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Solar Thermal Technologies

Solar thermal power is the extended form of solar energy or we can say that it’s the energy required for harnessing the solar energy.

How Solar Thermal Power works?

In this method energy from the sun is used to generate heat.Solar panels are used to gather heat from the sun so that heat is imprisoned and is used for water and space heating in buildings. These panels are located in such a way as to maximize absorption of heat from the sun during the day.The panels hold tubing through which water flow. This tubing is known as solar thermal collectors. In indirect method a non-hazardous anti freeze liquid is utilized instead of water.The sun warms the liquid and it shifts this heat to water held in a tank.

In solar thermal energy plants incoming radiation is trapped by large mirror fields they in turn concentrate the energy towards absorbers.The absorbers receive the concentrated radiation and transfers it thermally to the working medium. The heated fluid is operated as in power stations directly (in case steam or air is used as medium) or indirectly through a heat exchanging steam generator on the turbine unit which then drives the generator.

Different technologies for solar thermal power plants

Three different technologies for solar thermal power plants making useOf concentrating solar energy systems.

1 Parabolic troughs, 2 Central receivers (towers) and 3 Parabolic dishes.

1. Parabolic Trough

This system uses linear concentrators of parabolic shape which have highly efficient surfaces; it can be molded in angular movements towards the sun position and can focus the radiation onto a long line receiving absorber tube. The captivated energy is transferred by a working fluid, and the fluid is then piped to a conventional power conversion system.

2. Central Receivers

This system use heliostats to follow the sun by two axes mechanisms follow the elevation and azimuth angles with the function to reflect the sunlight from several heliostats (device used for tracking the movement of the sun) which are oriented around a tower and then focus it towards a central receiver which is at the top of the tower. This technology serves as an energy input to the power conversion system because it shift solar energy very efficiently by optical means of delivering highly concentrated sunlight to one central receiver unit.

3. Parabolic Dishes Systems

The reflectors focus sun rays on a dish-mounted receiver at its focal point. Within the receiver there is a medium for transferring the heat which uses the solar energy and shifts it to the power transformation setup, which could be mounted in one unit together with the receiver or at the ground. Dish collectors attain highest solar flux and the highest performance of all concentrator types with respect to peak solar concentration and of system efficiency due to its ideal optical configuration and two axis controls for tracking the sun. The dish technology is applicable to off-the-grid power generation, i.e. at remote places or at island situations.

Advantages

• Solar thermal products and structure are a smart choice for businesses. • The use of thermal products can accumulate our other natural resources and water for us. • Thermal heating systems are very useful for businesses like hotels and restaurants. • The annual cost of heating water can also be minimized by using thermal solar heating systems. For this reason many industries are replacing their energy sources from natural to solar thermal energy. • This is a new technology and the government is supporting it and provides ease by reducing taxes on these products. The use of solar thermal products gives a corporate image of the commercial users. • The systems are environment friendly and stop vast quantities of carbon dioxide, nitrous and sulfur dioxide from entering the atmosphere. • A Solar Hot Water system provides 50 to 70% of water heating nearly free each year. In the months of summer, the system can in reality provide almost all your hot water needs and approximately 50% for the rest of year. • Solar thermal systems will allow you to save money by improving your building energy rating lessening your energy bills. • Net metering, is that the extra energy produced during the day by solar source will make your meter run in the opposite direction, so that at night you use the extra energy you have produced in the day time. • Solar panels are low repairs and trustworthy systems.

Parabolic Dish

• Dispatchable peaking electricity • High conversion efficiencies • Modularity • Hybrid operation

Parabolic Trough

• Dispatch able peeking electricity • Hybrid operation

Future of Solar Thermal Power

For various commercial applications different technologies involved in the concentration are developing the new techniques and methods for the construction of the high flux solar system which would be of greater value as compared to the present ones. This high flux is efficiently used in commercial and business sectors. This type of solar thermal systems will certainly provide within the next decade a significant contribution to efficient and inexpensive, renewable and clean energy supply.

Solar Water Heater

Solar water heating systems use free heat from the sun to warm domestic hot water. A conventional boiler or immersion heater can be used to make the water hotter,  or to provide hot water when solar energy is unavailable.

Solar Water Heating (SWH) or Solar Hot Water (SHW) systems comprise several innovations and many mature renewable energy technologies that have been well established for many years. SWH has been widely used in Greece,  Turkey,  Israel,  Australia,  Japan,  Austria and China.

In a “close-coupled” SWH system the storage tank is horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank through thermosiphon flow. In a “pump-circulated” system the storage tank is ground or floor mounted and is below the level of the collectors; a circulating pump moves water or heat transfer fluid between the tank and the collectors.

SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is normally used to heat the water.

Solar Cookers

A solar cooker, or solar oven, is a device which uses the energy of sunlight to heat food or drink to cook it or sterilize it. High-tech versions, for example electric ovens powered by solar cells, are possible, and have some advantages such as being able to work in diffuse light. However at present they are very unusual because they are expensive. The vast majority of the solar cookers presently in use are relatively cheap, low-tech devices. Because they use no fuel and cost nothing to operate, many nonprofit organizations are promoting their use worldwide to help reduce fuel costs for low-income people, reduce air pollution and slow deforestation and desertification, caused by use of firewood for cooking. Solar cooking is a form of outdoor cooking and is often used in situations where minimal fuel consumption is important, or the danger of accidental fires is high.

Low-Tech Solar Cookers

There are a variety of types of solar cookers: over 65 major designs and hundreds of variations of them. The basic principles of solar cooker design are:

  • Concentrating Sunlight: A reflective mirror of polished glass, metal or metallised film is used to concentrate light and heat from the sun into a small cooking area, making the energy more concentrated and increasing its heating power.
  • Converting Light to Heat: A black or low reflectivity surface on a food container or the inside of a solar cooker will improve the effectiveness of turning light into heat. Light absorption converts the sun’s visible light into heat, substantially improving the effectiveness of the cooker.
  • Trapping Heat: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker. A plastic bag or tightly sealed glass cover will trap the hot air inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.
  • Greenhouse Effect: Glass transmits visible light but blocks infrared thermal radiation from escaping. This amplifies the heat trapping effect.

Types of Solar Cooker

The available designs of solar ovens fall into three main categories: the box, parabolic, and panel designs. The feature common to each oven design is the shiny reflective surface that directs the sun’s rays onto a dark cooking vessel. Each category has advantages when compared on their heating ability, ease of construction, ease and safety of use.

1. Box Cooker Solar Ovens 2. Reflective Panel Solar Cookers 3. Parabolic Solar Cookers

Box Solar Cooker

Box cookers are the most common type made for personal use. There are over several hundred thousand in India alone. Despite the name “box” cooker, they are made in both circular and rectangular shapes. They consist of an enclosed inner box covered with clear glass or plastic, a reflector, and insulation. There is a wide variety of patterns and plans for the box cooker. While they do not heat quickly, they do provide slow, even cooking and are extremely cheap to make. Box cookers are very easy and safe to use, and fairly easy to construct.

Box Solar Cooker

Panel Solar Cooker

Panel cookers are flat reflective panels which focus the sunlight onto a cooking vessel without the inner box common in box cookers. Panel cookers are the easiest and least costly to make, requiring just four reflective panels and a cooking vessel, but they are unstable in high winds and do not retain as much heat when the sun is hidden behind clouds. The diagram shows a panel cooker with a dark cooking vessel and thermometer all wrapped in a plastic oven bag.

Panel Cooker

Parabolic Solar Cooker

Reflective materials are used to concentrate light and heat from the sun into a small cooking area, making the Sun’s energy more concentrated and therefore more powerful, resulting in the fastest cooking times of all Cooker designs.

Parabolic cookers require more precision to focus the sunlight on the cooking vessel and are therefore the most complex design to build. If the sunlight is not focused exactly on the cooking vessel, the food will not cook efficiently.

Parabolic Cooker

 

Power Tower

The solar power tower (also known as ‘central tower’ power plants or ‘heliostat’ power plants or power towers) is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun’s rays upon a collector tower (the target). Concentrated solar thermal is seen as one viable solution for renewable, pollution free energy production with currently available technology.

Early designs used these focused rays to heat water, and used the resulting steam to power a turbine. Newer designs using liquid sodium have been demonstrated, and systems using molten salts (40% potassium nitrate, 60% sodium nitrate) as the working fluids are now in operation. These working fluids have high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. These designs allow power to be generated when the sun is not shining.

The central receiver solar concentrator design concentrates sunlight to a receiver mounted on a tower with the use of mirrors. The concentrated sunlight causes the solar central power tower to generate electricity. The receiver is also called a heat exchanger. This system uses hundreds of sun-reflecting mirrors.

The large size of the receivers makes them useful in utility scale applications. The concentrator works by pumping liquid salt at 290 degrees C from a cold storage tank. The salt pumps through the receiver. The receiver heats the salt to 565 degrees C. The salt leaves the receiver and goes to a hot tank for storage. Pumping hot salt through the steam-generating system produces the steam needed for the Rankine-cycle turbine system. As a result, power generates. After receiving enough power, the salt goes back to the cold storage tank.

Solar CRT uses proprietary software to control thousands of tracking mirrors, known as heliostats, to directly concentrate sunlight onto a boiler filled with water that sits atop a tower. When the sunlight hits the boiler, the water inside is heated and creates high temperature steam. Once produced, the steam is used either in a conventional turbine to produce electricity or in industrial process applications. In order to conserve precious water, the steam is air-cooled and piped back into the system in a closed-loop process.

Heliostats

Tracking mirrors, known as heliostats, are highly engineered and designed for accuracy, durability and longevity with minimal maintenance. The heliostat consists of two flat glass mirrors (supported by a lightweight steel support structure) that are mounted on a single pylon equipped with a computer-controlled drive system. This control system enables the heliostats to track the sun in two-directions, maximizing the collection of the sunlight while accurately aiming at the solar receiver. A 130 MW plant may utilize up to 60,000 heliostats, depending on land area and shape, and site-specific considerations. The low-impact design of the heliostat allows solar plant sites to accommodate a slope of up to 5%, avoid areas of sensitive habitat and eliminate the need for the concrete pads used with other solar thermal technologies, reducing the system’s environmental impact.

Solar Field Optimization Software and Control System

A proprietary solar field optimization software is used during the system design phase to determine the optimal position of each heliostat to maximize output and meet the customer’s power production profile. The technology also provides considerable design flexibility, allowing projects to be built on sites with irregular topographies and shape. Using actual site conditions and custom-built meteorological datasets, the software produces precise GPS-ready mappings ready for download to solar field installation crews.

Proprietary heliostat control software system, the Solar Field Integrated Control System (SFINCS), controls the heliostats arrayed in the solar field to track the sun and aim the sunlight onto the receiver. SFINCS performs a number of functions including:

Solar energy management, to focus the ideal amount of solar energy on the receiver at various times of the day to maximize electricity production while ensuring that the solar receiver’s flux and temperature limits are not exceeded.Solar field control, to provide aiming points on the solar receiver surface for each individual heliostat, as well as facilitating start-up and shutdown. Heliostat tracking maintenance, to calibrate the heliostats based on three-dimensional laser scanning and other photogrammetric methods.

At the core of the SFINCS are proprietary algorithms that perform real-time optimization of the distribution of energy across our solar receiver using real-time, heliostat-aiming and closed-loop feedback systems. In addition, SFINCS can automatically configure the heliostats to protect them from inclement weather.

Solar Receiver (Boiler)

The solar receiver is a standard utility-scale industrial boiler designed to be heated from the outside using concentrated solar radiation reflected onto the boiler by the heliostats. The boiler is designed to withstand the rigors of the daily cycling required in a solar power plant over the course of its lifetime, and is treated with a proprietary solar-absorptive coating to ensure that maximum solar energy is absorbed in the steam.

In electricity generation applications, the high-temperature, pressurized steam generated in the solar receiver is piped to a conventional steam turbine generator. The electricity generated is then delivered to the transmission grid for consumption.

In a solar-to-steam application, such as thermal enhanced oil recovery, the process is similar to generating electricity. However, for solar-to-steam applications, saturated steam is piped from the receiver to a heat exchanger to generate the process steam.

Parabolic Dish

A dish/Stirling system comprises of a parabolic dish concentrator, a thermal receiver and a Stirling engine/generator located at the focus of the dish. These systems use a parabolic dish concentrator tracking the sun and focusing solar energy into a cavity receiver where it is absorbed and transferred to a Stirling Engine/ generator. Though there are other engines like the brayton and organic Rankine cycle engines, Stirling engines are the most popular and widely used due to their high thermal to mechanical efficiencies and their potential for long term, low maintenance operations.

Dish/Stirling engines are modular in design i.e each system is a self contained power generator. This is an advantage because they can be arranged in plants ranging from a few kilowatts to tens of megawatts.

Over the last 15 years a number of Dish / Stirling systems from 5 to 50 Kw have been developed in United States, Germany, Japan and Russia. The few of the Dish / Stirling designs are

Advanco’s Vanguard System Schlaich-Bergermann und Partner McDonell-Douglas / Stirling Energy Systems Cummins Power Generation

Dish Stirling systems have successfully demonstrated the technical feasibility of generating electrical power. Future challenge lies in developing long lasting systems that can produce competitively priced electric power.

Parabolic Trough

A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day.

Alternatively the trough can be aligned on an east-west axis, this reduces the overall efficiency of the collector, due to cosine loss, but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinoxes with errors in the focusing of the light at other times during the year (the magnitude of this error varies throughout the day, taking a minimum value at solar noon). There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio. In order to increase the level of alignment, some measuring devices have also been invented.

Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on primary-secondary designs using nonimaging optics.

Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400°C. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV (Photovoltaic Cells) but less than Stirling dish concentrators.

Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of area varies enormously.

Parabolic Trough Solar Collector