Guest Editorial

J. Energy Resour. Technol. 2016;138(6):060301-060301-1. doi:10.1115/1.4034818.

This Special Issue of Journal of Energy Resources Technology (JERT) contains a selection of reviewed papers from the ECOS 2015 Conference,1 which was held in Pau, France, during June 29th–July 3rd, 2015. The ECOS series of annual conferences was established in 1987 by the ASME Energy Division and serves as a platform for engineers/scientists worldwide to meet annually for discussing developments in the field of energy conversion systems and related environmental, economic, and social issues. ECOS 2015 was its 28th conference.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Conversion/Systems

J. Energy Resour. Technol. 2016;138(6):061601-061601-14. doi:10.1115/1.4032240.

The objectives of this study are to (a) evaluate the exergy and energy demand for constructing a hydrofractured shale gas well and determine its typical exergy and energy returns on investment (ExROI and EROI), and (b) compute the gas flow and intrinsic exergy analysis in the shale gas matrix and created fractures. An exergy system analysis of construction of a typical U.S. shale gas well, which includes the processes and materials exergies (embodied exergy) for drilling, casing and cementing, and hydrofracturing (“fracking”), was conducted. A gas flow and intrinsic exergy numerical simulation and analysis in a gas-containing hydrofractured shale reservoir with its formed fractures was then performed, resulting in the time- and two-dimensional (2D) space-dependent pressure, velocity, and exergy loss fields in the matrix and fractures. The key results of the system analysis show that the total exergy consumption for constructing the typical hydrofractured shale gas well is 35.8 TJ, 49% of which is used for all the drilling needed for the well and casings and further 48% are used for the hydrofracturing. The embodied exergy of all construction materials is about 9.8% of the total exergy consumption. The ExROI for the typical range of shale gas wells in the U.S. was found to be 7.3–87.8. The embodied energy of manufactured materials is significantly larger than their exergy, so the total energy consumption is about 8% higher than the exergy consumption. The intrinsic exergy analysis showed, as expected, very slow (order of 10−9 m/s) gas flow velocities through the matrix, and consequently very small flow exergy losses. It clearly points to the desirability of exploring fracking methods that increase the number and length of effective fractures, and they increase well productivity with a relatively small flow exergy penalty.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):061602-061602-9. doi:10.1115/1.4034547.

Carbon capture and storage (CCS) represents a key solution to control the global warming reducing carbon dioxide emissions from coal-fired power plants. This study reports a comparative performance assessment of different power generation technologies, including ultrasupercritical (USC) pulverized coal combustion plant with postcombustion CO2 capture, integrated gasification combined cycle (IGCC) with precombustion CO2 capture, and oxy-coal combustion (OCC) unit. These three power plants have been studied considering traditional configuration, without CCS, and a more complex configuration with CO2 capture. These technologies (with and without CCS systems) have been compared from both the technical and economic points of view, considering a reference thermal input of 1000 MW. As for CO2 storage, the sequestration in saline aquifers has been considered. Whereas a conventional (without CCS) coal-fired USC power plant results to be more suitable than IGCC for power generation, IGCC becomes more competitive for CO2-free plants, being the precombustion CO2 capture system less expensive (from the energetic point of view) than the postcombustion one. In this scenario, oxy-coal combustion plant is currently not competitive with USC and IGCC, due to the low industrial experience, which means higher capital and operating costs and a lower plant operating reliability. But in a short-term future, a progressive diffusion of commercial-scale OCC plants will allow a reduction of capital costs and an improvement of the technology, with higher efficiency and reliability. This means that OCC promises to become competitive with USC and also with IGCC.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):061603-061603-9. doi:10.1115/1.4034589.

In an attempt to improve the performance of hybrid absorption and mechanical vapor compression refrigeration systems and to determine the fundamental reasons for such improvements, two configurations of the hybrid refrigeration cycle with a booster compressor at different positions of the cycle (between the evaporation and the absorber, or between the generator and the condenser) are simulated and analyzed. The interrelation between the two subcycles and the hybridization principle have been explored and clarified. An NH3/H2O-based hybrid cycle is the basis of this simulation. It was found that (1) the hybrid cycle performance is mainly governed by the interaction between its two subcycles of mechanical compression and thermal compression and their respective energy efficiencies, and (2) the hybrid cycle primary energy-based coefficient of performance (COP) was higher by up to 15% (without internal heat recuperation) as compared with the nonhybrid absorption cycle, (3) in comparison with the nonhybrid absorption and vapor compression cycles working in the same temperature regions, the more efficient use of low-temperature heat by cascade utilization of the two energy inputs (heat rate and mechanical power) with different energy quality, and the enhanced refrigeration ability of low-temperature heat are the basic reasons for the hybrid cycle performance improvement and significant energy saving, (4) the hybrid cycle achieves an exergy efficiency of 36.5%, which is 27% higher than that of the absorption cycle, and 4.5% higher than the vapor compression cycle, achieving a thermal-driving exergy efficiency of 37.5% and mechanical work saving ratio up to 64%.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):061604-061604-5. doi:10.1115/1.4034590.

The concept of the pro-ecological tax or support is proposed in this article. It could replace the existing subsidy mechanisms for renewable power technology or could represent the ecological taxes burdening the fabrication of consumer products based on nonrenewable resources, e.g., electricity from the conventional fossil fuel power plants. Moreover, the proposed concept for renewable power technologies could represent a new system of subsidy based on physical laws. The proposed pro-ecological tax or support is defined as proportional to the thermo-ecological cost (TEC). TEC expresses the cumulative consumption of nonrenewable exergy burdening the fabrication of the considered consumption of the product with the additional inclusion of necessity of compensating adverse environmental effects due to harmful waste products rejection. It is assumed that calculating new exergy tax or subsidy for electricity, the total cost burdening the final consumer remains the same. This principle determines the coefficient of proportionality between the cumulative consumption of nonrenewable exergy (TEC) and the value of the tax or subsidy. In the case of nonrenewable electricity generation, the TAX causes the largest price growth. The higher is the efficiency of power technology, and the lower is the TEC of fuel, the lower tax is provided. In this paper, the prices of electricity produced from renewable and nonrenewable resources are calculated, taking into account the unavoidable consumption of resources. The unavoidable exergy consumption is caused due to the power plant construction, which is distributed through the whole life cycle of the plant. Results of calculation of proposed subsidies are compared with the current Polish system for supporting renewable power technologies.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2016;138(6):062001-062001-8. doi:10.1115/1.4033399.

The large-scale production of renewable energy is limited by the intermittence nature of the renewable energy sources. Moreover, the electricity production of the thermal and nuclear power plants is not flexible with the electricity demand. Hence, the integration of energy storage technologies into the grid has become crucial as it creates a balance between supply and demand for electricity and protects thereby the electrical grid. Among the large-scale energy storage technologies, a novel adiabatic compressed air energy storage (A-CAES) system will be developed in this paper. This storage system is characterized, compared to the conventional compressed air energy storage (CAES) system, by the recovery and the reuse of the compression heat in order to improve the system efficiency and avoid the use of fossil fuel sources. This paper discusses a comparison between the static and dynamic modeling of the A-CAES system performed by a computer simulation using “Modelica.” Unlike the static model, the dynamic model takes into account the mechanical inertia of the turbomachinery (compressors and turbines) as well as the thermal inertia of the heat exchangers. Consequently, it enables studying the flexibility of the storage system and its ability to meet the electrical grid requirements (primary and secondary reserves) by evaluating the duration of the transient states. Furthermore, the comparison between the static and dynamic models permits to estimate the efficiency losses due to the transient evolutions.The results show that the storage system needs more than 2 min before being able to consume all the excess energy available on the electrical grid and more than 5 min before being able to produce all the energy required by the electrical grid. These time frames are due especially to the transient states (start-up) of the turbomachines. Finally, the system efficiency is 64.7%, the transient states of the system cause losses of 0.9%. These small losses are explained by the short duration of the transient states relative to that of the steady states (15 hrs).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):062002-062002-7. doi:10.1115/1.4033587.

Vortex-induced energy converters (VIECs) are attracting the attention of researchers looking for energy-harvesting systems in the marine environment. These energy converters, while probably less efficient than many other specialized devices, have very few moving parts and are particularly suitable for operation in harsh environments, such as those encountered in the ocean and in offshore platforms. The principle of operation of VIECs is tapping the transverse vibration of a blunt slender body immersed in a stream, induced by unsteady flow separation (Von Karman vortex street). The simplest device is an array of cylinders: under specific conditions and with careful design, it is possible to work close to resonance and thereby to obtain large amplitudes of oscillation, which are converted into electricity by suitable devices (linear electrical generators or piezoelectric cells). The system was developed experimentally at University of Michigan, with several patents pending and scientific material published on preliminary tests. Numerical simulations of system dynamics allow to simulate more realistic operating conditions and to perform the mechanical optimization of the system in relation to a specific sea location. A model of the system was thus developed, resulting in a nonlinear dynamic mathematical formulation; this last is solved in the time domain using matlab/simulink programming. The sensitivity of the efficiency to the main design variables is investigated. The results demonstrate that the efficiency and power density are not attractive for the typical Mediterranean Sea conditions; however, as energy can be harvested over large surfaces, the system appears to deserve attention.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):062003-062003-7. doi:10.1115/1.4033303.

The aim of this work is to study a binary Rankine process with a significantly higher efficiency compared to a conventional coal-fired power plant. This paper focuses on the design of the process and especially on an efficient combination of flue gas, potassium, and water streams in the components of the steam generator, such as economizers, evaporators, and superheaters, to decrease the overall exergy destruction. Based on a literature review, a base case for a coal-fired binary Rankine cycle with potassium and water as working fluids was developed and, in order to evaluate the thermodynamic quality of several variants, comparative exergy analyses were conducted. A simulation of the process and calculation of the values for the streams were carried out by using the flow-sheeting program CycleTempo, which simultaneously solves the mass and energy balances and contains property functions for the specific enthalpy and entropy of all the substances used. Necessary assumptions are predominantly based on literature data or they are discussed in the paper. We present the exergy analysis of the overall process that includes the flue gas streams as well as the potassium and water cycles. A design analysis and sensitivity studies show the effects of stream combinations and key parameters on the net efficiency, which is higher than 50%.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):062004-062004-10. doi:10.1115/1.4033677.

Mining operations are located in increasingly remote areas in order to search for relatively high-grade mineral deposits, despite the challenges that arise. These challenges are fundamentally logistic and directly impact the profitability of the remote operation. One of the main challenges is energy supply, since locations that lack a power grid, fuel pipelines, or adequate—if existing—road access have substantially increased energy-related operating costs. Today, a remote mine's energy costs add up to 40% of total operating expenses; this is in contrast with grid-connected, accessible mines, where the energy costs seldom reach 20% of the total. In searching for more cost-effective energy supply options, the present work uses the optimal mine site energy supply (OMSES) concept to optimize the design and operation schedule of a remote underground mine's energy supply system (ESS). Energy demand, weather, and economic data were collected and processed, emulating a remote mine in the Northwest Territories, Canada. The optimal energy system minimized the total cost of the energy supply, which included not only the operation cost but also the annuitized capital investment in equipment. Subsequently, the optimal system's design for the considered demands and environmental factors was subject to simulation and control optimization. Wind power was included in the formulation. Issues such as the necessary spinning reserve and the penetration curtailment, among others, were analyzed, both in the design and the control problems. The present work identified potential improvements for the integrated design (ID) and control of a remote mine's energy system, in particular when including a renewable energy resource with a considerable level of variability, i.e., wind. The optimal solution included the installation of two wind turbines (WTs), achieving 3% diesel savings with a 20% increase of investment compared with the conventional design. The model was validated with a real project—the Diavik Diamond Mine ESS, which included a wind farm with four turbines. A model predictive control (MPC) approach was chosen to optimize scheduling in a simulation with variable conditions of wind speed and ambient temperature; this proved to be a convenient method to assess the robustness of optimal designs. Results also confirmed the limitations of design optimization when uncertainties related to wind energy were ignored.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):062005-062005-11. doi:10.1115/1.4033623.

After air and water mixing, the process of gas compression in the downcomer shaft or pipe of a hydraulic air compressor is considered nearly isothermal due to (i) the mass flow rate of water being typically of three orders higher than that of the gas it compresses, (ii) water having a heat capacity approximately four times that of air, and (iii) the intimate contact and large heat transfer area between the gas phase and the liquid phase of the bubbly flow. A formulation for estimation of the efficiency of a closed- or open-loop hydraulic air compressor, expressed in terms of the principal hydraulic air compressor design variables, is presented. The influence of a hitherto underappreciated factor affecting the performance of these installations, such as the solubility of the gas being compressed in the water, is explored. A procedure for estimating the yield of compressed gas, accounting for these solubility losses, is explained and used to determine the mechanical efficiency of historical hydraulic air compressor installations from reported performance data. The result is a significant downward revision of hydraulic air compressor efficiency by approximately 20% points in comparison to most reported efficiencies. However, through manipulation of cosolute concentrations in the water, and the temperature of the water (through regulation of the ejection of compression heat), the mechanical efficiency can be increased to the formerly reported levels. The thermo-economic implication of these efficiency determinations is that in a modern context, hydraulic air compressors may be able to outperform conventional mechanical gas compression equipment.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2016;138(6):062006-062006-6. doi:10.1115/1.4034068.

The direct heat exchange network (direct flow mixing network) and the indirect heat exchange network (exchanger network) are two of the elements that constitute a water network where heat and mass are transferred. When designing these systems, it is important to consider different aspects such as thermodynamics and equipment costs. This paper analyzes different design options within the framework of heuristic methodologies on a case study taken from the open literature. Two design methodologies are compared on the basis of exergy losses. It also enunciates a series of considerations in heuristic design for the heat and mass exchange networks. A very helpful tool in relation with the considerations set out herein is the composite curve; special focus will be given during its construction. This paper shows how to incorporate the exergy component in design seeking to minimize the thermal irreversibility.

Commentary by Dr. Valentin Fuster

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