Avaliação exergética, energética, económica e ambiental de centrais de energia solar concentrada na Líbia

Doutoramento em Engenharia MecânicaThe PhD project addresses the potential of using concentrating solar power (CSP) plants as a viable alternative energy producing system in Libya. Exergetic, energetic, economic and environmental analyses are carried out for a particular type of CSP plants. The study, although it aims a particular type of CSP plant – 50 MW parabolic trough-CSP plant, it is sufficiently general to be applied to other configurations. The novelty of the study, in addition to modeling and analyzing the selected configuration, lies in the use of a state-of-the-art exergetic analysis combined with the Life Cycle Assessment (LCA). The modeling and simulation of the plant is carried out in chapter three and they are conducted into two parts, namely: power cycle and solar field. The computer model developed for the analysis of the plant is based on algebraic equations describing the power cycle and the solar field. The model was solved using the Engineering Equation Solver (EES) software; and is designed to define the properties at each state point of the plant and then, sequentially, to determine energy, efficiency and irreversibility for each component. The developed model has the potential of using in the preliminary design of CSPs and, in particular, for the configuration of the solar field based on existing commercial plants. Moreover, it has the ability of analyzing the energetic, economic and environmental feasibility of using CSPs in different regions of the world, which is illustrated for the Libyan region in this study. The overall feasibility scenario is completed through an hourly analysis on an annual basis in chapter Four. This analysis allows the comparison of different systems and, eventually, a particular selection, and it includes both the economic and energetic components using the “greenius” software. The analysis also examined the impact of project financing and incentives on the cost of energy. The main technological finding of this analysis is higher performance and lower levelized cost of electricity (LCE) for Libya as compared to Southern Europe (Spain). Therefore, Libya has the potential of becoming attractive for the establishment of CSPs in its territory and, in this way, to facilitate the target of several European initiatives that aim to import electricity generated by renewable sources from North African and Middle East countries. The analysis is presented a brief review of the current cost of energy and the potential of reducing the cost from parabolic trough- CSP plant. Exergetic and environmental life cycle assessment analyses are conducted for the selected plant in chapter Five; the objectives are 1) to assess the environmental impact and cost, in terms of exergy of the life cycle of the plant; 2) to find out the points of weakness in terms of irreversibility of the process; and 3) to verify whether solar power plants can reduce environmental impact and the cost of electricity generation by comparing them with fossil fuel plants, in particular, Natural Gas Combined Cycle (NGCC) plant and oil thermal power plant. The analysis also targets a thermoeconomic analysis using the specific exergy costing (SPECO) method to evaluate the level of the cost caused by exergy destruction. The main technological findings are that the most important contribution impact lies with the solar field, which reports a value of 79%; and the materials with the vi highest impact are: steel (47%), molten salt (25%) and synthetic oil (21%). The “Human Health” damage category presents the highest impact (69%) followed by the “Resource” damage category (24%). In addition, the highest exergy demand is linked to the steel (47%); and there is a considerable exergetic demand related to the molten salt and synthetic oil with values of 25% and 19%, respectively. Finally, in the comparison with fossil fuel power plants (NGCC and Oil), the CSP plant presents the lowest environmental impact, while the worst environmental performance is reported to the oil power plant followed by NGCC plant. The solar field presents the largest value of cost rate, where the boiler is a component with the highest cost rate among the power cycle components. The thermal storage allows the CSP plants to overcome solar irradiation transients, to respond to electricity demand independent of weather conditions, and to extend electricity production beyond the availability of daylight. Numerical analysis of the thermal transient response of a thermocline storage tank is carried out for the charging phase. The system of equations describing the numerical model is solved by using time-implicit and space-backward finite differences and which encoded within the Matlab environment. The analysis presented the following findings: the predictions agree well with the experiments for the time evolution of the thermocline region, particularly for the regions away from the top-inlet. The deviations observed in the near-region of the inlet are most likely due to the high-level of turbulence in this region due to the localized level of mixing resulting; a simple analytical model to take into consideration this increased turbulence level was developed and it leads to some improvement of the predictions; this approach requires practically no additional computational effort and it relates the effective thermal diffusivity to the mean effective velocity of the fluid at each particular height of the system. Altogether the study indicates that the selected parabolic trough-CSP plant has the edge over alternative competing technologies for locations where DNI is high and where land usage is not an issue, such as the shoreline of Libya.O projeto de Doutoramento aborda o potencial de usar centrais de energia solar concentrada (CSP) como um sistema de produção de energia alternativa disponível na Líbia. Uma análise nas vertentes exergética, energética, económica e ambiental foi realizada para um tipo particular destas centrais – um sistema de 50 MW com receção parabólica, porém ela é suficientemente geral para ser aplicada a outras configurações. A originalidade do estudo, para além da modelação e análise da configuração selecionada encontra-se na utilização do estado da arte em termos da análise exergética combinada com a avaliação do ciclo de vida (LCA). A modelação e simulação da central CSP selecionada são efetuadas no terceiro capítulo tendo em consideração as duas componentes: ciclo de potência e campo de coletores solar. O modelo computacional para a análise do sistema foi desenvolvido com base em equações algébricas que descrevem o sistema, e que são resolvidas usando o software EES. Deste modo, são definidas as propriedades em cada ponto de interesse para os diferentes elementos do sistema, o que assim permite determinar as energias, eficiências e irreversibilidades desses elementos. O modelo desenvolvido tem o potencial de se tornar uma ferramenta de grande utilidade para o projeto preliminar de engenharia de centrais CSP, e também para a avaliação da eventual reconfiguração de centrais elétricas solares comerciais em operação. Além disso, o modelo pode ser utilizado no estudo de viabilidade da operação de centrais CSP, através da análise energética, económica e ambiental, para regiões diferentes da que foi escolhida no presente estudo -Trípoli (Líbia). O cenário total da viabilidade da operação da central CSP é completado através da análise horária com base anual apresentada no quarto capítulo. Esta análise permite a comparação de diferentes sistemas e, eventualmente permite fazer a seleção com base nas componentes económicas e energéticas, que são determinadas dentro do contexto do software greenius. A análise também toma em conta o impacto de financiamento e incentivos dados aos projetos no custo da produção de energia. O principal resultado desta análise é a verificação que o desempenho é mais elevado, com o consequente menor custo nivelado da eletricidade, para a Líbia em comparação com o Sul da Europa (Espanha). Assim a Líbia tem o potencial de se tornar um candidato atrativo para o estabelecimento de centrais CSP com o objetivo, como foi considerado em várias iniciativas europeias, de exportar eletricidade gerada através de fontes de energia renováveis de países do Norte de África e Médio Oriente para a Europa. A análise apresenta uma breve revisão do custo corrente da eletricidade e o potencial para reduzir o custo da energia a partir da tecnologia de receção parabólica de centrais CSP. A avaliação do ciclo de vida com base exergética (ELCA) e a avaliação do ciclo de vida convencional são realizadas para a centrais CSP específicas no quinto capítulo. Os objetivos são 1) avaliar o impacto ambiental e custo, em termos de do ciclo iv de vida exergético do sistema; 2) identificar pontos fracos em termos da irreversibilidade dos processos; e 3) verificar se as centrais CSP podem reduzir o impacto ambiental e o custo de geração de eletricidade em comparação com centrais que consomem combustível fóssil. O capítulo ainda apresenta uma análise termoeconómica com base na metodologia do custo específico da exergia (SPECO), que avalia o custo relacionado com a destruição de exergia. A análise verificou que o impacto mais importante é a contribuição apresentada pelo campo solar (79%), e os materiais com maior impacto são: aço (47%), sal fundido (25%) e óleo sintético (21%). A análise ELCA mostra que a maior demanda de exergia é devida ao aço (47%); a análise existe uma considerável demanda de exergia relacionada com o sal fundido e ainda o óleo sintético. Em comparação com as centrais que consomem combustível fóssil (NGCC e óleo) a central sistema CSP apresenta menor impacto ambiental, enquanto o pior desempenho ambiental é o da central com queima de óleo seguida pela central a gás natural (NGCC). Na central CSP, o campo solar apresenta o custo mais elevado, enquanto o gerador de vapor, entre os componentes do ciclo de potência, apresenta o maior custo. O armazenamento de energia térmica permite que as centrais CSP superem a intermitência de radiação solar para responder à procura de energia elétrica independentemente das condições climáticas, e também possam estender a produção de eletricidade para além da disponibilidade da radiação solar diária. A análise numérica do transiente térmico de um sistema de armazenamento de gradiente térmico é realizada durante a fase de carregamento. O sistema de equações que descreve o modelo numérico é resolvido através da utilização de diferenças finitas implícitas no tempo usando o software Matlab. Os resultados da análise indicam que as previsões estão em boa concordância com os dados experimentais para a evolução no tempo da região de gradiente térmico, em particular para regiões mais afastadas da entrada. Nesta região os desvios observados são provavelmente causados pelo alto nível de turbulência devido à penetração do jato no seio do tanque de armazenamento. O modelo analítico simples para simular a turbulência que foi desenvolvido melhora os resultados. Esta abordagem não requer esforço computacional adicional e determina a difusidade térmica efetiva ao longo do tanque

Modelling and performance evaluation of a direct steam generation solar power system coupled with steam accumulator to meet electricity demands for a hospital under typical climate conditions in Libya

This study aims to build a dynamic model of a direct steam generation (DSG) solar power system coupled with a steam accumulator to meet electricity demands for a hospital under transient environmental conditions in Libya. The main components of the system are DSG parabolic trough collectors, a steam accumulator, a turbine, a condenser and a circulation pump. The system is modelled via using Simulink\Simscape software blocks with integrated MATLAB functions to run a dynamic simulation. As the simulation tool reflects the transient operation of the components, advanced control strategies were applied to the model. Using the proportional integral controller (PI controller), safe operation of the system is secured by pump flow rate control, safe turbine operation is provided by pressure control and power output is matched with the demand by using a throttle valve control. 1584 m2 solar collector area and 160 m3 total volume of pressurised steam tank are used in the simulation considering the electricity demand of the hospital and solar radiation in the location. The produced work output was controlled to match the demand profile of the hospital, which needs 200 kW in the peak period and 50 kW at the night. The designed system shows a maximum thermal efficiency of 23.5% for the operation condition

A Simulink/Simscape dynamic modelling study on shifting the power output profile of a direct steam generation solar power system with steam/water storage

Solar thermal power plants convert thermal energy from direct solar radiation into mechanical work and electricity via a thermodynamic cycle. This method produces renewable electricity. A direct steam generation technology (DSG), which generates steam directly in the solar field's absorber tubes and feeds it directly to the turbine or thermal storage, is one of the many options in solar thermal power plants. It has a number of intriguing advantages and is a promising technology. However, one of the major problems with the use of the direct steam generation technology is that it is affected by the fluctuation of solar radiation, which results in the production of fluctuating power output and no economic competition for long-term storage. The design of the control system is complicated by the steam generation system's challenging dynamic behaviour. It is primarily caused by the coexistence of two-phase flow in the absorber tubes and the natural transitory feature of solar radiation. In solar thermal power plants, thermal storage is a critical requirement. There are different types of heat storage systems in solar plants such as molten salts, concretes, phase change materials and steam accumulators. The steam accumulator has been adopted in Planta Solar 10 project. Steam accumulator is a viable option for decreasing the influence of changing irradiance on the power generation of solar thermal systems since they have fast reaction time and high discharge rates due to the rapid evaporation and condensation of water/steam under non-equilibrium conditions. This thesis aims to study the dynamic behaviour of a novel direct steam generation solar power system integrated with a steam/water accumulator for shifting its power output profile to match the electricity demand profile in a Libyan hospital as a case study. Another issue in DSG solar power systems is that expansion in turbine might happen in liquid-vapour two-phase region. Use of a cascade Rankine cycle can be a solution, in which an organic Rankine cycle (ORC) is a bottom cycle, while a steam Rankine cycle is a top cycle with a higher condensation temperature to mitigate the issue of wet expansion. The bottom cycle can also run separately using low-temperature residue steam/water in the accumulator to offer a further capacity in shifting power output profile. Therefore, the design of a cascade steam-organic Rankine cycle integrated with steam\water accumulator is studied from operating and thermodynamic perspectives, aiming to fully unlock the potential of such advanced high-efficiency cogeneration systems. This thesis presents a comprehensive literature review on Rankine cycle solar power generation systems regarding cycle configurations, thermal storage and working fluids. The design, validation and parametric study of a direct steam generation solar power system is conducted for Libyan climate conditions, particular with dynamic simulation of a steam/water accumulator. Moreover, a cascade steam-organic Rankine cycle solar power system integrated a steam accumulator was studied. Finally, the cascade steam-organic Rankine cycle solar power system integrated with two water accumulators was modelled. All the designs presented in this thesis was simulated by using Simulink\Simscape software in order to see the dynamic behaviour of systems under different climate conditions. As a case study, the electricity demand profile of a Libyan hospital was chosen as a target in the design of system. The results demonstrated that by using 1544m2 of parabolic trough solar collectors and 160m3 of steam storage tank with a proper control of flow rate, the power output profile of the system can meet the electricity demand of the Libyan hospital even at night. In the cascade steam-organic Rankine cycle solar power system coupled with the steam accumulator with and without using a recuperator, the findings reveal that the recuperative system outperforms the non-recuperative system in terms of power generation and thermal efficiency. In the cascade steam-organic Rankine cycle solar power system integrated with two water storage tanks, results show that the top cycle output is higher than ORC power output in the nominal operation mode. At the peak radiation time, the top cycle generates 136 kW, while the ORC generates 37 kW. In the discharging mode in the evening, a separate operation the bottom ORC system can still generate 31 kW for six hours based with using a 51 m3 hot water tank

A Simulink/Simscape dynamic modelling study on shifting the power output profile of a direct steam generation solar power system with steam/water storage

Solar thermal power plants convert thermal energy from direct solar radiation into mechanical work and electricity via a thermodynamic cycle. This method produces renewable electricity. A direct steam generation technology (DSG), which generates steam directly in the solar field's absorber tubes and feeds it directly to the turbine or thermal storage, is one of the many options in solar thermal power plants. It has a number of intriguing advantages and is a promising technology. However, one of the major problems with the use of the direct steam generation technology is that it is affected by the fluctuation of solar radiation, which results in the production of fluctuating power output and no economic competition for long-term storage. The design of the control system is complicated by the steam generation system's challenging dynamic behaviour. It is primarily caused by the coexistence of two-phase flow in the absorber tubes and the natural transitory feature of solar radiation. In solar thermal power plants, thermal storage is a critical requirement. There are different types of heat storage systems in solar plants such as molten salts, concretes, phase change materials and steam accumulators. The steam accumulator has been adopted in Planta Solar 10 project. Steam accumulator is a viable option for decreasing the influence of changing irradiance on the power generation of solar thermal systems since they have fast reaction time and high discharge rates due to the rapid evaporation and condensation of water/steam under non-equilibrium conditions. This thesis aims to study the dynamic behaviour of a novel direct steam generation solar power system integrated with a steam/water accumulator for shifting its power output profile to match the electricity demand profile in a Libyan hospital as a case study. Another issue in DSG solar power systems is that expansion in turbine might happen in liquid-vapour two-phase region. Use of a cascade Rankine cycle can be a solution, in which an organic Rankine cycle (ORC) is a bottom cycle, while a steam Rankine cycle is a top cycle with a higher condensation temperature to mitigate the issue of wet expansion. The bottom cycle can also run separately using low-temperature residue steam/water in the accumulator to offer a further capacity in shifting power output profile. Therefore, the design of a cascade steam-organic Rankine cycle integrated with steam\water accumulator is studied from operating and thermodynamic perspectives, aiming to fully unlock the potential of such advanced high-efficiency cogeneration systems. This thesis presents a comprehensive literature review on Rankine cycle solar power generation systems regarding cycle configurations, thermal storage and working fluids. The design, validation and parametric study of a direct steam generation solar power system is conducted for Libyan climate conditions, particular with dynamic simulation of a steam/water accumulator. Moreover, a cascade steam-organic Rankine cycle solar power system integrated a steam accumulator was studied. Finally, the cascade steam-organic Rankine cycle solar power system integrated with two water accumulators was modelled. All the designs presented in this thesis was simulated by using Simulink\Simscape software in order to see the dynamic behaviour of systems under different climate conditions. As a case study, the electricity demand profile of a Libyan hospital was chosen as a target in the design of system. The results demonstrated that by using 1544m2 of parabolic trough solar collectors and 160m3 of steam storage tank with a proper control of flow rate, the power output profile of the system can meet the electricity demand of the Libyan hospital even at night. In the cascade steam-organic Rankine cycle solar power system coupled with the steam accumulator with and without using a recuperator, the findings reveal that the recuperative system outperforms the non-recuperative system in terms of power generation and thermal efficiency. In the cascade steam-organic Rankine cycle solar power system integrated with two water storage tanks, results show that the top cycle output is higher than ORC power output in the nominal operation mode. At the peak radiation time, the top cycle generates 136 kW, while the ORC generates 37 kW. In the discharging mode in the evening, a separate operation the bottom ORC system can still generate 31 kW for six hours based with using a 51 m3 hot water tank

Predictive control strategies for optimizing temperature stability in instantaneous hot water systems

Domestic hot water production is responsible for a significant part of domestic energy consumption; instantaneous gas heating devices are widely used because they don?t require reservoirs, therefore have a competitive use/consumption ratio compared to other technologies. However, users' perception of comfort is severely affected by sudden changes in temperature outside the desired temperature. The instability of the water temperature with overshoots and undershoots is the most common disadvantage, which occurs mainly due to sudden changes in the water flow requested by users and the response delays inherent to the heating system. Traditional heat cell power controllers have difficulties in responding to these problems in a timely manner, as they don?t have the capacity to anticipate the effects of sudden variations in water flowrate. In this work, predictive control strategies were developed which, due to its predictive nature, allows anticipating and correcting the negative effects of sudden variations of water flowrate in the temperature. A comparative analysis of model based predictive controllers (MPCs), with and without adaptive function, with traditional controllers used in the tankless gas water heaters (TGWHs) was carried out. Tests in a simulated environment demonstrated better performances in the stabilization of temperature during sudden changes in water flowrates.publishe

Gain scheduling model predictive controller design for tankless gas water heaters with time-varying delay

One of the most significant drawbacks of tankless gas water heaters is the difficulty of controlling the outlet hot water temperature as changes in water demand cannot be predicted. These sudden changes in hot water flow rate, are associated with the system inhered delays, and are responsible for temperature overshoots and undershoots that severely affect the comfort perception of the user. In the present work, a linear model predictive controller (MPC) design is employed for the stabilization of a constrained nonlinear thermal process with time-varying delay, however, control performance degrades significantly when operating far from the linearization operating point. The idea is to design multiple linear MPC controllers, each with its linear state-space model and constant time delay, describing process dynamics at a specific level of operation. Gain scheduling MPC presents improved performance when compared with classic PID and combined feedforward-feedback controllers, reducing the settling time up to two thirds.publishe