Non conventional sources of Energy : Basics, Advantages, Disadvantages and challenges

On Mar 10, 2014 by Sunil Saharan At 4:03 PM | , | 1 comments »
From past few years we are continuously listening one term i.e. Renewable energy resources. Those who are related to Energy production and energy consumption field know what It means.

Non Conventional Resources: Meaning

Non conventional resources are those energy resources which can be used again and again. It means renewable resources are those which can be reused without much efforts and energy can be received at much lower cost. Such resources never ended. Hydro Energy, Solar Energy, Geo-thermal Energy, Tidal Energy, Bio-Energy are a few of renewable resources.

Non Conventional Resources: Advantages

There are a lot of advantages of non-conventional energy resources. A few of them are:
  • No Fuel Cost: Renewable energy resources provide energy continuously at negligible fuel cost and hence these are cheap source of energy which makes them favorable over other sources of energy.
  • Pollution Free and Eco-Friendly: These Non Conventional Energy Resources are pollution free and hence are Eco-friendly. This is one of the best thing to be considered in this age of global warming.
  • Simple Design: Renewable energy sources have simple plant design and hence easy to operate and no specialized workforce is requires in these plants.
  • Lower maintenance cost : Being simple in design the maintenance cost of these plants is very less and these plants can thus produce electricity at much lower cost than other plants.
  • Available at load center : These plants can be installed remotely and at locations completely off grid. Hence better choice for single plant use.

Non Conventional Resources: Challenges

These renewable sources also have a few disadvantages. Few of them are:
  • High Initial cost: Although running cost and maintenance cost of these plants is very low but installation cost for these plants is very high which is major disadvantage to these plants.
  • Low energy density: Low energy density is another problem these plants have. Energy per unit area is small in these plants which makes them big in size and hence large area is requires to install them.
  • Seasonal: These sources are seasonal in nature like Solar energy is not available in rainy and winter days. Hydro energy is not available in dry days.

SCR : Construction, Working and V-I characterstics

On Oct 18, 2013 by Sunil Saharan At 8:35 AM | , | 2 comments »

SCR : Construction and Symbol

SCR (Silicon Controlled Rectifier) is a four-layer (p-n-p-n) unidirectional device with three terminals, namely, anode (A), cathode (K) and gate (G) having three p-n junctions generally named as J1,J2,J3. The symbol and construction of SCR is shown below:


 

An SCR works in three modes. These modes are
Forward blocking mode
Forward Conduction mode
Reverse blocking mode
These modes are explained below

Forward blocking mode

In this mode of operation anode is made positive with respect to cathode and gate is kept at zero potential. In this connection arrangement shown above junction J1 and J3 are forward biased while J2 is reversed biased due to which current is blocked and only a small amount of current called leakage current will flows from anode to cathode due to minority charge carriers till applied voltage reaches its break over value at which junction J2 breakdown and at this break over voltage it starts conducting but below break over voltage it offers very high resistance to the flow of current through it and said to be in off state.





Forward Conduction mode

In this mode, SCR conducts currents from anode to cathode with a very small voltage drop across it (as the resistance of SCR reduced due to breakdown of J2, hence voltage drop across it decreases instantaneously). A SCR is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode.
In this mode, SCR is in on-state and behaves like a closed switch. Voltage drop across SCR in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen from the voltage drop increases slightly with an increase in anode current.



Reverse blocking mode



When cathode is made positive with respect to anode with gate open, SCR is reverse biased as shown here. Junctions J1 & J3 are seen to be reverse biased whereas junction J2 is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them.
A very small leakage current of the order of a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state, of the SCR. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly.
A large current associated with VBR gives rise to more losses in the SCR. This may lead to SCR damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a SCR does not exceed VBR. When reverse voltage applied across a SCR is less than VBR, the device offers high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.

The Complete V-I Curve is shown below:
SCR : Working and V-I Characterstics

Breakover Voltage:
When the gate of the Silicon Controlled Rectifier is open, then the minimum amount of forward voltage which is required for Silicon Controlled Rectifier to start conducting heavily is known as Breakover Voltage.          
Peak Reverse Voltage:
When Cathode Terminal of SCR is kept positive with respect to anode, then the maximum reverse voltage at which Silicon Controlled Rectifier remains in OFF state or doesn't conduct , is called Peak Reverse Voltage.
Holding Current:
When the gate terminal of SCR is kept open, then the maximum value of anode current at which Silicon Controlled Rectifier is turned OFF from conducting condition, is known as Holding Current.
Forward Current Rating:
The maximum value that an Silicon Controlled Rectifier(SCR) can bear or handle without burning is called Forward Current Rating.