Tuesday, February 21, 2006

High tunnel heat math ... is it worth it?

The cold over the last weekend got me calculating how much heat I would need for my vinifera to survive a cold snap. Although I'm sure the equation has several more terms than I care to account for, I think I can use a basic equation for a rough estimate.

(Surface AREA in SQFT )*(Temperature difference in F)*1.2 =BTUs of heat needed

The hoop house I intend to use will have an area of about (96*16*PI)+(PI*16*16)=6,534.5 SQFT

Let's try an example:
From the last weekend ... I spent 19 consecutive hours below 3 F The average Temperature over this period was about -8 F or an average temperature difference during this time of about 11 F.

So ..... (6,534.5*11*1.2)=86,255 BTU/hour needed to remain at 3F or higher.
I have two 60,000 BTU forced air kerosene heaters. They should be able to supply a total of 120,000 BTUs per hour. They can last about 12 hours of continuous heating.

So based on this ... I should be able to last a little less than 17 hours. These calculations do not take in account any thermal inertial due to heat retained in the ground or any additional insulation ... which I intend to use. I may have to get a couple of backup heaters ... or refuel more often.

Next, I have to build my computerized controllers .... luckily I have quite a bit of experience with this part. The entire computer controlled heating system should cost about $400 to build(including heaters). Fuel should cost about $4/hr. Even in a cold year, cheap insurance to keep the vines alive. This year it would have cost me about $100 bucks in fuel costs (worse case) to allow my 75 vinifera vines to survive the winter.

Extending the costs over 10 years .... assume on average 100 hours of heat per year over 10 years:
high tunnel=$4000 --- one time cost
heat=$4000 ---- ~$400 per year
Other misc =$500 --- sensors, computer controller ... tarps etc.
Assume 20 lbs/vine/year

So this system will add $0.56 per pound of grapes. Even if you go best/worst case ... I think you can go plus or minus a .25 per pound i.e. $.31 - $.81/lb . .... note vinifera shipped in from the west coast will cost any where from $0.65 to as much as $1.50 per pound (in small quantities) when shipping is included, so then this begins to look competitive. And hey.... they were grown in Wisconsin! And this does not take into account the reduced risks due to Frosts, Winter kill, reduced pests, and the costs associated with them. This is all theory so far ... we will see how reality plays out ... it may be far more harsh than I anticipate, but many of the numbers and costs I used I hope to beat.

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