Evaluation of Solar-Based Thermo-Mechanical Cooling in Hot Climate Conditions

  • Yusra Alkendi

Student thesis: Master's Thesis

Abstract

Conventional air-conditioning systems consume significant amounts of energy annually and contribute to high peak electricity demand in hot climates, such as in for example the Gulf Cooperating Council (GCC) region. Solar-assisted air-conditioning has the potential to reduce the energy consumption and economic and environmental impacts of space cooling, but is still insufficiently exploited in the GCC, where its application is considered in this Thesis. The overall objective of the present research is to contribute to the development and thermodynamic evaluation of sustainable small-scale residential solar-driven space cooling solutions for hot climates with high seasonal and daily fluctuations in outdoor air temperatures. A review of published research studies on solar-assisted refrigeration is undertaken, focusing on two promising types of solar thermo-mechanical refrigeration systems, namely solar Rankine and solarassisted ejector-based systems. From this review, thermodynamic cycle concepts having potential for space cooling in high outdoor air temperatures with pronounced seasonal/daily temperature fluctuations are identified. These concepts are integrated into a proposed solar collector-driven dual booster-assisted triple parallel ejector refrigeration system (SBTPERS) for application to space cooling in a representative small-scale residential building in the United Arab Emirates (UAE). The system incorporates as main thermodynamic enhancement features three parallel constant-pressure ejectors operated in critical mode, and two compressor boosters upstream of the secondary flow inlet of two ejectors from three. Candidate safe and environmentally acceptable working fluids (i.e., R11, R141b, R245fa, R600a) with suitable thermodynamic properties are identified for the SBTPERS. Numerical thermodynamic models for constant-pressure ejectors based on ideal and real gas assumptions, both for critical and sub-critical mode operations, are developed in Engineering Equation Solver (EES) and successfully validated against extensive experimental and numerical published data available for air, R11, R141b, and R245fa over a range of boundary conditions (i.e., generation, evaporation, and condensation temperatures). An organic Rankine cycle (ORC-VCR) thermodynamic model is also constructed in EES and successfully numerically validated. The SBTPERS model is then developed using the validated ejector, power/generator and refrigeration sub-systems modeling methodologies implemented in EES, and a solar collector sub-system model implemented in TRNSYS software. Using R245fa as working fluid, selected based on thermodynamic, safety and environmental considerations, the SBTPERS predicted annual-average thermal COP, solar fraction, and exergy efficiency are found to be 0.16, 30%, and 4.95%, respectively. The hourly-average system thermal COP, ejector entrainment ratio (for one of the three ejectors at a time), and solar fraction vary from 0.15 to 0.18, 0.20 to 0.24, and 18 to 91%, respectively, during an annual period, and the refrigeration subsystem exergy efficiency from 4.7 to 5.3%. Compared with a simple ejector refrigeration cycle including a single ejector (i.e., high-critical temperature ejector) with no compression booster, the SBTERS would improve annual-average COP, ejector entrainment ratio, and exergy efficiency by a factor of approximately 19 for each variable. The triple parallel ejector contributes to improve each variable by a factor of approximately 10 relative to a single ejector system with no boosters, while the two boosters contribute a further improvement factor of approximately 1.7-1.8, relative to a triple parallel ejector system without boosters. The SBTPERS system annual-average COP and exergy efficiency are found to be mostly sensitive to the generator outlet temperature/pressure, followed by the evaporator outlet temperature/pressure. Based on the results, the proposed SBMPERS system is a promising thermodynamic refrigeration cycle concept for small-scale residential air-conditioning in harsh climates with yearly elevated and fluctuating outdoor temperatures.
Date of AwardDec 2018
Original languageAmerican English
SupervisorValerie Eveloy (Supervisor)

Keywords

  • Solar air-conditioning
  • solar organic Rankine refrigeration
  • solar-assisted ejector refrigeration
  • solar thermo-mechanical refrigeration
  • renewable cooling
  • Engineering Equation Solver
  • TRNSYS.

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