Around the clock, power generation and power consumption must balance. Literally balance. But how does the power system work and which defence mechanisms prevent the power system from collapsing?
Over the past 20 years, Denmark has phased out many of the conventional power stations in favour of more CO2-friendly wind and solar power generation. However, sustainable energy fluctuates and is significantly more complicated to integrate.
Therefore, since these conventional and stable power stations are no longer available for the power balancing, we need to find new ways to balance the power generation with the actual power consumption.
One of these ways is seen in DIN Forsyning’s project Green District Heating: Two large seawater-based heat pumps are vital power balancing handles in the future grid-balancing.
Heat pumps and full bathtubs
In a recent article, we explained how the seawater-based heat pumps in Esbjerg are designed to be part of the future energy supply. We compared the power control agility and load flexibility of the heat pumps to the acceleration of a Formula 1 race car – i.e., how it must be able to brake abruptly and then accelerate within seconds.
The heat pumps also play an essential role in balancing the power grid. In short, the word balancing means that each second all year round the power consumption must match the power generation. At any given time, the power consumed by e.g. light bulbs, coffee machines, refrigerators, televisions, or heat pumps must match the power generated by power stations, solar panels, wind turbines, etc. If it does not match every single second, the power system will either experience overload or run out of power. In either case, the grid frequency will deviate from the required 50 Hertz.
Maintaining this balance is so vital that the load dispatchers at the power stations even read TV guides to know when there might be a sudden change in the power consumption: When Denmark – or another national football team - plays an international football match and the first half is played, the power consumption for e.g. coffee machines typically increases significantly during the break, and therefore the power stations seek to deliver precisely this increased power demand at the moment the referee blows the half-time whistle.
You can compare the grid-balancing to a full bathtub, which must be kept completely filled to the brim with water at all times, while a bunch of children constantly disturb the water level by opening/closing the tap and the plug in the drain. At the same time, the person responsible for the water level in the bathtub (i.e. the person responsible for the grid balance) desperately tries to keep the tub completely full by opening/closing taps and drains.
One united power grid requires power control handles
Continental Europe – from Denmark in the north to Portugal in the south – is connected through one united power grid. This means that, in principle, an imbalance between power consumption and power generation in for instance Portugal is measured or registered as a disturbance in Denmark. So, if the power generation from wind or solar in, for example, Denmark or Germany unexpectedly decreases or increases for a shorter or longer time - which it naturally does, as our wind and solar forecasts are inherently inaccurate – an immediate response from a power station and/or a power consumer is required.
An example: Let us assume that the power consumption is constant for the next 60 minutes. Suddenly clouds cover the solar panels, and the wind dies down, meaning that the solar panels and wind turbines produce less. The result is power shortage – i.e. lack of electricity/power in the grid. Previously, a power station would have responded immediately and supplied the missing power to restore the balance. This is how the system has worked for many years without major problems since all power stations were tailored and trimmed for this very purpose.
In recent years, however, a significantly part of the large central power stations have been decommissioned in Denmark and abroad, and therefore the former stability reserves no longer exist. Instead, we have introduced other smart "power control handles" to compensate for the missing power stations: Such a "handle" is the large seawater-based heat pump and its "assistant colleagues" – i.e. a mix of electric boilers, gas engines, and large pumps and/or other power consumers.
This is how the power control handles help if no power station is available
As mentioned above, power consumption and power generation must always balance. Should the power generation fall due to the weather, we now reduce or completely switch off the power consumption from a heat pump or a pump instead. This means that even though less district heating is generated, the district heating system is still functionable for a certain period because a 40,000 m3 heat storage tank with 90 °C hot water acts as a buffer for such situations. This heat storage tank is constantly charged/discharged as necessary and provides about 8 hours of district heating on a winter’s day and about an entire weekend’s consumption of district heating during the summertime.
The opposite happens if the wind turbines suddenly produce more because the wind speeds up. If too much power/electricity is thus pumped into the grid, we simply turn on a large electric boiler and increase the power consumption. The surplus heat is pumped into the heat storage tank and stored until the situation changes again.
Added Values designs the brain in the power balancing
We now know which "power control handles" are available to DIN Forsyning to help balance the power grid. Now it must merely work in practice. In Added Values, we are currently designing the power balancing controller, which is the brain behind the entire grid-balancing concept at DIN Forsyning when it comes to both power and district heating. The system is developed in Modelica, an advanced modelling language able to simulate the process dynamics of boilers, heat pumps, the heat storage tank, gas engines, pumps etc.
The power balancing controller continuously monitors the power grid, i.e. whether there is shortage or excess of electric power generation (black curve). In case of an imbalance, the power balancing controller steps in immediately and adjusts in the desired direction: This means that it either increases or decreases the load on the seawater-based heat pump (red curve), it starts up the electric boiler, or it utilizes the battery in the emergency supply system. The purpose of these interventions is to stabilize the power grid to avoid a power blackout.
For the power balancing controller to "pull" the power control handles on the heat pump, electric boiler, gas engine, etc., it needs to know the instantaneous limitations (maximum/minimum consumption, rate of change etc.) of each unit. For this purpose, the power balancing controller uses capacity diagrams of these units, comprising how much the load in the very moment can be increased or decreased without an engine becoming too hot, a storage tank being filled up, or without a valve stops working, etc. In other words, these specific components and units are continuously controlled in a certain way to compensate for inaccurate weather forecasts, for instance.
Ancillary services to the benefit of society
Needless to say: the power balancing controller is a crucial element in the stability of the power grid. Once the controller is fully developed, implemented and quality tested by us together with Picca A/S and DIN Forsyning, it must be tested and approved by Danish Transmission System Operator (TSO) Energinet too. After final approval, the power balance controller will be released for daily operation and thus contribute to ensuring the stability of the power grid. So, the power balancing controller supplies ancillary services to Energinet.
These ancillary services are essential to society. Instantaneously you can switch on/off a heat pump, an electric boiler, a gas engine, etc. to balance the power system. In other words, ancillary services are defence mechanisms against power blackouts.
Ancillary services are valuable, and DIN Forsyning can thus profit economically by supplying these ancillary services and thereby reduce the customers’ costs for district heating.
Admittedly, we need to save on hot water, but when next time you relax in a (lukewarm) tub, you might reflect on the importance of balancing the power grid and what helps determine why you sometimes save money by switching on your dishwasher at night.
Related articles by Added Values