The expansion device is the critical component of micro-to-small scale organic Rankine cycle (ORC) systems, substantially affecting system efficiency and cost. Low isentropic efficiency and ...lubrication requirements are the main issues associated with using volumetric expanders in ORC systems. Despite lubrication contributing to reducing internal leakages in an expander, it may compromise the performance of the ORC system by adversely affecting the evaporator’s thermal capacity. This study tests a recently developed and modified revolving vane expander (M-RVE) in a micro-scale ORC test rig by implementing an adjustable oil mass flow rate. The impact of the lubricant oil on the performance of the M-RVE prototype is investigated within a wide range of oil circulation rates (OCR). The results demonstrate a negligible improvement in the filling factor for OCRs higher than 1%. Moreover, the shaft power is not considerably sensitive to OCR, while the calculated isentropic efficiency of the expander improves with OCR. Furthermore, the impact of the lubricant oil on the performance of the evaporator is studied, assuming the exact OCR as the expander and measured temperature and pressure similar to the pure refrigerant for the lubricant-refrigerant mixture in the evaporator. The study shows that the evaporator capacity is penalized with OCR, especially for values higher than 1%. Hence, an OCR of about 1% is a good compromise, and it can be used as a guideline for designing revolving vane expanders for micro-scale ORC systems without a dedicated lubricant oil circuit.
Latent heat thermal energy storage (LHTES) systems allow us to effectively store and release the collected thermal energy from solar thermodynamic plants; however, room for improvements exists to ...increase their efficiency when in operation. For this reason, in this work, a smart management strategy of an innovative LHTES in a micro-scale concentrated solar combined heat and power plant is proposed and numerically investigated. The novel thermal storage system, as designed and built by the partners within the EU funded Innova MicroSolar project, is subdivided into six modules and consists of 3.8 tons of nitrate solar salt kNO3/NaNO3, whose melting temperature is in the range 216 ÷ 223 °C. In this study, the partitioning of the storage system on the performance of the integrated plant is evaluated by applying a smart energy management strategy based on a fuzzy logic approach. Compared to the single thermal energy storage (TES) configuration, the proposed strategy allows a reduction in storage thermal losses and improving of the plant’s overall efficiency especially in periods with limited solar irradiance. The yearly dynamic simulations carried out show that the electricity produced by the combined heat and power plant is increased by about 5%, while the defocus thermal losses in the solar plant are reduced by 30%.
In this paper four different detailed models of pipelines are proposed and compared to assess the thermal losses in small-scale concentrated solar combined heat and power plants. Indeed, previous ...numerical analyses carried out by some of the authors have revealed the high impact of pipelines on the performance of these plants because of their thermal inertia. Hence, in this work the proposed models are firstly compared to each other for varying temperature increase and mass flow rate. Such comparison shows that the one-dimensional (1D) longitudinal model is in good agreement with the results of the more detailed two-dimensional (2D) model at any temperature gradient for heat transfer fluid velocities higher than 0.1 m/s whilst the lumped model agrees only at velocities higher than 1 m/s. Then, the 1D longitudinal model is implemented in a quasi-steady-state Simulink model of an innovative microscale concentrated solar combined heat and power plant and its performances evaluated. Compared to the results obtained using the Simscape library model of the tube, the performances of the plant show appreciable discrepancies during the winter season. Indeed, whenever the longitudinal thermal gradient of the fluid inside the pipeline is high (as at part-load conditions in winter season), the lumped model becomes inaccurate with more than 20% of deviation of the thermal losses and 30% of the organic Rankine cycle (ORC) electric energy output with respect to the 1D longitudinal model. Therefore, the analysis proves that an hybrid model able to switch from a 1D longitudinal model to a zero-dimensional (0D) model with delay based on the fluid flow rate is recommended to obtain results accurate enough whilst limiting the computational efforts.
Organic Rankine cycle (ORC) systems are some of the most suitable technologies to produce electricity from low-temperature waste heat. In this study, a non-regenerative, micro-scale ORC system was ...tested in off-design conditions using R134a as the working fluid. The experimental data were then used to tune the semi-empirical models of the main components of the system. Eventually, the models were used in a component-oriented system solver to map the system electric performance at varying operating conditions. The analysis highlighted the non-negligible impact of the plunger pump on the system performance Indeed, the experimental results showed that the low pump efficiency in the investigated operating range can lead to negative net electric power in some working conditions. For most data points, the expander and the pump isentropic efficiencies are found in the approximate ranges of 35% to 55% and 17% to 34%, respectively. Furthermore, the maximum net electric power was about 200 W with a net electric efficiency of about 1.2%, thus also stressing the importance of a proper selection of the pump for waste heat recovery applications.
Intending to reduce the related environmental impact of olive oil wastewater while producing new by-products, this research paper proposes an innovative solution for the treatment of wastewater that ...combines microfiltration and ultrafiltration techniques with microalgae cultivation. Laboratory scale analysis and pilot scale operation have been performed to assess the techno-economic viability of the olive mill wastewater for Arthrospira platensis production. More precisely, growth rate, time of division, and characterization (lipids, carbohydrates, proteins, and so forth) of microalgae are evaluated. The results obtained from the techno-economic analysis show that the integration of the systems makes it possible to efficiently exploit the inorganic nutrients of the olive mill wastewater for the cultivation of Arthrospira platensis. In particular, the quality of the obtained biomass complies with the food grade regulations, whereas avoided costs for the olive mill wastewater disposal bring a reduction of 70% in the biomass production cost.
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•Membrane filtration and A. platensis cultivation is effective for the treatment of OMW•A. platensis growth rate is not inhibited by the phenols present in OMW•Avoided cost for OMW disposal reduces about 70% of the biomass production cost•The use of olive mill wastewater for microalgae production is potentially profitable
Currently, the olive oil industry is one of the most relevant agro-food sectors of the Southern Mediterranean area, but the overall production process generates a huge amount of wastewater with a high polluting and phytotoxic content. To reduce the environmental impact of wastewater generated, we propose a novel integrated treatment to obtain (1) organic fertilizer and clean water for agricultural purposes and (2) inorganic nutrients for Arthrospira platensis cultivation. The results of this study show that the integration of the different sub-systems makes it possible to efficiently exploit the inorganic nutrients of the olive mill wastewater to cultivate Arthrospira platensis.
This paper investigated the feasibility of an integrated process that recycles olive mill wastewater while concurrently producing Arthrospira platensis for food applications. From the experimental results obtained at laboratory and pilot scale, Arthrospira platensis produced using olive mill wastewater is feasible with food-grade quality and with a production cost lower than 25 €/kg.
•Modelling approaches for the analysis of micro and small-scales ORC are reviewed.•Peculiarities of micro and small-scales ORC are considered in conducting the review.•Advantages and disadvantages of ...different modelling approaches are provided.•Off-design modelling approaches of the ORC main components are presented.•Assumption-free models for system level modelling are discussed.
Organic Rankine cycle (ORC) systems are a technology capable of producing electricity and heat from a wide range of energy sources and are particularly well-suited for medium and low-temperature sources. However, an almost infinite number of technical solutions (cycle configurations, working fluids, components, etc.) can be adopted making the full experimental characterisation of ORC operations for each application unfeasible. To overcome the limitations of extensive experimental investigations, numerical tools are often adopted, thereby supporting the design and operation of these plants. Therefore, over the last two decades, many researchers have put their efforts into developing models to elucidate the design and off-design performances of ORC systems. In this paper, the different modelling approaches for the analysis of ORC systems are discussed and a conclusive review is performed concerning the micro and small-scale ORCs. In total, more than 160 works are reviewed with many of them related to models of volumetric machines and assumption-based system modelling. Semi-empirical models of expanders show good capabilities and accuracy (with errors below 5%) while spatial resolution methods for heat exchangers are used to better capture the dynamics of the system. However, only a limited number of papers (10) deal with assumption-free models of the systems to predict their performance considering the actual boundary conditions. In summary, the present review paper provides a clear overview of the advantages and disadvantages of each modelling approach at both component and system levels to provide insights for interested readers in the advanced simulation of micro and small-scale ORC systems.
•A HiL framework of a novel integrated solar mCHP plant is developed.•The HiL simulator is designed in Matlab/Simulink and coupled with the real controller of the plant.•The HiL simulator is used for ...control algorithms optimization and control strategy definition.•Optimal proportional and integral gains of the PI-based control are assessed for all the diverters.•The best control strategy to assure constant inlet temperature at the ORC consists in acting on the aperture of a diverter.
In this work a hardware-in-the-loop (HiL) simulator of a novel micro combined heat and power system is presented and its use for control algorithm optimization is demonstrated and discussed. The plant under investigation consists of a concentrated Linear Fresnel Reflectors solar field, a 2 kWe/18 kWt Organic Rankine Cycle unit and an advanced latent heat thermal energy storage tank equipped with reversible heat pipes as developed by a consortium of universities and companies within the EU funded project Innova Microsolar. A smart control unit manages their integration and monitors the operation of each subsystem.
In order to support the optimization of the control algorithms and the definition of the best control strategy of the micro-CHP plant at different working conditions, a simulation framework based on Matlab/Simulink has been developed by the authors and connected to the real control unit according to a HiL approach. Ad-hoc models of the different subsystems together with those of the components (i.e. valves and variable speed pumps) regulating the plant operation have been included.
The use of the HiL simulator has permitted to optimize the control logic of the integrated plant prior to its future commissioning, thus helping to overcome some of the technical and reliability issues occurring during the setup of the real system. In particular, the HiL has allowed: (i) to define the proportional and integral gains of the diverters in order to assure a robust and fast response of the plant during the switch among the different operation modes; (ii) to prove the limits of acting on the oil pump flow rate in assuring the nominal oil temperature at the inlet of the ORC unit, due to the inherent fluctuations caused by this control strategy; and (iii) to assess the best control strategy which is obtained by acting on the aperture of the diverter which controls the oil mass flow rate to the ORC unit. Hence, the scientific approach here proposed can be extended also to many other complex energy conversion systems in order to significantly reduce the potential critical issues during their commissioning.
Solar energy is widely recognized as one of the most attractive renewable energy sources to support the transition toward a decarbonized society. Use of low- and medium-temperature concentrated solar ...technologies makes decentralized power production of combined heating and power (CHP) an alternative to conventional energy conversion systems. However, because of the changes in solar radiation and the inertia of the different subsystems, the operation control of concentrated solar power (CSP) plants is fundamental to increasing their overall conversion efficiency and improving reliability. Therefore, in this study, the operation control of a micro-scale CHP plant consisting of a linear Fresnel reflector solar field, an organic Rankine cycle unit, and a phase change material thermal energy storage tank, as designed and built under the EU-funded Innova Microsolar project by a consortium of universities and companies, is investigated. In particular, a fuzzy logic control is developed in MATLAB/Simulink by the authors in order to (i) initially recognize the type of user according to the related energy consumption profile by means of a neural network and (ii) optimize the thermal-load-following approach by introducing a set of fuzzy rules to switch among the different operation modes. Annual simulations are performed by combining the plant with different thermal load profiles. In general, the analysis shows that that the proposed fuzzy logic control increases the contribution of the TES unit in supplying the ORC unit, while reducing the number of switches between the different OMs. Furthermore, when connected with a residential user load profile, the overall electrical and thermal energy production of the plant increases. Hence, the developed control logic proves to have good potential in increasing the energy efficiency of low- and medium-temperature concentrated solar ORC systems when integrated into the built environment.
•A small-scale CSP system previously tested has been re-engineered and modelled.•A model of the CSP system coupled with a compact desalination plant has been developed.•Performance of the system have ...been evaluated with different operating conditions.•The fresh water production reached a maximum of about 75 L/day in Crendi (Malta).•The integrated system has an interesting potential especially in remote and arid areas.
In this paper, the potential of coupling a small-scale non-imaging focusing heliostat CSP plant with a compact single effect thermal desalination plant is presented and an analytical model for assessing the overall performance is described.
The main novelty of this work relies on the study of two small-scale prototype units designed and tested by the authors. More precisely, the system consists of 25 hexagonal non-imaging heliostats for a total reflective area of 9.5 m2 with a peak power output of 6 kWt and of a single stage thermal desalination plant for fresh water production of few hundred liters per day.
In order to better evaluate the potential of such a coupling, three different locations in the Mediterranean area have been considered: Palma de Mallorca, Pantelleria and Crendi.
The simulation analysis proved interesting results. Compared to electrically driven desalination plant, the performance of the proposed plant is only slightly affected by the salt content of the treated water. In terms of fresh water production, the yearly fresh water production in Crendi is about 60% higher than in Pantelleria because of the higher DNI throughout the year. On the contrary, the maximum daily fresh water production differs to a smaller extent: it reaches a peak of about 70.5 L/day in Pantelleria and 75.0 L/day in Crendi whilst it is always <60 L in Palma.
Despite further improvements are needed to reduce the cost of the fresh water produced by similar small-scale CSP desalination plant to overcome the market uptake, the results of the analysis have shown that such a coupling could have a very interesting potential especially for small-scale applications in rural and remote areas where the fresh water demand ranges between 30 L and 60 L per day.
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