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2016 Stand Alone PV System for EV Charging MEMORANDUM JIGEESHA UPADHYAYA AND KABILAN SELVARANGAN 10/29/16 Memorandum To: Ron Roedel, CTO, Second Solar, Inc. From: Jigeesha Upadhyaya, Solar Industry Consultant Kabilan Selvarangan, Solar Industry Consultant Re: Standalone PV System to charge an EV Introduction This memorandum presents the design and analysis of a residential PV charging system for Chevy Bolt 2017, a new model of electric vehicle by Chevrolet. The charging station is independent of the existing residential electrical system as well as electrical grid i.e. all the output from PV is directed to charge the car. Chevy Bolt was selected because of its promising specifications at a very good cost. Based on our analysis we came up with a solution that says a 10kW PV system is required to charge the car to meet the daily average run of 50 miles. The details of this calculation are shown in the rest of this memorandum. Analysis The time has come that we, human beings, try to take the problem of climate change seriously and do our tiny bit by adapting to clean energy techniques in all spheres of life. Here, we focus the use of clean energy in the sector of transportation. We take an electric car, we use solar energy to charge the car which makes the carbon footprint almost zero. We cut on fossil fuels and use a clean energy source which is not just clean but will be available free of cost after a certain period of time. A miracle! We, next, focus on the engineering of the design that is involved to make that miracle happen. Following steps were taken for designing the PV system: a. Selecting the car: We selected 2017 Chevy Bolt EV as we found it gives a high range of 238 miles on a single charge and is comparatively cheaper than other EVs in the market. The table below gives the details necessary for our calculation: Electric Battery Miles Motor Vehicle Size on a Power Cost Charging 0-60mph Rate single charge 2017 60kWh 238 150kW $37,500 7.2kW/hr 7 seconds Chevy Bolt EV b. Determining the load per day: The first and foremost step before designing a PV system is to calculate the load requirement. According to average daily mile requirement of 50 miles by the car, we was calculated that a 12.6kWh of load is required every day. Now, since there are lots of other electronic components involved in building a PV Systems, we need to consider the losses to find the corrected load. To calculate the corrected load, following parameters were considered: a. Charge Controller Loss – 96% b. Battery Efficiency – 88% c. State of Charge – 80% d. Inverter loss – 95% e. Wire Loss – 98% f. Temperature Coefficient – 0.88 g. Derate Factor – 90% h. Temperature Multiplier – 1.04 i. Days of Autonomy - 2 After incorporating all these parameters in our load calculation, we get a total load requirement of 52kWh. If we divide this by the peak sun hours of Phoenix i.e. 6 hours we get the PV System size which is approximated as 9kW. Based on the system size and daily load requirement we figured out the other components. The components used are 30 300 watt modules, 3 MPPT Charge controllers, 24 6volt lead acid batteries, 10kW inverter and a Level 2 EVSE (Electric Vehicle Supply Equipment) to charge the car. Each of these components will be described below in detail: 1. Solar Modules: We have used ‘Vikram Solar Eldora’ modules. Based on the input voltage and current rating of the charge controller, we designed the best possible configuration of the modules and reached to conclude that we will be needing 10 modules, of 300W capacity each, in a string and three such strings in parallel connected to three different charge controllers. So a total of 30 modules are used in a 10*3 fashion. 2. MPPT Charge Controller: We used three ‘Schneider Conext MPPT 80 Solar PV Charge Controller’ for each 10 module string. The maximum input it can take is 600V and the maximum input the solar modules can output is 456V. In terms of current, each string will generate maximum of 8.8A and the input current limit for the charge controller is 28A. So all the values are justified. The output from the charge controller can be in two forms. It can either provide load to DC appliances or connect batteries and charge them with the PV module output. We will not be needing any DC appliance so we charge the lead acid batteries by providing them to 48V (battery bank voltage) and 80A. 3. Lead Acid Batteries: We decided we will be using 48 volt battery bank as generally it is advised to use 48 volt battery pack for systems larger than 3-4 kW. So we used 24 ‘Crown CR-430 6 volt Flooded L16 Batteries’. The depth of discharge for the batteries is 80% so we must not discharge the battery beyond 80% i.e. a 10kW battery bank becomes 8kW. If the battery is left at low charge for a long time, problems like sulfation occur which reduce battery life, hence battery must always be stored at full state-of-charge. To minimize batteries in parallel and cater to the voltage demand we chose to keep 3 batteries in parallel and 8 in series. 4. Inverter: Since the PV size is 9kW, we have to select the rating of the inverter higher than that so we selected ‘SolarEdge SE10kUS’ inverter with AC Power Output of 10000 Watts. The input and output voltage is well within limits. 5. Level 2 EVSE (Electric Vehicle Supply Equipment): It is very important to select an EVSE of the same power rating as the car requires. So we found ‘EvoCharge EVSE’ of 30A rating and 7.2kW power rating which matches with the car charging rate of 7.2kW per hour. EVSE basically converts the AC power from the inverter into DC to be fed into the car. The details of the analysis and calculation are shown in the Appendix Appendix Appendix A: Calculating the load and PV System Size Energy to run 238 miles Energy to run 50 miles 60kWh 12.6kWh 𝐿𝑜𝑎𝑑 ∗𝐷𝑜𝐴∗𝑇𝐵 12.6𝑘𝑤ℎ∗2∗1.04 Corrected load =𝐼𝐿 ∗𝑊𝐿 ∗𝐵𝐸∗𝐷𝑜𝐷∗𝑇𝐶∗𝐶𝐶𝐸∗𝐷𝐹 = 0.95∗0.98∗88∗.8∗.85∗.88∗.9 = 52kWh Where, IL - Inverter loss WL – Wire Loss BE – Battery Efficiency DOD – Depth of Discharge TC – Temperature Coefficient CCE – Charge Controller Efficiency DF – Derate Factor DoA – Days of Autonomy TB – Temperature of Battery Multiplier Corrected Load PV System Size = Peak Sun Hours = 52kWh 6h = 9kW Appendix B: Calculating the Battery Size and configuration Average Load Battery Capacity = Voltage of battery system = 52.064x1000 48 Total Capacity = 1085Ah Number of batteries in parallel = Capacity of single battery = System Voltage Number of batteries in series = Battery Voltage = 48 6 1085 430 =3 =8 So a combination of 3 parallel strings of 8 batteries in series will satisfy the battery requirement. Appendix C: Charge Controller Specifications and Module Configurations Specifications for Schneider Conext MPPT 80 600 Charge Controller: Nominal battery voltage 48V PV array operating voltage 195V to 550V Max. PV array open circuit voltage 600 V Array short-circuit current 35 A (28 A @ STC) Max. Output current 80 A Max. output power 4800 W (for 48 V battery) Max. power conversion efficiency 96%(for 48V battery) 9kW Output current from charge controller required to charge batteries = 48 = 187.5A But the rating of the selected charge controller allows a maximum of 80A So, we will require 3 of the charge controllers in parallel to cater to the current demand by the batteries. Power of PV Array 9kW Total number of modules required = Power of single module = 300W = 30 modules To connect 30 modules in parallel across 3 charge controller, the best way is to make 3 strings of 10 modules in series. Voltage per string = No. of modules*Voc = 10*46.7 = 467V < 550V So voltage is within limit of the charge controller. Appendix D: Inverter Selection To satisfy the PV System demand of 9kW, we used an inverter of 10kW. Specifications of SolarEdge SE10kUS is given below: Nominal AC Power Output 9980 @ 208V 10000 @240V Max. AC Power Output 10800 @ 208V 10950 @240V Vm(min) 240 Vin(max) 500 Max. Continuous Output Current 48 @ 208V 42 @ 240V Max. Input Voltage 500 Nom. DC Input Voltage 325 @ 208V / 350 @ 240V Max. Input Current 33 @ 208V 30.5 @ 240V Max. Input Short Circuit Current 45 Appendix E: EVSE Rating The EVSE rating should be in tally with the inverter. An EVSE should be able to handle the amount of current inverter supplies continuously. In our case, the output current from the inverter is 42A. So we chose an EVSE which can handle up to 50A of current. References 1. 2. 3. 4. 5. Battery University www.plugincars.com www.homepower.com www.altestore.com www.wholesalesolar.com