and Leibnitz.

We would have cell phones or been to the moon 100 years earlier, and sad to say, will also have nuclear bombs 100 years earlier.

The use of water to heat up a greenhouse is not technically correct. Rather, bodies of water inside a greenhouse are used as a temperature buffer to minimize extreme diurnal temperature fluctuations. Water has high thermal capacity, and is therefore a good heat source and heat sink for the greenhouse which can help alleviate the difference between highest and lowest air temperature.

While it is so easy to estimate the expected heat loss of a greenhouse and thus control its design and size up your heaters and air conditioners, given your dimensions, it is next to impossible to design the amount of water you would need to buffer your temperature inside a greenhouse. Your water bodies, in various barrels, shapes, sizes, kind of materials that hold them, and how they are arranged inside the greenhouse, and the interactions of plant shading, and other equipment's contribution to heating or cooling, would make it a horrendous workload computer modeling the physics and other engineering calculations for sizing the water bodies, and it won't be accurate for so little gain in the process. One cannot estimate ahead of time the amount of heat that the water bodies will exchange with the greenhouse. But rejoice that computing the total contributions of your water bodies after the fact is very straightforward and quite easy as Millet have shown you. You simply have to measure average water temperature of all the water bodies, calculate the difference between the average highest and lowest, multiply by heat capacity and instantly know how much heat exchanged happened. And if you are able to track that through time, you will know historically how much those water bodies in their particular arrangements have helped in the heat exchanges. While it is easy to measure the temperature, it is quite very hard to predict the temperature of the greenhouse as influenced by these water bodies.

So my recommendation would be as much water bodies as you can fit and still have a very usable greenhouse.

As to the amount of heat that you need to supply, it would be straight forward calculations from three major component losses (not heat exchanges within the greenhouse): sensible heat transfer, air mass transport (aka, air leakages), and net radiation losses. Usually, the empirical formulas are given by the greenhouse manufacturers to size up your heaters or coolers given your target minimum temperature. I would know that greenhouse manufacturers and designers have excellent agricultural engineers if the three major heat losses are considered and are shown in their brochures. Usually, only two major losses are considered when calculating the heating needs of greenhouses. And these are the sensible heat transfer and mass transport.

If you can't find the empirical formulas, I can estimate the heat losses for you based on the sensible heat and mass transport processes, and if you have net radiation data including the evening, I can make my calculations more reliable. The more accurate and correct detail you provide, the better the calculations.

Not knowing the exact other details, I'll have to assume the values. I will be only calculating during the night time when there is net radiation loss, and outside temperature is lower.
Sensible heat transfer: (am not including transfer between ground and the greenhouse as they are in the same category as buffering effect of water structure and am not including, heat losses from exposed frames)
Qc = Surface Area x Heat Transfer Coefficient x Temperature Difference
Surface Area of hoop house that is 12 feet wide, 20 ft long and 7 ft high = pi x (6 + 7)/2 x 20 + pi x 6 x 7 = 540.4 sq ft.
Heat Transfer coefficient of single layer 4 mils builder PE film = 1.15 BTU/sq ft/F deg/hr
Desired Temperature difference between greenhouse and worst time outside temperature = 15 F deg
(take note that Farenheight degree is entirely different than Degree Farentheit, the former iimplies temperature difference, the latter indicates temperature reading, so the writing of F deg is intentional)

Sensible heat loss during the worst time: 540.4 sq ft x 1.15 BTU/ sq ft/F deg/hr x 15 F deg = 9,322 BTU/hr.

Estimated heat loss from air mass transport is proportional to the volume of the greenhouse:
Qa = 0.22 x Air exchange coefficient x volume of greenhouse x temperature difference.
Volume = 1/2 x pi x 6 x 7 x 20 = 1319.5 cu ft.
Air exchange coefficient of well sealed newly constructed greenhouse from single layer PE = 0.70
Qa = 0.22 x 0.70 x 1319.5 x 15 = 3,049 Btu/hr.

Add all of them together, your total heat loss: 9322+3049= 12,371 BTU/hr.

And as to the water bodies, assuming they are covered and don't have a lot of evaporation, your heat exchanges with them is black body radiation, which they absorb from the sun during the day and release at night, and sensible heat transfer with air and whereever part of the greenhouse they are in contact with. Much more complex calculations, so no estimates published there.

But then, you can measure their effects in the buffering of temperature. You will notice that your low temperature in the greenhouse would be higher when these water bodies are present than without them, and if you have thermostatic controls of your heater, it will become as savings without those btohersome calculations, and you can only infer, after the fact, after the measurement.

Here is an interesting web site on greenhouse geothermal heating. It is of particular interest to me as it talks about geothermal heating of greenhouses in different areas. One area is in my area, Denver, Colorado, a cold winter area, with high greenhouse heating costs. - Millet

http://geoheat.oit.edu/bulletin/bull26-1/art2.pdf

Thanks JoeReal ... I ment , how much kw do I need to obtain that amount of Btu needed for my greenhouse