New publication: measuring residence time distributions in screw conveyor reactors

Charles Chamberlin, David Carter, and Arne Jacobson recently authored an article on measuring residence time distributions of wood chips in a screw conveyor reactor. A screw conveyor or auger makes use of a rotating helical blade inside of a tube or trough to move wood chips, sawdust, flour, or other granular materials through a reactor — such as a dryer, heater, cooler, gasifier, or torrefier. How much change in the materials takes place in such reactors depends on the average residence time and how variable that residence time is.

Internal view of a screw conveyor.

The screw conveyor in this Norris Thermal Technologies torrefier moves the woods chips from the inlet (on the left) to the outlet (on the right). The rate of rotation controls the residence time within the reactor. The reactor cover has been removed to show the screw.

This paper compares three alternative methods for measuring the residence time distribution of wood chips in a screw conveyor reactor using experimental results from a pilot scale torrefier:

  • addition of material to an empty reactor (step-up),
  • halting addition of material to a reactor under steady flow, (step-down), and
  • addition of a pulse of labelled material (i.e., a tracer) to a reactor under steady flow.

We found that all three methods yield residence time distributions that are approximately symmetrical and bell-shaped, but the distribution estimated from the pulse input of tracer exhibited a long trailing tail that was not detectable in either the step-up or step-down results. Second, we demonstrated that a normal probability plot provided a useful way to display and analyze the distributions obtained in the tracer experiments. Finally, we observed that all three methods yielded mean residence times that consistently differed from the nominal values, with the step-up method averaging 8% shorter, the pulse addition of tracer averaging 7% longer, and the step-down averaging 60% longer.

The article appeared in the August 2018 issue of Fuel Processing Technology and is available to download here in pdf.

Schatz History: the Trinidad photovoltaic array

From 1990 through 2016, a 9.2kW photovoltaic (PV) array located at the Humboldt State Telonicher Marine Laboratory in Trinidad, CA powered the Schatz Solar Hydrogen Project. This array provided the energy to aerate the Marine Laboratory aquaria either directly or indirectly through a Teledyne Energy Systems ALTUS™ 20 electrolyzer that produced hydrogen fuel for a proton exchange membrane fuel cell.

Peter Lehman and Charles Chamberlin stand outside the Schatz Solar Hydrogen Project

Peter Lehman and Charles Chamberlin

Charles Chamberlin and students at the foot of the Trinidad solar array

Students enact a fuel cell exchange as hydrogen and oxygen “molecules”

Located ~150 meters from the Pacific Ocean in a cool, marine climate, the array consisted of 192 ARCO M75 modules made with single-crystal silicon and rated at 48W. Prior to array installation in 1990, Jim Zoellick measured the current-voltage (I-V) curves for each module and discovered that even when new, the average peak power under full solar illumination at normal operating cell temperature was 39.9W – 14% below the 46.4W nameplate rating.

After 11 years of field exposure, Antonio Reis and Nate Coleman retested each module’s I-V curve in 2001 and found that the average peak power had fallen by 4.3% to 38.1W. They attributed the drop in performance to visually observable defects in the modules, especially the delamination of the modules’ ethylene vinyl acetate (EVA) encapsulant, leading to shading and corrosion of the cells and the occurrence of localized hot spots.

After 20 years of service, Marc Marshall, Mark Rocheleau, and Scott Rommel again retested each module’s I-V curve in 2010, and found that the average peak power had fallen an additional 12.4% to 33.4W and that the variation among the modules had dramatically increased.

After over 26 years of field service, the array was decommissioned in 2016. All of the 192 modules were removed and tested one last time by Jacob Rada, a M.S. candidate working at SERC. He found that the average peak power had fallen 22% since 1990, with an average degradation rate of 0.85%/yr, and that the variation among the modules had increased further since 2010. Each original module was discolored and showed some degree of delamination. (Read the paper…)

Two students test a PV module outside the Schatz Center

Andre Bernal and Jake Rada test a PV module

Having now been individually tested four times over 26 years, these are some of the oldest carefully monitored solar modules from a working array. Of the original 192 models, 188 modules were still in service after 26 years. One module was damaged in 1996 and was replaced with a module of similar size and performance characteristics. Three other modules were later replaced due to physical damage.

In the coming year, we will be hanging seven of the modules with their corresponding I-V curve data in the staircase of our main building, as a celebration of our Center’s first project and as a teaching tool for university students and visitors.

PV Module Testing Round Four


SERC volunteer Andre Bernal (left) and graduate student research assistant Jake Rada (right) measure the current and voltage curve of a photovoltaic module that has completed 26 years of service in the Schatz Solar Hydrogen Project. Each of the 192 modules in the project has been tested in 1990, 2001, 2010, and now again in 2016.



The Schatz PV Array – Twenty Years and Counting

Trinidad Solar Hydrogen Project

The Schatz Solar Hydrogen Project was installed in 1991 to demonstrate a combined solar PV and hydorgen fuel cell system. hydrogen for

Twenty years ago Jim Zoellick measured the performance of every one of the 192 photovoltaic modules just before they were installed in the Schatz Solar Hydrogen Project array. Nine years ago Antonio Reis and Nate Coleman retested every module after it had been in service for eleven years. This year Mark Rocheleau, Marc Marshall, and Scott Rommel tested every module for the third time after twenty years of service.

These painstaking measurements provide a unique opportunity to track the degradation of the performance of individual modules over twenty years of service in the cool, coastal, marine environment 150 meters from the Pacific Ocean at HSU’s Telonicher Marine Laboratory in Trinidad, CA.

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New SERC Building

New SERC Building Architectural Drawing

A rendering of the new SERC building.

This fall, we’ve taken two critical initial steps toward building a new facility for SERC on the Humboldt State University campus. The construction money we’ll need has been set aside by our foundation and the preliminary engineering site work has been completed.

The new facility will be built on a picturesque hillside site just west of the new Behavioral and Social Sciences building and north of the Campus Center for Appropriate Technology. The building will house laboratory space, a machine shop, faculty and staff offices, and a library/conference room in approximately 4,000 net square feet. It will replace our present home, which is severely space limited and located in a 60 year old building that has seen better days.

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