Volume 2 wind energy 2 19 – stand alone, hybrid systems

Volume 2 wind energy 2 19 – stand alone, hybrid systems Volume 2 wind energy 2 19 – stand alone, hybrid systems Volume 2 wind energy 2 19 – stand alone, hybrid systems Volume 2 wind energy 2 19 – stand alone, hybrid systems Volume 2 wind energy 2 19 – stand alone, hybrid systems

2.19 Stand-Alone, Hybrid Systems KA Kavadias, Technological Education Institute of Piraeus, Athens, Greece © 2012 Elsevier Ltd All rights reserved 2.19.1 2.19.2 2.19.3 2.19.4 2.19.4.1 2.19.4.2 2.19.4.3 2.19.4.4 2.19.5 2.19.5.1 2.19.5.2 2.19.5.3 2.19.5.4 2.19.5.5 2.19.6 2.19.6.1 2.19.6.2 2.19.7 References Further Reading Introduction Historical Development of Wind Stand-Alone Energy Systems Contribution of Wind in Stand-Alone Energy Systems System Configuration Wind Turbine Generator Storage System Unit Complementary Electric Generator Unit Auxiliary Electronic Equipment Stand-Alone Hybrid Systems Configurations Stand-Alone Wind Power Systems Stand-Alone Wind–Diesel Power Systems Stand-Alone Wind–Photovoltaic Power Systems Stand-Alone Wind–Hydro Power Systems Stand-Alone Wind–Hydrogen Power Systems Energy Storage in Wind Stand-Alone Energy Systems Design Parameters of Energy Storage Systems Short Description of Energy Storage Technologies Design, Simulation, and Evaluation Software Tools for Wind-Based Hybrid Energy Systems Glossary Hybrid power system A power system which uses multiple generation sources by incorporating different components such as generators, storage medium, power conditioning and system control in order to supply power to a remote consumer Loss of load hours (LOLH) Power reliability factor indicating the number of load failures in which the load demand exceeds the power supply on hourly based simulations 623 624 626 629 631 632 632 632 632 633 636 638 643 645 647 649 649 651 653 655 Loss of load probability (LOLP) Power reliability factor indicating the probability that instantaneous power demand will exceed the respective power supply for the time period analyzed Loss of power supply probability (LOPSP) Power reliability factor indicating the probability of insufficient power supply for a given period of time Stand alone energy system An electricity system which operates independently of the electricity transmission and distribution network or is not connected to it at all 2.19.1 Introduction Energy is indispensable for sustainable development and poverty reduction At present, there are 1.6 billion people in the world, mostly in rural areas, who have no access to electricity Another 2.5 billion people still rely on traditional fuels such as wood, dung, and agricultural residues to meet their daily heating and cooking needs, this, however, having serious impacts on the local environment and on people’s health [1, 2] Apart from the Third World and many of the developing countries that face serious problems of insufficient electrical network infrastructure, isolated electricity consumers who lack direct access to electrical networks and have limited political influence may be encountered in many regions of the developed countries as well In this context, stand-alone wind power systems, which have already been in use for hundreds of years, have proven to be a reliable and environmentally friendly technological solution for the electrification of remote consumers in areas with moderate or high wind potential Stand-alone electrical energy systems constitute the first applications of the implementation of renewable energy sources (RES) The first attempts at generating electricity from wind energy were directed toward providing energy independently in remote areas where there was no connection to the grid [3] By using a small wind turbine of only a few hundreds of watts and a storage medium, it was possible to cover the modest needs of electrical energy (in most cases only lighting) Since then, the term ‘stand-alone’ has defined an electricity system that operates independently of the electricity transmission and distribution network or is not connected to it at all The aim of such systems is to meet load demand in a direct way, keeping power generation and consumption as close as possible In recent years, the term ‘stand­ alone’ has mainly been used to describe power systems of up to tens of kilowatts, which mainly refer to domestic wind systems [4] Further, hybrid power systems suggest a concept the roots of which can be traced back many years, describing a power system that uses multiple generation sources by incorporating different components such as generators, storage medium, power Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00222-5 623 624 Stand-Alone, Hybrid Systems conditioning, and system control in order to supply power to remote communities [5] Wind hybrid power systems usually combine two or more forms of energy, resulting in a more efficient system overall In the basic form of wind hybrid systems, a wind turbine is combined with a small diesel engine (hybrid stand-alone system), or is connected to the local diesel power station as in the case of isolated power stations (autonomous hybrid system) such as in remote islands Contemporary small-scale wind hybrid energy systems are stand-alone systems which usually comprise a wind turbine, and a photovoltaic (PV) generator and the corresponding energy storage system In a stand-alone installation that consists only of a wind power energy system, on a short-term basis the wind turbine energy production should meet the power demand of the consumer Therefore, at any given moment there must be a balance between the energy production of the wind turbine and the respective load demand In order to attain power balance, either the wind turbine should be controlled accordingly or the power consumption should be adapted to the output of the wind turbine Hybrid energy systems are used in order to tackle the fundamental technical problems that arise from the dependence of the remote consumer on the stochastic energy yield of a wind turbine [6] Stand-alone hybrid energy systems are being used in a wide range of applications worldwide As already made clear, such applications normally concern remote consumers that either not have the choice of grid connection – as in isolated small islands – or live in entirely remote locations, far away from the nearest electrical grid, resulting in a grid connection cost that is extremely high In the aforementioned cases, installation of a properly sized wind hybrid energy system may sufficiently fulfill the energy demand The most common applications of stand-alone hybrid energy systems include winter or summer shelters, isolated farms, grid-isolated communities, telecommunication stations, small desalination systems, water pumping installations, electrification of lighthouses, and even far-off road lighting As already mentioned, the most common application of hybrid stand-alone power systems concerns satisfaction of the domestic electrical needs of consumers in remote locations where the available wind potential – either alone or combined with the local solar potential – is exploited Such installations are always supported by energy storage systems, which are essential to ensure energy autonomy Another significant parameter that should be taken into consideration is the load demand profile, which in combination with the renewable energy potential of the area constitutes the essential input data for the sizing and cost estimation of wind-based stand-alone systems In any case, energy security in stand-alone systems is accomplished with the inclusion of a conventional power generator (i.e., diesel generator) which, besides guaranteeing 100% energy demand satisfaction, could also contribute to lowering the size of the renewable energy devices and energy storage system components Remote islands can also be considered as application areas of stand-alone hybrid systems, because in most cases their weak microgrid operates on expensive fuel and therefore the exploitation of renewable energy sources is considered as vital for energy cost reduction In such isolated communities, however, certain problems need to be considered, mainly deriving from the minimum permissible contribution of renewable energy sources in the local microgrid [7] In order to fulfill such constraints, serious research efforts have been recorded during the last 15 years – and are still ongoing – achieving even 90% RES contribution in small island grids by implementation of RES-based hybrid energy solutions Regarding the applications of wind-based hybrid power systems in the telecommunication sector, RES have been identified as an energy solution for minimizing the operating expenses [8] Windy areas such as coastal locations and hills, where, in many cases, the telecommunication masts are installed, are ideal for wind stand-alone systems that are capable of minimizing the fuel consumption of diesel generators and thereby the operational cost of the installations Furthermore, RES exploitation can be achieved with the installation of a suitable storage system, which will absorb residual renewable energy and return it back for consumption when lower renewable energy production is encountered Most telecommunication transmitters require air-conditioning services during the summer periods, which makes the combination of wind energy generation with photovoltaic systems’ energy ideal, because during summer the availability of solar energy is quite high In combination with covering the electrification needs of remote or isolated consumers, stand-alone renewable energy systems can also contribute to the satisfaction of potable water needs through small desalination systems [9] Potable water shortage is often encountered in remote island regions, where, in cases where wind conditions in the area are favorable, wind-based stand-alone systems can be used in water desalination plants for the production of potable water The main constraints on the use of wind energy in such systems is the nonsteady power supply, which forces the desalination plants to operate in suboptimal conditions In order to overcome this undesirable way of operation, considerable energy storage capacity is necessary to support the installation Furthermore, the contribution of photovoltaic energy could be essential in many cases for realizing uninterruptible power supply to the desalination plant [10] Direct utilization of the mechanical energy produced by the wind turbine (shaft power), without converting it to electricity, is possible in water pumping systems [11] Wind-powered water pumps operate either in a direct manner, that is, directly attached to the turbine’s shaft, or through electricity generated by a typical small wind turbine Wind energy has been in use for centuries for water pumping, and even nowadays there is a large number of installations worldwide In cases where a high starting torque is necessary, a large number of blades are used similar to the older wind turbines In cases where the operation of a mechanical drive is used for pumping water, the placement of the wind turbine is restricted to be near the water reservoir, whereas in cases where wind electricity is supplied to the water pump, the wind turbine may be placed far away from the water reservoir for maximizing wind energy exploitation [12] Potential applications of wind water pumping systems include domestic water supply, community water supply, cattle watering, and irrigation [13] 2.19.2 Historical Development of Wind Stand-Alone Energy Systems Wind energy has been exploited for grinding grain or pumping water for at least 3000 years [6] The use of wind turbines for electricity generation can be traced back to the nineteenth century when an experimental wind turbine (Figure 1)[14] driving a Stand-Alone, Hybrid Systems 625 Figure The first electricity-producing wind turbine, installed in 1891 [14] dynamo was built by Poul La Cour in 1891 in Denmark [3] The remarkable fact is that La Cour at once tackled the problem of energy storage He used the direct current (DC) generated by his wind turbine for electrolysis and stored the hydrogen gas that was produced, establishing in this way the first wind stand-alone energy system Based on La Cour’s wind turbine model, by 1908 the Lykkegard company had built 72 electricity-generating wind turbines with power output ranging between 10 and 35 kW, which were used to supply energy to rural settlements For much of the twentieth century, wind turbines were being used to charge storage batteries which then were used to operate small appliances [15] The interest in electricity generation by means of wind power during the wind turbine evolution had seen some fluctuations following the diesel fuel cost fluctuations In periods when the fuel prices were rising, such as during World War I and World War II, the interest in wind power was growing Back then, the subject of environmental protection had not yet arisen and thus there was no association with energy production Despite the reduced interest in supplying wind energy to electricity networks, wind turbine manufacturers continued their efforts in building wind turbines for stand-alone applications In 1922 in the United States, Marcellus and Joseph Jacobs developed small wind turbines which became known as ‘wind chargers’ which were used for recharging batteries for power supply of rural settlements and remote houses In Germany until the 1930s, a total of 3600 American wind turbines were built under license, and most of them were used for pumping water but some of them were modified for electricity generation The first wind turbine feeding a local grid was installed in 1931 in the USSR in Balaklava The electricity generated was fed into a small grid which was supplied by a 20 MW steam power station The idea of using wind turbine electricity for supplying a grid network was also supported by Hermann Honneff in 1932, whose vision was a five-rotor wind turbine of 20 MW that was to generate electricity in combination with conventional power plants [16] Accordingly, in the United States in 1941 the world’s first large wind turbine was installed in Vermont [17, 18] Until the ‘oil price shock’ of 1973, the extremely low prices of conventional fuels held back the development of the wind energy sector, as the investors were not highly motivated to invest new funds in order to overcome the numerous technical problems and faults that had been encountered during the more practical operation of large wind turbines Therefore, all these years stand-alone wind power installations continued to be the main application of wind power After the energy crisis in 1973, the interest in wind turbine technology was rekindled giving a significant boost to the wind technology evolution Based on the traditional models of three-bladed rotors and grid-connected induction generators that dominated large-scale wind turbine models, Danish companies, which were active in agricultural machinery, began building small turbines to sell them to private owners or agricultural holdings (Figure 2) These small Figure Remote installation of a small wind turbine at a private electricity user’s holding in Denmark (1985) Photo by Rüth found in Reference [3] 626 Stand-Alone, Hybrid Systems Small wind turbine manufacturers (2009) 18 countries with or less companies; 31 Sweden; Spain; Netherlands; USA; 95 Germany; 16 China; 19 UK; 22 Canada; 24 Japan; 29 Figure Global distribution of small wind turbine manufacturers (number of companies per country) [20] wind turbines were also used by consumer associations to cover the electricity needs of small communities The installation of small wind turbines was supported by the Danish government through financing and legal regulations, and at the same time the pattern of rural Danish settlements with its many single farms generally favored the decentralized installation of wind turbines By 2001, about 150 000 Danish households were registered as owners of shares in wind turbines [19] According to the most recent data available, there is a growing interest in the small wind turbine (those of rated capacity < 100 kW) industry In the United States, the small wind turbine market grew by 15% in 2009 pushing their total installed capacity to 100 MW It is worth mentioning that almost 50% of the small wind turbines were installed during the last years of the industry’s 80 years of history In this context, the global sales of small wind turbines for the year 2009 were 15 500 units for off-grid connections with a total power of 7600 kW Another 5200 units were sold for on-grid connection with a total capacity of 34 400 kW Regarding the different size range of small wind turbines and their use, 100% of the wind turbines with rated power up to kW, 10% of those with rated power 1–10 kW, and < 1% of those with rated power 11–100 kW are used for off-grid applications According to Figure [20], about 40% of global small wind turbine manufacturers are located in the United States, 25% in Europe, 10% in Asia, and 25% in the rest of the world, indicating the worldwide interest in small wind turbine installations 2.19.3 Contribution of Wind in Stand-Alone Energy Systems Stand-alone power systems are mainly used in cases where there is no grid electricity available or the cost of connection to the local electricity grid is prohibitive Given that the minimum grid extension cost for low-voltage lines exceeds 10 000 € km−1 of grid line – a value which increases in cases of difficult access situations [21–23] – and that the already high cost of fuels increases even more with the remoteness of the location [24], remote consumers should try to exploit all alternative choices that are available in their area In such cases, renewable energy sources can provide the necessary electricity and thermal energy at a cost competitive to the corresponding electricity cost of the local network In the case of grid-connected wind parks, the area in which the park will be installed is selected according to the available wind speed values In these cases, measurements are taken for quite a long period of time (i.e., at least 12 months) in order to evaluate the wind potential of the area On the other hand, in the case of wind stand-alone systems, the area of installation is already given, and the owners are not willing to make time-consuming wind speed measurements for the estimation of the wind potential Therefore, the decision of whether to install a wind stand-alone system is taken in accordance with physical indications such as bending of trees and existence of old windmills in the area, as well as on the basis of the local habitants’ experience concerning wind patterns in the area In this context, the annual energy yield of a wind turbine depends on the operational characteristics of the wind turbine and the available wind potential in the area The wind potential of an area could be described to a good approximation by the Weibull function given in eqn [1] f uw ị ẳ k uw k − e C C � u �k w C ½1Š Stand-Alone, Hybrid Systems 627 where f(uw) is the Weibull distribution function; k is the shape factor; C is the scale factor; and uw is the wind speed The quality of the wind potential of the area depends, according to the Weibull distribution, on the scaling factor, C, which is proportional to the mean annual wind speed and the shape factor, k, which depends on how widely wind speeds are spread around the mean wind speed value (inversely proportional to standard deviation) Accordingly, with respect to the wind frequency distribution, the annual power production of a typical wind turbine for different wind speeds can be calculated as Pw ¼ ρAuw ηwt Cp ½2Š where ρ is the air density (kg m−3); A is the swept area of the rotor (m2); ηwt is the total electromechanical efficiency of the electrical and mechanical components of the wind turbine; and CP is the power coefficient of the wind turbine’s rotor For a wind turbine with a typical (simplified) wind power curve (Figure 4), the energy yield for different wind potential areas is presented in Figure According to Figure 5, the energy production of the wind turbine strongly depends more on the scale factor, C, which is related to the mean annual wind speed and less on the shape factor, k, which is related to the standard deviation of the wind speeds Further, if one considers a wind turbine that operates according to a typical power curve, the estimated energy performance per kilowatt of nominal wind turbine power is presented in Figure According to Figure 6, the energy production potential of a small wind turbine can reach up to 4200 kWh kW−1 for areas with a mean annual wind speed of m s−1 As already mentioned before, the power curve of the wind turbine is of great importance, as it describes the expected power generation for each wind speed at the hub height Therefore, the annual energy yield of a commercial wind turbine is expected to be different from the one estimated using the typical power curve Figure presents the percentage of annual energy yield per kilowatt of rated power of several commercial small wind turbines of different sizes relative to the typical power curve turbine According to Figure 7, the expected deviation between wind turbines with different power curves can be as high as 25% The data were collected from the official websites of the manufacturers [25–27] and the wind turbine database of Soft Energy Applications and Environmental Protection (SEA&ENVIPRO) Laboratory of TEI of Piraeus [28] The energy production of the previous figures presented refers to the capability of the wind turbine to produce energy in areas with specific wind potentials In stand-alone systems, the wind power generation does not always match the power demand of the consumer Therefore, not all wind energy generated will be absorbed by the demand The maximum wind energy absorption depends on the wind turbine energy production based on the wind potential of the area and the wind turbine selected, as well as on the corresponding power demand in accordance to the load profile of the consumer Thus, in order to estimate the wind energy absorption rate of a stand-alone power system the parameters that should be considered are Non-dimensionalized wind turbine power curve 1.2 1.0 P/P0 0.8 0.6 0.4 0.2 0.0 Figure Typical nondimensional wind turbine power curve 10 15 Wind speed (m s−1) 20 25 628 Stand-Alone, Hybrid Systems Annual energy yield of kW wind turbine in different wind potential areas 25 000 Energy yield (kWh) 20 000 k = 1.0 k = 1.5 k = 2.0 k = 2.5 15 000 10 000 000 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 Scaling factor "C" Figure Estimated annual energy yield of a kW wind turbine based on typical power curve, installed in different wind potential areas Energy performance of typical wind turbine Annual energy perfomance (kWh kW–1) 000 000 000 000 000 000 Mean annual wind speed (m s–1) Figure Mean annual expected energy production of a typical wind turbine at different wind potential areas (shape factor k = 2.0) • • • • the wind potential of the area, the type and the size of the wind turbine selected, the energy demand profile of the consumer, and the storage system size Kaldellis and Vlachos [29] presented a detailed case study indicating the influence of the above-mentioned parameters Based on a rural household load profile of a remote consumer [30] with an annual energy consumption of 4750 kWh, they estimated different optimum wind stand-alone configurations in respect of zero load rejections, for an area with a mean annual wind speed of 5.6 m s−1 (Kea island in Greece) According to their results, presented in Figure 8, one can clearly see that the wind energy utilized by the consumption is between 35% and 50% of the total wind energy produced during year of operation A wind turbine in combination with a PV power station and with the appropriate storage capacity can satisfy the energy demand of stand-alone power systems The addition of a PV power station to a wind stand-alone system can significantly increase the system’s reliability and its ability to cover the electricity needs of a remote consumer setting the diesel fuel generator as an emergency backup unit [31] In Figure 9, the possible combinations of wind turbine, photovoltaic station, and storage medium size are presented [31] Each proposed configuration is selected based on the zero-load-rejection condition and is capable of covering the annual needs (4750 kWh) of a remote consumer on an hourly-basis calculation According to Figure 9, the storage capacity strongly depends Annual energy yield related to typical wind turbine Stand-Alone, Hybrid Systems 629 Commercial small wind turbines' performance related to typical one 140% 120% 100% 80% 60% 40% 20% 0% Typical power curve (5 kW) Sudwind Aeolos-H AOC DANmark Aeolos-H Aeroman Aerostar HSW 30 UGE-4k N 1245 (1 kW) 15/50 11 (5 kW) 14.8–33 18 (30 kW) (4 kW) (45 kW) (50 kW) (20 kW) (33 kW) (36 kW) Commercial small wind turbines Figure Small commercial wind turbines’ annual energy performance relative to the wind turbine with a typical power curve at an area of mean annual wind speed of m s−1 and shape factor k = 2.0 KEA hybrid system-wind energy distribution 60% No diesel oil 100 kg y–1 diesel oil 50% 100 kg y–1 diesel oil 40% 30% 20% 10% 0% Rejected energy System loss System consumption Figure Application results of different wind stand-alone system configurations in the island of Kea in Greece capable of covering the energy demand of a rural consumer on the photovoltaic rating and wind turbine size selected It should be noted that the figure refers to an area with a mean annual wind speed of 5.5 m s−1 and an annual solar potential of 1650 kWh m−2 2.19.4 System Configuration Stand-alone hybrid energy systems have emerged as one of the most promising ways to handle the electrification requirements of numerous isolated consumers worldwide, including houses in the country, remote farms, shelters, telecommunication stations, small islands, lighthouses, and so on A typical hybrid energy system combines two or more electricity generation units, which in some cases, where high RES quality exists, can be based purely on RES, along with the appropriate energy storage system and the corresponding auxiliary electronic devices Typical stand-alone hybrid systems also utilize a small thermal power unit, which is used as a backup power system in cases when the RES power generator units along with the storage system cannot fulfill the energy demand 630 Stand-Alone, Hybrid Systems Energy autonomy configuration of wind-photovoltaic stand-alone system 25 000 Wind turbine's rated power No wind turbine kW kW 7.5 kW 10 kW 15 kW Storage capacity (Ah) 20 000 15 000 10 000 000 000 000 000 000 000 000 000 000 000 10 000 Photovoltaic station rated power (kW) Figure Energy autonomous wind–photovoltaic stand-alone system configurations in the island of Kea in Greece capable of covering the energy demand of a rural consumer UPS AC current flow DC current flow Wind turbine Data monitoring AC/DC rectifier In case of AC wind turbine Wind turbine’s charge controller DC loads DC switchboard PV array PV charge controller Inverter AC loads AC switchboard Diesel generator Battery bank Overall management control Figure 10 Typical hybrid RES-based stand-alone system [5, 32] Figure 10 [5, 32] presents the most common configuration of a small-scale RES hybrid stand-alone energy system which comprises a wind energy converter, a photovoltaic power station, a diesel generator used as a backup power provider, a battery bank unit for storing residual energy, and auxiliary electronic equipment which includes charge controllers, an inverter and the corresponding switchboards for alternative current (AC) and DC loads Contemporary stand-alone systems also include an overall system management unit capable of controling the power flow according to the instantaneous power generation and power demand Several wind-based hybrid energy configurations can be found in the international literature, which incorporate different combinations of renewable energy power production units, conventional electricity generation systems, and storage system Stand-Alone, Hybrid Systems 631 configurations The most well-known systems, apart from the one presented in Figure 10 are the following (the corresponding indicative references are given in brackets): • • • • wind–diesel systems [33–35] wind–hydro installations [36–39] wind–biomass-based installations [40, 41] wind–hydrogen/fuel cell hybrid energy systems [42, 43] Besides, different system configurations exist with regard to the energy storage medium used in each of the above combinations 2.19.4.1 Wind Turbine Generator The wind turbines used in stand-alone systems are often in the range of up to 50 kW [4], as the term ‘stand-alone system’ usually indicates small electricity systems up to the scale of a small community The amount of power a turbine will produce depends primarily on the diameter of its rotor, as it is the rotor diameter that determines the quantity of wind intercepted by the turbine Small wind turbines in comparison to large wind turbines operate at a higher rotational speed for the same wind speed; have a tail for the orientation of the nacelle; have significantly smaller tower heights and therefore experience lower average wind speeds; and comprise simpler and cheaper safety systems to withstand high wind speeds As far as the overspeed protection of small wind turbines is concerned, the most common safety mechanism is the turbine pitch-up or tilt-up and furling Pitching is more common on very small wind turbines Because wind speeds increase with height, the turbines are mounted on a tower In general, the higher the tower, the more power the wind system can produce The tower also keeps the turbine above the air turbulence that may exist close to the ground because of obstructions such as hills, buildings, and trees Note that relatively small investments in increasing the tower height can yield high rates of return in terms of increased power production The generator of a small wind turbine is one of the most important parts of the structure and strongly influences the energy performance and the reliability of the wind turbine Most small wind turbines use permanent-magnet generators, which not require external excitation They are simple to use, as they need only a rectifier to produce the DC voltage required for a battery, but, on the other hand, the magnets are easily broken and many are sensitive to temperature changes Similar to permanent-magnet generators are synchronous generators, which need a field current charge to produce the magnetic field, thus reducing their efficiency The permanent-magnet generators give satisfactory performance if they are connected through a rectifier to the batteries, whereas in case they are connected directly to an AC load with a constant frequency, the speed of the wind turbine should be constant The electrical output from the generator is usually three-phase AC with a variable voltage and frequency The correspond­ ing current is converted to DC using a rectifier and then to a fixed voltage and frequency as required in ordinary electricity-consuming appliances Regarding the wind turbine installation, there are two basic types of towers: self-supported (free standing) and guyed Most home wind power systems use a guyed tower Guyed towers, which are the least expensive, can consist of lattice sections, pipes, or tubing depending on the design, and are supported by guy wires (see Figure 11) They are easier to install than self-supported towers; however, because the guy radius must be between one-half and three-quarters of the tower height, guyed towers require considerable space to install them Tilt-down towers (which can be either self-supported or guyed) are more expensive, but they offer the consumer an easy way to perform maintenance on smaller, light-weight turbines, usually kW or less Lattice towers are easier to transport but tend to have a lower service life than pole towers Tubular towers require smaller foundations but are usually heavier than the other types, thus increasing the purchase and transportation costs In cases of wind turbines installed near the sea, hot-dipped galvanized tubular towers should be considered According to Wood [44], the optimum tower height for a small wind turbine is typically 18–33 m depending on the turbine size and wind potential of the area Guyed Figure 11 Basic types of wind turbine towers Self-supported Lattice 632 2.19.4.2 Stand-Alone, Hybrid Systems Storage System Unit Owing to the stochastic behavior of wind, wind generation cannot always provide a firm capacity to an autonomous electrical power system [45] In addition, the implied fluctuations can – in some cases – cause problems related to stability, harmonics, or flicker An energy storage system, when sized appropriately, can match the highly variable wind power production to a generally variable system demand, remarkably limiting the energy production cost (e.g., by generating capacity savings) In this context, the critical parameters concerning the storage systems potentially used in a wind hybrid installation include lifetime expectancy, energy efficiency, depth of discharge, and the initial and operational cost A short description of the most common storage systems is given in Section 2.19.6 2.19.4.3 Complementary Electric Generator Unit The quantity of energy that a wind power generator can produce strongly depends on the available wind potential at the installation area Although the total annual energy production might seem enough to cover the corresponding electrical energy needs, satisfaction of the load demand by the energy produced should be examined at least on an hour-by-hour basis The duration of calm spells is an important parameter that influences the decision about the choice of components and the size of a wind hybrid installation for a stand-alone system to provide constant electricity for consumption There could be situations where, although the calculated annual energy produced seems enough to cover a consumer’s power needs, long calm spells could cause a load failure In order to confront such situations, larger storage systems, which significantly increase the initial cost of the plant, are usually considered An interesting option is the installation of an additional, independent electric power generator, which reinforces the electricity production system Several studies have shown that a wind turbine in combination with a secondary power generator, which could also be based on renewable energy sources (e.g., photovoltaics) or could be conventional fuel-based generators (e.g., diesel or gas), can limit the energy storage system size and in many cases reduces energy production costs Another quite interesting option is the combination of the energy storage system with an alternative electric power generator unit An example of such an installation is the combination of a wind turbine coupled with an appropriate hydrogen production system based on electrolysis, to be used as energy storage, and a fuel cell unit that uses the hydrogen produced and stored during low energy demand to produce electric power during low or very high wind speeds An additional advantage of the specific installation is the opportunity to use the hydrogen produced as a fuel in appropriate devices (taking advantage of the heat produced by its combustion) or even as a fuel for hydrogen cars Of course, the currently low energy efficiency of the cycle, that is, from hydrogen production to the final power production by the fuel cell, should also be taken into consideration [43] Another option is the combination of a wind electric power generator with a small pump-hydro unit in which water is pumped from a lower water reservoir to a higher water reservoir during low energy demand situations, and returned through the hydro turbine to the lower reservoir during low or very high wind speeds [46] In such installations, the water stored could also be used to cover any water supply needs 2.19.4.4 Auxiliary Electronic Equipment The auxiliary electronic equipment needed to support a stand-alone wind hybrid system depends on the application type For example, the parts required for a wind turbine coupled with a pump-hydro storage system will be very different from those needed for a wind–diesel hybrid system Most manufacturers provide system packages that include all the necessary parts of the system Stand-alone systems, which in most cases are combined with batteries, also need a charge controller to prevent the batteries from overcharging or overdischarging Small wind turbines generate DC electricity When using standard appliances that use conven­ tional AC current, an inverter to convert the DC electricity from the batteries to AC is necessary The controller of the system ensures that there is a current limit in order to protect the generator and also to monitor the battery condition to avoid overcharged conditions In addition, it could be used as a primary overspeed protection system by reducing the blade speed Most wind turbine controllers have a current limit so as to protect the generator by limiting the power output of the wind turbine In this way, overheating of the generator can be avoided, protecting it from possible insulation and wires melting Such control is essential as the generator in small wind turbines is usually air-cooled; therefore, the generator’s current capability depends on air temperature, wind speed, and the thermal resistance from the wires to the air but also the heat loss of the generator The inverter is an essential part of the auxiliary electronic equipment, as it produces the correct voltage and frequency output required by the load Contemporary inverters also monitor the battery level, thus protecting the battery when the depth of discharge has been reached Some inverters produce a nearly square output, which is likely to cause more electromagnetic interference 2.19.5 Stand-Alone Hybrid Systems Configurations In this section, the most common commercial stand-alone hybrid system configurations are presented In order for the reader to have a comprehensive view of the opportunities given by the different combinations, scientific research results of the SEA&ENVIPRO Lab are used for the sizing integration of each configuration Those results refer to case studies in the isolated Aegean Sea islands in Greece, which possess high renewable energy source potential However, for forming an integrated concept, Section 2.19.7 presents other optimization tools that could also be used for the sizing of stand-alone hybrid power systems Stand-Alone, Hybrid Systems 641 Photovoltaic module operation curves 3.5 3.0 1000 W m–2 800 W m–2 Current I (A) 2.5 600 W m–2 400 W m–2 2.0 200 W m–2 1.5 1.0 0.5 0.0 15 10 20 25 Voltage (V) Figure 21 Operation curves of a photovoltaic module for different solar radiation values In cases (3) and (4) above, when the battery capacity is near the bottom limit, an electricity demand management plan should be applied; otherwise the load would be rejected In order to maximize the energy security of the system, a diesel engine could be added to act as a backup energy source in the extreme case that no renewable energy production is available and at the same time no energy is available in the storage system The sizing procedure of a wind–PV hybrid system is much more complicated than the above-mentioned stand-alone systems, as one has to match the stochastic wind generation and the fluctuating PV generation with the time distribution of load demand Generally, the PV systems comprise an array of PV modules that produce electricity, taking advantage of the existing solar radiation, according to the PV operational curves (Figure 21) The number, z, of PV panels is bounded as follows: Etot Etot z P ỵ 8760 CFPV 8760 CFPV ½11Š According to the charge controller voltage, Ucc, and the photovoltaic panels’ operation voltage, UPV, the necessary number of photovoltaic panels, z2, connected in series is estimated as: z2 ẳ Ucc UPV ẵ12 Therefore, z1 parallel strings of panels are required for the installation: z1 ¼ z z2 ½13Š Note that P+ is the nominal power of the module, CFPV is the photovoltaic installation’s capacity factor, and η* is the corresponding energy transformation coefficient, given that the PV production is not rectified In order to investigate the potential configurations of stand-alone wind–PV hybrid power systems, a more detailed sizing procedure should be followed More precisely, given that analytical data of the operational characteristics exist, integrated computa­ tional algorithms could be used for the estimation of the most appropriate system configuration to be chosen The analytical system sizing can considerably increase the stand-alone system’s reliability and decrease the installation cost and, furthermore, the long-term energy production cost The main inputs required for an analytical sizing procedure are • detailed wind speed, uw, measurements at hub height over a given time period (e.g., year); • detailed solar radiation, G, measurements over a given time period (e.g., year) usually at a horizontal plane; • ambient temperature, θa, and pressure data for the entire period analyzed; • operational characteristics of the wind turbine (at standard-day conditions); • operational characteristics (current, voltage) of the PV modules selected; • operational characteristics of all the other electronic devices of the installation; and • the electricity consumption profile on an hourly basis, being also dependent on the period of the year analyzed (winter, summer, or other) An example will be presented based on the research of Kaldellis et al [31], which strengthens the necessity of the analytical hybrid system dimension estimation in order to guarantee energy autonomy of a typical remote consumer In this study, the numerical 642 Stand-Alone, Hybrid Systems algorithm WT-PV, developed by SEA&ENVIPRO Lab, was used This algorithm estimates the combination of the required wind turbine size, Po; the number of photovoltaic panels, z, needed; and the corresponding battery capacity, Qmax, that will guarantee system energy autonomy for a given period of time WT-PV is based on the following steps: For the region analyzed, the wind turbine rated power, Po, is selected, taking values from a specific numerical range defined by the user Accordingly, the number, z, of PV panels, each with a peak power P+, is determined, based on the respective operational characteristics A battery capacity is selected, starting from a minimum value, while a maximum battery capacity limit also exists Battery capacity range can vary according to the user’s definitions For every time point of a given time period, the wind energy, Pw, produced by the wind turbine and the energy yield, PPV, of the photovoltaic generator are estimated, taking into account the existing wind speed, the available solar radiation, the ambient temperature and pressure, the selected wind turbine power curve, and the power curve of the photovoltaic panels (see Figure 21) The wind energy production is compared with the consumer’s energy demand, Pd If an energy surplus occurs (Pw > Pd), the energy surplus along with the energy produced by the PV generator is stored in the battery system and a new time point is analyzed Otherwise, the algorithm proceeds to the next step If (Pw < Pd), the energy deficit (Pd – Pw) is covered by the photovoltaic generator production Any energy surplus is stored to the batteries and a new time point is analyzed If this is not the case, the algorithm proceeds to the next step The energy deficit (Pd – Pw – PPV) is finally covered by the energy storage system, if the batteries are not near the lower capacity permitted limit (Q > Qmin) Accordingly, the algorithm is repeated from step (4) In case the battery is practically empty, the battery size is increased by a given quantity, provided the maximum battery capacity limit is not exceeded Then the complete analysis is repeated from step (3) If the maximum battery size is reached, the number of photovoltaic panels is increased and the algorithm proceeds to step (2) In case the maximum available number of photovoltaic panels is reached, a new wind turbine rated power is selected and the algorithm restarts After the analysis is completed, the distribution Qmax = Qmax(Po,z) is predicted, taking into account that every set of Qmax, Po, and z guarantees the energy autonomy of the remote consumer for the entire period analyzed The optimum configuration may be subsequently predicted on the basis of an appropriate criterion, for example, the minimum initial cost A case study in the island of Zakynthos in Greece presented by Zafirakis et al [49] is selected for the implementation of the above algorithm The wind–PV hybrid system should be able to cover the electricity needs of a typical remote consumer with a given consumption profile based on his seasonal electricity needs In this study, the peak power demand of the remote consumer does not exceed 3.5 kW, whereas the annual energy consumption reaches ∼4750 kWh The configuration was designed to be installed in Zakynthos island, which possesses medium solar energy potential (1500 kWh m−2 yr−1) and medium wind potential with mean annual wind speed values reaching m s−1 Figure 22 presents the different configurations that are capable of guaranteeing 100% energy autonomy totally based on renewable energy sources for year of operation Scenarios of wind–battery and PV–battery were also included by the authors for comparison According to their results, a stand-alone PV system would require at least 1250 Ah storage capacity combined with more than 10 kW of PV installation On the other hand, the wind stand-alone system requires significantly higher battery capacities, up to 6000 Ah, for the same wind turbine size (15 kW) The combination of both renewable energy sources could decrease the storage system requirements as well as the necessary size of the wind turbine and the photovoltaic installation Stand-alone configurations solutions for Zakynthos island 20 000 Wind-only 10 panels 20 panels 50 panels 75 panels 100 panels PV-only 18 000 Battery capacity (Ah) 16 000 14 000 12 000 10 000 8000 6000 4000 2000 0 1500 3000 4500 6000 7500 9000 10 500 12 000 13 500 15 000 Wind power (Watts) (or PV power (Watts) for the PV-only solution) Figure 22 Variation of a wind–PV hybrid system dimensions in Zakynthos island Stand-Alone, Hybrid Systems 643 Stand-alone configurations electricity production cost for Zakynthos island Electricity production cost (Euro kWh−1) 2.5 Wind-only 10 panels 20 panels 50 panels 2.1 75 panels 100 panels 1.9 PV-only 2.3 1.7 1.5 1.3 1.1 0.9 0.7 0.5 1500 3000 4500 6000 7500 9000 10 500 12 000 13 500 15 000 Wind power (Watts) (or PV power (Watts) for the PV-only solution) Figure 23 Electricity production cost of wind–PV stand-alone configurations for Zakynthos island As already mentioned, selection of the most appropriate configuration is normally decided on an economic basis: either the minimization of the initial installation cost or the optimization of the electricity production cost Figure 23 presents the electricity production cost for the different configurations of Figure 22 According to Figure 23, the minimum electricity production cost ranges between 0.6 € kWh−1 for a 500 W wind turbine and kW of PV, and 1.2 € kWh−1 for a PV-only installation of kW It is also important to note that for high wind turbine rated power (15 kW) the electricity production cost presents very narrow variations converging at about 1.1 € kWh−1 2.19.5.4 Stand-Alone Wind–Hydro Power Systems Complementarity of renewable energy sources can also be exploited in wind–hydro power systems, which are based on the exploitation of both wind potential and hydraulic power in order to enhance the reliability, energy quality, and stand-alone system performance In addition, the water storage capability of the hydroelectric system can significantly limit the intermittence of wind power generation Thus, a stand-alone wind–hydro power system does not essentially refer to the independent production of electricity by a hydro power installation or a wind turbine, both of which supply energy to a remote consumer The wind–hydro concept mainly refers to the integration of a wind power installation with a pumped hydro storage (PHS) system that will be able to absorb the residual wind energy during low-power demand periods and return it for consumption when wind power cannot satisfy the demand The implementation of wind power generation with PHS is targeting mainly the range of isolated communities in remote islands with no connection to any mainland grid rather than single consumers as indicated for the systems described in the previous sections The integration of wind power with PHS has been investigated for at least 20 years by numerous researchers [37, 46, 50, 51] Most of the cases analyzed refer to isolated islands with the target of minimizing the conventional fuel energy consumption and eliminating the negative environmental impacts Combined wind–hydro energy stations can contribute to the maximum RES penetration into the load demand, which, according to research results, can even exceed 90% A typical wind–hydro power system capable of fulfilling the energy needs of an isolated community is presented in Figure 24 More precisely, the hybrid system consists of • • • • one or more wind turbines, a small hydroelectric power plant, a water pump station, and two or more water reservoirs at elevations h1 and h2 (h1 > h2) working in a closed circuit along with the corresponding pipelines The hybrid wind–hydro power plant is usually supplemented by an existing autonomous power station (APS) which usually comprises conventional internal combustion engines The main objective of the wind–hydro station is the fulfillment of the energy demand by increasing the renewable energy source absorption and reducing the operation time of the local APS The sizing procedure of the wind–hydro power system includes sizing of the wind turbine and the hydro turbine, as well as the determination of the exact location, volume, and geometry of the water reservoirs along with the determination of the rated power and operational range of the water pumps and the water piping system dimensions (diameter, length) More precisely, the rated power of the water pumps may be determined by the maximum power of the wind turbines, as the water pump must have the capability to absorb the maximum power output of the wind turbines, whereas in the case of large-scale systems, the rated power of the pump depends on the frequency distribution of the wind park’s energy surplus; that is, 644 Stand-Alone, Hybrid Systems Upper reservoir (h1) APS Energy consumption Wind park (zWTs) Water pump station Lower reservoir (h2) Reversible hydraulic machines Figure 24 Combined wind–hydro installation for remote communities Ppump ¼ ρw g H V_ ηp ηel ½14Š where Ppump is the power required by the water pumps; H the pump head; V_ the volume flow rate; ηp the pump efficiency; ηel the electrical efficiency of the system; ρw the density of the water; and g the acceleration due to gravity.The static head, H, of the pump must satisfy the expression H ≥ h1 h2 ị ỵ Hf ẳ h1 h2 ị ỵ Kp V_ ẵ15 where Hf is the total hydraulic losses, both lengthwise and local, when the water reservoir is used for energy storage and Kp is the friction losses factor It should be noted that H and ηp depend on the operational characteristics of the selected pump The nominal power of the hydro installation results from the precondition that it covers the peak power demand of the system each time examined, with an optional future increase (of 20%) The exit power is given as _ ηH el Ph ẳ w g H V ẵ16 where V_ ′ is the flow rate of the turbine; H′ the hydro turbine head, ηH the turbine efficiency, and η′el the electrical efficiency of the system In addition, the following equation is also valid: _ H′≤ ðh1 − h2 Þ − H f ẳ h1 h2 ịKH V ½17Š where h is the hydrostatic head and δHf′ is the total hydraulic losses, both lengthwise and local, when the water circuit is used for energy production Note that H′ and ηH depend on the operational characteristics of the hydro turbine selected The dimensions of the upper water reservoir are defined by the available hydrostatic head, which depends on the relative elevation between the upper and the lower water reservoirs, and by the required levels of energy autonomy for the system For example, by selecting d0 days of energy autonomy, the useful volume Vo of the water reservoir is given as Vo ¼ Etot 24d0 ¼ Vmax − Vmin Δt ηΗ ηel ρw g H′ ½18Š where Etot is the total energy demand for the time duration of analysis, Δt, in hours (e.g., 8760 h for year); and Vmax and Vmin are the maximum and minimum storage capacity, respectively, of the upper water reservoir During a long-term energy balance analysis of a wind–hydro power system operation, the following operational situations may arise: The wind power produced is in excess of the energy demand of the system a) In that case, the energy surplus is stored through operation of the water pumping system in the upper reservoir Stand-Alone, Hybrid Systems 645 Renewable energy sources penetration in Karpathos island 100 RES Penetration (%) 95 90 85 80 = 1.0 = 1.5 = 2.0 = 2.5 = 3.0 75 70 65 60 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Number of WTs (z) Figure 25 Renewable energy sources penetration capability using the wind–hydro solution in the autonomous electrical system of Karpathos island b) In case the upper reservoir is full, the energy surplus is forwarded to other alternative uses, such as a water desalination plant The electrical power demand is higher than the wind park output a) In that case, the hydro turbines cover the power deficit b) In case the upper reservoir is almost empty, the internal combustion engines of the APS take over the power deficit, under a scheduled operational plan For estimating the optimum wind–hydro configuration, advanced numerical algorithms should be used, to analytically simulate the operation of different system size combinations By applying an analytical simulation procedure, Kaldellis and Kavadias [52] presented interesting results regarding the renewable energy possibilities in the electrification of remote islands The study, which is presented here, concerned a medium-sized Aegean Sea island (Karpathos), and the basic scope was the maximization of RES penetration The annual energy production of the local APS of the island was estimated at 24 400 MWh and the peak-load demand at ∼6500 kW, whereas the corresponding minimum value was 1400 kW The island has a very high wind potential, as the long-term annual mean wind speed approaches 9.6 m s−1, at 10 m According to their results, remarkable renewable energy penetration can be achieved (Figure 25) by increasing the number of wind turbines used and the size of the water reservoirs selected through the parameter which represents the number of days of energy storage autonomy Another interesting optimization approach for the economy enhancement of large wind–hydro installations concerns a planned hydro power production under a pattern of guaranteed energy by the hydro power system on a daily basis during the peak load demand hours In this way, high energy-purchase prices can be realized by selling power to the local autonomous grid during the peak load demand hours [46] Of course, in case the water stored in the upper reservoir is not enough for the fulfillment of the condition of guaranteed energy delivered to the local grid, the water pump absorbs the required energy from the grid during low-demand periods when the energy purchased price from the local grid is low 2.19.5.5 Stand-Alone Wind–Hydrogen Power Systems As already mentioned in Section 2.19.2, the first wind turbine installed in Denmark in 1891 was generating DC, which was used for electrolysis to produce hydrogen The hydrogen produced was used for gas lighting and later on for autogenous welding [3, 53] Since then wind energy was scarcely used for hydrogen production, and only during the last few decades did wind–hydrogen stand-alone systems become a reality In this context, such configurations, when in stand-alone system mode, exploit the energy surplus of the wind turbine to produce hydrogen, provided of course that the electricity demand of the consumption side has been satisfied Hydrogen as an energy carrier can be stored to overcome the daily and seasonal discrepancies between energy source availability and demand [54] In cases of load deficit or no available wind, the hydrogen is fed to a fuel cell device to produce electricity and satisfy the demand There are several issues that should be taken into consideration when using wind energy for the production of hydrogen More precisely, direct coupling of an electrolyzer with a wind turbine denotes intermittent operation and highly variable power output, which could cause the electrolyzer to operate at very low power rate resulting in the mixing of H2 and O2, which at such load levels permeate through the electrolyzer Also, intermittent power makes the electrolyzer operate at temperatures lower than the respective nominal, as some amount of time is required for the electrolyzer to reach its normal operating temperature [55] These problems in stand-alone power systems can be eliminated by the use of either a complementary renewable power source or a diesel generator, which may fill the power gaps during the electrolyzer’s 646 Stand-Alone, Hybrid Systems Electrolyser's operation curves 31 20 °C 30 °C 40 °C 50 °C 60 °C 29 Voltage (V) 27 25 23 21 19 10 15 Current (A) 20 25 30 Figure 26 Operational characteristics of electrolyzers for different power levels and temperatures, based on Reference [55] operation [56] In this case, an extended economic analysis should be undertaken in order for the system to be proved not only energy-efficient but also cost-effective (Figure 26) The use of an electrolyzer–fuel cell set as a storage option for wind stand-alone systems is still in its initial stage, although both electrolyzer and fuel cell technologies have achieved considerable progress during the last decades The main drawback of such configurations is the low energy efficiency of the charge–discharge cycle (round-trip efficiency), which is estimated to be between 30% and 40% The use of advanced electrolyzers can raise the efficiency even up to 60%, but, on the other hand, increased purchase cost should be taken into consideration A very interesting and viable application of wind–hydrogen stand-alone systems could be in configurations where a bulk energy storage system is necessary in order to absorb the rejected wind energy in remote electricity grids, similar to the case of wind–hydro power applications Islands where usually weak electrical grids exist often possess significantly high renewable energy potential However, the local autonomous electrical networks in most cases are unable to absorb the renewable energy produced The other barriers against renewable energy penetration in islands are caused mainly by the significant difference between energy production and demand As a result, significant amounts of renewable energy are rejected by the local grids Furthermore, one should not neglect the cost of electricity production in remote electrical grids, where in most cases the electricity production cost could even be times the corresponding selling price By storing the excess wind energy and using it during peak load demands, that is, when there is no wind available, renewable energy penetration limits may be bypassed and the disturbance of the local grid stability may be avoided The use of the stored energy at peak demand can also improve the operation of the autonomous power stations and reduce their operational cost A typical wind–hydrogen configuration capable of fulfilling the energy needs of an isolated community is presented in Figure 27 More precisely, the hybrid system consists of • one or more wind turbines; • a water purification unit to improve the quality of the water used; • a water storage tank to ensure that the process has adequate water in storage in case the water supply system is interrupted; • an electrolyte solution (in alkaline systems); • a hydrogen generation unit consisting of an electrolysis stack, a gas purification module, a dryer, and a heat removal system; • a hydrogen storage medium (It should be noted that in order to fill the hydrogen tank, a compressor may be required if the electrolyzer is not designed to provide high pressure [57]); • a fuel cell electricity generation unit; and • a power conditioning and control unit During the operation of a wind–hydrogen energy production installation, the following operational situations may arise: The wind power produced is in excess of the energy demand of the system a) In that case, the hydrogen production unit absorbs the energy surplus to produce and store hydrogen b) In case the available wind energy is more than the hydrogen production unit’s power capacity or less than the minimum power required, the surplus is transferred to low-priority loads Stand-Alone, Hybrid Systems 647 UPS Wind turbine Electricity consumption Rectifier Inverter Fuel cell syetem Compressor unit Control panel Water flow Hydrogen flow DC current AC current Feed water Electrolysis unit Hydrogen storage Figure 27 Integrated wind–hydrogen stand-alone installation The electrical power demand is more than the wind park output a) In that case, the fuel cell unit covers the power deficit b) In case there is not enough stored hydrogen, the internal combustion engines of the APS cover the power deficit, under a scheduled operation plan An alternative operation mode of a wind–hydrogen stand-alone system was suggested by Ntziachristos et al [58] In their study on a hybrid wind–fuel-cell power station, they consider that the electrolyzer should always remain in operation, and in cases where the wind turbine is not in operation, an internal loop should provide the electrolyzer with electrical power at standby levels from the fuel cell unit in order to avoid intermittent operation In addition, in such a case, the wind turbine’s electricity production is primarily supplied to the electrolyzer for producing and storing hydrogen The analysis was based on a case study for the Karpathos island in Greece Karpathos Island is a medium-size island in Greece with an autonomous electricity grid and possesses excellent wind potential The simulation procedure developed by Ntziachristos et al [58] introduced a level of hybridization, which indicates the ratio of wind energy delivered directly to the local grid to the energy delivered to the grid from the fuel cell Given that the fuel cell provides constant power to the grid, the level of hybridization also indicates the variation over the constant power delivered to the grid According to the results of their study, depending on the wind turbine selected and the preferred hybridization ratio, different electrolyzer rates and storage tank capacities can be realized In an attempt to define the energy production cost of a wind–hydrogen power system, Kavadias et al [59] investigated the option of installing electrolyzers in order to absorb the wind energy rejected by a remote local electricity network in the island of Crete in Greece Crete island is of great importance, as it has the largest autonomous electrical network in Greece Although the island possesses excellent wind potential and faces substantial energy demand fulfillment, wind energy cannot be fully exploited owing to the local electrical grid instability barriers [60] By an analysis made by Kaldellis et al [60], it was estimated that in a 25 MW wind park almost 10% of the annual production was rejected leading to an average income loss of 25 000 € MW−1 for the wind park owners on the island [61] According to their results, an optimum configuration could be achieved on the basis of minimum hydrogen production cost, which depends on the wind energy purchase price (Figure 28) 2.19.6 Energy Storage in Wind Stand-Alone Energy Systems In long-term operation, dependence on the wind turbine energy production leads to the question of security of supply (firm power) Because of the stochastic nature of wind, independent and firm power cannot be realized by means of wind turbines alone It requires a holistic supply concept which includes at least an energy storage system All efforts in striving for an autonomous energy system with the aid of renewable energy sources always end up with a requirement for energy storage The search for cost-effective energy storage is a theme pervading the whole range of these technologies To exaggerate, one might say that as soon as an economically viable solution for storing energy has been found, all energy problems concerning the utilization of renewable energy sources can be solved [3] 648 Stand-Alone, Hybrid Systems Hydrogen Production Cost 0.40 0.05 kWh–1 Euro kWh–1 0.35 0.15 kWh–1 0.30 0.25 0.20 0.15 0.10 10 12 14 Nominal power (MW) Figure 28 Hydrogen production cost for different electrolysis installation sizes The adoption of energy storage in stand-alone wind-based energy systems improves the reliability of energy supply and can contribute to the exploitation of the otherwise-wasted portion of renewable energy The main drawback of energy storage systems is the high initial cost and in some cases the increased losses during a charge–-discharge operation cycle But it should be noted that maximum exploitation of renewable energy sources across a wide range of stand-alone energy system applications is possible only via the utilization of energy storage systems A typical energy storage system configuration comprises an energy source, which in the case of stand-alone systems involves renewable energy (e.g., wind); power conversion components, which include the necessary energy source interface and the main power conversion system; control devices; and an energy storage medium The electricity generated by the renewable energy machine, which could be either AC or DC, passes through the necessary conversion stages to be stored in the appropriate form In cases of energy deficit, the required amount of energy is drawn from the storage medium to be converted to the appropriate form of electricity requested by the load demand The energy flow of a typical energy storage system is presented in Figure 29 The input energy delivered to the energy storage system during its charging phase gets reduced owing to distribution and conversion losses Distribution losses occur during the transfer of energy from the original energy source to the storage system, whereas conversion losses, which are the most important in majority of the cases, derive from the conversion of electrical energy to the form of energy required to charge the storage system During the discharging phase, similar losses occur when the energy is drawn from the storage medium and is converted to electricity to fulfill the load requirements Minor additional losses include self-discharge or idling losses that occur during standby or off-duty Distribution losses Conversion losses Distribution losses Input energy Output energy Self-discharge or idling losses Figure 29 Sankey diagram of a typical energy storage system [62] Conversion losses Stand-Alone, Hybrid Systems 649 100% 90% Cycle efficiency 80% 70% 60% 50% 40% 30% 20% 10% 0% r r ES i-ion SC eels a-S PHS VRB L/A AES PSB n-B i-Cd -HS l Ai N h L Z N FC eta C w M Fly SM Figure 30 Cycling efficiency of different energy storage systems [63] mode of the system The efficiency of a charge–discharge cycle is defined as the ratio of the storage system energy output to the energy input to the storage system Accordingly, Figure 30 presents the energy efficiency of the charging–discharging cycle of typical energy storage systems According to Figure 30, flywheels and electrical energy storage systems, along with Na-S and Li-ion batteries, exceed 80% efficiency rates whereas fuel cell hydrogen storage systems (FC-HS) and metal–air batteries drop below 50% The majority of the storage system technologies present efficiency rates between 60% and 80% 2.19.6.1 Design Parameters of Energy Storage Systems The power rating of an energy storage system is one of the main characteristics of the system The power rating usually results from the maximum power requirements of the electrical load on the consumption side during discharging and the most frequently appearing excess power on the input side during charging Accordingly, the size of the storage system is determined by the load demand and the energy source current and voltage requirements The power rating influences the energy storage capacity, which is determined by the power and autonomy period requirements as well as by the system’s efficiency and maximum depth of discharge Critical parameters of an energy storage system are also the discharge time and reaction time The discharge time can be thought of as a dependent variable interrelated with the available storage capacity and the system power rating The reaction time is inherent to the system, and short-reaction time energy storage systems can provide electricity instantly, whereas long-reaction time storage systems can only adjust to scheduled generation patterns that allow for a time interval between start-up and electricity production As mentioned before, self-discharge or idling losses are considered as minor energy losses in an energy storage system At the same time, however, self-discharge or idling losses determine the maximum storage duration and thus delimit the system’s application range There are cases in which additional energy is required to compensate for self-discharge losses or sustain certain conditions of operation required for some storage systems (e.g., temperature level or vacuum requirements) Moreover, aging mechanisms should also be taken into account, as they invoke long-term gradual degradation reducing the system service period expectancy During the selection of the energy storage system for integration into a stand-alone power installation, the energy and power density should also be taken into consideration, as the space to be occupied by the system may in certain cases be extremely large or the required volume may not be available Specific system boundaries are critical, and may be limited to storage media only or expanded so as to include power conversion systems and source/load interfaces 2.19.6.2 Short Description of Energy Storage Technologies The energy storage systems are classified according to the form in which energy is stored Therefore, the main categories are mechanical, chemical, and electrical storage systems The category of mechanical energy storage includes the PHS systems, the compressed air energy storage (CAES) systems, and the flywheels In the category of chemical energy storage systems one may consider batteries, which in turn include lead–acid, nickel–cadmium, sodium–sulfur, metal–air, and lithium-ion batteries Chemical storage also includes flow batteries (vanadium redox, polysulfide–bromine, and zinc–bromine batteries) as well as FC–HS systems Finally, the category of electrical energy storage includes the superconducting magnetic energy storage (SMES) and the supercapacitor energy storage systems 650 Stand-Alone, Hybrid Systems PHS is one of the most widely applicable bulk energy storage system, used in a wide range of installations all over the word (more than 100 GW have been installed worldwide) PHS may be coupled with wind turbines establishing the above-described wind–hydro power systems In a PHS, the energy to be stored is exploited by a water pump used to pump water up an elevation to where the upper reservoir exists and this is considered the charging process During the discharging process, water is released from the upper reservoir to an appropriate hydro turbine which operates connected to an electric generator The cycle efficiency of a typical PHS ranges between 65% and 77%, whereas the main drawback of the corresponding systems is the high investment cost The existence of at least two reservoirs at different elevations is also essential for feasible projects PHS is able to take up load within a few minutes and is characterized by a high rate of extracted energy CAES systems are also used for bulk energy storage During the charging process, the energy to be stored is used to pressurize air into an underground cavern, whereas during the discharging process the required amount of air is released from the cavern, heated using natural gas, and then supplied in the form of gases to a gas turbine where expansion takes place as in a typical Brayton–Joule cycle The main benefit of the CAES is the separation of the compression and generation stages Given that during charging– discharging cycle in a CAES the generation of kWh of electricity requires ∼0.75 kWh of electricity for the compressor and 4500 kJ of fuel for combustion, the overall efficiency of CAES is estimated to be on the order of 50% The feasibility of CAES installation depends on the availability of a cavern [86]; therefore, favorable sites and geological formations for underground storage are necessary The main advantages of CAES are that it has 2–3 times faster ramp rate than conventional units, its heat range at low capacity is stable, and it has considerably lower emissions than conventional gas turbines Flywheels are used mainly to ensure short-duration power quality and to provide a reliable option for UPS applications In a flywheel energy storage system, the energy is stored as kinetic energy by causing a disk or rotor to spin on its axis During discharging, that is, when power is required, the flywheel takes advantage of the rotor’s inertia and the stored kinetic energy is converted to electricity Modern flywheels consist of a rim attached to a shaft (rotating mass), which is supported by bearings and is connected to a motor/generator Flywheels are characterized by high power density, relatively low maintenance needs, high cycling rate, deep discharges, and high self-discharge rate Regarding chemical energy storage, batteries are considered the most common and representative technology They are the most widely adopted storage technique used in many RES-based applications Different battery types exist; each one has its own special characteristics, over a wide range of applications The most mature battery types are the lead–acid and nickel–cadmium batteries Lead–acid batteries are characterized by their considerable self-discharge rate, low maintenance requirements, low energy density, limited service period, low depth of discharge, and considerable environmental impacts Nickel–cadmium batteries are character­ ized by their higher energy density and self-discharge, deep discharge rate, longer service period, high capital cost, low efficiency rates, and quite severe environmental impacts More advanced battery technologies include sodium–sulfur, metal–air, and lithium-ion batteries For sodium–sulfur batteries, an operating temperature of 300 °C is required, meaning that heat supply is necessary On the other hand, sodium–sulfur batteries have no self-discharge, and efficiency and depth of discharge are quite high Lithium-ion batteries have high energy density, a considerable number of charge–discharge cycles, and deep discharge rates On the other hand, the main drawback of the technology is the high capital cost and the required protection circuits to maintain voltage and current within safety limits Finally, metal–air batteries are characterized by high energy density, low system performance, short service period, low self-discharge rate, and very low system cost Flow batteries store energy by means of a reversible chemical reaction The energy is stored in two liquid electrolyte solutions The energy capacity and the rated power of the system are independent of one another Energy capacity depends on the quantity of electrolytes used Flow batteries are used in a number of large-scale and stand-alone RES installations Different technologies of flow batteries exists (vanadium redox, polysulfide–bromine, zinc–bromine) which are characterized by the electrolytes used The efficiency of flow batteries ranges between 60% and 80%, with future prospects ensuring high cycling capacity and deep discharge rates Finally, the significant environmental impacts should also be considered as a drawback of the technology Production of hydrogen is one of the ideal methods for the absorption of intermittent RES such as wind energy In FC–HS systems, the renewable energy is converted to fuel (hydrogen), which is stored in appropriate storage tanks The storage capacity depends only on the amount of the hydrogen that need to be stored and is theoretically independent of the fuel cell’s nominal power During the discharging procedure, hydrogen is released from the storage tank and is fed to the fuel cell unit, which then generates electricity The main drawback of the FC–HS systems is the low charge–discharge cycle efficiency estimated to be between 30% and 40%, including the losses during both the electrolysis to produce hydrogen and the storage stage The advantages of the technology, on the other hand, include the low energy cost, the high energy density, and the negligible self-discharge rate The category of electrical energy storage includes SMES and supercapacitors SMES store energy in the magnetic field produced when DC flows through a superconducting coil Significant amounts of energy is required to keep the system within the operating temperature range of 50–77 K Supercapacitors’ operation is based on the same operational principle as that of conventional capacitors where energy storage occurs in an existing electric field Both these energy storage systems are destined for power quality applications supporting the short fluctuations of RES energy production rather than for storing residual energy Recapitulating, for the selection of the most appropriate energy storage system for a wind-based stand-alone system, one should take into consideration numerous characteristics among which are the power rating, required storage capacity, discharge time, self-discharge rating, mass and volume energy and power density, service period, cycle efficiency, maximum depth of discharge rate, response time of the storage system, and last, but not least, energy and power cost Figure 31 presents a classification of different energy storage systems in terms of energy capacity, rated power, and discharge time Pumped hydro, Stand-Alone, Hybrid Systems 651 Figure 31 Energy capacity, discharge time, and power ratings for different energy storage solutions [64] compressed air, and fuel cell hydrogen systems are ideal for applications of commodity storage, rapid reserve, and area control/ frequency responsive reserve On the other hand, flywheels, supercapacitors, and superconducting magnetic systems are suitable mainly for power quality/reliability and transmission system stability applications Typical batteries can be used in a wide range of applications from power quality improvement to energy management Flow batteries are appropriate for transmission and distribution deferral 2.19.7 Design, Simulation, and Evaluation Software Tools for Wind-Based Hybrid Energy Systems The future success of wind-based hybrid energy systems relies on continuous research, development, and demonstration of RES technologies, and is based on improvement in their operation performance, cost reduction, and improvement in their reliability In order to determine the optimum configuration, different combinations of components should be considered, based on a number of factors, for example, the specific area’s renewable energy potential, the load demand requirements, and the subjective preference factor of the designer or the user which may lead to favoring a particular system not necessarily constituting the optimum solution design The criteria used for the optimization of wind stand-alone hybrid systems are the same as the ones used for the optimization of any renewable energy-based hybrid system The system designer should best balance power reliability and system cost requirements with the intermittent characteristics of renewable energy, compelling the investigation of the system’s reliability during the design process In this context, the most common method used for the power reliability analysis of hybrid systems is the ‘loss of power supply probability (LOPSP)’ [65], in which the probability of insufficient power supply (i.e., the probability of the system’s maximum power output not satisfying the load demand) is recorded for a given period of time The ‘loss of load probability (LOLP)’ model [11, 66] is also widely used, in which the probability that instantaneous power demand will exceed the respective power supply is estimated for the time period analyzed Similarly, ‘loss of load hours (LOLH)’ [67] represents the number of load failures in which the load demand exceeds the power supply, based on simulations on an hourly basis Regarding the system cost analysis, several economic criteria can be used to determine the economic performance of hybrid energy systems The most commonly used criterion is the ‘net present value (NPV)’ [21, 68, 69], which is defined as the total present value of a time series of cash flows including the initial cost of the system, the replacement cost of major components of the installation, the maintenance and operation costs, and finally the revenues (estimated as the avoided expenses by the use of RES) Further, the ‘internal rate of return (IRR)’ [21, 70] also constitutes a criterion that can be used for the evaluation of the economic performance of hybrid energy systems More precisely, IRR is the discount rate that dictates NPV equal to zero during a given time period Besides, on top of that, one may also use more simplified economic criteria that also provide an indication of a hybrid system’s economic performance, such as the payback period and the economic efficiency [34, 71] In this context, different optimization techniques have been developed in order to define the renewable-based electrification system that will guarantee the lowest energy production cost with the maximum exploitation of the system’s capabilities, although, as already mentioned, system power reliability requirements should also be considered in the optimization methods applied At this point, what should also be underlined is the important role of the meteorological data sets – used as time series – in such commonly used simulation approaches More specifically, there are several methods presented by researchers on the basis of the available meteorological data sets, which include 652 Stand-Alone, Hybrid Systems • typical meteorological year’ [72], which includes a dataset of hourly values selected for specific months from different years from a long period of records; • long period meteorological data [33, 73], which requires at least a full year’s meteorological data to be available; • yearly average monthly method [74], in which monthly average values of the required meteorological parameters are used; • worst months method [74], where the most unfavorable month can be chosen for each system component; for example, the most unfavorable month for wind potential, the most unfavorable month for solar radiation, and so on; and • worst month method [75], in which the most unfavorable month is chosen for the entire system dimensions; that is, when the largest system size occurs as a result of the least favorable renewable potential in the month used The time series simulation method is the most widely used optimization routine method Given the computational power of contemporary computer systems, the significant computational effort required by the specific method is no longer a drawback In the time series simulation method, the resolution can vary from h to intervals – or even lower – depending on the depth of the analysis, with the time step effect being of critical importance for the calculation results (see, for example, Reference [76] for the sizing of a stand-alone solar energy system) In this context, the optimal sizing of hybrid energy systems can be obtained through linear programing, probabilistic approach, iterative techniques, dynamic programing, and multiobjective optimization techniques [68, 77, 78] A wide range of software tools have been developed to analyze the integration of renewable energy into different energy systems [79] The list can include more than 70 software tools, with many more being able to adjust or be programed so as to be used as optimization tools for the design of hybrid energy systems Nevertheless, this section will focus only on a few software tools that are both freely available to users and have been widely used in research papers The Hybrid Optimization Model for Electric Renewables (HOMER) software was developed by the US National Renewable Energy Laboratory [80–82] HOMER is a computer model that can be used for the design of micropower systems and facilitates the comparison of power generation technologies across a wide range of applications The software is capable of modeling a power system’s physical behavior and its lifecycle cost over its lifetime The user can compare many different design options based on their technical and economic characteristics HOMER can model off-grid and grid-connected micropower systems serving electric and thermal loads, and including any combination of PV modules, wind turbines, small hydro, biomass power, reciprocating-engine generators, microturbines, fuel cells, batteries, and hydrogen storage The required inputs of the software include load demand, hybrid system components, available resources to be exploited, economic indices, generator control dispatch strategy (load following, cycle charging), possible operational constraints, and finally the optimization variables used to build the set of all possible system configurations After the data input procedure, HOMER simulates the operation of the system by making energy balance calculations on an hourly basis for year For each hour, the electric load is compared to the energy the system can supply and HOMER decides how to operate the generators and whether to charge or discharge the storage medium If the system meets the load for the entire year, HOMER estimates the lifecycle cost of the system, accounting for the capital, replacement, operation and maintenance, fuel, and interest costs The main advantage of HOMER is that it includes an optimization module that automatically finds the combination of components that can serve the load at the lowest lifecycle cost The main disadvantage of HOMER is that it is mainly an economic model dedicated to system selection and pre-sizing, and cannot be used for system design requirements Hybrid2 software package is a combined probabilistic/time series computer model that helps the designer in sizing hybrid power systems and selecting operating options on the basis of overall system performance and economics [5, 77, 78, 83] The program uses time series data, which include site-specific conditions and load profiles The user is able to consider a number of system configurations and operating strategies in order to optimize the system design Two different simulation models are used in the program: the logistical models used for long-term performance predictions and component sizing, and the dynamic models used for component design and assessment of system stability Hybrid2 uses a time series simulation analysis over intervals typically ranging from 10 to h The basic inputs of the program include load demand, site and resources data, power system characteristics including the bus system and system components, the base case which will be used for comparison purposes, and the costs/economic parameters of the system The main advantage of Hybrid2 is that it is mainly a technical model dedicated to system design, so it can simulate some important technical constraints, including bus voltage levels, intrahour performance of components, and complex diesel generator dispatch strategies The weakness of Hybrid2 is that it does not include optimization and sensitivity analysis modules Finally, Hybrid Optimization by Genetic Algorithms (HOGA) is a program developed by the electrical engineering department of University of Zaragoza in Spain [84, 85] The optimization of the hybrid system is achieved through the minimization of the total system cost for the period analyzed, based on net present cost calculations The multiobjective optimization feature of the program is achieved by additional variables that may also be considered, such as carbon dioxide emissions or LOLP The program can be used – apart from the hybrid energy systems that generate electricity – to also examine the performance of hybrid systems that produce hydrogen or energy applied to water pumping loads The simulation is carried out using h intervals during which all parameters remain constant The program allows surplus energy management, which includes the options of either selling the energy to the local electricity grid or producing hydrogen in an electrolyzer and storing it in tanks Table summarizes the basic characteristics of the different tools that can be used for simulation or/and optimization of hybrid energy systems Stand-Alone, Hybrid Systems Table 653 Basic characteristics of hybrid system simulation tools HOMER Hybrid2 HOGA National Renewable Energy Laboratory, HOMER Energy LLC • Free to download • Simulation • Optimization • Multiple RES analysis capability Renewable Energy Research Laboratory, University of Massachusetts • Free to download • Simulation • Optimization • Multiple RES analysis capability • Economical optimization • Control strategies • Control strategies Electric Engineering Department, University of Zaragoza • Free to download • Simulation • Optimization • Multiple RES analysis capability • Multiobjective optimization • Control strategies Thus, as it may be concluded, users nowadays have the advantage of designing optimum wind stand-alone hybrid energy system configurations with the help of a wide range of free software tools and commercially available integrated programs, and the elaborate research and development on stand-alone applications carried out continuously will certainly result 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