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DC-Coupled Solar Plus Storage: Results from the Field
- 1. Battery Energy Storage System Integration on the DC Bus of a PV Farm Inverter Jason Handley, P.E. Director – Smart Grid Emerging Technology Copyright © 2018 Duke Energy All rights reserved.
- 2. Battery Energy Storage Systems – AC vs. DC Coupled page 2Copyright © 2018 Duke Energy Corporation. All rights reserved. AC-Coupled Hybrid PV + StorageDC-Coupled
- 3. Mount Holly Microgrid – DC Coupled Battery 100 kVA Parker Hannifin PV Inverter 150kW PV Farm 240kW/122kWh SAFT Battery 250kW Dynapower DC-DC Converter Recombiner Box Combiner Box Blocking Diode 480V - 315V XMFR 277V – 120V XMFR DC/AC Ratio Annual Energy AC Production Energy Lost to “Clipping” 1.0 163.06 MWh 0.0 MWh 1.3 193.86 MWh 1.8 MWh (0.9%) 1.5 217.24 MWh 11.0 MWh (4.9%) PV System Characteristics 1. South Facing system 2. 20˚ - tilt ground mount system 3. PAC = 100kW; PDC = 149.50kW 4. PV irradiance > 700 W/m2 5. GPS Coordinates: 35.29˚ Latitude DC Breakers and relays
- 4. Mount Holly Microgrid – One Line Diagram SAFT Mini – E 240kW 122kWh RCB Dynapower 250kW DC-DC Converter DPS - 250 ABB 200A DC Breaker Blocking Diode & CT 25 strings with 19 panels in series (total 150 kW)
- 5. Dynapower DC-DC Converter – DPS 250 Basic Constant Current Constant Power Constant Voltage Advanced Clipping Early Morning/Late Evening Capture Capacity Firming Ramp Rate Control PV Time Shifting Operating Modes
- 6. Operating Modes - Advanced page 6Copyright © 2018 Duke Energy Corporation. All rights reserved. Estimated energy that can be captured from clipping with 1.5 ILR: ~5% Early morning/Late evening capture: DC-DC Converter operates in MPPT mode, while PV inverter is OFF Capacity Firming enables the fixed user defined AC inverter output, regardless of the PV DC production
- 7. Irradiance – perfect sunny day page 7Copyright © 2018 Duke Energy Corporation. All rights reserved.
- 8. Mount Holly PV Farm Output – perfect sunny day page 8Copyright © 2018 Duke Energy Corporation. All rights reserved. Pmax (PV) E = 677.42 kWh E = 827.78 kWh = 150.37 kWh or 22.20 % PV Farm output (no effect on kW curve with DC-DC converter) Actual power stored in BESS= 85.4 kWh P (DC-DC) Energy stored in battery
- 9. page 9Copyright © 2018 Duke Energy Corporation. All rights reserved. PV Inverter OFF DC- DC Converter OFF MPPT CLIPPING OFF OFFMPPT MPPT OFF PV EMUL MPPT OFF OFF Early Morning Capture Late Evening Capture OFF Putting it all together
- 10. 500 kW 2,400 kWh DC COUPLED Solar Plus Storage 10Dynapower Confidential DC-COUPLED MOST ECONOMICAL SOLAR PLUS STORAGE • LESS EQUIPMENT — TRANSFORMERS, SWITCHGEAR • MORE ENERGY — CLIPPING RECAPTURE, LOW VOLTAGE HARVEST • FACTORY INTEGRATION WITH CENTRAL INVERTER MANUFACTURERS — FURTHER COST REDUCTIONS
- 11. CLIPPING PROFILE MA SMART Program Simulated clipping based on historic weather data. 2 MW AC Inverter ILR: 1.5
- 12. DC-COUPLED CLIPPED CHARGE PROFILE Simulated clipping power profile based on historic weather data. 2 MW AC Inverter ILR: 1.5 DC Coupled Storage: 1,000 kW 2,000 kWh
- 13. FINANCIAL/TECHNICAL ANALYSIS MASSACHUSETTS SMART PROGRAM 1,000 kW 2,000 kW2,000 kWh With the addition of DC coupled storage the customer will be able to capture 265,388 kWh of clipped energy per 2 MW system and 116,192 kWh of clipped energy on the 1 MW system. $1,496,866. ADDITIONAL REVENUE PER YEAR COMPARED TO AC-COUPLED STORAGE
- 14. MASSACHUSETTS SMART ESS ADDER MA SMART Program 14
- 15. PV Array Capacity (kW STC) 3000 2700 PV Inverter kW AC Rating 2000 1000 ILR 1.5 2.7 PV Only kWh Production 4,451,127.19 2,739,453 PV Only PPA $ 0.1553 $ 0.1553 PV Only Revenue $ 691,260.05 $ 425,437.05 ESS kW 1000 500 ESS kWh 2000 1000 Storage Hours 2 2 ESS PPA Adder (MA SMART) $ 0.0363 $ 0.0247 PV kWh Clipped 334,032.13 874,390.50 % Clipping Recaptured 79% 30% Clipping kWh Captured 265,388.32 262,317.15 PV + ESS PPA $ 0.1916 $ 0.1800 PV + ESS Revenue (AC coupled) $ 852,835.97 $ 493,101.54 PV + ESS (DC Coupled) $ 903,684.37 $ 540,318.63 DC Coupled ESS Cost $ 1,200,000.00 $ 600,000.00 DC Coupled Additional Annual Revenue Compared to PV only $ 212,424.32 $ 114,881.58 DC Coupled Revenue Compared AC Coupled Revenue $ 50,848.40 $ 47,217.09 Clipping Revenue Per kWDC $ 16.95 $ 17.49 DC Coupled ROI (years) Compared to PV Only 5.65 5.22 DC-COUPLED SUMMARY ANALYSIS MA SMART Program
- 16. 500 kW 2,400 kWh DC COUPLED Solar Plus Storage 16Dynapower Confidential DC-COUPLED MOST ECONOMICAL SOLAR PLUS STORAGE • LESS EQUIPMENT — TRANSFORMERS, SWITCHGEAR • MORE ENERGY — CLIPPING RECAPTURE, LOW VOLTAGE HARVEST • FACTORY INTEGRATION WITH CENTRAL INVERTER MANUFACTURERS — FURTHER COST REDUCTIONS
- 17. CLIPPING PROFILE MA SMART Program Simulated clipping based on historic weather data. 2 MW AC Inverter ILR: 1.5
- 18. DC-COUPLED CLIPPED CHARGE PROFILE Simulated clipping power profile based on historic weather data. 2 MW AC Inverter ILR: 1.5 DC Coupled Storage: 1,000 kW 2,000 kWh
- 19. FINANCIAL/TECHNICAL ANALYSIS MASSACHUSETTS SMART PROGRAM 1,000 kW 2,000 kW2,000 kWh With the addition of DC coupled storage the customer will be able to capture 265,388 kWh of clipped energy per 2 MW system and 116,192 kWh of clipped energy on the 1 MW system. $1,496,866. ADDITIONAL REVENUE PER YEAR COMPARED TO AC-COUPLED STORAGE
- 20. MASSACHUSETTS SMART ESS ADDER MA SMART Program 20
- 21. PV Array Capacity (kW STC) 3000 2700 PV Inverter kW AC Rating 2000 1000 ILR 1.5 2.7 PV Only kWh Production 4,451,127.19 2,739,453 PV Only PPA $ 0.1553 $ 0.1553 PV Only Revenue $ 691,260.05 $ 425,437.05 ESS kW 1000 500 ESS kWh 2000 1000 Storage Hours 2 2 ESS PPA Adder (MA SMART) $ 0.0363 $ 0.0247 PV kWh Clipped 334,032.13 874,390.50 % Clipping Recaptured 79% 30% Clipping kWh Captured 265,388.32 262,317.15 PV + ESS PPA $ 0.1916 $ 0.1800 PV + ESS Revenue (AC coupled) $ 852,835.97 $ 493,101.54 PV + ESS (DC Coupled) $ 903,684.37 $ 540,318.63 DC Coupled ESS Cost $ 1,200,000.00 $ 600,000.00 DC Coupled Additional Annual Revenue Compared to PV only $ 212,424.32 $ 114,881.58 DC Coupled Revenue Compared AC Coupled Revenue $ 50,848.40 $ 47,217.09 Clipping Revenue Per kWDC $ 16.95 $ 17.49 DC Coupled ROI (years) Compared to PV Only 5.65 5.22 DC-COUPLED SUMMARY ANALYSIS MA SMART Program
Editor's Notes
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- What problems are we trying to solve here? There are three ways to couple the BESS to the power system: AC, DC and hybrid. Depending on how BESS is coupled, the range of benefits changes. All potential use cases and problems that could potentially be solved are listed on the left side (there are total of 6 of them). Those are some of the challenges that our feeders are faced with high penetration of PVs. At Mount Holly, we have both AC and DC coupled BESS. Based on the graph above, DC coupled battery provides the most flexibility in terms of use cases.
- This table shows the operational characteristics of the PV farm at Mount Holly, which is a good representation of large PV systems at Duke Energy footprint (GPS coordinates, ILR ratio, etc…one of the differences is that some systems are not fixed tilt, so their analysis might be slightly different). As can be seen from this analysis, with 1.5 ILR ratio, the total yearly energy output with DC coupled BESS is ~5% more than the similar system without BESS. Note that this analysis was done by the third party.
- This slide shows all that was needed to be added in order to create DC coupled BESS. Parts in black are from the original PV system, and parts in red were additional parts that were added (5 strings, BESS, DC-DC converter, breakers, recombiner box, and blocking diode & CT
- Why we picked DynaPower is also because the number of operating modes – total of 8. They allow for flexibility and getting the maximum out of the overall solution. As you can see there are 3 basic and 5 advanced modes. Most of our operation is in Advanced mode range.
- This is one part of the overall optimization algorithm that shows the advanced operating modes that collect the maximum kWh from the PV system. This is composed of two parts. One part is early morning/late evening capture. PV inverter starts producing power when the voltage on DC side is larger than the voltage on AC side (which in our case is over 525Vdc). However, DC-DC converter can actually start charging the battery at 380V. So this is additional benefit to the overall efficiency, since this energy would otherwise be wasted if there was no DC-DC converter. Same goes for evening side where PV system shuts down and then DC-DC converter is used to charge the remaining kWh into battery. The middle of the day is clipping, when the most of kWh is stored in BESS. This is another advanced mode, where battery can be used to smooth the output of PV. So in this case, if the cloud comes over and PV output starts going down below 100kW, then battery is used to discharge to make up for the difference. If there is any excess of output of PV, then the system is in clipping mode, so kWh is stored in the battery. Towards the end of the day, we can enter capacity firming mode where we use the output of the battery to reduce the feeder peak.
- Here are some of the actual field results. The graph shown in this picture was recorded on a perfect sunny day and it shows the irradiance. As can be seen from the graph, the curve is almost perfect shape for the operating region of PV farm.
- The blue curve shows the output of the PV farm during this perfect day. PV inverter operates in MPPT mode and DC-DC converter operates in Advanced Mode (clipping only). As seen from the graph, the curve shows the expected output of the PV farm for the perfect sunny day. When the PV inverter reaches its maximum kW output, the DC-DC converter starts in Advanced mode and it stores the “clipping” portion of the curve in the battery. Note that in this case, in order to enable this functionality, we had to install a separate meter on the AC side of the PV inverter, and send this data to DC-DC converter in order to properly implement the Advanced “clipping mode”. As can be seen from the graph, the total energy produced from the PV system is 677.42kWh (note that this includes all system losses which are 2.4kW when PV is in not conducting), and max energy that could be stored in the battery is 150.37kWh, which is an increase of 22.2%. Our BESS is rated at 122kWh and normal operating SOC are 20% 90%, so our maximum stored energy was 85.4kWh (black line) .
- This slide shows how we change the operating modes of DC-DC converter and PV inverter for our preferred automated algorithm. This is the most critical part, because we must ensure that we do not put these devices in operating modes that might damage the equipment or cause faults on the system.