Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston expander isentropic efficiency increases at off-design operation by 10-16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away its nominal point, the offdesign ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, predicting system performance and can be used by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.

The use of annular fins in air-cooled heat exchangers is a well-known solution, commonly used in airconditioning and heat-recovery systems, for enhancing the air-side heat transfer. Although associated with additional material and manufacturing costs, custom-designed finned-tube heat exchangers can be cost-effective. In this article, the shape of the annular fins in a multi-row air heat exchanger is optimised in order to enhance performance without incurring a manufacturing cost penalty. The air-side heat transfer, pressure drop and entropy generation in a regular, four-row heat exchanger are predicted using a steady-state turbulent CFD model and validated against experimental data. The validated simulation tool is then used to perform model-based optimisation of the fin shapes. The originality of the proposed approach lies in optimising the shape of each fin row individually, resulting in a non-homogenous custom bundle of tubes. Evidence of this local-optimisation potential is first provided by a short preliminary study, followed by four distinct optimisation studies (with four distinct objective functions), aimed at addressing the major problems faced by designers. Response-surface methods-namely, NLPQL for single-objective and MOGA for multi-objective optimisations-are used to determine the optimum configuration for each optimisation strategy. It is shown that elliptical annular-shaped fins minimise the pressure drop and entropy generation, while circular-shaped fins at the entrance region (i.e., first row) can be employed to maximise heat transfer. The results also show that, for the scenario in which the total heat transfer rate is maximised and the pressure drop minimised, the pressure drop is reduced by up to 31%, the fin weight is reduced up to 23 %, with as little as a 14 % decrease in the total air-side heat transfer, relative to the case in which all the fins across the tube bundle are circular. Moreover, in all optimised cases, the entropy generation rate is also reduced, which shows a thermodynamic improvement in tube bundle performance.

The application of phase-change materials (PCMs) has received significant interest for use in thermal energy storage (TES) systems that can adjust the mismatch between the energy availability and demand. In the building sector, for example, PCMs can be used to reduce airconditioning energy consumption by increasing the thermal capacity of the walls. However, as promising this technology may be, the poor thermal conductivity of PCMs has acted as a barrier to its commercialization, with many heat-transfer enhancement solutions proposed in the literature, such as microencapsulation or metal foam inserts, being either too costly and/or complex. The present study focuses on a low-cost and highly practical solution, in which natural-convective heat transfer is enhanced by placing the PCM in an eccentric annulus within a horizontal double-pipe TES heat exchanger. This paper presents an annulus-eccentricity optimisation study, whereby the optimal radial and tangential eccentricities are determined to minimize the charging time of a PCM thermal energy store. The storage performance of several geometrical configurations is predicted using a computational fluid dynamics (CFD) model based on the enthalpy-porosity formulation. The optimal geometrical configuration is then determined with response surface methods. The horizontal double-pipe heat exchanger studied considered here is an annulus filled with N-eicosane as the PCM for initial studies. In presence of N-eicosane, for the concentric configuration (which is the baseline case), the charging is completed at Fo = 0.64, while the charging of optimum eccentric geometries with the quickest and slowest charging is completed at Fo = 0.09 and Fo = 2.31, respectively. In addition, an investigation on the discharging performance of the studied configurations with N-eicosane shows the quickest discharge occurs with the concentric annulus case at Fo = 0.99, while the discharge time of the proposed optimum annuli is about three times this value. In other words, the proposed optimum geometry with the quickest charging time charges~7.1 times faster but also discharges~3 times slower, which is ideal for a TES, especially when used as passive thermal storage systems in nearly zero-emission buildings. Complementary studies demonstrate that the proposed optimum configuration improves the TES performance also when employing other PCM types as well as various shell-to-tube diameter ratios.

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