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Can biogas-to-electricity waste heat be captured to help cool cows?

Tamara Scully Published on 31 March 2014

Cornell University graduate students Kristy Perano and Joe Usack

Anaerobic digesters are utilized on dairy farms across the country. Microbial action processes manure or other biodegradable waste products into solid and liquid components along with biogas.

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Digested solid manure components are then used as bedding or compost, and the liquids applied as a natural fertilizer. The resulting biogas is often used to power a generator to produce electricity.

While using captured biogas to power the electric grid is a nice idea, the reality is that only a small amount – about 30 percent – of the biogas energy value generated is actually converted into electricity. The majority is converted to heat, much of which is lost to the environment.

Reclaiming the heat
However, a multi-disciplinary team of Cornell professors and student researchers have combined forces in an attempt to re-capture this waste heat and re-use it in a chilled water system designed to keep cows cool.

The multi-faceted study, funded by New York State Energy Research and Development Authority, with in-kind support from the Cornell Pro-Dairy Program, will determine whether or not the economics of capturing energy, now lost as a byproduct of biogas-to-electricity systems, makes practical sense.

Professors Lars Angenent and Kifle Gebremedhin, along with Curt Gooch, dairy environmental systems engineer, Cornell Pro-Dairy Program, have combined efforts for the study.

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Conducted through the Department of Biological and Environmental Engineering, the study will determine if an absorptive chilling system powered by the re-claimed heat will work efficiently and effectively to keep cows cool and comfortable on the farm, reducing dairy cow heat stress.

“The premise is that excess heat from a biogas-fueled engine generator set can be captured and used to make chilled water using the principle of adsorption chilling,” Gooch says. “Since digesters need less heat in the summer than in winter to operate, and cows need cooling in the summer, the synergy makes sense to investigate.”

Cooling with heat
Using heat to cool things down may not sound like a good idea, but the waste heat can be converted to cold water with commercially available absorption chillers. Absorption chillers are not a new technology. They were developed in the 19th century to cool water using energy provided from a heat source via a thermochemical process.

For the study, graduate students and doctoral candidates Kristy Perano (working under Gebremedhin) and Joe Usack (working under Angenent) partnered together to investigate the potential of capturing and using waste heat to power an absorption chilling system.

In the system, cooled water is circulated through waterbeds. The water being pumped through the cows’ waterbeds allows their body temperature to be reduced via conductive cooling.

In order to convert standard commercial dairy waterbeds to a cooling system, the students cut the beds’ corners, inserted piping and designed a system where the cooled water is continually circulated and maintained at a temperature between 40 and 50ºF.

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The water is chilled via an absorption chilling system. Such a system could potentially be powered by re-captured heat.

Usack studied the amount of energy being lost by a biogas-to-electricity system at Sunnyside Farms in Venice, New York.

Whether the re-captured waste heat from this type of system can power an appropriate-sized cooling system for the milking herd and make the cost of the installation and operation of the system viable for dairy farmers remains to be determined.

“The economics of the whole system is being investigated as part of the overall project,” Gooch says. “That is exactly what we are studying to find out.”

Cow comfort
Gathering data and testing the theory that conductive cooling via a chilled mat system could effectively reduce heat stress was another issue the researchers needed to investigate.

The idea for utilizing cow chilling mats has been in development at Cornell for about 15 years, after waterbeds were released into the marketplace, according to the researchers. Kelley Bastian, a former graduate student of Gebremedhin, had previously studied conductive cooling. Perano developed the concept further for this research study.

In this part of the study, four cows were given standard waterbeds with no chilling, and four cows were given waterbeds with the chilled water circulating through to provide conductive cooling. Freshly cooled water, continually circulated through the system, allowed the chilled mats to keep working, offering ongoing cooling to the cows.

“We investigated two cooling levels and two thermal stress levels, and a control where cows were exposed to similar thermal stress levels but without cooling,” Gebremedhin says. “The control cows had waterbeds but no chilling. Everything else was the same. The experimental tests provided several comparisons.”

Kristy Perano monitors a cow’s heat stress level

Cows begin to feel heat stress, which can impact milk production, fertility rates and cow health, beginning around 70ºF. High levels of humidity heighten the heat stress response. Lower humidity is more tolerable, and heat stress isn’t seen until temperature are roughly around 75ºF if the percent of relative humidity remains low.

Multiple times each day, Perano and Usack monitored the animals’ respiratory rates, body temperatures, sweating rates and skin temperatures. This data demonstrated how the chilled mats affect the cows’ level of heat stress.

Another issue considered was the reaction of the cows to a reduced-bedding system as opposed to a generously bedded stall. A reduction in bedding is needed to allow the heat transfer from the cow to the chilled waterbed to occur, researchers explain.

“The study was performed in a controlled research setting. Subsequent work needs to be done to test and demonstrate results on commercial-scale systems,” Gooch says.

According to Gebremedhin, initial data from the cow response study “look promising.” The study of the cows’ responses to the chilled water system was completed over the summer of 2013.

The heat re-capture and absorption chiller portion of the study is ongoing. The feasibility of recapturing heat from biogas-to-electricity production and using it to effectively power a chilled mat system with the capacity to reduce dairy cow heat stress remains to be seen. PD

Tamara Scully, a freelance writer based in northwestern New Jersey, specializes in agricultural and food systems topics.

PHOTOS
TOP: Cornell University graduate students Kristy Perano and Joe Usack are researching the potential to cool cows via waterbeds, using waste heat from an anaerobic digester to power an absorption chilling system.

BOTTOM: Perano monitors a cow’s heat stress level. Further research is needed to determine if this system can effectively and economically cool cows. Photos courtesy of Cornell University.

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