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Gemini: A Non-Invasive, Energy-Harvesting True Power Meter Brad Campbell and Prabal Dutta University of Michigan RTSS’14 – December 5, 2014 Buildings are significant consumers of electricity worldwide 55.3% Source: Natural Resources Defense Council, “Constructing Change: Accelerating Energy Efficiency in India’s Buildings Market” Source: NIST,Trends “Measurement Science Roadmap for Zero-Net Source: JRC, “Electricity Consumption and Efficiency in European Union” Energy Buildings” 2 But we don’t know where the electricity goes inside of the building “However, sub-metered energy use data are not available for many buildings, often due to the high cost of metering and gathering data.” US Department of Energy Building Technologies Office “Technological areas requiring further development include sensing and measurement technologies.” US National Science Board August 2009 “To properly direct [building efficiency] efforts, planners must understand where energy…is consumed.” US National Science and Technology Council Committee on Technology, Oct 2011 3 Gemini: A true power meter to address this issue Circuit panel level energy-harvesting meter that measures true power Voltage Monitor Current Meter Two devices work together to calculate power 4 How can we determine the electricity breakdown? Meter every device Meter at the circuit level Single watch point solutions ? • High installation cost • Configuration • Defined sense points • Reasonable load fidelity • Extensive calibration • Scale issues 5 End goal: Measure true power P(t)=I(t)*V(t) Voltage Current Power 6 Existing commercial meters are expensive and hard to install Circuits Metering Circuits • Installation complexity • Labor cost • Infrastructure upgrade • Meter cost Metering • Easily >$1000 • Highly accurate 7 Research solutions address cost, installation, and size issues Interference Piezoelectromagnetic (PEM)2 Magnetometer1 Current Transformer Energy-Harvester (Monjolo)3 • Calibration and cross-talk issues • Not actually measuring true power • Accuracy issues et. al, “The Design and Evaluation of an End-User-Deployable, Whole House, Contactless Power Consumption Sensor” CHI’10 et. al, “Stick-On Piezoelectromagnetic AC Current Monitoring of Circuit Breaker Panels” Sensors’13 3DeBruin, et. al, “Monjolo: An Energy-Harvesting Energy Metering Architecture” SenSys’13 1Patel, 2Xu, 8 Bo x Fa M p o n C u t e n it om r o 1 r C pu 00 o m te % r C pu 75 o m te % p r C ute 50% om r 2 Po p u t 5 % Am we er I pl r S dle if i u p er p (L ly A m oud pl ) ifi er O sc illi TV sc o R pe C Ca o u t a Si ble ble er gn B B a l ox o x G (o e C ner ff) Bl FL at u- (2 or R 3 ay W Pl ) C T V a ye ab r le (Id Vo M le) r n od a e Al d o m ar F m an H Clo um c id k R ifie oo r m P r ba Sh i n t re er dd er C om Power Factor Not measuring true power leads to errors in non-unity power factor loads 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Many loads have power factors < 1 9 We are missing the sweet spot of panel meter designs Performance Cost `` Install Cost Unit Cost Size Online Calib. Acc. Cross Sensitivity True Power Update Rate Commercial Magnetometer PEM Monjolo • Clear cost vs. performance trade-offs 10 Panel power meter wishlist • Simple installation • Small devices – fit in existing panels • Retrofit • Low unit cost • DOE Wireless Metering Challenge: $100 • Revenue grade accuracy not required • 2-5% error is fine • No deployment-time calibration procedure 11 Gemini: energy-harvesting true power meter Calculates true power by distributing current and voltage acquisition. 5 cm Voltage Monitor Current Meter • Non-contact • Calculates true power • Energy-harvesting 12 Virtualizing the voltage channel A φ VAC<A,φ> Voltage Monitor Few meters with access to voltage channel Current Meter Many current sensors that synthesize the voltage channel locally 13 Voltage waveforms can be represented by one or a few Fourier Coefficients Voltage Current 40 W Incandescent AC Fan 3D Printer Refrigerator MacBook Pro Dimmed Light Bulb Ice Shaver Audio Receiver 14 Key: properly phase-aligning the current and voltage waveforms Voltage Monitor Current Meter Δ t • • SFD V? SFD V? A A single packet provides time synchronization Do not need time sync protocols A, Δt Δ t 15 The current sensor’s energy-harvesting power supply limits its computational runtime Duty cycle of 0.2% (load at 5 W) 3 mJ 1. Request Voltage 2. Sample Current 2 1 4 3 5 3. Receive Voltage 4. Calculate Power 5. Store Result 16 Results: average error 8.7%, average absolute error: 2.2 W Gemini Ground Truth Percent Error 3D Printer 80 70 80 40 20 30 40 60 40 40 30 20 10 0 10 50 20 20 0 80 60 Power (W) 60 100 0 50 0 0 5 10 Time (m) 80 90 60 60 40 30 20 0 Time (m) Power (W) 120 % Error Power (W) 100 10 20 30 40 50 60 70 80 20 25 30 LCD Monitor 150 0 15 Time (m) Desktop 0 % Error 100 90 80 70 60 50 40 30 20 10 0 100 80 60 40 % Error 9 8 7 6 5 4 3 2 1 0 % Error Power (W) Smartphone Charging 20 0 0 10 20 30 40 50 60 Time (m) 17 Timing errors between voltage and current have large effects on error 18 The effect of measuring voltage at a different location is minimal Impedance of the loads: V2 P= ZL 120 2 ZL = =17W 840 Voltage triangle: Calculate phase offset: æ ö æ 17 ö -1 VL f = tan ç ÷ = tan-1 ç ÷ = 88.3° è 0.5 ø è VR ø 1.7° (0.08 ms) voltage phase difference between the two loads V=IZ VL=IZL φ VR=IR 19 Approximating voltage with a single Fourier coefficient results in less than 2% error Power calculation error using synthesized voltage 20 The current sensor must use the correct voltage channel in its calculations Buildings typically have three phase power (three voltage channels offset by 120°). How does the sensor choose the correct voltage channel? Correct ±180° 120° 240° Heuristic: Try all three voltage phases and choose the largest value. 21 Revisiting the matrix of panel meter designs `` Install Cost Unit Cost Size Online Calib. Acc. Cross Sensitivity True Power Update Rate CT Based Magnetometer PEM Monjolo Load Proportional Gemini Gemini Limitations • • Sample rate • Possibility: add a single daisy chained wire for power to each current transformer Detecting broken nodes • • When load is off harvesting stops and updates stop Possibly look at historical trends to distinguish 22 Gemini is an accurate true power meter • Installable • Small, independent current transformers • Accurate • Virtualized voltage allows for sample-by-sample power calculations 23 Questions? Brad Campbell [email protected] http://bradcampbell.com http://lab11.eecs.umich.edu University of Michigan 24 Power meter design Energy-harvesting power supply • Measurement and communication Uses multiple Current Transformers • For harvesting and measurement • Working on a design for multiplexing a single 25