Source: ADVANCED COOLING TECHNOLOGIES, INC. submitted to
THERMAL REGULATION WITH SALT HYDRATES FOR BIODIGESTER ISOTHERMALITY
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
COMPLETE
Funding Source
SMALL BUSINESS GRANT
Reporting Frequency
Annual
Accession No.
1025901
Grant No.
2021-33530-34721
Cumulative Award Amt.
$99,988.00
Proposal No.
2021-00903
Multistate No.
(N/A)
Project Start Date
Jul 1, 2021
Project End Date
Jul 31, 2022
Grant Year
2021
Program Code
[8.12]- Small and Mid-Size Farms
Project Director
Charles, J.
Recipient Organization
ADVANCED COOLING TECHNOLOGIES, INC.
1046 NEW HOLLAND AVE
LANCASTER,PA 17601
Performing Department
Advanced Cooling Technologies
Non Technical Summary
To solve the digester thermal control problem, Advanced Cooling Technologies (ACT) is developing an energy storage material that can be wrapped around a digester. This material will absorb thermal energy from the sun during the day and release it to the digestate at night. The energy storage materials are very inexpensive but there are some challenges related to their long-term use. Fortunately, the project team has successfully solved these issues during past projects. Laboratory testing will be used to demonstrate the life of the developed materials followed by testing on a small biodigester for several months. By the end of the project, ACT hopes to demonstrate a low-cost, small-scale, biodigester which will allow many more small farms to take advantage of this renewable energy source. This will accelerate the transition to renewable energy sources, helping to minimize the ecological damage created by our current energy generation mix.?
Animal Health Component
20%
Research Effort Categories
Basic
0%
Applied
20%
Developmental
80%
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
40353702020100%
Knowledge Area
403 - Waste Disposal, Recycling, and Reuse;

Subject Of Investigation
5370 - Sewage and waste disposal facilities and systems;

Field Of Science
2020 - Engineering;
Goals / Objectives
The goal of this Phase 1 project is to increase the biogas production potential of small-scale anaerobic digesters (AD) without significant increases in system cost and/or complexity. If successful, this technology has the potential to make AD technology economically viable for small farms, increasing utilization of this renewable energy source. The technical goal of this project is to develop and successfully test a working small-scale anaerobic digester (AD) with passive thermal control across multiple seasons - especially the winter. AD bacteria require a stable, relatively warm digestate temperature to maximize their biogas generation rate. This is a challenge in small ADs where rapid ambient temperature changes and cold wintertime temperatures can easily deactivate the efficacy of the bacteria. This project will fix this problem by adding a low-cost phase change material (PCM) around the AD, which will act as a thermal buffer to rapid temperature changes. The PCM also serves as a thermal energy store that can absorb solar thermal energy during the day and release it into the AD throughout the night, forming a passive solar heating system that can maintain the digestate temperature throughout the winter. The development and performance demonstration of this passively heated and temperature-controlled AD is the primary goal of the project.The following objectives present a pathway to achieving the Phase 1 goal:Selection and thermal reliability testing of a hydrated salt PCM. A PCM that demonstrates a pathway to 25+ year performance through accelerated freeze/thaw tests is targeted. The PCM has a material cost target of ~$0.1/kg.Model, design, and fabricate a sub-scale proof-of-concept AD utilizing the selected PCM for passive thermal control. A system design that can be easily fabricated from inexpensive and widely available components is targeted.Successfully test the sub-scale AD with PCM thermal control and simulated solar heating over several months during the coldest seasons of the year. A daily digestate temperature variation of no more than ±2°C is targeted during passive solar heating of the digestor with integrated PCM thermal storage.Prepare an economic assessment of the proposed AD with passive solar heating and PCM thermal control. If a pathway towards the LCOG goal of $0.30/m3 is not realized in the Phase I prototype system, variations to the current design will be proposed for additional cost reductions to be examined experimentally during Phase II.
Project Methods
Four technical tasks make up the work plan for this Phase I project. These tasks surround selection and testing of the PCM, design and fabrication of the sub-scale AD, testing of the sub-scale AD, and an evaluation of AD performance followed by a cost analysis. A description of the task methods and success criteria is listed below.1. Selection and Testing of the PCMThe first step is the selection of an appropriate PCM with good long-term thermal stability and a low material cost. ACT has experience developing stable, low-cost hydrated salt PCMs, including a CaCl2·4H2O-based PCM which is a good fit for AD thermal regulation. A thorough literature review of additional low-cost PCMs will be also completed. The PCM for the AD system will be down-selected on their match with the following criteria:A material cost of less than $0.1/kgMelt temperature between 35 and 45°CLatent heat of fusion greater than 100kJ/kgSubcooling of less than 2°CA demonstrated pathway towards 25+ years of thermal stabilityOnce a suitable candidate PCM is selected it will first be characterized using differential scanning calorimetry (DSC). DSC can quickly measure both the melt temperature and latent heat of fusion of a 5-50mg PCM sample. Properties such as subcooling cannot be accurately measured using DSC due to the small sample size. For this, a drop calorimeter will be constructed to allow for testing of PCM samples between 50 and 100g. Drop calorimeter is a method previously used by ACT for simultaneous measurements of the melt temperature, latent heat of fusion, and subcooling of relatively large (50-200g) PCM samples. Both the DSC and calorimetry testing will be used to confirm the PCM meets the specified thermal criteria.2. Design and Fabrication of the Subscale Prototype DigesterA sub-scale prototype digestor with PCM thermal control and simulated solar heating will be designed using inexpensive, readily available components. To evaluate the system design, it will be modeled using a first-principles-based thermodynamics and heat transfer analysis. This model will be used to confirm that the proposed system can meet the desired design objectives and for future comparison with the experimental results provided by the prototype system. The sub-scale prototype will be designed to meet the following nominal operational targets:~275 gallon AD volumePCM mass capable of storing 12-24 hours of required heat with a digestate-to-ambient temperature difference of ~40°C.The AD will be designed around PVC and polyethylene components, which help minimize fabrication and materials costs while eliminating corrosion concerns for materials in prolonged contact with the hydrated salt PCM.A water heating system capable of supplying 100% of the thermal energy needs of the biodigester. This heating system will be operated such that it simulates the operation of a passive solar heating system. Heated water will be supplied to the PCM during the day with the water heating system turned off at night, allowing stored thermal energy in the PCM to maintain the digestate temperature.Temperature probes will be integrated into the test digestor to monitor PCM, water, and digestate temperatures throughout the test program.3. Testing of the Subscale Prototype DigesterThe completed subscale digester will be experimentally tested at the SolarCITIES homestead farm in Glenmoore, PA. The AD will be tested using locally-available influent and the following test objectives and methods are targeted:2-3 months of continuous testing, ideally with a portion of this testing falling within the coldest seasons of the year.The system will be filled with influent at a rate commensurate to its volume with the system level and solids percentage maintained at their optimal point throughout the test period.Temperature measurements of the digestate, PCM, heating water, and ambient air will be taken at a ~1 hour or less interval.4. Prototype Performance Evaluation and Cost AnalysisTest data from the subscale prototype digester will be analyzed by ACT to see how it compares to the performance predicted by the system thermal model. In addition to the thermal performance evaluation, a cost assessment of the prototype system will be conducted to inform the economic potential of PCM-regulated ADs for small farms. Key performance and economic targets for this analysis are as follows:Less than a ±2°C daily temperature variation of the digestate despite large daily temperature changes and differences between the digestate and ambient temperatures.Less than a ±2°C temperature deviation from the optimal digestate temperature for maximum gas production (~40°C) even at low ambient temperatures.High-temperature uniformity within the entire digestate tank with spatial variability of less than ±2°C.No measurable decrease in the thermal stability of the PCM throughout the test period.A complete economic assessment of the levelized cost of gas (LCOG) for the prototype AD with a proposed pathway towards an LCOG of less than $0.30/m3.An updated system design based on the thermal and economic findings from the sub-scale prototype, which is capable of achieving the LCOG target outlined above.

Progress 07/01/21 to 07/31/22

Outputs
Target Audience:The target audience for the PCM-enhanced biodigester is small to medium-sized farms or community groups. We have worked closely with SolarCITIES, who is a leading non-profit in the biodigester space. We have completed our test program at their homestead farm test site and are working with them to show off our technology to a broader audience. Changes/Problems:Several challenges were met during the Phase I program, with two key challenges bearing special consideration. Despite these challenges no key changes were made to the Phase I project plan or objectives due to these challenges. 1. PCM Containment Leakage Leaks were found in a few PCM bags after installation. It was determined that this was due to tears caused during transport. The bags were patched using tape, but as a result, ACT has determined that a more durable bag material should be used in the future. How this will be addressed in the future: During the testing and operation of the subscale PCM digester, it was noted that the fabrication and assembly of the PCM bags was an area for improvement for future iterations of this technology. The heat-sealable bags were not very durable for transport, and the size of the bags resulted in more PCM being used than was analytically determined to be necessary. As such, ACT sees an opportunity to develop robust, modular packaging for the PCM to be applied to existing biodigesters.As a ready-made option, ACT sees the thermal PCM solution being adopted into a dual-walled tank system. ACT will seek to work with a plastics manufacturer to develop the form of the dual-walled tank for fabrication. A dual-walled digester tank would be more robust than the modular PCM packages, as the PCM layer would be built into the system's walls. This product would be available as an off-the-shelf option for farms in cold-weather climates. 2. Selection of a PCM with a lower melting point Looking at the test data of the PCM-enhanced biodigester, the average temperatures of the PCM for each wall of the prototype digester rarely reach the PCM's melt temperature of ~40 ºC. From observations made during the field tests, it was noted that a small portion of the PCM would melt, typically in the PCM bags towards the top of each wall, but otherwise, the PCM remained in its solid state. While the use of the PCM has been shown to benefit the system, if the PCM does not appreciably melt, then the benefits of the PCM's thermal storage are not fully realized. How this will be addressed in the future: ACT suggests replacing the existing CaCl2·4H2O PCM withCaCl2·6H2O. The hexahydrate (6H2O) iteration of the calcium chloride (CaCl2)-based PCM has a lower melt temperature (~25 ºC) and has been fully characterized for stability by ACT. Given the scope of the Phase I effort, the CaCl2·4H2O could not be fully characterized, and as such, ACT cannot comment on its long-term performance stability. What opportunities for training and professional development has the project provided?ACT's core strengths include designing and fabricating advanced cooling systems, including heat pipes, HiK plates, two-phase cooling, and advanced heat exchangers with internal thermal storage. Phase change materials play a key role in ACT's core technologies, and have a strong interest in research and development in this area. This project provides ACT an opportunity to further develop advanced phase change materials that can benefit the targeted bio-energies market. The project has allowed one engineer and technician to develop their skills in the related work. The broader heat transfer and renewable energies communities benefit from this long-term program through journal and conference publications to disseminate impactful and innovative progress. There are plans to attend the International Biomass Conference & Expo and other reputable biomass conferences. Future publications will be developed based on the thermal performance test results. How have the results been disseminated to communities of interest?Key program details have been shared with the power and energy community. A good example of this is the online article by our program manager Elizabeth Seber in Power Magazine Online (https://www.powermag.com/small-scale-biodigesters-aregetting- a-big-improvement-in-thermal-control/). In addition, SolarCITIES has been working with their connections in the biodigester community to raise awareness of our PCM-enhanced biodigester technology and several stakeholders have expressed interest in it. In March 2023 Elizabeth Seber (ACT) will be presenting the Phase I program findings at thethe 2023 International Biomass Conference & Expo in Atlanta, Georgia, USA. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? The following key accomplishments were achieved during the first year of the program, which fulfills the Phase I goals: 1. A suitable hydrated salt PCM was selected with a price point of about $0.1/kg. This calcium chloride-based hydrated salt PCM is known to be stable during repeated freeze/thaw cycles and its thermal properties were verified using differential scanning calorimetry and drop calorimetry during Phase I. 2. A small-scale biodigester was designed with integrated PCM encapsulation bags circling the digestate tank. This digester was based on existing small-scale biodigesters fabricated by SolarCITIES and is very low-cost and simple to manufacture. The team fabricated the biodigester and fitted the PCM thermal energy storage to it along with an insulating greenhouse to enable passive solar heating of the PCM. 3. The PCM-enhanced biodigester was transported to the SolarCITIES test site, instrumented, and filled with digestate containing an existing bacterial colony. Despite beginning operation in February, the PCM biodigester began producing usable gas within a matter of weeks and by May was outproducing an adjacent control digester. Test data showed that when ambient temperatures fell the PCM-enhanced digester generated more gas than the control. At times the PCM digester generated 60% more gas than the control. Over a 5-day period, the prototype digester was found to maintainhigher internal temperatures than the control (not PCM-enhanced) digester. Despite large temperature deviations from day to night, the prototype digester has better temperature stability when compared to the control digester. In other words, the day-to-night digestate temperature swings are less. To take advantage of passive solar, a greenhouse placed around the prototype digester keeps the ambient air around the prototype warmer than the control. The greenhouse air is warmer than the ambient air by between 5°C at night and up to 10-15°C during the day. While the prototype's increased temperature stability could be attributed to the elevated air temperatures within the prototype digester greenhouse, it should be noted that the daily air temperature variation within the greenhouse is greater than for the exterior ambient air. In other words, the prototype digester is experiencing a greater ambient temperature swing than the control digester. Despite this, the prototype digester maintains a much more stable internal temperature. This is attributed to the use of the PCM insulation, which acts as thermal storage to keep the prototype digestate at its elevated temperature throughout the colder nights. 4.To determine the cost-benefit of the prototype PCM digester compared to the control digester, the improvement in biogas production was first determined. The percent difference in gas production between the prototype and control digesters showed that at higher ambient temperatures, each digester produced comparable amounts of gas, while at lower temperatures, the prototype digester produced more gas. This result is especially apparentat lower ambient temperatures (10-20 ºC) where the prototype digester produced substantially more biogas than the control digester. Since data was only gathered from May to July 2022, a curve fit of the data was used to extrapolate the performance of the prototype digester across an entire year. Using historical temperature data from Lititz, PA (a geographically close location to the Glenmoore, PA testing site), the digester's predicted excess gas production throughout the year was predicted. This prediction suggests that ACT's prototype digester would have produced around 10,473 L/yr more biogas than the control digester. At an estimated cost of $9.76 per thousand cubic feet as of August 2022 [10], assuming all biogas produced is sold at a 1:1 equivalent to natural gas, the prototype digester would only result in an additional revenue of $3.61 in 2021. It is important to note that the digester was operated and fed as needed by the homestead farm, where field testing was conducted. As both the prototype and control digesters were fed, they increased their gas production in the days following feeding. Janice Kelsey of Solar CITIES expressed that the digesters would produce enough gas for daily cooking if fed with the previous day's food waste. In this regard, while a $3.61 increase in revenue may not seem like much improvement, the prototype digester continues producing gas well into the winter months when the control digester has shut down, providing valuable biogas for on-farm use. In addition, neither the control nor prototype biodigester were operating at their maximum gas production capacity due to a lack of available feedstock. It is expected that at peak operation, the difference in gas production between the digesters would be greater at cooler temperatures. This gas production rate would scale as the digester size scales.

Publications

  • Type: Conference Papers and Presentations Status: Awaiting Publication Year Published: 2023 Citation: Elizabeth Seber, Josh Charles. "Development of Thermal Regulation for Biodigester Isothermality." 2023 International Biomass Conference & Expo, Atlanta, GA, Feb 28-Mar 2, 2023.


Progress 07/01/21 to 06/30/22

Outputs
Target Audience:We have worked closely with SolarCITIES, who is a leading non-profit in the biodigester space. We have completed our test program at their homestead farm test site and are working with them to show off our technology to a broader audience. Changes/Problems:Leaks were found in a few of the PCM bags after installation. It was determined that this was due to tears caused during transport. The bags were easily patched using tape, but as a result, ACT has determined that in the future, a more durable bag material should be used. What opportunities for training and professional development has the project provided? Nothing Reported How have the results been disseminated to communities of interest?Key program details have been shared with the power and energy community. A good example of this is the online article by our program manager Elizabeth Seber in Power Magazine Online (https://www.powermag.com/small-scale-biodigesters-are-getting-a-big-improvement-in-thermal-control/). In addition, SolarCITIES has been working with their connections in the biodigester community to raise awareness of our PCM-enhanced biodigester technology and several stakeholders have expressed interest in it. What do you plan to do during the next reporting period to accomplish the goals?During the final month of the program (July 2022), both the PCM-enhanced and control biodigesters continued operation withtemperature and gas production temperatures recorded for both digesters. This data will be used to develop future PCM-enhanced biodigester designs and validate the effectiveness of the technology.

Impacts
What was accomplished under these goals? The following key accomplishments were achieved during the first year of the program, which fulfills the Phase I goals: 1. A suitable hydrated salt PCM was selected with a price point of about $0.1/kg. This calcium chloride-based hydrated salt PCM is known to be stable during repeated freeze/thaw cycles and its thermal properties were verified using differential scanning calorimetry and drop calorimetry during Phase I. 2. A small-scale biodigester was designed with integrated PCM encapsulation bags circling the digestate tank. This digester was based on existing small-scale biodigesters fabricated by SolarCITIES and is very low cost and simple to manufacture. The team fabricated the biodigester and fitted the PCM thermal energy storage to it along with an insulating greenhouse to enable passive solar heating of the PCM. 3. The PCM-enhanced biodigester was transported to the SolarCITIES test site, instrumented, and filled with digestate containing an existing bacterial colony. Despite beginning operation in February, the PCM biodigester began producing usable gas within a matter of weeks and by May was outproducing an adjacent control digester. Test data showed that when ambient temperatures fellthe PCM-enhanced digester generated more gas than the control. At times the PCM digester generated 60% more gas than the control.

Publications

  • Type: Other Status: Published Year Published: 2022 Citation: Elizabeth Seber. "Small-Scale Biodigesters Are Getting a Big Improvement in Thermal Control." Published in Power, Aug 2022,