Pressure retarded osmosis (PRO) process and its significance for thermosyphon technology is discussed. In previous work the possibility to drive a thermosyphon by difference of densities from induced salinity gradients from evaporation at solar collectors with downward heat transfer was assessed. Nevertheless it was concluded that large evaporative areas or dilution with volatile compounds was mandatory. In this work it is shown that by taking advantage of the energy released during the spontaneous mixing of the low-concentration evaporative fraction and the high-salinity of the no-evaporated fraction which is generally referred as pressure-retarded osmosis (PRO) process, then the thermosyphon can runs with downward heat transfer (hot fluid flows down and cold fluid rises up) and using an evaporative surface area much more smaller and then eliminating the need for dilution with high volatile compounds.
The basis of a novel method for passive solar water heating homologous to the thermosiphon but driven by induced salinity, which causes a fluid to circulate without the need for a mechanical pump and with inverse natural convection (downward heat transfer), is outlined. The brinesiphon, like the thermosiphon, operates by harnessing the tendency of a less dense fluid to rise above a denser fluid, resulting in fluid motion through a collector, but with two exceptions: first, the
buoyancy is controlled by induced salinity gradients rather than thermal gradients, and second, as a result, natural convection is in the opposite direction than that in the homologous thermosiphon concept; i.e., hot fluid flows down, and cold fluid rises. A brinesyphon may be more suitable for solar domestic water heating systems than the thermosiphon because the direction of flow allows downward transfer from a solar collector to a lower storage tank without any type of mechanical pumping system.
The aim of this paper is to present an analysis of long term outdoor exposure of two thin film photovoltaic (TFPV) module technologies deployed in semi-arid climate in Saida city located in Algeria. The TFPV modules are: a-Si:H/µc-Si:H (micromorph) and copper indium selenide (CIS). The TFPV modules were characterised by measuring their I-V curves during three years under the same outdoor climate conditions, where the measurement of weather parameters were also performed. The goal of the analysis is to evaluate the degradation rates of the TFPV modules in semi-arid climate. The analysis is based on two techniques, the effective peak power of PV module and power irradiance technique. It was found that TFPV modules CIS and micromorph exhibit a degradation rate of -2.34%/year and -1.73%/year respectively. The calculated degradation rate for CIS technology is higher than those reported in the literature for locations in Europe and lower than those for locations with hot and humid conditions. In the opposite the degradation rate of the micromorph was lower than those given in the literature.
Faults in photovoltaic (PV) systems, which can result in energy loss, system shutdown or even serious safety breaches, are often difficult to avoid. Fault detection in such systems is imperative to improve their reliability, productivity, safety and efficiency. Here, an innovative model-based fault-detection approach for early detection of shading of PV modules and faults on the direct current (DC) side of PV systems is proposed. This approach combines the flexibility, and simplicity of a one-diode model with the extended capacity of an exponentially weighted moving average (EWMA) control chart to detect incipient changes in a PV system. The one-diode model, which is easily calibrated due to its limited calibration parameters, is used to predict the healthy PV array’s maximum power coordinates of current, voltage and power using measured temperatures and irradiances. Residuals, which capture the difference between the measurements and the predictions of the one-diode model, are generated and used as fault indicators. Then, the EWMA monitoring chart is applied on the uncorrelated residuals obtained from the one-diode model to detect and identify the type of fault. Actual data from the grid-connected PV system installed at the Renewable Energy Development Center, Algeria, are used to assess the performance of the proposed approach. Results show that the proposed approach successfully monitors the DC side of PV systems and detects temporary shading.
The recent increase of intermittent power generation plants connected to the electric power grids may stress the operation of power systems. So, grid codes started considering these power plants should con- tribute to the grid support functions. Recently, a power ramp rate limitation is being included in several grid codes, which is a challenge for photovoltaic installations due to the lack of inertia. This paper pre- sents a method to deal with the main grid code requirements considering a PV plant with an energy stor- age device, where a strict two-second time window ramp rate restriction is applied. A direct ramp rate control strategy is used, which includes a dynamic SOC control and battery support functionality for active power setpoint compliance. The control strategy is validated by simulations.
The present article assesses the study of the PV generator capability curves for use in large scale photovoltaic power plants (LS-PVPPs). For this purpose, the article focuses on three main aspects: (i) the modelling of the main components of the PV generator, (ii) the operational limits analysis of the PV array together with the inverter, and (iii) the capability curve analysis considering variable solar irradiance and temperature. To validate this study a PVPP of 1 MW is designed, modelled and simulated in DIgSILENT PowerFactory®. The results for each case scenario shows that the capability curve and the limitations are directly affected by: the solar irradiance, temperature, dc voltage, and the modulation index.
Silvestre, S.; Kichou, S.; Guglielminotti , L.; Nofuentes Garrido, Gustavo; Alonso-Abella, M. Solar energy Vol. 139, p. 599-607 DOI: 10.1016/j.solener.2016.10.030 Data de publicació: 2016-12-01 Article en revista
The present study analyses the degradation of thin film photovoltaic modules corresponding to four technologies: a-Si:H, a-Si:H/µc-Si:H, CIS and CdTe, under 5 years of outdoor long term exposure in Leganés, Spain. The period of outdoor exposure ranges from January 2011 to December 2015. The degradation rate and the stabilization period are analysed by using two different techniques. Moreover, the evolution of the fill factor and performance ratio is assessed. The CdTe module was found to have the highest degradation rate:-4.45 %/year, while the CIS module appears to be the most stable with a degradation rate of -1.04 %/year. The a-Si:H and a-Si:H/µc-Si:H modules present stabilization periods of 24 and 6 months respectively. The CdTe module degrades significantly for a period of 32 months, while the CIS module is the least degraded PV specimen over the whole experimental campaign.
This paper presents a new approach for automatic supervision and remote fault detection of grid connected photovoltaic (PV) systems by means of OPC technology-based monitoring. The use of standard OPC for monitoring enables data acquisition from a set of devices that use different communication protocols as inverters or other electronic devices present in PV systems enabling universal connectivity and interoperability. Using the OPC standard allows promoting interoperation of software objects in distributed-heterogeneous environments and also allows incorporating in the system remote supervision and diagnosis for the evaluation of grid connected PV facilities. The supervision system analyses the monitored data and evaluates the expected behaviour of main parameters of the PV array: Output voltage, current and power. The monitored data and evaluated parameters are used by the fault detection procedure in order to identify possible faults present in the PV system. The methodology presented has been experimentally validated in the supervision of a grid connected PV system located in Spain. Results obtained show that the combination of OPC monitoring along with the supervision and fault detection procedure is a robust tool that can be very useful in the field of remote supervision and diagnosis of grid connected PV systems. The RMSE between real monitored data and results obtained from the modelling of the PV array were below 3.6% for all parameters even in cloudy days.
In this study, heat extraction from both the gradient and heat storage zones of a salinity-gradient solar pond (SGSP) has been evaluated. For this purpose, an experimental solar pond pilot plant was constructed in 2009 in Barcelona (Spain). The structure of the pond is a cylindrical tank of 3-m height and 8 m diameter with a total area of 50 m(2). The main objective was to evaluate a heat-extraction system from the SGSP designed to enhance the system efficiency under different conditions. Thus, an in-pond heat exchanger covering all of the lateral wall area of the pond was installed, and its performance was compared with the traditional in-pond heat exchanger situated on the bottom of the pond. Heat extraction experiments were performed using both heat exchangers individually or both at the same time. The study covers the experiments performed at three different seasonal temperature conditions: winter (December), summer (July) and autumn (October and November). The variations of the temperature inside the pond during the heat extraction were measured and analyzed. The results demonstrated that the efficiency of the pond increases when the heat is removed from the lateral heat exchanger alone compared to either using the bottom heat exchanger or using both heat exchangers simultaneously
Thermocline storage concept is considered as a possible solution to reduce the cost of thermal storage in concentrated solar power (CSP) plants. Recently, a multi-layered solid-PCM (MLSPCM) concept—consisting of a thermocline-like tank combining layers of solid and phase change filler materials—has been proposed. This approach was observed to result in lower thermocline degradation throughout charge/discharge cycles, due to the thermal buffering effect of the PCM layers located at both ends of the tank. MLSPCM prototypes designed for a pilot scale plant were numerically tested and compared against other designs of single-tank thermocline systems, such as: solid-filled thermocline, tanks filled with a single encapsulated PCM and cascaded-PCM configurations. Results showed promising results of the MLSPCM configurations for their potential use in CSP plants.
In this work, the MLSPCM concept is used for designing a thermal energy storage (TES) system for a CSP plant with the dimensions and operating conditions of a parabolic trough plant of 50 MWe, similar to Andasol 1 (Granada, Spain). The performance evaluation of each of the proposed prototypes is virtually tested by means of a numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. Two sets of cases are considered, one with the objective of testing the TES systems individually, by defining specific operating conditions and taking the systems to a periodic steady state; and another, aiming to evaluate their performance after several days of operation in a CSP plant, in which the weather variability and the thermal behavior of the tank walls and foundation are simulated. Thermal performance parameters, such as total energy and exergy stored/released and the efficiency in the use of the storage capacity, are calculated and compared with those obtained by other thermocline-like configurations (single-solid and single-PCM), and with a reference 2-tank molten-salt system. Obtained results allow to continue considering the MLSPCM concept as an interesting alternative for thermal storage in CSP facilities.
Ranaboldo, M.; Domenech, B.; Reyes, G.; Ferrer-Martí, L.; Pastor, Rafael; García-Villoria, A. Solar energy Vol. 117, p. 268-281 DOI: 10.1016/j.solener.2015.05.005 Data de publicació: 2015-07-01 Article en revista
Despite various institutional efforts, about 22% of the total Nicaraguan population still do not have access to electricity. Due to the dispersed nature of many rural inhabitants, off-grid electrification systems that use renewable energy sources are a reliable and sustainable option to provide electricity to isolated communities. In this study, the design of an off-grid electrification project based on hybrid wind-photovoltaic systems in a rural community of Nicaragua is developed. Firstly the analysis of the location, energy and power demands of all users of the community is carried out. A detailed resource assessment is then developed by means of historical data, in-situ wind measurements and a specific micro-scale wind flow model. An optimization algorithm is utilized to support the design defining generation (number, type and location of generators, controllers, batteries and inverters) and distribution (electric networks) systems considering the detail of resource variations. The algorithm is modified in order to consider a long-term perspective and a sensitivity analysis is carried out considering different operation and maintenance costs' scenarios. The proposed design configuration combines solar home systems, solar based microgrids and wind based microgrids in order to connect concentrated groups of users taking advantage of best wind resource areas. (C) 2015 Elsevier Ltd. All rights reserved.
A model of a storage tank with an immersed serpentine heat exchanger is described and validated against experimental data available from the literature. The tank is modelled one dimensionally using the multi-node approach corrected by an energy conservative reversion elimination algorithm to prevent inverse gradient solutions to occur. A one dimensional model in the flow direction is also used for the serpentine based on control volume techniques. The serpentine is discretized in equal sized control volumes and the energy equation is solved in each of them. The energy exchanged between the serpentine and the tank is then introduced as an internal heat source of the tank multi-node. With this model the behaviour of tanks with internal serpentines can be predicted minimising tuning parameters to be derived from previous experimental analysis of the tank. Additionally, by an appropriate formulation of the governing equations in the serpentine control volumes, it is possible to handle complex internal fluid phenomena as coupling of the tank within a thermosyphone cycle or two phase flow.
A higher Low Convective Zone (LCZ) temperature is important for the Salt Gradient Solar Pond (SGSP) thermal engineering application, which is bound to bring a more extensive application. For this purpose, we studied on adding a layer of coal cinder at bottom of LCZ of solar pond, here coal cinder is the burning residues of coal. Temperature development of adding coal cinder at bottom of SGSP has been experimentally and theoretically studied. One dimensional transient numerical model was used to estimate the temperature development in SGSP. The outdoor experiments were used to qualitatively study the temperature evolution of different bottom treatments and porous materials influence, and also used to validate the mathematics model present in this paper. The results show that adding coal cinder at bottom of SGSP leads to a higher LCZ temperature than the traditional bottom treatment. Good consistency has been achieved in the simulation results and the experimental results. Finally, in order to estimate the effect of coal cinder used in large-scale solar pond, a numerical simulation was given and compared to the similar research. The results of this paper show that it can obviously increase LCZ temperature by adding coal cinder at bottom of LCZ, and as a cheap material with perfect thermal performance, it is suitable to be applied in practical SGSP. (C) 2014 Elsevier Ltd. All rights reserved.
A detailed numerical model for flat-plate solar thermal collectors based on one-dimensional finite volume techniques was recently presented, see Cadafalch (2009). The model considers a solar thermal device as a pile of components represented by one or several layers characterized by thermal inertia, internal energy generation and heat transfer to neighboring layers. A multi-layer model is then used to evaluate the full flat-plate solar thermal device. The model permits to investigate any configuration and material by combining appropriate layers. Standard components as opaque insulation, absorbers, air-gaps and glasses were addressed in Cadafalch (2009).Here, a numerical model to evaluate honeycomb-like transparent insulation material in the covers as a component of the multi-layer model is discussed in detail. The honeycomb is evaluated coupling radiation, convection and conduction phenomena. The discret ordinate method is used to evaluate media participation in thermal radiation.A comparison of numerical and experimental results is presented and discussed in order to show evidence of the model credibility.
In this work a three dimensional heat transfer analysis of honeycomb Transparent Insulation Materials (TIM) destined for improving the efficiency of flat plate solar collectors is performed. The cellular and repetitive nature of the TIM structure has allowed simplify the problem and simulate a single isolated cell with opaque and adiabatic walls. The combined heat transfer by radiation and conduction across the isolated cell is treated by means of the solution of the energy equation in its three dimensional form which is coupled to the Radiative Transfer Equation (RTE). The Finite Volume Method is used for the resolution of the RTE. The numerical results are compared to experimental measurements of the heat transfer coefficient on various honeycomb TIM given by different authors in the literature showing a reasonable agreement. The 3D simulations have allowed to study in detail the thermal behavior of the TIM and to understand the real physics of the problem. Finally, a parametric study is conducted in order to investigate the effect of the variation of the most relevant optical and dimensional parameters of the TIM on the heat transfer.
Concentrated solar power plants have attracted increasing interest from researchers and governments all over the world in recent years. An important part of these plants is the storage system which improves dispatchability and makes the plant more reliable. In this paper, a one-dimensional transient mathematical model for a single-tank thermocline thermal energy storage system is presented. The model used temperature dependent correlations to obtain the thermophysical properties for the heat transfer fluid and considered heat loss through the tank wall. The effect of variation in important system parameters like the type of heat transfer fluid, the storage temperature difference and the cycle cut-off criterion on system performance was investigated. The results suggest that two important aspects for assessing the performance of the system are the cyclic behaviour of the system and the time required to attain equilibrium conditions. These aspects directly influence the discharge capacity and discharge power of the storage system, and therefore play an essential role in understanding the start-up characteristics of the system and provide an insight regarding the availability of storage when designing the power cycle for concentrated solar power applications. It was also observed that the cycle durations and the time required to attain cyclic conditions are highly sensitive to not only the storage temperature difference, but also the cut-off temperature difference.
Giannakis, S.; Merino, A.; Darakas, E.; Escalas-Cañellas, A.; Pulgarin, C. Solar energy Vol. 98, num. Part C, p. 572-581 DOI: 10.1016/j.solener.2013.10.022 Data de publicació: 2013-12 Article en revista
In this study, the effect of light intermittence on solar disinfection of secondary treated wastewater was investigated. Synthetic secondary
effluent was spiked with Escherichia coli and submitted to 3 different light intermittence regimes by circulating the effluent between a dark storage tank and three in-series illuminated reactors. The relative influence of the recirculation rate on bacterial inactivation was studied, in short (3–7 min) light regimes and a dark-to-light ratio of 2.04. Lower recirculation rates resulted in poorer disinfection results, showing the detrimental effect of longer dark storage periods on the removal efficiency. Also, longer time intervals were employed in batch tests, to investigate the effect of 1, 2 and 3-h dark intervals, during recreated solar disinfection conditions; fourteen different scenarios were tested. Three hours of continuous or cumulative illumination were proven enough to provide the necessary dose to damage bacteria irreparably, while interruption during these hours favored bacterial resistance. Finally, absence of regrowth was observed in all cases that derived from samples with null bacterial counts. However, when a fraction of viable bacteria was present at the end of the solar treatment, survival was favored.
This paper describes the development, validation and use of a design and simulation tool for modeling the performance of a salinity gradient solar pond. An experimental solar pond pilot plant was constructed in central Catalonia (NE part of the Iberian Peninsula). The body of the pond is a cylindrical reinforced concrete tank, with 3m height, 8m diameter and total area of 50m2. The lateral tank wall has been insulated with 60mm of rock wool. The gradient in the solar pond was settled and maintained since 30 September 2009 to date. The developed tool was validated by comparing simulation results to experimental data collected from the experimental solar pond from November 2009 until August 2011. The resulting first-order differential equations describing the overall energy balance in the pond were solved numerically using a finite-difference method. The temperature profiles of the pond were properly described, especially at lower subzones of the non-convective zone (NCZ) and the lower convective zone (LCZ). The higher errors between experimental and predicted values were found in the upper convective zone (UCZ). Once validated, successfully, the model was used to predict the thermal performance of pre-industrial solar pond to be constructed and operated in Granada, SW of Spain. The thermal profiles predicted temperature differences between surface and bottom of around 40°C during summer time, with a maximum temperature of 75°C. The energy efficiency of the LCZ was anticipated to range between 12% and 25% along one year operation, resulting in 16% of incoming radiation to be extractable for site application.
This paper describes the development, validation and use of a design and simulation tool for modeling the performance of a salinity gradient solar pond. An experimental solar pond pilot plant was constructed in central Catalonia (NE part of the Iberian Peninsula). The body of the pond is a cylindrical reinforced concrete tank, with 3 m height, 8 m diameter and total area of 50 m2. The lateral tank wall has been insulated with 60 mm of rock wool. The gradient in the solar pond was settled and maintained since 30 September 2009 to date. The developed tool was validated by comparing simulation results to experimental data collected from the experimental solar pond from November 2009 until August 2011. The resulting first-order differential equations describing the overall energy balance in the pond were solved numerically using a finite-difference method. The temperature profiles of the pond were properly described, especially at lower subzones of the non-convective zone (NCZ) and the lower convective zone (LCZ). The higher errors between experimental and predicted values were found in the upper convective zone (UCZ). Once validated, successfully, the model was used to predict the thermal performance of pre-industrial solar pond to be constructed and operated in Granada, SW of Spain. The thermal profiles predicted temperature differences between surface and bottom of around 40 °C during summer time, with a maximum temperature of 75 °C. The energy efficiency of the LCZ was anticipated to range between 12% and 25% along one year operation, resulting in 16% of incoming radiation to be extractable for site application.
This paper presents a new maximum power point tracking algorithm for PV systems useful in case of non-uniform irradiance conditions.
This algorithm takes into account the number of bypass diodes in a PV string to calculate the voltage bands associated with the
peak power points that appear in the power–voltage characteristic of the PV system. The main contribution of this study is to state that
the global maximum power point can be tracked by considering only the possible voltage bands which can be found by using the proposed
analytical equation in a simple manner. The algorithm is based in the evaluation and analysis of these voltage bands and in the
selection of the PV system voltage related to the maximum power point of work. The proposed algorithm has been validated by means of
simulation and also in an experimental study.
This paper presents a new maximum power point tracking algorithm for PV systems useful in case of non-uniform irradiance conditions. This algorithm takes into account the number of bypass diodes in a PV string to calculate the voltage bands associated with the peak power points that appear in the power–voltage characteristic of the PV system. The main contribution of this study is to state that the global maximum power point can be tracked by considering only the possible voltage bands which can be found by using the proposed analytical equation in a simple manner. The algorithm is based in the evaluation and analysis of these voltage bands and in the selection of the PV system voltage related to the maximum power point of work. The proposed algorithm has been validated by means of simulation and also in an experimental study.
This paper presents a detailed characterization of the performance and dynamic behaviour of photovoltaic systems by using LabVIEW real-time interface system. The developed software tool integrates several types of instruments into a single system which is able to offer online measurements all data sources and comparison simulation results with monitored data in real-time. Comprehensive monitoring and analyzing of PV systems play a very important role. The proposed method is a low-cost solution to provide fast, secure and reliable system by making the system database-ready for performance analysis of PV systems. The proposed method is also applied to a grid connected PV system in the Centre de Developpement des Energies Renouvelables (CDER) in Algeria. The results show that there is a good agreement between the measured and simulation results values. The integration methodology of robust simulation and monitored data in real-time can be extended to study the fault diagnosis of a PV system.
Two collector models were analyzed under thermosyphon solar thermal system conditions: an extension of the physical model described by Duffie and Beckman (1991) and a modified correlation model based on the test efficiency curve obtained from European Standards. Special attention was paid to the body forces term of the momentum equation, a key aspect for thermosyphon system calculations. The models were verified and validated using a virtual test that numerically reproduces efficiency curves according to EN12975 (2006). A virtual test generated to represent thermosyphon unsteady system conditions was used to analyze model response under transient conditions. The Extended Duffie–Beckman model was shown to perform well when submitted to strong unsteady boundary conditions such as inlet fluid temperature, irradiance and mass flow rate. The model based on the efficiency curve was shown to work well for time steps larger than the collector residence time. However, for lower time steps, the model was found to be inaccurate due to the hypothesis of a single control volume for the fluid analysis. For the same reason, besides the assumption of a first order temperature profile in the fluid flow, the model was not capable to predict a physical behavior when submitted to strong variations of the fluid inlet temperature.
This paper proposes a simple diagnostic method to determine the number of open and short circuited PV modules in a string of a PV system by taking into account the economical factor, such as minimum number of sensors. The diagnostic algorithm has as inputs the irradiance level, the PV modules temperature, the number of PV modules present in the string analyzed and its output power. So, just temperature and irradiance sensors, as well as a power meter by string are needed in the monitoring system forming part of the fault diagnostic system. The proposed fault detection method has been successfully validated experimentally.
Reuss, M.; Benkert, S.; Aeberhard, A.; Martina, P.; Raush, G.; Renztell, B.; Nogari, S. Solar energy Vol. 59, num. 4-6, p. 259-270 DOI: 10.1016/S0038-092X(97)00013-3 Data de publicació: 1997 Article en revista