Review of energy system flexibility measures to enable high levels of variable renewable electricity
Introduction
Energy systems need flexibility to match with the energy demand which varies over time. This requirement is pronounced in electric energy systems in which demand and supply need to match at each time point. In a traditional power system, this requirement is handled through a portfolio of different kind of power plants, which together are able to provide the necessary flexibility in an aggregated way. Once variable renewable electricity is introduced in large amounts to the power system, new kind of flexibility measures are needed to balance the supply/demand mismatches, but issues may also arise in different parts of the energy system such as in the distribution and transmission networks [1], [2].
Large-scale schemes of renewable electricity, noticeably wind and solar power, are under way in several countries. Denmark plans to cover 100% of country’s energy demand with renewable energy (RE) [3], Germany has as a goal to meet 80% of the power demand through renewables by 2050 [4], and in several other countries increasing the RE share is under discussion or debate [5], [6], [7]. At the same time, the renewable electricity markets are growing fast, e.g. in the EU, wind and solar stood for more than half of all new power investments in 2013 [8]. On a longer term, by 2050, RE sources could stand for a major share of all global electricity production according to several studies and scenarios [9], [10], [11], [12]. Compared to today’s use of RE in power production, the variable RE power utilization (VRE) could increase an order of magnitude or even more by the middle of this century. The experiences from countries with a notable VRE share, such as Denmark, Ireland and Germany, clearly indicate challenges with the technical integration of VRE into the existing power system, but also problems with the market mechanisms associated. Therefore, improving the flexibility of the energy system in parallel with increasing the RE power share would be highly important.
There is a range of different approaches for increasing energy system flexibility, ranging from supply to demand side measures. Sometimes more flexibility could be accomplished through simply strengthening the power grid, enabling e.g. better spatial smoothing [13]. Recently, energy storage technologies have received much attention, in particular distributed and end-use side storage [14], [15], [16]. Storage would be useful with RE power [17], but it is often perceived somewhat optimistically as a generic solution to increasing flexibility, underestimating the scale in energy [18]. Different types of systemic innovations, e.g. considering the energy system as a whole and integrating power and thermal (heating/cooling) energy systems together, could considerably improve the integration of large-scale RE schemes [19], [20]. The concept Smart Grid involves a range of different energy technologies and ICT to better manage the power systems and increase their flexibility [21]. Many other options are available as well.
The purpose of this study is to present a broad review of available and future options to increase energy system flexibility measures to enable high levels of renewable energy. Several of these measures are applicable for any type of energy system or energy supply. We present solutions that are linked to the demand side, electricity network, power supply, and the electricity markets. The literature on individual measures or technologies for energy system flexibility is vast. Recently, a few reviews on the subject have been published [22], [23], [24], [25], [26], but with a more narrow scope, whereas here we strive for a broader coverage of the available options. In addition to presenting options for energy system flexibility, we also try to reflect these against large-scale RE utilization and integration whenever possible.
Section snippets
Defining flexibility
To operate properly, the power system needs to be in balance, i.e. power supply and demand in the electric grid has to match at each point of time. The electric system is built in such a way that it has up to a certain point a capability to cope with uncertainty and variability in both demand and supply of power. For example on the supply side, the kind of flexibility is accomplished through power plants with different response time. Introducing variable power generation such as wind and solar
Overview
Demand side management (DSM) comprises a broad set of means to affect the patterns and magnitude of end-use electricity consumption. It can be categorized to reducing (peak shaving, conservation) or increasing (valley filling, load growth) or rescheduling energy demand (load shifting), see Fig. 1 [30]. Load shifting requires some kind of an intermediate storage [31] and a utilization rate of less than 100% [32] as both an increase and a decrease of power demand need to be possible in this case.
Grid ancillary services
With increasing variable renewable power production, system stability issues will become more likely [22] which can be mitigated through grid ancillary services. These services are generic in nature, i.e. not necessarily bound to RE power use.
Grid ancillary services involve different time scales and requirements with regard to power and energy capacity. For example, a power quality service has to provide rapid response, but only for a short duration, making it a power-intensive service. On the
Energy storage
Energy storage is used to time-shift the delivery of power. This allows temporary mismatches between supply and demand of electricity, which makes it a valuable system tool. Energy storage has recently gained renewed interest due to advances in storage technology, increase in fossil fuel prices and increased penetration of renewable energy [150]. In previous chapters, the usefulness of storage for ancillary services was already mentioned. In this chapter, we will present different storage
Supply-side flexibility
The power balance of electric systems is normally handled by the supply side (e.g. power plants). With supply-side flexibility, we mean measures or technologies through which the output of power generation units can be modified to attain the power balance in the grid, e.g. when large amounts of variable RE power is in use.
Advanced technologies
In the next, we present future strategies for dealing with large-scale RE schemes in which surplus RE production is utilized for different purposes, i.e. combining new loads to the power systems such as heating or cooling demand, electric vehicles and power-to-gas schemes. In this way, wasting the electricity from curtailment of RE power could be reduced or even avoided.
Grid infrastructure
Sufficient transmission capability is essential for power system flexibility [67], [318]. Robust and well-designed grids with appropriate grid codes [258] can balance large local differences in supply and demand, offering strong spatial interconnections. Furthermore, well-functioning energy markets need well-functioning transmission lines. Three future developments in grid infrastructure, namely supergrids, smart grids, and microgrids will be discussed here.
Electricity markets
Though energy system flexibility is often perceived as a technological issue only, it has a strong link to the electricity market as well. For example, different power tariffs may enhance flexibility. On the other hand, fuel-less energy sources such as wind and solar power have a zero-marginal cost on the market, meaning that large-scale RE schemes would drop the average electricity price. Poor market design may limit access to technical flexibility in energy systems [258]. In the next, we will
Conclusions
The number of options to improve energy system flexibility when increasing the share of renewable power in electricity production is large. It is likely that this theme will draw even more interest in the years to come as the prospects for new renewable energy technologies [387], [388], [389], [390], [391] are positive and further price reductions are expected. Energy system flexibility is definitely a “hot topic” as demonstrated by the large number of references contained in this review (close
Acknowledgements
The financial support of the Academy of Finland (Grant 13269795) is gratefully acknowledged.
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