Other Renewable Sources of Energy

In this tutorial , we shall learn some of the other renewable energy sources like Solar, Small Hydro and Biomass and miscellaneous forms of energy. These forms of energy can be replenished  i.e. the fuel for producing energy is continuously available forever. Hence these are said as renewable sources. In the awake of dwindling coal and oil reserves there is a urgent need to identify future sources of Energy . Research and Education now focus on this aspect so as to address the issues. The tutorial will focus on explaining the principle behind the Energy conversion, the technology deployed along with a brief overview of pros and cons. The tutorial is also filled with neatly drawn graphic content to exemplify the concepts wherever necessary.

Solar Energy

The Sun is the primary source of energy for the major Renewable sources including Wind, Solar Photo Voltaic, Solar Thermal and Tidal Energy. Although wind and tidal doesn’t directly result from solar radiation they are considered to be originated due to Sun and are effected by Sun. Even the earth temperature is maintained constant with the Solar radiation and the trees produce food from photosynthesis where Solar radiation is mandatory. Research is now looking to produce a Artificial leaf which can produce Energy from the Solar Radiation

Now we will narrow into Solar Energy. Traditionally, man has been harnessing solar energy since ages for purpose of heating and lighting. After the invention of Steam turbine, it became possible to use solar heated liquid to generate electricity. Later after evolution in Semi conductor physics, photo voltaic cells were developed and thus PV generation through Silicon panes came into sustenance.

Electricity Generation from Solar Photovoltaic cells

The Electricity generation from Photovoltaic cell is accomplished due to the Photo Electric Effect of light. When light is incident on a metal, the electrons on the surface atoms absorbs are exposed to the radiation in energy of light. The energy in  the form of radiation is absorbed by a few electrons. Thus when energy is absorbed the total energy of electron increases. As a result it cannot rotate in the same orbit. It tends to shift to higher orbit and jumps from a lower orbit to a higher orbit. Generally, the photo electric effect is said to be observed when the electrons jumps from a valence band to conduction band. The conduction band is the band where electrons are free flowing and not under binding by atom.

A schematic of the photo electric effect is given below in the figure

Now, technically speaking, a electron volt is defined as energy required to drive a electron through a voltage difference of one volt. It is denoted by eV and is a unit of energy for measuring the energy gap between two bands of the metal ie (valence band and conduction Band) . For each of the metal this band gap varies and for some metals this band gap matches the energy in radiation. When this happens, photoelectric effect takes place. Thus only a few metals can generate electricity from the solar radiation due the band gap constraints.

The equation for the solar radiation is given as follows. Here E is the energy and h is plancks constant and υ is the frequency of the radiation.

E= hυ

Thus the energy generated from the radiation should be more or equal to band gap of the metal and the band gap of the metal is also known as work function and denoted as Φ. Thus we obtain the relation E> Φ or E= Φ. In most cases. The first equation is valid for most of cases and extra energy is dissipated as losses. It is to be noted that the silicon has a work function of 1.1 eV which is highly suitable for PV applications

Problem : Calculate whether a radiation of  50nm frequency would be sufficient to excite a electron of the silicon metal

Solution:

Solar Cell

As we have discussed above, silicon can be used to for the photovoltaic operation. Generation of electrons is not just sufficient for production of electricity, These electrons need to be trapped by proper mechanism so as to generate power from the input. Solar cells accomplish this task. It is basically a PN junction diode which has metal having photo electric effect as a N type material and dope with a P type semiconductor. The additional parts include the glass coating which is used to protect the cells from the harsh environments and the back cover to handle the cells and mount them on a structure. Most of the times, the cells are enclosed in a encapsulant which is a organic compound other than air. The practical model of a solar cell is as follows : Solar cells are of many types : thin film solar cell, mono crystalline cell, poly crystalline cell. These cells vary in construction and manufacturing methods. The fundamental difference is the lattice  organization of silicon.  Many other metals like Cd and Te are also in use.

Solar PV Plant lay out

Having discussed the basic of the solar cell, we now go ahead with the production of solar energy through a power plant. This is the exact point where a electrical student comes into picture. The vital parts of a solar PV plant are

1. The PV panels

2. Connection boxes and DC wiring

3. MPPT tracker

4. Inverter

5. Step up transformer

6. Switch yard , CBs and other misc equipment

7. Battery storage

PV panels : A basic solar cell discussed above produces energy in range of a few watts. Thus we need to connect the cells in series and parallel combination to get required voltage and current ratings. From basics, we understand to improve current , we add more cells to series and to improve voltage we add cells in parallel.  Commercially available panels are generally rated in kilowatts peak and are available in all ranges from 1kwp to 100kwp. Once such panels are chosen and  are acquired in large numbers to suit the rating of the plant.

Connection Boxes and DC wiring : Each panels have wires originating from every series and parallel combination. These wires are properly combined in a DC combination box and the DC wiring is used to connect all the panels output to a MPPT tracker.

Maximum Power Point Tracker :

The output of the solar cell is dynamic due to continually changnign radiation , cell temperatures and various other factors. The cell is a passive device which just generates the electrons. These electrons must be circulated by external circuit and the rate of flow of electrons increases, power increases. Thus the system must always tracks the maximum power point on and operate at that particular value. The following graph exemplifies the requirement of MPPT. The current changes for the irradiation and so is the power. Thus operating at the knee point where the power output is maximum as above shown in the graph is required to obtain maximum benefit from the investment. Technically, MPPT is DC-DC converter or a device with similar topology. Sometimes this is integrated with the inverter so as to save the space. These days commercial inverters are available with built in MPPT trackers.

Inverters : The energy produced by the solar panels is DC and we need an AC power to pump it into grid. Hence we use an inverter. Presently used inverter are manufactured with IGBTs and have an excellent efficiency of above 80%. Another interesting aspect here , as the power generated by inverter falls its efficiency falls. For example a 5MWp inverter operates at around 85% efficiency if the power being generated is 4MW. As the amount of generation due to solar is not constant always throughout the day, we generally deploy a inverter of lower rating so as to gain maximum output from the power plant. This means a 10 MWp power plant doesn’t have to install a inverter of capacity 10 MW. It goes for a 9.5MW or 9.7MW.

Also the inverters for Solar are custom made to suit the above problem and also the voltage and current from the panels varies continuously. Thus solar inverters are different from general purpose inverters.

Step up transformer : A transformer is required to boost the voltage of the power from the plant through inverter. Generally, the solar plants are being tied up to the grid through distribution substations. In such case the voltage must be around 33KV or 66KV. A inverter produces power only at about 440-600 depending on rating of the inverters. Thus a step up transformer is necessary to step uo the voltage to required rating.

Switch yard , CBs and other misc equipment : These are the generally used equipment for all electrical plants. Circuit breakers are required for protection. Other misc equipment includes SCADA, Metering infrastructure and Solar radiation measuring instruments.

Losses in Solar PV.

1. Temperature losses : The solar cells efficiency are rated at standard conditions and as the temperature of the panels rise, the efficiency falls. This is due to decrease in conductivity of the cells. The losses are   0.25%/0C - 0.5%/0C of the output depending upon the type of cell. As a result a cell operating at 200C higher than operating point will have a 10% losses

2. Irradiation losses : All the incident light is not absorbed in spite of best anti reflective coats. This losses accounts for the irradiation reflected by the panels .

3. MPPT and Inverter losses : While we track the MP point the system should operate at other points as well for a few time and also the system cannot check the dynamic change of parameters for every instant. Thus there are losses accounted for the MPPT which are assumed to be 1-2%. The inverter losses are around 2%

4. Transformer and Transmission line losses : These losses account for the losses in the t/f and the transmission lines use to connect the power to the substation.

Outlay of the plant

Solar Thermal

So far we have discussed some of the advanced technology to tap the solar power. But the solar thermal energy is being used over a century. Although there are several applications like water heating, room heating and cookers and etc which use direct heat energy, we shall now focus on electricity generation from the thermal energy.

The principle is similar to that of a thermal power plant. There the coal is burnt to turn the water into steam. Here we use the heat from the Sun. The heat energy from the sun when used directly cannot heat the water into steam. Hence , we use parabolic concentrators which concentrate the entire solar energy over a region into single point. Thus , we can heat water up to 1500C. Even this steam cannot be used perfectly to run steam turbine. Hence we generally use another fluid which is known as working fluid which can efficiently handle the heat and convert into the electrical energy in the turbine.

From the above discussion , we need to understand what a parabolic concentrator is. It is simply a reflective surface which is a paraboloid. It is as shown below. Generally, the pipes carrying the working fluid is placed at the focus of the parabola. Several hundreds of these are placed in series and are used to generate the heat which then runs a turbine.  

Some of the existing parabolic converter technologies are

1. Parabolic trough collector systems : This is the same technology which is discussed above.

2. Power tower system : In a power tower system , we have a tower which acts as a collector a numerous reflectors are placed in and around collector which reflect the solar radiation on the tower. This Is as shown below.

3. Parabolic dish type : This is similar to the parabolas as discussed previously. But the dish type is a very huge parabola which can be tracked in dual axis and produce large amount of heat.

Power tower system

The solar thermal plant Is relatively very cheap to construct compared to solar PV and also the efficiency is around 15-20% which can be compared on par with the solar pv. But the PV are very simple to construct and maintain whereas the thermal requires a lot of maintenance and running costs.

Barriers  

Solar thermal power plants need detailed feasibility study and technology identification along with proper solar radiation resource assessment. The current status of international technology and its availability and financial and commercial feasibility in the context of India is not clear.

Hydro Power stations

Hydro stations are developed from the early nineteenth century after the invention of water wheel. The hydro stations are a costly affair than the thermal because of huge costs involved in construction of dam, relocation and other costs. But the maintenance costs are less because no fuel is spent. The large scale power plants these days are built on the basis of multipurpose where water is further used for irrigation other purposes. Thus the overall cost per KW is reduced making these systems viable. It is also one of the cleanest form of energy like solar photovoltaic.

The outlay of the plant is as follows.