German Combined Power Plant Demonstrates Real-Time Integration of Renewables

A pilot project in Germany is showing how networking technology can create a 100-percent-uptime “virtual” power plant composed of widely-dispersed renewable energy sources.


Illustration of renewable resources networked in CPP

All images courtesy of Agentur für Erneuerbare Energien

Kurt Rohrig, project manager for the Combined Power Plant, a set of solar, wind, and biogas generation facilities linked to form a virtual power plant (VPP), says the project’s initial pilot proves that “Complete supply from 100 percent renewable energies is technically possible.” (“Renewable Energies Intelligently Networked,” Undine Ziller, Monitor Magazine for Electric Safety, January 2011) A paper from the German Renewable Energies Agency maintains that the Combined Power Plant is “just as reliable and powerful as a conventional large-scale power station.” (“Background Paper: The Combined Power Plant,” Informationskampagne für Erneuerbare Energien)

Getting Renewable Sources to Play Together Nicely

One of the most frequent criticisms of solar and wind power is that their contribution to electricity supply is unpredictable: The sun only shines when it shines, and the wind only blows when it blows. To integrate renewables into the power grid, you need a back-up of spinning reserves, non-renewable power sources that can be brought online at peak consumption periods or to take up the slack when renewable sources are down.

Not anymore, claims the group of scientists in Germany working on the Combined Power Plant at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). According to the Renewable Energies Agency, the project “dispels arguments that the availability of electricity from renewable energy sources is too dependent on meteorological influences.”

CPP control station at IWES

The Combined Power Plant links 36 geographically-dispersed wind, solar, biogas, and hydropower facilities in Germany into one jointly-controlled networked virtual power plant. Rohrig, deputy director of IWES, tells Undine Ziller, writing for Monitor Magazine for Electric Safety,

Behind the combined power plant is the idea of linking the fluctuating electricity generation from wind turbines and solar installations with combined heat and power plants so that the electricity demand can be covered around the clock and in any weather.

The 36 units comprising the virtual plant meet about one ten-thousandth of German’s current electricity requirements, enough to power about 12,000 households. The project is meant to demonstrate “in miniature what is also possible on a large scale,” says the Renewable Energies Agency background paper.

During field-test periods, operators control the virtual power plant from the project’s Central Control Unit (CCU) at IWES in Kassel, Germany. There they remotely monitor the power production of the wind farms and solar arrays connected to the plant. A first pilot project, Combined Power Plant 1, conducted from 2006 to 2008, developed and tested the control and networking infrastructure for the plant. Combined Power Plant 2, a 2011-2013 pilot project under way now, is designed to test whether the virtual power plant model can contribute to grid stability.

In the case of the Combined Power Plant, the fact that the individual units are geographically dispersed is a plus: Because the renewable sources are spread around the country, their various production levels will often balance one another out. If the wind isn’t blowing at the Pilsum offshore wind farm, chances are it is blowing at Nauen or Würselen. If the sun isn’t shining on one of the solar arrays, probably the wind is blowing at one of the wind farms. If the plant’s wind and solar facilities aren’t producing enough at a given time, the operators can bring a biogas plant online within seconds.

CPP dashboard concept

Under the Combined Power Plant model, if the renewables are producing too much (which creates its own problem for the grid), a hydropower storage facility kicks in; the excess electricity drives pumps that pump water uphill into a storage pool. The elevated water serves as potential-energy storage. The water can sit up there for a few hours or even a few days. When the energy is needed, the plant releases it and it flows downhill, driving turbines to turn the pool’s stored energy back into electricity.

In the Combined Power Plant pilot projects thus far, this pump-storage capability is only simulated. Although Germany does have pumped-storage plants, they are too large to be incorporated into the Combined Power Plant at its current scale. So during the pilot stage, the plant replicates the storage process by exchanging power through European grid interconnections.

Controlling a Combined Power Plant

In its background paper, the Renewable Energies Agency describes the two primary operating principles of the virtual plant as “anticipatory control and fine tuning.” The Combined Power Plant’s control center runs computer applications that help operators predict power generation and consumption through analysis of such data as historical consumption patterns and weather forecasts. Ziller explains how the plant’s system predicts demand and estimates supply needed:

[T]he load profile, i.e., the expected electricity demand in Germany, is forecast by scientists more than a year in advance, including all the fluctuations in demand over the day and between the seasons. Scaled at 1:10,000, this forecast forms the starting point for the control of a virtual power plant comprising three wind parks, 20 photovoltaic installations and three biogas turbines. The installations are distributed all over Germany and connected to the control centre in Kassel via the Internet. Measured values are retrieved, schedules calculated and individual power plants controlled via a central server.

Display of power demand helps controllers manage the CPP's outputContinuous monitoring of weather forecasts helps operators predict solar and wind production for the following days, and to plan a preliminary schedule for bringing biogas turbines online. Ziller writes that, “If the actual electricity generated from the wind and the sun varies despite an accurate weather forecast, their power output is adjusted using measured data accurate to the minute.”

Benefits of Virtual Power Plants

The concept of a virtual power plant is not unique to this pilot project in Germany, although the Combined Power Plant is groundbreaking, in that it incorporates only renewable power sources.

Peter Asmus, senior analyst with Pike Research, writes (“Microgrids, Virtual Power Plants and Our Distributed Energy Future,” The Electricity Journal, December 2010) that the ability to create VPPs is one of the benefits of utilities’ increasing investment in smart-grid technologies:

The beauty of the VPP is that it can optimize the entire system, and deliver much greater value, without the need for large capital investments in infrastructure and corresponding long lead times for implementation. Customer-owned generation sources, utility designed DR [demand response] and CPP [critical peak pricing] — and even plug-in electric vehicles (PHEV) — all become eligible candidates to help utilities solve grid balancing challenges…

Instead of building bigger and bigger physical power plants, software and other controls enable utilities to aggregate resources on a short-term basis according to proximity, cost, environmental performance, and/or other criteria. Like the microgrid model, VPPs are inherently flexible and modular. Since IT systems and corresponding software is the “glue” holding the VPP together, resources can easily be swapped in and out, depending on the ever changing requirements to keep grids in balance or to lower customer costs or displace dirty fossil generation during peak periods of demand.

IWES developed the Combined Power Plant project with support from the German Federal Ministry for the Environment, in partnership with Enercon GmBH for its expertise in wind energy, SolarWorld AG for its expertise in solar, and Shmack Biogas. Other partners in the initiative include Cube Engineering GmbH, Siemens AG, SMA Solar Technology AG, and ÖKOBiT GmbH.

Ziller writes that renewable energy sources, including wind, solar, biomass, hydro power, and geothermal energy, now supply 17 percent of Germany’s electricity demand. The federal government intends to increase that to 35 percent by 2020. Experts at Germany’s Environment Agency believe the country can achieve close to 100 percent renewable electricity generation by 2050.

Pilot projects such as the Combined Power Plant are helping the country develop the information-technology infrastructure it needs to meet those ambitious goals and still maintain a stable electric grid.

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