Guest Editorial

J. Energy Resour. Technol. 2018;140(2):020301-020301-1. doi:10.1115/1.4039139.

This special issue of the Journal of Energy Resources Technology (JERT) is dedicated to the 29th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS), which was held in Portoroz, Slovenia, from June 19 to 23, 2016. The ECOS series of annual conferences was established in 1987 by the ASME Advanced Energy Systems Division. These conferences offer a platform to engineers and scientists worldwide to meet annually and discuss developments in the fields of conversion, storage, and transportation of energy, particularly the related thermodynamic, environmental, economic, and social issues.

Commentary by Dr. Valentin Fuster

Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2017;140(2):020901-020901-10. doi:10.1115/1.4038462.

Turbine inlet temperatures (TIT) of 1500–2000 K have become a sort of standard for most modern advanced applications. First-stage blades are obviously the most exposed components to such hot gases, and thus they need proper cooling. In the preliminary design of the blades and their cooling system, designers must rely on simple models that can be further refined at a later stage, in order to have an approximate but valuable set of guidelines and to reach a feasible first-order configuration. In this paper, a simple lumped thermodynamic model of blade cooling is proposed. It is based on mass/energy balances and heat transfer correlations, and it predicts a one-dimensional temperature profile on the blade external surface along the chord for a given gas temperature profile, as well as the required cooling air flow rates to prevent blade material from creep. The greatest advantage of the model is that it can be easily adapted to any operating condition, process parameter, and blade geometry, which makes it well suited to the last technological trends, namely, the investigation of new cooling methods and alternative coolants instead of air. Therefore, the proposed model is expected to be a useful tool in the field of innovative gas turbine cycle analysis, replacing more computationally intensive and very time-consuming models.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020902-020902-10. doi:10.1115/1.4037900.

The economical and clean environment issues for a sustainable energy source at low temperature (LT) were considered and compared to natural gas technology as a fossil fuel source. The friendly environment refrigerants R410A, R407C, R717, R134a, and R600a were analyzed in an approximately 500 kW heating load output cascade heat pump. The heat pump was investigated at an intermediate temperature of 35 °C, high temperature (HT) cycle condenser at 70 °C, and compressors isentropic efficiency of 70%. All analyzed refrigerant pairs exhibited high heating season performance factor (HSPF), and it was ranged between 7 and 8.5. The thermal performance comparison revealed that the HSPF for R717/R600a showed the highest values among other refrigerant pairs. The results showed that at LT cycle evaporator temperature range of −10 to −2 °C, the natural gas technology revealed a higher season heating cost values than that of the heat pump plant by up to 10%. On the contrary at lower LT evaporator temperature, the heat pump plant technology exhibited a higher season heating cost lied in the range of 4–13.6% than that of the natural gas system. At compressors isentropic efficiency of 90%, the seasonal heating cost of the heat pump plant was lower than that of the natural gas technology by the range of 9–25% at test conditions. The mean seasonal CO2 amount released by the natural gas firing technology from all tested refrigerant pairs ranged between 2.1 and 2.5 times that of the heat pump plant technology for the investigated LT evaporator temperature range.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020903-020903-7. doi:10.1115/1.4038118.

Air pollution can have detrimental effects on gas turbine performance leading to blade fouling, which reduces power output and requires frequent cleanings. This issue is a fairly well-known phenomenon in the power industry. However, site selection for gas turbine installation on the basis of air quality is rarely part of the decision-making process, mainly due to lack of geographical options especially in an urban environment or perhaps due to a simple assumption that air quality at a local micro-level has no impact on the performance of the engine. In this paper, we perform a computational fluid dynamics (CFD) study on an area surrounding a combined heat and power (CHP) facility to assess the impact of local wind distribution on air quality and the performance of a gas turbine engine. Several aerodynamic properties are suggested as possible indicators of air quality and/or high airborne particulate concentration. These indicators are then compared to data collected at various points in and around the site. The results suggest that through post-processing of a simplified CFD simulation analyzing the adjacent terrain, a continuous map of field variables can be obtained and help designers locate future CHP or gas turbine power plants in regions of lower particulate concentrations. This, in turn, would greatly increase efficiency and cost-effectiveness of the proposed power plant.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020904-020904-7. doi:10.1115/1.4038197.

Renewable energy sources (RES) are quite capable to actively contribute to meet the today's energy demand. However, many of them have a time-dependent nature that constitutes their major disadvantage. To overcome this drawback, energy storage systems (ESS) need to be set up. In this way, the stored energy can be used in the absence of RES or under peak demand hours. High-temperature pumped thermal electricity storage (PTES) using packed bed constitutes an attractive solution but is characterized by high losses and irreversibilities. For this reason, in this paper, a new plant scheme is presented and its mathematical model built up. To predict the packed bed behavior, a one-dimensional two phase model of the hot and cold storages has been included, while the plant feasibility is evaluated using an energy and a cost analysis. Results show that the highest quantity of energy and round-trip efficiency are reached with a packed bed made of magnetite and titanium oxide.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020905-020905-9. doi:10.1115/1.4038312.

Large district heating networks greatly benefit from topological changes brought by the construction of loops. The overall effects of malfunctions are smoothed, making existing networks intrinsically robust. In this paper, we demonstrate the use of topology optimization to find the network layout that maximizes robustness under an investment constraint. The optimized design stems from a large ground structure that includes all the possible looping elements. The objective is an original robustness measure, that neither requires any probabilistic analysis of the input uncertainty nor the identification of bounds on stochastic variables. Our case study on the Turin district heating network confirms that robustness and cost are antagonist objectives: the optimized designs obtained by systematically relaxing the investment constraint lay on a smooth Pareto front. A sudden steepness variation divides the front in two different regions. For small investments topological modifications are observed, i.e., new branches appear continuously in the optimized layout as the investment increases. Here, large robustness improvements are possible. However, at high investments no topological modifications are visible and only limited robustness gains are obtained.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020906-020906-13. doi:10.1115/1.4036544.

The most critical component of an absorption heat transformer (AHT) is the absorber, by which the exothermic reaction is carried out, resulting in a useful thermal energy. This article proposed a model based on improving the performance of energy for an absorber with disks of graphite during the exothermic reaction, through an optimal strategy. Two models of artificial neural networks (ANN) were developed to predict the thermal energy, through two important factors: internal heat in the absorber (QAB) and the temperature of the working solution of the absorber outlet (TAB). Confronting the simulated and real data, a satisfactory agreement was appreciated, obtaining a mean absolute percentage error (MAPE) value of 0.24% to calculate QAB and of 0.17% to calculate TAB. Furthermore, from these ANN models, the inverse neural network (ANNi) allowed improves the thermal efficiency of the absorber (QAB and TAB). To find the optimal values, it was necessary to propose an objective function, where the genetic algorithms (GAs) were indicated. Finally, by applying the ANNi–GAs model, the optimized network configuration was to find an optimal value of concentrated solution of LiBr–H2O and the vapor inlet temperature to the absorber. The results obtained from the optimization allowed to reach a value of QAB from 1.77 kW to 2.44 kW, when a concentrated solution of LiBr–H2O at 59% was used and increased the value of TAB from 104.66 °C to 109.2 °C when a vapor inlet temperature of 73 °C was used.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020907-020907-8. doi:10.1115/1.4038461.

Combined heat and power (CHP) plants are efficient regarding fuel, costs, and emissions compared to the separate generation of heat and electricity. Sinking revenues from sales of electricity due to sinking market prices endanger the economically viable operation of the plants. The integration of heat pumps (HP) and thermal energy storages (TESs) represents an option to increase the flexibility of CHP plants so that electricity can be produced only when the market conditions are favorable. The investigated district heating system is located in Germany, where the electricity market is influenced by a high share of renewable energies. The price-based unit-commitment and dispatch problem is modeled as a mixed integer linear program (MILP) with a temporal resolution of 1 h and a planning horizon of 1 yr. This paper presents the optimal operation of a TES unit and a HP in combination with CHP plants as well as synergies or competitions between them. Coal and gas-fired CHP plants with back pressure or extraction condensing steam turbines (STs) are considered, and their results are compared to each other.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;140(2):020908-020908-10. doi:10.1115/1.4038378.

Liquid air energy storage (LAES) is a technology for bulk electricity storage in the form of liquid air with power output potentially above 10 MW and storage capacity of 100 s MWh. In this paper, we address the performance of LAES and the experimental evidences gathered through the first LAES pilot plant in the world developed by Highview power storage at Slough (London) and currently installed at the University of Birmingham (UK). We developed a numerical model of LAES plant and carried out an experimental campaign to gather new results which show the LAES operating principles, the reliability of the technology, the startup/shut down performance, and the influence of operational parameters. In summary, this work (a) contributes to the advancement of thermomechanical storage systems, (b) provides new experimental evidences and results for LAES technology, and (c) highlights the crucial aspects to necessarily improve the performance of LAES.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(2):020909-020909-12. doi:10.1115/1.4038666.

In the course of the “Energiewende,” the German electricity market is undergoing major changes. The state-aided priority of renewable generation has led to a significant decline in electricity prices. This reduces the profit margin of cogeneration units and increases the necessity of flexible operation to avoid electricity production when spot prices drop below marginal costs. In this work, a 100 MWel combined-cycle (CC) power plant supplying heat and power to a paper mill is investigated. Currently, the plant is operated heat-controlled and is therefore unable to react to changing electricity spot prices. With the integration of heat storage, the plant is enabled to switch to power-controlled mode. To evaluate the technical impact of the storage, the plant and a thermochemical MgO/Mg(OH)2 storage are modeled using the stationary process simulation tool ebsilon professional. Different operation modes are investigated and results are used to derive a mixed integer linear programming (MILP) model to optimize the operation of the plant/storage system. Using this method, the overall economic impact of the storage on the plant operation is quantified.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(2):020910-020910-7. doi:10.1115/1.4038930.

Nuclear energy is one of the possibilities ensuring energy security, environmental protection, and high energy efficiency. Among many newest solutions, special attention is paid to the medium size high-temperature gas-cooled reactors (HTGR) with wide possible applications in electric energy production and district heating systems. Actual progress can be observed in the literature and especially in new projects. The maximum outlet temperature of helium as the reactor cooling gas is about 1000 °C which results in the relatively low energy efficiency of the cycle not greater than 40–45% in comparison to 55–60% of modern conventional power plants fueled by natural gas or coal. A significant increase of energy efficiency of HTGR cycles can be achieved with the increase of helium temperature from the nuclear reactor using additional coolant heating even up to 1600 °C in heat exchanger/gas burner located before gas turbine. In this paper, new solution with additional coolant heating is presented. Thermodynamic analysis of the proposed solution with a comparison to the classical HTGR cycle will be presented showing a significant increase of energy efficiency up to about 66%.

Commentary by Dr. Valentin Fuster

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