Simulation of the operation of the Alqueva-Pedrógão system for the water supply and production of energy Ana Maria Gregório BARRONA; Rodrigo PROENÇA DE OLIVEIRA Instituto Superior Técnico Abstract The aim of this study is to analyze the capacity to satisfy the agricultural and energetic production water requirements in the Alqueva Multi-Purpose Project (AMPP) and to calculate the trade-off curves that exist between the satisfactions of the two needs. When evaluating the energy production capability, the pumpage system capacity was not considered. To make this analysis, balance equations were used to simulate the operations of the two main reservoirs, Alqueva and Pedrógão, and their capacities to satisfy the water demands. These equations were then programed in Excel, to allow a day-to-day analysis of the volume variations and satisfaction of the water demands according to different scenarios. The application of the equations used in the analysis involves the understanding of the complexity in the management of the two reservoirs. In the document it is presented a study of the Alqueva Multi- Purpose Project in all its complexity with the use of simple equations. The results, in the form of trade-off curves, reflect the relationship between the agricultural and energy production needs. We can observe in the results how the increase in the quantity of water for one need involves the sacrifice of the other. It is also possible to quantify this relationship, estimating the guarantee of the satisfaction of the water demands for different volumes of the water needs. The data used in this study was gathered from non-technical and other documents from the INAG, EDIA, REN and SNIRH websites. Some values of the flow series, the precipitation and evaporation rates were obtained from already existing studies. Key-wordsAlqueva Multi-Purpose Project, Energy, Irrigation, Trade-off curve, Optimization Introduction The AMPP is the largest hydroelectric project in Portugal. Source of debate, its development took several years and went through several governments. Even today is a source of controversy and debate, its development continues, in constant evolution adapting to the new factors that condition it. The AMPP provides water for three main purposes: urban use, agricultural and energy production. Of these three the agricultural and energy production use the greatest volume of water and a good management of hydraulic resources is required the satisfy both. The purpose of this dissertation is to analyze the capacity of the project to provide the water for agriculture purposes and for energy production. This capacity is presented in the form of trade-off curves.
General description of the project The region of Alentejo occupies a third of national Portuguese territory but it has the lowest population density and desertification is a constant threat. In this context, began the planning of the Alqueva Multi-Purpose Project (AMPP), whose purpose was the revitalization of the region by providing water to develop agriculture and the creation of a touristic space. The system is supposed to guarantee the water supply, even in years of extreme drought, serving an area covering 20 municipalities in the districts of Évora, Beja, Portalegre and Setúbal. Construction began in 1976 but due to a seventeen yearlong interruption, that only ended in 1995, with the creation of EDIA (Empresa de Desenvolvimento de Infraestruturas de Alqueva) it was only in 2002 that the floodgates were closed and the reservoir began to fill. Two years later the hydroelectric power plant began its production to the National Electric Grid. In 2006 was inaugurated the Pedrógão dam and hydroelectric power plant. This reservoir acts as Alqueva s support, in order for the storage-pump system to work. Despite its reduced capacity it is the second largest reservoir and power plant in the AMPP system. In 2012 the reinforcement for the Alqueva power plant began production for the National Electrical Grid, the Alqueva II hydroelectric power plant. With the new power plant the capacity to produce energy doubled. The AMPP irrigation system is divided in three sub-systems, Alqueva with 64 215 ha, Ardila with 31 161 ha and Pedrógão with 25 005 ha making the total area of irrigation 120 381 ha. This expansion is due to the overcapacity of the project. It was initially planned that the system would provide 6 hm3/ha/year but currently only supplies 3 hm3/ha/year expansion of the system and supplying water to neighboring reservoirs during drought years are being studied as possible ways to mitigate the problem. There are numerous infrastructures that support and assure the transport of water between the reservoirs and to their final destinations, among these there are: 69 dams, 47 pumping stations, 2 hydroelectric power stations, 5 mini-hydro power stations, and 20 reservoirs. However it is the Alqueva reservoir that is consider the mother of the entire system, since all of the subsystems develop from there. The main energy production components occur in the Alqueva power plant and in Alqueva II, each with 260 MW of installed power. The Pedrógão power plant is the second most potent, with 10MW of installed power. Several other small hydro power stations also contribute to the national grid. Simulation of the energy production in the AMPP Mathematical Model The model was elaborated in Excel and its purpose is to calculate the guarantees of supplying water for agriculture and energy production. The model has time period of 51 years, starting at 01/10/39 and ending in 30/09/89. The models provides a day-by-day simulation in order to calculate the results with more detail. The model s entry data are: the series of average daily affluent flow to the Alqueva reservoir, the average daily flow of the Ardila River, the environmental flow demands, the values of daily
evaporation and daily precipitation. There are two other variables not calculated by the model called the decision variables. The decision variables, controlled by the user, are the total water necessitates for agriculture (hm3/year) and the goal for energy production (GWh/year). The agricultural needs are distributed by the three subsystems in the following order, Alqueva is responsible for 60%, Ardila for 15% and Pedrógão for 25%. This distribution takes in account the area of each subsystem. The agricultural needs are only valid from May till September, as we consider that during the rest of the year the rain provides for any agricultural needs. During the months they are valid we consider them to be evenly distributed by the number of days. The energy needs are distributed by the Alqueva and Pedrógão dams, the Alqueva dam is responsible for the production of 90% of the energy needs and Pedrógão for the rest 10%. These needs are valid during the entire year and are evenly distributed, regardless of the conditions. Since the purpose of the AMPP is to revitalize the area by promoting agriculture in this model we consider that we can satisfy the energy production needs after we satisfy the agriculture needs. The model relies on several equations that calculate the behavior of the two reservoirs, Alqueva and Pedrógão, according to their circumstances, in terms of stored water volume and amount of water that can be used to satisfy the water demands. Of the three main needs, agriculture, energy production and ecological maintenance, only the first two have guarantees. We consider that the environmental needs are always fulfilled first. The results of the model constitute an approximation of how the reservoirs would behave in reality and estimate the demands that can be satisfied with these behaviors. Because the decision of quantity of water that is daily pumped back into the Alqueva reservoir is based on economic factors and the model did not have access to this data, the model does not include the water that is pumped back to the reservoir. Inputs Flow In the estimation of the number of flow tributary of the Alqueva for a stationary situation, coupled with the current state of water use in the basin of the Guadiana river, including in Spain, the records were used in several gauging stations located in the study area, provided by SNIRH and the results of a simulation study conducted by EDP, which estimated the flow generated in Spain for various scenarios of water use. In order to obtain a stationary series of the monthly tributary flow the values of the station of P.Mourão were affected by a coefficient in order to take into account a simulation done by COBA. Figure 1 represents the resulting monthly flow.
Evaporation The climate in Alentejo is characterized by humid and cold in winter and hot and dry during the summer. During summer there is considerable loss of water in the reservoirs. Due to the increase of surface water exposed, because of the reservoirs the loss of water to evaporation also increases. The volume of water evaporated depends on the area of the surface of the reservoir and the evaporation rate. The evaporation rate depends on several aspects like wind velocity, air temperature, humidity and solar radiation. The following chart illustrates the evaporation rate (mm) in the Alqueva and Pedrógão reservoirs (Oliveira 1994), these values will be used as the evaporation rate during the 51 years of analysis. Chart 1 - Evaporation rate (mm) Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep.. 97 50 26 33 44 87 116 168 204 167 244 164 Rainfall To calculate the amount of rainfall in the two reservoir, the data of the metrological station of Ponte de Mourão (taken from the SNIRH database) was analyzed during a period of 48 years. The following chart shows the average monthly rainfall (mm) during that time period. Chart 2 - Average rainfall (mm) Oct. Nov. Dec. Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. 55.5 73.4 76.7 68.4 66.8 71.0 55.5 37.2 23.1 5.16 3.95 25.3 2 9 9 7 3 6 4 9 4 Environmental flow Changes in the scheme of natural flows are one of the main consequences of anthropogenic changes in the environment, having a profound impact on aquatic ecosystems. Dyson et al. (2003) defines the ecological flow like water in the river needed to ensure the economic, environmental and social benefits downstream the dam, with the aim to guarantee the river's health and ecosystems that depend on it.
The original plans of maintaining the ecological flow of the EFMA were calculated using the methods of the wetted perimeter and incremental methodology. These values change throughout the year depending on the accumulated rainfall and weather forecasts. Thus the ecological flow rate used is the instantaneous minimum flow rate 3 m3 / s. Validation of the model Before presenting the results it is important to check the operation of the program. We will determine the validity of analyzing the volume observed program in two different cases. Case 1 In the first case we define agricultural needs of 700 hm3/year and the energy production goal is 100 GWh / year Figure 2 corresponds to the volume in the Alqueva reservoir. Figure 3 corresponds to the Pedrógão dam.
Case 2 Agricultural needs are 100 hm3 /year and the goal for the energy produced is 100 GWh / year. The following figure represents the initial volume in the Alqueva reservoir. The following figure is the representation of the initial volume in the Pedrógão reservoir. In the first case the minimum volume is slightly exceeded in Alqueva and Pedrógão, due to evaporation losses. In both cases the maximum amount is not exceeded. As Pedrógão volume depends on the volume discharged in Alqueva. In the first case there are erratic changes of volume, changing volume between the maximum and minimum depending on the capacity of Alqueva. In the second case, as the agricultural needs are smaller and Alqueva discharges more to Pedrógão, it keeps the volume near the maximum storage volume. Overall the model has the expected behavior and the values are within the possible limits. The trade-off curves between agricultural and energy needs
The following figure represents three trade-off curves with different guarantees: i) Both agricultural and energy production have a guarantee of 100%; ii) Agricultural needs are at 90% and energy production is at 80%; iii) Agricultural needs are at 80% and energy production is at 70%. As expected, as the agricultural needs grow, the lower the amount of energy that can be produced, because the available volume is smaller. This relationship is not linear but progressive, forming a curve. The energy produced by the model reaches 200 GWH / year, when the volume irrigation to 45 hm3 / year and then only with a guarantee of 70%. These values in no way correspond to what is actually produced in Alqueva. The energy produced by the model is apparently too low. The maximum model that can be produced on average per year is 220 GWh/year in Alqueva, and 36 GWh/year in Pedrógão, regardless of the estimated guarantee. Considering these circumstances, the average affluent volume to Alqueva is 1845 hm3, the average drop height at Alqueva is 51 m in Pedrogão is 24 m, we can estimate that the amount of energy that can be produced is on average a year, the system is 266 GWh, not counting the energy produced after pumping. Conclusions The EMFA is a complex project and its development caused profound impacts on the landscape, culture, the economy and the environment of Alentejo. The main objective of EFMA is the revitalization of the Alentejo, by providing water for irrigation to promote agriculture. Today the
production of energy also plays an important role in the enterprise, impacting the economy and the environment of the country. Both uses are important to the people who benefit from the project directly and for the country as a whole. As such it is important to understand and optimize the ability to satisfy both. The aim of this work is to analyze the relationship between water withdrawn for irrigation and discharged for energy production, by developing a mathematical model that simulates their relationship. The energy produced by this model does not account for the pumping capacity of EFMA and subsequent discharging water, and therefore the power generation amount is significantly less than what occurs in reality. Overall the program has fulfilled its purpose, establishing the relationship between the two needs. The Alqueva-Pedrógão system is very complex, and there is not exactly a default value to be imposed. For more accurate results would need to create a much more complex model that would take into account both the situation of each day as well as future situations and economic factors. References Alqueva, Características da área de regadio (obtido no site http://alqueva.com.pt/pt/#/regadio/areaem-exploracao/mapa-de-evolucao/10, consultado em Abril de 1015) Alqueva, Características das culturas em exploração (obtido no site http://alqueva.com.pt/pt/#/regadio/ocupacao-cultural/11, consultado na Abril de 2015) Alqueva, Ministério da agricultura e do mar. Quantidade e Qualidade da Água em Alqueva. Governo de Portugal António Pinheiro, António Moisés, Carlos Gaspar, Estação elevatória Pedrógão-margem esquerda e reforço de potência de Pedrógão. Conceção e principais caraterísticas das obras) Jornadas Técnicas APRH CNPGB, Características das barragens de Portugal (obtido no site http://cnpgb.apambiente.pt/gr_barragens/gbportugal/aa.htm, consultado em Agosto 2014) Consugal, Barragens de Amoreira, Brinches e Serpa Direção Regional de Agricultura e Pescas do Alentejo (2013). Caracterização agrícola do Alentejo Central. Ministério da agricultura e do mar Direção Geral de Energia e Geologia. Consumo de energia elétrica (obtido do site http://www.dgeg.pt/, consultado em Setembro 2015) EDIA, Caraterísticas das albufeiras e infraestruturas na região de Alqueva (obtido no site http://sigims.edia.pt/alqueva_storymap/, consultado em Julho 2014) EDIA (2005). Estudo de Impacto Ambiental da Rede Primária do Subsistema de Ardila EDIA, Gestão da rede primária do EFMA manutenção do regime de caudais ecológicos. O caso de Alqueva-Pedrógão. Jornadas Técnicas APRH EDIA (2014), Relatório de contas 2014 EDIA (2012), Relatório da atividades 4ºTrimestre 2012 EDIA (2013), Relatório da atividades 4ºTrimestre 2013 EDIA (2014), Relatório da atividades 4ºTrimestre 2014 EDIA (2015), Relatório da atividades 1ºTrimestre 2015
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