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Abstract This paper presents the exergy analysis of variable air volume (VAV) systems used in office buildings for air-conditioning. The mathematical models are developed using the engineering equations solver (EES) environment. The following indicators are presented: energy efficiency, exergy efficiency, and equivalent-CO 2 emissions due to the generation of electricity used by the VAV system.

An experimental investigation to develop and test a burner and a heat recovery system for thermophotovoltaic (TPV) applications is presented. Experimental data have been compared with theoretical calculations and considerations in the pre-design and design phases of the project to find the weakest point of the concept and to validate the expected performance. The TPV generator has been designed as a compact module in order to be used as a range extender in an electric car. The heat recovery system is the key element to increase the efficiency of the system. The heat recovery system presented in this paper is a rotary type regenerator that is very compact and has higher effectiveness in comparison with other types of regenerators with the same number of transfer units (NTU). The experimental data have been used to verify the numerical models used in the calculations for design of the regenerator matrix. A new version of the numerical model has been developed to take into account the variation of the thermal properties of the system with the temperature. Dimensions, weight, efficiency, emissions and high working temperatures have been the most important competitive constraints to observe for design of the system.

Abstract Lipids produced by microalgae are a promising biofuel feedstock. However, as most commercial mass production of microalgae is in open raceway ponds it is generally considered only a practical option in regions where year-round ambient temperatures remain above 15 °C. To address this issue it has been proposed to couple microalgae production with industries that produce large amounts of waste heat and carbon dioxide (CO2). The CO2 would provide a carbon source for the microalgae and the waste heat would allow year-round cultivation to be extended to regions that experience seasonal ambient temperatures well below 15 °C. To demonstrate this concept, a dynamic model has been constructed that predicts the impact on algal pond temperature from both bubbled-in off-gas and heat indirectly recovered from off-gas. Simulations were carried out for a variety of global locations using the quantity off-gas and waste energy from a smelter’s operations to determine the volume of microalgae that could be maintained above 15 °C. The results demonstrate the feasibility of year-round microalgae production in climates with relatively cold winter seasons.

Abstract In order to obtain high Stirling engine efficiencies, pressure losses in the regenerator must be minimized. In this study, numerical methods are used to determine the influence of misalignment and spacing on the pressure drop through stainless steel wire mesh regenerators. Microscope images were taken to discover the average wire misalignment found in wire mesh regenerators, and it was found that regardless of the intended stacking arrangement, the average misalignment was 44%–49%. A 3-dimensional CFD model was validated against previous empirical results, and used to determine the effect of wire mesh misalignment on the pressure drop for steady flow through a regenerator. A new friction factor correlation equation was developed to include the effect of wire misalignment in the form of tortuosity. It was found that if a perfectly aligned regenerator was manufactured, Stirling engine designers could expect a 38%–45% decrease in pressure losses. The numerical model was also used to determine the influence of axial spacing on the pressure drop, and it was found to be dependent on the misalignment, with pressure loss reductions only being realized in highly misaligned cases. As a result of this study, Stirling engine designers can confidently disregard alignment and spacing when assembling wire mesh regenerators, and can more accurately calculate the expected pressure losses.

Energy and exergy assessments are reported of a novel trigeneration system based on a solid oxide fuel cell (SOFC), for steady-state operation and using a zero-dimensional approach. The trigeneration system also includes a generator-absorber heat exchanger for cooling and a heat exchanger for the heating process. The influences of two significant SOFC parameters (current density and inlet flow temperature) on several variables are investigated. The results show that the energy efficiency is a minimum of 33% higher when using the trigeneration system compared with the SOFC power cycle. In addition, the maximum energy efficiencies are found to be 79% for the trigeneration system, 69% for the heating cogeneration, 58% for cooling cogeneration and 46% for electricity production. Moreover, the highest trigeneration exergy efficiency is almost 47% under the given conditions. It is also shown that, as SOFC current density increases, the exergy efficiencies decrease for the power cycle, cooling cogeneration, heating cogeneration and trigeneration. As current density increases, the trigeneration energy and exergy efficiencies decrease, and an optimal current density is observed to exist at which the net electrical power is a maximum. As SOFC inlet flow temperature increases, the trigeneration energy and exergy efficiencies and net electrical power increase to a peak and then decrease. The main exergy destructions occur in the air heat exchanger, the SOFC and the afterburner.

Abstract Performance and emissions experiments were conducted on a compression-ignition direct-injected natural gas engine (DING) equipped with a shielded glow plug ignition assist system. Tests were conducted at three different intake pressures (34.5 kPag, 68.9 kPag, 103.4 kPag), and four nominal (targeted) equivalence ratios (0.2, 0.3, 0.4, 0.5). CH4, NOx, CO and PM emissions were measured and analyzed, showing that emissions levels are influenced by the DING engine’s combustion modes and intake pressure. Premixed combustion which dominates at low equivalence ratios resulted in higher levels of CH4 and NOx emissions, while mixing-controlled combustion, which develops at higher equivalence ratios, resulted in elevated CO and PM levels. Higher intake pressure was found to improve all emissions levels. The most significant effect was the reduction of PM and CO emissions due to improved fuel charge mixing and air entrainment that results from a pressure-driven momentum increase of the engine’s air swirl field. Brake specific emissions and fuel consumption were estimated and compared against levels reported in the literature for dual-fuel port-injection, and High-Pressure Direct-Injection (HPDI) natural gas engines. The most significant finding was that the DING engine exhibits lower fuel consumption and PM emissions levels when compared to values reported in the literature for HPDI engines. The PM emissions advantage was attributed to a higher proportion of premixed combustion and the absence of a diesel pilot in DING engine operation. Lastly, PM size distributions were analyzed, showing that the DING engine produces PM that is smaller than PM of a conventional diesel engine, but similar to the PM reported in the literature for HPDI engines.

Abstract Disposal of waste plastic accumulated in landfills is critical from the environmental perspective. The energy embodied in waste plastic could be recovered by catalytic pyrolysis as waste plastic oil (WPO) and could be recycled as a fuel for diesel engines. This method presents a sustainable solution for waste plastic management as the gap between global plastic production and waste plastic generation keeps widening. The present study investigates the combined influence of EGR and injection timing on the combustion, performance and emission characteristics of a DI diesel engine fueled with neat WPO. Experiments were conducted at three injection timings (21°, 23° and 25°CA bTDC) and EGR rates (10, 20 and 30%) at the engine’s rated power output. When compared to diesel, the combustion event occurred closer to the TDC when the injection timing is delayed from 25°CA bTDC to 21°CA bTDC. The peak in-cylinder pressures and HRRs dropped gradually as the injection timing was delayed from 25°CA bTDC to 21°CA bTDC at all EGR rates. The engine delivered diesel-like fuel consumption with 5.1% higher brake thermal efficiency. NOx decreased up to 52.4% under 30% EGR when WPO was injected lately 21°CA bTDC. Smoke density remained lower by 46% and 9.5% for 10% and 20% EGR rates respectively for WPO only at early injection timing of 25°CA bTDC. HC and CO emissions stayed lower at early injection timing of 25°CA bTDC under 10% EGR. WPO injected at the advanced injection timing of 25°CA bTDC and low EGR rate of 10% was found to simultaneously reduce smoke and NOx by 46% and 38% respectively.