Keywords

1 Introduction

Renewable energy accounts for 26% of the world’s electricity production (IEA, 2020). Renewables in electricity generation provide environmental and economic benefits, which include the reduction of greenhouse gas emissions, mitigation of climate variability, and energy independence.

However, the intermittency of some renewable energy resources, such as solar and wind energy, is a major concern when the generation systems are connected to the grid. Therefore, several techniques are proposed in the scientific literature to address the issue of managing intermittent solar and wind energy resources: short, medium, and long-term forecasting of resource availability (Nkounga et al., 2018; Javed et al., 2020), geographical dispersion (spatial diversification) of production units (Liu et al., 2020), storage (Chatzivasileiadi et al., 2013), grid interconnection, and hybrid systems. In this chapter, we focus on storage and network interconnection techniques.

Energy storage options are numerous and include hydraulic pumping, fuel cell, flywheel, and the combinations battery/hydraulic pump, battery/supercapacitor, battery/fuel cell, battery/flywheel, and battery/flywheel/supercapacitor (Javed et al., 2020). However, a number of problems are prevalent during the operation of these storage systems: low predictability over time, lack of suitable location, high investment costs, low level of autonomy, short discharge time. These problems constitute additional obstacles to the integration of wind and solar energy systems into electricity networks beyond investment in power capacities.

In terms of capacities for electricity generation, solar photovoltaic and wind energy are among the most advanced renewable energy technologies that have been integrated into the main electricity grid in several regions of the world (Al-Shetwi et al., 2020). Connecting these systems to the centralized grid raises issues that include voltage fluctuation, reactive power, grid overload and harmonics distortions (Benzohra et al., 2020). These issues pose significant challenges in terms of power factor, storage management, energy forecasting and planning (Shafiullaha et al., 2018). These issues also raise the following question: How could solar and wind energy systems be successfully integrated into power grids over the long term and at low cost, while optimizing grid stability?

The objective of this study is to propose effective measures for managing the intermittency of solar and wind energy resources, the implementation of which would constitute a contribution to achieving the objectives of Sustainable Development Goal number 7 (SDG-7). The chapter includes five sections: Following the introduction, Sect. 2 describes the study's methodological approach. Section 3 presents the tools for the management of storage techniques, which include their configurations and the conditions of implementation for improved efficiency. Section 4 compiles discussions and recommendations. Finally, the conclusion and perspectives are presented in Sect. 5.

2 Methodological Approach

The study methodology is a systematic literature review followed by the definition of a protocol to address issues associated with intermittency of solar photovoltaic and wind energy in the literature. The protocol includes the following steps:

  • Classification and comparative analysis of power storage techniques.

  • Classification and comparative analysis of energy storage techniques.

  • Classification of storage techniques according to the planning horizon considered.

  • Identification of storage techniques considering investment costs, charging time, discharge time, required surface area, density per cubic metre related to each technique of storage for both power and energy storage.

  • Analysis of grid connection as backup option based on stochastic, deterministic, and hybrid management strategies (simultaneous use of stochastic and deterministic management techniques) suitable for solar photovoltaic and wind energy.

3 Results

Figure 10.1 displays a comparison of investment costs for different techniques of power storage. The blue and red bars represent the minimum and average investment costs for each type of storage, respectively. For power storage, hydraulic pumping, compressed air, hydrogen, and batteries have a relatively high investment cost per kilowatt compared to other techniques. Flywheel, magnetic conductivity and supercapacitor storage techniques have a lower investment cost per kilowatt; their minimum investment costs are between EUR 100 and 400 per kW.

Fig. 10.1
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Investment costs of power storage systems

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Flywheel and magnetic conductivity storage systems have similar investment costs that are relatively low, which explains why they are the techniques most widely used for power storage. Conventional batteries, in particular lead, nickel, lithium, and zinc-air batteries, are also technologies frequently used in storing power, despite their prohibitive costs. The cost of storage batteries varies according to the technology. The average cost of EUR 3000 per kW corresponds to lithium-ion technology and the lowest cost to sodium-nickel (NaNiCl) technology.

Figure 10.2 shows a comparison of investment costs for energy storage. The minimum investment cost is shown in blue, and the average investment cost is in red.

Fig. 10.2
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investment costs of energy storage systems

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For energy storage, hydraulic pumping, compressed air, and hydrogen feature the lowest investment costs for long-term energy storage. The flywheel, magnetic conductivity and supercapacitor have relatively high investment costs. Pumping requires an investment of EUR 60–150 per kWh, compressed air requires an investment of EUR 10–40 per kWh, and hydrogen requires an investment of EUR 1–15 per kWh. Conventional battery, here again, stands out with a cost that is relatively high and utilization that is frequent in energy systems. The battery investment cost depends on the technology; the minimum cost of EUR 50 per kWh corresponds to lead-acid batteries, and the average cost of EUR 1800 per kWh corresponds to lithium batteries. The latter feature the advantages of having much higher efficiency and autonomy period. Table 10.1 provides a classification of storage techniques according to discharge time, lifetime, self-discharge time and recharge time.

Table 10.1 Durability indicators of storage techniques (Chatzivasileiadi, Ampatzi, & IanKnight, 2013)
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The discharge time of storage options varies from 1 second to several days. The technologies featuring the lowest discharge times are supercapacitor, magnetic conductivity, and flywheel, with a limited duration of a few milliseconds for magnetic conductivity and supercapacitor, and a maximum duration of around 15 minutes for the flywheel.

Considering the service life of the storage techniques, hydraulic pumping can operate for one hundred years (one century). Conversely, hydrogen and battery storage have lifetimes comprised between 5 to 15 years and 3 to 30 years, respectively.

The recharge time in the fifth column of Table 10.1 shows instantaneous recharging for hydrogen storage. This makes it the fastest option. Conversely, conventional batteries can take up to 16 hours for a full recharge.

Table 10.2 provides a classification of storage options, considering production, density, and space requirements of the facilities.

Table 10.2 Indicators of energy storage (Chatzivasileiadi, Ampatzi, & IanKnight, 2013)
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Among the storage options in Table 10.2, magnetic conductivity requires the widest space. A production of 1–100 kWh requires 930 to 26,000 Wh/m2, compared to 20 Wh/m2 for 200 to 5000 kWh of a hydraulic pumping system. After hydraulic pumping, battery, and hydrogen are the least space-consuming storage systems. This characteristic provides them with value addition in long-term energy storage.

Table 10.3 provides a classification of storage systems, considering the power and density of the systems.

Table 10.3 Indicators of power storage (Mahmoud et al., 2020)
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Magnetic conductivity and supercapacitor have a relatively high density of 4000 to 120,000 kW/m3 for 0.01 to 1 megawatt power storage. Compressed air and hydraulic pumping systems have a relatively low-density compared to other techniques of storage. For a system of 100 to 5000 MW, a density of 0.1 to 0.2 kW/m3 is required for hydraulic pumping and for a system of 100 to 300 MW, a density of 0.2 to 0.6 kW/m3 is required for compressed air.

Table 10.4 presents techniques often used in managing the integration of solar and wind energy systems connected into the grid with storage. The management strategies are based on smart monitoring and control protocols, which are associated with constraints and objective functions.

Table 10.4 Energy management techniques (Bukar & Tan, 2019)
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Deterministic methods are management techniques based on linear programming, with numerical, analytical, iterative methods, and probabilistic computations, and graphic construction. Deterministic methods relate to genetic algorithms, particle swarm optimization, bee colonies, simulated annealing, biogeographic optimization, and imperialist competitive algorithms (Bukar & Tan, 2019). These techniques are used in the management or energy systems with storage and with or without grid connection. The appropriateness of these management techniques varies depending on the objective constraints and functions. In the effective management of intermittent solar photovoltaic and wind energy resources, each of these methods can apply objectively, depending on the objective functions. Table 10.4 presents some of these techniques considering the objective function and some constraints.

The hybrid (stochastic-deterministic) approach can also apply on some occasions. The elements presented in Table 10.4 for the management of the intermittency of solar and wind energy resources are structured based on established algorithmic protocols. The constraints represented are necessary to satisfy throughout the control process. The battery state of charge (SOC), energy demand, and the availability of solar and wind resources are basic indicators, which condition intervention in the network. Minimisation of costs, reactive power, and power factor are the constraints associated with the grid; the control strategy is successful when the grid constraints, resource constraints, and demand constraints are satisfied simultaneously.

4 Discussion of Results

The results observed in the previous section demonstrate that fluctuations in energy networks can be controlled despite the intermittency of solar and wind energy resources in the network. In the short term, high power systems can be associated with flywheel, magnetic conductivity and supercapacitors storage techniques. This observation complies with the recommendations of Chatzivasileiadi et al. (2013), which propose the use of flywheel (rotational energy), super magnetic conductivity and supercapacitors (in electrostatic form) storage technologies in the short term. These options should be prioritized in planning additional renewable energy capacities as a contribution to Sustainable Development Goal 7 (SDG-7). According to the European Patent Office quoting the International Energy Agency, between 189 and 305 GW of energy storage capacity will be needed by 2050 to mitigate the impact of connecting intermittent renewable energy power systems in energy networks (European Patent Office, n.d.).

With regard to long-term energy storage, hydraulic pumping offers interesting characteristics, especially for countries in the sub-Saharan Africa region, due to its relatively low investment cost and availability of the resource. This observation complies with the recommendations of Javed et al. (2020) and those of Mahmoud et al. (2020) for using this technology. This observation also explains the increasing popularity of hydraulic pumping in recent renewable energy generation systems (see Fig. 10.3). According to the International Hydropower Agency, 3.2 GW of hydraulic pumping storage was added between 2017 and 2018 (International Hydropower Association, 2018). According to Mahmoud et al. (2020), mechanical storage has an advantage in environmental impact, cost, and sustainability. Thus, compressed air and hydraulic pumping are relevant storage options to address the concerns that raise electricity generation with intermittent solar and wind energy resources in the region. Currently, only two power plants with compressed air storage are operational worldwide (110 MW in the USA and 290 MW in Germany), compared with about a hundred power plants associated with hydraulic pumping storage.

Fig. 10.3
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Capacity of storage systems installed worldwide (Javed et al., 2020)

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On connection to the grid as backup, the appropriate management strategy should consider the objective functions and the corresponding constraints, as proposed in Table 10.4. Control and command strategies defined considering this approach make it possible to anticipate potential energy deficit or surplus while addressing issues related to intermittencies such as load balancing, peak demand alleviation, cost minimisation, and energy efficiency (Mahmoud et al., 2020).

5 Conclusion

In this chapter, we explore different storage systems that could contribute to addressing the issues associated with the intermittency of solar photovoltaic and wind energy resources connected to the grid. The analysis of storage techniques considers, among other parameters, their investment costs, their durability, density, and space required.

The study shows that parameters such as power and energy density, available space, service life, charge, and discharge duration are key factors in the selection of the appropriate storage technology. In the short term, taking into account investment costs and power density per cubic meter, flywheel is the best option for power storage. For long-term energy storage, when only considering the investment cost, hydrogen appears as a good option. However, when the required surface area and power density per cubic metre are taken into account, hydraulic pumping is a better option.

The inclusion of these indicators in the selection of a storage system contributes to improving the efficiency and attractiveness of renewable energy systems, because it reduces the investment risks and uncertainties associated with grid stability due to the intermittency of these resources. However, the value of relevant indicators is context-specific. Therefore, the selection of renewable energy resources to feed into the grid and storage options should consider strategic management techniques and context-specific parameters such as available surface areas and desired scale of production.

This study is limited to the exploration of storage techniques in the management of intermittent solar and wind energy resources connected to the electricity grid. The extension of its results considering the integration of environmental and social factors such as the forecast of solar and wind energy potential and energy demands in a specific context could be envisioned after an adaptation of the protocol presented here. Indeed, the integration in the analysis of energy resources forecast methodologies can support selections of the appropriate storage techniques and grid management strategies for different communities.