Long-term automated simultaneous measurements at multiple locations within the same study site are often required to adequately characterize high spatial and temporal variability of the soil CO2 fluxes. The multiplexed LI-8100/8150 is a fully automated system dedicated to making such measurements using up to 16 soil chambers. Increase in the global flux network coverage creates a need for unattended measurements in remote locations where grid power is not available. Use of the “off-grid” power technology should be considered at remote sites to provide power for the LI-8100/ 8150 operation. One of the most developed of such technologiesis Photovoltaics, PV (use of solar cells to convert sunlight to electricity). PV systems consist of several key elements (e.g., solar panels, batteries, charge controllers, and cables), supporting structure (panel mounting, battery enclosure, etc.), optional equipment (combiner box, disconnects, battery meter, inverter for AC power), and require careful calculations and proper placement to function at maximum efficiency.
LI-8100/8150 POWER REQUIREMENTS
The first important item to consider before designing the PV system is the power requirements of the multiplexed LI-8100/8150 system. Table I shows normal and peak requirements for configurations with various numbers of chambers.
In regular sampling mode, when no chambers are moving, the multiplexed LI-100/8150 system would require between 22.5 and 35.0 Watts of energy. This mode lasts for several minutes at a time, depending on the specific user-programmed configuration. This is the period when one chamber is closed for sampling. After the sampling is completed, this chamber opens, while another closes, so that up to two chambers could be moving at the same time. During this process, which lasts for about 90 seconds, power requirements increase to 47.5 Watts. This is a part of the normal operation of the system, because movement of chambers could be relatively frequent (depending on the user-programmed configuration), and may constitute up to 50% of time (90-second chamber exchange followed by 90-second sampling).
Peak requirements in Table I are for the warm-up or re-start periods, when up to 4 chambers move at the same time, requiring up to 60 Watts. Such periods are rare, but should be taken into account when designing the PV system.
DESIGNING A PV SYSTEM
The main challenge for the off-grid PV system is to ensure that it can keep up with regular power demands, and can provide enough power during infrequent peak periods. Keys to the design of such a system are: (1) computing power demands at the study site, (2) evaluating how many batteries are needed to ensure operation at night and on overcast days, and (3) determining what solar panel array is required to satisfy power demands. Table II shows an example of step-by-step computations of these items for various numbers of chambers in the multiplexed LI-8100/8150 system.