Storage of electricity is a vital issue in the perspective of Bangladesh due to shortage of electricity in the national grid. Insufficient electricity supply causes the storage equipment to function inefficiently, implicating that they cannot charge up to their full capacity. This project proposes an idea about storage equipment that will utilize solar energy as well as system supply for the purpose of charging. Solar energy, a renewable source of energy will be available even if the system supply is off-line. So storage equipment will acquire electricity from solar system while lessening the consumption of supply electricity. An array of solar cell converts the solar energy into usable DC current. Batteries of different capacities can be used to store this depending on the size and type of the solar panel. A charging circuit is required for efficient use of solar energy due to fluctuation in radiation intensity. Another charging circuit is needed for the storage of supply electricity. As the stored electricity will support household loads, so DC current is to be converted into AC current by the means of an inverter circuit. Proper switching is necessary due to variation in sunlight intensity and operation during the period of load shedding.
Dual Mode Solar Home System 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. DMSHS can generate electricity in a clean and silent way. Dual Mode Solar Home System (DMSHS) will protect us from the future electricity crisis and increase of electricity prices. It is pollution less environmental process, so rapidly increase the consensus favor on this power system.
A PV system is a system in which a solar cell is used to convert solar radiation directly into electricity.
A PV system consists of the following components:
i) Solar cell array
ii) Charge Controller
iii) Storage System
iv) Inverter and Tracking System
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.
To provide Knowledge:
- About Dual Mode Solar Home System.
- About installation of Dual Mode Solar System.
- About the present condition of DMSHS in Bangladesh.
q Data collection from books, internet, manual and magazine.
q Visit of project site:
v Panna Group,Kamrangirchar, Dhaka.
v Altu Khan Jute Mills, Faridpur.
v BSTI Tajgaon, Dhaka.
1.5 Literature Preview
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 by the amount of time (measured in hours) that it is being ‘consumed. Electrical energy is measured in watt-hours (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 fails 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.
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 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 volt. age. 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
1.6 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 SHS an emergency source in time of need and also impinging dominating effect on the current power status of Bangladesh.
2.1 Case Studies
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. 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. Solar radiation-can be converted directly into electricity using semiconductor devices, which are known as photovoltaic PV ceils. The most commonly used material is silicon by diffusing phosphorus or boron into the silicon it is possible to create p-type 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 ceil 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. Therefore a lot of 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 fail sometimes only about 70% efficiency is left. The cost of the power conditioning depends on the need for AC or DC-voltages.
2.2Grid-connected photovoltaic systems
- Also known as utility interactive (Ul) system
- PV array pole-mounted, roof-mounted or integral part of building surface
- Building-integrated photovoltaic
- Power conditioning unit (PCU) inverts dc to ac for building and/or grid
- When PV power is insufficient, PCU draws from grid into building
- PCU may also function as MPPT
- System is robust without need of batteries
- Cost effectiveness as PV supplies power when grid power is most
- expensive during the day
2.3 Stand-alone system with energy storage
2.4 Off-Grid Solar Power system
An off-grid system requires a storage system for the electricity that you produce so that it will be available for times when there is no source of electricity. This storage system is one of the main features that distinguish an off-grid system from a grid-tied system. The other is a backup generator for long periods of cloud or calm.
The figure at the right shows the basic components for an off-grid system. A solar array and an optional wind turbine provide electricity to run the appliances in your home. Whatever you don’t need immediately is stored in the battery bank. Since you are completely reliant on your own resources the battery bank must be large enough to see you through at least 3 days without any solar or wind charging. This typically means very large battery cells forming a bank that requires a space that is a minimum of 2′ wide by 4′ long and 3′ high.
You will need to plan your energy use using a load analysis so that the charging system and battery bank is large enough to meet your needs. Heating your home in a cold climate will present some challenges. Some heating systems are difficult or prohibitively expensive to operate with an off-grid system. For example, you cannot run a geothermal system with off-grid power – the power requirements for the pumps are too large. Passive solar design and an in floor heating system are usually the best options for off-grid systems.
2.5 Grid-tied Solar Power System
A grid-tied system is connected to your electrical utility company’s power “grid”. The utility is now your backup generator. There is a basic monthly cost for a grid connection (usually around $25) but this is much less expensive than the $6000 or more for a generator and the fuel that the generator uses.
A grid-tied system often includes a net metering agreement. This means that when you produce extra power you can feed it back to the grid and receive a credit on your power bills for those times when you use more than you produce. Some utility companies may also pay you for your excess power, or buy power from you at higher than the going rate (this is called a Feed-In Tariff ).
Grid-tied Solar Power System
The grid now also becomes, in a sense, your battery bank. Because you feed back your excess power for a credit, it is effectively “stored” for you until you need it. Usually this means that you feed back extra power in the summer and then use the credits in the winter when you need the power for your heating system. This is much less expensive than buying and maintaining a battery bank.
Looking at the figure on the left you can see that the grid-tied system is the same as the off-grid system but without the battery bank and its charge controller and without the emergency generator.
2.6 A Cost Comparison
This means that it is much less expensive to set up and maintain a grid-tied system. It also means that most of your money is going to what you really wanted to buy in the first place – solar and/or wind power. The inverter system, which converts the DC solar power to normal household AC power, is the only other expense for materials. Installation is also less expensive if there is no battery bank, charge controller and generator to install.
This typically makes a residential grid-tied system at least $15,000 less than an off-grid system. So why would you want to invest in an off-grid system?
Off-grid systems are still cost competitive if you live sufficiently far from the closest grid connection. If you need to have power brought in it may cost you at least as much or more to connect to the grid as to pay for the batteries and generator required for an off-grid system. If it will cost you $20,000 or more to bring in power, for example, the off-grid system quickly pays for itself, especially since there will be no ongoing power bills.
Off-grid Battery Bank
2.7 A Benefits Comparison
Off-grid System Pros and Cons:
- Ideal for more remote situations where power is expensive to bring in.
- No power bills.
- No power outages.
- Self-sufficiency on a clean, renewable energy source.
- Batteries and generator are expensive and require maintenance.
- Lifetime for the batteries and generator (10 – 15 years) is less than for the solar array (35+ years) and wind turbine (20-25 years).
- No seasonal storage. Batteries can only store power for a few days and have a maximum capacity. When they are full, the rest of the power is wasted unless you can find an immediate use for it.
- Power use must be carefully planned.
Grid-tied System Pros and Cons:
- Easy backup from grid power.
- Eliminates need for expensive batteries and generator (which also requires fuel).
- Provides seasonal storage if a net metering or Feed-in Tariff program is available.
- Maintenance free for a solar power system (wind requires some maintenance and repair).
- Internet monitoring available with inverters designed to be used for individual solar panels (Example).
- You are providing clean energy to the grid.
- Power outages. When utility power goes out your system also goes out unless you invest in a battery bank. This is a requirement by the utility company and is for the safety of those repairing the system.
- You still have to pay the basic utility bill, just not for whatever power you’ve produced.
- You are still using non-renewable resources when there is no solar or wind.
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 Nickel-Cadmium (NiCd) batteries are also su itafale. 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 tire 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 temperature the shorter lifetime. Here too NiCd has better characteristics than lead acid.
2.8 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-type 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.
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 poly-crystalline and amorphous silicon are becoming more important.
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.
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 safer are slow and very energy and material consuming.
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 mono-crystalline. However combined with the use of cheaper silicon feedstock material, large cost reductions compared to conventional production methods are expected.
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 had 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.
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.
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.
2.9 The balance of system may comprise
- Array support structure
- Power conditioning
- Energy storage.
We will have a closer look at several elements of the balance-of-system.
The solar-cell modules rest on array support structure. The array support structure is generally made out of aluminum or steel struts, resting on a concrete foundation. Research is being done to develop low costs 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.
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.
2.10 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.
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 25 pC. When operating at higher temperatures the lifetime is shortening. At 40 pC the lifetime of a lead-acid battery reduced to about one third of the lifetime under standard conditions, while the lifetime of a NiCd battery reduced to about there quarters. Failures which can occur in the modules are broken connections, cracked solar calls 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 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 over-charging and excessively deep unloading. There are many controllers on the market that are not properly designed and may cause system failures.
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 ore applications are envisaged, or demand grows, or when additional funds become available. The original system in the mean time does not need to be replaces. When 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.
2.12 Maintenance and Reliability
Because a PV-system does not have moving parts and there fore no mechanical ware, maintenance requirements are minimal. The necessary maintenance comprises:
- Clearing 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.
- Maintenance and repair of the batteries.
Apart from the battery maintenance and (possibly) the collector cleaning a yearly service should be sufficient.
3.1 Types of batteries for PV use
The two most commonly used battery types in photovoltaic system are lead acid and Nickel cadmium. Each of these is made in a wide variety of construction for different uses. Nickel cadmium batteries are normally 3-4 times the price of lead acid battery. This is the main reason why lead acid batteries are by far the most common type used in PV systems.
Lead-acid and Nickel-cadmium batteries always have the following characteristics.
- Lead Acid
Nominal ocv: 2.0V/cell
Typical end voltage: 1.8V/cell
Relation of ocv to soc: variable but somewhat proportional
Nominal OCV: 1.2V/cell
Typical end voltage: 0.9-1.0V/cell
Relation of ocv to soc: none
The basic electrochemical reaction equation is a lead-acid battery can be written as follows:
Theory of operation:
3.2 Lead Acid Battery
Lead Acid Battery cells consist of a Lead (Pb) electrode and a Lead oxide (PbO2) electrode immersed in a solution of water and sulfuric acid (H2SO4). When the battery connected to a load, the Lead combines with the sulfuric acid to create Lead sulfate (PbSO4), and the Lead oxide combines with hydrogen and sulfuric acid to create Lead sulfate and water (H2O). As the battery discharges, the Lead sulfate builds up on the electrodes, and the water builds up in the sulfuric acid solution. When the battery is charged, the process reverses, with the Lead sulfate combining with water to build up Lead and Lead oxide on the electrodes.
Common example of Lead acid batteries is car batteries, alarm system backup batteries, and camcorder batteries. Lead acid batteries should never be fully discharged; this will effectively kill the battery, making it impossible to charge.
3.3 Features of Lead Acid Battery
Power-Sonic batteries use state of the art design, high grade materials, and a carefully controlled ;ate-making process to provide excellent output per cell. The high energy density results in superior poorer/volume and power/weight rations.
High Discharge Rate
Low internal resistance allows discharge currents of up to ten times the rated capacity of the battery. Relatively small batteries may thus be specified in applications requiring high peak currents.
Wide operating Temperature Range
Power-Sonic batteries may be discharged over a temperature range of -40oC to +60oC (-40oF to 140oF) and charged at temperatures ranging from -20oC to 50oC (4oF to + 122oF).
The high impact resistant battery case is made either of non-conductive ABS plastic or styrene. Large capacity batteries frequently have polypropylene cases. All of these case materials impart great resistance to shock, vibration, chemicals and heat.
Long Service Life
Under normal operating conditions, four of five years of dependable service life can be expected in stand-by applications, or between 200-1000 charge/discharge cycles depending on average depth of discharge.
3.4 Construction of Lead Acid Battery
Plate construction is the key to producing a good battery. Recognizing this, Power-Sonic utilizes the latest technology and equipment to cast grids from a lead-calcium alloy free of antimony. The small amount of calcium and tin in the grid alloy imparts strength to the plate and guarantees durability even in extensive cycle service. Lead oxide paste is added to the grid to form the electrically active material. In the charged state, the negative plate paste is pure lead and that of the positive lead oxide. Both of these are in a porous or spongy form to optimize surface area and thereby maximize capacity.
Power-Sonic separators are made of woven glass fiber cloth with high heat and oxidation resistance. The material further offers superior electrolyte absorption and retaining ability, as well as excellent ion conductivity.
Immobilized dilute sulfuric acid: H2SO4.
Case material is ABS, a high-impact proof plastic resin, styrene, or a polypropylene-polyethylene copolymer with resistance to chemicals and flammability.
Leak proof Design & Operational Safety
Power-Sonic batteries have been approved for shipment by air, both by D.O.T and I.A.T.A… U.L.’s component recognition program for emergency lighting and power batteries lists Power-Sonic under file numbers MH14318 and MH14838.
Depending on the model, batteries come either with AMP Fasten type terminals made of tin plated brass, post type terminals of the same composition with threaded nut and bolt hardware, or heavy duty flag terminals made of lead alloy. A special epoxy is used as sealing material surrounding the terminals.
In case of excessive gas pressure build-up inside the battery (usually caused by abnormal charging) the relief valve will open and relieve the pressure. The one-way valve not only ensures that no air gets into the battery where the oxygen would react with the plates causing internal discharge, but also represents an important safety device in the event of excessive overcharge. Vent release pressure is between 2-6 psi; the seal ring material is neoprene rubber.
Depending on model, the case sealing is tongue and groove with polyurethane, epoxy, or heat seal.
During the discharge portion of the reaction, lead dioxide (positive plate) and lead (negative plate) react with sulfuric acid to create lead sulfate, water and energy.
During the recharge phase of the reaction, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, oxide and sulfuric acid by an external electrical charging source.
To produce a truly maintenance-free battery, it is necessary that gases generated during overcharge are recombined in a so-called “oxygen cycle” should oxygen and hydrogen escape, a gradual drying out would occur, eventually affecting capacity and battery life. During charge, oxygen is generated at the positive and reacts with and partially discharges the sponge lead of the negative. As charging continues, this oxygen recombines with the hydrogen being generated by the negative, forming water. The water content of the electrolyte thus remains uncharged unless the charging rate is too high. In case of rapid generation of oxygen gas exceeding the absorbing capacity of the negative plate, the pressure relief valve will open to release excessive gas.
The Power-Sonic battery is protected against cell shorting by the addition of a buffering agent that insures the presence of acid ions even in a fully discharged state. The need for expensive circuitry in the design of a system to prevent deep discharge and possible cell shorting is thereby reduced considerably. Power-Sonic defines “deep discharge” as one that allows the battery voltage under load to go below the cut-off (or “final”) voltage of a full discharge. The recommended cutoff voltage varies with the discharge rate for a 6vot battery, for example, it is 5.25V at the 20 –hour (0.05C) rate. It is important to note that deep discharging a battery at high rates for short periods is not nearly as severe as discharging a battery at low rates for long periods of time. To clarify, let’s, analyze two examples.
- Battery A is discharged at the 1C rate to zero volts. “C” for a 4 AH battery, for example, is 4 amps. Full discharge is reached after about 30 minutes when the battery voltage drops to 1.5V/cell. At this point, only 50% of rated capacity has been discharged ( C amps X 0.5 hrs = 0.5C Amp. Hrs.) Continuing the discharge to zero volts will bring the total amount of discharge ampere-hours to approximately 75% because the rapidly declining voltage quickly reduces current flow to a trickle. The battery will recover easily from this type of deep discharge.
- Battery B is discharged at the 0.01C rate to zero volts. 0.01C foa a 4 AH battery is 40mA. Full discharge is reached after 100+ hours when the terminal voltage drops to 1.75 V/cell. At this point, the battery has already delivered 100% of its rated capacity (0.01 x 100 hrs = 1C Amp. Hrs.). Continuing the discharge to zero volts will keep the battery under load for another 4-5 days, squeezing out every bit of stored energy. This type of “deep” discharge is severe and is likely to damage the battery. The sooner a severely discharged battery is recharged, the battery its chances to fully recover.
The capacity of battery is the total amount of electrical energy available from a fully charged cell or cells. Its value depends on the discharge current, the temperature during discharge, the final (cut-off) voltage and the general history of the battery. Capacity, expressed in ampere-hours (AH) is the product of the current discharged and the length of discharge time. The rated capacity © o a Power-Sonic battery is measured by its performance over 20 hours of constant current discharge at a temperature of 680 F (200 C) to a cutoff voltage f 1.75 volts. As an example, Model PS-610 with a rated capacity of 1AH will deliver 50 mA (1/20 of 2AH, or 0.05C) for 20 hours before the voltage drops from 6.45 to 5.25 volts. By cycling the battery a few times or float charging it for a month or two, the highest level of capacity development is achieved. Power-Sonic batteries are fully charged before leaving the factory, but full capacity is realized only after the battery has been cycled a few time s or been on float charge for some time.
When a battery discharges at a constant rate, its capacity charges according to the amperage load. Capacity increases when the discharge current is less than the 20-hour rate and decreases when the current is higher. Shows capacity curves for major Power-Sonic battery models with different ampere-hour rating. Amperage is on the horizontal scale and the time elapsed is on hour the vertical scale; the product of these values is the capacity. Proper battery selection for a specific application can be made from this graph if the required selection for a specific application can be made from this graph if the required time and current are known. For example, to determine the proper capacity of a battery providing 3 amps for 20 minutes, locate the intersection of these values on the graph. The curve immediately above that point represents the battery which will meet the requirement.
3.5 Capacity Variation by Current Load
During discharge the voltage will decrease. The graphs in illustrate this for different discharge rate and ambient temperatures. “C” is the rated capacity of a battery: “C” for Model PS-610 (6V-1AH) is 1AH. By convention, rating of nearly all sealed-lead acid batteries, including Power-Sonic, is bases on a 20-hour (0.05C) discharge rate. An important feature of Power-Sonic batteries is shown in the discharge curves; namely, the voltage tends to remain high and almost constant for a relatively long period before declining to an end voltage.
Open circuit voltage varies according to ambient temperature and the remaining capacity of the battery. Generally, open circuit voltage is determined by the specific gravity of the electrolyte. Discharging a battery lowers the specific gravity. Consequently, it is possible to determine the approximate remaining capacity of a battery from the terminal voltage.
The open circuit voltage of a Power-Sonic battery is 2.15 V/cell when fully charged and 1.94 V/cell when completely discharged. As seen in under load, the battery can deliver useful energy at less than 1.94 V/cell, but after the load is removed the open circuit voltage will “bounce back” to voltages shown in figure 2.11, dependent upon residual capacity.
Actual capacity is a function of ambient temperature and rate of discharge. At 680F (20oC) rated capacity is 100%. The capacity increases slowly above this temperature and decreases as the temperature falls. Even at -40oF (-40oC), however, the Power-Sonic battery will still function at better thin 30% of its rated capacity when discharged at the 20-hour rate (0.05C). At any ambient temperature, the higher the rate of discharge, the lower the available capacity. This relationship is shown in Power-Sonic batteries may be discharged at temperatures ranging from -40oF (-40oC to 60oC) and charged at temperature from 4oF to 122oF (-20oC to 50oC). While raising ambient temperature increases capacity, it also decreases useful service life . It is estimated that battery life is halved for each 10oC above normal room temperature.
Shows the relationship between current and discharge time for different ambient temperatures.
Shelf life & Storage
Low internal resistance and special alloys is the electrodes assure a low self discharged rate and consequently, a long shelf life. If kept at 68oF (20oC), about 60-70% of the nominal capacity remains after one year of storage. One recharge per year is sufficient to maintain the original capacity of a battery not in use. The rate of self discharge varies with the ambient temperature. At room temperature it is about 3% per month. A low temperatures it is nearly negligible; at higher ambient temperatures self discharge increases.
To obtain maximum battery life and performance, batteries should be:
- Recharged as soon as possible after each use and not stored in a discharged state;
- Stored at 68oF (20oC) or lower, if possible and
- Recharged annually when not used.
3.6 Battery Life
The number of charge/discharge cycles depends on the capacity taken from the battery (a foundation of discharge rate ad depth of discharge), operating temperature and the charging method.
Stand by Use
The float service life, or life expectancy under continuous charge, depends on the frequency and depth of discharge, the charge voltage, and the ambient temperature. At a float voltage of 2.25V to 2.30V/cell and an ambient temperature of 60oF to 77oF (20oC TO 25oC) Power-Sonic batteries should last four to five years before the capacity drops to 60% of its original rating. Figure 2.16 indicated how capacity change over time.
The graph in Figure 2.17 shows life characteristics in float (standby) service for ambient temperatures ranging from 15oC to 55oC . If prevailing ambient temperatures are well above 20-25oC the life expectancy of this type of battery in float service depends greatly on temperature compensated charging. The typical temperature coefficient is – 2mV/cell/oC. The graph shown along side is based on temperature compensated charging.
To optimize battery life, it is recommended that the battery be disconnected from the load when the end voltage a function of the discharge rate is reached. It is the voltage point at which 100% of the usable capacity of the battery has been consumed or continuation of the discharge is useless because of the voltage dropping below useful levels. (see section on Deep Discharge on page 3) Discharging a sealed lead-acid battery below this voltage or leaving a battery connected to a load will impair the battery’s ability to accept a charge. To prevent potential over-discharge problems, voltage cut-off circuits as shown in figure2.18 may be used.
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.
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.
As a result of to 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 runway, and can destroy a battery in a little as a few hours.
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 sorted in a discharged state, or left on the shelf for too long, may initially appear to be “open circuited” or will accept for less current than normal. This is caused by a phenomenon called “sulfating”. 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.
During constant voltage or taper charging, the battery’s current acceptance decreases as voltage and state of charge increase. The battery is fully charged once the current stabilizes at a low level for a few hours.
Never charge or discharge a battery in a hermetically sealed 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. If in doubt or concepts of proper use and care are unclear, contact Power-Sonics’ departments for application engineering at 619-661-2020. Please note that there are two criteria for determining when a battery is fully charged: (1) the final current level and (2) the peak charging voltage while this current flows. Figure 13 depicts an example of typical charge characteristics for cycle service where charging in non-continuous and peak voltage can Therefore be higher.
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.
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, content potential charging is preferable.
Batteries in PV systems
When a PV module is connected to a suitable load. It will divert electrical power that is proportional to the strength of the light failing on its surface. In darkness,- of course, the module produces no power. Very few practical loads can be run directly this variable power availability (exceptions are some water pumps, fans, arid inverters that feed power into the local grid), in most Pv systems, the variable power from the Pv module (or modules) is fed into a storage battery and the bad takes whatever power it requires from this battery. The first principle in designing a PV battery charging system is that the average daily energy production (charging) should slightly exceed the average daily energy consumption
Batteries Provides three Important Function in PV System
Autonomy by meeting the load requirements at night or during overcast periods. Surge current capability by supplying when necessary, currents higher than the PV array can deliver, especially to start motors and other equipment, Voltage control preventing large and possibly damaging voltage fluctuation to the load.
3.8 Battery capacity
When a battery is being discharged its voltage falls as its capacity is being delivered, In PV systems, where low currents are normally drawn. This fall in voltage is quite small for both lead-add and nickel cadmium batteries until nearly all the capacity has been delivered. The total electrical power (Watts) delivered at any time during discharge of a battery is its voltage multiplied by the current being drawn. This power may be distributed between various loads, as well as overcoming the electrical resistance of cables. The total electrical energy delivered by a battery cover a certain period of discharge is measured in Watt-hours (Wh) or, for larger batteries, in (kWh), This is calculated by multiplying the Wnux at my time hy the number of hours for which that power was drawn, If the Watts change due to different! loads being switched on and off, then the total energy is calculated by adding together the WH for the different time periods. Mainly because the voltage when the battery is charging is higher than the voltage when it is discharging, the total energy required for recharging is higher than that which is delivered during the discharge .
The energy efficiency of a rechargeable battery is usually around 70-89% in PV system.
A lead add battery is 90-95%charged, almost no current is wasted in producing gases. But a nickel cadmium battery 80% charged the production of gases occur. The charge efficiency or Ah efficiency is often around 95% for lead -acid battery.
v 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 ceil or battery differences, charge the string in 24 volt battery groups through 3 constant current source with zener diode regulation across individual batteries or battery groups.
v To prevent problems arising from heat exchange between batteries connected series or parallel, it is advisable to provide air space of at least 0.4″ between batteries.
v Battery containers made of ABS plastic or styrene, can sustain damage if posed to organic solvents or adhesives.
v Recharge time depends on the depth of the preceding discharge and the put current of the charger. To determine the approximate recharge time of 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 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.
v For best results and generally acceptable performance and longevity, keep rating temperature range between -2ETC and +40°C.
v 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 lode. Disassembled batteries are hazardous waste and must be treated rdingly. It is unlawful to dispose of batteries except through a recycling center.
4.1 Power Inverter
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 was made to work in reverse, and thus was “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.
4.2 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, 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.
4.3 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.
4.4 MOSFET Based Single Phase Inverter
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 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 & short circuit (only a few mi 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 FETs was developed. By applying a suitable drive voltage to the Gate of an MOSFET, the resistance of the Drain-Source channel can be varied from an “OFF-resistance” of many hundreds of kD’s, effectively an open circuit, to an “O
resistances of less than ID, 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”
4.5 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 changes significantly.
To avoid such problems, most DC-AC inverters 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 pilses. 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.
4.6 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 occult and the low voltage input circuitry (including the battery leads). This path is usually via the auto town sensing, and possibly also the voltage or current sensing used for output regulation.
4.7 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 N AM D gates.
Considering a MOSFET transistor, If Vin is zero, the MOSFET is turned off and so no current flows through the
Fig 4.10: NAND gate using MOSFET
resistor, and , 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 usually described is through a transfer function which relates the output voltage as a function to the input voltage. From figure Rewriting the equation for Id
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.
In this project 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.
In Bangladesh where over half of the population is grid-connected, many parts of the country have frequent and lengthy power cuts. Solar-home-systems have huge potential to provide access to basic electrical services. System costs will decrease as the global PV market continues to grow, and the improving efficiency of solar panel and other accessories.
The strength of solar is the incredible variety and flexibility of applications, from small scale to big scale. So many organizations are providing their finance on less interest such as World Bank, German Bank,IDCOL etc to expand the Solar Home System all over the country.
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