How Molten Salt Heat Transfer Fluid in a Parabolic Trough Solar Collector

Research on thermal physics technology on parabolic trough collector systems plays an important part in the use of solar energy. The concept of the Parabolic Drough Solar Collector (PTSC) was thoroughly explored in this paper on the basis of how this plant operates and how it is used to produce and transfer heat. The efficiency of the molten salt PTSC was also examined numerically and the influences of the critical device parameters were analyzed. It has been observed that the temperature differential of the device decreases with an increase in heat transfer fluid depending on inlet velocity and temperature. Maintenance of the system has also been discussed based on the argument that the plant does not only require frequent maintenance and treatment but also needs to be inspected regularly so as to increase its efficiency in its given role. In addition, the results of the study show that the non-uniform transfer of solar energy affects the functioning of the PTSC. This paper seeks to briefly discuss how molten salt plays the role of a transfer fluid in a parabolic system.

Introduction

Engineers discover, innovate and create ideas that will enhance and improve the way of life for humans. Since the discovery of fossil fuels, there have been a lot of changes in the world and particularly the energy sector. As a result of this, there has been an advance in the manufacture and transport industries. However, with the high rate of fossil fuel depletion, there have risen additional needs to opt for other sources of energy more so the renewable sources of energy that do not only serve as a substitute for the fossil fuels but also serve as a long time solution to energy problems. It is important to note that this technology of transferring heat using molten salt in a parabolic trough collector was discovered while the energy sector was undergoing a lot of problems as far as power supply is concerned. This has proved to be an important source of both generation and transfer of energy.

Objectives

Engineers should make taking technology to a different level their main business like it is stated in the Saudi Arabian Vision 2030 goals. After the invention of the normal flat-type solar collector, a lot of improvements and enhancements took place and that overtook and replaced the flat-type design (Kearney, 2004). The main focus of this paper is on the Parabolic Solar Collector. According to Kearney, molten salt comprises of Sodium Nitrate and Potassium Nitrate in 60 and 40 percent composition respectively (Kearney, 2003). For this reason, a molten salt is the best solution to the increased electricity costs to replace oil. The aim objectives of this study are;
i) To discuss in detail the operation and the design of the PTSC.
ii) To shed light on the advancements that has been made on the replacement of oil with molten salt as the heat transfer fluid.

Theory

An example of molten salt Parabolic Trough is the Enel-owned molten salt demonstration trough in Priolo Gargallo, Sicily. It provides more than five megawatts of electric power to an already established power plant, achieves a temperature of 550 degrees Celsius and offers more than ten hours of full energy storage. The molten salt PTSC is an Italian-made technology developed at the Italian Research Center ENEA (Leitner, 2015). Additionally, there has been a new incentive to increase salt parabolic trough plants all over Italy. This process aims at having a storage capacity of more than 600 megawatts of power stored by 2017.

Discussion

Design of Parabolic Trough Solar Collector

For very many years, parabolic trough power stations have been the only technology involved with the use of solar energy to produce energy in power plants for commercial use. They have been in commercial use in California since 1985 where they have been used in large power plants. Electricity from parabolic trough power plants is produced using a steam turbine that is connected to a generator. The radiations from the sun are captured and reflected by a series of parabolic mirrors. This way adequate heat is generated that is used to produce steam. Molten salt Heat Transfer Fluid is usually easily corroded (Chang, 2015). Nonetheless, commercial salt does not contain chloride and perchlorates which are the most chemically active. Other corrosion resistant chemicals are however used in the collector. The PTSC contains three main components; the receiving tube carries the heat transfer fluid, the receiving panels are mirrors that concentrate solar radiation and the solar field piping are used to carry the molten salt from the storage locations to other parts of the plant.

Operation of the PTSC Plant

In the PTSC plant, the rays of sunlight are directed to the absorber tubes placed in the trough’s focal line in order to achieve high temperatures of between 350°C and 550°C. The surfaces of the trough are usually arranges in such a way that they track sunlight in one axis more so the north-south axis. If the radiation from the sun is adequate, it is able to produce enough energy to fill up the storage system adequately (Matino, 2015). The molten salt flows into the receiving tube and transfers the thermal energy to the steam turbine. Generally, the molten salt is heated to about 550°C by the rays of the sun and then passed through a number of exchangers to produce steam at very high temperatures. The generator then converts the steam to electrical energy. Using molten salt as the heat transfer fluid is advantageous over oil because; it does not need to be replaced after every cycle and after being heated at high temperatures, the molten salt occupies a relatively reduced volume, hence the size of the tanks can be reduced (Maccari, 2015). This saves on costs which in turn reduce the entire cost of this process making it more efficient and affordable.

Maintenance

i) Preventive Maintenance – This is basically maintenance on a weekly basis which involves cleaning the reflectors, lubricating the moving parts, replacing the heat transfer fluid and painting to resist corrosion.
ii) Predictive Maintenance – This involves inspecting the system for any potential failure. It includes inspecting the; misalignment at receiver tubes, broken glass tubes, the formation of calcite on the surface and replacing damaged alarms and sensors.
iii) Maintenance of corroded components is done by controlling fluid flow velocity, design, coatings, using the right material, using cathode protection and controlling the environment (Ruegamer, 2014).

Conclusion

Storage of heat using molten salt has been used in various fields across the world. It has been used as an alternative renewable energy which is environmental-friendly and which provides adequate storage facilities. This storage technique is used to produce electricity in high quantities as required just like fossil fuels and nuclear plants but with reduced environmental effects and costs. Moreover, solar enabled thermal plants with molten salt as the main fluid for transferring heat are able to operate day and night providing power for both grid and off-grid use. Molten salt thermal energy is the most reliable source as it provides power in relatively high quantities that meet different user needs adequately. It is also cost-effective compared to other sources of power, for example, nuclear energy, coal and natural gas. Generally, solar thermal energy storage has proved a very reliable source as it is able to produce more than twice the energy produced by other solar technologies.

Recommendations

Corrosion is the biggest disease that plagues the energy and fuel production sector. Parabolic Trough Solar Collector systems are not an exception considering the fact that corrosion occurs as a result of chemical reactions in the aqueous solution and the molten salt. This problem should be dealt with to ensure that the systems run clean and hence offer the most reliable form of service. Wire reinforced graphite should be alternated with fiberglass cores as they are less likely to corrode. Teflon washers should also be coated with fiber glass. Nevertheless, the graphite must be changed from time to time due to oxidation by the nitrate. The next step that should be taken is to put up a loop to repair and maintain the preheating system so as to ensure fewer breakages in the system.

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Appendices

List of Abbreviations
HTF - Heat Transfer Fluid
PTSC - Parabolic Trough Solar Collector
SEGS - Solar Energy Generating Systems