Energia e ambiente num mundo com muita gente

Muitos dos recursos que exploramos para o nosso dia a dia são finitos, isto é não se renovam (pelo menos à escala de tempo que é a nossa). Por outro lado, a Natureza tem uma capacidade finita para absorver os impactes que as actividades que desenvolvemos têm sobre ela. A Energia de origem fóssil é um excelente exemplo para ilustrar ambas as afirmações: os combustíveis fósseis aproximam-se da situação de pico de produção e a sua queima é o agente mais forte das alterações climáticas e de uma grande quantidade de outros impactes ambientais. Por outro lado, a procura de energia aumenta fortemente, em particular por causa da necessidade dos países em vias de desenvolvimento, com destaque para países como a China e a Índia. Este facto, em paralelo com o crescimento demografico, que no mínimo levará o mundo a ter uma população de 10 mil milhões de pessoas no final do século, tornam a pressão sobre a oferta e o nível dos impactes ambientais, verdadeiramente insustentáveis, quer o mundo evolua sem corrigir as assimetrias de consumo de energia per capita, quer faça um esforço tremendo para que este se reduza nos países industrializados e suba nos países em vias de desenvolvimento. É pois imperativo que o aumento de consumo global de energia se faça à custa de fontes de energia limpas (Energias Renováveis), em paralelo com uma redução inteligente de consumo (consumos evitados, eficiência energética), acabando mesmo esta estratégia, por estar na base do que se tem chamado de desenvolvimento sustentável. Considera-se que será mesmo necessário alterar de forma profunda a cultura consumista e desenvolvimentista "à outrance” que domina o mundo dito desenvolvido de hoje, para a adopção de um conjunto de valores de uma natureza completamente diferente e compatível com uma perspectiva operacional de desenvolvimento sustentável

OpenFOAM solver for 3d modelling of solar thermal volumetric receivers coupled to concentration systems

Over the last few years, the use of porous volumetric receivers in concentrated solar power (CSP) plants is being extensively investigated. In this work, a three dimensional solver is developed in OpenFOAM to model the solar radiation absorption, thermal and hydrodynamic performance of porous volumetric receivers coupled to solar concentration systems. The porous structure is assimilated to a continuous semi-transparent medium, and the volume averaged mass, momentum and energy conservation equations are solved using the local thermal non-equilibrium (LTNE) approach [1]. The absorbed solar radiation in the solid matrix structure is modelled by coupling a 3D in-house algorithm based on the Monte Carlo Ray Tracing (MCRT) method [2] with the CFD mesh, while the thermal radiation transfer is described by P1 spherical harmonics method. To test the model, a cylindrical receiver element (5 cm of diameter and 5 cm of height) made of open-cell SiC ceramic foam coupled to a parabolic dish with a concentration ratio of 500 is considered. The global model (MCRT and CFD) is designed to have as input the concentrated solar radiation and angle of incidence fields at the receiver inlet, and the main results are the spatial distributions of the absorbed solar radiation, temperature of the fluid and solid matrix structure and fluid velocity. The thermal efficiency, mean fluid temperature at the outlet and pressure drop across the receiver for the test conditions are 85.46%, 474.22 K and 103.10 Pa, respectively. The solver can be easily adapted to model the performance of porous volumetric receivers in different CSP systems.The authors acknowledge the support of the Portuguese National Science Foundation – FCT (Fundação para a Ciência e Tecnologia) – through the Grant No. SFRH/BD/115923/2016. The authors also acknowledge the funding provided by the European Union through the European Regional Development Fund, included in the COMPETE 2020 (Operational Program Competitiveness and Internationalization) through the ICT project (UID/GEO/04683/2013) with the reference POCI-01-0145-FEDER-007690. Acknowledgement are also addressed to LANIK ceramic foam company for the providing of the set of ceramic foam samples

New Optical Designs for Large Parabolic Troughs

The potential for cost reduction in parabolic troughs (PT) large collector fields is real and will be achieved in a variety of different ways. One problem certainly contributing to the costs of Solar Thermal Electricity (STE) PT fields is certainly the fact that large fields have a significant quantity of receiver lines and pipes bringing the heat transfer fluid to and off from them. The very large pipe length in large collector fields (for instance the 50MW fields in Spain) is a source of heat losses and parasitic losses due to significant pumping power, but also a source of other costs related to the number of pumps, to the amount of (costly) circulating fluid etc. In any given large field, receiver length and pipe length are determined by the aperture size of the PTs and one way to reduce these impacts on cost would be to increase aperture size. This has been the idea behind developments like the Ultimate Trough. In this paper new optical solutions are presented to obtain much larger troughs, using the same “standard” evacuated 70 mm inner radius tube, which in fact amounts to a substantial increase of concentration, but without sacrificing the acceptance angle of the optic. The Simultaneous Multiple Surface (SMS) method is used and practical solutions are obtained for apertures nearly close to twice the present standard of ≈ 6m width. The case of troughs for fixed receiver tubes is also discussed in this context. The solutions developed minimize transmission losses due to the glass cover and in that sense are an improvement on previous work. They also achieve a higher optical performance than other second stage solutions, because they are designed to eliminate optical losses through large gaps, something that is associated with the fact that the outer glass envelope has a much larger diameter than the inner receiver tube. The paper presents new examples of larger troughs with second stage concentrators, characterizing and comparing them with a “conventional” PT. The comparison is done for optical properties and for the energy collected on a sunny location (Faro, Portugal). The paper ends with a similar exercise done for fixed receiver troughs, an exercise that also leads to larger troughs (since it is done for the same 70 mm inner (evacuated) receiver tube) and concentration is increased. Again optical properties and energy performance are presented and compared with the conventional PT. The new solutions represent a potential reduction in field costs or even in O&M, as suggested, and this exercise will enable manufacturers the pondering of the manufacture of larger troughs (perhaps cheaper on a sqm basis) but with the extra cost of a secondary concentrator, knowing how much energy to expect from the adoption of solutions that benefit non-imaging optics design methods

Increasing the Cost Effectiveness of CSP Technologies Through the Development of a New CLFR “Etendue Matched” Collector

A new CLFR “Etendue Matched” is a promising CSP technology to achieve a better cost effectiveness with a lower levelized cost per m2 . This new technology can significantly reduce shading and blocking existing in a conventional LFR (Chaves, 2009; Chaves, 2010), while at the same time optimizing primary and secondary concentration to the limits allowed by first principles in optics. A preliminary evaluation of the optical and thermal performance has been performed (Canavarro, 2010, Horta 2011), and a configuration for a full scale CSP power plant proposed, with two secondary receivers (Fig.1), i.e a multiple receiver solution

Efficiency Improvement and Potential LCOE Reduction with an LFR-XX SMS Plant with Storage

Several forms of renewable energy have in the last decade achieved a remarkable technological improvement and cost effectiveness, including Solar Energy. Photovoltaic systems (PV) are reaching grid parity in the sunniest regions of Europe. Apart from PV, a promising technology is Concentrated Solar Power (CSP) with thermal storage, because it enables higher power plant availability throughout the year as well as dispatchability, a highly important and valuable feature for a power plant. However significant cost reductions are still to be achieved, for CSP to be competitive with conventional electricity production and with PV. In this paper a few technological improvements are described and a proposal is made to combine them in a system that is more efficient and has, simultaneously, the potential of being cheaper. The goal is to show the potential for advanced LFR concepts in combination with high temperature molten salt mixtures and energy storage as being also important contenders to the objective of bringing STE- Solar Thermal Electricity to the market in a competitive way. The concept presented and developed includes a higher ΔT of the heat transfer fluid (HTF), a particular storage strategy and a new optical concept based on a new Linear Fresnel Concentrator (LFR XX SMS-Simultaneous Multiple Surface, Fig.1,2, [1] enabling an important increase in the overall yearly solar to electricity conversion efficiency and generating an important potential for electricity cost reduction. It is shown that values approaching 10eurocents/kWh, for a 50MW plant with 7 hours storage capacity are possible in Southern Europe

Effect of thickness on the thermo-hydraulic performance of porous volumetric solar receivers with different internal geometries

In this work, the effect of thickness on the thermal and hydrodynamic performance of porous volumetric solar receivers made of open-cell silicon carbide (SiC) ceramic foam is investigated using an in-house detailed numerical model. The model is based in a Computational Fluid Dynamics (CFD) technique to solve the volume averaged mass, momentum and energy conservation equations, including the exchange of thermal radiation inside the receiver. A Monte Carlo Ray Tracing (MCRT) method was developed and then used to model the solar radiation transport in the porous media. Two optimised internal geometries (porosity and pores size) of the receiver with adiabatic side-walls are investigated for different thicknesses. Results show that the optimal thickness depends on the porosity and pores size and there is a value from which the thermal efficiency is nearly constant and the pressure drop always increase. It was also found that the thickness should be approximately between 5 and 7 cm for porosity and pores diameter between 0.85 and 0.90 and 3.0 mm and 4.5 mm, respectively, aiming to maximise thermal efficiency by decreasing the transmission losses of solar radiation, and to keep low pressure drop

Development and Test Results of a Calorimetric Technique for Solar Thermal Testing Loops, Enabling Mass Flow and Cp Measurements Independent from Fluid Properties of the HTF Used

Thermal heat transfer fluids (HTF) used in solar collectors (e.g. synthetic oils) are known to age and degrade [1]. This degradation is impossible to control, affecting the fluid heat transfer capacity and thus the ability of measuring the performance of an HTF heating device (e.g. a solar collector) based on known specific heat values. Collector testing is also crucially dependent on an accurate measurement of HTF mass flow rate. Such measurement relies on flow meters suitable for the accuracy, operating temperature and flow range requirements of the testing procedures, often an expensive and demanding component in particular when no-intrusive measurements are to be done ia a close circuit. For power measurement purposes, as those performed in solar collector testing procedures, a direct measurement of the product between specific heat and mass flow rather than a separate measurement of both quantities is suitable. A calorimetric technique delivering this direct measurement is thus a suitable strategy to overcome such difficulties with acceptable (and even higher) measurement accuracy. Solutions of this kind have already been proposed [2,3]. In this paper we revisit and improve the solution presented in [2] and demonstrate its usefulness in a solar collector testing loop, for temperatures up to 200 °C. A calorimeter prototype was thoroughly tested and calibrated with water as HTF (enabling accurate independent measurement of specific heat and mass flow rates values). Calorimeter calibration results where then used in the testing with thermal oil whose specific heat values were previously known from manufacturer and independent laboratory measurements. A comparison of Cp measured by the calorimeter with the value given by the HTF manufacturer is used to test the calorimeter capacities. The agreement achieved was very good. It is noted that the technique can be easily implemented in any high temperature loops, large or small

Vanadium Redox Flow Battery Modelling and PV Self-Consumption Management Strategy Optimization

This work aims to maximize the photovoltaic solar electricity’s self-consumption, through the development and validation of an equivalent electric model of a vanadium redox flow battery and its implementation in an energy management strategy. The first phase of the work presents the modelling of the 5.0 kW/60 kWh VRFB integrated in a solar photovoltaic microgrid - 3.5 kWp monocrystalline plus 3.2 kWp polycrystalline technology - at the University of Évora. The model is based in the equivalent electric circuit model built upon the consulted bibliographic references allowing to calculate the battery parameters on the desired power. It considers the auxiliary power consumption and operational parameters and despite its simplicity attains for a good match with experimental results. Upon its validation, the model is further enhanced as to better describe the VRFB real response in its regular operating conditions. Assessment of the enhanced model is based on key performance indicators such as selfconsumption rate, rate of battery usage or electric grid independence. In this work an approach to best fit the battery modelling and simultaneously the energy management strategy for a PV+VRFB system is presented, based on actual operating conditions and on a prescribed EMS goal

An approach to implement PV Self-Consumption and Ramp-Rate Control Algorithm using a Day-to-Day Forecast battery charging, with a Vanadium Redox Flow Battery

The variability of the solar resource is mainly caused by cloud passing, causing rapid power fluctuations on the output of photovoltaic (PV) systems. The fluctuations can negatively impact the electric grid, and smoothing techniques can be used as attempts to correct it. However, the integration of a PV+storage to deal with the extreme power ramps at a domestic/services scale is not explored in the literature, neither its effective combination with other energy management strategies (EMSs). This work is focused on using a battery energy storage unit to control the power output of the PV system, maintaining the ramp rate (RR) within a non-violation limit and within a battery state of charge (SoC) range, appropriate to perform this RR management at the domestic/services scale. For this purpose, the authors explore the vanadium redox flow battery (VRFB) technology. Based on model simulation, key-performance indicators (KPI) are studied and improved, and finally, experimental validation is carried out. A comparison among three EMSs is made: a self-consumption maximization (SCM), a SCM with ramp-rate control (SCM+RR), and the last strategy performing also night battery charging based on the day ahead weather forecast (SCM+RR+WF). The weather forecast allowed the battery SoC control, preparing it to carry out the RR control the next day. The results show that SCM+RR+WF, especially in wintertime, is an excellent approach to manage PV+battery systems. This strategy successfully controlled 100 % of the violating power ramps, obtaining also a self-consumption ratio (SCR) of 59 %, and a grid-relief factor (GRF) of 61 %.Comment: Keywords: PV solar energy; energy storage; self-consumption; ramp rate; VRF