A big amount of the pressure energy content in the natural gas distribution networks is wasted in throttling valves of pressure reduction stations (PRSs). Just a few energy recovery systems are ...currently installed in PRSs and are mostly composed of radial turboexpanders coupled with cogeneration internal combustion engines or gas-fired heaters providing the necessary preheating. This paper clarifies the reason for the scarce diffusion of energy recovery systems in PRSs and provides guidelines about the most feasible energy recovery technologies. Nine thousand PRSs are monitored and allocated into 12 classes, featuring different expansion ratios and available power. The focus is on PRSs with 1-to-20 expansion ratio and 1-to-500 kW available power. Three kinds of expanders are proposed in combination with different preheating systems based on boilers, heat pumps, or cogeneration engines. The goal is to identify, for each class, the most feasible combination by looking at the minimum payback period and maximum net present value. Results show that small size volumetric expanders with low expansion ratios and coupled with gas-fired heaters have the highest potential for large-scale deployment of energy recovery from PRSs. Moreover, the total recoverable energy using the feasible recovery systems is approximately 15% of the available energy.
Preheating is often required to prevent hydrate formation during the pressure reduction process in a natural gas distribution network’s pressure reduction station. This paper examines an energy ...recovery method to avoid the cost and energy consumption of this preheating. The primary aim is to assess the techno-economic feasibility of an energy recovery system based on the Ranque–Hilsch vortex tube coupled to a heat exchanger for large-scale application to the gas grid. To this end, a techno-economic model of the entire energy recovery system was included in an optimisation procedure. The resulting design minimises the payback period (PP) when the system is applied to the pressure reduction stations belonging to a particular gas grid. The pressure reduction stations always operate at an outlet pressure above atmospheric pressure. However, available performance models for the Ranque–Hilsch vortex tube do not permit prediction at backpressure operation. Therefore, a novel empirical model of the device is proposed, and a cost function derived from several manufacturer quotations is introduced for the first time, to evaluate the price of the Ranque–Hilsch vortex tubes. Finally, a nearly complete set of pressure reduction stations belonging to the Italian natural gas grid was chosen as a case study using actual operating parameters collected by each station’s grid manager. The results indicate that the environmental temperature strongly affects the technical and economic feasibility of the proposed energy recovery system. In general, pressure reduction stations operating at an ambient temperature above 0 °C are economically desirable candidates. In addition, the higher the energy recovery system convenience, the higher the flow rate and pressure drop managed by the station. In the Italian case study, 95% of preheating costs could be eliminated with a PP of fewer than 20 years. A 40% preheating cost saving is still possible if the maximum PP is limited to 10 years, and a small but non-negligible 3% of preheating costs could be eliminated with a PP of fewer than 4.5 years.
Planning the best path for the energy system decarbonization is currently one of the issues of high global interest. At the European level, the recent policies dealing with the transportation sector ...have decided to ban the registration of light-duty vehicles powered by internal combustion engines fed by fossil fuels, from 2035. Regardless of the official positions, the major players (industries, politicians, economic and statistical institutions, etc.) manifest several opinions on this decision. In this paper, a mathematical model of a nation’s energy system is used to evaluate the economic impact of this decision. The model considers a superstructure that incorporates all energy conversion and storage units, including the entire transportation sector. A series of succeeding simulations was run and each of them was constrained to the achievement of the decarbonization level fixed, year by year, by the European community road-map. For each simulation, an optimization algorithm searches for a less costly global energy system, by including/excluding from the energy system the energy conversion units, storage devices, using a Mixed Integer Linear approach. Three optimization scenarios were considered: (1) a “free” scenario in which the only constraint applied to the model is the achievement of the scheduled decarbonization targets; (2) an “e-fuels” scenario, in which all new non-battery-electric light-duty vehicles allowed after 2035 must be fed with e-fuels; (3) a “pure electric” scenario, in which all new light-duty vehicles allowed after 2035 are battery-electric vehicles. The comparison of the optimum solutions for the three scenarios demonstrates that the less costly transition to a fully renewable energy system decarbonizes the transportation sector only when the share of renewable energy sources exceeds 90%. E-fueled light-duty vehicles always turn out to be a less expensive alternative than the electric vehicles, mainly because of the very high cost of the energy supply infrastructure needed to charge the batteries. Most of all, the costs imposed to society by the “e-fuels” and “pure electric” light-duty-vehicle decarbonizing scenarios exceed by 20% and 60%, respectively, the “free” transition scenario.
The basic RANS-CFD analysis of the simplest radial-inflow turbine configuration is the subject of this paper. An original technique is here proposed to model the effect of the vaneless spiral casing ...using single-channel CFD calculations and providing an effective alternative to the more complex simulation of the 360-degree domain otherwise required to simulate this turbine configuration. The aim of the paper is to verify the effectiveness of the proposed modelling technique as a reliable engineering approach conceived to support the preliminary design phase of radial-inflow turbines with time-effective CFD calculations. To this end, the open-source CFD code MULTALL has been used to predict the aerodynamic performance of optimal designs of radial-inflow turbines with different specific speed and diameter and working with air as ideal gas. The MULTALL predictions are compared with the corresponding steady-state results obtained by calculations suited to the preliminary assessment of radial turbines designs performed on fully 360-degree turbine domains using the commercial code Star CCM+®. The investigation is conducted on two turbines that are designed in accordance with a widely validated method. The results show that the proposed CFD approach predicts well the trends and values of the aerodynamic performance of both the turbine designs: a 5% overestimation of the performance predicted by the fully 360-degree CFD models was never exceeded. The suggested turbine modelling approach implemented in MULTALL requires a three times lower computation time than the corresponding traditional 360-degree model.
Several configurations of supercritical CO2 (sCO2) power systems have been recently proposed as promising solutions for power generation from medium-temperature heat sources (300 °C – 700 °C). The ...paper exploits the potential of a methodology called “HEATSEP” to review and analyse critically the conceptual development of these configurations, when applied to waste heat recovery. The goal is to find a common thread in the evolution of all sCO2 configurations proposed in the literature for waste heat recovery, and search for new, more performing ones. As required by the HEATSEP method, the study is performed by separating each configuration in two parts: a part, named “basic configuration”, including all components that are necessary to realize the “basic concept” on which the system is based on, and the remaining part associated with the internal heat transfers and the heat exchanger network that realize them. The paper shows that all the “basic configurations” of sCO2 power systems presented in the literature are created by including one or more splitters after the compression process, and then re-joining the separate streams in different manners. All these configurations are included in a general evolution tree, showing how they have evolved logically and why. Starting from the most performing configurations in the literature, the HEATSEP method enables to identify possible power output improvements by exploiting heat transfers not yet considered. Results show that the possible power gains are very small (4 kW out of 480 kW) demonstrating that the literature configurations have negligible margins for improvements.
In the transition towards smart grid systems, a problem of increasing importance is the distributed generation of thermal and electric power at low cost and low environmental impact. This work ...proposes an innovative cogeneration system based on a biomass boiler and a micro-Organic Rankine Cycle (ORC) unit, suitable for application in isolated micro grids. The goal of this study is twofold: i) the analysis of the design choices that were made to achieve a good compromise between efficiency and cheapness of the micro-cogeneration system, and ii) the performance evaluation of the system for variations of key parameters, such as temperature and flow rate of the thermal oil, mass flow rate of the cooling water and operating assets of the ORC unit. Results of the in-depth experimental campaign show that 2250 rpm for the pump and 2300 rpm for the expander are the best combination of speeds to achieve the highest performance. They allow obtaining the maximum values of the ORC electric efficiency (7.3%), energy utilization factor of the cogeneration system (62%) and of the ORC unit (93%). With an oil temperature of about 150 °C, the achieved power production is 2530 W and the efficiency of the expander is 57%.
•The performance of a biomass-fired ORC system has been experimentally evaluated.•The design choices to build a market-ready micro-cogeneration unit are presented.•Key parameters are varied to assess their effect on the operation of the system.•The maximum net electric power and efficiency are 2.2 kW and 7.3%, respectively.
•The influence of the evaporator design on the ORC system dynamics is analysed.•Evaporators with different weights are simulated under realistic driving conditions.•Irregular heat source trends need ...evaporators with higher mass to dampen the fluctuations.•A weight increase of 70% entails a 11% peak reduction of the ORC net power output.
The use of organic Rankine cycle systems for waste heat recovery on heavy-duty vehicles is one of the most effective solutions to reduce the fuel consumption and the environmental pollution of heavy-duty transport. In this application, the variable driving conditions cause such systems to be operated with a highly fluctuating heat source, which must be primarily handled by properly designing the system components and, in particular, the evaporator. This paper investigates the effect of the design parameters of a fin-and-tube evaporator on the dynamic response of the organic Rankine cycle system. The goal is to understand and quantify the dampening effect given by the evaporator design parameters, which influence its weight, and, in turn, its dynamic time response. A finite-volume dynamic model of the evaporator is built in Dymola. Subsequently, the dynamic behaviour of the high-pressure part of the organic Rankine cycle system is simulated based on measurement data of the exhaust gas mass flow rate and temperature from a heavy-duty vehicle taken during a 45-min driving cycle. Simulations are carried out in MATLAB®/Simulink®, by importing the Dymola model as a functional mock-up unit. The results suggest that the larger the heat source fluctuations, the stronger the need to increase the evaporator weight to obtain appreciable dampening effects. The simultaneous variation of the inner diameter of the evaporator tube and the tube spacing leads to the highest dampening effect on the net power output, with a reduction of about 11% of the highest peak value (8220 W).
Energy communities are regulatory tools promoting aggregations of users to foster the shift towards a renewable distributed generation. First in the literature, this paper addresses together three ...main aspects affecting the convenience of these aggregations: the complementarity between generation and demand of different prosumers, the criterion allocating the operating costs of energy communities, and the application of demand-response programs. The goal is quantifying the relative weight of these aspects using Mixed-Integer Linear Programming to minimize the operating costs of citizen and renewable energy communities, where prosumers are connected to the grid as single entity, or separately. Incentive- or price-based demand-response programs and a novel cost allocation criterion, which rewards the members with the highest economic benefit in passing from simple consumers to prosumers, are applied to each community configuration. Results allow identifying general guidelines for the optimal economic operation of energy communities: i) complementarity may reduce costs by 15–20%, ii) a fairer cost allocation criterion may reduce the bills of prosumers using free-of-charge renewables by 20–30% compared to those using dispatchable sources, and iii) price-based demand-response may reduce community costs beyond 50%. Eventually, directions of further research, as the impact of energy communities on a national power system, are drawn.
•Three main aspects affect the aggregation of prosumers in energy communities.•Optimal complementarity of prosumers allows to reduce costs in the range 15–20%.•A novel cost allocation criterion for energy communities is proposed.•Price-based demand response reduces the renewable energy community costs up to 60%.•Citizen energy community has up to 39% lower costs than the renewable energy one.
•A comparison between energy communities modelled as aggregation of prosumers.•A multi-objective optimization of the design and operation of energy communities.•Demand Side Management as a powerful ...tool for improving the energy community design.•Reduction in the cost of energy and CO2 emissions of 14% and 24%, respectively.•Community demand management enhances the exploitation of renewable energy sources.
This paper focuses on three different configurations of energy communities (ECs) modelled as aggregations of local prosumers of renewable electric and thermal energy. The goal consists in improving the economic performance of the ECs while contributing to address the issue of climate change and fulfilling the existing energy demands or demands modified with smart strategies. Accordingly, type and size of the energy conversion and storage units of the prosumers included into ECs (design) and their operation are optimized with a bi-objective approach, considering investment and operation costs, and greenhouse gas emissions, both direct due to fuel burning, and indirect due to life cycle, as objectives to minimize. Moreover, two strategies of demand side management (DSM) are considered: a price-based demand response program applied downstream of the design optimization, and a new DSM model, which adapts the electricity demand to the renewable energy sources locally available, applied upstream of the design optimization. It results that the proposed DSM can ensure a better balancing between generation and demand profiles, thereby decreasing the stress on the electricity grid. Globally, ECs can reduce their energy expenditure of 14% and the overall CO2-equivalent emissions of 24% compared to the reference case of the simple consumers.
•Gas grid working with hydrogen/methane can make dispatchable renewable electricity.•No retrofitting of the grid is required until 50% of RES share.•Till 50% of RES share, it is convenient to use ...hydrogen only to shave the RES peaks.•Hydrogen injected in the grid allows converting renewable electricity into heat.•Gas grid can be used as unique energy storage system till 65% of RES share.
The temporal and geographical availability of renewable energy sources is highly variable, which imposes the importance of correct choices for energy storage and energy transport systems. This paper presents a smart strategy to utilize the natural gas distribution grid to transport and store the hydrogen. The goal is twofold: evaluating the capacity limits of the grid to accommodate “green hydrogen” for preset increasing shares of renewable energy sources (RESs) and determining at the same time the optimal mix of wind, photovoltaic (PV), biomethane and power-to-gas systems that minimizes the investment and operation costs. To this end, the energy supply system of an entire country is modelled and optimized considering the real characteristics and pressure levels of the gas grid, which is assumed to be the only storage mechanism of green hydrogen. The operational concept is to fill up the gas grid with hydrogen during the day and with natural gas during the night while always consuming the natural gas-hydrogen blend. Green hydrogen is generated by electrolysers powered by PVs, wind turbines and biomethane power systems. Results of the optimizations showed that: i) as long as the share of RES does not exceed 20%, there is no need to use the gas grid as RES storage system, ii) from 20 to 50% of RES share the gas grid receives the surplus of electricity in the peaks that would be necessary to “complete” the dispatchability of RES electricity, iii) above 50%, the excess of electricity in the peaks has to be used to generate the thermal energy required by the consumers. The gas grid can be used as unique renewable energy carrier and storage system up to 65% of RES share.
PDF
