Reduced Order Heat Exchanger Models for Low-to-Medium Temperature Range Solar Thermal Applications

Majumdar, Rudrodip and Saha, Sandip K (2019) Reduced Order Heat Exchanger Models for Low-to-Medium Temperature Range Solar Thermal Applications. In: Advances in Solar Energy Research. Part of the Energy, Environment, and Sustainability book series (ENENSU) . Springer Nature, Singapore, pp. 357-393. ISBN 978-981-13-3301-9

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Abstract: Pivotal heat transfer components of solar thermal systems may involve single phase flow of the working fluid in some unit (e.g. single phase solar collector), whereas, two phase flow of the working fluid occurs in the other units (e.g. two-phase solar collector coupled directly to the turbine, boilers and evaporators). Modelling of these systems are important to understand the heat transfer behavior, as well as, to develop the system level control among many other attributes. A detailed analysis of single phase and two phase systems is possible using mathematical models to characterize the fluid flow and heat transfer. Detailed description of the fluid flow and heat transfer become computationally very expensive with such models, and also a very high level of precision may not be required in large time (~few hours) simulations of the systems, as well as, in controlling the entire solar thermal power plant. Hence there is a need to develop computationally fast, low order dynamic models. Among many other modelling approaches, a particular class of heat exchanger model, namely the moving boundary lumped-parameter model, has emerged as an efficient and effective tool for simulating dynamic characteristics of the two-phase solar collectors and the evaporators, pertinent to organic Rankine cycle (ORC) systems. These models are efficient in locating the continuously moving working fluid phase change boundary without requiring any sophisticated, well-trained formulation pertinent to the starting solutions. Even a simplified, reduced order quasi-steady state model is capable of demonstrating moving boundary characteristics in a narrow evaporator tube that is employed to carry organic refrigerant (working fluid) into the two-phase ORC heat exchanger in the medium temperature solar thermal applications (~200 °C). The model is capable of predicting the variation in working fluid mass flow rates with time-varying temperature of the heat transfer fluid (usually a commercial thermic oil), that transfers energy into the organic refrigerant. The variation in wall temperatures of the evaporator tube for three distinct flow regimes of the working fluid (subcooled, two-phase and superheated) can be evaluated with changing average heat transfer fluid (HTF) temperature, corresponding to the varying levels of solar radiation incident on the collectors; and therefore, the model provides an avenue for ascertaining the practicability of the operating conditions based on the variation of the driving parameters values. By using the results of the quasi-steady model as the initial guess in the detailed dynamic model, the sharp transient characteristics can be explored by introducing time-dependent fluctuations in the subcooled refrigerant flow at the heat exchanger entry. Further complexity can be added to the modelling by incorporating axial variation in the heat transfer fluid temperature profile in the prevalent shell and tube heat exchanger. In this monograph, we will briefly discuss about the full-fledged numerical models followed by elaborate description on reduced order models.
Item Type: Book Chapter
Additional Information: Dr Majumdar's affiliation for this paper is Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai, India
Keywords: Reduced order modelling, Solar thermal system, Moving boundary model
Subjects: School of Natural and Engineering Sciences > Energy and Environment
Programmes > Energy and Environment Programme
Date Deposited: 11 Sep 2022 17:46
Last Modified: 11 Sep 2022 17:50
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    DOI: 10.1007/978-981-13-3302-6_12

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