As we know stand-alone (off grid) photovoltaic system is powering loads that are not connected to the utility grid.
Few main advantages is Gain energy independence & Access to electricity or reduce diesel generator costs for your home. Let’s first we focused on Technical Specifications of BOM.
- PV MODULES – PV modules must conform BIS Standards IEC 61215 / IS14286/IEC 61730. Each PV module must use a RF identification tag (RFID), which must contain the Month and year of the manufacture (separately for solar cells and module), Country of origin (separately for solar cells and module), I-V curve for the module, Peak Wattage, Im, Vm Temp. for the module.
- BATTERY BANK – The batteries shall be solar photovoltaic batteries of flooded electrolyte, low maintenance, lead Acid. The Storage batteries should conform IEC 61427 / IS 1651 / IS 13369 as per specifications. There must be Battery protection panel and contain suitable wooden rack, hydrometer, thermometer, cell tester and connecting leads etc.
- Power Conditioning Units (PCU) – PCU refers to combination of Solar charge controller and inverter and shall be supplied as integrated unit. PCU should conform IEC 61683, IEC 60068 as per specifications. while selecting PCU we must have to consider Switching device, Input voltage from PV array, Protections eg. (Short circuit protection, Input under voltage / Deep discharge of battery, Input surge voltage protection, Over current, Battery reverse polarity protection etc.), Cooling, Sine-wave output with THD at full load UPF and nominal input voltage, AC charger input & %Regulation.
- Charge Controller Unit (CCU) – The charge controller, controls the output of the solar array and prevents the batteries from being overcharged. There are two basic types available a PWM and a MPPT. Charge Controller Unit shall be dual input type from SPV & Grid Main Supply, however the input is fed from a SPV panel only for battery charging. The charge controller shall be preferably PWM type employing IGBT switching elements. Charge controller should conform IEC 62093 / IEC 60068 as per specification. while selecting CCU we must know following, Protections – (Short Circuit, Deep Discharge, Input Surge Voltage, Over Current (load), Battery Reverse Polarity, Solar array reverse polarity) Also there is requirements of following indication (String ‘ON’, Main ‘ON’, Charging ‘ON’, 80% Charged, 100% Charged, Charger Overload, Battery On Trickle. Battery disconnected / Fault Battery Reverse Polarity, Low Solar Power, System Fault and Charger over Temperature and Input Over / Under Voltage (for AC).
- DC DISTRIBUTION BOARD (DCDB) – A DCDB shall be provided in between PCU and Solar Array. It shall have MCCB of Suitable rating for connection and disconnection of array section. It shall have meters for measuring Array voltage and Array current.
- Inverters – Inverters shall be of very high quality having high efficiency and shall be completely compatible with the charge controller and distribution panel. Inverter should conform IEC 61683, IEC 60068 as per specifications. while selecting inverter consider following – Nominal Capacity (KVA/Kwp), Input Voltage, Efficiency, Overload Capacity, Regulation, Total Harmonic Distortion may be Less than 3%, Protection – (Over Voltage both at Input & Output, Over Current both at Input & Output, Over Frequency, Surge voltage inducted at output due to external source.)
- Cables & Wirings – Cables shall be conforming to IEC 60227/ IS 694 & IEC 60502/ IS 1554 & Voltage rating – ( 1,100V AC, 1,500V DC). For the DC cabling, Solar Cables, XLPE or XLPO insulated and sheathed, UV stabilised single core flexible copper cables shall be used. Multi-core cables shall not be used And For the AC cabling, PVC or XLPE insulated and PVC sheathed single or multi-core flexible copper cables shall be used. Generally above 5kwp system the minimum DC cable size shall be 6.0 mm2 copper. The minimum AC cable size shall be 4.0 mm2 copper. In three phase systems, the size of the neutral wire size shall be equal to the size of the phase wires. The following colour coding shall be used for cable wires: − DC positive: red (the outer PVC sheath can be black with a red line marking) − DC negative: black − AC single phase: Phase: red; neutral: black − AC three phase: Phases: red, yellow, blue; neutral: black − Earth wires: green.
- Earthing and lightning protection – Earthing is essential for the protection of the equipment & manpower. Two main grounds used in the power equipment’s are (System earth & Equipment earth). System earth is earth which is used to ground one leg of the circuit. For example in AC circuits the Neutral is earthed while in DC supply +ve is earthed. And equipment earth all the non-current carrying metal parts are bonded together and connected to earth to prevent shock to the man power & also the protection of the equipment in case of any accidental contact. The Earthing for array and distribution system & Power plant equipment shall be made with GI pipe and GI strip.
- MODULE MOUNTING STRUCTURE – Hot dip galvanized iron mounting structures may be used for mounting the modules / panels / arrays. The Mounting structure shall be so designed to withstand the wind speed of 150 km/ hour. The mounting structure steel shall be as per latest IS 2062: 1992 and galvanization of the mounting structure shall be in compliance of latest IS 4759 with thickness of 80 microns as per IS 5905. All fasteners shall be of Stainless steel – SS 304. The foundation for Module Mounting structures shall be 1:2:4 PCC Construction and must take care there shall be minimum necessary clearance between ground level and bottom edge of SPV modules.
Let’s we Look forward about steps to basic design of solar off-grid system.
Step 1 – Determine the Total Load.
As we know the Electrical energy usage is normally expressed in watt hours (Wh) or kilowatt hours ( kWh ).
( power of the appliance – W) x (Number of hours per day it will operate – Hours/day) = ( Energy consumed by that appliance per day – WH/day.)
To determine the daily energy usage for an appliance, multiply the power of the appliance by the number of hours per day it will operate. The result is the energy (Wh) consumed by that appliance per day.
Step 2 – Determine the sizing PCU & Battery bank.
- As we know System voltages are generally 12, 24 or 48 Volts. The actual voltage is determined by the requirements of the system.
- For example, if the batteries and the inverter are a long way from the energy source then a higher voltage may be required to minimise power loss in the cables. In larger systems 120V or 240V DC could be used, but these are not typical household systems.
Note – The recommended system voltage increases as the total load increases. For small daily loads, a 12V system voltage can be used. For intermediate daily loads, 24V is used and for larger loads 48V is used.
Step 3 – Determine the sizing of SPV needed.
The size of the PV array should be selected considering seasonal variation of – solar irradiation & the daily energy usage, Battery efficiency etc
The basic formula is Array Size Needed = (Load) x (MPPT Controller Loss) / (Sun Hours).
- For general calculation to determine number of modules in series, divide the system voltage by the nominal operating voltage of each module.
- For general calculation to determine the number of strings in parallel, the PV array output current required (in A) is divided by the output of each module (in A).
Note – Number of modules connected in series & parallel is depending upon system designer.
- Step 1 – Determine the Total Load.
AC Load Example, 230Vac
Load 1 – 100W x 4 Hours/day = 400 WH/day.
Load 2 – 50W x 6 Hours = 300 WH/day
Load 3 – 100w x 3 Hours/Day = 300 WH/day
Total Load = 1000 WH/day @ 230Vac.
- Step 2 – Determine the sizing PCU & Battery bank.
Proper battery bank sizing is important for the operation of the SPV. The basic calculation for this step is
(Load AH/day) = (Load WH/day) / Base Battery Voltage
(Load AH/day) = (1000) / 24V = 41.66 AH/D @ 24V
(AH Battery Bank Size Needed) = (Load AH/day) x (Days of Autonomy) / (Max DOD)
(AH Battery Bank Size Needed) = (41.66 AH/D) x (2) / (0.80) = 104.16 AH @ 24V of Battery Needed.
Let’s say we have 150AH, 12V Battery, we need two batteries in series connection.
Calculation 3 – Solar charge controller – It’s calculate based on PV module specification. If we consider
Pm = 100Wp, Isc = 6 Amps, Voc = 16.7Vdc.
Solar charge controller = (3 Pv Module * 6A) x 1.3 = 23.4 A i.e we select 30A, 12V charge controller.
Calculation 4 –Selection of inverter
Selection of Inverter depends on factors such as cost, surge requirements, power quality and for inverter/chargers, a reduction of the number of system components necessary. The selected inverter should be capable of supplying continuous power to all AC loads AND providing sufficient surge capability to start any loads that may surge when turned ON.
From the Total load (energy) assessment , a selected inverter must be capable of supplying 400VA continuous with a surge capability.
- Step 3 – Determine the sizing of SPV needed.
Array Size Needed = (Load) x (MPPT Controller Loss) / (Sun Hours)
Array Size Needed = (1000 x 1.10) / (4) = 275 watts of solar modules is needed.
If we using a 100 watt module we need 3 solar modules.
1) SPV – 100wp – 3Nos.
2) Battery required – 150AH, 12V. – 2 Nos.
3)Charge Controller – 12V, 30Amp.
4)Inverter – 400VA or greater. Other BOM as per system requirement.
Note – it’s just a simple design.. For bigger load we also have to consider different parameters as per requirements.