Mad Scientist Works For Greenhouse Heating Independence Down To -25F

greenhouse covered in snowMy friends have called me many things, some complimentary and some not so much but perhaps my finest moment came when a dear friend referred to me as me a “Mad Scientist.” Having spent 20 years working as a communication engineer it seemed only natural that when I retired I would finally go fishin’ or travel, but no… I built a lab where I could do solar energy research related to growing food in a self-contained environment. I guess once an engineer, always an engineer!

Soaring Raven Laboratory

SRL, Soaring Raven Laboratory, is located in south central Colorado near the foot of the Spanish Peaks at an elevation of 7,000 feet above sea level. The high country climate has dry warm summers with daily temperatures in the upper 80’s to low 90’s followed by long snowy winters. While it is not uncommon for winter temperatures to drop well below zero, Colorado is blessed with 265 or more sunny days a year making it an ideal location for testing solar energy applications and conducting related research.

The goal of SRL is to act as a platform for small scale solar energy research and education by focusing on low cost effective and sustainable grassroots solutions for residential energy needs.

The Greenhouse Lab

The lab was built around a self-contained 12’ x 24’ greenhouse that has the ability to obtain all of it energy requirements from the sun by the integration of both passive and active solar energy systems. It was constructed in the late fall of 2013 and became operational in December of that same year on the winter solstice.

In designing and locating the greenhouse there were a number of issues that needed to be overcome. The first was the placement of the greenhouse on a city lot already occupied by a home, detached studio/office and 10’ x 20’ shed. To complicate matters, the entire south end of the existing residence was covered by solar collectors so shading from the SRL greenhouse was of a major concern. However, after careful planning and some compromise the site was selected and a greenhouse sprouted from the earth!

The electrical power to operate the labs ventilation, circulating pumps and lighting systems comes from an array of solar panels. During sunny days excess power, not needed in the lab, is stored in a battery bank to operate an active hot air solar furnace for the house and provide limited amount of power for afterhours lighting.

The water required for the greenhouse is a combination of municipal provided and reclaimed gray water from the home. Currently, research and development is underway to move away from the city resource and use water collected from roof runoff and gray water.

One of the principal objectives of the SRL is to reach a level of complete heating independence down to -25F below zero. While inside temperatures would hardly be balmy in the greenhouse, plants would still be safe from freezing. Work still needs to be done in this area as during the winter of 2013, before the greenhouse was in full operation, the internal air temperature dropped to 34F when the outside temperature plummeted to -16F.

What’s Cookin’ in the lab?

At the lab we are currently testing a simple low cost hot water collector to boost the amount of passive solar energy that’s being stored in the 300 gallon theromass water tanks. Instead of using rigid copper pipe in the collector we are incorporating cross-linked polyethylene tubing, commonly called PEX or XLPE which is used frequently in domestic home water systems.greenhouse water battery bank

The advantage of using PEX is greatly reduced cost, as much as 75%, over the use of copper. Working with PEX is much less labor intensive than working with copper and should the system ever freeze, copper will rupture whereas PEX is much less vulnerable. On the downside, PEX does not as effectively transfer heat from the sun into the water that is circulating through the system as does copper. When exposed to sunlight PEX piping can degrade. Exposure to ultraviolet (UV) light in sunlight causes the internal molecular structure of PEX to break down. To counteract this effect the UV is being filtered out in our collectors before coming into contact with the tubing.

The PEX hot water collector is basically a loop of continuous tubing, perhaps a hundred feet long, snaking through a long shallow rectangular box painted black and covered with a sheet of polycarbonate. The PEX is connected to a low speed 200 GPH pump submerged in the theromass storage water tank. As the water circulates through the loops it’s heated by the sun. The pump is controlled by a thermostatic sensor located in the solar collector. The end result is the water in the tank stores the additional heat that in turn is released back into the greenhouse as needed.

PEX Tubing Solar Collectors Test

It was decided that a full scale test of the collector would be conducted in the winter of 2014-15. For the purpose of this test two of the four theromass storage tanks in the greenhouse were selected. Tank (A) would act as the control for the test. It would only receive its heat by passive solar collection from sunlight passing into the greenhouse alone, whereas tank (B) would be heated by passive solar energy, as tank (A), in combination with PEX solar collector.

November & December 2014 collector test “Alpha” results summary:

greenhouse inside 1In November, 2014 the PEX solar panel added 523,168 BTU’s (British Thermal Units) to the test tank (B). This energy was released back into the greenhouse in the form of radiant heat.

During the month there were 6 days of fully overcast skies and the PEX collector was inoperable during this period. Without the sun to power the collector or provide passive heating to the theromass heat storage test tank (B) the temperature dropped for a daily low on November 10th of 73.9 F to 62.6 F by November 13th. The control test tank (A) dropped from 62.6 F to 60.2 F over the same period. With nighttime outside air temperatures at -2.9 F the greenhouse temperature at the ground level dropped from 54.7 F to 39.7 F.

In the calculation of the energy used in the operation of the active solar collector inside the greenhouse during the November Collector Test Alpha the International Standard SI unit of energy is the joule and was used for this purpose. However, as the recording instrumentation within the greenhouse is only able report energy consumption in wattage it became necessary to collect the data in wattage before converting it into joules. One kilowatt hour is equal to 3.6 megajoules which is the amount of energy if work is done at an average rate of one thousand watts for one hour. This is reflected in the formula below:

kW . h=(3600s) [kW]=3600 [s] [ks/s] = 3600kJ=3.6MJ

greenhouse inside 2The pump that circulates the water through the collector into the test tank consumes 9 watts of power. At this rate it would take the pump 111.11 hours of operation to consume 1 kilowatt of power. At present, the daily runtime is not being collected for the pump but based on observations, on sunny days, the pump runs for about 6 continuous hours.

During the month of November the pump ran on 27 of 30 days. Assuming that it ran 6 hours daily the energy used would be 1.458 kWh or 5.248MJ (megajoules). The price of energy supplied by the local utility is at the rate of 15 cents per kilowatt hour. The cost to operate the pump for this 27 day period was a total of 22 cents.

In December the total BTU’s added to the greenhouse was slightly less than November at 521417. This is not surprising as there were a number of factors that played into this total; First, December had 10 overcast days while November had only 6. December has fewer hours per day of daylight than does November as it’s closer to the winter solstice, the shortest day of the year. In addition, the average daily outside temperatures is much lower in December than in November. The average for November was 51.7 F high and 24.7 F low. In December the average high was 45.7 F with an average low of 19.1 F.

On the last two days of December the outside temperature bottomed out at -12.9 F on 12/30/14 and -15.0 F on 12/31/14 under gray, sunless skies. The inside temperature of the greenhouse under these harsh condition dropped dangerously close to freezing with an overnight low of 32.6 F.

Conclusion of Test Alpha

While the PEX solar collector that was under observation collected more than one million BTU’s during the two full months of Collector Test Alpha, even with the help of a second PEX collector, this was not enough additional energy to protect the greenhouse from freezing should the nighttime temperatures drop below -15 F if combined with continuous gray, overcast skies for two days or longer.

The long term goal is to have the greenhouse be able to maintain inside nighttime temperatures no lower than 40 F with outside night time temperatures reaching the -25 F range combined with two days of overcast, sunless skies. It’s clear that modification will need to be made in future tests if the overall objective is to be reached.

Test Alpha-Bata

greenhouse tomoatoTest Alpha-Bata was a continuation of test Alpha when included all the data for the winter season 2014-15, starting October 22, 2014 and concluded on March 31, 2015. During this period test Tank (B) collected 2,650,386.6 Btu’s of heat over the control Tank (A).

2 ½ million Btu’s is a significant amount of energy at a very low overall cost. It’s estimated that the power to run the pump during the 161 days of the research project has a cost of about $1.60, or 16,564 Btu’s for each penny spent. The average daily Btu’s generated by the collector was 17,225.1

For those of you that are number freaks like me, I would be glad to send you a pdf file with the data. Just drop me an email.

Russel the Mad Scientist SoaringRavenLab at gmail dot com

greenhsoue at night lit up

Thanks to Russ Erganbright for participating in the [Grow] Network Writing Contest. We have over $1,500 in prizes lined up for the current writing contest, with more to come. Here is a list of the current pot of prizes:

– A 21.5 quart pressure canner from All American, a $380 value
– A Survival Still emergency water purification still, a $279 value
– 1 year of free membership in the [Grow] Network Core Community, a $240 value
– A copy of The Summer of Survival Complete Collection from Life Changes Be Ready, a $127 value
– 2 copies of the complete Home Grown Food Summit, valued at $67 each
– 3 free 3 month memberships in the [Grow] Network Core Community, valued at $60 each
– The complete 2014 Grow Your Own Food Summit interview series, a $47 value
– 4 copies of the Grow Your Own Groceries DVD video set, valued at $42 each
– A Bug Out Seed Kit from the Sustainable Seed Company, a $40 value
– 4 copies of the Alternatives To Dentists DVD video, valued at $32 each

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  • Andy UK-England

    We need more people and more information like this.
    This has got to be one of the best articles iv read, Maybe even the best one.

  • Bets

    Even the earliest ‘failed’ would be great for South Arkansas!

  • James

    why don’t you try a 24′ x 3′ x 2′ run of wood chips and let it turn into mulch..that will get hot enough to burn your skin and will likely provide all the heat you need. Having a good radian barrier as the base mght make it even more effective

    • Profile photo of Marjory Wildcraft

      James, I’ve always wondered about the efficiency of heating via compost. How many BTU’s does it throw off?

      Ditto for chickens and rabbits – could we heat greenhouses using small livestock?


    “For those of you that are number freaks like me, I would be glad to send you a pdf file with the data.” PLEASE! And thank you!

  • T Stone

    Very little money but very interested in solar heat and electricity. T Stone

  • Willie

    I’m building an experimental heated raised bed in my 16×32 greenhouse. The heated portion of the bed is about 18×5. I put 2″ thick insulation first then old metal roof panels and laid several rows of 3/4″ PVC piping down. I will fill the bed with soil and plan to build a solar heating panel with PVC as well to connect it to the pipe loop underneath the bed. A small fountain pump then circulates glycol through the system. A short hoop structure then on top of the bed hopefully extends my Upstate NY growing season.

  • Jose L

    I live in Florida so a green house will generate too much heat and most likely kill everything inside it. The problem I have here is sand since Florida is just an overgrown sandbar. So I’m trying an idea I found in the last summit series about using hay to add organic matter to the soil (sand) and get rid of weeds. I’ll let you know how this works out.

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