“Developing Automated, Energy Efficient Solar Heating for Biogas Digesters”

Project Final Report, 11/22/16

Project Director: Matt Steiman, College Farm

Student researchers: Emily Whitaker ’17 and Sean Jones ‘17

Summary: This project involved implementation of innovative solar heating systems for warming a new

biogas digester at the Dickinson College Farm. The success of this project was mixed. Major progress towards

the stated objectives was achieved throughout the summer research period. A handful of unforeseen roadblocks

and challenges impacted our ability to complete the project proposal’s stated goals and forced us to divert our

efforts into related work. The student researchers performed admirably at hands-on work, data analysis, and

outreach for the project. Advancements achieved through the summer work successfully increased sustainable

biogas production at the Dickinson farm and helped to increase notoriety of small scale renewable energy

education activities at the College.

Project goals (paraphrased from proposal):

Construction of second solar air heater to heat biodigester unit 1.

Connect air heaters to biodigester unit 1 via HVAC ductwork and fan system

Construction of biodigester unit 2, including solar water heating system

Automated controls for both heating systems

Investigate effectiveness of heating systems

Presentation of results / poster display

Roadblocks, challenges and resolutions: The following unforeseen factors impacted our ability to achieve all

stated goals and resulted in some diversion into related work:

Township permitting issues: Early this summer permitting disputes between the College and South Middleton

Township regarding farm construction projects came to a head, culminating in Township officials sending a

“cease and desist” letter to top College administrators. This action resulted in a freeze on all building activities

at the farm until matters could be smoothed over. Whether or not the farm biofuels projects would be rejected

by the Township was in question, so construction of our biodigester unit 2 which was planned for July had to be

shelved temporarily. Thankfully permitting disputes were settled (including a productive visit to the biofuels

site by local fire officials) in late July, but the delay did not allow enough time for custom fabrication of critical

pieces of the biogas system before the end of summer research and the beginning of the farm’s fall harvest

season. The custom pieces were ordered following the Township settlement, and construction of unit 2 has

been in progress since October. We anticipate completion in February of 2017. In the interim, based on critical

feedback from Tim Richwine in the College HVAC department, as well as further discussion with Hans Pfister,

we’ve decided to heat unit 2 with biogas-fired hot air routed through an improved delivery system, rather than

the solar hot water set up originally proposed. Trials heating with combusted biogas will commence in January.

Assessing heating effectiveness in summer: It proved to be difficult to study the effectiveness of solar air

heating of the biodigester in the summer months due to the ambient air temperature being very close to ideal

digester temperature, especially in the greenhouse. Our target temperature range for efficient digester

operations is 32 to 40 degrees C (90-104 F), and digester unit 1 was able to maintain that temperature in

summer without any supplemental heating, due to the effect of the greenhouse where it is stationed. With

minimal difference in temperature between the digester and the ambient air in the greenhouse, there was little

heat loss from the system and so efforts to measure the effectiveness of the solar heating components were

inconclusive. We did attempt to cool the digester by installing shade cloth on the greenhouse and removing all

inner insulating layers, but the high temperatures of summer prevailed. Thus meaningful data collection was

delayed until the colder weather of fall set in. Data collected throughout November were used in the analysis

provided below.

Technical equipment challenges: Our Vernier LabPro data logger failed mid season due to the hot moist

environment of the biodigester greenhouse. We’ve settled on Onset HOBO dataloggers as the more robust

option for field data collection – this device was used for the November data set and will be our chosen brand

going forward. Mice foiled some datalogging efforts by eating through temperature probe wires – we rectified

this by trapping and baiting with organic rodent baits. The automation system we chose for complex system

controls had a critical hardware error that we were unable to resolve even with persistent efforts by the student

researchers. Despite failure of the computerized system the students gained a working familiarity with machine

control language and system interfacing. We eventually settled on a simpler mechanical system which is

successfully controlling the solar air at present – automation of the solar air heating system was achieved, just

not in the form we had originally envisioned. Through all of these experiences the student researchers learned

valuable lessons in the bumpy road of real world project implementation.

Notable Achievements: Student researchers were busy daily with a full schedule of work, the scope of which

included heating system design and implementation, biogas processing and measurement, problem solving, data

collection and analysis, and outreach to student and public audiences. The following are noteworthy

achievements related to the summer research proposal.

Full implementation of solar heating for biodigester unit 1: A second solar air heater was constructed to

match the first one that the students had built during the spring semester. With guidance from campus HVAC

technician Tim Richwine, the two solar air heaters were connected in parallel and their outputs fed into a

network of steel pipes located beneath the first biodigester. After several attempts to control the heating system

with complex computer driven equipment, we achieved successful thermostatic control with a simpler

mechanical unit – the fan drawing air through the solar heaters now kicks on any time the air heaters rise about

90 degrees F, delivering hot air to the insulated space beneath the digester body.

Gas monitoring and cleaning equipment: In order to make any meaningful analysis of the biogas system

performance, we needed an accurate way to measure gas production over time. Prior to this summer gas

produced was measured via a crude system of estimation that was prone to wide margins of error and lost data.

After we secured an appropriately scaled natural gas metering device, the student researchers used scrap

materials present on the farm to build components to clean up the raw biogas prior to measurement. Raw

biogas contains high humidity and hydrogen sulfide, the combination of which would foul our gas meter over

time. A gas scrubber packed with “iron sponge” material was built to reduce hydrogen sulfide content, and a

series of condensation traps were installed upstream of the measuring device for daily manual removal of excess

water from the gas. Since mid-June all biogas produced by unit 1 has been accurately counted by the new gas

meter. Students and staff operating the digester record time stamped data points on a daily basis. Daily biogas

production averaged 3.2 cubic meters of biogas per day throughout the summer, which would result in about 6

kilowatt hours per day if converted to electricity in an engine generator.

Heater data collection and analysis: Solar air heater performance data collection activities began in summer,

but the most valuable data set was generated this fall after outside temperatures dropped into the freezing range.

A representative sample of solar heating during a cold period is depicted below in figure 1. During this

period, we compared the daily temperature change of the digester on sunny days - air heating system turned on

with two days of similar weather - the air heaters turned off. This was done in an attempt to assess the baseline

effect of the passive solar greenhouses surrounding the digester. The data show a net increase in digester

temperature (Td) of 1.8 degrees C over three sunny days with the air heater on, and a net decrease of 0.2

degrees C over two sunny days with the air heater off. Approximating the volume of the digester at 1000

gallons (3780 liters) of water, these temperature fluctuations represent a 3 day energy gain of about 27000

BTUs (6800 Kilocalories) with the air heater on, and a loss of 3000 BTUs (750 Kcal) with the air heater off.

Figure 1: Biodigester unit 1 solar air heating system performance, 11/2 to 11/10 2016. Red line represents

hot air fed into the digester pipe matrix from the air heaters, blue line represents cooler air leaving the pipe

matrix after passing its heat to the digester, and green line depicts the exterior temperature of the digester skin

at an insulated location. Peaks on days 1, 3,4,5 and 9 represent sunny days with the air heater fan set to

automatic. Day two was a cloudy day, air heater on auto. Days 6 and 7 were sunny days with the air heating

system turned off, day 8 was cloudy, heater turned off. Notice the gradual rise in digester temperature over the

sunny period with the air heater activated from days 1 to 5, and the subsequent decline on days 6 to 8 with the

air heater deactivated.

When the student researchers combined mass airflow readings taken from the air heater ductwork with the net

temperature drop of the heated air across the digester system, they derived an average value for solar air heater

input to the digester of 6814 BTU per hour and 40,885 BTU per day. (For comparison, an average electric

space heater produces about 4910 BTU per hour). So we can conclude that the solar air heaters are making a

meaningful contribution of heat to the digester system. However, when we compare the estimated daily solar

heat contribution of 40,885 BTU with the actual net energy increase of 27,000 BTUs over a sunny three-day

period, we can conclude that significant energy is being lost from the digester system due to convection,

conduction and radiation losses to the cooler environment. Thus while the efficiency of the solar air heaters

themselves is very high, the efficiency of the overall air heater / digester complex is estimated at only 30% at

present. This analysis is backed up by a second data logging run over a colder 9 day period in mid-November,

where we see a net drop in digester temperature of 1.5 degrees C despite the air heaters being activated and

many sunny days (See figure 2).

Air heater off

Cloudy day

Digester temp

Figure 2: Biodigester unit 1 solar air heating system performance, 11/11 to 11/20/16. The gradual decline of

the digester temperature (green line) by 1.5 degrees C despite relatively sunny weather over a nine-day period

indicates substantial heat loss to the environment. The outdoor low temperature was near or below freezing

each night during this interval.

We conclude that given the present level of insulation throughout the whole system, the solar air heaters alone

will not contribute enough energy to maintain the biodigester at an ideal operating temperature of about 32 C.

The system is working – at approximately 18 C the digester is still converting foodwaste to biogas as of

November 22, and we’ve never maintained active temperatures this late in the season over four years operating

the previous (unheated) biogas project. It’s just not working well enough. To maintain robust biogas

generation throughout the winter months we will either need to add a second source of energy input, more

insulation, or both. There are many possible points of energy leakage: to the soil and air around the digester,

from the heating pipe manifold before it reaches the digester, and around some exterior ductwork from the air

heater to the greenhouse. We will be tightening up these points of energy loss where possible. A recent visit

from Professor Ben Edwards revealed that the Earth Sciences department possesses an infrared imaging camera

– we hope to make use of this device over the winter to help us visually pinpoint the highest priority locations

for improving insulation. The data referenced above have informed our decision making for construction of

biodigester unit 2 – we are being as fastidious as possible at installing the in-ground insulation, and we chose to

purchase new insulation materials for the second digester, in contrast to recycling used insulation scrapped from

the theater scene shop for unit 1 (See figure 3).

Digester temp, day 0:

18.7 C Digester temp day 0:

18.1 C

Digester temp day 9:

16.6 C

Bucknell University assessment: During the summer research period, we partnered with Dr. Tom DiStefano

of Bucknell University’s Department of Civil and Environmental Engineering and his summer research

assistant Kimberly Shust to assess the effectiveness of our digester system. Thanks to the networking activities

of our project collaborator and promoter “Biogas” Bob Hamburg (a Bucknell alum) we secured Dr. DiStefano’s

interest in studying our particular digester, which uses a unique endcap sealing system of Hamburg’s design.

On his initial visit, Dr. DiStefano helped us calculate the appropriate feeding rate for our system, and convinced

us to feed the digester solely with ground food waste from the Dickinson cafeteria for the summer research

period. (Previously we preferred a blend of manure and food waste, but manure collection from the pastures is

cumbersome, and Bucknell has an interest in development of a food waste digester for their campus.) At each

feeding three times per week, our student researchers collected samples of everything coming into and out of

the digester (food waste, effluent, gas, and recirculation liquid) and shipped these up to Bucknell for analysis.

The conclusions of the study proved favorable: our digester system is effectively converting feedstocks to

biogas at a healthy rate, our system is producing high quality biogas (65% methane), and our system maintained

a favorable pH and internal buffering despite being fed strictly food scraps for a three month period. From 50

kilograms of food scraps we generated 7.5 cubic meters of biogas on average. The Bucknell team also

corroborated our previous calculations regarding potential energy yield of the college food waste resource: If

we convert all of the campus food scraps to biogas before composting, and burn this biogas in an electric

generator, we will generate an estimated 25,000 kilowatt hours of electricity – enough to negate the farm’s

power bill completely. The Bucknell University paper draft is attached as an addendum to this report.

Figure 3: In 2015 for digester unit 1 (left) we

installed pieces of used insulation recovered from

the College scene shop following a theater

performance. In 2016 we purchased new sheets of

insulation for digester unit 2 (right) in an attempt to

reduce possible heat losses to the ground. Unit 2

will utilize a floor of tightly stacked concrete

blocks for a broader hot air passageway with

increased thermal mass. New custom endcap for

unit 2 is also pictured.

Gas utilization: During down time between heating system research activities, we put the student team to work

solving inefficiencies in the overall biogas project. Chief among these was developing an effective way to

shuttle biogas from the point of production to the points of use. The biogas production of biodigester unit 1 was

so great early in the season that we sometimes needed to flare the gas rather than release unburnt methane to the

environment. Sean and Emily rectified this problem by building several large gas storage bags and lightweight

hand carts for moving the gas bags around. A 1.8

cubic meter gas bag (pictured below, on cart) will

power a single burner stove for 2 to 3 hours. By

outfitting the farm’s packing house, intern kitchen,

and farmer residence with gas stoves and the mobile

gas bags, we were able to utilize all of the gas surplus

for cooking needs. This substantially cut down the

amount of fossil energy (propane or electricity) the

farm purchased for cooking. Over the course of the

summer and fall, biodigester unit 1 produced almost

300 cubic meters of biogas, which if converted to

electricity would power the farm house for one

month.

Figure 4: Sean Jones with gas cart of his design.

Outreach presentations: This project resulted in several notable outreach presentations summarized below:

July: Students and staff host biogas field day for farmers in conjunction with PASA, Solar CITIES, and

Omega Alpha Recycling Systems. 30 paid attendees from several states

August: Pre-orientation workshop on biofuels hosted by student researchers

September: Biogas booth at Farm Frolics event staffed by student researchers

October: Physics Department presentation with poster by student researchers

October: Case Study presentation on Small Scale Biogas for Education at Association for Advancement

of Sustainability in Higher Education conference in Baltimore, co-presented by Matt and student team.

November: Guest lecture for environmental engineering students at Virginia Tech (Matt and Bob

Hamburg presenting).

February 2017, Small scale biogas presentation at PASA annual conference by Bob Hamburg, possibly

assisted by Matt.

Conclusion: Despite a few major roadblocks we are pleased with the success of this project overall. The

heating systems are still a work in progress – for every question we answer, a new one arises. However the

overall biogas system is functioning better and more efficiently than any previous project, and we are optimistic

that with persistence, adjustments and improved insulation we will achieve the goal of year-round biogas

production. Our collective understanding of small scale biogas system functionality expanded considerably

over the project period. The student researchers had many rewarding experiences and seemed quite genuine

when they reported at the AASHE conference that the project gave them a unique opportunity to use their

academic discipline for hands-on practical work outside the classroom.

Acknowledgements: Gratitude is due to the following parties:

The Center for Sustainability Education and Lindsey Lyons in particular for financial support and

patience with the busy and sometimes scattered farm team.

Ken Shultes for financial support and for negotiating the Township approval in a timely manner.

“Biogas” Bob Hamburg for physical efforts, technical guidance, and financial support

Hans Pfister for technical support, the solar air heater design, and providing two fantastic students.

Tom DiStefano and Kim Shust at Bucknell University for collaboration and tech support.

Biodigester Unit 1 (in greenhouse) and the Solar Air Heaters

Emily Whitaker ’17 and Sean

Jones ’17 at the AASHE

National Conference, Baltimore

October 2016