Download T11 Memorandum

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.
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5.
Battery University
www.plugincars.com
www.homepower.com
www.altestore.com
www.wholesalesolar.com