Final Report for GS06-052
Project Information
The sub-soil heat storage system known as the Subterrain Heating and Cooling System (SHCS) was shown to improve temperature condition and increase growth for soil plantings compared to a control greenhouse. It provided insufficient heat at night to dramatically elevate temperatures in an uninsulated greenhouse. It increased soil temperature 10F during March.
A liquid foam insulated greenhouse stayed 14F above the control overnight for low electric costs, but had technical problems. This or another kind of full envelope insulation is needed to make best use of solar heat storage systems. $75,000 in further funding was secured to continue research.
Tables, figures or graphs mentioned in this report are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or
sueblum@southernsare.org for a hard copy.
Introduction
STATEMENT OF PROBLEM:
The purpose of the affordable bioshelter project is to identify, test, and demonstrate the costs and performance capabilities of technologies with the potential to make solar greenhouses affordable. A solar greenhouse or bioshelter is characterized by its ability to collect and store solar heat, so that virtually no other heating is needed, even in winter. For the purpose of this project, “affordable bioshelter” is defined as a solar greenhouse that is economically competitive with hoop houses, having a payback period of five to ten years relative to the hoop.
More broadly, the purpose of this project is to contribute to a lower fossil fuel input and a more localized agricultural system for the high south and other climates where greenhouses are heated. Solar greenhouses are a technically and ecologically viable avenue for the development of a localized and diverse sustainable agriculture. Currently, wintertime trucking of foods to cold climates consumes vast amounts of fuel. The lack of an affordable local alternative is an impediment to a stronger relationship between local farmers and consumers. Although the fossil-fuel-heating-free capabilities of bioshelters are well proven, the high capital cost of solar greenhouses has prevented their commercial adoption (Todd, 1994). Instead, hoop houses, with propane heating costs now in thousands and climbing, dominate the greenhouse market in the high south (Boyette, 2006). Currently, lucrative winter crops such as hot-house tomatoes represent a dependence on rising operation costs for the producer, and a high fossil fuel input food source for consumers, even when purchased locally. As the energy-scarce future unfolds, affordable bioshelters must be ready for the challenge of replacing or retrofitting agricultural hoop houses. It is the purpose of this project to make a contribution toward that transition.
Specifically, this project will examine two promising technologies selected because of their potential to drastically cut the cost of high performance solar greenhouses (see figure 1). A controlled experiment with three mini-greenhouses will be conducted at the Appalachian State University Sustainable Development Farm to test their winter performance. These two technologies, Liquid-Foam Insulation (LFI) and Underground Heating and Cooling Systems (UHCS) are appropriate selections, since both have been working in demonstration projects since the year 2000 and have been proven technically feasible (Elliot, 2005; Cruickshank, 2005). However, neither has had its performance scientifically measured in a controlled experiment. This means the systems' performance in a given greenhouse cannot be estimated, and thus their economic viability cannot be assessed. Despite their apparent success in demonstration projects, without objective performance and economic evaluations, these systems represent an unacceptable risk to farmers.
It is the purpose of this project to complete such an objective performance evaluation and set it in an economic context. To facilitate technology transfer, the technical performance analysis will be fed into an economic analysis comparing the affordable bioshelter technologies with hoop houses and passive solar greenhouses. This analysis will also look at the viability of retrofitting existing hoop-houses. The purpose of measuring these technologies and placing them within an economic context presents this project with a well-defined and manageable problem and opportunity.
Figure 1
Liquid-Foam Insulation (LFI):
An LFI is a bubbling machine that generates a dense, insulating (R1/inch), and reflective soap-foam between two layers of plastic glazing, each on a hoop 10”-25” apart (Elliot, 2002). The soap solution re-circulates in a closed loop. By keeping the foam regenerated all night and allowing it to dissipate during daylight hours, full daylight and solar gain is possible without losing heat though glazing at night (ibid). This means that the heat stored in thermal mass by day is insulated at night at a rate approximately 3 times better than a traditional solar greenhouse which lacks night curtains (Clegg, 1979; Fisher, 1980; Carroll, 2006). This improved performance is coupled with the use of hoop construction, which is radically cheaper than the solid building construction typically utilized by solar greenhouses.
Underground Heating and Cooling System (UHCS):
UHCS replaces above-ground thermal mass, which consumes space and light resources, by turning the subsoil into the heat storage. Warm, moist greenhouse air is blown through 4” buried drain pipe. As the air cools underground, it drops most of its moisture and with it a large amount of latent heat, heat which is released during condensation. At night, when the air becomes cooler than the soil, the blown air delivers the heat stored in the soil by day. In addition to being a cost-effective form of heat storage, UHCS keeps the air dry and cool, aiding transpiration and reducing fungal growth. It keeps the soil warm and moist, aiding root growth, and functions.
- A numbered list of concise project objectives.
1. Design a controlled experiment in which the operation of SFI and UHCS in a greenhouse is validly compared to a control greenhouse.
2. Design a baseline passive solar hoop house for the control
a. design a hoop shape that is a better solar radiation collector than a typical shape hoop house
b. select an optimal amount of bellow grade insulation3. Build three identical miniature hoop houses out of donated bamboo, simulating the usual metal or PVC pipe construction.
4. Build a basic bubble generation machine, based on plans provided by Homestead Building Solutions Inc. (Elliot, 2005).
5. Design and install an underground piping network for the SHCS with the help of Going Concerns Limited (Cruickshank, 2005).
6. Install and calibrate data acquisition equipment in time for the January-February experimental period.
7. Conduct a series of experiments that will capture the necessary information to establish a meaningful preliminary assessment of LFI and UHCS.
8. Analyze the performance of each system and compare it to theoretically predicted performance, to analyze the performance of the two systems working together.
a. Establish the adaptability of existing theoretical performance calculations for solar greenhouses to apply to an LFI greenhouse.
b. Extrapolate from the data collected for the flow of energy into and out of storage in a SHCS, and graph these patterns of heat flow relative to the changing conditions in the greenhouse.9. Use the knowledge gained through the experiment and through working with the systems to develop a prototype affordable bioshelter with a reliable construction cost estimate.
10. Conduct an economic analysis comparing LFI and SHCS to a passive solar greenhouse and a hoop house.
Cooperators
Research
WORK COMPLETED
A robust bioshelter experimental research site was set up in which technologies can continue to be tested in different configurations and against a number of controls. Preliminary data was gathered during winter 2006-2007, primarily focusing on the performance of each technology on its own.
The major achievements to date are:
1. Located a test site with the needed services and good solar access.
2. Developed a greenhouse design that incorporates the desired features to be tested, as well as the capacity to gather accurate data and to control the experimental variables.
3. Built three greenhouses in a modular format, and transported them to the test site.
4. Installed two sub-soil heat storage systems.
5. Installed two 24x14 identical greenhouses incorporating the experimental systems in such a way that they could be turned off, and one greenhouse used as a control for the other.
6. Designed and installed a liquid foam generation and recapture system.
7. Designed, installed, and calibrated data acquisition systems in both greenhouses.
8. Designed and conducted experimental trials on the foam and the sub-soil heat storage.
9. Monitored, documented, and developed solutions to various implementation issues that arose during design and construction of the greenhouse systems.
Despite challenges and setbacks, particularly with the liquid foam insulation, the test site has yielded meaningful preliminary results this winter. While the original proposal only called for the testing of technologies separately, the improved experimental set-up is designed for a series of three-way comparisons with systems alone, and then in combination. We hope to complete these more detailed tests during the winter of 2007-2008.
EXPERIMENTAL DESIGN
GREENHOUSE: The design is an asymmetric Gothic arch with a 24’x14’ footprint. Two layers of polyethylene glazing are separated by a two-foot cavity for the foam. The end walls are super insulated with polyisocyanurate board. The exterior cladding is corrugated plastic and interior cladding is foil-faced poly bubbles. The structures are sealed to ensure a uniform blower-door test result in the two greenhouses. The design is scalable to a full-size commercial greenhouse. A third greenhouse is partially constructed and will be completed to allow for more revealing experiments using three-way comparisons.
SOLAR GAIN: the structures are E/W oriented. The south-facing glazing is set at 45° (the classic passive solar angle). The arch apex is set back toward the north to increase south-side surface area and to allow insulation of the north wall without blocking light. The north-side glazing is at an angle of 78° to reflect light onto the plants below it. The two, 7.5mil polyethylene layers transmit 81% of incident radiation. The IR coating is rated at 80% infrared radiation retention.
HEAT STORAGE SYSTEM (UHCS): The UHCS system consists of ADS 4” corrugated, perforated drain pipe buried in rows at depths of 1, 2, and 3 feet beneath the greenhouse, with a horizontal separation of two feet. The inlet manifold consists of a plastic barrel from which the pipes fan out and run across the length of the greenhouse to two exhaust boxes at the far end. A thermostatically controlled, 50-watt, 60 cfm fan circulates air. A heating thermostat controlling a circulation fan was set at 50°F to heat at night, and a cooling thermostat set at 75°F relayed to the same fan to cool during the day. Polystyrene insulation (R13.5) surrounds the perimeter to three feet below grade. Prior to installation, the subsoil was prepared in 4 parts local clay soil and 1 part river aggregate and compost, respectively, to achieve a homogeneous thermal mass.
LIQUID FOAM INSULATION (LFI): The LFI system consists of the inter-glazing cavity, a subsoil surfactant tank and recovery system, and foam generators. On demand, surfactant is pumped from the tank and sprayed onto a screen. A blower forces air through that screen, producing foam, until the cavity is filled with foam. As the foam drains of liquid, it is collected at the bottom of the cavity and directed back to the tank for reuse. Under typical conditions the cavity fills in 15 minutes, with refills required every 1-2 hours.
DATA ACQUISITION SYSTEM: Greenhouse temperatures were measured at heights of 8, 5, 3, and 0.5 feet inside the greenhouse using 10k thermistors and Hobo® U12 loggers. Subsoil temperature was measured in the same way at depths of 1 and 2 feet halfway between UHCS tubes. A Kestrel 41000 anemometer recorded air flow and humidity at the UHCS inlet manifold. Temperature and humidity were also measured at the UHCS inlets and outlets. A Hobo Microstation recorded outdoor temperature, humidity, ambient solar irradiance, and wind speed. The same cover crop was planted in both greenhouses as a gauge for the effects on plant growth.
EXPERIMENTAL METHODS
Performance of both the UHCS and LFI systems in one greenhouse was compared to the control greenhouse over several runs during late winter (January-March 2007). At least five days separated the runs to allow for soil temperature equilibration. In all cases the control greenhouse was operating as a traditional greenhouse. Cover crops were planted in each greenhouse before the experimental period. For two weeks the two greenhouses were kept in the same condition to ensure a valid control. The experimental periods were Jan.25-Feb.1 for the UHCS vs. control, Mar. 6-13 for UHCS with thermal mass vs. control, Feb.17-Mar.14 for long-term soil trends for UHCS with thermal mass, and Jan.17 and Mar.18 for LFI vs. control.
During the UHCS experiments the subsoil system was tested against the control. In both, pans of water were used to simulate the effect of transpiration. The UHCS system was also tested with a supplemental thermal storage consisting of 25 gallons of water in black-painted barrels at 1 gal/ft2 of south glazing, or 1/5 the optimum ratio for a passive solar greenhouse (Mazria, 1979).
During the LFI experiments, the cavity was typically filled to approximately 90% capacity and allowed to deplete to approximately 70% capacity, with an average thickness of two feet. At temperatures below approximately 25°F, foam would freeze and collapse so that its average thickness between refills varied from 3-10”.
RESULTS AND FINDINGS
UNDERGROUND HEATING AND COOLING SYSTEM (UHCS): The temperature difference between the UHCS greenhouse and the control is shown in Figure 1 in Appendix A. During the period shown, the UHCS greenhouse kept an average soil temperature of 50.6 between the tubes at two feet, or 3.1 degrees above the control, which averaged 48.5°F at the same depth. During this period, air temperatures at night were 2-3°F warmer in the experimental than in the control when the preceding day was sunny. The UHCS greenhouse was 3-4°F warmer than the control after a cloudy or partly cloudy day. Daytime highs in the experimental greenhouse were 2-4°F lower on some sunny days while no difference was observed on other days.
The modest increase in soil temperature and moderation of air temperatures had a dramatic effect on the planted cover crop. In the UHCS greenhouse, grasses grew to 4-5 inches tall by February 1st, compared to only 1-2” in the control. We estimated that there was about three times more biomass in the experimental unit than in the control.
Performance of the UHCS plus supplemental storage during early March, 2007 is shown in Figure 2 in Appendix A. During this period the experimental greenhouse had an average nighttime low of 8°F higher than the control, and a daytime high of 7.5°F lower than the control. During a longer period, the difference in soil temperature also grew from 2°F to 7.5°F, as shown in Figure 3 in Appendix A. Plants in the experimental greenhouse continued to dramatically outgrow those in the control.
LIQUID FOAM INSULATION (LFI): Performance of the foam insulation system is shown in Figure 4 in Appendix A. During the night of March 6 (9:00 pm-9:00 am), the experimental greenhouse maintained an average temperature 14°F above the control greenhouse. The system drew an average 400w throughout the night. The experiment was repeated with similar results on April 5th , accept that a 20% propylene-glycol solution was used, eliminating freezing and reduced refill frequencies, and cutting electric consumption by more than half. A separate experiment, the north wall was foamed during a cold sunny morning, and a temperature difference of 10°F was observed by some sensors, but not others. It was observed that filling the north side on a sunny day increased light intensity in the greenhouse by 38%.
Among the many observations taken, the following are particularly noteworthy. A liquid-to-air ratio was found to be 1:80 with the LFI in a 1.5% Dodecyl-benzenesulfonate- triethanolamine surfactant concentration. The foam generation was found to completely fill a 20-foot long bag in both a vertical and horizontal position. However, in the cavity the foam got “stuck” at 60-70% full if it flowed too slowly, or if it was filled in stages. The foam was also found to be vulnerable to quick collapse if exposed to cold dry air from an inflation fan, as well as to freezing and compression of the cavity by the wind. We discovered that the method we used to generate foam could not be automated with a timer without endangering the blower, because foam reached the blower intake at unpredictable times. An automated control system using LED light detectors was built and found to work, but has not been deployed since other problems, particularly leaks in the plastic cavity, remained as barriers to automation. In summary, we found that LFI is difficult to manage and to study. Our response to the challenges involved is discussed below.
Summary, Conclusions, and Recommendations
The function of passive solar greenhouses is well understood (Mazria, 1979; Santamouris, 1994). Given ample solar gains, insulation levels, and heat storage, solar greenhouses can be designed for 80% energy savings. While more complex than traditional designs, the technologies investigated in this study are capable of insulation and heat storage levels similar to traditional bioshelters.
Experimental results suggest that, with improvements, these technologies deserve further investigation into their performance and economic viability. The bioshelters team developed an understanding of the technologies and a network of advisors and collaborators, thus setting the stage for potential breakthroughs in affordable solar greenhouse design. Due to the short experimental period for the round 1 data collection reported here, all results are considered preliminary and all studies will be repeated in winter 2007-2008 with a longer duration.
The Underground Heating and Cooling System (UHCS) provides an easy to install, cost effective, and simple to operate heat storage system that is ready to deploy. It has functioned as designed with no problems since it was installed. The small 50-watt fan it uses makes the system particularly energy efficient. Analysis of the UHCS data reveals that during a typical day the system took up between 20-30% of the available solar energy, re-releasing 10-20% at night, while 5-10% went to increasing the soil temperature. This was derived by two methods, first by looking at soil temperatures and heat capacity of the moist soil, and second by examining the latent heat transfer. It is known that misting can increase relative humidity. Theoretically, misting near the UHCS intake will increase latent heat transfer. During a 3-day period when the soil was set to discharge heat at night but was not absorbing it by day, soil temperatures dropped an average of 1.25°F. Based on the above calculations, this accounts for a release of 25,000-38,000 Btus per night. If it were a 20x100 foot greenhouse the release could potentially be between 200,000-408,000 Btu/night, the equivalent of 2-4 gallons of propane (with current prices, $3.60-$7.20). Based on this calculation, the UHCS for this greenhouse, which costs $1,400 to install, would pay for itself in 2-4 years.
Despite this potential value from a thermal point of view, it is questionable whether the UHCS system is effective without added insulation since the energy released raises air temperatures only slightly. Although not yet experimentally verified, it is anticipated that the UHCS system will perform significantly better if used in an insulated greenhouse, because the UHCS will experience less of a heating load and the soil temperature will remain higher over the long term.
Ultimately it is not higher temperatures, but increased crop yields, that will increase revenue and be taken into account in payback calculations. The preliminary results regarding plant yields are very encouraging and substantiate a hypothesis (Cruickshank, 2007) to this effect. More data is needed to evaluate this preliminary conclusion. It is believed that root zone warming and moistening is partially responsible for the enhanced growth effect (ibid).
In summary, we have reached the following preliminary conclusions regarding the UHCS system:
1. The UHCS is able to elevate soil temperature in an insinuated greenhouse by 2-5°F with the solar radiation available in January at 36° latitude, and is able to moderate day/night swings by 2-5°F.
2. Small thermal improvement by the UHCS can lead to exponentially increased plant growth.
3. Substantive latent heat transfer into and out of soil can be measured.
4. There is a strong possibility of a payback period of less than five years.
Therefore, regarding the UHCS we recommend:
1. Further testing of the systems’ seasonal storage capabilities and effect on plant growth.
2. The study of adjustments to the system, such as misting to increase heat transfer.
3. That tubes be spaced more closely to increase surface area so the soil can reach a higher temperature to provide greater nightly heat exchange.
4. Exploration of the UHCS with other systems such as a supplementary soil heating systems.
The liquid foam insulation (LFI) system has been shown to be effective for nighttime insulation even when it partially collapses due to freezing. Serious practical difficulties with the entire LFI system were experienced, which did not allow for a sufficiently lengthy study of the technology. However, the results found in the trial were consistent with findings by previous studies (Shamim, 1995; Villeneuve, 2005). Evidence of the effect of filling the north wall during the day is inconclusive. Although some data show that LFI insulates in this application, contradictory data suggests something more complex is taking place. One possibility is that as sunlight warms the foam its R-value is reduced. Clear evidence was found of the reflective effect of the daytime application.
In contrast to the UHCS, the LFI system is technologically challenging and will require additional refinements. We have made contact with an aqueous foam research lab and others working with LFI, and the information gained has assisted with analysis and re-design of the LFI system. Although the system was not automated, Phase I provided the research team with considerable expertise in design and construction of LFI systems, and the Phase II proposal offers remedies to these challenges (Table 2).
The economic viability of a similar LFI product is currently being demonstrated by commercial launch of the new Sunarc Inc. liquid foam roof insulation retrofit system for gutter-connected greenhouses, which has been shown to achieve a 50% energy savings. Our system has the potential to push the envelope much further by using a cavity 2-3 times thicker for foam insulation of the roof and walls within a design capable of greater solar gain than existing greenhouse. This is most likely only possible with a new kind of greenhouse.
We estimate our design for a foam-insulated greenhouse to conservatively cost $10,000 extra to install for a 20x100-foot greenhouse. It takes about $5,000 to heat (at current propane prices) this size greenhouse in an average winter in the high south. Our design would need to reduce annual heating bills by 50% to obtain a four-year payback period. Since our insulation cavity and the coverage of the insulation are greater than that of Sunarc’s product, it is likely that savings would be greater than the 50% achieved by that system. Accounting for the estimated $700/year that it would take to run the compressor, based on a 15 minute fill every 2 hours, the payback at 50% fuel savings would be 5.5 years.
Table 2. LFI system problems and proposed solutions
Problem: Research shows R-values vary widely, from 0.3 to 3 per inch Solution: Keep foam dry so that conduction by water is minimized
Problem: Dry foam does not flow well. Solution: A mix of Dodecylbenzenesulfonate-triethanolamine salt with cocodiethanolamide and Dawn® (1% and 0.25% , respectively) was found to create foam capable of flowing to the whole cavity.
Problem: Foam evaporates in cold dry air on cold nights Do not inflate with ambient air; inject foam into un-inflated system or use inside air.
Freezing foam collapses Solution: Use propylene glycol. In a jar test, foam made with propylene glycol (freezing point -50°F) instead of water was compared to a water-based sample and found to have very similar properties.
Problem: Blower based system has inefficient, imprecise energy-intensive foam generation
Solution: A compressed air system used at UPENN aqueous foam lab was calculated to be 3-5 times more energy efficient. It is also able to produce foam more homogenously and with greater control of the liquid fraction10.
Problem: The LFI cavity is compressed by wind Solution: A second set of structural ribs is needed.
Problem: Automation currently not possible Use light sensors for controls, with secondary safety to ensure the air intake is not clogged; or, use an un-pressurized plastic cavity with ambient air intake and pressure release for the cavity.
Problem: Penetrations of the poly glazing compromise the envelope integrity
Solution: Use only bulkhead fittings designed for polyethylene, like those used with poly inflation fans.
In summary, we have reached the following preliminary conclusions regarding LFI:
1. The liquid foam used in this study insulates and helps keep a greenhouse warmer at night.
2. Liquid foam used in this study increases light intensity by about 1/3 in the greenhouse when used to fill the north side during a sunny day.
3. LFI is feasible, but requires improved design, for the application we are proposing.
4. There is a strong possibility for a high return on investment.
5. The LFI system needs continued research and development before it is ready for large-scale implementation.
Therefore, regarding the LFI system we recommend:
1. Continue research into ways to improve the energy efficiency of foam generation, and the insulation value of various foams.
2. A sturdy and highly reliable specialized greenhouse system be developed for this product.
Educational & Outreach Activities
Participation Summary:
Thesis: Testing Techonologies for Affordable Bioshelters
To be completed by May 2008
Reports on each technology, and one on technology combinations.
Interm Reports by February 2008, final reports by December 2008.
Project Outcomes
Impact of the Results/Outcomes
What demonstrable impacts has the project had to date?
• Produced pioneering data and knowledge on LFI and UHCS
• Initial economic analysis suggest favorable paybacks are possible, gives direction for further R&D
• A presentation and interactive session was conducted with the New River Organic Growers Association, which revealed all around interest by growers in increasing greenhouse production but reluctance to invest due to concerns about energy costs. A new research partnership with one of the farmers developed from these discussions.
• The experimental greenhouses were visited on the Watauga Extension's third annual solar greenhouse tour exposing local growers to our investigation.
• The Affordable Bioshelters Project gained national recognition and $75,000 in follow up funding as one of six winners of the 2007 EPA P3 Student Sustainable Design Competition.
What demonstrable impacts do you expect the project to have in the future?
Fall 2007
• We will retrofit a greenhouse at Lily Patch Farms with UHCS for an in-situ study
• The project will receive coverage in the local Extension newsletter.
• A basic project website will be set up
Spring through Fall 2008:
• We will produce and publish a set of reports and DIY manuals for growers. The reports will be scientifically based, and involve economic studies done in conjuncture with Appalachian State’s Entrepreneurship Institute. Interim reports will be published in the spring of 2008 and final reports in the fall following an outreach development and vetting workshop.
• A specialized Liquid Foam Insulated greenhouse prototype will be developed and tested
• An interactive website will be developed to help farmers explore greenhouse energy saving systems are appropriate for them.
• A paper will be submitted a peer-review journal and/or a magazine such as home power
Fall 2008
• Hold a dissemination vetting, and field development workshop
• Publish reports
Economic Analysis
Please see results and discussion section
Farmer Adoption
Farmers Directly Effected: Approximately a dozen farmers has been effected through direct exposure to our study. We have emphasized that our work is in progress and have not pushed farmers to adopt the technologies we are testing and developing. We are waiting until we complete our study so that we do not lead anyone astray.
Recommendations to farmers: Every greenhouse can be improved in a number of ways. There is a variety of option, a combination of which would be appropriate for your site. Insulation and heat storage should be increase together so that over heating is avoided and so that gain are multiplied. Existing greenhouse can be retrofit with sub-soil heat storages and a very simple retrofit greenhouse insulation system is being developed by engineers at the University of Manitoba. Contact us at affordablebioshelters@gmail.com, and we can help you make the right choices that will lead to significant energy savings for a reasonable up-front investment.
Letter of support in appendix: Letter from Charles Church; letter from Brad Hinkley
Areas needing additional study
Further study in this area is needed. Some of this research is being undertaken by our research group and others.
• The studies conducted during 2006-2007 should be verified by longer trials
• A reliable and durable LFI system needs to be developed
• Retrofits need to be studied
• Existing modeling software or new models need to be developed and verified for solar greenhouses
• Various technology combinations need to be studied
• An optimization study for various configurations of the UHCS needs to be done
WORK CITED
Boyette, Mike (2006) Greenhouse Energy: Alternatives, Conservation, Maintenance and
Structural Issues. Presentation at NC Cooperative Extension Regional Greenhouse Workshop. Mountain Horticulture Crops Research and Extension Center, Fletcher NC.
Boylan, Richard, (2006). Interview with Ag. Extension agent.. May 18 2006.
Clegg, P. & Watkins, D. (1979). The complete greenhouse book. Charlotte, VT: Garden
Way Publishing
Cruickshank, John (2007). Solar heated greenhouses with SHCS. Retrieved April 25, 2007
http://www.sunnyjohn.com/indexpages/shcs.htm
Cruickshank, John (2005). Interview, November 2005.
Elliot, R. (2002) Liquid solar insulation and shading system: Changing microclimates to reduce
climate change. In Conference Proceedings: IFOAM world scientific forum.
Elliot, R. (2005). Interview, December 2005
Elliot, R. (2005). Greenhouses. Retrieved May 25 2006. http://www.homesol.ca/greenhouses.php
Fisher, R. & Yanda, B. (1980). The food and heat producing solar greenhouse: Design,
construction, operation. Santa Fe, NM. John Muir Publications, Inc.
Mazaria (1979) The Passive Solar Energy Book. Rodale Press. Emmuas, PA.
Shamim Tariq, P.E. et al. (1995) An experimental Study of heat Transfer
Through Liquid Foam”. ASHRAE Tiaasactbns: Research
Santamouris, M., Balaras, C. A., Dascalaki, E. (1994). Passive solar agricultural greenhouses: A worldwide classification and evaluation of technologies and systems used for heating purposes. Solar Energy, 53, 411-26.
Todd, J. (1994). Eco-cities to living machines: Principles of ecological design. New York: North Atlantic Books.
Villeneuve J.. de Halleux D.. Gosselin A. Amar D. (2005). Concept of Dynamic Liquid Foam Insulation for Greenhouse Insulation and the Assessment of Its Energy Consumption and Agronomic Performances. In Proceedings IC on Greensys Eds: G. van Strate et al. Acta Hort. 691 ISHS