Feasibility study of evaporative cooling and flushing WC using rainwater in a public building in the city of Gothenburg, Sweden

Abstract

[ES] Hoy en día, en el sector de la edificación se utilizan grandes cantidades de agua potable para diversos fines y funciones, como agua caliente para la ducha, la bañera y el lavabo y las cisternas de los inodoros. El cambio climático está contribuyendo notablemente a experimentar la escasez de agua en algunas partes del mundo. Sin embargo, actualmente, existen diferentes posibilidades tecnológicas innovadoras de sistemas de agua que podrían disminuir tanto los requisitos de potencia como el uso de energía y la carga sobre los diferentes recursos hídricos, las redes de distribución y la gestión de las aguas residuales, acercándose así al ODS 6 "Agua limpia y saneamiento". A su vez, el calentamiento global está afectando críticamente a las necesidades de refrigeración, convirtiéndolas en una cuestión de importancia primordial. Dado que los últimos siete años han sido los más calurosos jamás registrados, el consumo energético implicado en la refrigeración de espacios ha más que triplicado el valor de 1990 y el mayor crecimiento anual de la demanda de tuvo lugar en el año 2021, representando prácticamente el 16% (alrededor de 2000 TWh) del consumo final de energía dentro del sector de la construcción (IEA, 2022). Por tanto, existe una urgente necesidad de desarrollar nuevas e innovadoras medidas y tecnologías con el fin de reducir considerablemente el consumo de energía y conseguir así resultados más limpios y eficientes y así estar alineados con el ¿Net Zero Scenario¿ para 2050, donde el consumo energético en el sector de la edificación descendería un 25% respecto a la situación actual (IEA, 2022). Como consecuencia, se mejorarían y perfeccionarían algunos Objetivos de Desarrollo Sostenible como "Energía asequible y no contaminante" (ODS 7), "Industria, innovación e infraestructura" (ODS 9), "Ciudades y comunidades sostenibles" (ODS 11) y "Consumo y producción responsables" (ODS 12). El objetivo principal de este proyecto es estudiar y evaluar la viabilidad de instalar un sistema de refrigeración por evaporación que utiliza agua de lluvia en un edificio público de Gotemburgo (Suecia). Esta agua de lluvia se utilizará también en las cisternas de los aseos del edificio. Gotemburgo puede considerarse un lugar adecuado para implantar el sistema ya que mantiene un régimen de lluvias relativamente moderado a lo largo de todo el año, existiendo por tanto la posibilidad de aprovechar sus precipitaciones. El término viabilidad podría incluir un estudio tanto del consumo de agua como el de energía, un análisis de la calidad del agua y la optimización de algunos componentes del sistema tales como el tamaño de los depósitos. Tras realizar una revisión bibliográfica, se modelará el sistema en el software Equation Engineering Solver (EES) con todos sus respectivos componentes, se simulará y comenzará la evaluación estudiando tanto los aspectos hídricos como los energéticos. Una vez se tenga una primera idea del funcionamiento y el rendimiento del sistema, se interpretarán y analizarán los resultados y se investigarán ciertos parámetros para intentar optimizar el modelo.

[EN] The project "Feasibility of evaporative cooling and toilet flushing using rainwater in a public building" combines rainwater harvesting with the evaporative cooling technology and toilet flushing and explores the practicality and viability of implementing this innovative system in the city of Gothenburg, Sweden. This study comprehensively investigates various aspects related to water and energy, and the entire system is computationally modelled using the Equation Engineering Solver (EES) software. The primary objective of the project is to assess the feasibility of utilizing harvested rainwater either for employing evaporative cooling for temperature control or for flushing toilets in a commercial building. By leveraging these sustainable practices, the project aims to reduce both water consumption and energy usage, thereby promoting environmental conservation and decreasing the ecological footprint. The study involves an in-depth analysis of several key aspects. Firstly, the local climate and rainfall patterns in Gothenburg are examined to evaluate the availability and adequacy of rainwater as a resource. Secondly, evaporative cooling technology is extensively investigated to determine its efficiency in cooling indoor spaces. The thermal dynamics and cooling potential of the system are analyzed, taking into account factors such as temperature differentials, humidity levels, and air circulation. The Equation Engineering Solver software is employed to simulate and compute the system's performance under varying conditions, allowing for predictions and optimization. For doing that, three cases with different conditions concerning the priorities of cooling and flushing necessities and the water volume in the two existing tanks in the installation have been developed in order to analyse the benefits they would bring in terms of energy and water savings. The comprehensive analysis of the technical and environmental feasibility of the three cases of study provides some advantageous results. Starting by analyzing the water aspect, in the first case, the one that prioritizes the cooling aspect while the water to fill the toilet flushing necessities is provided just if in the second tank there is any water left, a total of 37% and 10% of the requirements would be filled for the AHU and the toilet flushing respectively. Secondly, the case that still prioritizes the cooling demand but water for toilet flushing is provided as long as the storage tank is half full, meets 69% and 8% of the demand of the AHU and the flushing respectively. Laying aside the flushing aspect considering simply cooling in the third case, 75% of the supply within the AHU is met. When it comes to the thermal part, on the one hand, fractions provided by the new evaporative system are 22%, 32% and 49% respectively. This is due to the fact that almost every month in the three cases studied aims for extra cooling since the system does not hold the quantity of water required and therefore, an extra supply from the current cooling device is needed. Nevertheless, the system brings energy savings of 1005.3, 1510.4 and 2022.4 kWh per year. Furthermore, the energy consumption, in the three cases is very similar and low. The conclusions of the project, from a technical point of view, despite considering the fact that the requirements are not entirely met with actual medium contribution fractions either within the water and cooling aspects, with the existing water availability, the tanks and the pumps sizing and the existing AHU, the system is reliable and is capable to perform properly. Therefore, it can be concluded that it is technically feasible. Concerning the environmental feasibility, the key point of the study, depending on the case, the system would bring different savings. On the one hand, the water savings are 40.3, 47.0 and 8.2 m3 yearly for every case respectively, concluding that the second case is the one that would entail the highest water savings to the system. On the other hand, the energy savings hold values of 1005.3, 1510.4 and 2022.4 kWh annually, with the third case bringing the greatest value. Consequently, the three cases can be considered environmentally feasible since all of them achieve the overall objective of the project, the reduction of water and energy usage. However, depending on the savings’ preferences, either the second case with the highest reduction in water or the third case with the highest decrease in energy could be considered the best ones. From a general point of view, the -4- second case could be the most suitable to set since it is the one that saves more water and moreover, it brings an energy reduction between the average values in the first and in the third case. These outcomes provide valuable insights and inform decision-making processes for the successful implementation and adoption of the system, and they guide future implementation strategies and contributions to sustainable urban development.