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The paper presents development of a utility interface solar power converter to supplement deficit in Grid power supply for a water pumping system used in rural home of Indian villages. The power supply system comprises of solar (PV) array, PWM converter incorporating PWM control strategy, energy storage battery devices, submersible pump and water storage tank(s) etc. The model of the system has been designed for its optimal operation and a prototype solar power converter unit has been developed to drive a ½ hp pump motor. The Life cycle cost evaluation of the solar power converter has been done and compared with conventional DG set. This has resulted in a cost effective system with a 60% - 70% grid power saving.
Keywords: Photovoltaic; solar pump; PWM inverter; Diesel Generator.
Water is the basic need of all living being. Approximately 30% of world population lack access to water for drinking, livestock and irrigation. Traditional technology used to access the water from available sources like bore well or open well employ water pump to lift water and store it in an overhead tank(s). These pumps are either powered by conventional grid supply or alternative Diesel Generating (DG) set etc. but higher cost of fuel consumed by DG set and non availability of adequate grid supply have forced scientist and engineer to think for either a supplementary or alternative renewable energy source to produce electrical power[1,2,3,4,5,6,]. Solar photovoltaic energy is one of the potential source is preferred due to availability of free sun fuel ,straight forward technology, lower maintenance with reliable operation etc. Though the solar modules (cells) are expensive but efforts are being made to use it not only for power conversion but also for building the exterior wall or covering the roof of pump farm houses or water pump Houses.
Further, the demand of electricity is increasing day by day by the growing population where as grid supply extension has almost become standstill due to its limited resources like fossil fuel etc. and its further expansion is not possible due to various technical and economic reasons. This has motivated the researchers to develop utility interface solar power converter to generate power which can meet the increasing energy demand of houses located specially in rural sector of the country connected to weak grid supply sources. The system can work even as a standalone device in the grid less village areas. Data acquisition of demand based load profile were accessed .The computational analysis for optimal design of the component has been done and prototype Inverter model has been developed and tested for its dynamic Performance. The proposed system is able to bring an energy saving up to a maximum value of 60-80% of power drawn from utility supply in these rural houses.
The modeling and design of components of water pumping system include the following modules
· PV cell
· Battery back-up source
· PWM Inverter
· Intelligent power controller
· Pump for water storage in an overhead tank etc.
The present study highlights the following:
· Study of user demand of pump and lightening load profile in a rural house of Indian villages.
· Optimal design of solar power converter module consisting of PV array, Battery, PWM Inverter and Pump-Motor etc.
· Prototype development of utility interface solar adaptive Power converter unit.
· Life Cycle Cost Analysis of the prototype solar converter system and its cost comparison with DG set.
· Social Impact of use of solar powered converter on rural development in Indian Villages.
The solar energy is harnessed through photovoltaic cell and converted into utility grade AC power using PWM inverter. The system adapts to meet the varying load profile under two modes of its operation. The schematic diagram of the proposed scheme is shown on Figure (1).
Bore well/Open well
Figure (1): Model of Solar Power Converter and Pump Motor
Figure (2): Block and Circuit Model of utility interface Solar Power Converter System
The converter works in bidirectional mode and performs both under charging and inverter mode of operation. The push pull configured centre tapped converter unit is switched ON and OFF alternatively in push pull mode by transistorized switching Power devices to produce ac power from the DC source.
· Mode 1: High Insolation / low Insolation with more than 50% state of charge (SOC) of the Battery : The PV charges the battery bank as well as feed lighting load through PWM inverter.(S2 remain ON)
· Mode 2: Low battery condition (less than 50% SOC) : Grid or DG (in absence of Grid ) is connected to load and charges the Battery till it becomes fully charged (S1 or S3 become ON)
The switching of these power devices are controlled through PWM base drive pulses and thus minimizes the power loss. The intelligent controllers monitor and control the system parameters and perform the various power management control tasks like:
· PV power management
· Battery management
· Load power management
2. OPTIMAL DESIGN OF SYSTEM COMPONENTS
2.1 PV Sizing
The empirical formula based on energy balance equation has been used to compute the optimal size of PV module for critical limit of load as stated below:
(1) S.F * Hour Sun P rating Cell PV avg €½
(Where sun hour = 6.2 for area under study, S.F. = 1.2 for cloudy weather)
(2) hrs 24/ P P 24hr 0hr L avg ƒ¥ €½
The average load power (Pavg) is computed over 24 hrs taking the load value as constant for every 1 hour interval.
The optimal number of PV module = (3) Module PV Standard Rating PV
2.2 Battery Sizing
Battery stores the energy to a maximum value as per average load power requirement.
The battery capacity = (4) SOC* 12V Power Load Average
Where SOC (State of Charge) of Battery = 50%
2.3 Pump Motor
The pump is driven by ac motor whose optimal value can be computed by the following expression
Motor Power = (5) ¨P H
Where HP = Hydraulic power of pump [W]
¨ = Efficiency of pump
The hydraulic power HP can be computed by the following expression
HP = Q ² g H (6)
Where, Q = discharge rate, ² = density of water 1000kg/m3, g = acceleration due to gravity 9.81 m/s2,
H = dynamic Head (m)
2.4 Inverter Module
The inverter produces AC power output with DC power input .The efficiency of Inverter depends on harmonic content in ac output power which depends on the number of PWM switching pulses i.e. N approximated to a sine wave in both the half cycle of output AC wave. The PWM pulses are generated through Microprocessor/computer software program embedded in single chip of integrated circuit.
3. PWM CONTROL ALGORITHM OF PV CONVERTER
The PV converter produces PWM sinusoidal pulses using Direct PWM modulation control strategy. The pulse width (Pi ) of PWM pulses approximating to an equivalent sine wave is computed from the control algorithm (equation 7) as stated below:
(7) 2N 1) - i (2 Sin * 22N 180 P i ° €½
Where, Pi = PWM pulse width of i th pulses
N = number of PWM Pulses within one half cycle approximating to a sine wave
i = 1, 2....N
The simulated PWM pulses for a typical representative value of N=3 is shown on Figure (3).
Figure (3): Simulated Direct modulated Sinusoidal PWM pulses for N = 3 in one half cycles (10ms=180 Deg),
Frequency = 50 Hz (Scale X = Degree; Y= Pulse Voltage x 5V)
4. HARDWARE IMPLEMENTATION
The algorithm of software program for generation of N number of PWM pulses has been implemented through 16 bit Microprocessor (8086). The pulse width and Notch width timings are computed from the switching angles of PWM pulses and are loaded in the timer (peripheral device) unit of Microprocessor and outputted through ports interfaced with Microprocessor. The programs flow-chart (Figure (4)) is as depicted below:
Figure (4): Flow Chart of PWM Pulse Generation Program
5. TECHNICAL SPECIFICATION
A prototype solar converter unit of the system has been designed and developed as depicted in Table (1).
Table (1): Technical Specification of Prototype Sample of Solar Converter system PV Cell
12V, 75Wp @ STP
Battery (Lead Acid)
500VA, 220V, 50Hz PWM Sine Wave
Pump (Self priming ¼ HP submersible pump for dynamic head of 10 meters ,CFL etc.
220V ±10%, 50Hz