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RTU Single CT Method

RTU Single CT Method. Reid Hart, Associate Director, Tech Research, PECI Kevin Campbell, Engineer, PECI 12/15/2011 – RTF RTUG Meeting. Unitary HVAC Unit CT Method for Annual Energy Use Calculation. Research Funded by: Bonneville Power Administration

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RTU Single CT Method

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  1. RTU Single CT Method Reid Hart, Associate Director, Tech Research, PECI Kevin Campbell, Engineer, PECI 12/15/2011 – RTF RTUG Meeting

  2. Unitary HVAC Unit CT Method forAnnual Energy Use Calculation Research Funded by: Bonneville Power Administration Helpful Feedback: NBI, EWEB, & RTF Staff

  3. Motivation • True Power Method http://blogs.cuit.columbia.edu/greendc/files/2009/10/wattnode_bbspx.jpg

  4. CT Method • Materials • Current Transformer (1) • Data Logger • Advantages • No voltage connection • Less equipment • Does not require licensed electrician http://www.tootoo.com/buy-split_core_current_transformer/

  5. CT Method Goals • Investigate relationship between power and high leg current • Focus on variations based on capacity, vintage and manufacture • Create multiple models that predict power from single phase current • Quantify error associated with a spot power measurement for model calibration.

  6. CT Method - Data • 44 RTUs at two sites in Washington with Dreamwatts monitoring System • Six weeks of minute data during June/July and Sept/Oct used for analysis • Focused on eleven RTUs of varying capacity, vintage and manufacturer

  7. Power Factor

  8. Power factor non-linear

  9. Power FactorNearly Linear in Range of Interest

  10. CT Method - Development • Identify high leg of current for each RTU • Determine relationship between power and high leg of current for each mode of operation • Off - Constant • Fan Only - Constant • Compressor – Linear • Determine regression slope for each RTU • Categorize regression slopes and create aggregate model

  11. Regression slopes

  12. RTU Regression Slopes Three Phase (3Φ) by Vintage Manufacturer or Size groupings did not improve alignment; a 3Φ meta model used

  13. Calibrating the Model:Single Spot Measurement High Spot Measurement Low Spot Measurement

  14. Project Annual Energy Use • Use annual energy signature sheets • Six units annualized

  15. Error In Annual Energy Use Unfortunately, neither % of RLA nor OAT get us to the middle of the cloud reliably

  16. Single CT Method Recommendations • Useful to track persistence of savings • Error from spot measurement timing is a bias rather than random • Percentage savings is likely reasonably accurate • Use as diagnostic tool to determine which RTUs are not performing well • Savings must be substantial (greater than 20% in order to ensure savings are not in the noise)

  17. OAT Method Also Investigated • Compressor operation vs. OAT • Requires reliable compressor signal • Requires spot measurement • Less accurate than single CT • Likely useful as % savings or persistence approach • Needs more investigation into drivers of slope for RTU groups (vintage, charge, size, etc.)

  18. CT Method Future Work • Determine how to improve the accuracy of regression calibration to individual units. • Two point spot measurement? • Artificially increase condenser temp and load compressor to produce second current-power point • Field testing on units studied worthwhile • Investigate larger sample to find independent variables that establish current to power relationship: Charge Condition; cycling rate; vintage; size; manufacturer

  19. Questions?

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