Balancing energy from wind and photovoltaic farms
In order to ensure the security of the electricity supply, the transmission system operators (TSOs) rely on so-called ancillary services. One of these ancillary services is the balancing power. The plants that provide balancing power equalise the imbalance between the generation and consumption so that the grid frequency remains stable. The balancing power is available in three different qualities that differ mainly in terms of their activation rate. The primary balancing power must be fully available within 30 seconds, the secondary balancing power in five minutes and the minute reserve in 15 minutes.
The expansion of renewable energies, especially intermittent generators, has created two challenges with respect to the balancing power: firstly, the demand for balancing power is increasing owing to the rise in forecasting errors caused by the expansion of wind power and photovoltaics; secondly, there is a decrease in the number of conventional power plants that have until now provided most of the balancing power. It is therefore important that wind and solar farms also provide balancing power so that these challenges can be met. Wind and solar farms in Germany currently do not provide any balancing power. In addition, the provision of balancing power by wind and solar farms will help to avoid the expensive ramping up and down of power plants and the construction of storage capacities. Furthermore, this could also increase competition in the balancing market, which in turn should reduce costs.
In the "Control energy through wind turbines" and "Combined Power Plant 2" projects funded by the German Federal Ministry for the Environment, work has already been conducted on achieving balancing power by wind and solar farms. To this end, tools and methods have been developed, such as the new method for verifying possible infeeds, which has also been demonstrated in a field test. However, the methods and tools developed in these projects have two drawbacks: on the one hand they did not meet all the requirements and on the other they were based largely on the existing regulations, mainly because it was intended to demonstrate the feasibility. It is therefore important to develop tools and methods that allow optimal use of wind and solar farms in the balancing power market in terms of the overall system.
Precise knowledge of possible infeeds
In order that photovoltaic systems can feed balancing power into the grid, precise knowledge of the possible infeed is required. The possible infeed corresponds to the power that a PV solar farm would have fed into the grid if it had not been throttled. Until now there has been no method for meeting the balancing power requirements in terms of the preciseness and the temporal resolution. If the forecasts were accordingly precise, photovoltaics could also participate in the balancing power market. With an installed capacity of around 30 GW, PV has – at least theoretically – sufficient balancing power reserves when there are good weather conditions. The information on the possible infeed is required by the transmission system operators in order to verify that the providers could provide negative balancing power or meet a demand to provide positive balancing power. In addition, this also makes it possible to determine whether balancing power was actually provided.
Risk-based bidding strategies in the balancing power market
To enable wind and solar farms to provide balancing power just as economically and reliably as conventional power plants requires risk-based bidding strategies. The rules for procuring balancing power stipulate that the balancing power offered must be 100% reliable. Since the operation of technical systems is always fraught with uncertainty, which is also borne out by the experiences of the TSOs regarding the reliability of the existing providers, this means that the providers need risk-based bidding strategies. The bidding strategies used by existing providers have been rules of thumb. These are aimed at safeguarding the largest plant in the pool or, for example, 10% - 30% of the offered balancing power, whereby they achieve close to 100% reliability. These rules of thumb have, however, two drawbacks. On the one hand there is the danger that too little or too much is safeguarded, since no plant-specific default probabilities are taken into account. This negatively affects either the security or the economic viability. Another disadvantage is that these rules of thumb do not apply to wind and solar farms, since there is a certain amount of forecast inaccuracy for the farms in terms of the power available for the periods offered. Therefore not just potential suppliers of balancing power using wind and photovoltaic farms but also existing providers are considerably interested in risk-based bidding strategies that, based on probabilistic forecasting, provide an ideal combination of efficiency and security.
Solutions for the information and communication technology for virtual power plants
There are currently no commercial ICT solutions for providing balancing power using virtual power plants with intermittent generators. However, only these solutions will make it at all possible to tap the potential offered by the farms providing more than 60 GW of wind and solar energy. In developing the information and communications technology (ICT), the primary focus is on the speed, security and stability of the data transmission. In addition, the engineers want to develop generic interfaces for communicating with the transmission system operators and for probabilistic forecasts for the purpose of preparing bids that take into account the data security.
08/2014 – 07/2016
Fraunhofer-Institut für Windenergie und Energiesystemtechnik (IWES)
34119 Kassel, Germany