Year-Round Renewably Heated and Powered Greenhouse

Final Report for FNC99-256

Project Type: Farmer/Rancher
Funds awarded in 1999: $4,997.00
Projected End Date: 12/31/1999
Region: North Central
State: Minnesota
Project Coordinator:
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Project Information


The goal of this project was to design and build a greenhouse that operated year-round without requiring consumption of purchased fuels or electricity. Although there would be wood back-up available, it was intended to avoid burning wood fuels for heat only, instead incorporating into the design a means to capture heat from wood fuel being used in other farm processing such as maple syrup production, pasteurization, canning or sauce making. The central theme of this project is thermal banking, capturing heat produced from whatever source available whether it be solar gain, active composting, or waste heat from any other process and storing it in thermal mass for gradual release as needed, The thermal mass would be the floor of the greenhouse isolated from the exterior by 2” rigid foam insulation and/or water stored in a large salvaged cheese vat. Storage would be accomplished actively with a simple solar voltaic system running a large furnace fan and small water circulation pumps collecting the day-time solar gain for use at night. The initial cost of building this greenhouse was considerably higher than a standard greenhouse with conventional heat, but the upfront costs were to reflect the future avoided costs of purchased inputs in the form of power, fuel or plastic films.

A considerable amount of effort and inputs went into constructing the thermal storage system before the above ground structures were even begun. The site was leveled and topsoil piled for future use in the growing bed mix. Orientation was east/west with a 15’ by 60’ section planned on the north side for a two-story processing building and workspace that included sap and cider processing, controlled climate mushroom fruiting room, greenhouse germination chambers, and a 60’ solar food dryer running the length of the ridge of the greenhouse. Once the site was leveled and staked, we hired a back-hoe and dug four trenches 4’ wide by 3’ deep and 60’long. A similar trench perpendicular across the center of the trenches was also dug splitting what would eventually be the growing beds into two halves in order to increase efficiency of the heat distribution system. Six inches of 1” plus smooth rock was placed in the bottom. Eight “U” shaped loops of 4” smooth perforated PVC drain tile were laid on top of the rock with one end of the loop connected into a header of 10” schedule 80 PVC sewer pipe running the 30’ length of the center perpendicular trench. Although Polyethylene is a more benign material than PVC and would have been our preference in plastic material, it was available only in corrugated drain tile. We found in past attempts at building this kind of thermal ground storage system that the ends of the beds were not heated very well, most likely due to much greater resistance to air flow by the corrugated materials. (PVC and vinyl lead to production of dioxin in the environment both at production stage and at disposal if by incineration.) The remaining legs of each of the 8 “U”s were connected into two 4” nonperforated PVC returns that culminated in two 8’ long stand-pipes pointing back toward the ridge for connection to supplemental fans if needed. These standpipes were capped for the initial part of the study and used for monitoring temperatures in the return air deep in the beds through remote sensors tapped into the standpipes and placed deep in the bed. In addition, tees were placed in various locations at the ends of the loops for temperature and pressure monitoring. These were capped as well to maintain air pressure, forcing warmed, moist air to perfuse through the gravel and the soil mix in its return to the ridge of the greenhouse. (See diagram 1 of bed layout.) [Editor's Note: To see this and other diagrams, please contact the NCR-SARE office at: [email protected] or 1-800-529-1342.] The array of air channels was then covered with 1 foot of smooth 1” plus rock which was then covered with used Remay row cover to prevent infiltration by soil mix. The topsoil was then mixed with sand and one-year-old composted manure and bedding from our livestock, poultry, and draft horses. We covered the fabric with a few inches of soil mix and laid out a serpentine pattern with 1/2" Wirsbo in-floor radiant tubing, one loop for each bed coming back to a manifold with valves for controlling the flow to each bed. We filled the trenches to ground level with soil mix and later raised the beds 6” using white oak 3” x 6”s (untreated) after construction of the above ground greenhouse was completed. (See Diagram 2.)

We were unable to find a metal frame greenhouse that suited our high-ridge and insulated north workspace requirements so we opted to use pole barn style farming. 6” x 6” square posts, 2x10” rafters on 2’ centers, 2” x 4” perlins and a welded metal support truss down the center of the 32’ span at the splice point of the rafters. Twin wall polycarbonate was used on roof and side wall glazings as well as on the vertical ridge air collection section’s south facing wall. Tempered glass panels, split from salvaged thermopanes and arranged in an overlapping configuration on the south wall, allows us to slide every other one in front of its adjacent panel giving us 50% opening of the south wall for summer ventilation and access for harvest from the front of the greenhouse. The vertical ridge collection area was constructed to allow venting into the attic ridge vents of the insulated north section or into the insulated space for heating work spaces. These latter vents also allow access into the vertical ridge for screens filled with sliced veggies, fruit, or herbs creating a 60’ long solar food dryer. (See diagram 3.) 8” galvanized stove pipe painted flat black runs horizontally in the vertical ridge collection space with tees every 10’. The center tee connects into 14’ of vertical 8” stove pipe, again painted flat black, that runs down to a squirrel cage furnace fan that is enclosed in a forced air shroud with two other shuttered inlets, one connected to a hood system from the sap boiler and the other pulling warm air from above a wood cook stove. The outlet of the furnace fan is connected to six Unisolar solar panels on the roof. A linear booster was included between motor and panels to protect the motor and increase efficiency. No other thermostats or controls were installed and the system works well. (One quarter HP proved to be undersized for our fan at colder temperatures and low light conditions. Our fan had greased bearings instead of oiled bushings and became stiff when cold. Our blower unit was not initially in a warm enough space.) The brighter the sun shines, the more air the fan moved. To utilize other heat sources, supplementary fans connected to stored electricity in battery banks will be installed to utilize heat at times when the sun is not shining.

We are presently growing frost tolerant crops like salad mix, braising greens and herbs in the winter, but have been getting good production on cucumbers, tomatoes, eggplants, and peppers planted in the beds in March with very little supplemental heat. We expect to lengthen our growing season considerably when the straw bale walls on the north section are in place. (The first floor still has a temporary double layer of plastic separating it from the greenhouse after our first straw bale walls went to the fields as mulch due to getting wet.)

There is a direct correlation between bed temperature and overnight lows in the greenhouse. It appears that bed temperatures above 40 degrees keep our greenhouse crops from freezing, although the stratification of air temperatures in the greenhouse is not as much as I had hoped. There are only a few degrees difference between temperatures in the plant zone and that at 3’ height or 6’ height with the air temperature being warmer down low near the bed. A pull-out cover at 6’ during very cold weather would greatly benefit at night. Placing tees at ground level in the stand-pipes with perforated 4” drain tile laid on top of the beds, opened in the evening for convective dispersal of stored heat from the beds would most likely raise the temperatures a few more degrees. The water storage and solar water heating system is not yet operational, as construction, the air moving system, and collecting the data in the midst of the hectic pace of supporting our efforts from production of this small, diversified farm system has absorbed all of our available time and resources. I have attempted to analyze the bed temperature/greenhouse temperature data and came to the following conclusions:
1. As we raised the deep bed temperatures, we also raised the night time temperatures at 6” depth and of surface air temperatures
2. We were able to keep the surface air temperatures 2 degrees to 3 degrees warmer than the temperatures at 3’ above ground by warming the bed.
3. Plants grew better in warmer soil. We had no more frost problems once the beds reached temperatures in the mid 40’s.
4. Beginning with unheated frozen beds, we were able to thaw the beds in a little more than a week blowing unsupplemented solar gained heat into the beds. Temperatures at 20” depth and in return ducts at 30” depth raised about one degree per day on average days and up to four degrees per day when sunny, and ridge temperatures reached 100 degrees to 110 degrees. We found in February and March, it was beneficial to supplement wood heated air into the blower system on overcast days and extremely cold nights when temperatures dropped as far as 12 degrees below zero outside. We began to stoke the wood stove at night whenever temperatures dropped below 25 degrees through March to give added protection for the tomatoes, cucumbers, eggplants, and peppers that were transplanted into the beds at the end of that month.

In the final analysis, the system has considerable merit and potential. The concepts are sound and our system will be upgraded and fine-tuned as we learn more and are able to dedicate the necessary time and resources. If we had some hindsight wishes to make, they would be as follows:
1. A metal truss/perlin roofing system to allow more light in when the sun is at lower angles in early morning or late afternoon. The 2” x 10s” blocks too much light. Perhaps 3’ on center rafters or 2” x 8”s if wood is used would greatly improve the situation.
2. Bury the in-floor radiant tubing closer to the surface of the beds bringing it just below the depth of tiller lines or spading forks. Perhaps one foot to 14” below the surface.
3. More standpipes, one at each end of the eight half beds for monitoring temperatures and pressure for purposes of design analysis, and for releasing stored solar gain at bed surface during the night,
4.I would have preferred thermoplane glazings all around. With seconds available at a reasonable price it was only the mounting hardware and construction details that directed us to the less permanent (20 yr) and less thermally efficient twin-wall polycarbonate. One advantage to the poly-carbonate is that it comes in sheets 6’ wide and 30’ or more lengths (custom cut). It screws on (with gasketed neoprene washers) in a day or so.
5. We were under-funded for such an ambitious project. Adverse weather for several years limited available capital and time as did negative developments in the organic marketplace due to certification politics and economics.

Things we are very happy with:
1. The over-all design of the structure with the long steep roof slope and the vertical ridge collection sheds the snow readily in the winter (be ready to plow snow from the south--lower--side as it has piled 6’ deep for us.) The natural air flow of heated air into the ridge system amazes us even in partly cloudy or extremely cold weather.
2. Having drive-through doors on either end allows us to straddle one of the middle beds with wagons and tractor. This allows us to harden off plants on hay-wagons, pushing them in at night if the temperature drops or in the event of adverse weather. It also allows us to easily bring in compost for incorporation into beds.
3. The north section promises to become the operations center of the farm. With addition of a dug-into-the hill ice-house and root cellar our washing packing operations could be moved adjacent and wash water recycled for irrigation or sprinkling logs for mushroom culture.
4. This project moves our farm many steps closer to energy and weather independence in this time of global climate change and energy insecurity. Our goal of utilizing our own resources for energy production of the farm, “in-house” use of this energy, and maximum recycling of energy are coming closer to reality.

We have pictures and slides of all phases of construction and growing of greenhouse crops. We have done three field day tours of the greenhouse, plan one more this summer and one during the coming winter. We are willing to give presentations on this greenhouse project in the future for a modest honorarium and travel expenses. More detailed drawings of the above ground structure with dimensions and construction details could be made available at a modest cost.

SARE has formed a unique partnership with Cooking Up A Story, an online television show about people, food, and sustainable living. Our partnership will enable us to expand the range and depth of their stories and how-to videos, and to profile successful farmers whose practices have benefited from SARE grants and SARE research to improve their own economic viability, and stewardship of the land. NCR-SARE features have included Steven Schwen.



Participation Summary
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.