Solar Energy in USA

This paper contain the solar energy aspect in USA. Solar electric generation have been commercialized for about 25 years. But the utilization of solar electric grids is very poor. The reason behind this failure is that the total cost per kilowatt-hour from the solar generators substantially exceed the cost per kilowatt-hour for electric generated at central generating Stations powered by burning coal, oil, gas or by nuclear power plants [18].

In this project, we are designing a series controller. The most simple series controller is the series-interrupting type.

Which disable the current flow into batteries when they are full. The charge controller constantly monitors battery voltage, and disconnects or open-circuits the array in series once the battery reaches the regulation voltage set point. When battery voltage drops to the array reconnect voltage set point, the array and battery are reconnected, and the cycle repeats.

Assume that the battery is fully charged when the terminal voltage reaches 14 volts with a specific charging current. Assume also that when the terminal voltage reaches 14 volts, the array will be disconnected somehow from the batteries and that when the terminal voltage falls below 14 volts, the array will be reconnected.

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Now note that when the array is disconnected from the terminals, the terminal voltage will drop below 14 volts, since there is no further voltage drop across the battery internal resistance. The controller thus assumes that the battery is not yet charged and the battery is once again connected to the PV array, which causes the terminal voltage to exceed 14 volts, which causes the array to be disconnected.

This oscillatory process continues until ultimately the battery becomes overcharged or until additional circuitry in the controller senses the oscillation and decreases the charging current. One way to eliminate overcharging resulting from the oscillatory process is to introduce hysteresis into the circuit, so that the array will not reconnect to the batteries until the batteries have discharged somewhat.

Figure1.3: Hysteresis loop in charge controller for voltage sensing.

Now consider the discharge part of the cycle. Assume the battery terminal voltage drops below the prescribed minimum level. If the controller disconnects the load, the battery terminal voltage will rise above the minimum and the load will turn on again, and once again, an oscillatory condition exists. Thus, once again an application for hysteresis is identified.

Generally, charge controller is divided into 3 main portions, which are Arduino UNO microcontroller, input parts and output parts. The input part for the charge controller is battery voltage sensing circuit. It is used to detect the input voltage, send to the Arduino processor to analyze according to the program which is uploaded inside its memory. The output part is consist of battery-panel connect/disconnect circuit, sources load connect/ disconnect circuit, low voltage warning, fully charged indicator and normal indication.

Crystal oscillator soldered on Arduino development board provide a clock signal to microcontroller Atmega 328. This provides a square wave signal which determine the time required for each T state. As in general Arduino board has 16Mhz frequency crystal hence takes 1/16 usec to run 1 T state [7] Two power MOSFETs (IRF250) are used as solid-state switch for the panel-battery line and battery-load line. LEDs of differing colors are used to display the system status. LEDs of differing colors are used to display the system status.

Here, Arduino UNO microcontroller is used to control the operation of charging control and data acquisition task in this thesis. Arduino UNO contains one analog input pin & two I/O pins which are suitable for the development of the charge controller. Pin A0 is used to perform the analog to digital conversion, which is used in input parts like battery voltage sensing circuit and current sensing circuit. Pin 9 controls disconnect or reconnect operations for photovoltaic panel or load and provide the information of battery charging status. Pin 3 is used as to show charging/discharging of battery at different battery voltage. Block diagram of charge controller is shown in Figure 1.4.

Figure 1.4 Block diagram of a charge controller

The other features of this designed charge controller are listed as below.

· Solar charging current: 100 Amps continuous

· Nominal battery voltage: 24V

· Photovoltaic panel voltage ratings: Up to 3 KWP

· Battery type: lead-acid

· Hysteresis Control Option

· Over voltage and under voltage protection

· Excess temperature protection

· Battery voltage status indication by LEDs