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Monthly Archives: January 2013

solar tower with heliostatsolar troughStirling dishSolar power plant in Queensland (annexure 1)It is a fact that solar energy is emerging as a key source of future energy as the climate change debate is raging all over the world. The solar radiation can meet world’s energy need completely in a benign way and offer a clear alternative to fossil fuels. However the solar technology is still in a growing state with new technologies and solutions emerging. Though PV solar is a proven technology the levelised cost from such plants is still much higher than fossil fuel powered plants. This is because the initial investment of a PV solar plant is much higher compared to fossil fuel based power plants. For example the cost of a gas based power plant can be set up at less than $1000/Kw while the cost of PV solar is still around $ 7000 and above. However solar thermal is emerging as an alternative to PV solar. The basic difference between these two technologies is  PV solar converts light energy of the sun directly into electricity and stores in a battery for future usage; solar thermal plants use  reflectors (collectors)  to focus the solar light to heat a thermic fluid or molten salt to a high temperature. The high temperature thermic fluid or molten salt is used to generate steam to run a steam turbine using Rankine cycle or heat a compressed air to run a gas turbine using Brayton cycle to generate electricity. Solar towers using heliostat and mirrors are predicted to offer  the lowest cost of solar energy in the near future as the cost of Heliostats are reduced and molten salts with highest eutectic points are developed. The high eutectic point molten salts are likely to transform a range of industries for high temperature applications. When solar thermal plants with molten salt storage can approach temperature of 800C, many fossil fuel applications can be substituted with solar energy. For example, it is expected by using solar thermal energy 24×7 in Sulfur-Iodine cycle, Hydrogen can be generated on a large commercial-scale at a cost @2.90/Kg.Research and developments are focused to achieve the above and it may soon become a commercial reality in the near future.

“The innovative aspect of CSP (concentrated solar power) is that it captures and concentrates the sun’s energy to provide the heat required to generate electricity, and not using fossil fuels or nuclear reactions. Another attribute of CSP plants is that they can be equipped with a heat storage system to generate electricity even when the sky is cloudy or after sunset. This significantly increases the CSP capacity factor compared with solar photovoltaics and, more importantly, enables the production of dispatchable electricity, which can facilitate both grid integration and economic competitiveness. CSP technologies therefore benefit from advances in solar concentrator and thermal storage technologies, while other components of the CSP plants are based on rather mature technologies and cannot expect to see rapid cost reductions. CSP technologies are not currently widely deployed. A total of 354 MW of capacity was installed between 1985 and 1991 in California and has been operating commercially since then. After a hiatus in interest between 1990 and 2000, interest in CSP has been growing over the past ten years. A number of new plants have been brought on line since 2006 (Muller- Steinhagen, 2011) as a result of declining investment costs and LCOE, as well as new support policies. Spain is now the largest producer of CSP electricity and there are several very large CSP plants planned or under construction in the United States and North Africa. CSP plants can be broken down into two groups, based on whether the solar collectors concentrate the sun rays along a focal line or on a single focal point (with much higher concentration factors). Line-focusing systems include parabolic trough and linear Fresnel plants and have single-axis tracking systems. Point-focusing systems include solar dish systems and solar tower plants and include two-axis tracking systems to concentrate the power of the sun.

Parabolic trough collector technology:

The parabolic trough collectors (PTC) consist of solar collectors (mirrors), heat receivers and support structures. The parabolic-shaped mirrors are constructed by forming a sheet of reflective material into a parabolic shape that concentrates incoming sunlight onto a central receiver tube at the focal line of the collector. The arrays of mirrors can be 100 meters (m) long or more, with the curved aperture of 5 m to 6 m. A single-axis tracking mechanism is used to orient both solar collectors and heat receivers toward the sun (A.T. Kearney and ESTELA, 2010). PTC are usually aligned North-South and track the sun as it moves from East to West to maximize the collection of energy. The receiver comprises the absorber tube (usually metal) inside an evacuated glass envelope. The absorber tube is generally a coated stainless steel tube, with a spectrally selective coating that absorbs the solar (short wave) irradiation well, but emits very little infrared (long wave) radiation. This helps to reduce heat loss. Evacuated glass tubes are used because they help to reduce heat losses.

A heat transfer fluid (HTF) is circulated through the absorber tubes to collect the solar energy and transfer it to the steam generator or to the heat storage system, if any. Most existing parabolic troughs use synthetic oils as the heat transfer fluid, which are stable up to 400°C. New plants under demonstration use molten salt at 540°C either for heat transfer and/or as the thermal storage medium. High temperature molten salt may considerably improve the thermal storage performance. At the end of 2010, around 1 220 MW of installed CSP capacity used the parabolic trough technology and accounted for virtually all of today’s installed

CSP capacity. As a result, parabolic troughs are the CSP technology with the most commercial operating experience (Turchi, et al., 2010).

Linear Fresnel collector technology:

 Linear Fresnel collectors (LFCs) are similar to parabolic trough collectors, but use a series of long flat, or slightly curved, mirrors placed at different angles to concentrate the sunlight on either side of a fixed receiver (located several meters above the primary mirror field). Each line of mirrors is equipped with a single-axis tracking system and is optimized individually to ensure that sunlight is always concentrated on the fixed receiver. The receiver consists of a long, selectively coated absorber tube.

Unlike parabolic trough collectors, the focal line of Fresnel collectors is distorted by astigmatism. This requires a mirror above the tube (a secondary reflector) to refocus the rays missing the tube, or several parallel tubes forming a multi-tube receiver that is wide enough to capture most of the focused sunlight without a secondary reflector. The main advantages of linear Fresnel CSP systems compared to parabolic trough systems are that:

LFCs can use cheaper flat glass mirrors, which are a standard mass-produced commodity;LFCs require less steel and concrete, as the metal support structure is lighter. This also makes the assembly process easier.

»»The wind loads on LFCs are smaller, resulting in better structural stability, reduced optical losses and less mirror-glass breakage; and.

»»The mirror surface per receiver is higher in LFCs than in PTCs, which is important, given that the receiver is the most expensive component in both PTC and in LFCs.

These advantages need to be balanced against the fact that the optical efficiency of LFC solar fields (referring to direct solar irradiation on the cumulated mirror aperture) is lower than that of PTC solar fields due to the geometric properties of LFCs. The problem is that the receiver is fixed and in the morning and afternoon cosine losses are high compared to PTC. Despite these drawbacks, the relative simplicity of the LFC system means that it may be cheaper to manufacture and install than PTC CSP plants. However, it remains to be seen if costs per kWh are lower. Additionally, given that LFCs are generally proposed to use direct steam generation, adding thermal energy storage is likely to be more expensive.

Solar to Electricity technology:

Solar tower technologies use a ground-based field of mirrors to focus direct solar irradiation onto a receiver mounted high on a central tower where the light is captured and converted into heat. The heat drives a thermodynamic cycle, in most cases a water-steam cycle, to generate electric power. The solar field consists of many of computer-controlled mirrors, called heliostats that track the sun individually in two axes. These mirrors reflect the sunlight onto the central receiver where a fluid is heated up. Solar towers can achieve higher temperatures than parabolic trough and linear Fresnel systems; because more sunlight can be concentrated on a single receiver and the heat losses at that point can be minimized. Current solar towers use water/steam, air or molten salt to transport the heat to the heat-exchanger/steam turbine system. Depending on the receiver design and the working fluid, the upper working temperatures can range from 250°C to perhaps as high 1 000°C for future plants, although temperatures of around 600°C will be the norm with current molten salt designs. The typical size of today’s solar power plants ranges from 10 MW to 50 MW (Emerging Energy Research, 2010). The solar field size required increases with annual electricity generation desired, which leads to a greater distance between the receiver and the outer mirrors of the solar field. This results in increasing optical losses due to atmospheric absorption, unavoidable angular mirror deviation due to imperfections in the mirrors and slight errors in mirror tracking.

Solar towers can use synthetic oils or molten salt as the heat transfer fluid and the storage medium for the thermal energy storage. Synthetic oils limit the operating temperature to around 390°C, limiting the efficiency of the steam cycle. Molten salt raises the potential operating temperature to between 550 and 650°C, enough to allow higher efficiency supercritical steam cycles although the higher investment costs for these steam turbines may be a constraint. An alternative is direct steam generation (DSG), which eliminates the need and cost of heat transfer fluids, but this is at an early stage of development and storage concepts for use with DSG still need to be demonstrated and perfected.

Solar towers have a number of potential advantages, which mean that they could soon become the preferred CSP technology. The main advantages are that:

»»The higher temperatures can potentially allow greater efficiency of the steam cycle and reduce water consumption for cooling the condenser;

»»The higher temperature also makes the use of thermal energy storage more attractive in order to achieve schedulable power generation; and

»»Higher temperatures will also allow greater temperature differentials in the storage system, reducing costs or allowing greater storage for the same cost.

The key advantage is the opportunity to use thermal energy storage to raise capacity factors and allow a flexible generation strategy to maximize the value of the electricity generated, as well as to achieve higher efficiency levels. Given this advantage and others, if costs can be reduced and operating experience gained, solar towers could potentially achieve significant market share in the future, despite PTC systems having dominated the market to date. Solar tower technology is still under demonstration, with 50 MW scale plant in operation, but could in the long-run provide cheaper electricity than trough and dish systems (CSP Today, 2008). However, the lack of commercial experience means that this is by no means certain and deploying solar towers today includes significant technical and financial risks.

Sterling dish technology:

The Stirling dish system consists of a parabolic dish shaped concentrator (like a satellite dish) that reflects direct solar irradiation onto a receiver at the focal point of the dish. The receiver may be a Stirling engine (dish/ engine systems) or a micro-turbine. Stirling dish systems require the sun to be tracked in two axes, but the high energy concentration onto a single point can yield very high temperatures. Stirling dish systems are yet to be deployed at any scale. Most research is now focused on using a Stirling engine in combination with a generator unit, located at the focal point of the dish, to transform the thermal power to electricity. There are currently two types of Stirling engines: Kinematic and free piston. Kinematic engines work with hydrogen as a working fluid and have higher efficiencies than free piston engines. Free piston engines work with helium and do not produce friction during operation, which enables a reduction in required maintenance. The main advantages of Stirling dish CSP technologies are that:

»»The location of the generator – typically, in the receiver of each dish – helps reduce heat losses and means that the individual dish-generating capacity is small, extremely modular (typical sizes range from 5 to 50 kW) and are suitable for distributed generation;

»»Stirling dish technologies are capable of achieving the highest efficiency of all type of CSP systems

»»Stirling dishes use dry cooling and do not need large cooling systems or cooling towers, allowing CSP to provide electricity in water-constrained regions; and

»»Stirling dishes, given their small foot print and the fact they are self-contained, can be placed on slopes or uneven terrain, unlike PTC, LFC and solar towers. These advantages mean that Stirling dish technologies could meet an economically valuable niche in many regions, even though the levelised cost of electricity is likely to be higher than other CSP technologies. Apart from costs, another challenge is that dish systems cannot easily use storage. Stirling dish systems are still at the demonstration stage and the cost of mass-produced systems remains unclear. With their high degree of scalability and small size, stirling dish systems will be an alternative to solar photovoltaics in arid regions.”

(Source : IRENA 2012)

 

solar absorption chillersAir conditioning makes up bulk of the power usage, especially in tropical countries where the sun is shining almost throughout the year and the humidity levels are high. It makes a perfect sense to use solar heat to cool homes, business and factories. Many air-conditioning systems are commercially available using simple roof top PV solar panels to generate electric power to run an electric window air-conditioners. This system uses commercially available solar panels and window air-conditioners and uses solar power to generate electricity to run the compressor and the blower in the air-con unit. This system requires large storage battery to store adequate electricity to run your air-conditioners for specified period. Otherwise it requires a large area of solar panels to meet the demand. The efficiency of such systems can be improved using DC operated compressors and fans. However, renewable energy such as solar is still expensive to run air-conditioners because of high initial investment cost, though it may be economical in the long run as the cost of solar panels and accessories slowly come down over a time. Moreover such systems are limited to small air condition capacities.

solar chillers-typical apacitiessolar absorption chillerFor large air-conditioning requirements such as business and factories, we need a system that uses solar heat directly to air-condition the premises with higher efficiency and thermal storage capabilities. Designing such a system is not very difficult because most of the components necessary to install such systems are readily available. One can install an air-conditioning system based on 100% solar thermal heat with molten salt thermal storage. Alternatively, a hybrid system can be installed based on solar heat without a thermal storage but using   city gas supply. Many countries use gas for heating during winter seasons but do not use gas during summer. These countries can use a hybrid (solar-gas) system to air-condition their premises and avoid peak electric usage during summer seasons thereby avoiding electrical black-outs. The advantage with such system is they can also be used for heating the premises during winter season. With changing climate due to global warming many warm countries like India also experiences cold temperatures during winter season. For example New Delhi in India has experienced a sharp drop in temperature up to 15-20c during winter from earlier winters.

Solar cooling systems to date have used waste heat gas absorption chiller heaters, which utilize the waste heat from cogeneration systems (CGS) for the cold water. However, these chiller heaters with their established technologies are devices designed for the effective use stable CGS high-temperature waste heat, so they cannot accommodate the preferential use of solar heat when solar hot water temperatures suddenly change from large variations in the heat collector temperatures due to changes in the weather. The new solar absorption chiller heaters are now specially designed for the effective use of low-temperature solar heat to address this problem and improve the energy conservation effect from solar cooling system. Hot water at less than 90C can be used for such systems and typical chillers with their rated specification are shown in the figures.solar trough

The efficiency of the system can be vastly improved by using parabolic solar concentrators, up to 27 times higher than ordinary flat plate solar collectors resulting in conversion efficiency up to 85% in heating and cooling. By selecting a natural refrigerant such as R717 we can save the environment from ozone depletion. Such systems offer flexibility to use exhaust heat, natural gas along with solar thermal storage up to 220C (phase transition temperature).The system offers an attractive return on investment, electricity savings and Carbon pollution reduction. The system can be designed from 5TR up to 200TR refrigeration capacity for 100% solar and up to 1000TR for a solar-gas hybrid systems. The solar thermal system with molten salt storage is versatile in its application because the same system can be designed for heating or cooling or on-site power generation for continuous applications.

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Plastics have become an integral part of our lives. Plastic  constitutes about 12% of Municipal solid wastes generated in USA,a sharp increase from just 1% in 1960 to the current level. Increasing usage of plastics have created  environmental issues such as increased energy and water usage, emission of greenhouse gases and finally waste disposal and health issues. Many countries are now trying to cut the waste disposal problems by reducing usage, recovering  fuels from plastics and recycling.However  a large quantity of plastics are still returned to landfills  creating long-term health problems.

“According to EPA :

  • 31 million tons of plastic waste were generated in 2010, representing 12.4 percent of total MSW.
  • In 2010, the United States generated almost 14 million tons of plastics as containers and packaging, almost 11 million tons as durable goods, such as appliances, and almost 7 million tons as non-durable goods, such as plates and cups.
  • Only 8 percent of the total plastic waste generated in 2010 was recovered for recycling.
  • In 2010, the category of plastics which includes bags, sacks, and wraps was recycled at almost 12 percent.
  • Plastics also are found in automobiles, but recycling of these materials is counted separately from the MSW recycling rate.

How Plastics Are Made

Plastics can be divided in to two major categories: thermosets and thermoplastics. A thermoset solidifies or “sets” irreversibly when heated. They are useful for their durability and strength, and are used primarily in automobiles and construction applications. Other uses are adhesives, inks, and coatings.

A thermoplastic softens when exposed to heat and returns to original condition at room temperature. Thermoplastics can easily be shaped and molded into products such as milk jugs, floor coverings, credit cards, and carpet fibers.

According to the American Chemistry Council, about 1,800 US businesses handle or reclaim post-consumer plastics. Plastics from MSW are usually collected from curbside recycling bins or drop-off sites. Then, they go to a material recovery facility, where the materials are sorted into broad categories (plastics, paper, glass, etc.). The resulting mixed plastics are sorted by plastic type, baled, and sent to a reclaiming facility. At the facility, any trash or dirt is sorted out, then the plastic is washed and ground into small flakes. A flotation tank then further separates contaminants, based on their different densities. Flakes are then dried, melted, filtered, and formed into pellets. The pellets are shipped to product manufacturing plants, where they are made into new plastic products.

Resin Identification Code

The resin identification coding system for plastic, represented by the numbers on the bottom of plastic containers, was introduced by SPI, the plastics industry trade association, in 1988. Municipal recycling programs traditionally target packaging containers, and the SPI coding system offered a way to identify the resin content of bottles and containers commonly found in the residential waste stream. Plastic household containers are usually marked with a number that indicates the type of plastic. Consumers can then use this information to determine whether certain plastic types are collected for recycling in their area. Contrary to common belief, just because a plastic product has the resin number in a triangle, which looks very similar to the recycling symbol, it does not mean it is collected for recycling.

SPI Resin Identification Code

1

2

3

4

5

6

7

Type of Resin Content

PET

HDPE

Vinyl

LDPE

PP

PS

OTHER

Markets for Recovered Plastics

Markets for some recycled plastic resins, such as PET and HDPE, are stable and even expanding in the United States. Currently, the US has the capacity to be recycling plastics at a greater rate. The capacity to process post-consumer plastics and the market demand for recovered plastic resin exceeds the amount of post-consumer plastics recovered from the waste stream. The primary market for recycled PET bottles continues to be fiber for carpet and textiles, while the primary market for recycled HDPE is bottles, according to the American Chemistry Council.

Looking forward, new end uses for recycled PET bottles might include coating for corrugated paper and other natural fibers to make waterproof products like shipping containers. PET can even be recycled into clothing, such as fleece jackets. Recovered HDPE can be manufactured into recycled-content landscape and garden products, such as lawn chairs and garden edging.

Source Reduction

Source reduction is the process of reducing the amount of waste that is generated. The plastics industry has successfully been able to reduce the amount of material needed to make packaging for consumer products. Plastic packaging is generally more lightweight than its alternatives, such as glass, paper, or metal. Lighter weight materials require less fuel to transport and result in less material in the waste stream.”

Source : EPA.

 

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