FACE Engineering & Operations
Brookhaven provides engineering and operations support to the DOE/BER FACE Facility and other FACE and climate change experiments [ see our support page ]. Brookhaven will provide support for facility design and equipment specification. Once operational, we can provide monitoring of system performance and offer management and operations guidance. We also provide and maintain the control software for the FACE system, monitor performance and maintenance issues at affiliated FACE sites, and distribute solutions, work-arounds and lessons learned as appropriate.
The use of open-air gas releases to treat large scale plots has been undertaken since the 1970’s. Hartwell Allen coined the name Free Air Carbon Dioxide Enrichment and the acronym FACE to describe a range of experiments that used open air releases to study the ecological effects of increasing atmospheric concentrations of carbon dioxide (Allen, 1992). Early designs used line source releases, and depended on consistent wind speed and direction or long term averaging to obtain a uniform treatment. In the 1980’s circular systems with computer aided feedback control were designed to allow open-air exposures to sulfur dioxide, nitrogen oxides, and ozone (Mooi and van der Zalm 1985; McLeod et al 1985; Hendrey & Miglietta, 2006).
Researchers at Brookhaven National Laboratory studied these emerging technologies and added several improvements. BNL incorporated one second data collection with rapid feedback to the control hardware using tunable, time filtered, Proportional-Integral-Derivative (PID) algorithms, with additional feed-forward components for wind speed to tighten system performance. Spatial and temporal variability within the plot volume was reduced by pre-diluting the treatment gas before releasing it into the treatment plot.
The number of vent pipes opened at the ends of the treatment arc were adjusted to reduce high treatment gas concentrations near the cross-wind edges of the plots, and an alternative release pattern was developed for fumigation under low wind conditions. This work resulted in a new FACE design that could provide uniform exposures of CO2 and other gases to large field plots (Hendrey et al 1992). The first full scale experiment using this design was conducted in a cotton field in Yazoo City, MS in 1987. This design was expanded for use with cotton and other field crops in Maricopa, AZ (Lewin et al 1994, Nagy et al 1994), mature forests (Hendrey et al 1999) and for studying both elevated ozone and CO2 levels on forest species (Dickson et al 2000).
Design of a Typical Brookhaven FACE Site
The BNL FACE system hardware consists of a high-volume blower, a plenum or wide ring-shaped pipe for air distribution, 24 to 32 vertical standing vent pipes for emitting CO2 into the exposure volume, and facilities to store and vaporize large quantities of liquid carbon dioxide. The CO2 used in FACE experiments is typically obtained as a by-product from the manufacture of agricultural fertilizer or from the digestion of organic matter. Food-grade, liquefied CO2 is delivered to the FACE site by truck and transferred to an insulated and pressurized receiving tank.
Tank pressure is maintained at 1725 kPa to keep the CO2 in a liquid state. A refrigeration unit and an electric heater are used to maintain this pressure regardless of demand for CO2 by the FACE control system. Liquid CO2 is piped from the storage tank to heat exchangers which vaporize the CO2 as needed. The gaseous CO2 is then channeled through pipes and pressure regulators to the FACE plot.
Controlling CO2 Concentration
Regulation of CO2 concentrations within the FACE treatment plot is accomplished through the use of a custom software program that collects information from multiple sensors and sends control signals to electrically actuated relays and control valves that regulate the quantities and placement of the CO2 released into the treatment plots. Wind speed and direction are measured near the top of the canopy by a sensitive cup anemometer and wind vane. Air is sampled from a control point at the top of the canopy in the center of the array and pumped to a non-dispersive infrared gas analyzer that continuously monitors the CO2 concentration.
The computer program collects data from these instruments every second and integrates the data streams over time constants of 15 to 60 seconds, depending on the data type and its use. The control program feeds this information into a tunable, PID algorithm with additional feed-forward components for wind speed to compute the appropriate CO2 release rate. A gas metering valve regulates the quantity of CO2 injected into the plot. Actual CO2 flow is measured by an electronic flow sensor for comparison to the computed demand and documentation of the quantities of CO2 released into each plot. The program also uses wind speed and direction readings to determine which vent pipe valves should be open.
Measured and computed variables, are recorded in one minute time intervals for future analysis by the system operations team and other facility users. An independent monitoring system measures and records the 3 dimensional distribution of CO2 within the treatment plot volumes to document both spatial and temporal variability and provide an independent check on the accuracy of the control system.
Fan and Plenum
A radial fan is used to provide additional air flow within the plenum to reduce the CO2 concentration to approximately 30,000 ppm as it leaves the emitter holes on the pipes and enters the outer edges of the treatment plots. A toroidal plenum carries the CO2-enriched air from the fan to the vertical vent pipes. This plenum is typically assembled outside of the circle of the vent pipes to minimize its impact on the study area.
Vertical Vent Pipes
Injection of CO2-enriched air into a FACE plot takes place at the vertical vent pipes (VVPs). VVPs extend from the ground surface to above the plant canopy and are evenly spaced in a circle around each FACE treatment plot. Valves at the bases of the VVPs are opened or closed as needed to release CO2-enriched air from the upwind side of the plot. A "feathering" of the quantity of CO2 injected into a plot near the cross-wind edges is achieved by keeping the second VVP from each end of an arc of 12 upwind VVPs closed (i.e., only 10 VVPs open).
Emitter ports on each VVP are arranged in triplets of holes, with one hole directly on a radial line facing the plot center and the other two set at 60o to each side of center. These triplets of emitter ports are spaced along the length of the vent pipe from the ground to the top of the canopy. Since the wind profile changes greatly with elevation from ground level to above the top of the canopy, the vertical configuration of the emitter ports is adjusted empirically to achieve optimal three-dimensional distribution of [CO2] within the experimental plots.
Under low-wind conditions it is difficult to determine wind direction, and the actual wind direction can rapidly fluctuate. Therefore, if the wind speed drops below the anemometer threshold for 20 seconds, directional control is terminated and every other VVP around the FACE ring is opened. This insures that CO2 will be released upwind of the plot regardless of the actual wind direction.
Last Modified: February 1, 2008