SUN IN CITY

In 1972, first CdTe/CdS solar cell was reported. It is a type of photovoltaic that uses Cadmium Telluride (CdTe) and Cadmium Sulphide (CdS) thin film for light absorption and charge extraction for generating electricity using the photovoltaic effect. Two major companies that are manufacturing CdTe solar cells are "Antec" in Germany and "First Solar" in US.

CdTe (Cadmium Telluride) solar cell belongs to the family of chalcogenides (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe). CdTe is formed by reaction of Cd and Te vapours and then deposition on surface using sputtering, thermal evaporation and other physical vapor deposition techniques. CdTe is a direct band gap semiconductor and has high absorption coefficient. As absorption coefficient is inversely proportional to absorption length, only a few micron thick layer of cadmium telluride is required to absorb visible spectrum of sunlight. Band gap of CdTe is 1.45 eV which is most appropriate for solar cell applications because wavelength corresponding to 1.45 eV comes out to be 855 nm (λ = hc/E = 1240/1.45 = 855). Thus, whole visible region will be absorbed and wavelength above 855 nm will not be absorbed.

CdTe Solar Cell Parameters

Efficiency (η) : 21 %

Open Circuit voltage (V_OC) : 0.88 V

Short Circuit Current Density (J_sc) : 30 mA/cm²

Fill Factor (FF) : 80 %

Mobility of Electrons (μ_e) : 500-1000 cm² / Vs

Mobility of Holes (μ_h) : 50-80 cm² / Vs

Melting Temperature : 1365 K
Absorption Coefficient (600 nm) : >5 * 10⁵ / cm
Density : 6.2 g / cm³

CdTe Solar Cell Structure

Figure 1. CdTe Solar Cell Structure

Figure 1 represents superstrate structure of CdTe solar cell. Superstrate structure is the one in which light enters through the substrate itself. In CdTe solar cell front contact is made by TCO (Transparent Conducting Oxide) and is also called (Window Layer). TCO layer should be thick enough so that it can prevent the impurities from the substrate. On TCO CdS layer is deposited (100 nm) followed by deposition of CdTe layer (5 micro meter). Once the junction is formed device needs to be activated by chlorine treatment using CdCl2. In this treatment CdCl2 is dripped on to CdTe coated substrate and then heating it at 450 degree Celsius for 15 minutes. CdCl2 treatment lowers down the defect density and give rise to holes, promoting P-type character. The last step involves the formation of back contact. Figure 2 represents sequence wise manufacturing process of CdTe solar cell.

Figure 2. Manufacturing process of CdTe Solar Cell

CdTe is used as P-type semiconductor and CdS is used as N-type semiconductor leading to the formation of junction and depletion region as shown in figure. Band gap of CdTe and CdS is 1.45 eV and 2.42 eV. As the major portion of the solar spectrum is absorbed by CdTe, CdS is mainly used as partner layer to form the cell and is referred as "Buffer Layer". To enhance the efficiency of solar cell active layer band gap should align with that of buffer layer band gap. In cadmium telluride solar cell, CdS is highly doped and CdTe is lightly doped so that depletion layer portion is extended deep inside CdTe side and recombination will be less probable due to the presence of an electric field in the depletion region as shown in figure 3.

Figure 3. CdTe Solar Cell Energy Band Diagram

Issues with CdTe Solar Cell

1.) Intermixing of CdS-CdTe layer.
2.) Toxic nature of Cd.
3.) Bandgap can not be tailored.

In the early stage of technology, amorphous silicon was found to be an interesting material because with doping it can be made P-type or N-type and provided an opportunity for the formation of junction and leading to the formation of solar cells. But after introducing sunlight radiation on solar cell, efficiency decreases and this effect is called Staebler Wronski effect.

Large number of defects around 10²³ / cm³is present in amorphous silicon due to large number of dangling bonds. There are several missing bonds and these missing bonds are called dangling bonds. Dangling bond reduces the performance because they act as traps and they absorb the carriers. This problem was solved by making hydrogenated amorphous silicon (a-Si: H). Hydrogen passivation during the manufacturing process creates hydrogen bond (with dangling bond) and reduces the defect densities ( to around 10¹⁶/ cm³ and improves the device performance. With this improvement, a lot of interest began in building in amorphous silicon and the manufacturing process started.

For solar applications hydrogenated amorphous silicon (a-Si: H) showed high absorption coefficient in the visible range of solar spectrum (AM 1.5G). As the absorption coefficient is inversely proportional to the absorption length, only a few micron layer thickness of hydrogenated amorphous silicon is required to absorb 90% of the visible spectrum. In 1976 first hydrogenated amorphous Silicon (a-Si: H) solar cell was developed by “Carlson and Wronski” with an efficiency of 2.4 %.

Figure 1 Energy Band Structure

Figure 1 shows energy band of crystalline silicon and hydrogenated amorphous silicon (a-Si: H). It is observed that in case of crystalline silicon there is tetrahedral configuration with SP³ hybridization which indicates “no trap states”. But in case of hydrogenated amorphous silicon (a-Si: H) instead of tetrahedral configuration, bonds with different length and angles are present which are less stable and are called as “weak bonds”. When energy is given to material in the form of solar radiation, these week bonds break and causes degradation in inefficiency. This effect is called Staebler Wronski effect. It is fortunate enough that efficiency does not drop continuously after light exposure. After some time, it gets saturated to a value of 80 to 90% of the initial value.

Gallium Arsenide (GaAs) is a compound of two basic elements; Gallium and Arsenic. It is III-V direct bandgap semiconductor with 1.41 eV band gap, so relatively it can absorb (AM 1.5G spectrum) light more efficiently. This band gap can be varied by Aluminium (Al) or Indium (In) like AlGaAs or InGaAs. First GaAs solar cell was used in space application for Venera 3 mission and then till now, its efficiency has reached up to 29.1 % by Alta Devices. To check the efficiency of all types of solar cells, click here.

GaAs has several advantages over Silicon such as higher temperature applications and higher carrier mobility. But is 5 to 10 times expensive than silicon and due to direct bandgap radiative recombination is also dominating.

Gallium Arsenide (GaAs) unit cell representation

In other semiconducting materials that are being used in solar applications, efficiency reduces with increase in temperature. But GaAs has a low-temperature coefficient, thus it is not much affected by increasing temperature. In GaAs elements used for N-type doping are Selenium (Se), Silicon (Si) and Tin (Sn) while elements used for P-type doping are Zinc (Zn), Magnesium (Mg) and Carbon (C). With lower resistance and higher carrier mobility, GaAs creates less noise in electronic circuits at high frequencies. Although being expensive GaAs finds it's applications in other technologies like satellites, military applications and ICs.

Note

Mobility
Mobility is defined as how quickly the charge carrier can move within the semiconductor. In GaAs electron mobility (μ_e) and hole mobility (μ_h) is in the range of
(μ_e) = 5000 to 10000 V/cm²
(μ_h) = 100 to 400 V/cm²

Carrier Life Time
It is the average time taken by the minority carrier in the excited state to recombine. In GaAs electron lifetime (T_e) and hole lifetime (T_h) is in the range of
(T_e) = 1 microsecond to few nanoseconds
(T_h) = few nanoseconds.

For solar applications, GaAs solar cell provides high mobility and high carrier lifetime which cause lower recombination possibilities and higher efficiency is obtained.

Solar radiation that earth receives varies inversely with the square of the distance between the sun and earth. Due to continuous motion of the earth in the orbit, radiation coming on earth also varies.

Extra-Terrestrial Solar Radiation

It is solar radiation just outside the earth’s atmosphere. The figure shown below represents extra-terrestrial solar irradiation.

Two terms are commonly used while studying solar spectrum namely “Irradiance” and “Insolation”

Irradiance: It refers to the solar power received by the collector per unit area and its unit will be kW/m²/nm.

Insolation: It refers to the energy received by the collector per unit area over a given period of time and its unit will be kWhr/m²/nm.

Figure: Reduction in radiation at the earth's surface because of absorbption and scattering of radiation by the ozone layer, CO₂, water vapour and dust particles present in the atmosphere. (Source: https://rredc.nrel.gov/solar//spectra/am1.5/)

Solar Spectrum atEarth’s Surface

After entering into the atmosphere about 6% of the irradiation is reflected and around 16% irradiation is absorbed and scattered by the ozone layer, CO₂, water vapour and dust particles present in the atmosphere. Radiations that gets scattered in the atmosphere are called Diffused Radiation and radiation that gets reflected from the ground is called Albedo Radiation.

Airmass

Airmass gives an idea of the path length through which spectral irradiation passes through the atmosphere before reaching the earth’s surface. This length varies continuously due to the varying position of the sun with respect to earth. To avoid confusions with varying path lengths, standard Airmass coefficients (AM0, AM1, AM1.5, AM2) are made to compare different solar panels and same is used worldwide.

Airmass Coefficient

It is defined as the direct optical path length of the insolation through the atmosphere. It is expressed relative to the zenith path length which is normal to the horizon plane at sea level.

Air mass approximation is given by

AM = 1/Cos(z)

Where z is zenith angle in degree.

AM0 represents radiation spectrum just outside the earth’s atmosphere.

When sun is exactly at overhead position, spectrum is referred to as AM1. In this case, sun radiations travel the shortest distance and is considered unity(1).

When sun is at 48.5 degree from zenith, spectrum is referred to as AM1.5. In this case obviously, the radiations will have to travel a longer distance in the atmosphere.

Example

For air mass =1.5 (AM1.5), calculate zenith angle (z).

AM = 1/Cos(z)

1.5 = 1/Cos(z)

z = 48.5.

Both Solar Energy and Biomass Energy have their individual strengths and weaknesses. Solar energy requires very less maintenance but is limited only for 5-6 hours per day on an average. Biomass energy has an advantage of dispatch ability but biomass prices vary on a wide scale every year. Solar PV-Biomass Hybrid power plant which will enhance the potential of the plant by balancing their individual strengths and weaknesses.

Sunlight takes 8 minutes 20 seconds to reach earth's surface and it is almost 3078 times the energy that we need to sustain. So, our SUN is seen as most abundant renewable energy source.

Average global power demand is considered to be 16 TW. Surface power density is a tool which helps us evaluate land requirement for each renewable energy resource. Table given below shows range of densities that represent an average power density of all locations on earth.

Source	W/m2
Hydro	8-10
Wind	2-3
Solar	20-30
Bio	0.1-0.3
Geo	0.005-0.015
Tidal	2.5-3.5
Wave	20-30

Solar Energy is economically viable and competitive in market. A recent report in International Energy Agency (IEA) states that photo-voltaic and solar thermal technologies will fulfill 27% demand by 2050. Dropping cost of solar cell and continuous innovation is making solar technology more attractive. Innovations have been made in past years and efficiency has also reached beyond limits, thus promising a future with clean energy.

Softbank CEO, Masayoshi Son

Masayoshi Son, is a Japanese businessmen and also the CEO of SoftBank. Son was named world's 45th most powerful person by Forbes Magazine's "List of world's most powerful people 2013".

Soudi Crown Prince Mohammad bin Salman

Mohammad bin Salman is the crown prince of Saudi Arabia which is the second most important position in the country. He was appointed Crown Prince in June 2017.

Masayoshi Son and Mohammad bin Salman signed MoU to create world's largest solar power generation project. During a press conference in New York they said that they are expecting to build solar plant of 200 GW capacity in the country by 2030. In 2019 two plants of capacity 3 GW and 4.2 GW will be launched.

Estimated cost of this project is $200 billion through 2030. This large scale solar plant will thus provide a support to domestic solar equipment manufacturing industry. This project would almost triple the electricity generation capacity in the country.

China's Tengger Desert Solar Park is currently the biggest in the world with capacity of 1500 MW. Being the largest exporter of petrolium, this is a significant move by Saudi Arabia.

Characteristics of Solar Panels

Open circuit voltage V_oc can be defined as maximum output voltage at zero output current which means whole current flows through the diode (R is infinite). Short circuit current I_sc can be defined as maximum output current at zero output voltage which means whole current flows through the load (R is zero). As we can see in figure 1,2 and 3 that I_sc varies linearly in proportion with the illumination and V_oc varies logarithmically with illumination. On the other hand I_sc varies little with temperature and V_oc is inversely proportional to temperature.

ICICI Lombard General Insurance made an announcement today that solar park developers can now be covered under the insurance scheme and stated

" The standalone insurance product gives comfort to lenders and investors and is a perfect solution to support project finance. The product will be distributed through the company’s branches and affiliated agencies. "

Alok Agarwal, Executive Director, ICICI Lombard General Insurance told that " The viability of solar power projects depends largely on the performance of solar modules. Also, the risks associated with this industry are different compared to the conventional power generation sources, such as thermal power plants. Thus, a comprehensive solar panel insurance product would help in covering the various risks associated with developing, building, operating, owning and investing in solar power projects. "

With the increase in Solar Industry all across the country to achieve the target of 100 GW solar energy by 2022, this insurance scheme will contribute a lot.

Agarwal added " This is a long-term product in which a solar park developer is protected for at least 15 years from the start date of commercial operation of solar parks. This will make the energy transition cheaper and generate more economic value. To enable this transition, we must frame the right policies, and must embrace the new era of solar power and uphold our promise to provide a cleaner environment for our future generations. "

Key problems such as degradation of PV modules, material ageing and under performance of PV modules will be covered in this scheme.

Target of 100 GW Solar energy by 2022 is set up by Government of India, motivating Indian solar PV panel manufacturers but but cheaper chinese solar modules have created a burden on these companies and have risked the employment of thousand of workers. Due to this pressure continuity on Indian manufacturers we may see increase in bad loans next year.

Occupational Safety and Health Administration (OSHA) is an agency for safety and health legislation and have specified certain mandatory measures for ensuring safety at work place. OSHA is an act passed by United States in 1970. Many countries including India follows safety measures provided by OSHA.

Guidelines of OSHA are divided into four categories :

1) Eye and face protection.

2) Head protection

3) Hand Protection

4) Foot Protection

Thus employers are the one who are responsible for providing safe and healthful workplace to their workers by following OSHA guidelines.

What is Net Metering ??

Net Metering is an agreement signed by the system owner and utility which allows the owner to buy or sell the energy from utility using Net Meter which tracks energy exchange.

An Example

During early morning or evening hours when there is no sun, energy required to fulfill the load is taken from the utility grid and net-meter moves forward. While in day time, during peak sun hours the load is less and energy generated is in excess. This excess energy is fed back to the grid and net-meter moves backward.

NOTE- Electricity generated needs to be fed in real time as electricity travels with speed of light.

Advantage of using Net Meter

1) Financial Benefits - Per unit cost of excess energy generated is paid to the user by utility.

2) No need of battery backup - Excess energy from PV panels need not to be stored in battery as it can be fed directly to grid. Thus there is no battery capital cost and maintenance cost.

Feasibility of Net Metering System

Usually DISCOM that are active in your area do a feasibility check of your system under certain guidelines such as - The total capacity of solar plant to be installed should not be more than 30% capacity of the distribution transformer.

Other guidelines vary as per local DISCOM's criteria.

For the past few years, much research has been conducted regarding standalone and grid-connected Renewable Energy (RE) sources all over the world. The HOMER tool has been used to analyze a hybrid electric supply system (hydro/PV/wind/ biomass) and to find the optimum sizing of components for a diesel-based RE system. The name HOMER is an abbreviation of “Hybrid Optimization Model for Electrical Renewable” and it is developed by U.S. National Renewable Energy Laboratory (NREL). HOMER Pro allows simulation of various combinations of Solar PV modules, Wind turbines, and Biomass-based generators. Proper functioning of HOMER requires an understanding of its three core capabilities –

Simulation: At its core, HOMER is a simulation model. HOMER simulates a viable system for all possible combinations of the equipment that is to be considered. Depending on the problem, HOMER may simulate hundreds or even thousands of systems.

Optimization: The optimization step follows all simulations. HOMER sorts all the simulated systems and filter them according to criteria defined by a user so that the best possible fits can be observed.

Sensitivity analysis: This is a step that HOMER allows to model the impact of variables that are beyond our control, such as wind speed, fuel costs, etc., and see how the optimal system changes with these variations.

HOMER PRO is a good tool........ DO check it out.

To get familiar below is a screenshot of HOMER PRO home screen.

Government announce various schemes to provide continuous power supply to rural India. Still, hundreds of villages use chimney lamps. ‘Electrified’ villages still waiting for power. In many villages, electricity poles have been installed, but lines have not been laid. In order to improve energy security in such places we need “DC Solar Micro Grid” for providing free electricity which will be free upto a particular limit and then chargeable. Energy is a strategic research priority and we must be committed to deliver solutions to the global energy challenge.

Smart Grids is a broad concept that covers the entire electricity supply chain and is characterized by the use of technologies to intelligently integrate the generation, transmission and consumption of electricity. We define the concept of Smart Grids as one that embraces all measures in support of immediate and future integration of automation and control technologies into local, national or regional electricity infrastructure. The concept aims to optimize grid systems and their operation, integrate high levels of renewable energy penetration, and improve the reliability and efficiency of electricity supply. In addition to being smart, this system guarantees access to modern energy services without marginalizing the poor.

Concept :
The concept aims to optimize grid systems and their operation, integrate high levels of renewable energy penetration, and improve the reliability and efficiency of electricity supply. In addition to being smart, effective and economical measures will be taken in order to guarantee access to modern energy services without marginalizing the poor.

Applying the Concept :

Distribution Design : Distribution using smart sensors, flexible and intelligent switches and interrupters at critical points on distribution circuits will minimize the extent of outages and increase the speed of restoration, while keeping cost increases at a minimum. Smart distribution technologies will be especially important for addressing rural electrification needs and minimize connection costs.

DC Micro Grid : Cost is further reduce through the implementation of (DC) micro‐grids. While losses can be reduced through saving layers of DC/AC power conversion, the more expensive protective devices required for fault management and control, such as coordinated power converters, add complexity and outweigh some of the potential savings.

Demand Side Management : This usually affects the poorest electricity consumers the most. The DC load such as LED Bulb, LED Tube, DC Fan, DC TV each with 12V rating will be used which is of lower wattage rating, thus reducing the demand. This could also encourage people to adopt energy efficient practices for peak times, either because of higher tariffs or dependency on batteries.

Charging Through Prepaid System : While rural electrification is a priority, millions of people live near the grid but cannot afford a connection. For these people, charging stations ensure a minimum level of access to electricity services. For eg. Anyone can got to the recharge shop and ask for electricity and can buy it if its free daily consumption is over.

Before understanding Tilt Angle, we should get familiar with Latitude and Longitude.

Latitude and Longitude are imaginary lines that are drawn on maps for locating different places on earth. Latitude is the distance toward north or south of the equator and longitude is the distance toward east or west of prime meridian. Both equator and prime meridian can be seen in the figure below.

Tilt angle is the latitude of the destination where you want to install the solar plant. In summer when sun is available for longer duration, Tilt Angle is Latitude - 15 degree.

And in winters when sun is available for shorter duration the Tilt Angle is Latitude + 15 degree.

If you live above the equator line then you should point your panels due south. If you live below equator line then your panels should be pointed north.

1. Solar PV Panel

It converts sun’s rays into electricity by exciting electrons in silicon cells using the photons of light from sun.

2. Battery Bank

For Solar plant lead acid deep-cycle storage batteries are used for storing the energy generated by PV panels for providing backup to the system when sun is not present. DOD is the complement of State of Charge (SOC). It describes the degree to which a battery is emptied relative to its total capacity. This affects the length of the battery’s operational life, as well as the total number of kilowatt-hours it will be able to store over its lifetime. DOD should be taken as 50%. If a lead-acid battery is discharged 100% every time its electrolyte will quickly degrade compared to if it were only discharged to a maximum of 50%.
3. MPPT Charge Controller

Maximum Power Point Tracker (MPPT) is a kind of charge controller that utilizes the solar panel power to its maximum potential. The MPPT fools the panels by giving output with different voltage and current which will allow more power to go into the batteries. Output voltage and current from the solar panel is monitored by MPPT and operating point that will deliver that maximum amount of power to the batteries will be determined. MPPT can accurately track the always-changing operating point where the power is at its maximum, thus the efficiency of the solar cell will be increased. Many algorithms have been developed for tracking maximum power point of a PV generator such as Perturb and Observe (P&O), Incremental Conductance, and fuzzy logic based tracking techniques. These algorithms vary in effectiveness, complexity, convergence speed, sensors required and cost.

4. Power Converter

HRES that contains both AC and DC elements requires a converter. It acts as both rectifier and inverter accordingly.

1. First Generation Wafer Based Silicon Cells

Silicon wafer based technology is the oldest and most popular due to its highest power efficiency. First generation solar cells are of two types Mono-crystalline and Polycrystalline solar cell.

1.1. Mono-crystalline Solar Cell

Mono-crystalline cells are made by cutting a block of silicon that has been grown from a single crystal. These are oldest technology based solar cells. Efficiency of these cells varies from 14% to 17.5%. Performance of these cells is not good at high temperatures but occupies significantly less volume to produce same amount of power. These are two times more expensive when compared to thin film solar cells. Area required for 1 kW solar panel is 6 to 9 m².

1.2. Poly-crystalline Solar Cell

Polly-crystalline cells are made by cutting from multifaceted silicon crystal This technology is comparatively an economical choice. Efficiency of these cells varies from 12% to 14%. Similarly as in Mono-crystalline solar cell performance is not good at high temperatures but occupies significantly less volume to produce same amount of power. It is also two times more expensive when compared to thin film solar cells. Area required for 1 kW solar panel is 8 to 9 m².

2. Second Generation Thin Film Solar Cell

Second Generation Thin Film Solar Cell cells are much more economical when compared to first generation silicon wafer solar cells. Thin film solar cells are not made up of any crystal. They are made by depositing a thin layer of silicon that is deposited on a base material like metal or glass. Light absorbing layer of silicon wafer cell is around 350 m thick while light absorbing layer of thin film solar cell is quite thin .

2.1. Amorphous Silicon Thin Film Solar Cell

Amorphous Silicon thin film solar cells are comparatively less costlier and easily available all around. “Amorphous” means there is no definite arrangement of atoms or no definite structure. Temperature performance of these cells is good at both low as well as high temperatures and efficiency of these cells varies from 4% to 8%. These cells require less installation time and require much larger space. Area required for 1 kW solar panel is 13 to 20 m².

2.2. CdTe Thin Film Solar Cell

For making a practically economically viable PV system Cadmium Telluride (CdTe) as it is 50% less costlier than conventional silicon cell and it’s efficiency varies from 9% to 11% . Area required for 1 kW solar panel is 11 to 13 m². This cell technology is toxic due to the presence of Cd which is a heavy metal and toxic. Thus manufacturing is limited due to environmental hazards associated with Cd..

3. Third Generation Solar Cells

It is the new technology that has evolved in the market now-a-days but are not studied deeply.

3.1. Nano Crystal Based Solar Cell

These are generally known as Quantum Dots (QD). QD refers to the crystal having small size of the order of nano meters such as porous Si. Efficiency of these cells varies from 7% to 8%. These cells have excellent thermal stability and offer a wide range of product designs. It is also 50% less expensive than Si cells and requires less installation time and large space.

3.2. Dye Sensitized Solar Cell

Many studies are going on Dye Sensitized solar cell (DSSC) for improving it’s performance efficiency. These cells use dye molecules between different electrodes. These cells consists of four components – semiconductor electrode, a dye sensitizer, redox mediator and counter electrode. Efficiency of these cells is around 10% and performance is not good at high temperature conditions.

3.3. Polymer Solar Cell

Polymer solar cells (PSC) are flexible in nature because of the presence of polymer substrate. Efficiency of these cells varies from 3% to 10% and performance of these cells is not good at high temperatures. Due to its flexibility it offers wide range of product designs and requires less installation time.

3.4. Concentrated Solar Cell

In Concentrated solar cell (CSC) technology large amount of solar energy is concentrated over a tiny region of a solar cell. The principle of this technology is based on optics. When solar radiations are concentrated on a point, ample amount of heat energy is generated and converted into electricity using integrated power generator. Efficiency of this cell is 40% and offers excellent thermal stability.