سیستم های ذخیره سازی پمپ آبی مجهز بادی برای جزیره های راه دور: تجزیه و تحلیل حساسیت کامل بر اساس دیدگاه های اقتصادی

The electrification of the non-interconnected Greek islands is mainly based on Autonomous Power Stations (APSs) that are characterized by considerably high electricity production cost, whilst, in several cases, problems related with power shortage are encountered. At the same time, the contribution of wind energy is significantly restricted due to electrical grid limitations imposed to “secure” the stability of the local network and thus resulting in significant rejected wind energy amounts. On the basis of sensitivity analysis, the present study evaluates the techno-economic viability of a system that incorporates the simultaneous operation of existing and new wind farms (WFs) with pumped storage and hydro turbines, which are able to provide the electrical grid of a remote island with guaranteed energy amounts during the peak load demand hours on a daily basis. The performance of the system is simulated during a selected time period for various system configurations and an attempt is made to localize the optimum solution by calculating various financial indices. Emphasis is given on the conduction of an extensive sensitivity analysis considering three main variables (i.e. produced energy selling price, the percentage of state subsidization and the price of the wind energy surplus bought from the already existing WFs) taking also into account several constraints of the national legislation. Based on the most economically viable (payback period quite less than 10 years) configuration derived (24 MW WFs, 15 MW water pumping system, 13.5 MW hydro turbines), the contribution of renewable energy increases by almost 15% (in absolute terms) compared to current conditions, reaching about 25% of the island’s energy consumption pattern. The proposed analysis may be equally well applied to every remote island possessing remarkable wind potential and appropriate topography.

The energy production of the non-interconnected Greek islands is mainly based on Autonomous Power Stations (APSs) which consume conventional fossil fuels, while the contribution of Renewable Energy Sources (RESs) (mainly wind) accounts for only 9% of the total electricity generation in these regions. Specifically, in 2008, the total electricity production was approximately 6250 GW h from which only 580 GW h derived from renewables [1]. The Greek Public Power Corporation (PPC), being the exclusive supplier and practically the sole producer of the energy deriving from fossil fuels combustion, faces considerable electrification problems related to power shortage, seasonal load demand variations, weak transmission electricity networks and outdated thermal power units. Moreover, the electrification of the islands is closely associated with very high energy production costs that may in many cases exceed – during peak hours – the 200 €/MW h, much depending though on oil prices since the fuel cost sharing accounts for more than 50% [2] and [3]. On the other hand, despite the high wind potential encountered in many Greek island regions, the wind energy contribution to the electrification of these areas is significantly restricted mainly due to existing technical barriers, which protect the autonomous electrical grids from possible instability problems [4] and [5]. The intermittent nature of the wind in combination with remarkable fluctuations of daily and seasonal electrical load demand have lead to strict wind energy penetration limits [6] and [7], thus making it difficult to achieve higher than 15% wind energy contribution in autonomous electrical networks [8] and [9]. To overcome the intermittency and uncertainty of wind and to establish wind power as a more reliable technology able to remarkably contribute to the electrification of remote areas, a combination of hydro and wind power generation by means of pumped-storage [10], [11] and [12] under economically viable terms seems to be the most advantageous solution [13], [14], [15] and [16]. In this context, the techno-economic performance of a wind-based pumped-hydro storage (WPHS) system [17] and [18], capable to exploit rejected wind energy amounts from both existing and new wind farm (WF) installations is examined, using as a case study the island of Lesbos, Greece. The operation of the system is based on the condition of guaranteed energy supply to the local island’s grid on a daily basis during the peak load demand hours when the marginal electricity production cost of the conventional power station is very high. The WPHS system exploits an amount of the wind energy surplus but in case that the water stored in the upper reservoir is not enough for the fulfilment of the condition of guaranteed energy production, the water pumps absorb any energy required by the grid during low demand periods (i.e. nights), when the energy production cost of the conventional power station is low. In this context, the operation of the WPHS system is simulated for various system configurations with a use of an integrated computational algorithm [19]. Particularly, the developed numerical code is applied for different values of daily guaranteed energy delivered to the island’s local electrical grid at peak hours and thus several operational modes of the proposed system result [17]. In the current work, emphasis is given on the economic behavior of the proposed project based on extensive sensitivity analysis by taking also into consideration various limitations and valuation criteria. Finally, an optimum system configuration is proposed which represents a cost competitive solution compared with the current electricity consumption patterns of Lesbos island.

The optimum sizing of a WPHS system able to provide the local grid of the island of Lesbos with guaranteed hydroelectric energy amounts during the peak load demand periods, was analyzed in detail. Several – technically acceptable – combinations of the key system’s components (i.e. WFs, hydro turbines, pumps, reservoirs) have been derived by the application of a numerical algorithm and an attempt has been made to obtain the optimum solution by calculating various financial indices such as the net present value, payback period and energy production cost. The optimum solution was selected based on the maximization of the net present value, while also taking into account various other constraints and parameters of the existing Greek legislation. Moreover, with the use of extensive sensitivity analyses, the great dependence of the investment’s financial indices on parameters such as the produced energy selling price, state subsidization and price of the wind energy surplus bought by the existing WFs was pointed out. According to the results obtained, the following main conclusions may be drawn: The most economically viable solution (giving up to npv20 = 3.2 and payback period less than 10 years, depending on the financial scenario adopted) is found to be the recovery of a considerable amount (i.e. 65%) of excess energy (i.e. Erejected = 38,000 MW h) produced by 24 MW WFs (9 MW existing + 15 MW new) with the use of 15 MW water pumping system, 13.5 MW hydro turbines’ nominal power, 396,727 m3 minimum upper reservoir volume and 2.5 m tubes diameter. It is worth mentioning that for any subsidization scenario (γ = 0%, 20% or 40%), any price of the energy bought by the existing WFs, (View the MathML sourcecWFO∗ = 0, 30 or adapted) and produced energy selling price from 100 to 200 €/MW h, the energy production cost of the WPHS system is fairly less than the “peak” energy production cost of the thermal power units (i.e. 250 €/MW h) and from the avoidable associated fuel cost (i.e. 150 €/MW h). Particularly, the energy production cost of the optimum solution fluctuates between 10 and 135 €/MW h, depending on the financial scenario adopted. At this point, it should be noted that, the energy production cost of the proposed project may be easily estimated at the beginning of its implementation and does not retain risks during its lifetime, as is the case of fossil fuels. Based on the optimum financial scenario, after the installation of such a project, RES contribution increases by 12% (in absolute terms) compared to current conditions, exceeding 23% to the island’s energy balance. On top of that, it is worth mentioning that if it were to install only WFs (new installations of the order of 15 MW) without pumped-storage, RES contribution in the energy consumption of the island would be hardly over 15%. Nevertheless, the proposed solution is indicative and constitutes an initial approach of the problem as it depends on a large number of factors that may change such as geographical parameters (e.g. area limitations) and evaluation criteria (e.g. maximization of the npv). In this context, what should be underlined is that if one neglects the economic performance of such a system and evaluates its behavior only from an energy point of view (e.g. maximization of the wind energy absorption for storage, minimization of conventional energy requirements by the local grid, other energy considerations), the results could differ considerably. However, such evaluation is included in previous works of the authors [17], [18] and [19]. In any case however, from the present analysis it became inarguable that under certain conditions, WPHS systems may considerably contribute to the reduction of the current high energy production costs, by taking advantage of the excellent wind potential which is encountered in most of the Greek islands and up to now is only partially exploited. Furthermore, this system constitutes a solution to the problem of the restricted wind energy contribution to the islands’ energy balance, while, at the same time, encourages future investments in wind energy applications since a considerable amount of the expected wind energy rejection is exploited and thus creating additional revenues for the WFs’ investors. Also, benefits for the autonomous electrical grids such as power and frequency control and energy reserve services may be obtained. Finally, the socio-economic benefits of such systems should not be neglected since they contribute to regional development by creating local job opportunities and reducing harmful emissions related with the fossil fuel combustion.