Modelled and Observed Operation of a Passive Melting Technology for Photovoltaic Arrays

Transcription

1 Modelled and Observed Operation of a Passive Melting Technology for Photovoltaic Arrays Michael Ross and Eric Usher Energy Diversification Research Laboratory Presentation from

2 Partners: uenergy Diversification Research Laboratory (PV for the North) uscience Institute of the Northwest Territories utn Conseil uenvironmental testing at Siemens Solar Industries

3 Photovoltaic (PV) Systems Sun PV Panel Load Battery

4 The Problem upv systems are remote and must be very reliable uunder certain conditions, snow and rime accumulate on the arrays uthe electrical output or the array drops uthe system fails, requiring expensive maintenance visits

5 Existing Solutions: uoversize the battery bank: costly usteep panel tilt angles uflushmount panels for commshells uicephobic coatings: wear off

6 The TN Conseil Solution uground-reflected sunlight warms back of panel and melts accumulation upassive technology Sun uno moving parts upatented Panel Absorber Foil Airspace Clear Lexan Back Cover Reflective Snowcover

7 Snow and Rime Accumulation: Literature uunsuccessfully tried to model accumulation using hourly meteorological data urime: Too site specific for available data usnow: Robust criteria could not be formulated

8 Wet Snow Accumulation on Vertical Panels Two criteria (Makkonen, 1989): Wet-bulb temperature greater than 0 ºC udemonstrates why steep panel tilt angles are effective If sticking efficiency is known, can estimate growth rate using only observed visibility

9 Further Accumulation on Panels Already Covered in Snow Hopper Filled With Snow Snow Falling from Hopper Angle of Repose Snow forming Cone uangle of repose analogous to tilt angle of snow-covered PV panel

10 Further Accumulation on Panels Already Covered in Snow Angle of Repose Degrees Angle of Repose Temperature (ºC) from Kuroiwa et al., 1967

11 Snow Accumulation on Off-Vertical Panels (Kobayashi, 1987) usnow particles bounce/slide down array surface u Islands of accumulated snow form uaccumulation from the bottom of the panel upward ucollection efficiency low below -3 ºC, high above -1 ºC

12 Snow and Rime Accumulation: Field Reports urime: mountain-top systems can fail for months, requiring battery banks with three months autonomy or more usnow: only occasionally a problem uglaze: possibly a problem in Newfoundland uhoarfrost: generally not a problem

13 PV System Siting in Rime-Prone Areas upresently, panels mounted vertically and placed at most windswept site uinstallers believe that wind and gravity will detach rime uless windy sites better ulower tilt angles may be better

14 Monitoring of the TN Conseil Technology utwo panels installed in the EDRL s rooftop array upanel temperatures 15 to 30 ºC higher usnow melts significantly faster utemperature gradients due to convection

15 Actual Operation, March 6, :00 AM 11:00 AM 1:10 PM 3:20 PM

16 Modelling Solar Radiation Incident on the Rear Face of the Panel uground-reflected radiation is dominant (85 to 95% of total) urear face receives approximate 25 % of the radiation on the front face uarray shadow on the ground is not significant for small arrays

17 Modelling the Thermal Performance usteady-state energy balance model ulexan cover limits heat loss, permitting temperature to rise to melting point uabsorber foil provides heat for melting

18 Conditions Required to Initiate Melting of Dense Rime Rime thickne s s of 2, 5, a nd 10 cm; winds pe e ds of 2, 5, a nd 10 m/s. Blue line s a re for pa ne ls with TN Cons e il te chnology, Re d line s a re for unmodifie d pa ne ls. Fro nt Ins o la tio n o n Rime S urfa c e (W/m^2)

19 Rate of Melting for Dense Rime 0.7 Blue line s a re for TN Cons e il modifie d pa ne ls; re d line s a re for unmodifie d pa ne ls W/m2 600 W/m W/m2 600 W/m W/m Ambie nt Te mp (C)

20 Simulation of Snow and Rime Removal uassume a accumulation exists on January 1: how long will it take to melt off? uused hourly weather data for four sites utwo criteria: panel reaches melting point ( shedding ) or complete melting occurs

21 Simulation Results ureduces snow removal times by 80 to 90% for Bagotville, Québec site ureduces rime removal times by 20 to 40% for mountainous sites in B.C. and Newfoundland upoorer performance in Arctic umany assumptions required: results only good for rough comparison

22 Continuing Work utesting design improvements at EDRL uinstallation and monitoring at rimeprone sites: rime accumulation on rear face?

23 Conclusions urime is a serious problem for PV panels at some sites utn Conseil technology is highly successful at snow removal; shows promise for rime removal uprediction of accumulation with existing data and models was difficult

24 Questions?