I'd like to make a commercial solar greenhouse, for the production of
annual and perennial bedding plants.
The problem is that the heat storage must be separated from the plant
growing space to get uniform enough temperatures.
Growers find that for many species a delta T between day and night
temps greater than about 10F will cause the internode (distance between
pairs of leaves) to increase, making the plant leggy.
Most plants need warm temperatures to germinate then cooler temps to
continue growing. Some plants prefer different temps to grow.
During daylight it is necessary to ventilate. This can be reduced if
you supply auxilary CO2, but cannot be eliminated as other gases (worst
one is ethylene, second is ammonia) will rise to toxic (for the plants)
levels. Both are bad news in tens of parts per million.
The green house does not have be warm all year. It is sufficient if
part (1/4?) can be warm enough for seeding by 1 Feb, and the rest by 1
March,
Ideally the plumbing doesn't have to be drained for the winter -- the
house will stay above freezing even in December.
Consider a half hoop house, about 18' wide and 100' long, built against
an insulated wall.
Would this work:
I=insulation. Insulated wall on N side of space.
p = 1/2" poly zigzaging up the wall, flow is overall from bottom to
top, so that top of greenhouse space acts as preheater. I figure with
a 16' ceiling I will get some stratification no matter what I do.)
g = glazing, glass for the upper part, poly film for the arches of the
green house itself.
b=barrels of water, (Oops -- if the back wall is 20' tall, this means
10 psi on the barrels, just from head -- I don't think a barrel will
take that. Ok, we run copper pipe through the barrels, or a LOT more
plastic pipe. Sigh. I wanted to make this cheap.)
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IIIIp g
IIIIp g
IIIIp g
IIIIp g <--collector
IIIIp g
IIIIp g
IIIIp g
IIIIp g
IIIIp g ggggggg
IIIIp ggggggg
IIIIp ggggggg <- greenhouse
IIIIp gggggg
IIIIp gggggg
IIIIp ggggg
IIIIp ggggg
IIII gggg
IIII gggg
IIII ggg
IIII ggg
IIII gg
IIII gg
IIII g
IIII g
IIIIIIIIII IIIIIIIIIII IIIIIIIIIII IIIIIII
IIIIbbbbbI IbbbbbbbbbI IbbbbbbbbbI IbbbbbI
IIIIbbbbbI IbbbbbbbbbI IbbbbbbbbbI IbbbbbI
IIIIbbbbbI IbbbbbbbbbI IbbbbbbbbbI IbbbbbI
IIIIbbbbbI IbbbbbbbbbI IbbbbbbbbbI IbbbbbI
There is no air exchange between the greenhouse and the collector
space.
In winter heating mode:
During the day hot air stratifies near the top of the greenhouse.
Ventilation is done 4' below the peak with the usual vent tubes, but
with the air directed downward.
Water runs through a couple miles of 1/2" pipe fastened to the wall.
(One run
every 3/4" vertical inch. If the collecting space is 10 feet high we
need 90 x length of the greenhouse. It is manifolded to lengths of 6
times the length of the greenhouse. This allows a gentle bend at each
end. 1/2" pipe is chosen because it is cheap, and has much thinner
walls than 1" pipe.
Resulting hot water is circulated through the barrels, which also
provide the bench supports.
Suppose that during the day this system can be used to bring the barrel
temperature up to about 125 F. Insulate the barrels to R10 (2" poly
styrene foam.)
(If the green house target temp is 65 F this means we have conductive
transfer of 60 degrees/ R10 -- about 6 btu/hr per square foot of
covering. The covering for the benches is somewhat larger than the
greenhouse covering. But double film poly will be about R2, so the
net effect is that the temperature drop across the poly will be about
1/3 of the drop across the barrel insulation, or about 20F. So
passively it will keep the greenhouse fine until outside temps are down
to about 45 -- means that until the temp gets down to that level we
have to keep the vents open.)
Now when the outside temperature drops, we run small fans that expel
air from the barrel space into the green house space, with separate
thermostats for each fan or group of fans. By using many small fans, we
can put partition sheets up, and keep it cool for pansies, and warm for
tomatoes.
On 1 Feb our typical night time temperature is about -18C or about 0F
In cross section a 2' length of greenhouse will have roughly 2 x 30 `
square feet of poly film. If we allow a 5F temp drop at night, we have
a 60 degree differential, which will require 60 F * 60 sqft /2 = 1800
BTU per hour. Our 2' section of green house has 6 barrels in it, with
6 x 450 lbs of water = 1800 lbs of water. So the water temp would need
to drop 1 degree per hour to keep the greenhouse warm. Depending on
how hard we want to push air in and out, this allows either some
outside ventilation all night (a good thing) and/or several days heat
storage for cloudy days, and/or cushion for exceptionally cold nights.
Now lets look at the collector: Feb 1 has a day length of about 10
hours. Let us assume 5 hours of usable sunlight at 180 BTU/ft^2/hr
That works out to 900 BTU/square foot/ day. (Valid at 54 degrees
latitude?) If our collection zone is 10' high then in our 2' chunk we
have 20 square feet of collection = 18000 btu/day 18000 btu will warm
up 1800 lbs of water by 10F
We have a problem: The greenhouse will cool for 19 hours (24 -5)
which will cool the barrels 20 degrees, but will warm up only 10
degrees. We need more collector.
Answer: In Feb we are mostly seeding flats the green house doesn't
all have to be at temperature. If we only heat 1/3 of the greenhouse
we have enough storage. Plus, while seeds germinate, they don't need
much ventilation.
Question: Will the greenhouse stay above freezing in the winter.
Worst case: Temperature of -30 on december 21.
Basically we've assumed both inside and outside temps have dropped by
the same amount. We still get about 1800 BTU/hr.
But now we only have 8 hours of daylight, of which we can use 4, and
the heat content is lower -- what, 160BTU/sqft/hour. So we only get
500 BTU/sqft/day. We're only going to collect 10,000 BTU (20 sqft x
500) but we're spending 36000 (20 hours at 1800) We're losing ground.
Turn it around the other way, we can support about a 15F temperature
differential in December. What to do:
1. Could fill the barrels with brine instead of water, and use glycol
for the circulating solution. Brine is corrosive, glycol is expensive
and poisonous. I don't like this solution.
2. Could run well water through the system. Start it through the
barrels, then the pipe rack and let it exit the system at 0.5 F. Well
water here is about 50 F. If we started this when the barrel temp
reach 50 F, we would get nearly perfect efficiency. To provice the
missing 26000 btu would take 26000/18 about 1500 lbs of water per 2'
section. -- about 200 gallons, or about 10 gallons per hour per 2'
section. A 100' greenhouse would require 500 gal/hour, which is about
2 gallons per minute more than my well will provide.
May not be that bad however. If we drain the pipe rack the barrels will
take days and days to freeze. So the greenhouse itself gets cold, but
the plumbing does not. (Does mean we have to be careful in how we lay
out the plumbing.) And remember this is worst case. Usually we get
our cold weather at times of more sun.
In summer cooling mode -- Vent strips opened at top and bottom of
collector box, so that there is a solar chimney running the length of
the greenhouse. A similar strip is opened at the base of the green
house. (Details on how to do this in a way that is reasonably easy to
do -- what zize?
Thoughts?
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