Supplemental Heating of High Tunnels by Energy-Producing Compost Piles
A compost heater is a system that conveys heat from a compost pile to another location. At Sunspot Urban Farm, we have built two compost piles (2015 and 2016) for conveying heat to a nearby high tunnel (inexpensive passive solar structure to intensify production). In 2015, the pile failed to maintain heat long enough to begin conveying heat to the high tunnel. In 2016, the pile, having design features different from the 2015 pile (see Table 1), has maintained heat and has conveyed heat to the high tunnel via a hydronic delivery system. This report describes the construction of the 2016 pile and hydronic system (Photos 1 through 30) and preliminary results of conveying heat to warm the soil within the high tunnel.
1. Build a compost heater and hydronic system for delivery of hot water to soil in a high tunnel. 2. Monitor and evaluate the heat delivery of the compost water heater to warm the soil in a high tunnel during winter and spring months.
Building the 2016 Compost Heater We met with our technical advisor Addy Elliott of Colorado State University on October 13, 2016, to examine the failed 2015 pile and to discuss an improved design for the 2016 pile. We agreed to these design changes: larger pile; better mixed and wetted materials; and methods for injecting air and water deeply into the pile periodically after construction (see Table 1). We built the new heater on October 15, 2016 (Photos 1-30), incorporating the new design features (Table 1). The pile’s dimensions are 20’ diameter and 6’ high, including the wall of insulating hay bales. Internal to the insulating layer, the pile is 17.5’ diameter. The pile consists of a mix of hardwood chips (75%), horse manure (20%), and sawdust (5%) (Photo 11). These three materials were mixed thoroughly by a skid steer immediately prior to construction of the pile. During this mixing, the materials were thoroughly wetted for four hours, using two garden hoses (Photos 9-10). This process of thoroughly mixing and wetting the materials, which was not performed in the building of the first pile in 2015, is thought by us to be the primary cause of the sustained heat of the 2016 compost heater (see Figure 1). Prior to adding the mix of decomposable materials, a single central coil-tower of ¾” polyethylene tubing was wrapped around a 5’ high column of wire fencing held in place by 8 T-posts (4-posts of 8’ and 4 posts of 6’ pounded into the ground); the fencing was securely wired to the T-posts (Photos 4-8). The coil-tower is 8.5’ diameter. The ¾” tubing was purchased in 100’ rolls (much easier to manipulate than the 300’ rolls used in the 2015 pile). Eleven rolls were used, for 1,100’ of tubing for the coil-tower. Individual rolls were interconnected by grey, straight plastic barbed irrigation connectors secured by screw-style hose clamps (Photo 5). The tubing was secured to the fencing by 12” zip-ties (Photo 6). The tubing coil begins 18” above ground, for two reasons: a 100’ perforated, flexible plastic 4” diameter tube was laid in a coil at the base of the pile for passive aeration of the pile; and a foundation of decomposable mix would eventually be poured at the base of the coil-tower for warming the water-filled tubing (Photo 4). Four feet of ¾” polyethylene tubing contains 2 cups of water; therefore, the coil-tower holds 34.375 gallons of water. The tubing of this closed system coil is linked to the high tunnel through an underground triplewall pipe containing two lines of tubing (Photos 2 and 3): the line entering the pile (the “cold” line) enters at the base of the coil and spirals to the top; as it ascends, it becomes the “hot” line. Once the “hot” line is at the top of the coil, it runs down through the center of the heat exchange coil to exit the pile via the triplewall pipe. The triplewall pipe consists of a 4” diameter hard, plastic tube; the tube is 10’ long, and 2 tubes were used. The “hot” and “cold” lines run conterminously in the tubing, and each line is insulated with a 1” thick foam sleeve. The tube containing the lines is buried in an 18” trench, filled with dirt, and covered with the original dirt plus a line of hay bales (Photo 20 insert). A note on this step: It is important to clearly label the “hot” and “cold” lines (depicted in Photo 5) and to feed them into the tubing prior to building the coil, doing so only after completing these three steps: duct-taping each line closed, duct-taping the lines together to unify them, and duct-taping the ends of the foam sleeve to the lines so the sleeve will not peel off when the unified lines are forced through the triplewall pipe. Prior to adding the mix of decomposable materials, a hay bale wall was built to outline what would become the circumference of the pile (Photos 4, 8, and 13), initially 3 bales high (ultimately 5 bales high). Into the space created by the hay bale wall, a skid steer successively dumped the pre-mixed and pre-wetted materials, first filling within the coil-tower and then around the coil-tower (Photos 12 and 13). Three people spread and compacted the compost material as it was dumped. As the materials were piled higher, 2 soaker hoses (a 200’ and a 100’ hose) were wound around and within the heat exchange coil (Photos 14 and 15), and six 10’ perforated triplewall pipes (hard, plastic 4” diameter tubes with 2 rows each of 10 holes; Photo 16); these pipes were added roughly equidistantly throughout the pile. Each of these tubes terminates at the hay bale wall for allowing the adding of water or air (Photo 17 and 18); each tube has a removable cap. At 3’ above the ground a second perforated, flexible 4” diameter tube (identical to the one on the ground beneath the coil-tower) was placed, with one end terminating at the hay bale wall, for aeration. As the pile reached the desired height of 6’, more tiers of hay bales were added. Once 6’ was reached, the pile was capped with 18” of tree leaves and loose hay (Photo 20). Note: After construction, the pile has continually settled, so many additional leaves and hay have been added to maintain the top of the pile at the level of the top tier of hay bales. All post-construction adjustments of air (passive and active; Photo 18), water, and insulation to the pile are listed in Table 2. To monitor the success of the pile, we measured its temperature repeatedly, beginning day one. We inserted three compost thermometers of 3’ to 4’ long at these strategic locations: the center (4’), along the heat exchange coil (3’), and at the edge of the pile (3’). When inserting the probes, we were careful not to puncture the polyethylene tubing or soaker hoses; for future piles, we recommend deciding during construction where the probe thermometers will be placed to insure no punctures. Temperature readings spiked initially to 170°F and have since remained near 120°F (Figure 1). Hydronic System: Conveying Heat to a High Tunnel Bed The “hot” and “cold” lines from the compost heater enter the west end of a 20’x 50’ high tunnel (Photo 21). One garden bed 40’ long is plumbed with ¾” polyethylene irrigation tubing buried 3” below the surface (Photos 27, 28, and 29). The tubing runs the entire length of the bed, running west to east, then 2’ north, and then east to west to return to the west wall where it began. This in-bed plumbing is connected to the “hot” and “cold” lines coming from the compost heater through the buried triplewall pipe. This closed loop system is powered by a Taco circulator pump (Photo 22, #4; Photo 23). On December 9, 2016 we worked with certified plumbers to set up and engage the system (Photo 21). Two pumps have been trialed. The first was 1/30 horsepower and was used for half a month (Photo 23). Because the horsepower proved too low to move the water quickly enough, we replaced it on December 23, 2016 with a larger Taco circulator pump of 1/8 horsepower. To charge the system with water, we installed an inlet fitting with a ball valve that Tees into the “hot” line (Photo 22, #2; Photo 24 (top panel)). To remove air from the system as water is introduced, we installed a bleeder valve (Photo 22, # 6; Photo 25), to which we attached a short garden hose (Photo 22, #7). Also, to control the rate of flow and to shut the system down if necessary, we installed a ball valve in the “hot” line just prior to the location of the pump. To measure the heating effect of the hydronic system (not the pile), we attached three strap-on temperature gauges (Photo 26) to the tubing near the pump: one on the “hot” line just upstream of the pump; one on the “hot” line just downstream of the pump; and one on the “cold” line just downstream of the bed and immediately prior to exiting the high tunnel and re-entering the underground triplewall pipe. To further measure the heating effect, we placed 6” probe thermometers in the soil near the irrigation tubing that is buried 3” in the bed (Photo 30). For comparison purposes, we placed the same kind of probe thermometers in an adjacent bed (control bed) that is not connected to a compost heater system.
Impacts and Contributions/Outcomes
Temperature Results in the High Tunnel Bed To date, the buried ¾“ irrigation tubing running the length of a 40’ bed (treatment bed) has no measurable effect on the overall soil temperature, when compared to the adjacent 40’ control bed that contains no such tubing. On most days, the soil temperatures of the two beds were within a few degrees difference. In some cases the control bed had warmer soil than the treatment bed. For both beds, soil temperature was positively correlated with ambient temperature in the high tunnel: on sunny days, soil temperatures rose; on cloudy days, they fell. This high tunnel has an insulated perimeter (foam board to a depth of 3’) and on average has soil temperatures that are 4-5°F warmer than soil in the adjacent high tunnel to its south (see Photo 1). The temperature of water flowing through the irrigation tubing is measured indirectly via the strap-on thermometers described in the previous section; the measurement is “indirect” because they are attached to the outside of the tubing and do not contact the water (Photo 26). These three measurements are regularly taken and recorded. Using the 1/30 horsepower pump, the “cold” return line frequently had readings near freezing (32°F), although the “hot” line regularly ran above 100°F, indicating that in the 40’ garden bed the tubing dissipated about 70°F. To discover how quickly the heat dissipated, we took additional temperatures on December 16. Three measurements were taken on the “cold” return line in the garden bed. These measurements occurred at the east end, midway, and just prior to exiting the bed (to return to the compost heater). The measurements were taken by removing the soil from above the line in those three locations, strapping on a strap-on thermometer, waiting several minutes for the reading to stabilize, and then recording the number. The readings were 58°F (east end), 59°F (midway), and 58°F (west end). The “cold” line outside the bed, however, was 40°F, meaning that nearly a 20°F reduction occurred immediately upon leaving the garden bed. A likely explanation is that this section of the “cold” return line is permanently exposed to the ambient temperatures and thus does not receive the insulating effect of the surrounding soil; thus, an insulator of some kind is needed for the “cold” return line prior to its reentering the buried triplewall tube. On this same day, in contrast to the buried polyethylene tube temperatures of 58°F (midway), adjacent soil temperatures midway on this date averaged 42.5°F. Thus, although hot water is circulating in the hydronic system it has yet to ameliorate adjacent soil temperatures. The temperatures of the “hot” supply line immediately entering the high tunnel varied by pump power. With the 1/30 horsepower pump, these measurements were generally around 50°F. Meanwhile, the readings just downstream of the pump were generally close to 100°F, indicating that the pump itself was warming the water. Because of these results, we replaced the small pump with a 1/8 horsepower pump, to move the water more quickly through the system. With this larger pump, the “hot” line entering the high tunnel was warmer, near 80°F; the temperatures just downstream of the pump tended to be similar, indicating that the 1/8 horsepower pump’s heat is having less of a heating effect. After 2 days with the 1/8 HP circulating pump, we measured the soil temperature at four points where the “hot” supply line first entered the bed, in a series of 1’, 2’, 3’, and 4’ from the start of the bed. The results indicate that the heating system had an effect, but the effect diminished quickly. The results from the “hot” line (80°F when first entering the bed) were soil temperatures of 50.5°F at 1’, 50°F at 2’, 46.4°F at 3’, and 44.2°F at 4’ (shown in Photo 30). Given that plants require a soil temperature of at least 50°F to access soil nutrients, these preliminary measurements indicate either of two conclusions: the hydronic system is insufficient to warm the bed; or, the system needs more time to transfer the heat from the polyethylene tubing to the surrounding soil. In the coming weeks, we will repeat this measurement procedure to learn which conclusion is justified. Evaluation of the Compost Heater System The system can be expensive to build if materials are not sourced for free. In 2015 we sourced free wood chips but in 2016 we were unable to secure the needed quantity of small chips by our construction date so we purchased them from a local company. Delivery of materials (small woodchips and horse manure) and operation of a skid steer cost about $1,000 combined. Cost of hay bales, irrigation tubing, circulation pumps, fittings, connectors, and other materials also cost about $1,000. Granted, some of these materials could be reused for subsequent compost heaters. If most materials had to be purchased for the compost heater, then it might be less expensive to heat a high tunnel with electric heat, from solar or other power sources. To date, the compost heater has not improved the soil temperatures except at the beginning of the bed but we continue to experiment with the hydronic system. The Taco circulator pumps generate significant heat (about 130°F on the surface of the housing over the motor of the 1/8 horsepower pump), and the high tunnel temperatures in the northern Colorado winter consistently fall below freezing at night. Thus, the exposed part of the irrigation tubing, between the wall of the high tunnel and the start of the bed, needs insulation from the cold; yet, this is where the pump is located, and because the pump generates heat, there is a trade-off between heavily insulating the tubing and keeping the pump adequately vented so it does not melt or burn the insulation. This is a constant concern that could be addressed by constructing an insulated pump house. Because the “cold” return line prior to re-entering the buried conduit is typically near freezing, there is concern that the system will lock up, causing the pump to burn out or overheat and cause an electrical fire. There is also a worry that the system will leak, causing the pump to have insufficient water to move, and thereby burn out or overheat. Due to the buried nature of the heater system, there is no way to check for leaks, except near the pump. Charging the system with water was straightforward and easy, except for two complications. First, air had to be removed from the system by a simultaneous method of injecting water into the system at one port and bleeding air off at another port. Although this method worked, and it was obvious when air exited the lines, it was not obvious when all air had been removed. Second, the connections to the pump tended to leak slightly. Stopping these leaks completely was difficult but finally achieved. Shutting down the system was facilitated by having a ball valve in the “hot” line just prior to the pump (Photo 22, #3). The original purpose of this ball valve was to vary the rate at which water flowed through the system; we never used the valve for this purpose because we experienced no conditions warranting a slowing down of the system; if anything, we needed the system to speed up. However, the ball valve did prove valuable when we needed to shut down a pump or replace a pump. By closing this valve, water remained in the 1,100’ irrigation tubing and did not leak out while we manipulated pumps. This valve also proved valuable when we needed to stop the pump for a week when we were out of state. A small amount of water drained from the line downstream of the pump; to ensure that no water exits the system from the “cold” line, we recommend placing a second ball valve just downstream of the pump. Education and outreach In 2016, we have performed educational outreach in five ways. First, the project has been described in three college classrooms at Front Range Community College and Colorado State University. Second, our urban farm website now contains reports of this education and research project (https://sunspoturbanfarm.squarespace.com/research). Third, in relation to construction of the system, we educated those persons who helped with construction and who provided us materials and supplies for construction. Fourth, in casual conversations with friends and acquaintances, we have described the system many times. Finally, we are just beginning to dialogue with other growers who are also experimenting with compost water heater systems and hope to synthesize this conversation in our 2018 report. Project Personnel Rod D. Adams and Amy A. Yackel Adams Sunspot Urban Farm 1008 Sunset Avenue Fort Collins, CO 80521 Website: sunspoturbanfarm.squarespace.com Email: firstname.lastname@example.org Addy Elliot Technical Advisor Soil and Crop Sciences Colorado State University Fort Collins, CO 80523 Email: email@example.com Acknowledgments This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 140867021 through the Western Sustainable Agriculture Research and Education program under sub-award number [FW15-057]. USDA is an equal opportunity employer and service provider. 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. We thank Addy Elliot for valuable project input and the following individuals for their volunteer work in constructing the heater: Natasha Varian, Joel Berger, Matt Jones, Leslie Kraynak, Jeff Davis, and Patrick Gazely. Woodchips were sourced from Jordan’s Tree Moving & Maintenance, Inc and horse manure from Brian Fisher. Local artisan wood turner, Steve Germaine, generously provided barrels of sawdust for the project from his beautifully crafted bowls, pots, and vessels (http://www.stevegermainewoodbowls.com).
Sunspot Urban Farm
1008 Sunset Avenue
Fort Collins, CO 80521
Office Phone: 9705565942
Colorado State University
Soil and Crop Sciences
Fort Collins, CO 80523
Office Phone: (805) 704-8198
1207 Castlerock Drive
Fort Collins, CO 80521