Browsing by Author "Nithyanandam, Karthik"
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- Analysis and Optimization of a Novel Hexagonal Waveguide Concentrator for Solar Thermal ApplicationsKant, Karunesh; Nithyanandam, Karthik; Pitchumani, Ranga (MDPI, 2021-04-12)This paper analyzes a novel, cost-effective planar waveguide solar concentrator design that is inspired by cellular hexagonal structures in nature with the benefits of facile installation and low operation and maintenance cost. A coupled thermal and optical analysis of solar irradiation through an ideal hexagonal waveguide concentrator integrated with a linear receiver is presented, along with a cost analysis methodology, to establish the upper limit of performance. The techno-economic model, coupled with numerical optimization, is used to determine designs that maximized power density and minimized the cost of heat in the temperature range of 100–250 °C, which constitutes more than half of the industrial process heat demand. Depending on the incident solar irradiation and the application temperature, the cost of heat for the optimal design configuration ranged between 0.1–0.27 $/W and 0.075–0.18 $/W for waveguide made of ZK7 glass and polycarbonate, respectively. A techno-economic analysis showed the potential of the technology to achieve cost as low as 80 $/m2 and 61 $/m2 for waveguide made of ZK7 glass and polycarbonate material, respectively, which is less than half the cost of state-of-the-art parabolic trough concentrators. Overall, the hexagonal waveguide solar concentrator technology shows immense potential for decarbonizing the industrial process heat and thermal desalination sectors.
- Investigations on Latent Thermal Energy Storage for Concentrating Solar PowerNithyanandam, Karthik (Virginia Tech, 2013-06-10)Thermal energy storage (TES) in a concentrating solar power (CSP) plant allows for continuous operation even during times when solar radiation is not available, thus providing a reliable output to the grid. Energy can be stored either as sensible heat or latent heat, of which latent heat storage is advantageous due to its high volumetric energy density and the high Rankine cycle efficiency owing to the isothermal operation of latent thermal energy storage (LTES) system. Storing heat in the form of latent heat of fusion of a phase change material (PCM), in addition to sensible heat, significantly increases the energy density, thus potentially reducing the storage size and cost. However, a major technical barrier to the use of latent thermal energy of PCM is the high thermal resistance to energy transfer due to the intrinsically low thermal conductivity of PCMs, which is a particularly acute constraint during the energy discharge. Secondly, for integration of TES in CSP plants, it is imperative that the cyclic exergetic efficiency be high, among other requirements, to ensure that the energy extracted from the system is at the maximum possible temperature to achieve higher cycle conversion efficiency in the power block. The first objective is addressed through computational modeling and simulation to quantify the effectiveness of two different approaches to reduce the thermal resistance of PCM in a LTES, viz. (a) developing innovative, inexpensive and passive heat transfer devices that efficiently transfer large amount of energy between the PCM and heat transfer fluid (HTF) and (b) increase the heat transfer area of interaction between the HTF and PCM by incorporating the PCM mixture in small capsules using suitable encapsulation techniques. The second portion of the research focuses on numerical modeling of large scale latent thermal storage systems integrated to a CSP plant with the aforementioned enhancement techniques and cascaded with more than one PCM to maximize the exergetic efficiency. Based on systematic parametric analysis on the various performance metrics of the two types of LTES, feasible operating regimes and design parameters are identified to meet the U.S. Department of Energy SunShot Initiative requirements including storage cost < $15/kWht and exergetic efficiency > 95%, for a minimum storage capacity of 14 h, in order to reduce subsidy-free levelized cost of electricity (LCE) of CSP plants from 21¢/kWh (2010 baseline) to 6¢/kWh, to be on par with the LCE associated with fossil fuel plants.
- Using Encapsulated Phase Change Salts for Concentrated Solar Power PlantMathur, A.; Kasetty, R.; Oxley, J.; Mendez, J.; Nithyanandam, Karthik (Elsevier, 2014-06-01)Storing thermal energy as latent heat of fusion in phase change material (PCM), such as inorganic salt mixtures, can improve the energy density by as much as 50% while reducing the cost by over 40%. However, to discharge stored energy from PCMs, which has low thermal conductivity, requires a large heat transfer area which drives up the cost. Fortunately, salts encapsulated into small capsules can provide high specific surface area thus alleviating this problem. However, a technical barrier with encapsulating salts is that when it is produced, a void must be created inside the shell to allow for expansion of salt when it is heated above its melting point to 550 °C. Terrafore's method to economically create this void consists of using a sacrificial polymer which is coated as the middle layer between the salt prill and the shell material. The polymer is selected such that it decomposes much below the melting point of salt to gas leaving a void in the capsule. Salts with different melting points are encapsulated using the same recipe and contained in a packed bed consisting of salts with progressively higher melting points from bottom to top of the tank. This container serves as a cascaded energy storage medium to store heat transferred from the sensible heat energy collected in solar collectors. Mathematical models indicate that over 90% of salt in the capsules undergo phase change improving energy density by over 50% from a sensible-only thermal storage. Another advantage of this method is that it requires only a single tank as opposed to the two-tanks used in a sensible heat storage, thereby reducing the cost from a nominal $27 per kWht to $16 per kWht and coming close to the SunShot goal for thermal storage of $15 per kWht.