Penetration Of Renewable Power Generation Research Articles

Coordinated control for EV aggregators and power plants in frequency regulation considering time-varying delays

Nowadays, large scale intermittent renewable energy is being integrated to power systems as a solution for the low-carbon development worldwide. With the increasing penetration of renewable power generation, system frequency stability is becoming more and more serious. To increase the utilization of renewable energy, electric vehicles (EVs) are suggested to participate in load frequency control (LFC) through aggregators due to their vehicle-to-grid (V2G) capability and quick response characteristic, which is denoted as EV-LFC controller in this paper. In order to fully take the advantages of EVs in the LFC, this paper presents a coordinated control strategy between EV-LFC controller and traditional power plants based LFC (PP-LFC) controller for frequency regulation. In this strategy, the EV-LFC has a priority in response than the PP-LFC when the system deviation violates its acceptable range. However, the LFC integrating EVs is with inevitable time delays due to the data and control signal transmission. Meanwhile, the system inertia uncertainty caused by renewable energy in power system may also cause instability problem. For this reason, an improved robust stability criterion is proposed to estimate the asymptotically stable for LFC system considering the inertia uncertainty and time-varying delays simultaneously. Additionally, a PI controller for EV-LFC controller is used to enhance the system frequency stability. Finally, the effect of increasing EVs number on the frequency stability is investigated, which may guide system operator to utilize EVs to the LFC properly. Case studies are carried out based on a simplified Great Britain (GB) power system. Simulation results show that the proposed coordination strategy can not only provide effective frequency regulation, but also reduce the output of traditional power plants, in which the inertia uncertainty and time delays are properly considered.

Discussion Paper: Addressing Intermittency with Dispatchable Solar and Variable Supply Electric Power Services

As measured standardly by levelized cost of energy (LCOE), onshore wind and utility scale solar PV (photovoltaic) have been rapidly declining in cost and are now reported to be the cheapest ways of generating electric power. While this is indisputably a welcome development, the fact that renewables in general --- wind and solar in particular --- are variable and available only intermittently makes LCOE a quite incomplete, perhaps even misleading, measure of value because it neglects the system cost of accommodating renewables for the sake of meeting demand and keeping the grid in balance. The additional cost of building and employing peaker plants (mainly fueled by natural gas), which serve as backup and reserve and hence are often idle, has led many observers to pessimistic conclusions about the economic viability of very high levels of penetration by renewable sources of energy. This paper fully acknowledges the problem. We present an ensemble of several ideas that plausibly may be useful for mitigating the intermittency problem accompanying high levels of penetration of renewable power generation, particularly solar PV (photovoltaic). The ensemble, presented as a proposed setup or institution, includes provision for dispatchable solar PV (and by extension wind power) as well as provision for a market for what we call variable supply electric power. This is described in the paper and contrasted with existing services, which may be described as guaranteed supply. Further, we confront our proposed setup with three years of insolation data for Philadelphia, PA. This leads to a number of quantified measures of variability that should be useful for system design and management. This is an exploratory paper, aiming at an initial vetting of these ideas. Very much remains to be done to investigate them fully.

Reconstructing Operating Reserve: Flexibility for Sustainable Power Systems

Traditionally, the planning of operating reserve has been done in terms of capacity and average constant ramping requirements, whereas the newly emerging concept of power system flexibility puts emphasis on resources maneuverability, as well as accurately capturing the intra-hourly variability and uncertainty resulting from significant penetration of renewable power generation. However, the traditional reserve paradigm is deemed impeding to the notion of flexibility, whereas there is yet to be a proper way of defining power system flexibility. To that end, we rethink the fundamental meaning of reserve with respect to the emerging concept of flexibility and present a new flexibility modeling framework. We characterize flexibility provision and flexibility requirements via dynamical envelopes that can reflect the higher order dynamics of power system resources and those of variability and uncertainty. We assert that flexibility adequacy is directly related to how well the aggregate flexibility envelope formed by flexibility resources encloses the flexibility requirement envelope and its dynamics over operational planning horizons. An optimal flexibility planning problem with envelopes is formulated, followed by examples involving unit commitment and economic dispatch.

On the importance of reducing the energetic and material demands of electrical energy storage

Two prominent low-carbon energy resources, wind and sunlight, depend on weather. As the percentage of electricity supply from these sources increases, grid operators will need to employ strategies and technologies, including energy storage, to balance supply with demand. We quantify energy and material resource requirements for currently available energy storage technologies: lithium ion (Li-ion), sodium sulfur (NaS) and lead-acid (PbA) batteries; vanadium redox (VRB) and zinc-bromine (ZnBr) flow batteries; and geologic pumped hydroelectric storage (PHS) and compressed air energy storage (CAES). By introducing new concepts, including energy stored on invested (ESOI), we map research avenues that could expedite the development and deployment of grid-scale energy storage. ESOI incorporates several storage attributes instead of isolated properties, like efficiency or energy density. Calculations indicate that electrochemical storage technologies will impinge on global energy supplies for scale up — PHS and CAES are less energy intensive by 100 fold. Using ESOI we show that an increase in electrochemical storage cycle life by tenfold would greatly relax energetic constraints for grid-storage and improve cost competitiveness. We find that annual material resource production places tight limits on Li-ion, VRB and PHS development and loose limits on NaS and CAES. This analysis indicates that energy storage could provide some grid flexibility but its build up will require decades. Reducing financial cost is not sufficient for creating a scalable energy storage infrastructure. Most importantly, for grid integrated storage, cycle life must be improved to improve the scalability of battery technologies. As a result of the constraints on energy storage described here, increasing grid flexibility as the penetration of renewable power generation increases will require employing several additional techniques including demand-side management, flexible generation from base-load facilities and natural gas firming.