Getting wind and sun onto the grid


Wind and solar PV capacity has grown very rapidly in many countries, thanks to supportive policy, and dramatic falls in technology cost. By the end of 2015, these technologies – collectively referred to as variable renewable energy (VRE) – had reached double-digit shares of annual electricity generation in ten countries.

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In Denmark their share in electricity generation has risen to around 50%, and was around 20% in Ireland, Spain and Germany, in all cases without compromising the reliability of electricity supply. Despite this evidence, discussion of VRE integration is often still marred by misconceptions, myths, and in cases even misinformation.

Commonly heard claims include that electricity storage is prerequisite to integrate VRE, and that conventional generators are exposed to very high additional cost as VRE share grows. Such claims can distract decision-makers from the real, though ultimately manageable issues; if unchecked they can bring VRE deployment to a juddering halt.

This manual, written for policy makers and staff in energy ministries and regulatory bodies, has two main objectives: firstly to clarify the true challenges faced in the early days of VRE deployment; and secondly to signal how these can be mitigated and managed successfully.

It reveals how measures to maintain cost-effectiveness and reliability of the power system differ over four stages of VRE deployment. These phases are differentiated by an increasing impact of growing VRE capacity on power systems, providing a useful framework for prioritisation of tasks, which may otherwise be presented as a wall of challenges at the outset of deployment.


Fig. 1: Annual VRE generation shares in selected countries and correspondence to different VRE phases, 2015.


Phase one

Phase one is very simple: VRE capacity has no noticeable impact on the system. Where wind or solar plants are installed in a system that is much bigger than those first plants, their output and variability go unnoticed compared to daily variations in power demand. Examples of countries in Phase one of VRE deployment at present include Indonesia, South Africa and Mexico; annual VRE shares in these countries reach up to around 3% in annual electricity generation.

Phase two

In phase two, VRE has noticeable impact, but by upgrading some operational practices this can be managed quite easily. For example, forecasting of VRE plant output can be done so that flexible power plants can balance their variability, along with that of electricity demand, more efficiently.

There is no single threshold in terms of energy share; when a power system will enter phase two depends on its own properties. For example, ranging from 3% to almost 15% VRE share of energy, countries in phase two at present include Chile, China, Brazil, India, New Zealand, Australia, the Netherlands, Sweden, Austria and Belgium.

Phase three

It is phase three that sees the first really significant integration challenges, as the impact of variability is felt both in terms of overall system operation, and by other power plants. Power system flexibility now comes to the fore. The term flexibility in this context describes the ability of the power system to respond to uncertainty and variability in the supply-demand balance, in the timescale of minutes to hours, for example providing power from other sources when the wind drops.

Today, the two main flexible resources are dispatchable power plants and the transmission grid; but demand side options and new storage technologies are likely to grow in importance in the medium-term. Examples of countries considered to be in phase three of VRE deployment include Italy, the United Kingdom, Greece, Spain, Portugal and Germany; the VRE penetration in these countries ranges from 15 to 25% in annual generation.

Phase four

New challenges emerge in phase four. These are highly technical and may be less intuitive in nature than flexibility, relating instead to the stability of the power system. The stability of a power system is its resilience in the face of events that might disturb its normal operation on very short timescales (a few seconds and less). Countries that are seeing challenges primarily related to this phase include Ireland and Denmark, with an annual VRE share of around 25 to 50% in annual generation.

This manual focuses on the first two phases, in which most countries find themselves today; the flexibility aspect of phase three is discussed briefly also, because advance planning in this regard is critical.

In phase one the integration challenges are small but two aspects are important. Firstly, proper assessment is needed of the impact of those first few VRE plants on the grid at their point of connection; and secondly a set of rules appropriate to VRE plants and governing their operation (grid connection rules) needs to be in place. Integration tasks in phase two are more onerous, although international experiences prove that they are entirely manageable.

  • The grid connection code identified in phase one needs to keep pace with the level of VRE deployment. Due to their often highly technical nature, grid codes rarely receive adequate policy attention. However, the majority of security of supply concerns with VRE in recent years have resulted from a failure to anticipate them in the grid code; while they have been subsequently resolved by amendments to it.

Prominent examples include low-voltage ride through capabilities, which were first discovered to be an issue in Spain in the mid-2000s, and the “50,2 Hz” problem in Germany. Both issues have been resolved via re-programming of VRE power plants. A grid code that is updated with an eye to international experiences, with strong stakeholder buy-in and effective enforcement, is critical to security of supply.

  • The output of wind and solar power plants must be reflected in the wider planning of power system operation. The system operator – the all-important institution in the integration context – must have visibility (data) of what these power plants are doing in real-time, so it can plan the operation of dispatchable power plants accordingly.

The system operator must also be able to curtail a proportion of VRE output at critical moments; this is crucial for the operator to be able to perform its primary objective of upholding security of supply. For example, the Spanish system operator has dedicated a section of its control centre to monitor and control VRE output effectively.

  • The absence of an effective system for forecasting the output of wind and solar plants, in contrast, will not jeopardise supply security, but it will make integration very expensive, as the system operator will have to maintain disproportionately large reserves against variability.
  • It is important to establish a systematic approach to maximising the use of existing network assets, as well as planned expansion of the grid, to resolve bottlenecks. Where VRE power plants are small and dispersed over the low voltage grid (e.g. rooftop solar PV), managing the interface between the (high voltage) transmission network and (lower voltage) local networks emerges as a priority. The latter is becoming a priority in regions including Australia (South Australia), Germany (Bavaria), United States (Hawaii) and Italy (Puglia).
  • Finally, important steps should be taken to adapt VRE power plants to the needs of the wider power system (i.e. not just vice versa). International experiences show that a well-balanced portfolio of wind and solar PV power plants, for example, can have complementary electricity output profiles, which may enable the better use of existing grid assets.

Choice of location can have important benefits also: a dispersed portfolio will have a smoother overall output than if plants are geographically concentrated, and will therefore be easier to manage. Concentration of plant has led to issues in regions including Tamil Nadu in India, and South Australia.

In phase three, the first priority is to make fully available the flexibility that exists already in the power system, so that it can better accommodate the variability of VRE plants. Changing the way in which conventional power plants operate often represents the easiest flexibility gain, and their full potential can be unlocked with a combination of changes to market design – essentially, how electricity is traded – and technical upgrades.

System integration is but part of the whole range of challenges that arise in the deployment of VRE power plants. Some of these others include the design of renewable energy policy frameworks, measures to kick-start a domestic renewable energy market, and questions around the design of wholesale and retail electricity markets.

These can be found in the annexes as well as in other recent IEA publications. This manual focuses on the integration challenges as they can be expected to arise. It presents examples of where and how they have been encountered and resolved, and provides explicit recommendations as to how newcomers to VRE deployment should proceed.


This executive summary is republished here with permission.

Click here to download the full report

Contact Marc-Antoine Eyl-Mazzega, IEA,



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