بهره برداری از جریان آب به طور موقت توسط نیروگاه ترکیبی PV-برق آبی

The paper presents a new type of Renewable Energy Sources (RES) suitable for exploitation watercourse with periodical-temporary water flow. This innovative solution consist of Hydroelectric Plant (HEP) and solar Photovoltaic (PV) generator working together as one hybrid power plant, producing green energy with the same characteristics as classical hydroelectric plants. The main objective of this hybrid solution is achievement of optimal renewable energy production in order to increase the share of RES in an Electricity Power System (EPS). As a paradigm of such exploitation of RES, the example of HEP Zavrelje/Dubrovnik in Croatia was used, where it was ascertained that the proposed solution of hybrid PV-HEP system is natural, realistic and very acceptable, which enhances the characteristics of both energy sources. The application of such hybrid systems would increase the share of high quality RES in energy systems.

In order to reduce air pollution due to CO2 emission from energy sector, European Union (EU) foresees up to 20% of Renewable Energy Sources (RES) in covering total energy consumption by the year 2020 [1]. The predicted share of RES in production of electric energy only is 33.8%. On the other hand, the AIP scenario of EREC forecast of Renewable Energy Scenario by 2040 [2], foresees 82% of RES in total production of electric energy. The construction of large energy capacities based on renewable energy sources will be required in order to achieve these objectives. However, the problem of exploitation of RES lies in the fact that, unlike the conventional sources and hydro energy, renewable energy sources cannot guarantee safe and continuous electric energy supply to consumers. Inherent characteristics of major renewable energy based electricity generation systems based on wind and solar energy are intermittency and non-controllability [3]. Therefore, although the annual energy balance can contain sufficient energy from RES for full covering of energy consumption, RES cannot continuously provide electric power to consumers with sufficient reliability [4]. Obviously, considerable effort will be required to achieve EU objectives with conventional and innovative solutions of RES. One of the promising innovative solutions presented in this paper is exploitation of periodical water flows, i.e. hydro energy potentials combined with Solar Energy (SE). SE has no possibilities of continuous production, as it depends exclusively on daily solar radiation. That is why solar energy has to be stored in order to be available constantly. On the other hand, hydro energy can be produced continuously if there are suitable water resources. Water can be stored easily, thus providing continuous production of energy. That is why the promising solution of the problem is the use of pump storages as Electric Energy Storage (EES) for SE production. Pump storage is a mature technology with large volume, long storage period, high efficiency and reliability, while capital cost per unit of energy is low [5], [6] and [7]. Climate and hydrological conditions in the Mediterranean part of Croatia, as well as in areas with similar climate, create watercourses which have high flow in rainy winter periods and dry out or have significantly lower flow in dry periods, Fig. 1(a) [8]. Evidently, exploitation of water flow for energy production only in winter period is generally possible, but there is the question of its cost effectiveness and acceptability of seasonal energy production for an Electric Power System (EPS). On the other hand, in Fig. 1(b) it can be seen that the distribution of solar energy (measurement from 2004 to 2008; Pyranometer, Kipp & Zonen, CM 11, [9]) at the same location is fully compatible and replenished with energy from seasonal watercourse. Full-size image (79 K) Fig. 1. Hybrid concept with main input resources: (a) Monthly average daily flows (m3/s) of the Zavrelje spring [5] and (b) solar radiation at location Zavrelje/Dubrovnik, Croatia [6]. Figure options The exploitation of such hydro energy resources has traditionally been exploited by constructing seasonal reservoirs [10]. Unfortunately, on pronouncedly karstic hydro geological terrain, as in Croatian Mediterranean river basin, as well as many other locations, the construction of large reservoirs is generally very difficult and an ecologically sensitive issue. Unlike large reservoirs, the smaller ones are more frequently built, mostly for irrigation purposes. The promising solution is implementation of Solar Hydroelectric Plant (SHE) in dry periods [11] and [12], and conventional hydroelectric plants in winter period. This is a hybrid concept where HEP and PV generator continuously operate together as a unique energy production unit-system, Fig. 2. Storage volume depends on the period of flow balancing and production of hydro energy. Therefore, the hybrid system will have minimum volume of storage for the case of daily balancing. Any longer period of balancing (per week, seasonally, annually) provides a safer and better power supply but requires a greater storage volume. Full-size image (33 K) Fig. 2. Operation concept of hybrid solar hydroelectric plant (PV-HEP). Figure options The SHE concept is similar to pump storage hydroelectric plant with two pipes between lower and upper storage. Such concept of SHE with two pipes provides continuous energy production regardless the natural water flow into the headwater pool. The main objective of building this hybrid energy system (SHE = PV + HEP) should be maximization of reliable renewable energy production and therefore satisfying the European Union (EU) requirements [1]. Equal objectives are sought through various combinations of RES and hydro power plant operation [13] and [14]. The price of energy of the proposed hybrid system would be competitive in relation to other RES (wind, solar, geothermal, etc.). The bigger the flow and the longer the duration of flow of seasonal watercourse into upper pool, the more cost effective is the system. This paper explains and analyses this new concept of exploitation of solar and hydro energy based on periodical water flow.

It can be seen that by hybrid operation annual energy production in the existing HEP increases significantly, particularly during dry months. The benefit of joint work is significant not only regarding amount of energy production but also regarding reliability of production. In this way, the available HE is fully utilized for the production of hydro energy, and any excess daily energy from PV generator, which is placed directly into the EPS for the case when natural flow of water is sufficient for operation of HE, or if the storage is full. In this way we can increase the percentage of renewable energy production of high quality, because PV-HE system has accumulation which ensures continuous system operation and as a source of energy is equal to conventional sources. Because of this, subsidies provided for the development of RES are much more useful and the produced energy is more acceptable for EPS. Naturally, there is also the question of cost effectiveness of PV power plant upgrade to the existing HE with seasonal inflows. Based on the basic information presented in this paper, potential investors can analyze the profitability of investment. Namely, it is possible to analyze the relationship between investment and the investment profit from the sale of energy for specific conditions, taking into account the subsidies as global and local environmental aspects of the solution. Smaller PV generators are more productive because they provide more energy so that the earnings per installed power are higher and therefore acceptable as a solution. Bigger storage volume is more expensive than smaller, but provides greater production of hydro energy and safety of operation of the entire system, and therefore is preferable especially if the difference in the production of hydro energy is greater than the costs of building a larger storage. The obtained results (Fig. 5 and Fig. 6) show that the optimal construction of PV generator is of about 12 GW, and storage of about 286,000 m3, because it exploits in the best way the natural and artificial flow of water into the upper storage. This PV generator power has a very small surplus of intermittent energy (2.51% of total produced) and uses nearly all energy to exploit the existing HE. In summer it provides the total daily volume of water needed for continuous production of hydro energy and in winter it compensates water deficit. In case of using smaller storage, the excess intermittent energy is almost half of the total energy produced (41.46%). A more detailed B/C analysis (beyond the scope of this paper) should determine the profitability of investment in this system. However, if we start from the growing needs for energy production from RES and international obligations, then the energy of the hybrid system is necessary, regardless the profitability of investment, but is also of much higher quality and more reliable to use than energy from intermittent RES.