Solar
Loads on a greenhouse project. Solar
Loads on a greenhouse project.
Related
Items
Air
Speed
Plant
Transpiration
As stated in the
preamble on the Fequently Asked Question sheet there are many items
that will effect the total solar loading on your greenhouse which
result in the selection and sizing of the cooling/ventilation
systems.
The primary tools/devises/methods used to offset the solar loads are
primarily:
The items that effect the solar loading on your structure are:
In the Previous vent write-ups we have been asked how we arrived at the 15,000,000 BTU/hour total solar load.
The project information from the example is as follows:"Say a grower is planning on building a 10 bay gutter connect project that is located near Fort Wayne, Indiana. The project is comprised of 30' wide gutter houses each 240 feet long for a total area of 72000 sq. ft . and wishes to use only roofing vents for cooling.
Environmental Data for the site is 92 deg. F DB/73 deg. F wet bulb summer outdoor design-4 deg. F winter outdoor design, the project is located at 41 deg. F north latitude at a elevation of 793 feet above sea level."
At our office we use a computer model that allows hourly calculations but here by hand we will demonstrate the method.
Again using ASHRAE standards, this is the rational.
At 40 deg. N latitude solar load imposed on the greenhouse structure will be predominately roof load. At 40 deg. north latitude the greatest solar loading will occur on June 21 at 12:00 PM. Based on Simpson's rule with time interval equal to 10 minutes. ( ASHRAE - fundamentals/fenestration ) the load is 267 BTU's per sq. ft. (826 watts/M2). This is used for our max. load calculations.
Note: We assumed no shading factor ( SC ) other than the cladding product which is 0.91 and based on a clear day.
Chart One
Area (sq. Ft. ) SHGF SC/trans factor Load-Btus/hr East Wall
2880 41 0.91 107,452 West Wall
2880 41 0.91 107,452 North Wall
4500 38 0.91 155,610 South Wall
4500 95 0.91 389,025 Roof
72000 267 0.91 17,493,840 Slope factor on roof
28800 (38+95)/2 .091 1,723,680 Total Solar Heat gain
18,253,377
Note: The above chart is assuming it to be a perfectly clear day, free of clouds and haze, and in the real world this seldom happens so we like to apply a haze/cloud factor, as illustrated below.
June 21: 12:00 PM, 40 deg. N latitude
Chart 2
Corrected
Area (sq. Ft. ) SHGF SC/trans factor Load-Btus/hr East Wall
2880 41 0.77 90,921 West Wall
2880 41 0.77 90,921 North Wall
4500 38 0.77 131,670 South Wall
4500 95 0.77 32,9175 Roof
72000 267 0.77 14,802,480 Adjusted Solar Heat Gain
15,445,167 Note: The above chart would closer reflect an average typical day.
Now to throw a real kicker.
Let's move this
entire greenhouse to Logan Utah.
According to weather data ( again as published in ASHRAE - fundamentals ) Logan has a 1% summer design of 93 deg. F DB design which is not that different than Fort Wayne at 92 deg. F DB. Both projects are located at about 41 deg. N latitude.So would it be safe to say the loads will be the same !!!!!!!!!
If you did you would be incorrect.
The greenhouse in Logan Utah would be faced with a higher solar load just by virtue of the increased elevation from sea level. ....... Logan is 4,775 feet about sea level. Whereas, Fort Wayne is 791 feet. ...... Solar loading increases 0.07% every 1000 feet in elevation. So the solar loading at Logan would be that much greater.
Rule of Thumb
Typically, ( again by
rule of thumb ), plant transpiration ( we're assuming the house is
full of crop ), will look after 1/2 of the solar gain. So the
ventilation and cooling systems will need to look after 7,700,000
BTU's of heat. For those that wish this expressed in refrigeration
terms this is a 641 Tons mechanical cooling load.
Just to back track a bit. Lets assume the grower wished to use fan
cooling. What would the total airflow rate be if they wished to
maintain a spacial temperature of no more than 10 deg. F from
ambient.
Using Q = H / 60 X C * D ( ti-to)
where H = 7,700,000 BTUs/hr
C= specific heat of air = 0.245 BTU/Lb X F
D = density of air 0.075 lbs/ft 3
Ti - To = 10 def. Fwhich yields an air flow rate of 698,412 CFM of air flow.
Check using .... the Rule of thumb air flow rate of 1 AC/min based on 8 foot height.
= 72,000 sq. ft. X 8 feet = 576,000 CFM
By now you should
realize that the "old rule of thumb can leave you short" in
cases. 698,000 CFM verses 576,000 CFM. ( The old rule of thumb may
work well in a greenhouse with soil or gravel floors - where an
additional evaporative cooling base is created. However, in modern
houses with covered floors and drip irrigation, this evaporative
cooling base is not available. ). Basically ... if you short yourself
on cooling and ventilation, the crop will suffer stress simply by the
increased in transpiration rates. " They ... the plants are spending
all their energy just to keep cool , not growing or producing fruit
etc. "
Daily Load Profile Curve
For those interested the chart below is a sample of a the cooling
load profile curve for the roof only on the above project. The values
calculated is solar loading in BTU's/hr. Only the months of Jan to
Aug have been listed.
LATITUDE
What would happen if
we moved this project further north say up to 48 deg. north
latitude.
June 21: 12:00 PM, 48 deg. N latitude
Chart 4
Corrected
Area (sq. Ft. ) SHGF SC/trans factor Load-Btus/hr East Wall
2880 40 0.77 88,704 West Wall
2880 40 0.77 88,704 North Wall
4500 37 0.77 128,205 South Wall
4500 134 0.77 464,310 Roof
72000 252 0.77 13,970,880 Adjusted Solar Heat Gain
14,740,803 Note: The above chart would closer reflect a typical day. Compare this with Chart 2.
Even though we have moved this structure further north, the solar loading really didn't fall that much. The roof loading sure dropped, but take a look at how the south wall gain increased. Interesting.
Air
Speed
Air speed influences many factors in plant growth. These include
transpiration and evaporation, leaf temperature and carbon dioxide.
In general air speeds of 20 to 50 feet per minute across the leaf
surfaces facilitate the uptake of available carbon dioxide, promotes
transpiration/evaporation and reduces leaf temperature. At high air
speeds 100-200 feet per minute plant growth is inhibited.
Here's a way to check the air speed.
Say a greenhouse structure is comprised of 21' wide X 10' feet high
under the gutter gutter houses. Each house is ventilated with an
exhaust fan rated at 19,000 CFM air flow. The exhaust fans are
located at mid point of the height under the gutter.
The air flow rate
would be 19,000 / (21 X 10) = 90 feet per minute, assuming a bench
type crop.
If it was a tall vine like crop such as tomatoes or cucumbers, the
face area could be as high as 180 feet per minute.
In the case of the tomato house, it would be unwise to add more air
flow for cooling, if cooling was problem since all that would occur
is greater mechanical induces stress on the crop. here the grower may
wise to consider evaporative cooling of high pressure fog cooling to
reduce the heat gain of the structure or by adding some form of
shading which would also limit the solar gains.
Plant Transpiration
In the project sample above, assume the grower is wishing to
raise tomatoes. The plant density that they are targeting for a one
plant per four sq. feet. Therefore, in the 72,000 sq. feet of
greenhouse structure, they should try to acheive 18,000 plant spaces.
Generally it is accepted to provide up to 1.0 USGal (4 litres) per
plant per day.
Now lets assumed the old 50% rule of thumb ( ie 50% of the daily
solar load is offset by plant transpiration. If you look at chart 4
for the month of June, the total ( average/projected ) dailey solar
load is 124,462,800 BTU's. 50% of this load is assumed to be
controlled by plant transpiration. So 50% of 124,462,800 is
62,231,400 BTU's.
To offset this load would require the evaporation of 59,268 lbs of
water ( 7,115 USGals or 26,933 litres ) or 0.39 USgal/plant ( 1.4
lires/plant ). So almost 1/2 of the water uptake by the plants on
this day is just just for cooling. The question to ask is ...... gees
not much growing or production is left avaiable for the plants .....
????. USing the 50% rule.
AGROPONIC INDUSTRIES LTD.
908 RANCHVIEW CRES. NW., CALGARY, ALBERTA, CANADA, T3G1P9
ph (403)241-8234, fax (403)241-8234
Email: agropon@agroponic.com, ULR: agroponic.com
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