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

innovative coupling of cogeneration units with fire tube boilers thermo fluid dynamics of the fire tubes

Nowadays the thermal energy demand in the industrial sector is usually satisfied by means of fire tube boilers while electricity is supplied from the grid. Alternatively cogeneration units could be adopted for thermal and electrical energy self-production, whilst installing boilers only as back-up units. However, even when cogeneration is profitable, it is not widespread because industries are usually unwilling to accept cogeneration plants for reliability and high investment costs issues. In this work a system aimed at overcoming the above mentioned market difficulties is proposed. It consists of an innovative coupling of a combined heat and power unit with a modified fire tube boiler. In particular, a CFD analysis was carried out by the authors in order to address the most critical aspects related with the coupling of the two systems. More precisely, the following aspects were evaluated in detail: (i) pressure losses of the exhausts going from the prime mover to the boiler due to the sudden cross-section area variations; (ii) thermal power recoverable from the exhausts in the tubes of the boiler; (iii) dependence of the system on the final users' specification

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%

Analysis of labour market needs for engineers with enhanced knowledge in sustainable renewable energy solutions in the built environment in some Asian countries

Despite the rapid growth in the uptake of renewable energy technologies, the educational profile and the skills gained at University level do not always comply with the practical needs of the organisations working in the field. Furthermore, even though the residential sector has very high potential in curbing its CO2 emissions worldwide thus meeting the challenging goals set out by the international agreements, such reduction has been limited so far. Within this context, the 'Skybelt' project, co-funded by the EU under the framework of the Erasmus + programme aims at enhancing in several Universities of Asia and Europe the engineering skills of students of all level for application of sustainable renewable energy solutions in the built environment. With the target of increasing the employability of graduates and the impact of the project, a survey on the labour market needs for specialists with enhanced knowledge and skills in the topic of the project has been conducted in the related Asian countries. Hence, relevant industries, labour market organisations and other stakeholders have been interviewed and the main results of this analysis is reported in the present paper. As first outcome of this activity, the obtained results have been considered in the selection of the modules to be improved according to a student centred study approach

Modelling approaches of micro and small-scale organic Rankine cycle systems: A critical review

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 150 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

Experimental analysis of a pulse tube based new prototype for cells cryopreservation

Cells cryopreservation is crucial for the treatment of several diseases, but the survival rate of the cells is significantly affected by the cooling process. Currently, programmable freezers based on liquid nitrogen technology are usually adopted but these solutions may cause the death of the cells due to undesired crystallization, membrane damage or osmotic shock. In the recent years, pulse tube refrigerators have attracted a lot of interest in many applications because of their intrinsic characteristics. Despite more gradual, the cooling rate of a similar refrigerator needs to be carefully controlled to meet the desired requirements of cells cryopreservation. Therefore, at the premises of Sapienza University of Rome a pulse tube-based prototype has been designed for cells cryopreservation and an experimental tests campaign has been conducted to assess the performance of the system for the scope. A new control logic, able to adjust the supplied voltage to electric heaters for the conditioning of the temperature inside the stand tubes, has been implemented and different configurations evaluated with cooling rate varying in the range 0.5°C/min to 1.5°C/min. The analysis has shown that the proposed control logic is able to cool down the stem cells in all the investigated range with a maximum temperature difference between the mean temperature of the tubes and the theoretical temperature of −7.65°C for the configuration with copper plate and −4.09°C for the configuration with aluminium plate which represents a safe condition. On the contrary, the copper plate allows approximating better the real cooling curve with the theoretical one and achieving a lower temperature variance at cooling rates higher than 1.25°C/min. Although some further efforts are needed to tune the system up, the present work has demonstrated that a pulse tube refrigerator can be technically and commercially adopted as a viable solution for stem cells cryopreservation

Experimental modeling of a lubricated, open drive scroll expander for micro-scale organic Rankine cycle systems

Scroll compressors converted into expanders represent a good choice in micro-to-small-scale organic Rankine cycle (ORC) applications. Several studies have tested and modeled scroll expanders in ORC systems in the last decade to assess their performance and reliability in off-design conditions. In this work, SANDEN TRS090F scroll compressor is converted into an expander and tested in a micro-scale ORC system using R134a for power generation from low-grade heat. The experimental data are used to modify the available semi-empirical model in the literature considering a polytropic expansion, a more detailed suction pressure drop model, and a variable loss power correlated to the expander pressure ratio. In addition, the two known expander geometrical parameters, the built-in volume ratio, and the swept volume are considered as fixed inputs to the model instead of being determined as part of the model results. In general, the results of the analysis show that the proposed model can predict the expander's overall performance with good accuracy in different operating conditions. The maximum deviation between the model results and most of the measurements is 5% for the mass flow rate and the shaft power, 15% for the overall isentropic efficiency, and 3 K for the discharge temperature. Then, the impact of the different losses is presented, and finally, the validated model is used to generate the performance maps of the studied expander at different working conditions

Assumption-free modeling of a micro-scale organic Rankine cycle system based on a mass-sensitive method

Organic Rankine cycle (ORC) systems are one of the most suitable technologies to produce electricity from low-temperature sources. In this paper, the main components of a non-regenerative, micro-scale ORC unit are modeled using the experimental results. These components are then used as functions in the system-level solver developed in MATLAB© to predict the performances of the system at off-design conditions. The proposed system solver is based on a novel approach, in which no assumptions are made about the system's state, and only the components’ specifications and the real system boundaries that an operator encounters are adopted as inputs. To this end, the conservation of mass is considered in addition to the conservation of energy in the modeling of the system. Using the assumption-free model, the performances of the ORC system are mapped in the range of the experimental data considering the pump and the expander speeds as variables. The results show that the optimum system net electric performance is achieved at the pump and the expander speeds of 400 rpm and 900 rpm approximately. However, the pump is prone to the risk of cavitation due to low subcooling at the condenser outlet at this condition. Moreover, zero superheating is calculated at the expander suction that is not recommended for its operation. Hence, the developed assumption-free, object-oriented, mass-sensitive model has led to the full understanding of the system limitations and losses in the case of waste heat recovery applications. The proposed approach could be extended also to other ORC systems thus mapping their performances at off-design conditions without making artificial assumptions

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