Overview of the current state of DC power distribution and its benefits.  Delivered at a seminar at National Tsing Hua University, Taiwan

Gerber, Daniel

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eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleDC Power Distribution in BuildingsPermalinkhttps://escholarship.org/uc/item/6b4446v1AuthorGerber, DanielPublication Date2019-08-11 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaDC Power Distributionin BuildingsDaniel Gerber, dgerb@lbl.govLawrence Berkeley National LabsBerkeley, CA, USA4/8/20War of the Currents:Why We Chose AC• Back then, AC made sense• Transformers require AC• AC generation: coal, nuclear, gas• AC loads: fixed speed motors, incandescent lamp, resistive heatingTesla EdisonWhy We Chose ACTransformers require AC• Long-distance power distribution requires high voltage to overcome I2R wire loss• In the 1880s, voltage conversion required transformersTeslaWhy We Chose ACAC Generation• Coal, nuclear, and natural gas plants contain turbines connected rotating equipment• Induces current in stator (or rotor) coils• Rotor (or stator) is locked to centralized grid at 60 HzTeslaWhy We Chose ACTraditional AC Loads• Traditional loads interface well with AC• Incandescent lights• Resistive electric heating• Fixed speed motor loads such as compressors, fans, machineryTeslaWhy DC Distribution?Power Electronics• Allows for DC conversion• Often more economical than 60 Hz transformer• Allows precise current controlEdisonWhy DC Distribution?DC Generation• Renewable Generation: solar and wind are natively DC• Electrical storage: batteries are natively DC• Reduces DC/AC conversions in buildings with onsite generation and storageEdisonWhy DC Distribution?DC Loads• Modern loads are internally DC• LEDs lighting• Electronics• EV charging• Variable speed BLDC motors in HVAC and water heating• Induction stoves• Many DC power standards: USB, EthernetEdison• Higher efficiency in Zero Net Energy (ZNE) buildings with large solar and storage capacity• Simple power electronics: better cost and reliability• Reliable microgrid islanding through power electronics allows for low-cost disaster resiliency• Improved power quality• Communications via combined data and power• Plug loads safe to touch, allows cost reduction in wiringWhy DC Distribution?Advantages over AC:Efficiency, Cost, ReliabilityEdisonOffice Building with AC Distribution1. Maximum Power Point Tracking (MPPT) Inverter2. Battery Inverter3. Load Packaged Rectifier (all loads are internally DC)120 V  48 V   3 0 V 120 V 120/208 V120/208 VLow Voltage DC Loads120/208 V  380 V High Voltage DC LoadsNative-DC Loads1233Voltage Domains:120V AC48V DC380V DCOffice Building with DC DistributionDirect-DC Loads 380 V   3 0 V  380 V  380 V High Voltage DC Loads 380 V  380 V Low Voltage DC Loads 48 V 1243120/208 V1. DC MPPT converter2. DC Charge Controller3. Grid Tie Inverter4. DC Distribution ConverterVoltage Domains:120V AC48V DC380V DCUS Department of EnergyClean Energy Research Center (CERC) ProgramResearch Goal: Compare AC and DC buildings• Simulations to determine efficiency savings• Conduct techno-economic analysis• Experimental validationEfficiency Results• 12% baseline efficiency savings with DC• Most savings with large solar and battery• Dominant AC loss: wall adapters• Dominant DC loss: grid-tie inverterUS Department of EnergyClean Energy Research Center (CERC) ProgramResearch Goal: Compare AC and DC buildings• Simulations to determine efficiency savings• Conduct techno-economic analysis• Experimental validationTechno-Economic Analysis• Results determined from market cost data, grid tariffs, and Monte-Carlo analysis• First cost is higher for DC• With significant efficiency savings, the payback period is less than a yearCalifornia Energy Commission (CEC)Direct DC Plug Loads for ZNE BuildingsTask Lamp15 V USB-C~5% W savedResearch Goals• Modify AC plug loads for direct-DC input• Demonstrate savings in consumption and costZone Light, 380 V DC, 6% W savedRefrigerator, 380 V DC, 1% W saved(since original doesn’t have PFC)Bath Fan48 V PoE8-15% W savedNREL/LBNLDC Design ToolResearch Goals• Develop an Energy Design and Scoping Tool for DC systems• Target audience: building planners, designers, and engineers who are considering deployment of DC distribution systems• Extends DOE’s tools: EnergyPlus and the OpenStudio• Enable user to assess and compare the energy efficiency and life-cycle cost of a design• Validate the DC Design Tool using collected experimental and field dataNREL/LBLDC Field TestingXingye SolarShenzhenIBR BuildingShenzhenIBEW BuildingSan LeandroMarriott SinclairFort WorthResearch Goals• Establish evaluation methods and metrics for DC-systems• Measure and evaluate the performance of several buildings with new DC distribution installations• Assess technical barriers inhibiting robust adoption of DC systems• Identify opportunities to optimize DC-system performanceConverter AC vs DC Loss AnalysisAC vs. DC Boost Converters: A DetailedConduction Loss ComparisonDaniel L GerberBuilding Technology and Urban SystemsLawrence Berkeley LabsBerkeley, CA, USAdgerb@lbl.govFariborz MusaviEngineering and Computer ScienceWashington State UniversityVancouver, WA, USAfariborz.musavi@wsu.eduAbstract—Studies have shown the efficiency benefits of DC dis-tribution systems are largely due to the superior performance ofDC/DC converters. Nonetheless, these studies are often based onproduct data that differs widely in manufacturer and operatingvoltage. This work develops a rigorous loss model to theoreticallycompare the efficiency of a DC/DC and an AC/DC PFC boostconverter. It ensures each converter has the same componentsand equivalent operating voltages. The results show AC boostconverters below 500 W to have 2.9 to 4.2 times the loss of DC.Keywords—DC microgrid, boost converter, loss model, powerfactor correctionI. INTRODUCTION AND MOTIVATIONDirect current (DC) distribution has taken the spotlight inmicrogrid research due to the proliferation of solar generation,battery storage, and internally-DC loads. Recent studies andexperiments compare the system efficiency of an AC andDC microgrid [1]–[8]. For the commercial building sector,the reported savings with DC vary widely from 2% [1] toas much as 19% [8]. In general, the reported savings arehighly dependent on the system converter efficiencies, thesystem topology and voltage levels, and the coincidence ofgeneration and load. In 2017, Gerber et al. performed a seriesof highly-detailed parametric simulations that varied solarand storage capacity in equivalent AC and DC buildings. Anextensive loss analysis revealed low-power AC/DC convertersto contribute the most loss in AC buildings [3]. For example,the peak efficiency of a high-quality AC/DC LED driver is94%, whereas a DC/DC LED driver is typically 98% [4].These studies have many limitations, the most significantbeing nonequivalent models for AC and DC converters. Thesemodels often use efficiency curves from product data, whichcan be limited or biased, especially for the less-common DCproducts. In addition, previous studies compare AC and DCsystems at very different distribution voltages (AC 120 Vrmsor 480 Vrms, and DC 48 V or 380 V). Such a comparisonfavors the system with the higher distribution voltage, sincehigh-voltage converters tend to be more efficient. Ideally, thesestudies would compare AC and DC systems at the samevoltage, but it would be difficult to find product data for oneof the systems.This work develops a rigorous loss model to theoreticallycompare the efficiency of equivalent AC and DC convertersU.S. Department of Energy Contract No. DE-AC02-05CH11231at the same voltage. Although several previous works haveanalyzed and established loss models for the DC/DC [9]–[12] and AC/DC PFC [13]–[20] boost converters, they eachhave their own methods and formulae, making an analyticcomparison difficult. In addition, many of them neglect es-sential components such as the input bridge drop and outputcapacitor equivalent series resistance (ESR). This is the firstwork to establish a set of formulae that compare the lossbetween an AC and DC boost converter, both of which have thesame components and equivalent operating voltages. AlthoughDC/DC converters are already known to be more efficient, thiswork reports exactly how much more.II. DERIVING SWITCHING CONVERTER LOSS MODELSThis work models the resistive and diode loss elements inthe boost converter. The process requires solving for the aver-age and RMS currents through each component. As shown inFigure 1, these include the inductor (IL,rms), switch (IQ,rms),boost diode (ID,rms, ID,ave), output capacitor (IC,rms), andthe two active bridge diodes (IB,rms, IB,ave). The averageresistive loss PLoss,R in the inductor, switch, and capacitor ismodeled asPLoss,R = RR ⇤ I2rms, (1)where RR is the inductor copper resistance, switch on-stateresistance, or capacitor ESR, respectively. The average diodeloss PLoss,D in the boost and bridge diodes is approximatedas a fixed diode drop VD and series resistor RD:PLoss,D = VD ⇤ Iave +RD ⇤ I2rms. (2)VOUTQCLD LoadVIN B+~_~Fig. 1: The AC PFC boost converter model includes the inputinductor L, two of the bridge diodes B, the switch Q, the boostdiode D, and the output capacitor C. The DC boost converterhas an input at the inductor, and bypasses the diode bridge.TABLE IV: DC/DC Boost Model Validation Currents (A):Po = 250 W, Vo = 350 V, and Vpk = 170 VParameter Model(simple)Model(ripple) Simulation ExperimentIL,rmsIB,rms1.466 1.485 1.485 1.552IB,avg 1.466 1.466 1.466 1.452IQ,rms 1.056 1.070 1.070 1.041ID,rms 1.017 1.030 1.030 1.105ID,avg 0.705 0.705 0.705 0.717IC,rms 0.732 0.751 0.751   Fig. 3: Experimental prototype for a boost converter withPo = 250 W, Vo = 350 V, and Vpk = 170 V.The loss comparison is illustrated by the efficiency curvesin Figure 4. The AC boost has 2.9 to 4.2 times the loss ofthe DC boost in the range of 100 W to 500 W. Note thatthis model does not account for switching loss, which wouldnormally decrease the efficiency at low power. A loss analysisin Figure 5 reveals the switch and diode bridge to be the mainsources of loss, given the components in Table V.VI. CONCLUSIONSystem efficiency comparisons between AC and DC of-ten fail to compare converters with equivalent voltages andcomponents. This work develops a rigirous formula-based lossmodel to compare an AC and DC boost converter. The modelis based on component currents, which are validated throughboth simulation and experiment. The results show AC boostconverters below 500 W to have 2.9 to 4.2 times the loss ofDC. Nonetheless, there are various other DC/DC topologiesthat are even more efficient than the boost, including resonantand switched-capacitor converters. Therefore, this study servesmore as a baseline for an AC and DC comparison. Future workinvolves extending the comparison to other types of converters.TABLE V: Components in PrototypeComponent Part Number ParametersInductor (2x) Bourns1140-821K-RCRR = 0.154(each inductor)Diode Bridge Diodes Inc.GBU804VD = 1RD = 0.028(each diode)Switch VishayIRF840PBF RR = 0.85Boost Diode CreeC3D04060FVD = 0.81RD = 0.13Capacitor (10x) TKDB43501A6107M000RR = 1.33(each cap)Boost PFCControllerTIUCC28019   Boost DCDCControllerArduinoUno   Fig. 4: Efficiency curves that compare an AC and DCboost converter with the same components and input voltageVpk = 170 V. The model only accounts for conduction losses.Fig. 5: Loss analysis for the AC and DC boost converters withinput voltage Vpk = 170 V and output voltage Vo = 400 V.• Gaps in Prior Research• Converter efficiency based on product data• Hard to compare AC and DC• Requires a lot of data, which is often unavailable• Comparing different voltage levels, eg. 120 V AC to 48 V DC• Different components with different parasitics• Project Goal• Develop a detailed boost converter loss model• Compare AC and DC boost converter with the same voltage and same componentsNon-technical Barriers to Adoption of DC Power• Lack of DC loads, due to lack of DC buildings• Chicken and egg problem• Lack of standards in voltage and connectors• IEC is working on this• Designers and electricians don’t understand DC• Incorrect safety concerns• Optimal design for cost and efficiencyAreas for Further Technical  Research in DC Power• Protection and fault interruption• Solid-state, hybrid, converter blocking• Topology• Pulsed• Bipolar• SST• Control and stability• Current sharing (primary, secondary)• Communications, demand response (tertiary)Protection and Fault Interruption• Problems with mechanical breakers for DC• No zero-crossing to extinguish arcs• Mechanical breakers too slow, need < 1 ms• Possible solutions• Solid-state breakers - use MOSFET etc. to block• Very fast interruption, but on-resistance losses• Hybrid breakers – parallel solid-state and mechanical• Low on-resistance but slightly slower (~1 ms)• Converter blocking – use DC/DC converters as protection• Free functionality• Most solutions are more expensivethan AC breakers• Still require series mechanical disconnectTopology• Pulsed - Voltserver• Pulsing power allows high voltage distribution without conduit• Only transfer power after digital handshake• Bipolar Distribution (+/0/-)• Increase power transfer capacity; thinner wiring• Pole-neutral loads are resilient to faults on the opposite pole• SST in Buildings• Multiport coupling of solar, storage, grid• Isolation between circuits; required for behind-the-meter transactive powerBus Stability and Control• Current sharing• Primary control – decentralized• Often uses droop control to regulate bus• Virtual series resistance: Vref = 380 – iout*Rdroop• Secondary control – distributed• Units communicate to adjust droop parameters to account for network parasitics• Optimal control for economics or energy• Tertiary control – centralized• Demand response• Price-based control• Grid servicesThank you!dgerb@lbl.gov

DC Power Distribution in Buildings

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DC Power Distribution in Buildings

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