Grid Connected Residential PV and Two Years Producing Solar Electricity in Delaware

Transcription

1 Grid Connected Residential PV and Two Years Producing Solar Electricity in Delaware (and dispelling some Solar Myths) Steve Hegedus Institute of Energy Conversion University of Delaware Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#1

2 Outline 1. Major markets and grid connected applications of PV 2. Available sunlight - concept of average daily sun-hours 3. Effect of array orientation - tilt, south-facing, shading 4. Effect of temperature 5. Shading - series connected PV cells 6. Inverter - critical component 7. My system System sizing and output prediction, comparison to load Wiring, layout, installation Cost 8. Other PV grid connection issues and recent developments Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#2

3 Trends in PV Markets: MW P capacity installed each year total world PV grid connected resid+comm communications rural off-grid central >0.5 MW Grid connected PV driving market growth Centralized (util) PV Negligible until recent, Rapid growth in EU of projects >10 MW Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#3

4 1.8 kw Solar Array in Newark, DE Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#4

5 Grid connected design issues Net metering is enabling policy Utility buys back energy at selling price (reversible meter) Grid is your back-up and your battery Full credit for excess daily production, nothing wasted Either maximum PV you can afford or maximum fit on roof Partial or total offset house/business load Aesthetics of increasing marketing value (color, form factor, stand-off) Few systems w/battery to store excess, backup select loads Improve reliability, security, grid to charge/equalize battery Increase cost, complexity, less efficient Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#5

6 Some terminology: Power vs Energy Power: output of module or array in Watts (W) or kilowatts (kw) or megawatts (MW) under Standard Test Conditions STC= 1 sun or 1000 W/m2 of AM1.5 sunlight, module at 25 C, A 200 W module produces 200W only under specific conditions Power is instantaneous output from the PV Energy output of module or array in kw-hrs is what you want Energy is power integrated over time Energy is what you pay for on the electric bill (13.5 cents/kw-hr) Energy is output over period of varying sunlight, angle, temperature Goal is to calculate daily/monthly/annual energy from module knowing PV STC and location and tilt Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#6

7 Solar array output (kw) You want ENERGY (kw-hrs) produced but PV modules are rated in peak POWER (kwatt) at standard test condition 1 sun?? 1 sun 3-6 hrs 6 am 12 pm 6 pm Blue curve is hourly PV power output Area under blue curve is daily kw-hrs Same area as red square Width of red square is equiv hours of full sunlight (PV prod max output) Height of red square is output under 1 sun Widely available parameter for various tilts, tracking options on monthly basis To calculate daily PV energy (kw-hrs) produced, mult size of array in kw by # hours of equivalent full sun for your orientation times 0.8 for losses ac kw-hrs! Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#7

8 Solar Hours: notable concept worth repeating The number of solar hours is equivalent to the number of hours of full STC sunlight at 1000 Watt/m 2 So it is equivalent to number of kilowatt-hours/m 2 Allows simple and direct conversion between STC power (# kilowatts) of PV and energy produced (# kilowatt-hrs of energy) Account for all system losses with single derating factor (same as Performance Ratio) Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#8

9 Example: simple but accurate calculation Assume a 1.8 kw rated PV system (9-200 STC modules) installed in Delaware at 45 tilt Annual daily average hours of peak sun for DE = 4.6 hours Assume no shadowing, oriented due south Assume 80% derating factor for many small system losses 6% for inverter efficiency, 7% for temperature, 2% resistance See PV Watts for complete list and range of losses So, 1.8 kw x 4.6 hrs/day x 0.8 x 365 days = 2400 kw-hrs/year Comparison: my system produced 2350 kw-hrs (2007), 2244 kw-hrs (2008) Actually have 3% shading, so 2400 x 0.97= 2330 kw-hrs/year Installer (Solar Pathfinder chart) predicted 2282 kw-hrs/year Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#9

10 Solar only works where its sunny and hot, right? Wilmington has 72% as much annual sunlight as Phoenix Sunlight is relatively evenly dispersed over large fraction of US Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#10

11 Solar cells work better in summer? Module Power (Watts) module temperature (F) PV module rated at 200 Watt standard test condition (77 F) Percentage full output (%) outdoor air temperature (F) Output decreases about 0.3% for each degree F (-0.5%/C) Module temperature deg F higher than air temperature

12 Wait, it gets even better- Wilmington DE vs Phoenix AZ Compared to 25 C STC rating, actual temperature costs: Module in Wilmington loses about 7% Module in Phoenix loses about 13% Wilmington has only 72% as much sunlight as Phoenix So PV module in Wilmington can generate 78% as much energy as in Phoenix! Modules in Wilmington and Minneapolis MN have same annual output (+3% difference in sun hours, -3% due to T)

13 But what about the pitch and orientation of my roof? Do PV modules work if they aren t facing exactly towards the sun? Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#13

14 Solar radiation on a tilted surface Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#14

15 Does my roof have the right slope for solar? monthly available insolation for different tilt ang (Wilmington lat=40 N) 6 Wilmington Annual Average Irradiance vs Tilt Angle 6 kw-hr/m2/day (flat) 2 25 (Lat-15) 55 (Lat+15) 1 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Conclusion: Tilt is almost irrelevant (for annual output)!! Annl Average (kw-hrs/m2/day) Tilt Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#15

16 Does my roof face the right direction? Effect of azimuth angle (from South to East facing) for various roof tilt 5 Daily Average Sun Hours East Roof pitch Azimuth Angle (180= South, 90=East) Southern orientation more important for steeper roof tilt. But even for 40 tilt, only 12% annual loss for ±60 of South South Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#16

17 But the sun doesn t shine the same every day. What about variability in sunlight? Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#17

18 Variability in irradiance (solar hours) Wilmington DE data (horizontal tilt) March 21: AVG=3.92 SD=1.34 March : AVG=3.84 SD=0.30 Yearly : AVG=4.02 SD= Wilmington DE Solar Data March 21/March/Year solar irradiance (kw-hrs/m2/day) or hours of full sun/day March 21 March avg year avg. year Off-grid stand-alone system design must account for minimum solar hours not annual average! Grid connected system only cares about average hours. Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#18

19 Monthly variation in solar hours: clouds 8 7 monthly average insolation (sun hours) for latitude tilt angle Avg kw-hr/m2/day MIAMI, 25 PHX, 33 WILM, 40 MINN, Month PHX MIAMI WILM MINN Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#19

20 So, is there any limitation to installing solar panels on my roof? Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#20

21 Tree shadowing most common problem in Newark Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#21

22 And in California (those darn redwoods ) Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#22

23 Beware of even small shadows: series connected Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#23

24 Why only 1.8 kw? Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#24

25 Series connected solar cells in a module GE 200W module 54 cells in series V MP =26.3 V V/cell=0.49 V I MP = 7.6 A I/cell=7.6 A Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#25

26 Series connected cells and shading Shadowing 1 cell out of row on 1 module (6 cells) 1 module (54 cells) Cover 1 module with porch screen Decrease in Watts (approx) 6% 20% 20% 15% Decrease in power much greater than just power lost from that cell or module. Shadowed module actually consuming power, converting it to heat ( hot spots ) Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#26

27 Sun charts: predictable hourly, daily and monthly motion of sun across sky Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#27

28 Shading and Solar Travel: Solar Pathfinder Widely used tool by PV professionals Maps shadows onto monthly solar trajectory grid (latitude specific) Calculate monthly and annual shadowing Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#28

29 Solar Pathfinder Analysis and Report: Losses due to Tilt and Shading: Calc monthly and annual output for optimum and actual tilt (99%), shading (97%), savings Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#29

30 What else do I need besides the PV module? Balance of Systems (BOS) Inverter (convert DC to AC power) Wiring, conduit, utility disconnects, meters Mounting rails on roof PV is about 60-80% of installed grid connected system cost Complete systems for grid connection now widely available Installers have preferred manufacturer, select compatible components for different size systems Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#30

31 Inverter: brains of the system Converts DC from PV into AC for home use or sell to grid Efficiency 94-96% Typically 220V in US Critical element for utility safety Shuts off <2sec when grid down Or 88%<V AC <110% Or freq: 59.3<f<60.5 Hz Off > 5 min after disturbance Negligible EM interference Generate AC only when grid connected Keeps array at maximum power About 10-15% of total $ About /W Major source of poor reliability!!! AC to house DC in from PV Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#31

32 Xantrex 3 kw inverter efficiency Peak Eff. ~ 95%, (Euro standards 94%) Peaks at about half the rated power Cost about $0.80/W Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#32

33 Inverter-to-PV ratio : sizing factor SF Common to undersize inverter SF=P INV /P PV Inverters $800/kW, much cheaper than PV, so not just cost PV DC power rarely operates at rated (PR= ) But sometimes PV exceeds inverter rating Inverter thermal protection avoid damage SF=70-150% allow inverter limit losses <1% many locations Limited choice of inverter sizes; cost effective to choose SF<1 even if occasionally overloaded. Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#33

34 Inverter-to-PV: sizing factor, meas vs calc?????? Top L: am (squares) pm (triangle) Top R: Vdc for SF=55% Bot R: Vdc for SF=100% Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#34

35 Effect of oversizing PV: inverter protects itself MPPT increases V, move out on I-V curve, limit P=constant Decrease P even more to protect inverter from overheating Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#35

36 Various grid tied residential-size inverters Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#36

37 Whats maximum power tracking? Module Power (Watts) Module power for different intensity (suns) and temperature sun, T=25 C 0.75 Sun, T=25 C 0.5 sun, T=25 1 sun, T=60 C 0.5 sun, T=60 C Volts Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#37

38 9-200 W roof mounted GE modules Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#38

39 Hegedus house DC and AC wiring diagram Note dc and ac disconnect, grounding Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#39

40 Feet and mounting rail installation Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#40

41 Rooftop junction box, ground wire and conduit Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#41

42 Mounting clamps and rail Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#42

43 First module installed Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#43

44 Torquing down the bolts and clamps Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#44

45 Outside: wiring, conduit and disconnects AC to or From Grid! DC From PV DC AC net meter AC To inverter In basement From inverter In basement Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#45

46 Inside: install inverter (Xantrex/GE), meter, connect to breaker box 2.5 kw Inverter 220 V output Meter Original Breaker box PV connect In existing 220V slot DC from PV Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#46

47 Hegedus House 1.8 kw system energy estimate Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#47

48 Monthly PV system output (ac): measured vs predicted comparison of predicted (Solar Pathfinder, DRF= and measured monthly PV output 300 % difference installer-predicted and actual PV energy o monthly PV energy (kwhr) Calc PathFinder Measured 2007 Measured 2008 Measured Month % difference Month Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#48

49 Daily output: March and June March 2007 daily kw-hrs output Hegedus system Expected value 7.2 kw-hrs 14 June 2007 daily kw-hrs output Hegedus system Expected value 8.7 kw-hrs daily kw-hrs daily kw-hrs snow on modules day of the month day of the month Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#49

50 Daily output: Sept and Dec September 2007 daily kw-hrs out Hegedus system Expected value 7.7 kw-hrs 14 December 2007 daily kw-hrs outp Hegedus system Expected value 4.4 kw-hrs daily kw-hrs daily kw-hrs day of the month day of the month Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#50

51 Watts produced by solar array Hourly data from two cloudless days Daily output for two clear "cloudless" days for 116 Manns Ave Newark 1700 W max ac expected Feb 17, 2007 May 30, :00 8:00 10:00 12:00 14:00 16:00 18:00 time (hrs) hours daylight PV online (hours) kw-hrs Max Temp( F) 2/ More solar power in winter? Cold and Clear!!! 5/

52 City of Newark electric bill before and after PV electric bill before (3 year avg) and after PV (2007) 80 before PV 60 after PV 2007 electric bill ($) Month Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#52

53 What fraction of total power from PV? percentage of PV power (%) Fraction of house electric demand supplied by PV Annual average = 72% ratio PV/house 2007 ratio PV/house 2008 ratio PV/house Month After 2 years, PV supplied 72% of our house load Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#53

54 Cost vs system size for local installer (2007) PV System Costs vs Size $ / W PVs 13 PVs 16 PVs 28 PVs array power (kw) Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#54

55 Costs* of 3 local PV systems: effect of size Size kw # PV Mods Invert er kw Ratio PV/Inv Total cost $/W install $/W total $ , , ,000 * Divide all costs by 2 for after-rebate cost; PV about $6.25/W Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#55

56 System size and cost: California Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#56

57 So, what s the payback time? 1.8 kw system in DE x 4.5 hrs sunlight/day x kw-hrs/day Price of electricity= 15 cents/kw-hr (conservative for 20 year!!) So PV offsets $445/yr of electricity costs (with net metering) $10/W installed, 50% State rebate, $2000 Fed $8000 net cost 20 year life leads to $400/year annual cost (simple, no deprec, maint, etc) Approx 17 year payback (assuming no increase in electric rates!!) PV electricity costs about same as grid electricity it offset Neglects maintenance, rising rates, Renewable Energy Credits If RECs sell for est 200 $/MWhr, 2 kw system earns $500/yr, reduces payback time to <8 yrs Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#57

58 Renewable Energy Credits Selling attribute of clean energy not the kw-hr Minimum unit of 1 MW-hr Sold to utilities, states to meet their mandated Renewable Energy Portfolio or other internal goals Lots of paperwork My installer just sold his customers RECs for us at about $200/MW-hr (or 20 cents/kw-hr which is more than the price of the energy!!) Check for $450 Will reduce payback time by more than 2X (<8 years) if steady Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#58

59 Long Term Performance and Maintenance PV modules warrantee to provide 80% of initial rated performance after 20 or 25 years Expect 1% degradation in output per year Inverters warranteed 5-10 years, might get 8-15 years Experts recommend annual inspection for loose wires, high resistance, dirt etc ($200) Hose them off after extended dry spell (dust, pollen, bird poop) Western desert locations 5-7% annual losses due to dust, 2-3% with one rinsing Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#59

60 Know your PV system (trust but verify!) I recommend DAILY recording of system output Constantly aware of sunlight, clouds, system output Should get highest output at noon on bright sunny cold day; compare to expected output Compare to similar days in recent past Compare monthly output to predicted kw-hrs Be an active PV owner: educated, involved, aware Most inverters offer computer interface to monitor system Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#60

61 Don t have good roof exposure? No problem! 2.4 kw ground mounted array, Newark Rt 896 Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#61

62 PV rooftop array as standard feature in new developments (SMUD in central CA) Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#62

63 1. PV works just fine in Delaware Summary 2. Orientation and tilt nearly irrelevant (within 10%) 3. Avoiding shadowing is critical 4. Small system can meet significant fraction of load if large electric appliances are avoided (AC, electric dryer, range, etc) 5. Installation was hassle free 6. Presently supplying 72% of household demand with PV Steve Hegedus, Institute of Energy Conversion ELEG620 April 5, 2009 slide#63