Multi-objective thermo-economic optimization of biomass retrofit for an existing solar organic Rankine cycle power plant based on NSGA-II

Non-dominated sorting genetic algorithm (NSGA-II) was deployed in this paper for multi-objective thermo-economic optimization of biomass retrofit for an existing solar organic Rankine cycle (ORC) power plant. The existing plant consists of a field of linear Fresnel collectors (LFC), integrated directly with two-tank thermal energy storage (TES) system, which interfaces with ORC power block. The real solar-ORC plant currently runs at Ottana, Italy, albeit with some technical challenges basically due to inconsistent availability of solar irradiation. In order to upgrade the plant, a novel scheme had been proposed to install a biomass unit in parallel to the solar field, such that both LFC/TES and biomass furnace could directly and independently satisfy fractional thermal input requirement of the ORC. Being a retrofit system, existing design parameters of all the already operating units were imposed as equality constraints in this study, and the combustion excess air, as well as pinch point temperature difference of furnace heat exchangers that optimize the hybrid plant were investigated. Results showed that biomass mass flow rate of 0.133 kg/s and investment cost rate of 57 €/h are optimal for the studied biomass retrofit scheme. At this optimum point, excess air was obtained as 56%, furnace heater pinch point temperature difference as 28.8 °C and air pre-heater pinch point temperature difference as 38.5 °C. More generally, results showed that excess air value of less than 100%, furnace heater pinch point temperature difference of less than 80 °C, and air pre-heater pinch point temperature difference of less than 80 °C would optimize the studied biomass retrofit scheme. Keywords: Solar-Biomass power plant, Organic Rankine cycle, Hybrid renewable energy, Multi-objective optimization, Non-dominated sorting genetic algorithm (NSGA-II), Power plant retrofi

Use of weather forecast for increasing the self-consumption rate of home solar systems: An Italian case study

With the aim of increasing the self-consumption rate of grid-connected Photovoltaic (PV) home systems, two main options can be implemented: the inclusion of an energy storage system, in particular a battery bank, and the adoption of a Demand Side Management (DSM) strategy. However, both the reshaping of the load consumption curve with the displacement of deferrable loads and the optimal management of the battery bank require estimation of the daily PV generation profile. The assessment of the on-site energy production can be carried out based on weather forecast data. However, the latter are characterized by uncertainty, which may affect the achievable self-consumption rate. This work investigates the influence of weather forecast errors on the performance of home PV systems equipped with a battery bank and characterized by a certain share of deferrable loads. Two different weather forecast services are considered, referring to the annual meteorological conditions occurring in Rome, and energy consumption data for 150 different households are analysed. The self-consumption rate is maximized by solving a suitable optimization problem, while different combinations of relative battery capacity, PV-to-load ratio and share of deferrable loads are considered. Two different approachesâ\u80\u94deterministic and stochasticâ\u80\u94are adopted and compared with an ideal approach where the PV generation profile is perfectly forecasted. The results show that the adoption of the deterministic approach leads to a reduction in the achievable self-consumption rate in the range of 0.5â\u80\u934.5% compared to the ideal approach. The adoption of a stochastic approach further reduces the deviations from the ideal case, especially in the case of consumption profiles with a high share of deferrable loads. Finally, a preliminary economic analysis proves that the use of a battery bank is not yet a cost-effective solution and a price reduction of the current battery prices is therefore required

Modeling and simulation of an isolated hybrid micro-grid with hydrogen production and storage

Abstract This work relates the study of system performance in operational conditions for an isolated micro-grid powered by a photovoltaic system and a wind turbine. The electricity produced and not used by the user will be accumulated in two different storage systems: a battery bank and a hydrogen storage system composed of two PEM electrolyzers, four pressurized tanks and a PEM fuel cell. One of the main problems to be solved in the development of isolated micro-grids is the management of the various devices and energy flows to optimize their functioning, in particular in relation to the load profile and power produced by renewable energy systems depending on weather conditions. For this reason, through the development and implementation of a specific simulation program, three different energy management systems were studied to evaluate the best strategy for effectively satisfying user requirements and optimizing overall system efficiency

Experimentally-validated models for the off-design simulation of a medium-size solar organic Rankine cycle unit

Organic Rankine Cycle is an efficient and reliable technology for the thermal-to-electricity conversion of low-grade heat sources but the variability in boundary conditions often forces these systems to operate at off-design conditions. The development of reliable models for the performance prediction of organic Rankine cycle power systems under off-design conditions is therefore crucial for system-level integration and control implementation. In this paper, a mathematical model for the evaluation of the expected performance of organic Rankine cycle power units in a large range of operating conditions based on experimental data collected in a medium-size solar organic Rankine cycle power plant is presented. Two different empirical approaches for the performance prediction of heat exchangers and machines, namely, constant-efficiency and correlated-based approaches, are proposed and compared. In addition, empirical correlations based on experimental data are proposed for the preliminary assessment of the energy demanded during the start-up phase and the corresponding duration. Results demonstrate that a good achievement in terms of accuracy of the model and reliability of the simulation performance can be obtained by using a constant-efficiency approach, with average errors lower than 5% and 2.5 K for the expected net power and outlet oil temperature respectively. The use of polynomial correlations leads to a more accurate estimation of the performance parameters used for evaporator and the turbine (in particular the evaporator heat effectiveness and the isentropic and electromechanical efficiency for the turbine), which strongly affect the main output variables of the model and, at the same time, are remarkably influenced by the operating conditions. A reduction in the average error in the prediction of the net power and outlet temperature of the heat transfer fluid to about 4% and 1.5 K respectively is therefore achieved by this approach. Average errors of 18.5% and 12.5% are achieved for the start-up time and the corresponding energy absorbed, respectively. Although the results obtained in terms of accuracy could be improved, these correlations can give an initial indication about the duration and energy required during this phase

Optimal integration of hydrogen-based energy storage systems in photovoltaic microgrids: a techno-economic assessment

The feasibility and cost-effectiveness of hydrogen-based microgrids in facilities, such as public buildings and small- and medium-sized enterprises, provided by photovoltaic (PV) plants and characterized by low electric demand during weekends, were investigated in this paper. Starting from the experience of the microgrid being built at the Renewable Energy Facility of Sardegna Ricerche (Italy), which, among various energy production and storage systems, includes a hydrogen storage system, a modeling of the hydrogen-based microgrid was developed. The model was used to analyze the expected performance of the microgrid considering different load profiles and equipment sizes. Finally, the microgrid cost-effectiveness was evaluated using a preliminary economic analysis. The results demonstrate that an effective design can be achieved with a PV system sized for an annual energy production 20% higher than the annual energy requested by the user and a hydrogen generator size 60% of the PV nominal power size. This configuration leads to a self-sufficiency rate of about 80% and, without public grants, a levelized cost of energy comparable with the cost of electricity in Italy can be achieved with a reduction of at least 25–40% of the current initial costs charged for the whole plant, depending on the load profile shape

Thermocline vs. two-tank direct thermal storage system for concentrating solar power plants: A comparative techno-economic assessment

This paper concerns the ongoing studies on a Concentrated Solar Power (CSP) plant in operation in Ottana (Italy), comprising a 629 kW organic Rankine cycle (ORC) unit fed by a linear Fresnel solar field. Hexamethyldisiloxane (MM) and “Therminol SP-I” are used respectively as ORC working fluid and heat transfer fluid in the solar receivers. A two-tank direct Thermal Energy Storage (TES) system is currently integrated in the CSP plant, serving as a direct interface between solar field and ORC. With the view of improving the solar facility, two alternative TES configurations were proposed in this study: a one-tank packed-bed TES system using silica as solid storage media and another similar one including encapsulated phase-change material (molten salt). Comprehensive mathematical models were developed for simulating daily behaviour as well as for assessing yearly performance of the various TES technologies. Furthermore, a preliminary economic analysis was carried out. Results showed poorer response of the one-tank TES system to large fluctuations in the ORC inlet fluid temperature, leading to reduction in the mean ORC efficiency (18.2% as against 19.7% obtained with the two-tank TES). Conversely, higher energy storage density and lower thermal losses were obtained adopting the one-tank TES, resulting in about 5% more annual solar energy yield. Invariably, equivalent annual ORC energy production of 0.92 GWh/year was obtained for the three TES configurations. Additionally, adopting a one-tank TES system meant that the purchase costs of a second tank and its storage medium (thermal oil) could be saved, resulting in investment costs about 45% lower and, ultimately, levelized cost of storage about 48% lower than what obtains in the two-tank TES system

Life Cycle Analysis of a Hydrogen Valley with multiple end-users

This paper aims to evaluate the environmental impact along the overall life cycle of the various components of a Hydrogen Valley with multiple end-users fed by green hydrogen. As case study, a hydrogen valley including a MW-scale electrolyser powered by different percentages of energy supplied by a wind farm and/or a photovoltaic plant, and an H2 storage section is considered. The H2 produced is used to feed a fleet of fuel cell electric vehicles and a stationary fuel cell, while the residue H2 is injected in a natural gas pipeline considering a maximum safety limit of 5%vol. When the safety limit is reached, the H2 overproduction can be used to produce biomethane through a biological hydrogen methanation process. With the aim of analysing the actual contribution of these hydrogen-based ecosystems towards more sustainable energy systems, a Life Cycle Analysis of the hydrogen valley is carried out. The results show that the final use of hydrogen for fuel cell electric vehicles produces the most valuable environmental benefits. Moreover, Hydrogen Valley solutions integrated with photovoltaic plants allows to maximize the use of H2 in fuel cell electric vehicles and therefore are the most valuable choice from an environmental point of view

Impacts of renewable energy resources on effectiveness of grid‐integrated systems: succinct review of current challenges and potential solution strategies

This study is aimed at a succinct review of practical impacts of grid integration of renewable energy systems on effectiveness of power networks, as well as often employed state‐of-the‐art solution strategies. The renewable energy resources focused on include solar energy, wind energy, biomass energy and geothermal energy, as well as renewable hydrogen/fuel cells, which, although not classified purely as renewable resources, are a famous energy carrier vital for future energy sustainability. Although several world energy outlooks have suggested that the renewable resources available worldwide are sufficient to satisfy global energy needs in multiples of thousands, the different challenges often associated with practical exploitation have made this assertion an illusion to date. Thus, more research efforts are required to synthesize the nature of these challenges as well as viable solution strategies, hence, the need for this review study. First, brief overviews are provided for each of the studied renewable energy sources. Next, challenges and solution strategies associated with each of them at generation phase are discussed, with reference to power grid integration. Thereafter, challenges and common solution strategies at the grid/electrical interface are discussed for each of the renewable resources. Finally, expert opinions are provided, comprising a number of aphorisms deducible from the review study, which reveal knowledge gaps in the field and potential roadmap for future research. In particular, these opinions include the essential roles that renewable hydrogen will play in future energy systems; the need for multi‐sectoral coupling, specifically by promoting electric vehicle usage and integration with renewable‐based power grids; the need for cheaper energy storage devices, attainable possibly by using abandoned electric vehicle batteries for electrical storage, and by further development of advanced thermal energy storage systems (overviews of state‐of‐the‐art thermal and electrochemical energy storage are also provided); amongst others

Integration of pumped thermal energy storage systems based on Brayton cycle with CSP plants

In this paper, the integration of Brayton cycle PTES systems with Concentrating solar power (CSP) plants is proposed and investigated. Specific mathematical models were developed to simulate the PTES and CSP sections as well as to calculate the thermal profiles of the different TES storage tanks during the charging and discharging phases. As case study, an integrated PTES-CSP system using argon as working fluid and characterized by a nominal power of 5 MW and a nominal storage capacity of 4 equivalent hours of operation is considered. The influence of the main design parameters on two performance indexes, namely, the charge-to-discharge efficiencies of the sole PTES section and the integrated PTES-CSP plant, have been investigated. The results demonstrate that the use of high values of pressure ratio is beneficial for the charge-to-discharge efficiency of the integrated plant, even if too high operating pressures could be detrimental for the design of the solar receiver and the high temperature storage tank. The low temperature TES is a critical component due to its cryogenic operating conditions, but an increase in the minimum temperature should be achieved by increasing the inlet temperature of the LP compressor. A sensitivity analysis on the compressor and turbine efficiencies, maximum and minimum temperatures, circuit pressure drop and working fluid has been carried out. Finally, a feasible design of the PTES-CSP system with a PTES roundtrip efficiency of nearly 52% and a charge-to-discharge efficiency of the integrated PTES-CSP plant of about 36% was proposed

Experimental and Numerical Dynamic Investigation of an ORC System for Waste Heat Recovery Applications in Transportation Sector

ORC power units represent a promising technology for the recovery of waste heat in Internal Combustion Engines (ICEs), allowing to reduce emissions while keeping ICE performance close to expectations. However, the intrinsic transient nature of exhaust gases represents a challenge since it leads ORCs to often work in off-design conditions. It then becomes relevant to study their transient response to optimize performance and prevent main components from operating at inadequate conditions. To assess this aspect, an experimental dynamic analysis was carried out on an ORC-based power unit bottomed to a 3 L Diesel ICE. The adoption of a scroll expander and the control of the pump revolution speed allow a wide operability of the ORC. Indeed, the refrigerant mass flow rate can be adapted according to the exhaust gas thermal power availability in order to increase thermal power recovery from exhaust gases. The experimental data confirmed that when the expander speed is not regulated, it is possible to control the cycle maximum pressure by acting on the refrigerant flow rate. The experimental data have also been used to validate a model developed to extend the analysis beyond the experimental operating limits. It was seen that a 30% mass flow rate increase allowed to raise the plant power from 750 W to 830 W