An Energy-Monitoring System for Stanford University's Leslie Shao-ming Sun Field Station     by John Scofield, Associate Professor of Physics         Designing the Energy Monitoring System The energy-monitoring system was designed and installed in close collaboration with several Stanford University employees. These included Dr. Philippe Cohen, Bryan Palmintier, graduate student, Cary Tronson, building manager, and Scott Gould, energy engineer for Facilities Operations. The challenge was to design an energy-monitoring system that would measure the building's total energy consumption and the amount of energy produced by the PV array, as well as other variables to ensure precise characterization of the PV array performance. The system would also need to provide real-time data for display on the World Wide Web and on a kiosk display located at the building entrance. Initial expectations also called for the system to monitor exactly where in the building energy was being used, but budget constraints prevented this, so we designed a flexible monitoring system that could be expanded with additional sensors in a later phase. Sunlight provides much of the building's energy, directly supplying light and heat and indirectly supplying electricity generated by the rooftop photovoltaic array. Two additional "pipes" bring energy to and from the building: wires connect the building to the local power company, Pacific Gas and Electric (PG&E), and a gas pipe connects the building to a liquid propane gas storage tank. Propane is used to supplement the building's heat, as well as for domestic hot water. Electric energy may be purchased from or sold to the grid. A list of desired sensors was assembled and prioritized. Initially, we decided to monitor 14 different sensors and record their measurements to a database at one-minute intervals for subsequent retrieval and analysis. Five of the sensors were chosen to characterize building energy consumption. The other nine characterize performance of the PV array. All the sensors were connected to a Campbell Scientific model cr23x data logger (right). This device is similar to those employed by Assistant Professor John Petersen for monitoring environmental variables at the Lewis Center. Both Petersen and Michael Murray '03 were extremely helpful to me in sharing information they had learned working with Campbell data loggers. The logger, which can measure and store output from dozens of sensors, is connected to the Internet, and its data can be retrieved by authorized computers. Battery backup and internal memory allow the logger to store more than four weeks of data from the 14 sensors without outside intervention. In practice, the managing computer retrieves data every two minutes for display on the Internet and archives the data at one-hour intervals. Both real-time and trend data are regularly posted online (see below). Summary data, updated automatically every two minutes, are condensed to one web page and displayed on a kiosk at the building entrance. This page may also be accessed off-site. The real-time measurements of the 14 sensors are displayed on the dashboard in the upper-left corner of the kiosk. A larger image of the dashboard is shown here. The first three sensors capture energy flows into and out of the building. The first measures bi-directional energy flow to and from the PG&E electric grid. The second monitors the energy generated by the PV system, and the third monitors propane gas usage. The building's energy consumption and the amount of this energy furnished by the photovoltaic array can be determined by combining these sensor readings. The last three sensors displayed on the dashboard measure the power consumed by the HVAC pumps and general lighting circuits in the east and west wings of the building.     The month-by-month electric energy summary is shown above. The red line indicates building energy consumption, the yellow bar indicates the energy supplied by the PV array, and the blue bar tracks the amount of electricity purchased from the grid.   Since the monitoring system began operation on April 4, 2003, the PV array has generated about 90 percent of the electric energy consumed by the building. (The propane flow sensor was not installed until mid-June, but propane usage has been minimal during these warm months.) From the perspective of PV, spring and summer are the good seasons, with long days and little need for heat. In the next six months, photovoltaic energy production will decrease and building energy consumption will increase. It will be interesting to see how well the solar hot-water-heating system works and how much propane will be required for heating. Page 1 | Page 2 | Page 3 | Page 4