PCM based thermal storage system/plant models for building energy system analysis

Student thesis: Doctoral Thesis

Abstract

Energy storage is an important measure in efforts to improve buildings' energy efficiency and reduce CO2 emissions from the building sector. Affording opportunities for better utilisation and management of building's energy and reducing wastages, it can serve as a vital mitigant to the negative impact of electrification of energy systems, which the UK government has chosen as means of achieving its goal of reducing CO2 emissions by 80% by the year 2050. Energy storage with phase change materials, though capable of improving energy systems' performance and reducing building energy demand, present challenges that necessitates pre-analysis and study for which virtualisation and simulation with numerical models is the most cost effective and risk-averse method. This requires use/application of whole building modelling applications like ESP-r that have capabilities to model PCMs. Existing PCM models in the software lack ability to represent current available physical PCM based thermal energy storage systems. These form the basis for the work reported herein, which discusses solution methods and their implementations. Models of two PCM thermal stores were developed using the enthalpy formulation method, which is more reliable and devoid of many of the problems associated with the heat capacity method. The thermal stores were modelled as shell and tube heat exchangers where the PCM, contained in the shell, was treated as one of the heat transfer fluids with the main heat transfer fluid flowing through the tube/pipes. Energy conservation equations for the PCM were formulated in terms of the heat balance over a volume of material and time interval leading to development of representative equations for heat exchange between PCM and heat transfer fluid, and temperature and phase changes experienced by the phase change material. An algorithm was developed employing iterative schemes to solve the discretised energy equations. The source code and algorithms were verified using structured walk throughs, debugging and consistency tests, and sanity test from ESP-r simulation of building model containing the plant models. They were also validated by comparing results from running simulation of the models with results reported for models of similar phase change thermal storage systems by Hosseini et al. (2012), Hosseini, Rahimi, & Bahrampoury, (2014) and Seddegh et al (2016). Results from the verification tests show that the models were accurate to within expected range and for the intended purpose while results from validation show close agreement with reference materials. The models were also subjected to sensitivity analysis aimed at assessing impact of grid sizes and convergence criteria on models' performance. These parameters, unlike other variable parameters of the model source codes, are determined internally within the codes while the others are determined by ESP-r. Results show that the models' outputs, HTF and PCM, are unaffected by changes in both the grid size and convergence criteria. The models' sensitivity to the convergence criteria is in regard to the code processing time. Additional sensitivity analysis was done for the fabric integrated thermal store to assess the source code's sensitivity to type of materials with which PCM cells at extreme ends of the PCM block interacted. Results show that outputs are unaffected by type of materials with which PCM cells at extreme ends of the PCM block interacted. Installation/implementation of the models' source codes in ESP-r was afterwards done by writing algorithms for each plant model in line with source codes source formatting and description in ESP-r incorporating the software's common variables and parameters, building up plant database entries and editing necessary code compilation and execution codes and programs
Date of Award29 Sep 2021
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorNicolas Kelly (Supervisor) & William Dempster (Supervisor)

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