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DUAL MODE IPS UTILIZING, SOLAR ENERGY & SYATEM SUPPLY CHAPTER 1 INTRODUCTION 1.1 Introduction Electrical energy could be considered as the most convenient form of energy. An important aspect of electricity is the flexibility; it is very easy to carry electricity from one place to other by using conductors. Electrical energy is much cheaper compared to other forms of energy. It is an inevitable component in all sectors of the modern world. Fossil fuels provide around 66% of the world's electrical power, and 95% of the world's total energy demands (including heating, transport, electricity generation and other uses). Coal provides around 28% of energy, oil provides 40% and natural gases provide about 20%. A concern is that the fossil fuels are being used up at an increasing rate, and that they will soon run out. If these fossil fuels were to run out now there would not be a suitable replacement for them that is equally as efficient at producing the same amount of energy. Storage of electrical energy is also a matter of great importance. Electrical energy storage in batteries and electrochemical capacitors will be vital for any future success in the global effort to shift energy usage away from fossil fuels. Electrical energy could be stored routinely in electro-chemical bonds within batteries that power countless portable and transportable devices. 1.2 Introduction to IPS 1.2.1 What is IPS An IPS is an electrical device that is capable of storing electrical energy as well as converting direct current (DC) into alternating current (AC) where the converted AC can be of any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. 1.2.2 Why Using IPS IPS have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. IPS is commonly used to supply AC power from DC sources (batteries). The electrical IPS is a high-power electronic oscillator. IPS is designed to meet power requirements of Home/office appliances like Light, Fan, TV, Video Player, Audio- Player, Fax, PABX, etc and can be plugged-in directly with the mains supply. It is also suitable for households, Business centers, Offices, Conference rooms, Restaurants, Medical facilities departments, Testing labs & Apartments etc.
Features • • • • •
Fully Automatic. Requires no Fuel/Lubricant for maintenance. Noiseless and Pollution free. Built-in over load and under voltage protection. Built in over charge and Battery Low Volt Disconnect Facility.
1.3.1 Solar Energy At the present moment two methods exist by which sunlight can be converted into directly usable energy: conversion to warmth (thermal energy) and conversion to electricity (photovoltaic energy). In the first method, for example, sunlight is absorbed by a blackened surface, which then warms up. If air or water is passed alongside or through this warmed surface, it too will be warmed. In this way the warmth can be transported to wherever it is needed. For storage, an insulated chamber is usually employed, From which, for example, hot water can be drawn. This, in brief, is a principle of thermal conversion. In photovoltaic conversion, sunlight falling onto a 'solar cell' induces an electrical tension; a number of cells combined in a panel are capable of generating electric current. 1.3.2 Solar Electricity Solar energy technology is used on both small and large scales to produce electricity. A unique advantage of small-scale solar energy systems is that, if they include storage devices, they may eliminate the need to connect to the electric grid. 1.3.3 Uses Solar energy has many uses. It can be used to provide heat, light or to generate electricity. Passive solar energy refers to the collection of heat and light; passive solar design, for instance, uses the sun’s energy to make homes and buildings more energy-efficient by eliminating the need for daytime lighting and reducing the amount of energy needed for heating and cooling. Active solar energy refers to storing and converting this energy for other uses, either as photovoltaic (PV) electricity or thermal energy. There are many reasons of solar power being an important addition in solar panel history to any household. It helps in fighting environment and climate change as well as helping in saving the traditional sources of energy. Installing solar panels will give a significant boost to our electricity supply. The electricity bill will be reduced significantly and the cost of initial installation of a solar energy system will pay itself off over time. Home solar power systems can be added be households and it can be used to use solar energy to heat our water supply. It is very viable natural energy technology to use if we are seeking a significant reduction to our electricity bill proven in solar panel history. It will dramatically reduce the price of heating based on the capacity and technologies used in the systems. We can install a lower capacity system. We can make use of solar power panels that can be found in a variety of shapes, sizes, formats and integrated technologies. 2
The solar power cells are dependent on the output of energy we require. It can also provide lighting in remote locations to our power supply, like our garden or a shed. It will remove the hassle of digging up the ground and laying the appropriate cable. We can also install lights in the garden with the help of solar panels. 1.4.1 Electrical Energy Electrical energy is the scientific form of electricity, and refers to the flow of power or the flow of charges along a conductor to create energy. Electrical energy is known to be a secondary source of energy, which means that we obtain electrical energy through the conversion of other forms of energy. These other forms of energy are known as the primary sources of energy and can be used from coal, nuclear energy, natural gas, or oil. The primary sources from which we create electrical energy can be either non-renewable forms of energy or renewable forms of energy. Electrical energy however is neither non-renewable nor renewable. The electrical energy that an appliance or device consumes can be determined only if we know how long (time) it consumes electrical power at a specific rate (power). To find the amount of energy consumed, we multiply the rate of energy consumption (measured in watts) by the amount of time (measured in hours) that it is being consumed. Electrical energy is measured in Watt-hours (Wh). Energy = power x time E = P x t or E = W x h = Wh Electrical energy is a standard part of nature, and today it is our most widely used form of energy. Many towns and cities were developed beside waterfalls which are known to be primary sources of mechanical energy. Wheels would be built in the waterfalls and the falls would turn the wheels in order to create energy that fueled the cities and towns. Before this type of electrical energy generation was developed, homes would be lit with candles and kerosene lamps, and would be warmed with coal or wood-burning stoves. It is important to understand that electrical energy is not a kind of energy in and of itself, but it is rather a form of transferring energy from one object or element to another. The energy that is being transferred is the electrical energy. In order for electrical energy to transfer at all, it must have a conductor or a circuit that will enable the transfer of the energy. Electrical energy will occur when electric charges are moving or changing position from one element or object to another. When the electrical energy is moved, it is frequently stored in what we know of today as batteries or energy cells. 1.4.2 Electrical Power Power (P) is a measure of the rate of doing work or the rate at which energy is converted. Electrical power is the rate at which electricity is produced or consumed. Using the water 3
analogy, electric power is the combination of the water pressure (voltage) and the rate of flow (current) that results in the ability to do work. Electrical power is defined as the amount of electric current flowing due to an applied voltage. It is the amount of electricity required to start or operate a load for one second. Electrical power is measured in watts (W). The formula is: Power = voltage x current P=VxI Or, W = V x A 1.5 Dual Mode IPS It is the concept of an IPS in which a DC battery is charged by supply electricity and the solar energy enabling it to function as an energy source by the application of proper circuitry. A regular IPS is used for emergency supply of electricity when the system supply is not available which is very much common in Bangladesh and available at different rating and capacity. Due to utilizing only system supply this type of IPS do not make any positive influence on the power crisis in Bangladesh. But dual mode IPS is an idea of an IPS that will utilize two major source of energy acting as an emergency source in time of need and also impinging dominating effect on the current power status of Bangladesh. 1.6 Power Status in Bangladesh After the independence of Bangladesh in 1971, in 1972 Bangladesh Power Development Board (BPDB) was created to look after the same function. Dhaka Electric Supply, headed by a Chief Engineer under BPDB used to control the electricity distribution and sales in Greater Dhaka District area up to September 1991. To improve services to the consumers and to enhance revenue collection by reducing the prevailing high system loss, Dhaka Electric Supply Authority (DESA) was created by an ordinance promulgated by the President of the Peoples Republic of Bangladesh in 1990. The President of the Govt. of Peoples Republic of Bangladesh ordered for establishment of the Dhaka Electric Supply Authority (DESA) by promulgation of ordinance No. 6 of 1990 on 6th March. (published in the Bangladesh Gazette, Additional issue on 14th March, 1990). Act No. 36 of 1990 for establishment of the Dhaka Electric Supply Authority (DESA) was issued (published in Bangladesh Gazette, Additional issue, 23rd June 1990) in superceding the ordinance no. 6 of 1990. 1.6.1 Effect of Current Shortage of Electricity in Bangladesh
Bangladesh is losing at least 3.5% of Gross Domestic product (GDP) due to the shortage of Power supply. Total losses reach to Taka 130000 Million in this year. If the government fails to increase the capacity of power supply by new production, the loss of economy will grow up day by day. According to a research report of Centre for Policy Dialogue (CPD, A civil society think tank, the size of GDP would be enlarge 3.5% compare to current status. The loss of past year GDP was Taka12, 000 crore, equal to 3.2% of GDP, due to power crisis. It will reach 3.5 % of GDP in this year, which is more than Taka13, 000 crore. The main victim of power shortage is commercial activities of the country. Business and Commercial activities of the country is boosting every year. But power crisis is hampering the growth of this sector. Total loss of this sector has reached to Taka 7,000 crore in this year. Impacts of power shortage in industrial sector reach to double by only two years. In 2008-09 total loss of this sector was Taka 4,000 crore. It will be reached to Taka 5,000 crore this year. In addition Export oriented industry of the country is fighting with power crisis. Factories all over the country do not get power supply minimum 4 hours in production time. 30% of production of Readymade garment (RMG), 76% export earning sector of the country, has decreased lack of power supply. Production cost of RMG also increasing and Bangladesh loosing competitiveness in world market according to statistics of Bangladesh Knitwear Manufacturers and Exporters Association (BKMEA). CPD statistics shows, Total lost in agriculture sector reached to Taka 950 crore in this year. It was Taka 518 crore in previous year. 625 irrigation pump has damaged in northern area of the country last year due to load shading. 1.6.2 Load Shedding When the supplying company receives more demand for electrical power than its generating or transmission or installed capacity can deliver, the company has to resort to rationing of the available electricity to its customers. This act is called load shedding. 1.6.3 Load Shedding Perspective of Bangladesh Shortage of Electricity Shortage of electricity may be considered in two forms. Firstly, reviewing the scenario of per capita electricity consumption and percentage of population having access to electricity in Bangladesh compared to other countries and secondly, determining gap between demand and supply of electricity in perspective of country's economic situation and GDP growth. Demand for electricity is increasing with the improvement of living standard, increase of agricultural production, development of industries as well as overall development of the country; but due to the failure in the last few years to increase electricity generation capacity proportionately to the demand, there exists 1500-1800 Megawatt electricity shortage at present. Especially a huge shortage exists during the evening peak demand. Due to the crisis 5
of gas supply, lack of necessary maintenance and rehabilitation of old power plants, it is not possible to utilize the total installed capacity. The shortage of electricity can be from the load-shedding made during the peak demand (5800 MW) of summer which is about 1800 Megawatt each day. Power Sector: An update (April 2010)
1.6.4 How to Overcome Load Shedding We can temporally overcome from the situation by using
Generator IPS UPS
1.7 Objective of Our Project/ Thesis To establish our concept we need following basic structure 1. 2. 3. 4.
Solar energy Supply electricity Charging DC battery Design of DC to AC inverter
1.8 Organization of Thesis Due to frequent load shedding in Bangladesh, the minimum time required in charging up the IPS battery is not sufficient. Hence the efficient operation of IPS becomes impeded. After facing the problem of load shedding, we come to enlarge our idea about an IPS that will provide continuous power supply. Here we proposed a concept of building a system that will store electrical energy using the solar energy and system supply. During load shedding IPS will be charged by solar energy through solar panel as well as acting as an emergency power supply. Implementation of such IPS will decline the consumption of electricity from national grid and also it effectively utilizes a renewable source of energy which is free of cost and available everywhere. 6
CHAPTER 2 ELECTRICITY FROM THE SUN 1. Introduction
In a solar cell light is converted into (electricity by means of the so called photovoltaic (PV) effect. PV is still enjoying large research and development efforts in order to produce more efficient and cheaper solar cells. But solar electricity is already economically feasible compared to other energy sources for a number of applications. In the past, inadequate system design and sizing of system components has led to unfavorable experiences. However in recent years PV has proved to be reliable if sufficient attention is paid to the design. In this chapter a closer look will be taken at those situations in which PV comes into consideration. Subsequently some characteristics of a PV-system are discussed and some attention is paid to those aspects which are important in designing a system. Finally some interesting applications will be examined.
Fig.3: Some solar panels 2.1 The case for PV In rural areas and at remote locations solar electricity is an energy source which can be a good alternative to other energy sources. At the moment PV is used mostly for small appliances, such as refrigerators, telecommunication and navigation equipment. Nevertheless in California (USA) A large PV plant has been built which feeds the grid. In general however the use of PV in large projects and grid connected applications is too expensive to be of interest. When a grid is available, a connection with the grid is generally preferred to the installation of a PV-system. When the reliability of the energy supply is crucial PV can offer a solution. This is for instance the case in vaccine refrigeration. When the vaccine temperature becomes too high, the vaccine can no longer be used for immunization. In many developing countries the performance of the grid is very poor, so PV might even be of interest in areas where a grid is available. The alternatives include back-up generators, accumulator batteries and other renewable energy sources. The production of solar cells requires high technology. Only a few developing countries, i.e. India and Brazil, have production facilities for solar cells. Most countries can produce only the non-PV-system parts. Another obstacle for the use of PV in developing countries is the high initial investment cost. Nevertheless PV has some specific advantages which make it attractive for the developing world. 8
PV can be used for very small appliances, where other sources, for instance diesel, are operating far below their nominal capacity and are therefore relatively expensive.
Thanks to the modular character it is easy to enlarge a system to provide more energy or add more applications to it.
PV has no moving parts, therefore repair is almost never needed and the maintenance requirements are moderate.
When well designed a system operates very reliable.
No fuel is needed.
The design of a PV-system, modules together with modules storage and electronic equipment, has to be carried out very carefully. Many of the unfortunate experiences with PV were related to a bad system design. PV can be very reliable as has been demonstrated in telecommunication projects where PV was chosen specifically because of its reliability, the same applies to buoys along the rivers and coasts. Even in The Netherlands with its small amount of sunshine and its large electricity grid, newly installed buoys are all PV powered. 2.2 Solar cells Solar radiation can be converted directly into electricity using semiconductor devices, which are known as photovoltaic (PV) cells. The most commonly used material is silicon. By diffusing phosphorus or boron into the silicon it is possible to create p- and n-type silicon, each with its own electrical characteristics. A thin silicon wafer is divided into two layers. Both layers are provided with metallic contacts. When sunlight falls upon the solar cell a part of the light is absorbed. The energy of the light releases electrons inside the silicon. When both sides of the cell are connected an electric current will start flowing. The size of the current depends upon the intensity of the incoming radiation. Not all the energy of the light is converted into electrical energy.
Fig.4: Schematic cross view of a silicon solar cell There are a number of semiconductor materials from which solar cells can be made. Until recently the most commonly used was mono-crystalline silicon. At the moment polycrystalline and amorphous silicon are becoming more important. Table 2 gives the theoretical and achieved conversion efficiencies for a few types of solar cell materials.
Table 2: Efficiency of solar cells under standard irradiation (at sea level) for several semiconductor materials. The theoretical efficiency and the achieved efficiencies under laboratory conditions and in industrial production are given. Instead of falling directly onto the flat plate modules, the sunlight can be concentrated first by the use of lenses or mirrors. The concentrated sunlight can be focused on a solar cell, which increases the efficiency of the cell. In this way a record efficiency of 31% was recently achieved for a silicon-gallium arsenide tandem cell. This method enables a reduction of the costs of the array but on the other hand extra costs are incurred by the lenses; the system as a whole also becomes more complex. The technology for concentrating sunlight is still under research and is not commercially available. 2.2.1 Mono-crystalline Silicon Mono-crystalline silicon solar cell technology is based on the semiconductor technology used in the transistor and integrated circuit industry. Using mono-crystalline silicon wafers solar cells can be manufactured with a conversion efficiency of 13 - 15%. The conventional processes employed to obtain single crystal wafers are slow and very energy and material consuming. 2.2.2 Polycrystalline Silicon Mono-crystalline silicon is gradually being replaced by polycrystalline silicon (sometimes also called semi-crystalline silicon). Polycrystalline silicon can be produced at lower costs. The efficiency of polycrystalline cells is 1 to 2% lower than the efficiency of monocrystalline. However combined with the use of cheaper silicon feedstock material, large cost reductions compared to conventional production methods are expected.
2.2.3 Amorphous Silicon Another option to reduce the costs of the cells is the use of amorphous silicon solar cells. These cells are very thin, and thus use very little material. Amorphous silicon has made considerable progress. The first cells were produced in 1974. In 1985 the market share already had reached 30%. Commercial applications have been found in pocket calculators, watches and battery chargers. One of the problems of amorphous silicon at the moment is the degradation of the cells. The cell efficiency decreases when light is falling upon the cell, especially during the first months of operation. 2.2.4 Other Materials Besides silicon other materials are under research for use as solar cells. CuInSe2, CdTe and GaAs look very promising in the long term, but in the coming years large-scale application of these types of cells is not expected. The same applies to stacked solar cells. In these structures two or more cells with different characteristics are combined in order to utilize as much of the solar energy as possible. 2.3 Balance of System All components of the system together, besides the modules, are called the balance-of system (BOS). The composition of the balance-of-system depends on the kind of application and on the location of the PV-system. The balance-of-system may comprise: • • • •
Array support structure Connections/wiring Power conditioning Energy storage.
We will have a closer look at several elements of the balance-of-system. 2.3.1 Array Support Structure The solar-cell modules rest on a array support structure. The array support structure is generally made out of aluminium or steel struts, resting on a concrete foundation. Research is being done to develop low cost constructions of wood and bamboo. Another way of reducing costs is to mount the modules on the roofs of buildings. At the moment only limited experience with this kind of construction has been gained. 11
At present most systems have fixed arrays. In case of a tracking system it must keep the modules in an optimal orientation towards the sun. There are several options. •
Seasonally-adjusted tilt. A few times a year the arrays can be adjusted to the elevation of the sun.
Single-axis or two-axis tracking. A drive mechanism keeps the modules in the direction of the sun during the whole day. The array structure can rotate in one or two directions.
Fig.5: Schematic of PV-system for household electrification 2.3.2 Power Conditioning The power conditioning can be composed of the following elements: • • • • •
Controllers Maximum power point tracking DC-AC converters Interface between the PV-system and the grid Electronic protection of the system.
The maximum power point tracking ensures that at any given moment, with any given amount of sunlight and any given cell temperature the maximum power is extracted from the modules. In general electricity is supplied as AC (alternating current). Therefore a lot of 12
equipment has been developed for AC-application. The PV modules, however, supply DC (direct current)- power. The consequence is that a choice has to be made between the use of DC-apparatus, not available for all appliances, and the installation of an inverter to convert DC into AC. To connect a PV-system with the grid, a special interface is needed including a DC-AC inverter. To obtain the highest possible system efficiency it is important to lose only small amounts of energy in the power conditioning. At the moment an efficiency of 95% is possible. When the system is not working on full power the efficiency of the power conditioning does fall sometimes only about 70% efficiency is left. The cost of the power conditioning depends on the need for AC or DC-voltages. 2.4 Energy Storage If electrical power is required when the sun is not shining or if there is a short peak demand, for instance to start an electric motor, some form of energy storage is needed or a back-up supply from a diesel or gasoline generator must be provided. When a PV-system is used to pump up water, in many cases the choice will be to store water instead of electricity. Several types of storage batteries are available; the lead acid battery is the most common, but NickelCadmium (NiCd) batteries are also suitable. The operation of the batteries requires much attention during the design of a PV-system. In a battery a certain amount of energy can be stored: this is the capacity of the battery. Lead acid batteries can only be discharged to 30% of the total capacity. From a technical point of view deeper discharge is possible, but the lifetime of the batteries then decreases dramatically. Moreover the total capacity of the battery will decline. Batteries can also be overcharged. This also has a bad influence on the performance of the battery. To keep the state of charge of the battery within the allowed range a battery controller can be used. This controller is part of the power conditioning. A NiCd battery has a better performance. Its design makes it impossible to overcharge or discharge the NiCd-batteries to deeply. Also 100% of the capacity can be used. However NiCd batteries are at the moment (1989) two to three times as expensive as lead acid ones. Many different batteries are available. A distinction can be made between open and closed batteries. The hermetically closed batteries need no refilling, because the water cannot evaporate. Therefore closed batteries in general require less maintenance than open ones. For uses in developing countries it is often better to transport the battery and the acid apart, so the battery will not age during the often lengthy transport time. When air mail is used, it is not even permitted to transport ready-to-use batteries. Because of safety precautions battery and acid have to be transported separately. The number of charge/discharge cycles specifies the lifetime of the battery. Another factor of importance to the lifetime is the temperature in which the battery has to operate. The higher the temperature the shorter the lifetime. Here too NiCd has better characteristics than lead acid.
2.5 PV-System Characteristics
2.5.1 Lifetime Since PV-systems have been used only relatively shortly for terrestrial applications, there is little experience on the lifetime. The lifetime of the modules which are commercially available at the moment (crystalline silicon) and of the power conditioning is expected to be about 15 years. A longer lifetime is thought to be not unlikely. Batteries have an expected lifetime of five to ten years at a temperature of 25ĂžC. When operating at higher temperatures the lifetime is shortening. At 40ĂžC the lifetime of a lead-acid battery reduces to about one third of the lifetime under standard conditions, while the lifetime of a NiCd battery reduces to about three quarters. Failures which can occur in the modules are broken connections, cracked solar cells and corrosion. Moisture penetration is the biggest problem in the long term. Apart from the possibility of sudden failure, the performance of the cells will slowly decline over the years, but this effect is relatively small. Bad connections between the 14
modules and the other components of the system can reduce the performance of the system as a whole. Most systems have a battery controller to prevent the batteries against overcharging and excessively deep unloading. There are many controllers on the market that are not properly designed and may cause system failures. 2.5.2 Modularity PV-systems can easily be scaled to the electricity demand. A single module provides enough energy to light one house, a number of modules can provide enough energy for an entire medical centre. A new system could begin with one or two modules for the most urgent purposes. The system can be expanded when more applications are envisaged, or demand grows, or when additional funds become available. The original system in the mean time does not need to be replaced. When the expansion does take place the composition of the whole system, modules, storage and power conditioning, must be taken into consideration, in order to maintain an optimal performance. 2.5.3 Maintenance and Reliability Because a PV-system does not have moving parts and therefore no mechanical wear, maintenance requirements are minimal. The necessary maintenance comprises: • • • • • •
Cleaning the collector surface Electrical check on the modules; the wiring/connections and the power conditioning Visual inspection of the modules for broken cells or surface, humidity, electrical connections and so on Visual inspection of the mechanical connections and the supporting structure, especially on corrosion Repairing or changing broken parts, Maintenance and repair of the batteries.
Apart from the battery maintenance and (possibly) the collector cleaning a yearly service should be sufficient. 2.6 Battery Chargers In developing countries a large number of batteries are used for radios, cassettes, flashlights and other applications. Often non-rechargeable batteries are used. Sometimes batteries can be recharged or car batteries (lead acid) are used. People often have to travel long distances to a charging station to reload these batteries.
Fig.6: Solar panel connection A battery charging station on PV is a very simple system. Only PV-modules are needed. Storage of energy is not necessary. To charge one battery in one day a module of about 200 Wp is needed. In several countries PV charging stations have proven to be a profitable option for local traders. A charging station can be part of a larger system by which the owner can earn some money to offset, for instance, his own battery costs.
2.6.1 Solar Based Battery Charging Circuit To make this project successful we’ve used the circuit which is cheap to implement and its working principle is much easier. Apparatus we needed: 1. Resistances…………….……………………….……120R, 100R, 1K, 50K 2. Variable Resistance…………………………………5K 3. Diodes………………….……...…………………….1N4007 4. Capacitors………………….………………………..0.1uF (poly), 100uF 5. Zener diodes………………………………………...6V-1W 6. Transistor…………………...……………………....TIP122 7. Voltage Regulator………….…………………….....LM317 6. LED ..………………………..…………………..….Red 2.6.2 Circuit Diagram of Battery Charge
Fig.7: Circuit diagram of battery charging circuit 2.6.3 Working Principle The circuit uses a variable voltage regulator IC LM 317 to set the output voltage steady around 16 volts. Variable resistor VR controls the output voltage. When the solar panel generates current, D1 forward biases and Regulator IC gets input current. Its output voltage depends on the setting of VR and the output current is controlled by R1.This current passes through D2 and R3. When the output voltage is above (as set by VR) 6volts, Zener diode ZD2 conducts and gives stable 6 volts for charging. Charging current depends on R1 and R3. Around 250 to 300 miliampere current will be available for charging. Green LED indicates charging status. When the battery attains full voltage around 5.9 volts, Zener diode ZD1 conducts and T1 forward biases. This drains the output current from the regulator IC through T1 and charging process stops. When the battery voltage reduces below 5.9 volts, ZD1 turns off and battery charging starts again.
2.7 Solar Power Residential One way to save your bill is to save energy in your home. Another way is to find alternative energy sources. Solar power can be one of the choices which can be considered seriously. There are many other ways that you can use to have home energy saving. This article will talk about if it is difficult to make solar power residential. There are many reasons that you should consider making your own solar power residential including: 17
It is less expensive than buying the whole solar power system which can possibly cost you many thousands dollars. It can be made out of the simple materials that can be bought at the local hardware stores. There are step-by-step guides to make solar power panels by your self. You can find out one easily on the net. The method is not difficult. You can enjoy doing that and have fun with your kids. The cost is approximately within a couple of hundred dollars. It is estimated that you will be able to cut your energy bill by fifty per cent. You can use solar power for many purposes for example, making electricity, cooking and heating. There are also other choices of making solar power residential. For example, you can do it by combining solar panel kits. They are the tool kits to make solar panel. You can use that but the effectiveness will depend on the brand used. In addition, the cost will be more expensive than doing the whole by your self. Solar power residential is not difficult to make and it is not expensive. If you know how to do it, you can enjoy the saving of your electricity bill. 2.8 Charging Dependable performance and long service life depend upon correct charging. Faulty procedures or inadequate charging equipment result in decreased battery life and/or unsatisfactory performance. The selection of suitable charging circuits and methods is as important as choosing the right battery for the application. General To charge a Power-Sonic battery, a DC voltage higher than the open-circuit voltage of 2.15 is applied to the terminals of the battery. Depending on the state of charge, the cell may temporarily be lower (after discharge) or higher (right after charging) than 2.15 volts. After some time, however, it should level off at about 2.15 volts per cell. Power-Sonic batteries may be charged by using any of the conventional charging techniques. To obtain maximum service life and capacity, along with acceptable recharge time and economy, constant voltage-current limited charging is recommended. During charge, the lead sulfate of the positive plate becomes lead dioxide. As the battery reaches full charge, the positive plate begins generating dioxide causing a sudden rise in voltage. A constant voltage charge, therefore, allows detection of this voltage increase and thus control of the charge amount.
2.8.1 Overcharging As a result of too high a charge voltage excessive current will flow into the battery after reaching full charge causing decomposition of water in the electrolyte and, hence, premature aging. At high rates of overcharge a battery will progressively heat up. As it gets hotter, it will accept more current, heating up even further. This is called thermal runaway, and can destroy a battery in as little as a few hours.
2.8.2 Undercharging If too low a charge voltage is applied, the current flow will essentially stop before the battery is fully charged. This allows some of the lead sulfate to remain on the electrodes which will eventually reduce capacity. Batteries which are stored in a discharged state, or left on the shelf for too long, may initially appear to be “open circuited” or will accept far less current than normal. This is caused by a phenomenon called “sulfation”. When this occurs, leave the charger connected to the battery. Usually, the battery will start to accept increasing amounts of current until a normal current level is reached. If there is no response, even to charge voltages above recommended levels, the battery may have been in a discharged state for too long to recover.
2.8.3 Charging Methods Selecting the appropriate charging method depends on the intended use (cyclic or float service), economic considerations, recharge time, anticipated frequency and depth of discharge, and expected service life. The key goal of any charging method is to control the charge current at the end of the charge.
Taper Charging This is the simplest, least expensive charging method. Either quasi-constant voltage or quasi constant current characteristics can be built into the charger through combination of transformer, diode and resistance. Of the two, constant potential charging is preferable. Typical taper chargers are comprised of small transformer- rectifier circuits wherein the transformer is so designed that the current is limited to the maximum initial charge current for the battery. This current is held constant until the terminal voltage and resultant current demand reach a point at which the charge current begins to fall. Although this type of Fig.8: Semi constant current charging circuit charger can provide a relatively fast recharge, it is basically a constant current device and the charge voltage may be driven too high. Therefore, it must be disconnected, usually within 12-24 hours, or after 100-120% of the preceding discharge has been returned. It is also sensitive to line voltage variations which can cause over- or under-charging. Consequently, this charging method can only be used in cyclic applications Fig.9: Semi constant current charging characteristics
Figure 15 shows an example of a typical diagram and Figure 16 the resultant charge characteristics for this type of basically unregulated charger. Constant Current Charging Fig.10: Constant current charging circuit
Constant current charging is suited for applications where discharged ampere-hour of the preceding discharge cycle are known. Charge time and charge quantity can easily be calculated; however an expensive circuit is necessary to obtain a highly accurate constant current. Monitoring of charge voltage or limiting of charge time is necessary to avoid excessive overcharge. While this charging method is very effective for recovering the capacity of a battery that has been stored for an extended period of time, or for occasional overcharging to equalize cell capacities, it lacks specific properties required in todayâ€™s electronic environment. An example of a constant current charge circuit is shown in Figure 17 and the charge characteristics for this type of charger in Figure 18. Constant Voltage Charging Constant current/constant voltage charging is the best method to charge Power-Sonic batteries. Depending on the application, batteries may be charged either on a continuous or non-continuous basis. In applications where standby power is required to operate when the AC power has been interrupted, continuous float charging is recommended. Non-continuous
Fig.11: Constant current charging characteristics
cyclic charging is used primarily with portable equipment where charging on an intermittent basis is appropriate. The constant current/constant voltage charge method applies a constant voltage to the battery and limits the initial charge current. It is necessary to set the charge voltage according to specified charge and temperature characteristics. Inaccurate voltage settings cause over- or under-charge. 20
This charging method can be used for both cyclic and standby applications. Figures 19 and 20 illustrate examples of a constant current/ constant voltage charging circuit and charging characteristics, respectively. The circuit diagram includes a temperature compensation feature for charge voltage to ensure optimum charging conditions regardless of changes in ambient temperature.
Fig.12: Constant current constant voltage charging circuit
Fig.13: Constant current constant voltage charging characteristics
2.10 Applications Notes Power-Sonic rechargeable sealed lead-acid batteries are designed to provide years of dependable service. Adherence to the following guidelines in system design will ensure that battery life is maximized and operation is trouble-free. • Continuous over-or undercharging is the single worst enemy of a lead-acid battery. Caution should be exercised to ensure that the charger is disconnected after cycle charging, or that the float voltage is set correctly. • Batteries should not be stored in a discharged state or at elevated temperatures. If a battery has been discharged for some time or the load was left on indefinitely, it may not readily take a charge. To overcome this, leave the charger connected and the battery should eventually begin to accept charge. • Avoid exposing batteries to heat! Care should be taken to place batteries away from heatemitting components. If close proximity is unavoidable, provide ventilation. Service life is shortened considerably at ambients above 30°C. • Although Power-Sonic batteries have a low self-discharge rate which permits storage of a fully charged battery for up to a year, it is recommended that a battery be charged 6-9 months after receipt to account for storage from the date of manufacture to the date of purchase. Otherwise, permanent loss of capacity might occur as a result of sulfation. To prolong shelf life without charging, store batteries at 50°F (10°C) or less, 21
• Fasten batteries tightly and make provisions for shock absorption if exposure to shock or vibration is likely. • Although it is possible to charge Power-Sonic batteries rapidly, i.e. in 6-7 hrs., it is not normally recommended. Unlimited current charging can cause increased off-gassing and premature drying. It can also produce internal heating and hot spots resulting in shortened service life. Too high a charge current will cause a battery to get progressively hotter. This can lead to “thermal runaway” and can destroy a battery in as little as a few hours. • Caution: Never charge or discharge a battery in an airtight enclosure. Batteries generate a mixture of gases internally. Given the right set of circumstances such as extreme overcharging or shorting of the battery, these gases might vent into the enclosure and create the potential for an explosion when ignited by a spark. Generally, ventilation inherent in most enclosures is sufficient to avoid problems. • Do not place batteries in close proximity to objects which can produce sparks or flames, and do not charge batteries in an inverted position. • When charging batteries in series (positive terminal of one battery is connected to the negative terminal of another), all batteries in the string will receive the same amount of charge current, though individual battery voltages may vary. • When charging batteries in parallel (positive terminals are connected to the positive terminal and negative terminals to the negative), all batteries in the string will receive the same charge voltage but the charge current each battery receives will vary until equalization is reached. • High voltage strings of batteries in series should be limited to twenty 6 volt or ten 12 volt batteries when a single constant voltage charger is connected across the entire string. Differences in capacity can cause some batteries to overcharge while others remain undercharged thus causing premature aging of batteries. It is, therefore, not advisable to mix batteries of different capacities, make, or age in a series string. To minimize the effects of cell or battery differences, charge the string in 24 volt battery groups through a constant current source with zener diode regulation across individual batteries or battery groups. • To prevent problems arising from heat exchange between batteries connected in series or parallel, it is advisable to provide air space of at least 0.4” (10mm) between batteries. • Battery containers made of ABS plastic or styrene, can sustain damage if exposed to organic solvents or adhesives. • Recharge time depends on the depth of the preceding discharge and the output current of the charger. To determine the approximate recharge time of a fully discharged battery, divide the battery’s capacity (amp. hrs.) by the rated output of the charger (amps.) and multiply the resulting number of hours by a factor of 1.75 to compensate for the declining output current during charge. If the amount of ampere-hour, discharged from the battery is known, use it instead of the battery’s capacity to make the calculation. 22
• For best results and generally acceptable performance and longevity, keep operating temperature range between -20°C and +40°C. • Do not attempt to disassemble batteries. Contact with sulfuric acid may cause harm. Should it occurs, wash skin or clothes with liberal amounts of water. Do not throw batteries into fire, batteries so disposed may rupture or explode. Disassembled batteries are hazardous waste and must be treated accordingly. It is unlawful to dispose of batteries except through a recycling center. CHAPTER 3 ELECTRIC SUPPLY BASE IPS CHARGER 3.1 Rectifier Rectification is the process of converting an alternating voltage or alternating current into direct voltage or direct current. The device used for rectification is called rectifier. Rectifiers are mainly two types, half wave rectifier and full wave rectifier. 3.1.1 Half Wave Rectifier Half wave rectifier is a circuit which rectifies only one of the halves of the ac cycle. During the half cycles when P is positive and N is negative, the diode is forward biased and will conduct. When P is negative and N is positive, the diode is reverse biased and will not conduct. Efficiency of the half wave rectifier will be about 40.6%. 3.1.2 Full Wave Rectifier
23 Fig.14: Full wave rectifier
Full wave rectifier is a circuit which rectifies both half cycles of the A.C. When P of 1st diode is positive, the 1st diode is forward biased and will conduct. Now the 2nd diode will not conduct as it is reverse biased. In all the half cycles either of the two diodes will be conducting. The efficiency of a full wave rectifier is about 81.2 %, twice the efficiency of a half wave rectifier.
Fig.15: Half wave rectifier
3.2 Ripple Factor The ripple factor for a Full Wave Rectifier is given by,
The average voltage or the dc voltage available across the load resistance is,
RMS value of the voltage at the load resistance is,
3.3 Efficiency Efficiency is the ratio of the dc output power to ac input power,
The maximum efficiency of a Full Wave Rectifier is 81.2%. 3.4 Form Factor Form factor is defined as the ratio of the rms value of the output voltage to the average value of the output voltage.
3.5 Peak Factor Peak factor is defined as the ratio of the peak value of the output voltage to the rms value of the output voltage.
Peak inverse voltage for Full Wave Rectifier is 2Vm because the entire secondary voltage appears across the non-conducting diode. 3.6 Rectifier with Filter The output of the Full Wave Rectifier contains both ac and dc components. A majority of the applications, which cannot tolerate a high value ripple, necessitates further processing of the rectified output. The undesirable ac components, the ripple, can be minimized using filters. The output of the rectifier is fed as input to the filter. The output of the filter is not a perfect dc, but it also contains small ac components. Some important filters are • • • •
Inductor Filter Capacitor Filter LC Filter CLC or p Filter
3.7 Inductor Filter When the output of the rectifier passes through an inductor, it blocks the ac component and allows only the dc component to reach the load.
Ripple factor of given by,
the inductor filter is
The above equation shows that ripple will decrease when L is increased and RL is decreased. Thus the inductor filter is more effective only when the load current is high. The larger value of the inductor can reduce the ripple and at the same time the output dc voltage will be lowered as the inductor has a higher dc resistance. The operation of the inductor filter depends on its property to oppose any change of current passing through it. To analyze this filter for full wave, the Fourier series can be written as
The dc component = Assuming the third and higher terms contribute little output, the output voltage is,
The diode, choke and transformer resistances can be neglected since they are very small compared with RL. Therefore the dc component of current
The impedance of series combination of L and RL at 2w is,
Therefore for the ac component,
So, the resulting current i is given by,
The ripple factor which can be defined as the ratio of the rms value of the ripple to the dc value of the wave is
If, Then If the load resistance is infinity, then the ripple factor is .0471. This is slightly less than the value of 0.482. The difference being attributable to the omission of higher harmonics. It is clear that the inductor filter should only be used where RL is consistently small. 3.8 Capacitor Filter The property of a capacitor is that it allows ac component and blocks dc component. The operation of the capacitor filter is to short the ripple to ground but leave the dc to appear at output when it is connected across the pulsating dc voltage. During the positive half cycle, the capacitor charges up o the peak vale of the transformer secondary voltage, Vm and will try to maintain this value as the full wave input drops to zero. Capacitor will discharge Fig.16: Capacitor Filter and characteristics
through RL slowly until the transformer secondary voltage again increase to a value greater than the capacitor voltage. The diode conducts for a period, which depends on the capacitor voltage. The diode will conduct when the transformer secondary voltage becomes more than the diode voltage. This is called the cut in voltage. The diode stops conducting when the transformer voltage becomes less than the diode voltage. This is called cut out voltage. Referring to the figure, with slight approximation the ripple voltage can be assumed as triangular. From the cut-in point to the cut-out point, whatever charge the capacitor acquires is equal to the charge the capacitor has lost during the period of non-conduction. If the value of the capacitor is fairly large, or the value of the load resistance is very large, then it can be assumed that the time T2 is equal to half the periodic time of the waveform.
The ripple waveform will be triangular and its rms value is given by,
The ripple may be decreased by increasing C or RL with a resulting increase in the dc output voltage. 3.9 Charging Circuit:
Fig.16: Charging Circuit 28
This voltage regulator circuit diagram can deliver up to 3A at 12V output voltage. It can replace the regulator IC type 7812. The circuit can be employed on occasions when a current of more that 3A is demanded for regulator. IC regulators of such high current rating are pretty hard to find. The transformer steps down mains voltage to 12rms & the rectifier bridge D1 rectifies it to produce a DC voltage. The C1 filters the rectifier output and produces a DC level. The series pass transistor Q1 is biased by resistor R1 Since zener diode D1 is under breakdown region the voltage across it will be 12V. So the total output voltage will be steady 12.7 V(theoretically). That is the zener voltage plus base emitter voltage of Q1.Here transistor Q1 will conduct the excess current required . The transformer T1 can be as 220V primary;12V/5A secondary step down transformer. The capacitors must be rated at least 25V. By changing the value of the Zener diode, different output voltages can be obtained from the circuit. CHAPTER 4 INVERTER AND SWITCHING 4.1 INTRODUCTION An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. It is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. batteries. Inverter is suitable for: • • • • • • •
Electric drills, fret saws, circular saws, electric chain saws, grinders Vacuum cleaners, coffee machines, irons, dryers, mixers, sewing machines, electric razors, etc. Lamps, energy-savings lamps Electronic devices, e.g. music amplifiers, battery chargers Computers and accessories, UPS Televisions and radios Ham radio transmitters, high voltage generators, among other things
Commonly specified inverter characteristics • • • • • •
Input voltage: Range and nominal Output power Output Voltage Regulation of output voltage and frequency vs load and input voltage Output frequency accuracy Load power factor 29
• • • • • •
Output waveform Harmonic distortion of output Overall efficiency vs loading Operating environment Size and weight Protection required
Technological advances have led to very sophisticated, solid-state inverters. From 100 to over 5,000 watts, ultra-efficient, with all sorts of advantages. Some of these use less than 10% of the energy consumed when fully loaded and way less than 1% at lesser inputs to run their own components 4.2 Output of Power Inverter Power inverters produce one of three different types of wave output: • • •
Square Wave Modified Sine Wave Pure Sine Wave
A type of electrical inverter that produces a square wave output that consists of a DC source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off in correct sequence, at a certain frequency. The square wave inverter is the simplest and the least expensive type of inverter. The second category consists of relatively inexpensive units, producing modified sine-wave outputs, which could logically be called “modified square waves” instead. They are basically square waves with some dead spots between positive and negative half-cycles. Switching techniques rather than linear circuits are used in the power stage, because switching techniques are more efficient and thus less expensive. These inverters require no high-frequency switching, as the switching takes place at line frequency. The modified sine inverter is different from a pure sine power inverter because the wave is in more of a step wave and because appliances are not specifically designed to work with this type of inverter. Although many appliances will still work with a modified sine inverter, some may not work as efficiently. As such, it may take more power to run appliances with a modified sine inverter.The pure sine inverter, which is also referred to as a "true" sine wave, utilizes sine wave in order to provide your appliances with power. A sine wave, which is produced by rotating AC machinery, is the type of wave that is generally provided by the utility company with the help of a generator. 4.3 MOSFET Based Single Phase Inverter 4.3.1 MOSFET as a Switch MOSFET is used as the electronic switches, because it makes the most efficient high-current switches. N-channel, Enhancement-mode MOSFET operates using a positive input voltage and has an extremely high input resistance (almost infinite) making it possible to interface 30
with nearly any logic gate or driver capable of producing a positive output. When it off it is virtually an open circuit, yet when it is on it is very close to a short circuit (only a few milliohms). So, very little power is wasted as heat. Also, due to this very high input (Gate) resistance many different MOSFET's can be paralleled together until required current handling limit is achieved. In DC-AC inverters designed to deliver high power, there are a no. of MOSFET connected to each side of the transformer primary, to share the heavy current. However because MOSFETS are essentially connected in parallel they behave like very high-power MOSFETs, able to switch many tens of amps. Connecting together various MOSFET's enables to switch high current or high voltage loads, but doing so becomes expensive and impractical in both components and circuit board space. To overcome this problem Power Field Effect Transistors or Power FET's was developed. By applying a suitable drive voltage to the Gate of an MOSFET, the resistance of the DrainSource channel can be varied from an "OFF-resistance" of many hundreds of kΩ's, effectively an open circuit, to an "ON-resistance" of less than 1Ω, effectively a short circuit. MOSFET can be turned "ON" fast or slow, or to pass high currents or low currents. This ability to turn the MOSFET "ON" and "OFF" allows the device to be used as a very efficient switch with switching speeds much faster than standard bipolar junction transistors. But when using MOSFET's to switch either inductive or capacitive loads some form of protection is required to prevent the MOSFET device from becoming damaged. If the resistive load was to be replaced by an inductive load such as a coil or solenoid, a "Flywheel" diode would be required in parallel with the load to protect the MOSFET from any back-emf. Driving an inductive load has the opposite effect from driving a capacitive load. For example, a capacitor without an electrical charge is a short circuit, resulting in a high "inrush" of current and when voltage from an inductive load is removed a large reverse voltage build up as the magnetic field collapses, resulting in an induced back-emf in the windings of the inductor. For the power MOSFET to operate as an analogue switching device, it needs to be switched between its "Cut-off Region" where V(GS) = 0 and its "Saturation Region" where V(GS)(on) = +ve. The power dissipated in the MOSFET (PD) depends upon the current flowing through the channel at saturation and also the "ON-resistance". Apparatus • • • • • • •
IC………………………………………………… TLC 494 Diode………………………………………….…. 1N4007. MOSFET………………………………………… IRF 540. Resistor……………………………………….... 10k, 22k, 100k, 470. Potentiometer………………………………….… 1k. Capacitor……………………………………….... 0.1uF, 0.22uF, 0.01uF. Transformer……………………………………... 12-0-12:220V.
4.4 TLC 494 TLC 549 is a 16 pin IC that consists of a 555 timer. It could me made to operate in both mono-stable and astable mode.
4.4.1 555 Timer The 8-pin 555 timer must be one of the most useful ICs ever made. 555 timer is very reliable, low cost and easy to use in variety of application. It can operate from supply voltage of +5V to +18V, making it compatible with both TTL(TransistorTransistor logic) circuits and op-amp circuits. The 555 timer is considered as a functional block that contains two comparators, two transistors, three equal resistor. A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.
Fig.17: Pin diagram of 555timer
Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor should be connected across the +Vs and 0V supply near the 555. 4.4.2 Inputs of 555 Trigger input When it is less then lower threshold this makes the output high. It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance above 2M. Threshold input When greater then upper threshold is this makes the output low. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance above 10M. Reset input
When it is less than about 0.7V this makes the output low , overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k.
Control input It can be used to adjust the threshold. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.
4.4.3 Astable Operation An astable circuit produces a square wave, this is a digital waveform with sharp transitions between low (0V) and high (+Vs). The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'. The time period (T) of the square wave is the time for one complete cycle. T = 0.7 × (R1 + 2R2) × C1
Fig.18: Astable operation of 555 timer
The time period can be split into two parts: T = Tm + Ts Mark time (output high): Tm = 0.7 × (R1 + R2) × C1 Space time (output low): Ts = 0.7 × R2 × C1
With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches 2/3Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V. The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again. This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V. An astable can be used to provide the clock signal for circuits such as counters. 33
A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezoelectric transducer with a low frequency of less than 20Hz will produce a series of and this can be used to make a simple metronome. An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a loudspeaker or piezoelectric transducer. The sound is suitable for buzzes and beeps. The natural (resonant) frequency of most piezoelectric transducers is about 3kHz and this will make them produce a particularly loud sound. 4.5 Circuit Diagram of Inverter
Fig.19: Inverter Circuit 4.5.1 Working Principle TLC 494 produces periodic pulses using the 555 timer in astable operation. Pulse train V1 is shown in fig. Pulse train is rectified and only positive half cycle appears on T1. Output from T1 is again inverted. A logic is implemented so that pulses become slightly displaced in time. These pulses are the applied to MOSFETs. It is necessary to allow a small interval between powering the transformer one way and powering it the other, the switches are pairs of MOSFETS and if they switch simultaneously there would be a risk of shorting out the power supply. Hence, a small wait is needed to avoid this problem. This wait time is called "Dead Time" and is necessary to avoid transistor shoot-through. 34
The above arrangement is push-pull scheme. The term push–pull is sometimes used to generally refer to any inverter with bidirectional excitation of the transformer. However, push–pull more commonly refers to a two-switch topology with a split primary winding. The operation of the circuit means that both MOSFETs are actually pushing, and the pulling is done by a low pass filter (coil) in general, and by a center tap of the transformer in the converter application. But because the transistors push in an alternating fashion, the arrangement is called a push–pull scheme.
DC from the battery is converted into AC very simply, by using a pair of power MOSFETs (Q1 and Q2) acting as very efficient electronic switches. The positive 10V DC from the battery is connected to the centre-tap of the transformer primary, while each MOSFET is connected between one end of the primary and earth (battery negative). So by switching on Q1, the battery current can be made to flow through the top half of the primary and to earth via Q1. Conversely by switching on Q2 instead, the current is made to flow the opposite way through the lower half of the primary and to earth. Therefore by switching the two MOSFETs on alternately, the current is made to flow first in one half of the primary and then in the other, producing an alternating magnetic flux in the transformers core. As a result a corresponding AC voltage is induced in the transformers secondary winding, and as the secondary has more number of turns in the primary, the induced AC voltage is much higher. Because the switching MOSFETs are simply being turned on and off, this inverter does not produce AC of the pure sinewave as the AC power mains. The output waveform is essentially alternating rectangular pulses as shown in the fig. 35
4.6 Improved Design An improved design is achieved by varying the width of the rectangular pulses, to control the form factor and hence the RMS value of the output voltage. This is called pulse width modulation (PWM), and is usually done by having a feedback system which senses the inverter output voltage (or load current). When this feedback senses that the load on the inverter output has increased, the inverter control circuitry acts to increase the width of the pulses which turn on the MOSFETs. So the MOSFETs turn on for longer each half-cycle, automatically correcting the RMS value of the output to compensate for any droop in peakto-peak output. The resulting regulation is usually capable of keeping the RMS value close to constant, for loads up to the inverterâ€™s full rated output power. However this approach does have limitations, mainly because it can generally only increase the pulse width to a certain point. This may not be sufficient to allow the inverter to deliver enough RMS output voltage in short-term overload or surge conditions. When many types of appliance are first turned on, they draw a startup current which is many times greater than the current drawn when they are running. This type of surge can overload the inverter, and its protection circuitry may shut it down to prevent damage to the transformer and MOSFETs. Incorporating special feature like soft start circuitry allows the inverter to cope with this type of short load current surge. The output voltage and power may drop, but at least the inverter keeps operating and allows the appliance to start up. a. Voltage Spikes Another complication is that due to fairly high harmonic content in the output of inverters high inductive load can develop fairly high voltage spikes due to inductive back EMF. These spikes can be transformed back into the primary of the inverterâ€™s transformer, where they have the potential to damage the MOSFETs and their driving circuitry. The risk of damage is fairly small during the actual power pulses of each cycle, because at these times one end of the primary is effectively earthed. Transformer action thus prevents the other end from rising higher than about twice the battery voltage. However from Fig.2, it is seen that there are times during every cycle of operation when neither of the switching MOSFETs is conducting, the flats between the rectangular pulses. At these times that the spikes can produce excessive voltage across the MOSFETs, and potentially cause damage. For this reason that many inverters have a pair of high-power zener diodes connected across the MOSFETs. The zeners conduct heavily as soon as the voltage rises excessively, protecting the MOSFETs from damage. Another approach is to have high-power standard diodes connected from each end of the primary to a large electrolytic capacitor, which becomes charged up to twice the battery voltage. When the ends of the primary attempt to rise higher than this voltage, the diodes conduct and allow the capacitor to absorb the spike energy. These type of protection allows the inverter to be tolerant of moderately inductive loads. However they may not be able to cope with heavy loads that are also strongly inductive. Quite apart from the generation of voltage spikes, heavily inductive loads tend to demand 36
current which is strongly shifted in phase relative to the inverters output voltage pulses. This makes it hard for the inverter to cope, because the only energy available to the load between the pulses is that stored in the transformer.
4.7 Frequency stability Although most appliances and tools designed for mains power can tolerate a small variation in supply frequency, they can malfunction, overheat or even be damaged if the frequency changes significantly. To avoid such problems, most DC-AC inverters include circuitry to ensure that the inverters output frequency stays very close to the nominal mains frequency 50Hz. In some inverters this is achieved by using a quartz crystal oscillator and divider system to generate the master timing for the MOSFET drive pulses. Others simply use a fairly stable oscillator with R-C timing, fed via a voltage regulator to ensure that the oscillator frequency does not change even if the battery voltage varies quite widely.
4.8 Inverters & safety Couple of important safety aspects should be maintained using a DC-AC inverter. A low power inverter rated at a mere 60 watts has an output which is potentially fatal. Such an inverter can typically deliver up to about 360mA at 230V, which is over ten times the current level needed to stop human heart and cause fatal fibrillation. There is also another kind of safety risk associated with inverters, which arises from the fact that in many inverters, there is a direct electrical path between the mains-voltage output circuit and the low voltage input circuitry (including the battery leads). This path is usually via the auto turn-on sensing, and possibly also the voltage or current sensing used for output regulation.
4.9 Dual Mode Switching In this project NAND gate is used for dual mode switching. NAND gate is the opposite of the digital AND gate. A low output results only if both the inputs to the gate are high. If one or both inputs are low, a high output results. The NAND gate is a universal gate in the sense that any boolean function can be implemented by NAND gates. Fig.20: NAND gate dual Considering a MOSFET transistor, If Vin is zero, the MOSFET is turned off as and somode no switch current flows through the resistor, and Vout = Vdd, a high. If Vin is high (assuming that VT for the MOSFET is significantly less than Vin) then the transistor is turned on, and if R and W/L are chosen so that enough current flows through R to drop most of Vdd across it, then
Vout will be low. The way this is usually described is through a transfer function which relates the output voltage as a function to the input voltage. From figure Vdd = IdRd + Vds Rewriting the equation for Id Id = (Vdd/ Rd)- (Vds /Rd) This is called a load-line equation, that shows Id varies linearly with Vds (with a negative slope) and has a vertical off-set of Vdd /Rd.
4.9.1 Relay A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set of contacts. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways: â€˘
Normally-open (NO) contacts connect the circuit when the relay is activated and the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact.
Normally-closed (NC) contacts disconnect the circuit when the relay is activated and the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact.
Change-over (CO), or double-throw, contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact. If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact.
4.9.2 Light Dependent Resistor (LDR) A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor. A photoresistor is made of a high resistance semiconductor. If light falling 38
on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron conduct electricity, thereby lowering resistance. A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire band gap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band, since the electrons do not have as far to jump, lower energy photons are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms there will be extra electrons available for conduction.
4.9.3 Working Principle Battery 1& and battery 2 is being charged from solar charging circuit and system supply charging circuit. Both batteries are connected through NO relay where relay contacts are open when not energized. Relay coils are energized from the output of the NAND gate V0. Diodes are connected with sources so that batteries do no discharge. Both solar supply and system supply, apart from charging the batteries also energizes the MOSFETs Q1 and Q2. When both supply is on MOSFETs are turned on and V0 is zero, and no current goes to coils of the relays. When one of the sources are turned off then V0 is almost equal to Vcc. So the relay(1-3) becomes energized and contact is closed. Both batteries becomes connected in series and goes to the inverter input. When the intensity of sunlight decreases relays might operate. This type of false operation is manipulated by using a LDR. When light intensity is high the current flows through the
Fig.21: Switching Circuit LDR, at the moment of intensity fluctuation relay4 will get energized and operate. So the inferior intensity of sunlight will not hinder the switching a night time.
CHAPTER 5 Conclusion We have faced some problem during working on dual mode IPS. The problems are given below and tried to give a solution to them 1.The mosfets used in rectifier circuit (solar part), can be burned out if they are on at the same time. >>To overcome the burning out probability of the mosfets, we have used some time delay between the operating time of the mosfets. 2. Harmonics were developed at the output of the IPS. >>A low pass filter is used to stable the output frequency. We have tried to make the IPS cost effective. And hence we have used two batteries; one is charging with solar energy and another with supply energy. So our dependency on supply energy is reducing. If one battery is used for charging, then at a time it can use only solar energy or supply energy. And a huge solar panel is needed to provide sufficient backup time as well as less charging time. The use of this type solar panel is very costly. The output frequency of the IPS can be varied within a wide range. The equation of the speed of synchronous motor isNs=120f/p Where, Ns= output speed of the motor f= input supply frequency of motor p= no. of poles Here from the equation, we can see that the speed of the motor is dependent only on the supply frequency as we know the no of poles for a motor is constant. So by varying the input supply frequency, we can vary the speed of the motor. 40
One of our battery is charged with solar energy. We know that the output of solar energy is dependent on the intensity of the sunlight. So, the charging time of the battery is dependent on the sunlight for a specific solar panel. For sunny day it will take less time and a huge time is needed to charge up the battery in a dull day. If the battery is not fully charged due to the lack of sunlight, it cannot contribute to supply the output for a long time. By improving the switching system of the IPS, we can recover the problem. If the switching system is designed such that, if the solar energy is not available/insufficient both of the batteries will be charged by the system supply. REFERENCES GATE, Solar energy. Status report, Vieweg, Braunschweig, FR Germany, 1986. Refff: http://www.ifpaenergyconference.com/Electrical-Energy.html Reff: Posted by jimso on November 27th, 2008 under Uncategorized Tags: solar power residential