A research group from Italy’s University of Bologna has simulated adding a floating PV (FPV) plant to an existing pumped-storage hydropower (HP) plant in the Swiss pre-alpine region. The Etzelwerk is an open-loop pumped-storage hydropower plant, which uses the difference in elevation of about 480 m between Lake Sihl and Lake Zurich to generate electricity for the Swiss Federal Railways (SBB).
“We analyzed the impacts of solar-hydro hybridization through a multi-disciplinary framework, particularly focusing on implications for water resources management,” corresponding author Domenico Micocci told pv magazine. “Moreover, the research was carried out over a long-term simulation horizon of 38 years. This allowed for better characterization of hydro-climatic conditions that drive water availability and PV production.”
According to the researchers, the Etzelwerk plant currently consists of seven Pelton turbines, which can process up to 34 m3/s, providing an installed capacity of 120 MW. In addition, three 5-stage pumps, with a total capacity of 54 MW, are able to pump water back from Lake Zurich to Lake Sihl.
In their simulation, the team has assumed an FPV plant covering 10% of Lake Sihl. Assuming a monocrystalline panel with a peak power of 375 MW and efficiency of 20.4%, on an area of 0.315 km2, the nominal capacity of the FPV site was 64.12 MW. The hydrological model was based on meteorological data over the period 1981-2018, while demand levels were supplied to the researchers by the SBB.
“We simulated the operations of the hybrid HP-FPV plant according to three different scenarios,” explained the academics. “NoPV was the base scenario, with no PV to support HP. In the PV1 scenario, the FPV plant is introduced and solar energy, when available, contributes to fulfilling the demand and/or is used to pump water from Lake Zurich to Lake Sihl and/or is sold in case of excess. The PV2 scenario is similar to PV1, but assuming that 50% of the water saved thanks to PV production can be released from the reservoir to support downstream conditions during low-flow periods.”
The results of the simulation showed that the addition of FPV to the plant had increased the total energy production by about 20%. In the case of NoPV, hydropower has supplied a yearly average of 256.6 GWh, PV1 has provided a total of 319.1 GWh, and PV2 has supplied 315.2 GWh. In PV1, the HP had supplied 257.7 GWh, and the PV contributed 61.4 GWh; while in the PV2 scenario, it was 254.1 GWh and 61.1 GWh, respectively.
“Thanks to hybridization, the plant fails to satisfy the demand much less than the conventional HP plant,” added the team. “The shortage index, which is related to the system reliability, decreases from 11.28% (scenario NoPV) to 3.24% (scenario PV1) and 3.53% (scenario PV2). Scenarios PV1 and PV2 do not significantly differ: additional releases therefore seem not to deeply affect the system reliability.”
The analysis also showed that the additional release of water in scenario PV2 increases the mean monthly discharge provided downstream, from 14% in May to about 50% between June and August. “Our simulations show that there seems to be a potential to provide additional environmental releases to the downstream river reach, without strongly invalidating other benefits already pointed out in previous studies, such as the increase of energy production and the improvement in the reliability of power supply,” concluded Micocci.
The research findings were presented in “Hybridization of an Alpine pumped-storage hydropower plant with floating solar photovoltaics: a study from the water resource perspective,” published in Renewable Energy. Scientists from Italy’s University of Bologna and the Swiss Federal Institute for Forest, Snow and Landscape Research have contributed to the study.
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