Modeling a Pump Storage System for São Miguel

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1 Modeling a Pump Storage System for São Miguel C. Ponte 1 Instituto Superior Técnico, Lisbon, Portugal A successful energy system requires a commitment of several variables, which lead to a sustainable system. This paper models several scenarios for a pump storage system in São Miguel, with different investments in renewable energies regarding EDA s proposes to find the best solutions to build the storage system taking into account its limitations. In order for this to happen, the economic, environmental and energetic compromises are going to be the main variables in the study, to find the best solutions for 2015 and The concept of a pump storage system is to pump water from a lower reservoir to an upper one when there is excess electricity production from renewable sources, and turbine the other way around when it is needed. The implementation of this project is essential to reach the objectives of the MIT- Portugal Green Islands Project, local Government and EDA S.A.. Key words: System, Energy, Storage, Renewable, Costs and Environment Introduction The investments in renewable energies and in energetic efficiency increase are more and more an objective and a need to undertake in order to decrease the pollutant gas emissions and the dependence on fossil fuels. In 2008, electricity production from renewable energies in Azores was 28% and today the local Government and EDA are projecting to invest strongly in this areas with the main objective of increasing the electricity production from renewable energy sources in all the Archipelago up to 75% until 2020 (EDA Informa Vol ). São Miguel Island is a great place with enormous potential in renewable energies. Nowadays geothermic, river hydro and wind are the technologies we can find on this island. However, a biomass power plant is being studied and is considered in this paper. It is not possible to import or export electricity, as an isolated system, so this energy system is very complex and difficult to manage. Knowing that, the load diagram of São Miguel is a characteristic curve, which has a high peek during the day and a low peek during night and also varies according to the days of the week and seasons. This is one of the obstacles to overcome in the future. Rendering the load diagram as stable as possible is an important goal to reach. There are 2 types of renewable energies, the ones that have a continuous production, and the ones who s production is variable, because they depend on the environment conditions. 1 Mechanical Engineer Master Student, Instituto Superior Técnico, Av. Rovisco Pais, Lisbon, Portugal, crispim.ponte@ist.utl.pt 1

2 These 2 types don t have flexibility to adapt to the variation of load, so this work, today, is done by the thermal power plant that guarantees the flexibility to the energy system needed to adjust the load variations during a day. This is really important, since one of the objectives of this pump storage system is to help and contribute to this load control, substituting the thermal power plant as maximum as possible. Taking into account the load diagram, the increase of renewable energy power in this system will only be sustainable until it reaches the low peek of load; otherwise energy is going to be wasted. So the idea is to increase the renewable energy power above today s lower peek and consume that excess amount with this pump storage system, due to the pump consumption at night. In the higher peeks this system will turbine from the upper to a lower reservoir to produce electricity. This system will not only be useful for the energy system, allowing it to strongly increase the renewable energy percentage in the system and reducing the use of the thermal power plant, but also it will attenuate the eutrophication problem that Furnas lagoon is facing, due to water circulation, since the lower reservoir is going to be the lagoon. Other environmental aspects were also studied, like the lagoon s water level and erosion from the turbine discharge. Several scenarios were modeled to study this power plant in order to find the best solutions for this system to become sustainable and viable. Energy Storage Systems There are a wide variety of energy storage technologies with their own specific characteristics, costs, technological maturity and applications. As renewable energies have a variable or a continuous production, it is difficult to manage and control the electric system, so that storage systems can manage these irregularities and yet increase the percentage of renewables in the system. Nowadays, energy storage systems are facing a revolution and are starting to be a realistic option for: 1. Restructuring electric markets; 2. Integrating renewable sources; 3. Improving energy quality; 4. Helping to increase the distribution of energy production; 5. Helping to satisfy the net to accomplish environmental requirements. There are 11 energy storage technologies that are important or are being developed: Pumped Hydroelectric Energy Storage PHES; Underground Pumped Hydroelectric Energy Storage UPHES; Compressed Air Energy Storage CAES; Battery Energy Storage BES; Flow Batteries Energy Storage FBES; Flywheel Energy Storage FES; Hydrogen Energy Storage System HESS; Supercapacitor Energy Storage SCES; Superconducting Magnetic Energy Storage SMES; 2

3 Thermal Energy Storage TES; Electric Vehicles Evs. In this paper a detailed analysis is only for PHES since it is the technology studied. Pump storage systems and being used since 1929 (Cheung 2003) to level the daily load in the electric net. The first PHES that uses seawater was built in 1999, but it has a big disadvantage - corrosion (Hulihara 1998). This system consists of 2 reservoirs in different heights and an installation of pumps/turbines. In low electricity demanding periods the excess of production by renewable energies is availed to pump the water from the lower to the upper reservoir. In the high electricity demanding periods that storage water is discharged for turbines to produce electricity. In Figure 1 is presented a schematic form of this system: Figure 1 - Schema of PHES (Murphy 2011) Its efficiency is between 70% and 85%, however variable speed machines are now being used to improve this (Anagnostopoulos 2007) and it is limited by the pump/turbine facilities (Gonzalez 2004). It can work as a supplier of electricity, frequency regulator and load regulation in pumping and turbining. The overall costs are $600/kW (Gonzalez 2004) and $2000/kW (Baxter 2006) depending on several aspects. However, new equipment with variable speed that can increase in 15% to 20% its capacity and improve in 3% its efficiency is being developed (Baxter 2006). Case Study São Miguel is the biggest Island from Azores archipelago. It has 4 different volcanoes and in the crater of Furnas volcano we find a beautiful lagoon with a lot of local and unique vegetation. This region is well known because of the Furnas lagoon with natural boiling water and their own particular gastronomy. It s at this lagoon that EDA, Azorean electricity production company, wants to build the energy storage pump system. Until this date, 3 different projects that study this power plant were developed. Two of them were technical projects where the engineers studied different solutions for the power plants construction and the other objective was to understand the impact of the energy storage pump system in the actual energy system running in São Miguel. This paper used the power plant design from the Estudo de Viabilidade para Central Hidroeléctrica Reversível da Lagoa das Furnas TPF Planage Nueva Econoler Spain October 2011 to model the pump storage system in the 3

4 software, which had the better efficiency. In this project 3 Pelton turbines were used with a total power of 11,49 MW and 5 pump groups, each group with 1 booster pump and 1 lifting pump, with a maximum power of 10,5 MW. Moreover, this project designs 2 different tubing, one for pumping and the other one for turbine, which allows the power plant to turbine and pump at the same time, for net regulation purposes. The other project regarding the study on the impact in the energy system was very important not only to understand today s power implemented in each power plant, but also forecasted the electricity demands until However, these projects are missing the modeling and analysis of the pump storage system to understand how or when it is viable and sustainable in the compromise of 3 variables: economic, environment and energy. Software tool: Energy Plan There are several software tools with different characteristics to model and analyze an energetic system. However, just a few tools are capable to model all variables of an energetic system. In order for this, the objectives of the modulation need to be established first and then the best tool to model the system has to be chosen. In this case the main objectives are: Quantify the penetration of renewable energies in São Miguel s energy system; Study the economic, environmental and technical/energetic sustainability and viability of the energy storage pump system. Take into account a list of software tools, EnergyPlan was picked to model this system, since it has a simple interface organized in tabs, it is easy to learn to model, the software download is free and its results are constantly published in academic articles. Figure 2 shows a scheme that describes the modeling inputs for this energy storage pump system: Figure 2 EnergyPlan modeling scheme for this system 4

5 Scenarios Energy Analysis To validate this study several model scenarios were needed to include different renewable power plants investment in 2015 and 2020 and to understand witch ideas were more suitable to reach the company s objective. This paper is only going to make reference to the most critical and important results. In each scenario 3 different technical options are studied. Firstly, 3 different scenarios were modeled regarding the year of 2010 with 23 MW of geothermal (reference scenario), year of 2015 and 2020 with 9 MW of wind and 23 MW of geothermal. In these scenarios there are no differences between each technical option in each scenario and their results claim that the energy storage pump system is not used. Therefore, there is no excess electricity production to be used by the pump system. It is demanding to invest more in the renewable capacity of the energy system of São Miguel so the pump system may become sustainable. Nevertheless, with the introduction of wind in 2015 and 2020 it s important to decrease the use of the thermal power plant. Secondly, 3 more different scenarios were modeled regarding: Year of 2015 with 37 MW of geothermal, 9 MW of wind and 4,33 MW at low load (22h-7h) and 6,48 MW of biomass, scenario 2; Year of 2015 with 37 MW of geothermal, 9 MW of wind and 6,48 MW of biomass, scenario 3; Year of 2020 with 37 MW of geothermal, 20 MW of wind and 6,48 MW of biomass scenario 4. The 3 technical options are: 1. Turbine power 11,49 MW, pump power 10,5 MW and a reservoir of m 3 ; 2. Turbine power 11,49 MW, pump power 10,5 MW and a reservoir of m 3 ; 3. Turbine power 5 MW, pump power 6,85 MW and a reservoir of m 3. For these scenarios the results were really interesting as we can see at Table 1: Wind River Hydro CSHP PP2 Geo Pump Turbine CEEP % CEEP RES ,35 19,22 44,91 82,14 258,01 8,83 6,84 0,14 0, ,49 80, ,83 19,28 44,91 81,68 258,01 9,43 7,30 0,07 0, ,02 80, ,72 19,26 44,91 82,58 258,01 8,27 6,40 0,1 0, ,89 80, ,41 19,09 53,01 76,04 258,01 12,16 9,41 0,31 0, ,52 81, ,26 19,25 53,01 75,15 258,01 13,31 10,31 0,17 0, ,53 81, ,25 19,11 53,01 76,85 258,01 11,12 8,61 0,22 0, , ,29 19,03 53,01 68,92 258,01 12,68 9,81 0,29 0, ,34 83, ,26 19,13 53,01 66,49 258,01 15,82 12,25 0,22 0, ,41 83, ,11 19,11 53,01 69,72 258,01 11,64 9,01 0,23 0, ,24 83,5 Table 1 - Energetic results of scenarios 2, 3 and 4 In red are the maximum results and in blue the minimum results. Each scenario pretends to have maximum values except in the production of the thermal power plant and in the excess of electricity, which depends on the company manager. Technical option 2 always shows to be the best solution between the 3 options modeled. Between scenario 2 and 3 despite the small increase of renewable electricity production, the difference of biomass is very important RES % 5

6 to increase the use of the energy storage system and to decrease the use of the thermal power plant. However, the CEEP value also increases and this amount is wasted, but it is important to have CEEP because it gives to the system capability to keep the storage system working well, regarding the increase of electricity demand forecasted by EDA for the following years. Scenario 4 is important to understand that despite the increase of electricity demand, the injection of 11 MW of wind is enough to cover that increase and to even have less use of the thermal power plant than the scenario 3. Furthermore, all 3 scenarios overcome the percentage of renewables objective. Thirdly, several other scenarios were tested to understand some particularities in the previous scenarios. Three more scenarios were modeled regarding the year of 2020 to understand which investment was important to maintain the sustainability of the storage system. From the results it is concluded that it is important to have 37 MW of geothermal and at least one more investment, 11 MW of wind or 6,48 MW of biomass, otherwise the use of the energy storage pump system is too low. However, in these cases the value of CEEP is low, so it only guarantees sustainability to the storage system in a very short period until CEEP reaches zero with the increase of electricity demands. Also 2 more scenarios were modeled where we can understand from scenario number 3 and 4 when CEEP becomes zero, moment when the storage system is no longer sustainable. Scenario 3 is for 2029/2030 and scenario 4 is for 2048, supposing there would be no more investments. At last were modeled 2 more scenarios, scenario 11 regarding scenario 2 and scenario 12 regarding scenario 3, where it is possible to understand the impact of not investing in 37 MW in geothermal but investing in 29 MW. In Table 2 we can see the results: Scenarios Wind River Hydro CSHP PP2 Geo Pump Turbine CEEP RES ,35 19,22 44,91 82,14 258,01 8,83 6,84 0,14 357,49 80, ,83 19,28 44,91 81,68 258,01 9,43 7,3 0,07 358,02 80, ,72 19,26 44,91 82,58 258,01 8,27 6,4 0,1 356,89 80, ,57 19,33 44,91 134,84 202,22 1,66 1, , ,57 19,33 44,91 134,84 202,22 1,67 1, , ,53 19,33 44,91 134,87 202,22 1,63 1, , ,41 19,09 53,01 76,04 258,01 12,16 9,41 0,31 364,52 81, ,26 19,25 53,01 75,15 258,01 13,31 10,31 0,17 365,53 81, ,25 19,11 53,01 76,85 258,01 11,12 8,61 0,22 363, ,53 19,33 53,01 127,17 202,22 3,41 2, ,1 70, ,57 19,33 53,01 127,14 202,22 3,45 2, ,14 70, ,41 19,32 53,01 127,27 202,22 3,28 2, ,96 70,5 Table 2 Energetic results of scenarios 2, 11, 3, and 13 In this test is important to understand that the volume of the upper reservoir is insensible for 29 MW of geothermal and for 37 MW it is. Also there is a very large increase of the use of the thermal power plant, there is no excess of electricity and therefore the use of the storage system is low. Furthermore the percentage of renewables in the system is below the RES % 6

7 objective. In order that, 29 MW of geothermal is not as sustainable as the investment of 37 MW is. Economic and Environmental analysis In this analysis it was assumed that the costs of the reservoirs of and m 3 were the same. It is expected that the variation of the use of the thermal power plant above analyzed is going to effect in the same way the fuel consumption costs, the CO 2 emissions and its costs. The scenario analysis was made the same as the energetic analysis. In order that, in the first 3 scenarios there is a very interesting result that is the investment in 9 MW of wind decreases the annual total cost regarding the reference scenario. So the energetic results are bad for the storage system, but the economic and environmental results are in favor. For the second 3 scenarios the economic and environmental results lead to the same conclusion as the energetic results, the strong investment in renewable energies decreases greatly the total annual cost. It is interesting that the total annual cost of scenario 3 is less than the scenario 2, which indicates that the investment is compensated, but from the scenario 3 to scenario 4 it is the opposite. So the investment in 11 MW of wind does not compensate in the economic point of view regarding scenario 3, but in the technical option 2 it compensates regarding scenario 2. For the third 5 scenarios, in the first 3 the total annual cost and the emissions are lower if we invest in the biomass power plant instead of wind. In the second 2 scenarios it is very interesting that the total annual cost for these scenarios in extreme conditions are lower than the reference scenario. In the last 2 scenarios the results were expected. Despite the large increase of the total annual costs regarding scenarios 2 and 3, the scenario with 29 MW regarding scenario 3 is better in the economic and environmental point of view. Conclusions and recommendations Several conclusions were already made along this paper. However it is important to state that scenario 3 is the best scenario in the energetic, economic and environmental point of view. This is the scenario that gives sustainability to the energy system and to the energy storage system and guarantees its work in the following years. Scenario 4 is also sustainable, in an energetic and environmental point of view but economically it doesn t compensate the investment regarding the scenario 3. However it does compensates for the technical option 2 regarding the scenario 2. For the volume of the reservoir, the m 3 is always a better solution, except when the investment in geothermal is 29 MW instead of 37 MW. In this case there is a difference between the 2 volumes here considered. 7

8 A very interesting conclusion is that it is more and more expensive to increase the renewable percentage in the system as we can see in the Table 3 and Figure 3: Scenarios %RES Total annual cost in thousands of , , , , , , , , , , Table 3 Reference table to figure % RES Total Annual Cost in thousand of Figure 3 - %RES vs Total Annual Costs References Anagnostopoulos, J. S. & Papantonis, D. E. Pumping station design for a pumped-storage wind-hydro power plant. Energy Conversion and Management, Baxter, R. Energy Storage - A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation, Cheung, K. Y., Cheung, S. T., Nvin De Silva, R. G., Juvonen, M. P., Singh, R. & Woo, J. J. Long-Scale Energy Storage Systems. London: Imperial College, EDA Informa Vol O presente e o futuro. Vols Ponta Delgada: Electricidade dos Açores, Gonzalez, A., O'Gallachóir, B., McKeogh, E. & Lynch, K. Study of Electricity Storage Technologies and Theis Potential to Adress Wind Energy Intermittency in Ireland. Final 8

9 Report, Department of Civil and Environmental Engineering, University College Cork, University College Cork, Hulihara, T., Imano, H. & Oshima, K. Development of Pump Turbine for Seawater Pumped - Storage Power Plant. Hitachi Review, Murphy, Tom. Do the Math. 15 de 11 de (acedido em 03 de 10 de 2012). 9

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