Solar-Thermal-Electrical energy – an alternative

We are in a crucial time. Energy crisis starts to affect our daily life. If the world economy expands to meet the aspirations of countries around the globe, energy demand is likely to increase the efficiency of energy use. Given adequate support, renewable energy technologies can meet much of the growing demand at prices lower than those usually forecast for conventional energy. By the middle of the 21st century, renewable sources of energy could account for three-fifths of the world’s electricity market and two-fifths of the market for fuels used directly.
The market for renewable energy depends on energy sources: heating and cooling, lighting, transportation etc. this demand, in turn, depends on economic and population growth and on the efficiency of energy use.
Solar-thermal-electric generation systems use sunlight to heat fluids. This system typically that follow the sun. The cost of generating power is significantly higher than the cost of base load electricity. With technological improvements, annual average solar-to-electric efficiencies could increase from the current 13% to 17% or more. Higher efficiencies and cost reductions for tracking mirror systems and other expensive components may reduce overall costs per kilowatt.
Deep inside the sun, temperatures of 16X106 0C and pressures of 68X109 atoms prevail. Solar energy is produced in this environment by nuclear reactions in hydrogen is turned into helium. The mass defect is turned into energy according to Einstein’s equation E=mc2.
The relative motions of the sun and earth are not simple, but they are systematic and thus predictable. Once a year the earth moves around the sun in an orbit that is slightly elliptical in shape. As a result of this shape, the earth is little farther from the sun in July than it is in January; this causes a small variation in the amount of solar energy the earth receives. This solar energy is converted into thermal and electrical energy.
Ref: 1. Renewable Energy – sources for fuels and electricity Ed. By Thomas B. Johansson and others; Island press, California, 1993
2. Encyclopedia Britannica:
3. Appropriate Technology Vol. 28 No.1
4. Direct Normal Solar Radiation Manual 1982, Solar Energy Research Institute, SERI/SP-281-1651. October.
Principle: (a) Solar Thermal conversion technology (STC)
Solar thermal conversion employs the use of concentrating solar collectors that focus and convert the sun’s energy to very high temperatures, approximate 1000C-10000C.

The collector captures and concentrates solar radiation, which are them delivered to the receiver. The receiver absorbs the concentrated sunlight, transferred its heat energy to a working fluid. The transport-storage system passes the fluid from the receiver to the power-conversion system; in some solar-thermal plants a portion of the thermal energy is stored for later use. The performance of a solar-thermal power plant reflects the efficiency and viability of its four principal systems. The efficiency of each component in a solar thermal system can be calculated. The efficiency of the collector system depends on, for example, on the reflectivity of its mirrors and the optical effectiveness of its geometry. The receiver’s efficiency is the ratio of energy absorbed by the working fluid to the energy incident on the receiver. Efficiency of the transport-storage system is determined by the ratio of thermal energy delivered to the power-conversion system to thermal energy absorbed in the receiver.
Although solar-thermal plants are capital-intensive, they are relatively cheap to operate because fuel costs are almost the capital costs are typically dominated by the collector system, which effectively represents the plant’s lifetime fuel supply. The next most expensive item is the power-conversion system, followed by the receiver and transport-storage system. Additional costs for land, structures, controls and small.
Solar thermal system come in three types, parabolic troughs, parabolic dishes, and the central receiver or power tower.

Parabolic troughs are independent, closely placed curved reflector units that track the sun, concentrate the light, and feed the resulting heat into a fluid and circulates through a central location. At the central location, steam is produced via one or more heat exchangers and use to drive a turbine to produce electricity. The used steam is then condensed, put through a series of steps, and eventually retuned as a fluid to the troughs to be reused. The temperature of the fluid can reach 4000C, and efficiency ranges when it comes to solar thermal systems.
Environmental concerns with parabolic trough technology include water availability waste water disposed, spices and emissions from the heat transfer fluid, and heavy land use by an application that does not allow for simultaneous use for another purpose. These systems are best suited for large, grid connected applications between 30 to 300 MW.
The parabolic dishes are modular bowl-shaped reflectors that stand as single units or in fields. Each dish features a mirror arranges that reflects and concentrates the sun light onto a receiver located at the focus of the dish. The working fluid running through each unit is heated as high as 3000C.
Each dish can produce 10 to 30 KW. Environmental concerts are minimal and primarily involve the possibility of very small oil, coolant, grease leaks etc. water in only required for washing mirrors.
Power-tower stands in the middle of a circular field of sun-tracking mirrors, which reflect the light onto the tower-mounted receiver. A working fluid is heated as it passes through the receiver. If molten salt is used as the fluid, the salt can reach temperatures above 10000C. It is suit for large power facilities generating between 30 to 200 MW.
Solar Photovoltaic cells generate electricity-converting sunlight directly using a method that differs fundamentally from the heat engines used in almost all other modes of electricity generation. The sun’s energy is collected and converted by solar panels with a certain surface area. The PV cell is usually made of semiconductor materials based on silicon (Si). A thin layer of n-type semiconductor material is deposited over a thicker layer of p-type material. Sunlight is used to free electrons from the bonds of the p-material. This increases the difference in charge between the materials which is measured as a voltage. When a load device is connected, a direct current is created from the N-material, repeating the cycle. The light energy is said to be a continuous bombardment of tiny particles of energy called photons. The photons collide with electrons, thus imparting their energy. A PV cell typically produces about 0.5V, and as high as 6 A in bright sun. solar cells, formed into module and panels, are rated at peak output power in full sunlight in the middle of the day. Much research has been done in the area of semiconductor materials for solar cells in an attempt to increase efficiency and decrease overall cost.
? Selection of PV cell materials :
Selection of cell materials is most important to increase to increase efficiency of a solar cell. Those very widely in manufacturing cost, conversion efficiency and manufacturing processes.

i. Single-Crystal Silicon (x-Si) cells :
This is the oldest and most expensive but most efficient technologies is known as x-Si technology. Conversion efficiency is almost 20%.
ii. Ribbon-Silicon Cells :
A ribbon cell is a single-crystal silicon cell that is manufactured in a process similar to metal extrusion. The efficiency for ribbon cells is usually mostly closer to 10%.
iii. Ploy crystalline- Silicon (p-Si) cells :
This type of cells is less costly to manufacture and has somewhat lower conversion efficiency than single-crystal silicon cells. It has the conversion efficiency usually 12-13%.
iv. Amorphous-Silicon (a-Si) cells :
It is a glassy alloy of silicon and hydrogen Amorphous-silicon photovoltaic modules can now converts sunlight into electricity with initial efficiency about 10%.
Today a-Si is a major commercial solar-cell material. It has spanned and exceptionally wide-range of applications.
? System design :
The cost of PV power depends primarily on two factors : the amount of sunlight available at a site and the cost of the module.
Solar resources :
The energy available for a PV module depends on latitude and climate and on whether the module is fixed or tracking. Systems that follow the sun capture significantly more energy than fixed systems. Tracking devices have the additional advantage of providing large amounts of power in the late afternoon when demand for electricity typically peaks. In desert areas, the amount of energy available to a tracking module in 50 percent greater than the energy available to a fixed module.
When limited amount of lands available, the tilt and spacing arrays become important. Overall, lower tilt angle allowed 30 percent more array area on the roof without shading and produced more energy.
Power conversion : for most applications, electricity produced by a PV array must be converted from direct current to alternating current through and inverter before it can be used. It is expected that as markets develop prices of inverter and control systems will fall. Now, the insulated gate bipolar transistors and MOS controlled transistors now being introduced into advanced motor control-system, so the costs of power conditioners has been reduced. The efficiency of power conditioners has also improved on the order of 96 percent approximately.
Overhead costs :
Overhead charges are associated with a number of items, including environmental impact analysis, legal fees, planning, contingencies etc. Typically, overhead costs for a mature, commercial process represent from 5 to 10 percent of total costs. These costs for small PV systems can be much higher than for large systems because of infrastructure, transportation, retail and other expenses.
Efficiency issues : The output of installed PV modules is typically 80 percent of what is measured under standard laboratory test condition. Most modules are less efficient as the temperature increases. The effect of temperature on their efficiency depends on the type of cells used.
Discussion : The solar radiation that reaches the earth’s surface consists of wavelengths ranging from 0.3 to 2.4 km. For most solar applications, the radiations in the visible range (0.38 to 0.78 km) and the near infrared (0.78 to about 2 mm) is the most important.
The total solar radiation at the ground consists of three components.
a) Direct beam radiation, which comes from the sun is a straight line and can cast shadows.
b) Diffuse radiation, which comes from the entire sky. It has been scattered one or more times by particles or molecules in the air. On a very clear day, the diffuse component amounts to 10 to 20% of the total solar radiation.
c) Reflected radiation, which comes from nearby surfaces such as building, ground etc.
The main points which are to be noted that the traditional fuels if have been lost, then civilization will face a big question. To protect our society it is that time to aware everybody for alternative energy. India will produce using non-conventional source about 20% of its demand.
If we analysis the solar radiation in three major cities in India, we will see the opportunity of using solar radiation.

City Lat. Long. Max.
Mumbai 18056’N 72050’E 81% - 460 Cal/cm2 day
Kolkata 32030’N 88020’E 80% - 490 Cal/cm2 day
Delhi 28040’N 77015’E 81% - 630 Cal/cm2 day
Chennai 1305’N 80015’E 83% - 510 Cal/cm2 day