Figure 1: Ancillary services used by Energinet. Source: Ramboll, but inspired by Energinet
Heat pumps have been stabilizing the power system for years by planning production according to the day-ahead prices. As an example, if a storm from the West is expected in Denmark, the hourly day-ahead prices are expected to be low due to the abundant amount of wind in the power system. The consequence is that all DH companies plan to operate heat pumps at the same time and thereby absorb the expected excess power production. The heat production can be supplied to thermal storage systems, which means that at increasing prices for electricity (meaning limited RE in the system), the DH companies supply heat from earlier stored heat and avoid production from boilers or other technologies. Seen from the power system, the production to thermal storage has the same effect as if it were a battery. Therefore, it is often referred to as a “virtual battery”. In the intraday market planning, imbalances will occur for different reasons, such as unexpected changes in output from RE production, in particular large solar PV plants. To balance the energy system, the TSO procures different kinds of ancillary services, each with a specific purpose in the balancing of the grid. The main difference between these services is the time scale for the required service, as seen in Figure 1. The TSO always prioritizes technologies for balancing with the lowest cost. Examples of available technologies for the TSO are:
Electric boilers, incl. thermal storage for up- and downregulation (upregulation requires that the electric boilers are planned in operation) Increased renewable integration drives the need for technologies that can balance the system. To reduce the need for balancing services, flexibility in electricity use is important. Heat pumps, electric boilers, and the production of green hydrogen are highly flexible technologies since thermal storage can be included, which allows for time- independent electricity consumption and heat supply. Presently, individual heat pumps have limited flexibility since they need to be in operation regardless of the electricity prices and can consequently put high pressure on the electricity system, requiring that in-efficient fossil-based power plants be operated. Therefore, in general, individual heat pumps should be avoided unless they show a clear socio-economic benefit compared to DH. The figure above shows the different tools for the TSO to maintain the balance in the power grid, in the very short-term: Fast Frequency Reserve (FRR), to the longer-term: Manual Frequency Restoration Reserve (FRR). Services can be either manual (m) or automatic (a) In the two extremes in the figure, some services are required for the ultra-short period (FRR as indicated in the above figure), whereas others are used for the longer period (mFRR as indicated in the figure). While electrical batteries are very suited for very short-term regulation, they are expensive to install, have conversion losses, require space, and have a limited energy content. Electric boilers connected to DH systems with thermal storage cannot
CHP plants, incl. thermal storage for up- and downregulation
Electrical batteries for up- and downregulation
Heat pumps, incl. thermal storage for up- and downregulation. Upregulation requires that the heat pumps are planned in operation.
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