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One of the key problems with integrating renewable energy in the power grid is the inherent variability and uncertainty. One way of mitigating this issue is using complementary energy sources, that when combined reduce variability and increase system reliability or optimize resource utilization. In this paper, various metrics for renewable energy complementarity are reviewed. Definitions, methodologies and interpretations of the metrics are provided.

A particular focus is placed on the implications of the metrics for wave energy. The temporal characteristics of wave energy is often similar to that of wind power, with a time delay. Therefore, combining wave energy with wind energy as well as other energy sources offer opportunities for synergetic co-location strategies.

The paper acts as a reference map for when to use which complementarity metric, and what the metric means. Our finding underscores the importance of leveraging energy complementarity, and aids to avoid pitfalls in wrongly assessing complementarity.

In this study, the impact of data time resolution on long-term voltage stability assessment of a power grid with high penetration of wind-solar hybrid power plants is investigated. Historical and synthetic wind data as well as solar irradiance are used to calculate power output from hypothetical offshore wind-solar hybrid power plants, geographically located off the coast of Massachusetts, USA. The results show that using hourly input data can overestimate the long-term voltage stability, compared with using minute data. However, the relative difference in terms of voltage mean value and standard deviation is marginal whilst the most significant difference is the intensity of the voltage fluctuations. The main drawback of using high-resolution data is the execution time, increasing proportionally with the number of time steps. Thus, it is argued that the choice of da ta time resolution should be based on the aspects of long-term voltage stability and the size of the power grid to be studied.

Flexibility has increasingly gained attention within the field of electrification and energy transition where a common objective is to reduce the electricity consumption peaks. However, flexibility can increase the risk of grid congestion depending on where and when and it is used, thus an overall system perspective needs to be considered to ensure an effective energy transition. This paper presents a framework to assess electricity users' contributions to grid load peaks by splitting electricity consumption data into subsets based on time and temperature. The data in each subset is separately correlated with the grid load using three correlation measures to assess how the user's consumption changes at the same time as typical grid peak loads occur. The framework is implemented on four different types of business activities at Uppsala municipality in Sweden, which is a large public entity, to explore their behaviors and assess their grid peak load contributions. The results of this study conclude that all four activities generally contribute to the grid peak loads, but that differences exist. These differences are not visible without splitting the data, and not doing so can lead to unrepresentative conclusions. The presented framework can identify activities that contribute the most to unfavorable grid peaks, providing a tool for decision-makers to enable an accelerated energy transition.

As the share of renewable energy sources in the energy mix increases, weather-dependent variations in several time scales will have a significant impact on the power system. One way of mitigating these variations is to co-locate complementary energy sources at the same location. In this study, the complementarity between offshore floating photovoltaics, wave, and wind power is analyzed and the grid impact of such co-located energy sources is addressed using capacity credit. Additionally, the possibility of installing supplementary generation capacity within existing offshore wind power farms is investigated. It is found that co-locating wave power with offshore wind results in increased capacity credit compared to stand-alone wind power farms and that in all analyzed cases, the capacity credit of the co-located energy sources exceeds the capacity credit contribution of the separate energy sources. Co-locating photovoltaics with offshore wind brings little benefit to the capacity credit, but shows potential in increasing the utilization of the transmission cable.

In the past decade, the global share of fossil-based electricity generation has decreased from 67% to 61% in favor of renewable alternatives. To achieve global goals, a continued extensive expansion of electricity generation from renewable energy sources is necessary. Offshore wind power is expected to constitute a significant portion of this additional generation capability. However, intermittent energy generation like wind or solar power has negative impacts on the electricity grid due to its inherently variable and non-dispatchable nature. Furthermore, energy generation from renewable energy sources is characterized by low utilization and requirement of large geographical areas.

One way to mitigate several of these negative aspects is by co-locating energy sources with complementary characteristics. Combining different types of complementary renewable energy sources can reduce overall variability, increase transmission system utilization, and decrease land use. This thesis addresses several aspects of grid integration of offshore co-located energy sources, primarily, offshore wind power, floating solar power, and wave power. One question analyzed in several of the included studies is the optimal combination of energy sources for co-location to achieve the lowest variability.

Another aspect investigated is the capacity credit for a hybrid park consisting of co-located energy generation compared to the capacity credit for a wind farm. In a case study for the Netherlands, the capacity credit for combined wave and wind power is higher than for wind power alone. Additionally, the complementarity of renewable energy sources is analyzed and explained.

The Polish Exclusive Economic Zone (EEZ) in the Baltic Sea is an area of increasing strategic importance for Poland 's pursuit of renewable energy, especially offshore wind. This research investigates the complementarity among solar, wind, and wave energy resources within the Polish EEZ to examine these energy sources ' temporal dynamics, correlations, and extremes. The primary data source corresponds to a 31-year hourly time series dataset from the ERA5 reanalysis, whose reliability was evaluated through performance metrics. The results from complementarity metrics indicate varying levels of association among the three variable renewable energy resources (VRES) in the EEZ, spanning from weak similarity to weak complementarity. The findings of this research indicate that blackouts are most probable at offshore locations during winter and autumn for renewable power systems integrating wind and solar energy, with over 70 % of occurrences within these seasons. The investigation of extreme events highlights critical elements when evaluating VRES and their complementarity. This understanding aids in effectively planning and managing renewable energy systems, ensuring resilience and reliability under challenging weather conditions. Furthermore, while the complementarity may be consistent throughout the entire Polish EEZ, the feasibility and cost of implementing hybrid power systems can significantly vary between locations.

The power grid faces a rising challenge of increasing variability due to the integration of intermittent renewable energy sources (RES) and the connection of more and new types of loads. This development heightens the risk of both capacity shortage and grid congestion, addressing the need to complement traditional grid extension, which is expensive and can take a long time. A promising approach is load flexibility, which is the ability of an electricity user to adjust its consumption during a set time interval. This study proposes a Flexibility Value Index (FVI) to rank electricity users based on the value of their potential flexibility. The FVI utilizes three indicators derived from a user's consumption and the local grid's load. The FVI is demonstrated on seven test profiles, followed by ranking five different types of users from Uppsala Municipality, Sweden, during winter working days. The study reveals a spread in the FVI, and the ranked list enables a public entity or a grid owner to focus resources on the users that can potentially realize the most flexibility. Furthermore, the FVI can be utilized on the production from RES, indicating which might be a suitable match to enhance the grid's hosting capacity.

An important aspect in designing co-located wind and solar photovoltaic hybrid power plants is the sizing of the energy converters to achieve as efficient power smoothening as possible. In this study, the ratio of wind- and photovoltaic energy converters in a hybrid power plant is determined by minimizing the overall stored energy that is needed to facilitate constant power output. Using Fourier transform the variability is isolated at predefined time scales that are relevant for grid integration. For the investigated time scales, energy and power ratings for energy storages are determined to counteract the variability. The resulting configuration is the one that is able to achieve constant power output with minimum stored energy. It is shown that co-locating wind- and photovoltaic energy converters smoothen seasonal energy generation, and reduce the energy storage need in both the diurnal and seasonal time scales. A case study for south-eastern Sweden is presented where the wind- \& solar hybrid plant configuration that minimizes the energy storage need and therefore most closely resembles constant output power is determined. It is found that a ratio of approximately 40-45\% wind power in the hybrid power plant yields the lowest need for energy storage. The presented method is valid for any number of co-located energy sources, and can also be extended to sizing of hybrid power systems.

This paper discusses the article "Assessing temporal complementarity between three variable energy sources through correlation and compromise programming." The discussed paper proposes a novel method of assessing comple-mentarity between three energy sources using correlation, compromise programming, and normalization. The method is then used to calculate a complementarity index which is applied to a case study in Poland. However, upon inspection, the normalization of the index overestimates the complementarity potential. This issue is dis-cussed in detail in this paper, and an alternative way of calculating the index is proposed, eliminating the issue of overestimating complementarity.

Co-locating renewable energy sources such aswave power, solar photovoltaic and wind power, forminga hybrid power plant, may reduce the overall variability,increase the utilization of the transmission system, and reduce the needed physical area. An important topic to address regarding the formation of hybrid power plants is which energy sources to co-locate, and to what proportions these energy sources should be included in the hybrid power plant. In this study, offshore hybrid power plants are analyzed in the North Sea region. By minimizing the plant variability the proportions of each energy source are found, forming the plant with minimal need for energy storage for constant power output operation. The added grid value of such plants is analyzed in terms of electrical infrastructure utilization and power production ramping. It is found that in conditions suitable for the wave energy converter used in the study, a plant configuration of 23% wave power, 22% wind power and 55% solar minimizes the need for energy storage. It is shown that the inclusion of wave power in a hybrid power plant lowers ramping of power generation, increases the capacity factor and provides an overall higher grid value compared to stand-alone installations.