U.S. Secretary of Agriculture Tom Vilsack declared a goal in 2011 of bringing one new manure-based anaerobic digestion (AD) system on-line per week; however, there are some real challenges that need to be overcome to make this goal a reality. To help achieve this goal, the economics of farm-based AD systems need to be improved. Making better use of AD surplus heat (mostly released to the ambient atmosphere), improving generator efficiency, and electricity use are real opportunity that needs to be flushed out. Greenhouses in the Northeast and similar climates are energy-intensive and therefore an excellent candidate to partner with AD systems to utilize their engine waste heat and electrical energy.
Heat and electricity represent a major expense for greenhouse growers (on the order of $10 to $20 per square foot of greenhouse space annually, when supplemental lighting is installed and used). Greenhouses can make use of the excess heat from AD to provide the necessary growing temperature for year-round production. Excess electricity from biogas-fueled combined heat and power (CHP) systems can be used to power supplemental lighting systems and ventilate greenhouse during warm weather, which are necessary to keep production constant throughout the year.
To aid in the development of digester/greenhouse synergies, we have undertaken a project to develop a series of user friendly computer programs. These programs allow users to develop estimates of heat and electricity production/demand from both existing and proposed digester and greenhouse systems, which in turn allows them to make informed decisions about sizing and the economics of such synergies.
To validate the models used in the digester and greenhouse computer programs we installed heat and power monitoring equipment on 4 dairy digester sytems and 2 commercial greenhouses. Data from these farms was compared to predictions from the models.
The first computer program predicts the magnitude and timing of heat and electricity production from a dairy anaerobic digester based on farm and digester physical and operational characteristics. This program can provide a user with an estimate of heat and electricity available for on farm operations, or as a basis for determining the feasibility of situating a greenhouse operation nearby.
The second program predicts the magnitude and timing of the heat and electricity demand of a commercial greenhouse based on the size and location of the greenhouse, as well as operational and management characteristics.
The third program combines the two first programs into an AD/GH model, complex enough to handle varying biomass inputs to the digester and required outputs (to operate the digester and associated greenhouse). The completed AD/GH model will allow the development and examination of different operational strategies by: 1) varying the feedstock of the digester to boost biogas production (perhaps necessary during high demand seasons), 2) varying the greenhouse production cycle to maintain a consistent level of production year-round through temperature variation, and supplemental lighting. Developing and testing of operational strategies though modeling will increase the applicability of the concept to a larger range of operation (digester and/or greenhouse) sizes.
Preliminary runs of the programs have shown that one of our participating dairy operations (with approximately 4,000 cows) and no co-digestion, produces enough surplus heat to support a 10,000 square foot greenhouse, capable of producing 2,000 heads of lettuce per day. This greenhouse is approximately 60% larger than one of the commercial greenhouses we monitored.
However, other considerations such as reliability of the heat and power availability will need to be taken into consideration when sizing and designing a greenhouse to operate using electricity and waste heat from a digester system.
This project seeks only to provide a tool to greenhouse users and developers to help them make informed decisions about locating and designing anaerobic digesters, greenhouses and their synergies. The viability of greenhouse operations also greatly depends on the marketing and operational costs (labor and non-energy production costs) which would need to be closely considered before making a decision about the value of reduced heating and electricity costs.
We are in the final year of the project, and the focus now is on outreach and extension activities. We will use the program to develop several summary tables, based on using the AD/GH model, with ranges of digester size and feedstock input, to estimate both the size of greenhouse that can be supported entirely or in part by varying digester size, style and feedstock. Further, greenhouse crop and production strategies will be examined to see how energy requirements are affected. Economic cost-benefit analysis of the total annualized costs for the various digester/greenhouse combinations will be investigated.
The programs will be made available on the Cornell PRO-DAIRY dairy environmental systems web site: www.manuremangement.cornell.edu. Accompanying materials such as a user manual, and examples will be posted to increase usability of the programs.
Tim Shelford, Research Associate, Cornell PRO-DAIRY Program firstname.lastname@example.org
Curt Gooch, Senior Extension Associate, Cornell PRO-DAIRY Program
The computer programs associated with this project will be available for download at the Cornell PRO-DAIRY website, www.manuremangement.cornell.edu.
First and foremost, we thank the four dairy farms and two greenhouse operations that have allowed us to outfit their systems with instrumentation needed to collect critical data needed for this project. Also, we would like to acknowledge the support of the USDA through their Hatch and Smith-Lever grant programs for the support of this project. We wish to thank the New York State Energy Research and Development Authority who provided funding to purchase the instrumentation equipment used in this project.
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