demand and securing frequency and voltage on the grid are managed only by such large production units.
However, the implementation of cleaner technologies, such as renewable energy, CHP, and energy conservation, is necessary to secure future renewable energy systems. Consequently, such distributed production units sooner or later need to contribute to the task of securing a balance between electricity production and consumer demands. The article presents technical designs of potential future flexible energy systems, which will be able both to balance production and demand and to fulfill voltage and frequency stability requirements to the grid. Again, the analysis is based on the reference scenario for West Denmark by the year 2020.
Electricity Balance and Grid Stability
Non-Active Components Active Components
Non-Active Components Active Components
Figure 5.19 shows the starting point of the analysis. The task of securing a balance between electricity production and consumer demand has so far only been managed by large power stations. However, small-scale CHP units have the technical potential for solving some of the balancing problems. When the analysis was made, small and medium-sized CHP stations did not participate directly in balancing wind power in Denmark. These stations, however, did contribute to the balancing of fluctuations in the demand. CHP stations have been paid through a triple-tariff system, with high payment between morning and late afternoon, reflecting a high electricity demand during this period, and low payment during night hours, weekends, and holidays.
Consequently, the Danish CHP units have been designed with relatively high production and heat storage capacities, making it possible to produce mainly during the high-tariff period. When electricity sales prices are high, the CHP unit operates at full capacity and stores excess heat in the heat storage. When prices are low, the CHP unit stops, and heat for district heating is supplied from the storage. Until 2004, such regulation ability was not used to integrate fluctuations in renewable energy.
Small CHP units can be used to balance the fluctuating output of wind power. The heat storage facilities of the CHP station are important features of this technique. Provided that the excess heat production can be stored for future use, the CHP station is able to increase electricity production when and if required for balancing activities, without economic penalty.
In the current Danish system, both tasks are solved primarily by large power stations. In some countries, large hydropower stations participate in fulfilling the task as well. When the share of distributed generation is small, balance requirements will not be compromised. However, when the share increases, the balance may be at stake. Consequently, the system design has defined limits to the integration of CHP and RES. In Figure 5.19, small and medium-sized CHP stations are illustrated as distributed generation components operating in accordance with a fixed triple tariff.
To identify limits and possible solutions to increasing the shares of RES and CHP, three potential future systems have been analyzed and compared with the existing system. Figure 5.20 compares a reference (system 0) with three potential future alternatives:
System 0 (reference) is characterized by the following:
• Distributed generation units are operating according to fluctuations in heat demand during the season in question and to a fixed triple tariff during the period of one week.
• RES (i.e., wind turbines) are operated according to fluctuations in the wind.
• Centralized power stations, including large CHP stations, are operated to secure a balance between electricity demands and production while still meeting seasonal fluctuations in district heating demand.
• The task of securing frequency and voltage stability is left solely to the centralized units, under the restriction that the production of these units must always correspond to at least 30 percent of the total electricity production; the production must always be at least 350 MW (in order to have the necessary units operating).
This system represents the present (prior to 2004) system in Denmark.
System 1 (activating small and medium-sized CHP stations): The system is the same as the reference apart from the fact that all CHP stations are operated to balance both heat and electricity production. If the electricity production exceeds the demand, parts of the CHP units are replaced by boilers. The heat storage capacities are used to minimize such replacements. The system has been analyzed both in a situation in which small and medium-sized CHP stations do not participate in the grid-stabilizing task (System 1A) and in a situation in which they do participate (System 1B). System 2 (adding heat pumps): In System 1, CHP production is replaced by heat production from boilers in periods of excess electricity production, and consequently, the fuel efficiency is decreased. The idea of System 2 is to compensate for the increase in fuel consumption by adding heat pumps to the system. Furthermore, heat pumps increase the flexibility of the system because they can consume electricity at hours of excess production and, at the same time, replace the heat production of CHP units. Again, the system has been analyzed both in a situation in which the small
System 0: Existing
System 2: CHP Units and Heat Pumps
System 1 : Activating DG CHP-units
System 3: Activating RES via Additional Demand
RES (Renewable Energy Sources)
Centralized CHP and Power Plants
DG (Distributed Generation)
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