Journal of Energy Engineering

Underground spaces normally are at cooler air temperature than ambient air temperature because of earth-sheltered walls. This cooler air can be circulated to occupied spaces using a solar chimney effect to exhaust air from the basement to where it is needed. This work aims to study the viability of using natural ventilation induced by a Trombe wall to draw fresh air from underground basement floors for space cooling in the dry desert climate. Outdoor air is delivered to the basement through an earth tube. A numerical model integrating thermal models of the basement space and the occupied zone, and the Trombe wall is used to predict the air temperature variation with time in the occupied space air temperature and to predict thermal comfort. The feasibility of implementing the proposed system is assessed in a case study of a residence in the inland dry desert climate of Lebanon during the summer. It was found that the proposed system achieved thermally comfortable conditions at 80% acceptability for a considerable number of hours without the need for mechanical ventilation (83.3% in June and 58.3% in August). The electric energy consumption of the system over summer (June through September) was estimated at $7.71\mathrm{kW}·\mathrm{h}/{\mathrm{m}}^2$ of floor area.

This study investigates the counterflow combustion of an aluminum dust cloud in a heterogeneous system of discrete heat sources. A numerical thermal model was developed to estimate the burning velocity at different concentrations of the dust cloud and oxidizer. The proposed model considers the effects of heat transfer mechanisms, and the calculation of the flame speed is based on a point-source approach. The model equations were derived and solved first to study the single-particle combustion. Later, to investigate the dust cloud combustion, the superposition principle was applied to include the effects of particles. The burning velocity and flame stabilization point were studied at different particle diameters, and the effect of counterflow strain rate was analyzed as well. Moreover, the minimum ignition energy was obtained as a function of dust cloud concentration. The predicted results for the burning velocity were in reasonable agreement with experimental data from the literature.

The accuracy of energy estimation using building energy models is contingent on user inputs. Depending on the situation, energy models may use plug-load densities that are unsubstantiated for the specific building type and space conditions. A common standard in the literature lists plug-load density for educational facilities as $1\mathrm{W}/\mathrm{s}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{o}\mathrm{t}(\mathrm{sf})$. However, plug-loads have changed because of technological advancements and the frequency and types of equipment usage. It is crucial to develop plug-load densities at the space level based on their functionality, especially with the current very limited knowledge of spaces and their equipment energy usage. This is particularly true for educational buildings situated on university campuses. They house a variety of spaces, such as classrooms, offices, study rooms, imaging rooms, auditoriums, lounges, teaching laboratories, etc. Through detailed sensing and monitoring of two educational buildings on a university campus, this paper discusses the methodology to calculate space-level plug-load densities. Furthermore, this paper compares the plug-load density values of university campus educational buildings calculated in this study with previous studies.

When applied for desulfurization in the coal combustion process, traditional sulfur sorbents have very low sorbent utilization and normally have a negative effect on combustion efficiency. A new type of sulfur sorbent, calcium peroxide (${\mathrm{CaO}}_2$), synthesized from natural limestone, is proposed and its effects on desulfurization and combustion efficiency are investigated in this paper. The first series of experiments were carried out in a drop tube furnace (DTF). The results showed that ${\mathrm{CaO}}_2$ captured more sulfur than limestone and improved the burnout behavior of coal. To investigate the sulfur removal efficiency of ${\mathrm{CaO}}_2$, the second experiment was carried out in a fixed bed. The results indicated that the sulfur capture capability of ${\mathrm{CaO}}_2$ definitely exceeds that of limestone no matter whether in a low or high temperature range. Moreover, the combustion behavior of coal with and without ${\mathrm{CaO}}_2$ addition was investigated using thermogravity (TG) and differential scanning calorimetric (DSC) analysis. An increase of combustion reactivity and exothermic behavior were determined during the coal combustion process. Furthermore, the possible accelerative mechanism of ${\mathrm{CaO}}_2$ on desulfurization and combustion efficiency is proposed based on further experimental evidence.

Microwave-assisted ferric chloride (${\mathrm{FeCl}}_3$) pretreated rice straw was utilized for reducing sugars production and ethanol production with free and immobilized cells of Trichoderma viride and Sacharomyces cerevisiae. The maximum reducing sugars concentration ($3.46\mathrm{g}/\mathrm{L}$) and $Y_{R/r}$ (reducing sugars yield/rice straw consumption; $0.17\mathrm{g}/\mathrm{g}$) from pretreated rice straw with free cells of Trichoderma viride at 84 h were much higher than those from untreated rice straw ($1.24\mathrm{g}/\mathrm{L}$, $0.06\mathrm{g}/\mathrm{g}$). Untreated and pretreated rice straw was converted to ethanol by free cells of Trichoderma viride and Sacharomyces cerevisiae. The maximum ethanol concentration from pretreated rice straw was $2.14\mathrm{g}/\mathrm{L}$ representing the $Y_{E/r}$ (ethanol yield/rice straw consumption) of $0.11\mathrm{g}/\mathrm{g}$, which was higher than those from untreated rice straw ($1.13\mathrm{g}/\mathrm{L}$, $0.06\mathrm{g}/\mathrm{g}$). Pretreated rice straw was used to produce ethanol by free cells of Trichoderma viride and immobilized Sacharomyces cerevisiae, and by co-immobilized cells of Trichoderma viride and Sacharomyces cerevisiae. The maximum ethanol concentration obtained by co-immobilized Trichoderma viride and Sacharomyces cerevisiae was $2.75\mathrm{g}/\mathrm{L}$ with the $Y_{E/r}$ of $0.14\mathrm{g}/\mathrm{g}$ at 84 h, which was higher than those achieved by free cells of Trichoderma viride and immobilized Sacharomyces cerevisiae ($2.36\mathrm{g}/\mathrm{L}$, $0.12\mathrm{g}/\mathrm{g}$). Co-immobilized Trichoderma viride and Sacharomyces cerevisiae were reused five times in repeated batch experiments. The ethanol concentration and the $Y_{E/r}$ in each batch were above $2.75\mathrm{g}/\mathrm{L}$ and $0.14\mathrm{g}/\mathrm{g}$ after 84 h, respectively. This proved the vitality and stability of co-immobilized Trichoderma viride and Sacharomyces cerevisiae.

This work evaluates experimentally on a comparative basis the effects of using three customary biofuels on the cyclic variability (irregularity) of combustion and emissions balance in a single-cylinder, light-duty, direct-injection diesel engine run at three loads. Blends of fossil diesel with up to 15% (by volume) ethanol and 24% $n$-butanol or diethyl ether (DEE) are investigated. Related experimental study including heat release diagrams reported by the authors for these blends in the same engine disclosed the differentiation in performance and emissions of these biofuel blends from running the engine with neat fossil diesel. Given that low ignition quality fuels, as the present biofuels, mainly at high blending ratios may give rise to unstable engine functioning and hence detrimental performance, this work examines on a comparative basis the strength of combustion cycle-to-cycle variations as revealed in the measured cylinder pressure diagrams. The latter are analyzed with respect to maximum pressures, maximum rates of pressure rise, mean effective pressures (indicated), and ignition delays, by using statistical tools for mean values and coefficients of variation. Apart from the examination and interpretation of the engine stability results, this study also explores the balance of all four regulated exhaust emissions, namely the defeat of the smoke-nitrogen oxides trade-off and the interesting carbon monoxide–unburned hydrocarbons adverse coupling (trade-off) realized when alcohols or diethyl ether are used in the blends.

Nanofluids belong to a new generation of heat transfer fluids. Their thermal properties make them suitable to be employed in high-performance energy systems. In this paper a new setup for investigating the interactions between microwaves and nanofluids is presented. This is a new issue in this field and only one other experimental campaign has been carried out in the scientific world so far. The design of this experimental setup together with the preliminary results on two different water-based nanofluids (${\mathrm{Al}}_2{\mathrm{O}}_3$ and CuO nanofluids) opens a new frontier in the field of heat transfer in nanofluids.

This study aims to examine the combustion process in a spark-ignition (SI) engine, and the correlation between the in-cylinder pressure, local gas temperature, and ion current under different operating conditions (i.e., spark timing, engine speed, equivalence ratio, throttle position). A commercial fast thermocouple was modified in order to measure both the local gas temperature and the ion current, and placed inside the combustion chamber. The fast thermocouple was charged on its stainless steel sheath isolated from the combustion chamber by an external 64-V direct current (DC) source. All three stages of the ion current as well as the local gas temperature were successfully measured in the experiment using an ion-current sensor integrated to the fast thermocouple. A significant positive relation between periods of combustion, ion-current signals, and local gas temperature was observed. A new correlation between local gas temperature and ion-current signals was obtained with an ion-current sensor mounted to the fast response thermocouple for combustion control with the ion-current method, which appears to be quite significant. The new ion-current sensor, which was developed for the purposes of this study, can be conveniently used as a combustion control sensor in the management of internal combustion engines.

A series of starting tests was conducted on a turbocharged diesel engine to investigate combustion instability for various coolant temperatures, ranging from 20 to 80°C, and fuel blends (neat diesel, blend of diesel with 30% biodiesel, and blend of diesel with 25% $n$-butanol). A statistical analysis was performed in order to quantify the effects of the coolant temperature and fuel properties on the extent of the instability phenomena. As expected, the engine thermal status was found to play an influencing role, with cold starting leading to combustion instability; a difference up to 38 bar between successive cycles was documented. The biodiesel blend exhibited higher instability and the $n$-butanol one showed higher absolute pressures but somewhat more stable operation. Both biofuel blends led to higher in-cylinder pressure irregularities compared to the neat diesel operation. For all examined blends, instability phenomena were still apparent even after several seconds from the engine start-up.

A great deal of energy is consumed annually for domestic heating and cooling. Meanwhile, the policies adopted for the reduction of ${\mathrm{CO}}_2$ emissions and fossil fuel consumption have led to the development of technological solutions based on renewable energy such as biomass and solar, along with the implementation of efficient multigeneration systems. In this study, two trigeneration systems, a microscale system based on a combined organic Rankine cycle (ORC) and a vapor compression cycle (VCC), incorporating a biomass boiler and parabolic trough collectors (PTC), are economically analyzed and evaluated against a heat pump (HP) powered by photovoltaic (PV) panels (PV heat pump). The thermal loads of a building located in Crete, Greece, are considered for the analysis. The efficiency of the ORC ranges between 3.7 and 10.05% for different organic fluids and conditions, while the cogeneration efficiency is up to 73.5%. For the PV heat pump, an average efficiency equal to 15% was calculated. The payback periods of the ORC-VCC and the PV heat pump systems were estimated to be 12.3 and 12.1 years, respectively.

In a deregulated electric power system, it is important to distribute transmission losses between market participants fairly. This paper presents an effective method for active loss allocation by tracing power flow. In this method, after obtaining the alternating current (AC) load flow results of the system, it is first converted to a system in which the power flow is constant along each branch. A new matrix (i.e., power adjacency matrix) is then constructed for the system. The matrix is decoupled, and matrix operations are carried out using the submatrices to derive three other matrices. The contributions of the generators and loads of the main system to active losses of its branches and the shares of the generators in active power consumption of the loads are specified using these matrices. The proposed method is implemented on a four-bus system and several standard test systems. The results show that the method can be applied to any transmission system, and the loss allocation results are preferable to that of previous methods.

Herein is presented an optimization of electrolyte pumping systems in industrial vanadium redox flow batteries (VRBs). The main advantages of the VRB relate to its prolonged service life, outstanding dynamic response, and flexible controllability during charge/discharge processes. In spite of their exceptional electrical parameters, VRBs suffer from significant energy requirements in the hydraulic pumping system needed for circulation of electrolyte from external tanks to battery cells. The industrial VRB equipped for this purpose with centrifugal driven by controllable electric motors hydraulic pumps were made from plastic components and able to work in the aggressive acid electrolyte. Electrolyte flow is carried out through the entire hydraulic system and battery cells containing carbon electrodes and providing electrochemical reduction-oxidation reactions of vanadium ions. Currently, electrodes in the VRB are made from carbon felt material because its porous structure possesses high specific surfaces which provide more reaction sites for the electrochemical reaction and thus a sufficient current (power). In spite of the significant electrode surface area, intensive electrolyte flow is required for eliminating effects of concentrated polarization of inactive vanadium ions in the vicinity of the electrodes’ surface causing a diminution of battery output power. However, carbon felt has significant hydrodynamic resistance, diminishing the overall efficiency of VRBs. This disadvantage could be significantly decreased by the correct selection (optimization) of electrolyte flow rate using as a criterion, a minimum energy surplus comprising hydrodynamic and electrical losses. In the already manufactured VRB, such optimization could be done by the appropriate control of the electrical motors driving electrolyte pumps. However, manufacturers do not inform end-users regarding all main battery components, and partly it looks as an aggregate is assembled from black-boxes. Therefore, this optimization problem seems a rather challenging task requiring a special approach. The present paper proposes an optimization of the available VRB hydraulic system including build-in centrifugal pumps and driving electrical motors based on experimentally determined battery internal electrical resistance and hydraulic losses. The purpose of this optimization is to obtain maximum VRB efficiency.

Identifying the optimal capital structure or optimal debt ratio for a physical project is a critical issue for decision makers who aim to maximize project worth. The debt ratio initially designed by the project company is finally determined during loan negotiations between project company and loan providers. With simulated data from a power plant project, this study conducted a two-party game model to examine the optimal capital structure with various combinations of bargaining powers between project company and loan providers. The results showed that the combination of bargaining powers between participants in the loan negotiation significantly affects the optimal debt ratio. The debt ratio was higher when the project company had more influence on the bargain, and it reached the highest level when the two parties had equal bargaining power.

This paper describes the simulation of a wood gasification household boiler operation during the period that synthesis gas (syngas), which is produced after gasification, is further combusted for the production of heat that is transferred to the boiler heating surfaces. The particular boiler was designed by an Austrian company for the benefit of a Greek company based in northern Greece, which is currently manufacturing the boiler to test and further optimize it. The nominal fuel power of the boiler is approximately $50{\mathrm{kW}}_{\mathrm{th}}$ and its efficiency is about 90 percent (specifically 88.98 percent). The study is oriented toward the use of computational fluid dynamics (CFD) software as a supporting tool during the design phase of a gasification boiler and focuses on the combustion characteristics and the main gas emission concentrations expected at its outlet. In this study, the boiler is simulated under nominal operating conditions, assuming steady-state conditions as these have been defined by the Austrian company. Due to the highly transient and complex nature of wood log gasification, an ad-hoc thermodynamic model describing the associated characteristics of syngas under steady-state conditions has been developed and applied in the proposed CFD approach in order to overcome the highly complex modeling of syngas production. The numerical results are compared against the corresponding 0-D design data, since no available experimental data exist, to ensure the validity of the proposed model. The scope of this work is to achieve a better and deeper understanding of the expected emission characteristics at the boiler outlet and a better estimation of the heat transfer distribution among all different heat exchanging surfaces. However, in the near future, it is expected that the boiler will be tested at the premises of Thermodynamiki S.A (KOMBI) to identify possible discrepancies between the 3D CFD model results and the experimental values.

A major problem for pulverized coal combustion is emissions such as ${\mathrm{CO}}_2$, CO, and ${\mathrm{NO}}_x$. In the oxy-fuel combustion method, a ${\mathrm{CO}}_2─{\mathrm{H}}_2\mathrm{O}─{\mathrm{O}}_2─{\mathrm{N}}_2$ mixture is used for the pulverized coal combustion. In oxy-fuel combustion systems, fuel is combusted in pure oxygen rather than air. Thus, the oxy-fuel combustion process produces low CO and ${\mathrm{NO}}_x$ emissions while air is not suitable for combustion in the oxy-fuel process. In this study, the oxy-fuel combustion method has been investigated for different fuel (pulverized coal) mass flow rates and different mixture recycle rates in terms of ${\mathrm{O}}_2\%/{\mathrm{CO}}_2\%/{\mathrm{H}}_2\mathrm{O}\%$, ${\mathrm{N}}_2\%$ in a lab-scale furnace. The oxy-fuel combustion method has also been investigated for different mixture rates burned of pulverized coal in the lab-scale furnace. In this analysis, the effect of different recirculated flue gas (FG) mixture rates, in terms of ${\mathrm{CO}}_2$, ${\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{O}}_2$ mass/molar fractions, on the flame temperature and ${\mathrm{CO}}_2/{\mathrm{NO}}_x$ emissions was investigated. Numerical analyses using computational fluid dynamics (CFD) of the pulverized coal combustion for different fuel mass flow rates and the FG mass flow rates have been conducted using commercial software. CFD analysis of the oxy-fuel combustion process has been performed by using a realizable $k_{\epsilon }$ turbulence model, multiple surface reactions of char combustion, single rate devolatilization, discrete ordinates (DO) radiation and second-order upwind discretization methods. The firing system of the furnace consists of a three-stage combustion burner. Through the primary stage, coal and recirculated FG streams are injected, while the secondary and tertiary stages inject oxidant medium containing 95% mass fraction and 5% mass fraction ${\mathrm{N}}_2$. The secondary stage fluid flow is a jet flow while the tertiary stage flow is a swirl flow. The effect of different recirculated FG mixture rates was investigated, in terms of ${\mathrm{CO}}_2\%$, ${\mathrm{H}}_2\mathrm{O}\%$, ${\mathrm{O}}_2\%$ ($0/0/21$, $59.5/37.5/3$, $77.6/19.4/3$, $97/0/3$) mass/molar fractions, on the flame temperature and ${\mathrm{CO}}_2/{\mathrm{NO}}_x$ emissions.

The work presents a comparative study of the fuel-air mixing and flame stabilization characteristics of two, high velocity ratio, coaxial jet configurations comprising either a plain circular arrangement or a convoluted trailing edge nozzle. The high velocity ratio, coaxial stream interaction results in a stable central recirculation zone at a distance downstream of the nozzle exit where a range of flame configurations can be successfully stabilized under suitable flow and mixing conditions. In this configuration, the reacting front region is supplied with both a radial and an axial propane-air mixture gradient which are formulated through a central upstream premixer duct and the coannular jet interaction downstream of the exit plane. Depending on the nozzle shape, plain circular or convoluted, a variety of partially premixed turbulent flame configurations can be obtained at a position detached farther from the nozzle exit. Measurements of mixture concentrations, mean temperatures, and flame chemiluminescence images of ${\mathrm{O}\mathrm{H}}^{*}$ and ${\mathrm{CH}}^{*}$ provided information for the interpretation of the relative variations in flame structure and burner performance. Preliminary supporting computational investigations for the cold flow fuel-air mixing patterns and the accompanying mixing wake axial velocities have also been conducted to give an insight into the predominant aerodynamic phenomena involved. The combined methodology helped to elucidate some aspects of the influence of the variable inlet fuel-air mixture distributions and their interaction with the high velocity ratio coaxial jet mixing topologies on the diverse flame configurations studied.

A novel combined cycle system comprising of a supercritical ${\mathrm{CO}}_2({\mathrm{sCO}}_2)$ cycle and a Kalina cycle is proposed in this paper. Exergoeconomic comparison between the combined ${\mathrm{sCO}}_2/\mathrm{Kalina}$ cycle and the ${\mathrm{sCO}}_2$ cycle is investigated. The exergoeconomic models are established, and detailed parametric analyses are conducted for both cycles to reveal the influence of selected decision variables on cycle performance based on the self-build thermodynamic simulation platform. Besides, the parameters of both systems are optimized utilizing the exergetic efficiency and total product unit cost as the objective functions through genetic algorithm (GA). The results reveal that there is an optimal main compressor pressure ratio, an evaporating pressure, and a Kalina turbine inlet temperature that contribute to better system performance. A lower pinch point temperature difference in evaporator and a higher ammonia water mass fraction are beneficial to operation. The nuclear reactor and the ${\mathrm{sCO}}_2$ turbine are the most and second important components from the view of exergoeconomics. The total product unit cost and exergetic efficiency of the combined ${\mathrm{sCO}}_2/\mathrm{Kalina}$ cycle are 5.50% lower and 8.02% higher than the values for the stand-alone ${\mathrm{sCO}}_2$ cycle.

In this paper the performance of a supercritical organic Rankine cycle (ORC) with parabolic rough collectors (PTCs) system is presented using R500 as an organic working fluid driven by the heat transfer fluid (HTF) Therminol VP-1 in a typical environment of Abu Dhabi, United Arab Emirates. An optimization for different working fluids and different HTFs at different locations for electrical power generation is performed. Performance evaluation for off-design conditions, covering the range of part load and overload to the design point, has been carried out using appropriate software. The results show that the locations with higher solar radiation have the best performance. It is found that Jeddah, Saudi Arabia, has the highest solar radiation compared with Munich, Germany; Irbid, Jordan; Riyadh, Saudi Arabia; and Abu Dhabi, United Arab Emirates. Moreover, the organic working fluid with the highest thermal efficiency under the same conditions has the best performance. Five potential applicable organic working fluids are analyzed and it is found that ammonia has the highest thermal efficiency versus R141b, propane, R143a, and R500. Sensitivity analysis shows that a 10% drop in the global radiation ($I_g$) causes a 13.75% increase in the number of collectors ($N$), where the electrical power produced by the generator ($P_{\mathrm{net}}$) and the overall efficiency for the whole system (${\eta }_{\mathrm{ov}}$) decreases by 11.25 and 1.25%, respectively.

During the last decade, the Leadership in Energy and Environmental Design (LEED) rating system has embodied the efforts of the U.S. Green Building Council (USGBC) to recognize buildings designed to achieve superior performance in several areas including energy consumption. Given the emergent interest in improving buildings’ energy efficiency, researchers have generated predictive physical and data-driven models for energy consumption. Although the physical approaches aiming to calculate the energy consumption behavior at the building level are accurate, the necessity of continuously inspecting and gathering data for all the input parameters often makes these approaches impractical in some applications. The objective of this study is to introduce a novel assessment method that investigates the correlation between LEED certification and the actual energy consumption by investigating a case study of LEED-certified research buildings in climate zone 2B. The research approach first consists of developing and testing eight performance models for 13 non-LEED buildings, and then inspecting the fit of the best model to five comparable LEED certified buildings. Heating, cooling, and electricity data are collected from all buildings, in addition to multiple weather, time, and building characteristics variables. The data are used to generate several regression models that predict the energy consumption of conventional research buildings. The results show the differences in energy use between LEED and non-LEED buildings are not as large as anticipated. This paper contributes to the body of knowledge by first introducing a novel generic assessment method for LEED buildings and second applying this method to a case study sample of LEED-certified research buildings in climate zone 2B. The findings of this paper support the actual performance assessment of LEED buildings by helping practitioners make better decisions throughout the project life. In order to ensure a fair and representative assessment of the LEED certification system, future studies are invited to adopt this new method while comparing the actual performance of LEED buildings to that of non-LEED +buildings.