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Digester Gas Used to Generate
Heat and Power while Reducing Pollution
Waitsfield, VT— The Essex Junction Wastewater Treatment
facility in Essex Junction, Vermont has selected Northern Power Systems
to engineer, build and install a $245,000 on-site power system that will
burn methane gas produced by wastewater processing to generate electricity
and heat for the facility. The new cogeneration system will produce over
400,000 kWh of electrical output per year, equivalent to 41% of the facility's
current annual demand. At the same time, the system will reduce the plant's
CO2 emissions by over 500,000 pounds--the equivalent of eliminating 42
cars from the road per year. As part of its commitment to removing financing
obstacles to such environmentally sound systems, Northern helped the Essex
Junction facility obtain grant and rebate assistance for the project from
various outside sources.
The Essex Junction Wastewater Treatment system will employ a new controls
method--the first of its kind for use in a biogas cogeneration application--developed
by Northern Power Systems for the project. The controls method will enable
the facility to conduct peak shaving (reducing metered demand by boosting
output kW during short, higher demand times) while monitoring the power
and heat requirements of the site. The system is expected to yield a total
annual electric utility savings of about $30,000.
Environmental Benefits and Cost Savings
The Essex Junction plant will realize significant environmental benefits
and energy cost savings by blending the methane with natural gas, which
also serves as a back-up gas source in the event of digester maintenance
or interruption of biogas supply. Modulating the mix allows the microturbines
to conduct peak shaving to a greater extent than if running on methane
alone, automatically increasing the output of the microturbines to meet
the additional electrical needs. Blending also improves the reliability
of the system by stabilizing the fuel input into the turbines.
Through cogeneration, the recovered heat from the turbines will be used
to heat the digester tanks and will offset 100% of the need to run a boiler
for this same purpose.
Broad Appeal to Similar Wastewater Treatment Facilities
According to Dan Reicher, executive vice president of Northern Power Systems
and former U.S. Assistant Secretary of Energy, the system developed with
the Essex Junction facility has widespread applicability nationwide. Reicher,
who led a $1.2 billion program to advance energy efficiency, renewable
energy and distributed generation, said, "An EPA Clean Water Needs
Survey indicates that there are some 3,300 wastewater treatment facilities
similar to the Essex Junction plant in the United States. From delivering
basic electricity, to cost reductions and reduced environmental impact,
this type of system design delivers terrific value for anaerobic digestion
wastewater treatment facilities," he added.
The average cost per kWh produced by the system will be 2.46 cents (including
annual maintenance expenses), yielding a 6.37-cent savings when compared
to the current utility rate. In addition to the energy savings, peak shaving
will accrue $4,500 in savings, thereby yielding a total annual electric
utility savings of about $30,000.
Based on a national utility average of 1.3 pounds of CO2 emissions per
kWh generated, the cogeneration system will offset over half a million
pounds of CO2 emissions per year (the equivalent of removing approximately
42 cars from the road per year).