Elsevier

Energy

Volume 33, Issue 9, September 2008, Pages 1453-1460
Energy

Geographic aggregation and wind power output variance in Denmark

https://doi.org/10.1016/j.energy.2008.04.016Get rights and content

Abstract

At modest penetration, wind power merely substitutes electricity generated typically at thermal power plants. In this case, wind power only provides economic benefits in terms of saved marginal fuel and operation and maintenance costs. At higher penetrations, it becomes increasingly important for the energy system to be able to operate without costly reserve capacity awaiting fluctuations in demand or wind power generation. Existing transmission interconnections have mainly been established in order to assist in reducing the reserve capacity of thermal power systems. While indeed relevant in thermal systems, this is typically even more important in renewable energy-based systems, in which fluctuations to a large extent are uncontrollable. This makes interconnected systems an interesting option for integrating electricity produced from such energy sources. Using a Danish example, this article demonstrates how different demand and wind production variations in different geographical areas assist in evening out fluctuations and reducing imbalances in systems with high penetrations of wind power. By exploiting these variations, the needs for reserve capacity and condensing mode power generation are reduced. However, the article also demonstrates that there are limits to what can be gained on this account.

Introduction

The transition from fossil fuel-based power generation to power generation based on fluctuating renewable energy sources, such as wind, sun, and wave power, poses a challenge to the operation of electricity systems. The cogeneration of power and heat, power and cooling or of power and desalinated water impose problems on the systems's load-following capabilities. Electricity consumption development adds another dimension to the issue. Traditional electric engines decrease their power uptake if generators are overloaded, thus causing the frequency to drop and thereby relieving the generators of some load. With many electric engines operated by use of frequency converters, loads are not relieved but rather kept at a constant level.

Different options exist for the integration of fluctuating renewable energy:

  • electricity storage which enables the storage of electricity from periods with surplus production to periods of deficits,

  • improved control of the rest of the system, and

  • interconnection with other systems.

Electricity storage is a widely considered solution which immediately suggests itself. However, apart from pumped hydro, efficiencies have hitherto proven too low for economic feasibility, as demonstrated by, e.g., Salgi et al. in Refs. [1], [2]. Thus, although constituting a valid option, it may not be the most feasible one. Some do find that storages are economically interesting—e.g., Kaldellis and Zafirakis [3]—but this might be attributed to specific local conditions, as for instance, high electricity production costs of the system in question.

Many analyses have treated flexible energy systems, where, e.g., combined heat and power plants (CHP plants) producing district heating are regulated more actively through the use of heat storages and are thereby able to move power production independently from heat demands, as exemplified in Refs. [4], [5], [6], [7], [8], [9]. Relocation, as Blarke and Lund designate this in Ref. [10], naturally has the advantage of not being impacted by the low cycle efficiencies of most storage systems.

The integration of fluctuating renewable energy by means of interconnection relies on either the magnitude of the system into which the renewable energy source is integrated or the natural variability of production and demand in the larger area of which the system forms part. With the droop of large systems being measured in several GW per Hz, even a large wind farm may be integrated into such systems assuming adequate grid capacity. Rather than relying on the sheer magnitude of the spinning capacity in the system, natural variations in wind over larger areas may also be utilised in the integration of weather-dependent renewable energy sources. This is the focus of this article.

The world has many transnational grid interconnections—but also a number of systems disconnected from others or only connected via direct current (DC) lines. These systems are not synchronised and do not have the immediate frequency-controlled load balancing, which interconnected alternating current (AC) systems have. Scandinavia has one system (Nordel); after having been split up into two for a number of years following the Balkan Wars, most of continental Europe now has one system (the union for the co-ordination of transmission of electricity—UCTE); and North America has several. There are tendencies in the direction of larger interconnections, as exemplified by the member states of the Cooperation Council for the Arab States of the Gulf which will eventually connect to the Mediterranean Middle East and Europe through Turkey and through the Arab-Maghreb line to North Africa and Spain. Though such distances are beyond what is readily technically feasible in terms of power exchange, they do emphasise the interconnection trend. While this interconnection scheme primarily has the objective to reduce reserve capacity requirements, as discussed by, e.g., Bowen et al. [11], it may also have a positive effect on the possibility of exploiting renewable energy sources.

Apart from most notably electricity production based on solid renewable fuels and hydro power, most other renewable energy sources are intermittent by nature and they inherently require either reserve capacity or other means to deal with fluctuations. In general, the smaller the system, the fewer the plants; the smaller the variation in energy sources and the geographic extension of the area in question, the larger the need for reserve capacity or a flexible system to accommodate fluctuations. This is clear from Fig. 1, which shows the wind production in the first week of 2006 of the entire Western Danish system as well as of a single well-located near-shore 2.3 MW wind turbine. The variations of the large system are much smaller than those of the single wind turbine. The standard deviation of the production of the single wind turbine underlines this at 32% of the maximum output compared to only 22% of the maximum output for the standard deviation of the entire Western Danish system for the year 2006.

Interconnection schemes are therefore seen as measures required for the integration of fluctuating renewable energy sources, see Ref. [12]. They are also very relevant in the Danish context where wind power, in the years 2004–2006, has corresponded to between 16.8% and 18.5% of the electricity end use [13].

Dependence on international connections does pose a potential threat to the security of supply for technical as well as political reasons. Merely using neighbouring countries as energy suppliers or as providers of reserve capacity may thus render a country vulnerable. A current example of potential political threats is the Russian supply of natural gas to Europe (see Ref. [14]). In the case of electricity, the issue is even graver, since this system, in contrast to the natural gas system, has no inherent buffer or storage. However, there is also a long-standing tradition for electrical interconnections, e.g., as early as in 1951, Eastern Denmark was connected to Sweden by submarine high voltage AC (HVAC) cables [15]. Likewise, at a broader European level, the UCTE was established more than 50 years ago to coordinate international electricity transmission in Europe [16]. There is, thus, a long-standing tradition for international cooperation within electricity supply and mutual reserve capacity building.

In line with the European Union's adoption of a stringent Kyoto-derived carbon dioxide emission reduction target, Denmark has pursued an ambitious energy policy. This has resulted in a complex energy system with many sources of energy and many interdependencies between sources, demands and conversion systems. In addition, however, Western Denmark has a connection of 1200 MW AC capacity to Germany, a 1100-MW high voltage DC (HVDC) connection to Norway and a 600-MW HVDC connection to Sweden. Eastern Denmark has a connection to Sweden of a total capacity of 1900 and 600 MW connection to Germany. Though not mutually connected (see Fig. 2), each of the two non-synchronised areas of Denmark thus has strong ties abroad, which contribute to power balancing and the reduction of reserve capacity needs. The international connections of Denmark are summarised in Table 1.

In addition to the issue of mere generating capacity, another function of the system is to provide ancillary services (basically grid stability). This issue is given increasing attention by utilities and research communities addressing the integration of fluctuating electricity sources. This aspect is increasingly important, as ancillary services have traditionally been supplied by large power plants. With stronger reliance on distributed generation technologies, the systems must be able to maintain their power of resistance against grid disturbances without depending on these ancillary service providers of the past.

Section snippets

Scope of the article

The scope of this article is to assess the amount of condensing mode power generation and reserve capacity required in the Western Danish electricity system. The analyses are made under different assumptions regarding the hourly variation of wind power and electricity demand for a 1-year modelling period. These so-called variation profiles differ as a consequence of areas being interconnected or not. The analyses also assume different degrees of wind power development and installed wind power

Time variations of demands and productions

Both production and consumption vary according to a diurnal cycle, a weekly cycle and a seasonal cycle. The diurnal cycle of demand is due to the timing of meal preparation, industrial activity, need for illumination, etc. The weekly demand cycle is influenced by the reduced demands of weekend-closed companies, institutions and organisations, while the seasonal demand cycle reflects changing needs for illumination, heating and cooling at high latitudes.

The production system has to follow demand

Energy system scenario

The analyses in this article take their point of departure in an energy system scenario for the year 2020 used in analyses by the Danish Energy Authority [17], [18]. Demands are thus the expected result of continuing present trends and policies. However, the amount of on-shore and off-shore wind capacities corresponds to the present level, despite the fact that future increases are expected especially in terms of off-shore wind. However, going beyond the current level of approximately 20% wind

Conclusions

The results of this article demonstrate that increasing the geographical extension of the area in which fluctuating wind power is being exploited reduces the average need for reserve capacity in the form of power plants operating in condensing mode. While the analyses have focused on one single source of renewable energy, i.e., wind power, they indicate that analyses of energy systems encompassing more unrelated energy sources or areas with larger geographic distributions would lead to a

Acknowledgement

This article is a substantially revised and extended version of a paper presented at the PowerGEN Middle East, Abu Dhabi 2006.

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