Development of a non-industrial waste water heat recovery unit using PCMs

  • Abdur Rehman Mazhar

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

The harnessing of heat from waste water in non-industrial buildings is of utmost importance to balance the demands of space with those of hot water heating in order to make future buildings more energy efficient, to increment the share of third generation renewable technologies in the energy consumption of buildings and to potentially make future buildings both a source and sink of decentralised heat in low temperature fourth generation district heating (DH) networks.

This research develops a methodology to design a waste water heat harnessing exchanger for non-industrial setups, incorporating phase change materials (PCMs). In this thesis, the methodology is applied to waste water heat harnessing in residential buildings’ appliances, with the associated boundary conditions. Heat is stored from the incoming waste water into the PCM and transferred back to cold water (CW). Such a heat exchanger (HE) decouples demand and supply by providing high-density latent thermal storage whilst incorporating heat recovery and storage in a single unit. This passive technique has considerable advantages over the commercially available technologies in terms of energy efficiency, economic and environmental benefits, as highlighted in the literature review. The boundary conditions and flow patterns of waste water flow in residential buildings are also established with an extensive literature review as the basis for the subsequent research. At the same time, the applications of PCMs used in passive waste heat harnessing applications are also put forward.

This research was divided into three main phases. Initially, the PCM having the optimum thermal characteristics and melt temperature with regard to the boundary conditions of this application was selected using a Particle Swarm Optimisation (PSO) algorithm coupled with the classical 2-phase analytical solution, developed in MATLAB. Secondly, the optimal geometric parameters of a corrugated pipe were selected to enhance heat extraction internal to the pipe for the flowing fluids. A Computational Fluid Dynamics (CFD) simulation developed in Star-CCM was validated based on an experimental setup to carry out a parametric study to select such a corrugated pipe. Finally, to propagate the heat fully into the PCM, an external thermal conductivity enhancement technique using high conductive fins was selected in a similar CFD and experimental setup. Based on the outcome of these three research phases, a full scale HE was developed and assessed for usage in conjunction with conventional appliances in residential buildings.

In residential appliances, the average outgoing waste or grey water (GW) is at about 325K (51.85°C) while the incoming fresh CW is at 285K (11.85°C), both at a mass-flow rate of 0.1kg/s. Based on these boundary conditions, in the first part of the research, a PCM with a melt temperature of 298K (24.85°C) was chosen to provide the optimal thermal performance both in terms of melting and freezing. In the second part of the research, a corrugated pipe having a rib height ‘e’ of 4.5mm with a pitch ‘p’ of 30mm was selected to provide the best enhancement in terms of internal heat transfer. Finally, based on the third part of the research, 1mm thick copper fins with a pitch of 10mm and dimensions of 40×90mm provided the best external heat transfer enhancement within the PCM. The design was based on the bottleneck case of PCM freezing, which is about 60-70% more-time consuming than melting for the same amount of heat transfer. Based on this configuration, the highest possible increment in CW temperature was achieved with a complete PCM phase change resulting in the maximum amount of latent heat transfer for both melting and freezing. Additionally, the holistic design of a HE with three multi-cascaded PCMs is predicted to increment the CW temperature by15K. In a typical four-member household, this device would save 4,687kWh of energy annually with a payback time on the investment of 4.77 years in lieu of present-day gas prices.

Based on the outcomes of this research, the design of such a HE for any non-industrial building can be established, especially for commercial ones that have a higher potential of heat harnessing. Additionally, usage of the harnessed heat has versatility in its potential applications that are not limited to CW heating, as in this thesis.
Date of AwardApr 2020
Original languageEnglish
Awarding Institution
  • Coventry University
SupervisorAshish Shukla (Supervisor) & Shuli Liu (Supervisor)

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