Testing Technologies for Affordable Bioshelters

View the project final report

Commodities

Practices

• Crop Production: greenhouses, high tunnels or hoop houses
• Energy: solar energy

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.

1. 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 insulation

   3. 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.