Mathematical modelling of operation modes and performance evaluation of an innovative small-scale concentrated solar organic Rankine cycle plant

In this paper an innovative small-scale concentrated solar 2 kWe organic Rankine cycle plant coupled with a phase change material storage tank equipped with reversible heat pipes is investigated using a simulation analysis. The plant, intended for residential applications, is going to be built and tested under the European funded H2020 Innova MicroSolar project executed by the consortium of several Universities and industrial organizations, led by Northumbria University. The authors of this work used the design of the integrated system, developed by the consortium, to preliminary estimate the overall performance of the system in order to provide useful information for its forthcoming real operation. In particular, according to the varying ambient conditions, the influence of different operation modes of the prototype plant are evaluated. The dynamic simulation analysis has shown an interesting performance of the system in terms of annual operating hours, power production and conversion efficiencies. More precisely, the organic Rankine cycle unit is able to operate for more than 3100 h/year, achieving the design performance when solar power is sufficiently high, producing about 5100 kWhe/year. For the considered operating set-point temperatures of the thermal energy storage, the plant is able to reach high conversion efficiency also when the organic Rankine cycle unit is supplied by discharging the energy stored in the storage tank, for about 800 h/year. Hence, the work has provided some useful insights into the best working conditions of such micro combined heat and power system to be integrated in residential buildings. Moreover, the analysis could serve as a general guide for the design and optimization of the mutual interactions of the different subsystems in small-scale concentrated solar organic Rankine cycle plants

Black box modelling of a latent heat thermal energy storage system coupled with heat pipes

This paper presents black box models to represent a LHTESS (Latent Heat Thermal Energy Storage System) coupled with heat pipes, aimed at increasing the storage performance and at decreasing the time of charging/discharging. The presented storage system is part of a micro solar CHP plant and the developed model is intended to be used in the simulation tool of the overall system, thus it has to be accurate but also fast computing. Black box data driven models are considered, trained by means of numerical data obtained from a white box detailed model of the LHTESS and heat pipes system. A year round simulation of the system during its normal operation within the micro solar CHP plant is used as dataset. Then the black box models are trained and finally validated on these data. Results show the need for a black box model that can take into account the different seasonal performance of the LHTESS. In this analysis the best fit was achieved by means of Random Forest models with an accuracy higher than 90%

THERMODYNAMIC ANALYSIS OF THE OFF-DESIGN PERFORMANCE OF A MICRO SOLAR ORGANIC RANKINE CYCLE TRIGENERATION SYSTEM

Organic Rankine Cycle (ORC) systems are considered as one of the most suitable technologies to produce electricity from low-temperature sources. ORC units can efficiently convert low-temperature solar energy into electric and thermal power. Independently from the solar technology used, the hourly and the seasonal fluctuations of solar energy entail challenging dynamic effects and bring these systems to operate in off-design conditions. Such effects are even more influential at micro-to-small scales granting paramount importance to the comprehensive understanding of their behavior. In this study, the annual performances of a 4 kWe/50 kWth solar ORC trigenerative system for residential applications are numerically investigated. Four different modeling approaches commonly used in annual system-level simulations of ORC systems are compared. These models differ in the system-level modeling approach and the components modeling method. The analysis has shown that the simplest ORC model results in the lowest discrepancy compared to the model with the least assumption, in which the components are modeled empirically, and the high and low pressures of the system are found iteratively. The difference between the produced electric energy using the four models is significantly higher in hot months, in which the average temperature of the water tank is high due to the requirements of the vapor generator of the absorption chiller. In this case, the expander pressure ratio drops drastically depending on the system model algorithm, which affects the produced electric power depending on the adopted expander model. On the contrary, the discrepancy between the models for the produced thermal energy is negligible

Fuzzy logic energy management strategy of a multiple latent heat thermal storage in a small-scale concentrated solar power plant

none4noLatent 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%.noneTascioni R.; Arteconi A.; Zotto L.D.; Cioccolanti L.Tascioni, R.; Arteconi, A.; Zotto, L. D.; Cioccolanti, L

Development of a hardware-in-the-loop simulator for small-scale concentrated solar combined heat and power system

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

Numerical investigation of pipelines modeling in small-scale concentrated solar combined heat and power plants

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

Development of a smart control unit for small-scale concentrated solar combined heat and power systems for residential applications

Solar energy has a significant potential for future power generation but its intermittent and variable nature results in fluctuations of the operational performance of solar power plants. Despite thermal energy storage (TES) systems improving the flexibility and the sustainability of the performance of Concentrated Solar Power (CSP) plants, smart management is required to deal with the complex dynamic variations in the behaviour and interaction of the different plant's subsystems. In this paper, the design, manufacture, and validation of a smart control unit with extended capabilities for a small-scale CSP combined heat and power (CHP) system are described. More precisely, the control unit has been designed to control and optimise the operation of a micro-scale Organic Rankine Cycle (ORC) unit coupled with Linear Fresnel Reflectors solar field and an advanced latent heat thermal energy storage tank which were developed by a consortium of universities and companies in the framework of the EU-funded project “Innova Microsolar”. In parallel to the designing and building the smart control unit, an advanced simulator has been developed in Matlab/Simulink® to investigate the performance of the plant for a wide range of varying ambient and operating conditions. The simulation framework has been connected to the real control unit according to a hardware-in-the-loop (HiL) approach to optimise the control logic of the integrated plant to overcome potential technical and reliability issues during the commissioning of the plant. The developed hardware and the proposed scientific approach can be extended to a wide range of complex solar energy systems equipped with TES and to be integrated into the built environment