Renewable Energy

Advantages and Disadvantages of Grid Connected System

2011-05-06T03:01:00.000+01:00

There are two types of grid connect systems, those without battery backup and those with battery backup.

Grid connected systems without a battery consist of two main components, a solar PV array and a grid-connect inverter. If the utility grid fails then the solar PV array has no way of providing power as the inverter will automatically disconnect itself from the utility grid.

Grid-tied systems with a battery backup also have an array and a grid-tied inverter, but include the addition of a battery bank and charge controller. Systems with a battery bank can provide power in the event of the electricity (utility) grid failure.

There are many advantages of using a grid connected system

a) Simple to install;
b) High efficiency;
c) reliable;
d) flexible;

If we examine a typical household installation, please refer to the diagram below a number of essential components comprise the solar photovoltaic system.

1. Solar panels - one or more modules wired together to generate a specific voltage and current
2. Combiner (junction) Box - allows termination of the solar panels
3. Grid tie inverter - device that converts DC (direct current) to AC (alternating current).
4. import / export meter - records energy generated and consumed
5. Grid connect point - distribution board connection
6. Electrical load - appliances that are powered by alternating current.

In a typical grid-tied solar system, the DC electricity produced by the solar array.

Ifs fed by cables into a combiner (junction) box where the solar panels are terminated and connected together. A cable from the combiner box feeds the DC electricity into a grid-tied inverter. The inverter converts the DC electricity into AC electricity which is used by the appliances or fed into the grid. The output from the inverter is fed via a fuse directly into the main distribution (fuse board).

Difference between Stand Alone and Grid Connected Systems

2011-05-06T02:59:00.002+01:00

A standalone system is the one which is not connected to the power grid. In contrast, the PV systems connected to the grid are called grid connected PV system. The primary difference between the standalone and grid connected systems comes from the storage feature, which is direct consequence of their connection with the grid. While the standalone PV systems usually have a provision of energy storage or deficit energy produce by the PV source in grid connected systems is supply or drawn from the grid.

b) Grid Connected Systems

2011-05-06T02:58:00.000+01:00

The solar power systems which are connected directly to the grid (connected to an electricity transmission and distribution system) is known as grid connected systems.

Such system is shown in the above figure. In these types of systems electricity generated is used in homes or offices and excessive power is fed back to the grid. Grid connected systems are designed to replace all or a portion of the building’s total electricity needs. The energy generated by such a system is used first within the home, and surplus energy can then be sold back to the grid. The surplus energy “spins the electric meter backwards” when the energy produced by the solar PV system is greater than that being consumed by the home. In actual fact the utility company will normally install an export meters which measure the amount of electricity generated by the solar PV system.

a) Stand Alone System

2011-05-06T02:45:00.000+01:00

A standalone system is the one which is not connected to the power grid. It usually have a provision of energy storage or deficit energy produce by the PV source in grid connected systems is supply or drawn from the grid. As standalone system is not connected to the grid, it must have battery support to supplement the load requirements during the night hours or otherwise. A standalone system can be divided into five categories which are given below:

i) DC Coupled PV System

It is the simplest possible standalone configuration where PV panels are directly connected to the load. The capacity of the PV panels is designed in such a way that it is sufficient to run the load during sunshine hours. As power available to the load varies according to the weather condition, this is called an unregulated system. There is no arrangement for the energy storage to have night time load operation. A figure below describes such system.

DC coupled PV System

ii) Standalone System with DC Load

The block diagram of the configuration is given below. It is similar to unregulated PV system except that the electronic control circuitry is inserted between load and PV panel. This circuitry can be a voltage or current regulator or a maximum power point tracker. In order to achieve the smooth operation of load this circuit is introduced.

Standalone system with DC load

iii) Standalone System with Battery and DC Load

This type of PV system configuration is used when night time operation of the load is required. It requires energy from the battery to operate. The block diagram of such system is given below.

Standalone System with Battery and DC Load

Batteries in a PV system contribute to the recurring cost as the life of the batteries is significantly shorter than the life of the PV modules. The battery life is affected by the manner it is used like overcharging or over discharging reduced the life of battery.

iv) Standalone with AC/DC Load

It is similar to the standalone system with battery and dc load, the only difference is that now AC load is connected with the system. The block diagram of such system is shown below.

Standalone with AC/DC Load
In this configuration an inverter is used in order to convert DC power to AC power which is coming from battery. This type is suitable for domestic and commercial applications.

v) Hybrid System with AC/DC Load

The block diagram of this type of configuration is given below. Whenever there is more than one type of power source connected in a system, the configuration is known as hybrid PV system.

Hybrid System with AC/DC Load

If the load is operated during the night time, when the sunlight is not available, large battery bank will be required to store enough energy. Supplying the load with wind or biomass will be much more economical because no energy storage is required.

Types of PhotoVoltaic System

2011-04-23T18:22:00.000+01:00

Usually photovoltaic system is divided according to following:

1) Functional requirements

2) Component Configuration

3) Type of Load

By keeping in mind these things we can classified it into two main systems .i.e.

a) Stand Alone System

b) Grid Connected System

A PhotoVoltaic System

2011-04-13T11:13:00.000+01:00

A photovoltaic system consists of number of arrays (combination of modules called an array).

A PV generator with mechanical support and sun tracking system.

Batteries or other storage devices.

A charge controller that not only prevents of overcharging of batteries but also prevents the discharging of batteries in night to arrays.

A rechargeable battery is used to store the direct current generated by the solar modules during day and discharging of battery at night if needed.

An inverter is used to to convert the direct current to alternating current in order to supply it to homes and businesses. This inverter consists of a circuit breaker for over discharging.

Above figure shows a typical PV system.

Equivalent Model of a PV cell

2011-04-13T06:32:00.000+01:00

An equivalent circuit of a PV cell is shown below

A diode shunt connected across the current source representing the diffusion current through the p-n junction.
Internal series and parallel resistances are represented by Rs and Rsh respectively.

This equivalent circuit of a pv cell can be connected in series or parallel to form a PV module as it is explain earlier.

Answers

2011-04-13T06:12:00.000+01:00

1) Well, actually that never happens. You can plot the i-v curve of a resistor on the same plot as the photovoltaic cell. It will be a straight line through the origin, with positive slope -- in accordance with Ohm's Law.

Where the two curves intersect will be the operating point for that particular load resistance. This happens in the "+v,+i" quadrant, so it will have voltage less than the open-circuit voltage and current less than the short-circuit current.

2) That's a general property of all power supplies. Open circuit means just that, i.e. the load resistance becomes (for all practical purposes) infinite. i=v/R=v/∞=0

Incidently, the i-v curve of a photovoltaic cell is related to that of a diode. Take a diode's i-v curve and flip it about the v-axis (equivalent to defining the current's polarity in the opposite sense). Then shift the curve upward by an amount equal to the short-circuit current. Works for photodiodes as well as photovoltaic cells, as they are the same thing but with different applications.

Questions

2011-04-13T06:10:00.000+01:00

Here arises two questions

1) Whats happens when the resistance load of a circuit powered by a PV cell generates a higher voltage then that produced by the PV cell (does band gap get affected)?

2) Why does current drop near the Open Circuit Voltage? Surely the generation of current depends on the irradiance on the PV cell? So why does current stop at the Voc point?

I-V Characteristics of a PV module

2011-04-13T06:06:00.000+01:00

The current and voltage are the two main attributes of a photovoltaic module. The performance of a pv module can be determine by these descriptors. The I-V characteristics represents an infinite number of direct current and voltage operating points and varies with solar radiation and cell temperature.

As we know that power is the product of voltage and current,therefore, maximum power can be extracted at a point where voltage and current is maximum as shown in the above graph.

Module

2011-04-09T22:20:00.000+01:00

When number of cells are connected together in series or parallel combination a module is form. If cells are connected in series it produces high voltage where as cells in parallel produces more electric current. The interconnected solar cells are usually embedded in transparent Ethyl-Vinyl-Acetate, fitted with an aluminum or stainless steel frame and covered with transparent glass on the front side.

The typical power ratings of such solar modules are between 10 Wpeak and 100 Wpeak. The characteristic data refer to the standard test conditions of 1000 W/m² solar radiation at a cell temperature of 25° Celsius. The manufacturer's standard warranty of ten or more years is quite long and shows the high quality standards and life expectancy of today's products.

Photo Voltaic Materials

2010-09-16T20:27:00.000+01:00

A solar cell made up of by different materials. The main aim of making solar cell is to keep price minimum while extracting maximum power from it. The materials used for making a solar cell is discussed below.

1) Crystalline Silicon : Nowadays solar cells present in market are usually made up off this material. Early forms of silicon photovoltaic cells were very expensive because of difficulties in the industrial preparation of high-grade silicon. very pure single-crystals of silicon need to be grown as cylindrical ingots, about 10 micrometers diameter, in order to maximum the cell exposure area. This is known as "monocrystaline silicon".
Recent development in this has been to grow silicon crystal in the form of ribbon rather than an ingot. This process generates less pure silicon than the traditional method and the efficiency is in between 10-12%.

2) Polycrystalline Silicon : It consists of a small silicon crystals oriented in a thin layer of polycrystalline material. The solar cells made up of this material is less expensive than the crystalline silicon on the cost of efficiency.

3) Amorphous Silicon : In this type of silicon there is no regular crystal structure therefore they defects in bonding and structure. Amorphous silicon can be deposited onto backing material in very thin films around 01 micrometers thickness. This reduces the amount of usage of silicon material resulting in reduction in mass production cost. Although they are cheaper but have less efficiency than the crystalline silicon.

How Solar Cell Works

2010-08-31T12:32:00.000+01:00

Bohr presented his theory of atomic model in 1913 that is more stable than that of Rutherford theory. According to his theory

Electron with mass (Mc) revolves around the nucleus in an orbit with radius (rn) and angular frequency (ωn). The centrifugal force produces is given by
F = Mc . rn. ωn^2
Electrons are held in an orbit around nucleus of an atom with a coulomb force
F = q 1 q 2 / 4 π ε r^2

These two forces are responsible for the movement of electron in an orbit.

Apart from Bohr's model, there is another theory called Plank's Theorem can be use to help an electron to remain in its orbit. According to this theory, electrons remain in an orbit, where angular momentum is an integral multiple of plank's constant.

An electron in an orbit contains energy given by En = mc * rn^2* ωn^2.

Now to move electron from its orbit to higher orbit, it is necessary to provide energy to that electron greater than that of given in the above equation from outside.

2010-07-28T09:07:00.002+01:00

Advantages of Photovoltaic Generation

2010-07-24T18:27:00.002+01:00

* There is no moving part so that little maintenance is required.

* They utilize an infinitely renewable and pollution free power source.

* The cells are reliable and long lasting with no harmful waste products.

* The cells are usually made of silicon which is one of earth’s most abundant and cheap materials.

* They have high power-to-weight ratio which is required in aerospace applications .

Energy from the Sun

2010-07-24T18:13:00.000+01:00

Around 174 petawatts of solar radition is recieved by earth at the upper atmosphere Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet .

Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection . When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle . The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind , cyclones and anti-cyclones . Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C . By photosynthesis green plants convert solar energy into chemical energy , which produces food, wood and the biomass from which fossil fuels are derived.

Genaration of Electricity through Solar Energy

2010-07-24T18:03:00.000+01:00

Solar energy, radiant light and heat from the sun , has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation , along with secondary solar-powered resources such as wind and wave power , hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.
Solar powered electrical generation relies on heat engines and photovoltaics . Solar energy's uses are limited only by human ingenuity. A partial list of solar applications includes space heating and cooling through solar architecture , potable water via distillation and disinfection , daylighting , solar hot water , solar cooking , and high temperature process heat for industrial purposes.To harvest the solar energy, the most common way is to use solar panels .
Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

Renewable Energy Resources

2010-07-20T03:24:00.000+01:00

The sources which can be reused to generate energy or electricity is commonly known as renewable energy sources. These sources include solar energy, wind energy, tidal energy, hydral power, energy from biomass and geothermal energy. Below is the chart given for the renewable energy sources in UK.

Renewable energy resources used in UK between 1990 and 1998, and the equivalent amount of oil.

Renewable energy resources	Equivalent amount in oil (thousands of tonnes)
	1990	1996	1997	1998
Active solar heating	6.4	8.6	9.0	9.5
Onshore wind	0.8	41.8	57.2	76.1
Hydroelectric	447.7	289.0	354.8	449.3
Landfill gas	79.8	247.2	316.0	400.8
Sewage sludge digestion	138.2	190.2	191.5	184.1
Wood	174.1	709.7	710.3	710.3
Straw	71.7	71.7	71.7	71.7
Municipal solid waste	160.0	368.7	427.0	567.7
Other biofuels	80.6	175.7	182.1	185.5
Total	1,159.3	2,102.6	2,319.6	2,655.0

Blogger Buzz: Blogger integrates with Amazon Associates

2010-07-20T02:27:00.000+01:00

Blogger Buzz: Blogger integrates with Amazon Associates