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Estudo numérico da influência de parâmetros funcionais num modelo de armazenamento de energia térmica em tanque único
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O interesse no armazenamento de energia térmica, inserido nas centrais eléctricas de Energia Solar Concentrada (centrais de CSP), tem aumentado, devido à despachabilidade que o sistema de armazenamento lhes confere. A despachabilidade conferida pelo armazenamento de energia térmica melhora a integração das centrais de CSP na rede e é, também, importante para a integração na rede eléctrica da electricidade proveniente da energia eólica e a energia solar fotovoltaica [1]. Além disso, a inclusão de um sistema de armazenamento numa central CSP aumenta o seu factor de capacidade e a sua competitividade económica.
O armazenamento de energia térmica pode ser efectuado ou num sistema de dois tanques, ou num sistema de tanque único. No sistema de dois tanques, existe um tanque que armazena fluido quente e outro fluido frio. Por sua vez, no sistema de tanque único armazenam-se num só tanque dois níveis de temperatura diferentes, existindo, assim, uma zona quente e uma zona fria. De forma a melhorar a estratificação da temperatura no interior do tanque, este encontra-se cheio com material sólido (leito do tanque). Os dois materiais (o material sólido e o fluido de armazenamento) constituem o material de armazenamento de energia do tanque (HSM). O fluido à temperatura máxima é armazenado no topo do tanque, enquanto o fluido à temperatura mínima é armazenado na base do mesmo. Deste modo, na zona quente o material de armazenamento está à temperatura máxima, e na zona fria está à temperatura mínima. A zona do HSM, com temperaturas intermédias entre a máxima e a mínima, designa-se por thermocline. Neste trabalho, estudaram-se alguns aspectos da operação de um sistema de armazenamento de energia térmica em tanque único.
O objectivo deste trabalho é proceder aos estudos da parametrização das seguintes propriedades: a degradação do thermocline, a perda de carga verificada no leito do tanque e a inclusão do ullage space no balanço de energia do tanque, sendo que este último acabou por não ser realizado, devido à análise energética efectuada preliminarmente. Os estudos foram feitos através de um modelo unidimensional que realizou os cálculos da temperatura (criando um perfil de temperaturas) e da perda de carga, ao longo da altura do tanque, considerando as propriedades térmicas dos materiais (massa volúmica, calor específico e condutividade térmica), com dependência da temperatura.
Para o estudo da degradação do thermocline, foram calculados os perfis de temperatura resultantes de sucessivas operações de carga e descarga. A partir destes perfis, foi possível determinar a taxa de degradação do thermocline. Além disso, determinou-se a variação da altura do thermocline e, consequentemente, a variação da eficiência de armazenamento de energia térmica em função da duração e do número de operações de carga e descarga.
Para o estudo da perda de carga do fluido, utilizou-se a equação de Ergun para um leito de partículas não esféricas e de diferente volume que constituía o leito do tanque, tendo-se, por isso, calculado o diâmetro médio da partícula representativa. Além disso, no cálculo da perda de carga, também foram consideradas as propriedades térmicas dos materiais (massa volúmica, e viscosidade dinâmica), com dependência da temperatura.
Através dos estudos efectuados, é explicada a maior degradação do thermocline nas primeiras operações, e é analisada a influência da porosidade e do caudal na degradação do mesmo. Apresentam-se também os resultados da perda de carga verificada, que permitem a noção da sua ordem de grandeza e da sua variação consoante o tanque está a ser carregado ou descarregado.
The interest in thermal storage systems in CSP power plants has increased due to the dispatchability they give to the plants. This dispatchability improves the CSP plant’s integration on the electrical grid and helps to compensate for the lack of dispatchability of wind and solar PV plants [1]. Additionally, the inclusion of a thermal storage system in a CSP plant increases its capacity factor and its economic competitiveness. This work regards some aspects of the operation of a single tank thermal storage system. Contrary to the two-tank system, where one tank storages hot fluid and the other storages cold fluid, in the single tank system the storage of the two levels of temperature takes place inside the single tank. Moreover, unlike the two-tank system, the single tank is filled with solid filler material (the solid filler material improves the thermal stratification on the single tank). At the upper part of the tank, the heat storage material (both the solid filler material and the storage fluid) is at a high temperature since the hot fluid is stored at the top. Below the hot temperature zone, there is a low temperature zone where the cold fluid is stored. The zone of the heat storage material with intermediate temperatures between the high and low temperatures (maximum and minimum temperatures) is called the thermocline. The aim of this work was to study the parameterization of the following properties: the degradation of the thermocline, the pressure drop, and the inclusion of the ullage space on the tank’s energetic balance. This last topic was not further studied due to the preliminary energetic analysis made. The studies were made using a one-dimensional model that calculates the parameters temperature (creating a temperature profile) and pressure drop along the tank’s height, considering the thermal properties of the materials (volumic mass, specific heat and thermal conductivity) as a function of the temperature. The degradation of the thermocline was studied through the temperature profiles that resulted from the modeled series of charges and discharges. Through these profiles, the thermocline’s degradation rate was determined. Also, its increasing height was determined and, consequently, the variation of the thermal storage efficiency as a function of the operation’s duration and number of operations of charge and discharge. Another goal of this work was to study the pressure drop of the fluid flow in the filler material. As the tank’s filler was composed of different volume non-spherical particles, a mean particle diameter was calculated. Then, the pressure drop was determined by the Ergun’s equation. Moreover, the pressure drop’s calculations also considered the thermal properties of the materials (volumic mass, specific heat and thermal conductivity) depending on the temperature. The results of the studies explain the faster degradation of the thermocline in the first operations, and analyses the influence of the porosity and mass flow rate on its degradation. Also, the results obtained for the pressure drop allow a notion of its magnitude and variation depending on if the tank is being charged or discharged.
The interest in thermal storage systems in CSP power plants has increased due to the dispatchability they give to the plants. This dispatchability improves the CSP plant’s integration on the electrical grid and helps to compensate for the lack of dispatchability of wind and solar PV plants [1]. Additionally, the inclusion of a thermal storage system in a CSP plant increases its capacity factor and its economic competitiveness. This work regards some aspects of the operation of a single tank thermal storage system. Contrary to the two-tank system, where one tank storages hot fluid and the other storages cold fluid, in the single tank system the storage of the two levels of temperature takes place inside the single tank. Moreover, unlike the two-tank system, the single tank is filled with solid filler material (the solid filler material improves the thermal stratification on the single tank). At the upper part of the tank, the heat storage material (both the solid filler material and the storage fluid) is at a high temperature since the hot fluid is stored at the top. Below the hot temperature zone, there is a low temperature zone where the cold fluid is stored. The zone of the heat storage material with intermediate temperatures between the high and low temperatures (maximum and minimum temperatures) is called the thermocline. The aim of this work was to study the parameterization of the following properties: the degradation of the thermocline, the pressure drop, and the inclusion of the ullage space on the tank’s energetic balance. This last topic was not further studied due to the preliminary energetic analysis made. The studies were made using a one-dimensional model that calculates the parameters temperature (creating a temperature profile) and pressure drop along the tank’s height, considering the thermal properties of the materials (volumic mass, specific heat and thermal conductivity) as a function of the temperature. The degradation of the thermocline was studied through the temperature profiles that resulted from the modeled series of charges and discharges. Through these profiles, the thermocline’s degradation rate was determined. Also, its increasing height was determined and, consequently, the variation of the thermal storage efficiency as a function of the operation’s duration and number of operations of charge and discharge. Another goal of this work was to study the pressure drop of the fluid flow in the filler material. As the tank’s filler was composed of different volume non-spherical particles, a mean particle diameter was calculated. Then, the pressure drop was determined by the Ergun’s equation. Moreover, the pressure drop’s calculations also considered the thermal properties of the materials (volumic mass, specific heat and thermal conductivity) depending on the temperature. The results of the studies explain the faster degradation of the thermocline in the first operations, and analyses the influence of the porosity and mass flow rate on its degradation. Also, the results obtained for the pressure drop allow a notion of its magnitude and variation depending on if the tank is being charged or discharged.
Descrição
Tese de mestrado integrado em Engenharia da Energia e do Ambiente, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2016
Palavras-chave
Tanque único Thermocline Modelo unidimensional Degradação do thermocline Perda de carga Teses de mestrado - 2016