IRJET- Power Management System for Electric Vehicle Charging Stations using F...
SEC Final
1. Design Project -2
Battery-Backup PV System in Mumbai
Team 05
Ajay Renganathan, Aakash Bhansali,
Naveen Kadimcherla
SEC 598 – Photovoltaic systems
engineering
Date: 12/10/2015
2. ii
Table of Contents
Executive Summary........................................................................................................................ v
Introduction..................................................................................................................................... 1
Work Description............................................................................................................................ 2
1) Design process.................................................................................................................. 2
(a) Site selection:..................................................................................................................... 2
(b) Evaluation of the solar resource:....................................................................................... 4
(c) Load analysis:.................................................................................................................... 5
(d) Inverter sizing:................................................................................................................... 5
(e) Battery sizing:.................................................................................................................... 6
(f) Array sizing and determination of annual output: ............................................................. 6
2) System Design.................................................................................................................. 8
(a) Description of system........................................................................................................ 8
(b) Block diagram ................................................................................................................. 10
(c) Wiring diagram................................................................................................................ 10
(d) Details of selected components (for data sheets see Appendix C).................................. 11
(e) Compliance with Indian NEC ......................................................................................... 22
3) Costs............................................................................................................................... 23
(a) Bill of materials ............................................................................................................... 23
(b) LCOE .............................................................................................................................. 24
(c) Annual and cumulative return ......................................................................................... 26
Conclusions................................................................................................................................... 29
Appendices.................................................................................................................................... 31
Appendix A – Battery sizing calculations .................................................................................... 31
Appendix B- Array sizing calculations......................................................................................... 32
Appendix C- Manufacturer’s Data sheets..................................................................................... 32
Appendix D – Overall system efficiency calculation ................................................................... 38
3. iii
List of Figures
Figure 1: Rooftop area 1 ................................................................................................................. 2
Figure 2: Rooftop area 2 ................................................................................................................. 3
Figure 3: Rooftop area 3 ................................................................................................................. 3
Figure 4: India Solar resource NREL ............................................................................................. 4
Figure 5: PVWatts results............................................................................................................... 7
Figure 6: Rooftop area 2 ................................................................................................................. 8
Figure 7: PV system diagram (Source: Enphase) ........................................................................ 10
Figure 8: Wiring Diagram............................................................................................................. 10
Figure 9: REC Twin peak string comparison ............................................................................... 12
Figure 10: Enphase module compatibility.................................................................................... 12
Figure 11: Lead acid vs Lithium Iron Phosphate (Source: Iron Edison) ...................................... 15
Figure 12: Wire selection chart..................................................................................................... 17
Figure 13: Voltage drop between Junction Box and Sub Panel ................................................... 18
Figure 14: Voltage drop between Sub Panel to Inverter charger and between Inverter charger and
Main Panel .................................................................................................................................... 18
Figure 15: Pergola structure.......................................................................................................... 19
Figure 16: Top view...................................................................................................................... 20
Figure 17: Side view..................................................................................................................... 20
Figure 18: Front view.................................................................................................................... 20
Figure 19: Overall system............................................................................................................. 21
Figure 20: Annual Return ............................................................................................................. 27
Figure 21: Cumulative return........................................................................................................ 28
4. iv
List of Tables
Table Caption Page number
Table 1 Load analysis 5
Table 2 Balance of system list 21
Table 3 Bill of materials 23
Table 4
Tata Power Utility tariff
rates
24
Table 5 Annual Return 26
5. v
Executive Summary
A battery backup photovoltaic system is one of the possible solutions to ensure
uninterrupted availability of the power to the customer, especially in countries experiencing
frequent power cuts like India. During the day, the PV array produces power which is used to
operate essential loads, charge a battery, or feed excess power back into the electric grid. When
the grid goes down, the energy stored in the battery is used to run the critical loads.
This project is about designing an AC coupled battery backup grid tied PV system for an
apartment in Mumbai, India with a gym facility that operates between the hours of 6 am -10 am
and 6 pm – 10 pm. It will provide backup power to the gym facility through batteries and help in
reducing the electricity costs of running this gym over the long term. To provide enough AC
energy to entirely power the gym for 8 months out of 12, it is estimated that an 11.76 kW PV array
was required and any excess power produced, if any, shall be fed into the grid via net metering.
The system consists of 280W polycrystalline modules, Lithium Iron Phosphate batteries,
Inverter/Chargers and microinverters.
The total cost of the system after applying 30% incentive is $52,598.7. The installed cost
per watt is $4.48/W. The levelized cost of energy is $0.46/kWh and the payback period was 16
years.
6. 1
Introduction
India is rapidly moving towards becoming one of the leading nations in the cause of
reduced carbon emissions. India has recently initiated work on the Global Solar Alliance and is
aiming for research in solar technologies and helping developing nations to equip with solar. The
Central Government of India has recently reinstated its 30% tax credit thus placing itself as a
lucrative market. The residential complexes now a days have various amenities like gym, air
conditioned lounges, pools, multiple elevators etc. However, certain complexes experience
frequent power cuts, with power not being available for long periods of time.
One solution to this, is the improvement of the grid infrastructure by the electric utility.
However, this is not an easy solution, and neither are the utilities currently making progress on
this in a timely manner. There is another solution, and that is, a battery backup photovoltaic system.
During the day, the PV array produces power which is used to operate essential loads, charge a
battery, or feed excess power back into the electric grid. When the grid goes down, the energy
stored in the battery is used to run the critical loads.
In this project, we decided to design a battery backup PV system to sustain the loads in a
gym in a residential complex in the city of Mumbai. A few key reasons for choosing this city
include: presence of a net metering policy, over 300 days of sunlight per year, and a renewable
energy policy aimed at creating 14,400 MW of fresh grid-connected installed capacity by 2019-
20[1].
This apartment has a gym facility which incurs a major portion of its total monthly
electricity bill. The gym loads operate between the hours of 6 am -10 am and 6 pm – 10 pm. The
objectives of this project are to provide backup power to the gym facility through batteries and to
reduce the electricity costs of running this gym over the long term. Based on our load analysis and
PVWatts results, we determined that an 11.76 kW PV array was required. This system provides
enough AC energy to entirely power the gym for 8 months out of 12. The excess power produced
is fed into the grid via net metering.
7. 2
In this report, we first describe our design process, system design, and the rationale behind
it. Following this, we discuss the components in our system and why we chose them. Finally, we
describe the economic analysis, payback period, and future scope.
Work Description
1) Design process
Designing a battery backup grid connected PV system involves a series of steps:
(a) Site selection:
The apartment rooftop was inspected to determine the available space. PVWatts was used to
draw the system on the satellite image of the location. Based on this, PVWatts displayed the
available space and maximum array size. Here, three different areas were considered. The final
selection of one of the options was done after load and shading analysis
Option 1:
Figure 1: Rooftop area 1
9. 4
(b) Evaluation of the solar resource:
Based on historical data from the national renewable energy laboratories, we analyzed the solar
resource, or the peak sun hours. Here, it can be observed that Mumbai has an average of 5 - 5.5
peak sun hours.
Figure 4: India Solar resource NREL
10. 5
(c) Load analysis:
In this step, the total daily power consumption of all the gym equipment was calculated. The
kW rating of each gym appliance was multiplied by the number of hours of usage to get the daily
kWh. Using this, the monthly kWh was calculated.
Table 1 : Load analysis
Appliance Power rating
(a)
Quantity
(b)
Hours on
(c)
Power consumed
(a) x (b) x (c)
Treadmill 1.5 kW 2 8 24 kWh/day
Cycle 0.75 kW 2 8 12 kWh/day
Fluorescent Lights 0.04 kW 18 8 5.76 kWh/day
Standing Fan 0.1 kW 6 8 4.8 kWh/day
Ceiling Fan 0.07 kW 1 8 0.56 kWh/day
Total = 47.12 kWh/day
Maximum monthly kWh = 47.12 x 30 = 1413.6 kWh/month
Maximum annual kWh = 1413.6 x 12 = 16,963.2 kWh/year
(d) Inverter sizing:
Before going into inverter selection, we would like to point out that battery backup grid
tied PV systems are of two types: DC coupled and AC coupled. A DC coupled system consists of
a charge controller and only one grid tied inverter, whereas an AC coupled system consists of two
inverters, one for the PV array, and another which is grid tied and used to charge to battery.
We chose to go with an AC coupled system with a microinverter to convert the PV array
output and an inverter/charger to charge the battery. The voltage and frequency in India are 240V
and 50Hz. Each of the gym appliances were rated between 220-240V. The inverter and
microinverter were chosen keeping these points in mind. The process of inverter selection and the
reasons for choosing AC coupling will be explained in detail under the “System design” section.
11. 6
(e) Battery sizing:
Based on the daily kWh consumption (47.12 kWh/day), we calculated the minimum required Ah
of the battery. The procedure to determine the Ah is shown below.
Step 1: kWh/day (battery) = kWh/day (loads) ÷ inverter/charger efficiency ÷ microinverter
efficiency ÷ wiring efficiency.
Step 2: Wh/day = kWh/day x 1000
Step 3: Ah/day = Wh/day ÷ nominal battery voltage
Step 4: Finally, considering a depth of discharge (DoD) of 80%:
Minimum required Ah/day = Ah/day ÷ 0.8
Based on these calculations, the minimum required Ah was 1400 Ah/day. For details of these
calculations, please see Appendix A.
(f) Array sizing and determination of annual output:
In this step, the battery to load Ah (step 3 value from previous section) was used to determine the
PV array size. The steps involved in this are shown below.
Step 1: Assuming the battery charging and discharging losses to be 10%, this Ah value was divided
by 0.9.
Step 2: The value obtained in step 1 is multiplied by the nominal battery voltage to get Wh/day.
Step 3: The Wh/day is divided by the peak sun hours in Mumbai to get the array size in watts.
Note: See Appendix B for the details of these calculations
From these steps, the array size determined was 11.7 kW. This value was entered into PVWatts,
which estimated the monthly AC energy production (kWh) for Mumbai, India. The values for each
month were compared to the monthly power requirements (1413.6 kWh). The tilt angle chosen
was 20°, in order to focus on maximizing annual production.
12. 7
Figure 5: PVWatts results
Note: Under system losses, the mismatch and shading loss % was changed due to the use of
microinverters (see Inverter)
As seen from the figure, the array produces enough power to run the loads on all months except
for June, July, August, and September.
13. 8
2) System Design
(a) Description of system
Based on the total area occupied by all of the modules (including spacing to avoid shading), we
decided to choose option 2 as our location. Here, the maximum area available was 190 𝑚2
.
Figure 6: Rooftop area 2
To install the PV system, a pergola will be constructed on the parking lot. The mounting system
will be installed on the pergola at a tilt angle of 20ᵒ due South.
In this apartment, the designed system will power the gym loads which operate between
the hours of 6 am -10 am and 6 pm – 10 pm. We decided to design an AC coupled battery backup
PV system. At the PV array side, we used micro-inverters instead of string inverters, and at the
battery side we used an inverter/charger. The system is single phase 240V and 50Hz.
In AC-coupled systems, the DC power from the array is first converted to AC by a
batteryless inverter, to be used by the AC loads through an AC load panel. Any unused energy is
used by a separate battery-based inverter/charger that either converts the AC to DC to charge the
14. 9
batteries, or, if it is a grid-tied system, it can also pass through to additional AC loads and/or the
grid. Also, if there isn’t enough PV energy to supply all the critical loads, the inverter-charger adds
grid energy.
The reason it is called “coupled” is that when the grid power is out, the battery inverter
“tricks” the batteryless inverter into feeding power. In simple terms, the battery based inverter
provides a “grid tied AC waveform” even when the grid isn’t present, which ensures that the
batteryless inverter syncs with it and stays ON in the absence of the grid.
The reasons for choosing an AC coupled system over DC coupled are as follows:
It is more efficient, as the batteryless inverter (the microinverter in our case) does the
majority of the power conversion. The efficiency of this inverter ranges from 96-98%, as
compared to the battery based inverter with 90-95% [2].
It allows adding battery backup to an existing batteryless grid tied PV system without
changing the existing wiring.
Low voltage, high current DC connections are minimized. This leads to cheaper and easier
installation [3].
Improved array-to-grid efficiency due to the removal of a conversion step.
In an AC-Coupled system, the array is connected to the grid through a PV inverter:
DC (array) PV Inverter AC (grid/load)
In a DC-coupled system, the array is connected to the grid first through a charge controller
and then through a battery grid-tie inverter/charger:
DC (array) Charge Controller DC bus Battery Grid-tie Inverter AC (grid)
The dual conversion results in reduced conversion efficiency [3].
16. 11
(d) Details of selected components (for data sheets see Appendix C)
(i) Module: REC 280TP
The module selected was the REC Twin Peak 280W Polycrystalline module. In order to
obtain an 11.76 kW system we required 42 of these modules (42 x 280 = 11760W).
The total space occupied by all the modules = 42 x 1.64 m2
= 68.8 m2
The modules were arranged in landscape mode with 2 rows of 21 modules each. However,
electrically, there are 3 series circuits of 14 modules each. The arrangement is such that there won’t
be any shading from adjacent modules. Due to this, we don’t require any minimal spacing between
the modules (see mounting for more details).
Reasons for choosing:
This was one of the few polycrystalline modules of 280W, which was compatible with the
Enphase M250 microinverter.
As compared to other 280W modules, the REC 280TP was much cheaper, costing only as
much as a 260W module. The price per module was $248.
The REC module uses unique half-cut cells that lead to a lower fall in power output when
shaded, as compared to the all other manufacturer’s modules.
High efficiency of 17%.
17. 12
Figure 9: REC Twin peak string comparison
REC TwinPeak modules are split into two twin sections which generate electricity
independent to each other, but combine again before the current exits the module. This helps them
to continue producing electricity in the non-shaded section even at times of reduced irradiance on
the module, increasing overall energy yield and installation profitability.
(ii) Inverter: Enphase M250
The inverter chosen was the Enphase M250 microinverter. This is compatible with 60 cell
modules only and gives a maximum output of 250W. After entering the REC module details, it
was found that the M250 was compatible with the REC module (60 cells).
Figure 10: Enphase module compatibility
19. 14
(iii) Battery: Iron Edison Lithium Iron Phosphate
The batteries chosen were Iron Edison Lithium Iron Phosphate batteries. The battery
voltage chosen was 48V. In order to meet our Ah requirement of 1402 Ah/day, we connected two
batteries in parallel. One was 48V, 1000Ah and the other was 48V, 400Ah.
These batteries also come with overcurrent protection, a battery disconnect, and a battery
management system. The battery management system monitors cell voltage and system voltage,
which is critical for Lithium Iron Phosphate batteries.
These batteries have an actual voltage of 52V and a maximum charging voltage of 56.8V.
Reasons for choosing Lithium Iron Phosphate batteries:
Low-Temperature Capacity: The storage capacity of Lead Acid (LA) batteries drop by
50% at -4°F, compared to 8% with LFP (Lithium Iron Phosphate). Keeping lead-acid
batteries warm so that they maintain reasonable capacity in cold climates can be
challenging, giving LFPs an advantage [6].
LFPs have about one-quarter the internal resistance (impedance) of LA batteries, which
reduces battery energy lost to heat [6].
Self-Discharge: At room temperature, idle (stored or disconnected) LA batteries lose 5%
to 15% of their electrical capacity per month, compared to 1% to 3% for LFPs [6].
Lifetime: Regularly used and properly maintained common deep-cycle LA batteries have
an average lifespan of about five years; LFP batteries have an estimated longevity of 10
years—half the frequency of LA battery replacement [6].
Even though Lead acid batteries have a low up front cost, than LFPs, their lifetime kWh
can be higher.
20. 15
Figure 11: Lead acid vs Lithium Iron Phosphate (Source: Iron Edison)
(iv) Inverter/charger: Schneider XW+
An Inverter/charger plays the role of a battery charger as well as an inverter. The Schneider
XW+ was chosen as the inverter/charger. One reason was because it was one out of only two
inverter/chargers compatible with LFP batteries at present. Furthermore, the inverter can be used
in AC coupled systems. It is compatible with India’s voltage and frequency of 240V, 50Hz, and
can handle the maximum current of 57.22A (see wires).
As the Schneider XW+ does not come with a single inverter/charger capable of handling
11.76kW, we connect two inverter/chargers in parallel. The inverters connected in parallel are the
XW+ 5548NA and the XW+ 6848NA. This allows the combination of the both to produce a
maximum array power of 12,300 kW (5500 + 6800 = 12,300).
The inverter/charger is programmed to charge the 48V battery at 54V and the sell voltage
of 54.5V. This is within the voltage limits of the battery (57V max) and the inverter/charger (64V
max).
21. 16
When the battery terminal voltage reaches the bulk voltage limit (programmable), then charging
enters the absorption stage. The inverter/charger bulk voltage is set just below the sell voltage so
that the inverter will only charge the batteries if their voltage drops below the sell voltage.
Working:
As the battery reaches sell voltage, current from the PV array shifts to the inverter for
inversion and powering the loads. The excess is sold to the utility. If the terminal voltage of the
battery falls below the sell voltage, then current from the PV array flows to the battery, to raise it
above the sell voltage again.
(v) Wiring:
Figure 12: Wiring lengths
22. 17
According to India’s NEC, the current handling capability of the wires should be 125% of the total
output current. The total output current from the microinverters was determined in the inverter
section to be 45.78A. Therefore, all of the wires should be able to handle 45.78 x 1.25= 57.22 A.
The electrical system in India supports 230V, 50Hz. The table shown below allows us to choose
the number of inverters per circuit and then select a length, which in the end leads us to the wire
size.
So, according to our design we chose the following parameters:
# of inverters/circuit = 14
Length of one way wire in feet = 168
Thus, the required wire size is #6.
Based on the wire size, we calculated the overall %VD for the wiring throughout the system. We
used a reliable calculator [8] and input the details such as the voltage and current.
Maximum voltage to be handled by all the systems is 240Volts
Maximum Current to be handled by all the systems is 57.22 Amperes.
Figure 12: Wire selection chart [7]
23. 18
1. Voltage drop between Junction box and Sub Panel
2. Voltage drop between Sub Panel to Inverter charger and between Inverter charger and Main
Panel
Figure 13: Voltage drop between Junction Box and Sub Panel [8]
Figure 14: Voltage drop between Sub Panel to Inverter charger and between Inverter charger and Main Panel [8]
24. 19
Since the distance between the Inverter charger and the main panel is the same as that between the
sub panel and the inverter charger, the voltage drop is the same. The total voltage drop of the
system was 2.1%. So, after all the considerations we decided to go ahead with a #6 Copper wire
made by Cerrowire.
(vi) Mounting:
To install the PV system, a pergola will be constructed on the parking lot. The mounting
system will be installed on the pergola at a tilt angle of 20ᵒ due South. This type of arrangement
avoids self-shading by the modules. The mounting selected was Nuevosol Energy’s Nuevo-fix.
The PV mounting system components require no field welding, drilling or other on-site fabrication,
which leads to faster solar panel installation.
An initial visualization of the system is shown in the images below.
Figure 15: Pergola structure
26. 21
Figure 19: Overall system
(vii) Balance of system:
Table 2: Balance of system list
Material Company and Model No
PV system performance meter ABB CDD
Combiner box Midnite Solar MNPV6
Sub Panel Murray LC002GS
Communications Gateway Enphase Envoy
Bi-Directional meter Analog Electric meter
Microinverter cable Enphase engage cable
Wiring Cerrowire THHN
Conduit SOUTHWIRE FO3750050M ALUMINUM FLEX CONDUIT
27. 22
As determined in the wires section, the balance of system components should be able to handle
57.22A.
A subpanel is used wherein critical loads are connected.
Reason for using the subpanel:
If the PV array output of the batteryless inverter is connected to the main distribution panel
instead of the sub panel, the energy from the array and batteryless inverter cannot be used by the
backed-up power system because during a grid outage it becomes isolated and will not have AC
line-voltage present, which it needs to work. So, when retrofitting to an AC-coupled system with
battery backup, the batteryless inverter output circuit will have to be moved to a new specific
backed-up load panel (sub panel), along with the household circuits that you want to operate during
an outage. Relocating those household circuits to a subpanel needs to happen in any retrofit,
regardless if the system is AC- or DC-coupled. The inverter–charger can charge batteries from
either the PV array or the utility if available.
(e) Compliance with Indian NEC
Annual total solar electricity produced = 17,231 KWh
Annual total electricity used = 16,963.2 KWh
Ratio = 17,231 / 16,963.2 = 1.01
This is within the 15% regulation (≤1.15) set by the state of Maharashtra.
Furthermore, the total voltage drop is within the 3% limit specified by the Indian NEC.
28. 23
3) Costs
(a) Bill of materials
Table 3: Bill of materials
Material Company and Model No
Price per
unit ($/unit)
No. Of
Units
Total
Price
(a) (b) (a)x(b)
PV Module REC 280TP 248 42 10416
Microinverter Enphase M250 155 42 6510
PV system
performance meter
ABB CDD 319.99 2 639.98
Combiner box Midnite Solar MNPV6 169.94 1 169.94
Sub Panel Murray LC002GS 13.77 1 13.77
Communications
Gateway
Enphase Envoy 485 1 485
Inverter Charger-1
Schneider Electric CONEXT XW+
6848 INVERTER/CHARGER
4564 1 4564
Inverter Charger-2
Schneider Electric CONEXT XW+
5548 INVERTER/CHARGER
3875 1 3875
Battery-1 Iron Edison LiFePO4 48V, 1000Ah 31344 1 31344
Battery-2 Iron Edison LiFePO4 48V, 400Ah 15150 1 15150
Bi-Directional meter Analog Electric meter 40 1 40
Microinverter cable Enphase engage cable 30 3 90
Wiring Cerrowire THHN 65.97 5 329.85
Conduit
SOUTHWIRE FO3750050M
ALUMINUM FLEX CONDUIT
26.26 4 105.04
Mounting Nuevesol NuevoFix 13.12 42 551
Total cost of the system* = $75,283.8
Incentive from Central Government of India (30%) = $22,585.1
Costs after incentives = $52,598.7
*These costs are tentative.
29. 24
(b) LCOE
Annual production = 17,231 kWh
Installed cost per Watt = 4.48$/W
Tata Power utility is the responsible entity to provide the power to the apartments where the gym
is located. They have different slabs of tariff rates.
Table 4: Tata Power Utility Tariff rates
We lack the information about the overall billing structure of the apartment. Since we know
that our load has a demand of more than 1000 units we would go with the highlighted slab rate and
the excess units will be carried on to the next billing period. Also our system annually produces
more units than required, but again due to lack of information regarding the billing structure we
assume that those units would account for other load demands (apart from gym). So we have
decided to take the tariff rate to be Rs12.5 per unit and escalating at 14.71%.
Since all our calculations have been in dollars we decided to convert the selected tariff rate to
dollars at $1 is equal to Rs 67. This conversion rate will remain constant throughout the system
lifetime period for easiness of analysis.
30. 25
Average tariff rate = $0.19/unit
Per watt (W) installed cost = $4.48/W
System size = 11.6kW
Investment in installed system cost = 11.6*1000*4.5 = $52,920
Annual Production = 17,231 kWh
Year 1 Revenue (based on the savings) from the system = 0.13*17231 = $2263.176
The system would require maintenance. So we have decided to take the maintenance cost to be
1% of the system cost. Thus the O&M cost comes to $753.
Since the batteries are for emergency we are unable to determine the actual life cycle of
the batteries, so we go with the warranty of the batteries i.e. 7 years for timely replacement. Every
7 years an amount of $46,494 has to be invested into the system. This cost is not eligible for central
government incentive.
In the LCOE and payback period analysis we are ignoring accelerated depreciation,
depreciation of dollar exchange rate and the salvage value of batteries after their replacement. The
system owner i.e. the apartment community is investing the amount after deduction of government
incentive from the actual system cost.
The lifetime for the system is 20 years. The lifetime cost of the system is initial investment
plus O&M cost for 20 years plus $92,998 for 2 time battery replacement. The overall lifetime cost
of the system is $159,990. The lifetime production by the system is 344,620kWh.
Levelized cost of energy is
$𝟏𝟓𝟗𝟗𝟗𝟎
𝟑𝟒𝟒𝟔𝟐𝟎𝐤𝐖𝐡
= $0.46/kWh.
33. 28
Figure 21: Cumulative return
The total battery cost accounts for approximately 62% of the initial system cost. Moreover these
costs recur every 7 years thus accounting for 185% of the overall system lifetime cost. That is the
primary reason for the system to pay itself around 16th
year.
-100000
-50000
0
50000
100000
150000
200000
0 5 10 15 20 25
Balance
Year
Cummulative return
34. 29
Conclusions
The team successfully designed an 11.76kW grid tied battery backup system to power a
Gym for 8 hours in an apartment in Mumbai. The loads require 16,963 kWh annually. The
designed system produces 17,231 kWh per year (as per PVwatts). The PV system consists of 42
REC 280W PV modules (efficiency 17%) with a microinverter connected to each module. The use
of microinverters reduces shading power losses, eliminates mismatch losses and provides the
option of adding on more modules. Furthermore, the Enphase envoy communications gateway
allows individual monitoring of each module, allowing power monitoring as well as fault
detection.
Due to the simple design of the module arrangement there is no need for minimum spacing
between each row, thus terminating shading loss by the neighboring module. Also, there is no
potential shade from the surrounding environment. The soiling loss has been accounted at 3%.
This loss percentage includes all potential threats like soil, bird waste and leaves. The Lithium Iron
Phosphate batteries are ac coupled by means of inverter chargers. The arrangement consists of a
parallel combination of a 1000 Ah and 400 Ah battery, which is charged by a parallel combination
of a 6.8kW and 5.5kW Schneider XW+ inverter/charger. After accounting all the losses the system
has an overall annual efficiency of 13% (see Appendix D).
The levelized cost of energy is $0.46/kWh. The system pays itself in the 16th year. The
reason for late payback is because of high battery cost. The batteries are the priciest items in our
system. They account for more than 60% of the total system installed cost. Also they have to be
replaced every 7 years thus accounting for a recurring investment of around $46,500. This
recurring investment slows down the payback period.
The team would recommend reducing the battery backup hours from 8 hours to 3 hours.
This would negate the need for 2 batteries and two inverter chargers. Also an on-site evaluation
has to be done. Till now the site evaluation was based on Google images. The wring length was
estimated. Thus if we can get the correct site details we can design an accurate pergola and
mounting system and determine the accurate construction cost and wiring loss.
35. 30
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[2] Z. Yewdall, 'AC Coupling - Methods', Home Power, no. 162, 2014.
[3] Schneider Electric, 'AC Coupling of Inverters: Forming an AC-Coupled system with Conext™
XW+/SW Inverter/Chargers and Conext CL/RL/TL/TX PV Inverters', Schneider Electric, 2015.
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http://www.pv-magazine.com/archive/articles/beitrag/smart-moves _100014710/618/#axzz3rzVsxk6s.
[5] S. MacAlpine and C. Deline, 'Modeling Microinverters and DC Power Optimizers in PVWatts',
NREL, 2015.
[6] R. Richmond, 'Lithium-Ion Batteries for Off-Grid Systems', Home Power, no. 153, 2013.
[7] Enphase Energy, 'Circuit Calculations for the M250 Microinverter', 2013.
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37. 32
Appendix B- Array sizing calculations
Step 1: Assuming the battery charging and discharging losses to be 10%, this Ah value was divided
by 0.9.
1121.9 ÷ 0.9 = 1246.55 Ah/day
Step 2: The value obtained in step 1 is multiplied by the nominal battery voltage to get Wh/day.
1246.55 x 48 = 59834.6 Wh/day
Step 3: The Wh/day is divided by the peak sun hours in Mumbai to get the array size in watts.
Array size (W) = 59834.6 ÷ 5.1 = 11732 W
Appendix C- Manufacturer’s Data sheets
(i) Module – REC 280TP
