Farmers in western North Carolina struggle with a short growing season and limited acreage. Over the last few years, Appalachian State University’s (ASU) Nexus project has been working to help farmers extend their growing season by developing sustainable greenhouse heating solutions from biomass waste streams. Nexus is a project of the Appalachian Energy Center and the ASU’s Department of Sustainable Technology and the Built Environment (STBE), and is based at the Watauga County Landfill in Boone, North Carolina. The project is developing integrated greenhouse heating systems that incorporate solar thermal collection, biochar production and heat capture, Microscale Anaerobic Digestion (AD), and sophisticated thermal storage and delivery. The end goal is to transfer these technologies to local farms to offset or eliminate fossil fuel greenhouse heating requirements and emissions while providing waste management and quality soil amendments.
Nexus is analyzing a variety of techniques for optimizing biogas production in the temperate climate of this high-altitude region. Insulated and earth-sheltered digester designs are being assessed as well as the incorporation of sustainable supplementary heat sources. Passive solar thermal heating and compost-based heat capture systems are being used. All of the AD trials utilize a dairy cow manure slurry with about 8 percent total solids. System optimization and economic analysis are still in process, but the practicability of these systems has been demonstrated. Also being assessed is the feasibility for small farm biogas upgrading, utilizing a simplified water scrubbing technique. Nexus is finishing installation of its biochar and solar thermal systems at two partner farms and is developing an AD system with a local small-scale goat dairy in the coming year.

Solar Thermal Digester

The most reliable cold climate digestion system that the Nexus team utilizes is a solar thermal design (Figure 1) developed by Zach Dowell, a former STBE graduate student. It includes a 175-gallon upright cylindrical AD tank encased in a 4- by 4-ft. plywood box that is air-sealed and super insulated with closed cell foam and blown-in cellulose with an R29 fiberglass insulated top (Figure 2). A 50-ft. heat exchanger coil of half-inch copper tubing circulates a 50 percent propylene-glycol antifreeze solution through the bottom of the digester from a 4- by 8-ft. flat plate solar thermal collector installed below the digester. This design facilitates a form of passive circulation known as thermo-syphoning, whereby water, heated in the collector, rises to the digester, then falls back to the collector as it cools. It continues to cycle this way, eliminating the energy and maintenance requirements of pump circulation. A Taco zone valve is installed in the solar thermal line with a digital controller that prevents the digester temperature from exceeding 95°F.
Biogas is transmitted through half-inch PEX tubing from the digester to an inverted 30-gal. collection barrel submerged in a 55-gal. barrel containing the same glycol solution. The 30-gal. barrel, which rises out as it fills with gas, has a valved outlet hose to supply gas to a burner or to a larger storage vessel.
This solar thermal design was able to regulate temperature significantly higher than ambient and produce biogas without active energy inputs during cold conditions that would normally halt gas production (Dowell, 2010). Figure 3 shows the relationship of digester temperature and gas production in the solar-heated digester.
The Nexus project team is currently utilizing a modified version of Dowell’s design at the department’s on-campus biofuels research facility for development of a  microscale biogas upgrading system. This digester is also used to introduce students to the science and practice of AD for biogas energy production.

Compost Heat Exchanger Design

The first digester installed was based on the early Taiwanese-style plug flow designs of graduate researcher Katherine Heléne Klavon at the University of Maryland under the guidance of Professor Stephanie Lansing (Klavon, 2011). A 700-gal. PVC bag was fabricated to serve as the digester, and placed inside an earth-sheltered 42-in. dual-wall HDPE drainage culvert insulated on the interior with 2 inches of XPS foam board (R10) below the bag, and a half-inch of XPS on top (Figure 4).
Based on the work of Jean Pain, a compost-based heat delivery system was designed to increase digester temperature during cold weather for improved bacterial activity in the AD and biogas production for supplemental greenhouse heating fuel (Mother Earth News, 1980). The heat of decomposition within an adjacent composting pile was captured by a buried half-inch PEX tubing coil for transfer to the AD. A 300-ft. length of half-inch PEX heat exchanger (HX) in the AD transmitted heat from a 12-ft. diameter by 6-ft. high static composting pile comprised only of fresh wood chips (~25 cubic yards). The pile contained a 300-ft. half-inch PEX coil heat exchanger.
Air circulation within the aerobically decomposing pile was achieved by running 4 lengths of 4-in. flexible drainage pipe from inside the pile out in the direction of the prevailing winds. The pile was enclosed with a perimeter of 70 straw bales. Temperature regulated valves and a Grundfos circulation pump controlled flow of HX fluid (a 50% propylene glycol solution) through the system and ensured that HX fluid in the digester never fell below that of the HX fluid in the composting pile. Heat from the composting pile was captured by the digester to improve biogas production during cold conditions.
Once active (it took about 2 weeks for the temperature to go from ~15°C (59°F) to 48°C (118°F), the composting pile maintained a minimum internal temperature of 110°F (43°C) for over 6 months, even during the winter. When the HX was operational, temperature within the digester was pushed above ambient levels into the mesophilic range. Figure 5 shows digester and composting pile temperatures during and after a one-day period of operation. Biogas with a >60 percent methane content was sampled from this system during October 2016.

Compost Enclosed Digester

To improve composting heat transfer and reduce energy and maintenance requirements for a compost-heated system, the Nexus project is now testing the thermal performance of Jean Pain’s compost-enclosed AD design (Mother Earth News, 1980). Utilizing Solar Cities’ digester design, a 330-gal Intermediate Bulk Container (IBC) based digester was built and buried in a 23 cubic yard fresh wood chip composting pile (Figure 6) (Solar Cities, 2017). Feedstock is top fed through a 3-in. diameter Uni-seal fitted PVC pipe; effluent exits through a lower top-mounted 2-in. PVC pipe. Gas is collected through an additional 2-in. top-mounted PVC pipe and exits through a half-inch valved pipe fitted to the top of the 2-in. section. The surrounding composting pile is identical in design to the previous HX system apart from the digester being located within the pile.
Thermal data from the first two weeks of installation show that digester temperature has been raised well above ambient (>85°F) (Figure 7). If this composting pile performs as well as the previous one, this thermal data suggests that the digester temperature will be maintained for a 6-month period above 80°F (26.7°C).

Microscale Biogas Upgrading

A low cost  microscale biogas upgrading system is in development. Many farms in the developed world are located in temperate climates where biogas production is difficult during the cold months because the process requires heat (Dana, 2010). Efficient biogas storage could allow temperate climate farmers to take advantage of optimal gas production during the warmest months by stockpiling surplus to help meet winter heating loads. The Nexus project is looking into compressed LPG tank storage or sufficiently durable bag storage for this purpose. However, efficient storage of biogas requires that the raw gas be upgraded first (Vijay, 2013).
Water scrubbing of biogas is an economical approach, practiced widely at the industrial scale. But commercial systems are not available for small-scale farms in western North Carolina (Petersson, 2009; Dana, 2010). Based on developing world experiments with simplified, low-pressure water scrubbing, the Nexus team is assessing the economic viability of a water scrubbing system for  microscale applications. This bench-scale study will assess the optimal carbon dioxide (CO2) removal efficiency of a small ambient pressure water column at varying gas and water replacement rates. Data from this experiment may be used to model the minimum resource requirements and economics for a  microscale farm site system.
The Nexus project’s recent partnership with Heritage Homestead (a local goat dairy) provides an exciting new opportunity for assessing these technologies at scale over the coming years. Ongoing assessment and refinement of these  microscale cold climate AD strategies — and working with local farmers to perform on-site research — improves the project’s ability to evaluate the economic viability of these systems at scale. Results to date suggest their practical feasibility.
Barry Febos is a biofuels research assistant in the Department of Sustainable Technology and the Built Environment (STBE) at Appalachian State University (ASU), and a member of the Nexus research project (febosb@appstate.edu). Dr. James Houser is an Associate Professor in ASU’s STBE Department, where his focus is biofuels and sustainable water management. His background is in water quality, renewable energy system design and modeling complex systems.