How much electricity can we save by using direct current circuits in homes?

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1 How much electricity can we save by using direct current circuits in homes? Brock Glasgo, Department of Engineering and Public Policy, Advisors: Inês Lima Azevedo, Chris Hendrickson

2 What is Direct Current? Direct Current (DC) Alternating Current (AC) Voltage, Current Voltage, Current Batteries, digital circuits Nearly all grid electricity All distributed electricity Inverter (87-94%) Rectifier (93-97%) 2

3 Overview Goal Assess the technical energy and economic savings of a transition to DC-powered homes using a unique dataset of actual home load data submetered in Austin, TX Why DC? We now have semiconductor-based power electronics that function as DC-DC transformers Consistent growth in distributed generation sources that generate DC Growing fraction of DC use in buildings CFL and LED lighting All consumer electronics Variable speed motors 3

4 Monitored Load Data Provided by the Pecan Street Research Institute Home 1192 Home 555 Home 6072 Sample contains ~700 volunteer homes monitored at 1-minute intervals (aggregated to 15-minute) Whole-home-, circuit- and appliance level Survey results, audit records, intervention records, etc. Monitoring from January 2012 through present Homeowners work with Pecan Street to choose which appliances/circuits to monitor Gantt charts used to show data quality and availability across time

5 Data Validation Annual Energy Consumption (MWh) Outliers Maximum 75% Median 25% 0 WikiEnergy (340) RECS 2009 (549) Austin Energy 2012 (1) Minimum Total Use RECS sample filters single-family, detached homes in Texas with central AC 5

6 Appliance Classification Existing Load Resistance Heating Appliances Oven, range, electric clothes dryer, dishwasher, electric water heater % of Sample Home Use % of RECS Use 5% 13% EV Charging Electric vehicle charging 11% 0% Other (electronics) All electronics, plug loads, lighting 34% 41% AC Motors Refrigeration Kitchen disposal, clothes washer, central air supply fan, gas clothes dryer, vent hood fan Air conditioning condensing unit, refrigerator, freezer, wine cooler 15% 5% 35% 41% 6

7 Technical Feasibility & Savings Existing Load Proposed DC Alternative Energy Savings Resistance Heating No Change No Change EV Charging Eliminate (1) rectification stage 3-7% Other (electronics) Eliminate (1) rectification stage 3-7% Other (lighting) Eliminate (1) rectification stage and convert to LED 7-28% AC Motors Brushless DC (BLDC) Motors 5-15% Refrigeration Variable speed BLDC compressors %

8 Existing AC Home Power Supply PV Array MPPT DC/AC Refrigeration Resistance Heat AC/DC DC/DC EV Charging AC/DC AC/DC DC/DC DC/AC Other (Electronic) Other (Lighting) Grid AC Motors 8

9 Simulated DC Home Power Supply PV Array MPPT DC/DC BLDC Refrigeration Charge Control Battery AC/DC DC/DC Resistance Heat EV Charging AC/DC DC/DC Other (Electronic) AC/DC AC/DC DC/AC Other (Lighting) Grid AC Motors 9

10 Simulated Home Power Supply PV Array MPPT BLDC Refrigeration Charge Control Battery DC/DC DC/DC DC/DC Resistance Heat EV Charging Other (Electronic) Other (Lighting) Grid AC/DC BLDC Motors 10

11 Key Assumptions Efficiencies of first-generation residential DC appliances maintain current niche market efficiencies (boats, campers, military, etc.) DC circuits will have the greatest price increase for: Circuit breakers Air conditioning condensing units Central air supply fans Refrigerators Similar degradation of AC-DC and DC-DC power supply efficiencies at part load Recent tests confirm this 120V standard is adopted DC will be implemented in new construction only 11

12 Monte Carlo Simulation Monte Carlo Parameters Component Efficiencies Component Costs Monitored Load Data Fixed Parameters Utility Rates DC Home Configurations Intermediate Results AC Home AC Equipment Energy Use Size DC Equipment Size DC Home Energy Use AC Home Energy Cost AC Equipment Cost DC Equipment Cost DC Home Energy Cost Final Results Energy / Emissions Savings (%, kwh/yr, tco 2 /yr) Cost Savings (%, $/yr) 12

13 Results Energy Savings 13

14 Results Cost Savings 14

15 Cost Effectiveness 15

16 Quantitative Conclusions Direct-DC distribution of PV energy is a feasible means of generating energy and emissions savings Single DC circuits serving brushless DC condensing units match (exceed) the cost-effectiveness of traditional PV arrays Storage for excess solar PV energy not cost effective in any of the simulated configurations 16

17 Regulators Perspective National Electric Code (ANSI 1 /NFPA 2 ) does not specify AC or DC under 600V, so no changes appear necessary at the distribution level Any home with a PV array already has a DC circuit Adoption of 120VDC standard maintains existing wiring losses maintains similar wiring/switch/breaker costs 1 American National Standards Institute 2 National Fire Protection Association 17

18 Utilities Perspective All changes occur downstream of meter Net metering programs credit generation per AC kwh Assumes a conversion loss and undervalues direct-dc consumption Utility solar installation incentives programs are administered based on installed AC capacity Unclear whether DC systems would be eligible Specific subsidies for direct-dc systems would be premature Lack of voltage standard, residential-specific appliances Brushless DC condensing units present the best opportunity 18

19 Industry s Perspective Energy savings potential justifies residential pilot projects Utilities require AHRI 1 certification and performance guarantees for condensing unit rebates Certification would allow early adopters of DC condensing units the same benefit available to less efficient AC units Experience with DC circuits (power plants, commercial buildings, telecom) will have to transition to a broader population of engineers, electricians, and building inspectors 1 Air-conditioning, Heating, and Refrigeration Institute 19

20 Acknowledgements CMU s Department of Engineering and Public Policy (EPP) Center for Climate and Energy Decision Making (CEDM) Carnegie Mellon Electricity Industry Center (CEIC) The Pecan Street Research Institute 20