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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)

 

 

All existing power generation technologies including nuclear power plants uses heat generation as a starting point. The heat is used to generate steam which acts as a motive force to run an alternator to produces electricity. We combust fossil fuels such as coal oil and gas to generate above heat which also emits greenhouse gases such as oxides of Carbon and Nitrogen. As I have disused in my earlier article, we did not develop a technology to generate heat without combusting a fossil fuel earlier. This was due to cheap and easy availability of fossil fuel. The potential danger of emitting greenhouse gases into the atmosphere was not realized until recently when scientists pointed out the consequences of carbon build up in the atmosphere. The growth of population and industries around the world pushed the demand for fossil fuels over a period which enhanced the Carbon build up in the atmosphere.

But now Concentrated Solar Power (CSP) systems have been developed to capture the heat of the sun more efficiently and the potential temperature of solar thermal can reach up to 550. This dramatic improvement is the efficiency of solar thermal has opened up new avenues of power generation as well as other applications. “CSP is being widely commercialized and the CSP market has seen about 740 MW of generating capacity added between 2007 and the end of 2010. More than half of this (about 478 MW) was installed during 2010, bringing the global total to 1095 MW. Spain added 400 MW in 2010, taking the global lead with a total of 632 MW, while the US ended the year with 509 MW after adding 78 MW, including two fossil–CSP hybrid plants”. (Ref: Wikipedia)

“CSP growth is expected to continue at a fast pace. As of April 2011, another 946 MW of capacity was under construction in Spain with total new capacity of 1,789 MW expected to be in operation by the end of 2013. A further 1.5 GW of parabolic-trough and power-tower plants were under construction in the US, and contracts signed for at least another 6.2 GW. Interest is also notable in North Africa and the Middle East, as well as India and China. The global market has been dominated by parabolic-trough plants, which account for 90 percent of CSP plants.As of 9 September 2009, the cost of building a CSP station was typically about US$2.50 to $4 per  watt, the fuel (the sun’s radiation) is free. Thus a 250 MW CSP station would have cost $600–1000 million to build. That works out to $0.12 to $0.18/kwt. New CSP stations may be economically competitive with fossil fuels. Nathaniel Bullard,” a solar analyst at Bloomberg

New Energy Finance, has calculated that the cost of electricity at the Ivanpah Solar Power Facility, a project under construction in Southern California, will be lower than that from  photovoltaic power and about the same as that from natural gas  However, in November 2011, Google announced that they would not invest further in CSP projects due to the rapid price decline of photovoltaics. Google spent $168 million on Bright Source IRENA has published on June 2012 a series of studies titled: “Renewable Energy Cost Analysis”. The CSP study shows the cost of both building and operation of CSP plants. Costs are expected to decrease, but there are insufficient installations to clearly establish the learning curve. As of March 2012, there was 1.9 GW of CSP installed, with 1.8 GW of that being parabolic trough” Ref: Wikipedia.

One Canadian company has demonstrated to generate Hydrogen from water using a catalytic thermolysis using sun’s high temepertaure.The same company has also demonstrated generating base load power using conventional steam turbine by  CSP using parabolic troughs. They store sun’s thermal energy using a proprietary thermic fluid and use them during night times to generate continuous power. The company offers to set up CSP plants of various capacities from 15Mw up to 500Mw.

 

 

 

 

 

 

 

We  acknowledge that solar energy is a potential renewable energy source of the future. The total energy need of the world is projected in the next 40 years to be 30 TW (terra watts) and only solar energy has a potential to meet the above demand. However, harnessing sun’s energy to its fullest potential is still a long way to go. Concentrated solar power (CSP) offers a greater hope to fill this gap. The main reason is the cost  advantage of CSP compared to PV solar and energy storage technologies and their costs.

The cost of PV solar has steadily decreased in the past few years. Though the cost of solar cell has come down to $0.75 per watt, the overall cost of the PV system is still around $ 3.00 per watt. This is due to the cost of encapsulation; interconnect wiring, mounting of panels, inverters and battery bank. The overall cost of the system will not come down drastically beyond a point. This makes PV solar still more expensive compared to conventional power generation using fossil fuels. People can understand the value of renewable energy and impending dangers of global warming due to greenhouse gases, but the final cost of energy will decide the future of energy sources.

In PV solar the sun’s light energy is directly converted into Electricity, but storing such energy using batteries have certain limitations. PV solar is suitable for small-scale operations but it may not be cost-effective for large-scale base load power generation. The best option will be to harness the sun’s thermal energy and store them and use them to generate power using the conventional and established methods such as steam or gas turbines. Once we generate thermal energy of required capacity then we have number of technologies to harness them into  useful forms. As we mentioned earlier, the thermal energy can trigger a chemical reaction such as formation of Ammonia by reaction between Hydrogen and Nitrogen under pressure, which will release a large amount of thermal energy by exothermic reaction. Such heat can be used to generate steam to run a stem turbine to generate power. The resulting ammonia can be split with concentrated solar power (CSP) into Hydrogen and Nitrogen and the above process can be repeated.

The same system can also be used to split commercial Ammonia into Hydrogen and Nitrogen. The resulting Hydrogen can be separated and stored under pressure. This Hydrogen can be used to fuel Fuel cell cars such as Honda FXC or to generate small-scale power for homes and offices.

By using CSP, there is potential of cost savings as much as 70% compared to PV solar system for the same capacity power generation on a larger scale. Focusing sun’s energy using large diameter parabolic troughs and concentrators, one can generate high temperatures.  Dishes can typically vary in size and configuration from a small diameter of perhaps 1 meter to much larger structures of a dozen or more meters in diameter.  Point focus dish concentrators are mounted on tracking systems that track the sun in two axes, directly pointing at the sun, and the receiver is attached to the dish at the focal point so that as the dish moves, the receiver moves with it.  These point focus systems can generate high temperatures exceeding 800ºC and even 1,800ºC.

The temperature required to run a steam turbine does not exceed 290C and it is quite possible to store thermal energy using mixture of molten salts with high Eutectic points and use them to generate steam. Such large-scale energy storage using lead-acid batteries and power generation using PV solar may not be economical. But it will be economical and technically feasible to harness solar thermal energy using CSP for large-scale base load power generation. It is estimated that the cost of such CSP will compete with traditional power generation using coal or oil in the near future.CSP has potential to generate cost-effective clean power as well as a fuel for transportation.

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