# Dynamic Simulation of Data Center Cooling Operation and Power Outages **Summary**: Data centers need reliable cooling because nearly all the electrical power consumed by IT hardware is ultimately released as heat. Different temperature levels and cooling methods are used depending on the hardware and cooling concepts. Alongside conventional air cooling, direct liquid cooling is becoming increasingly relevant, particularly at high power densities. Free cooling offers an energy-efficient operating option. It releases waste heat to the ambient air without using a vapor-compression chiller. Whether this is possible depends on factors including the outdoor temperature as well as the required cooling water temperatures. A dynamic simulation model can be used to investigate how cooling responds to changing conditions and how a power outage affects operation. The following application example examines both scenarios using a simplified data center. ## Simulation Model Structure The model represents the data center as an electrical and a thermal load. Individual servers and airflow within the building are not modeled in detail. The focus is on the interaction between cooling equipment, controls, and the electrical supply. The cooling system consists of two hydraulic separators, a heat exchanger, a vapor-compression chiller, and dry coolers. The hydraulic separators decouple the cooling circuits, while their water volume also provides thermal storage. The heat exchanger and chiller are arranged in parallel, providing two alternative cooling paths. The dry coolers ultimately release the waste heat to the ambient air. The data center’s cooling water supply is designed for a supply temperature of 17 °C and a nominal return temperature of 22 °C. Dynamic controls adjust cooling operation to the current temperature conditions. ![Data center model diagram with named elements](/images/sites/green-city-library/pages/solutions/data-center-cooling/model_display.webp) *Figure 1: Simplified simulation model of the data center, showing the main components and the electrical, thermal energy as well as information flows.* A grid connection and a diesel generator are modeled for the electrical supply. This allows both normal grid-connected operation and island operation during a grid outage to be investigated. All components are secured either via the general supply or the emergency supply. The model was built using the [**GreenCity Simulation Library**](https://www.green-city-library.de/contact/). The library provides prebuilt components for the dynamic simulation of energy systems. Combining these components makes it possible to represent more complex system configurations with a manageable modeling effort and extend them with custom control strategies. ## Temperature-Dependent Cooling Operation During normal operation, the cooling water temperature should remain as close as possible to its setpoint despite changes in thermal load and outdoor temperature. Two operating modes are available for this purpose. At low outdoor temperatures, cooling takes place through the heat exchanger. Waste heat passes from the data center’s cooling circuit to the heat rejection circuit and is released to the ambient air through the dry coolers. A vapor-compression chiller is not required in this mode. As the outdoor temperature rises, the temperature difference between the two cooling circuits decreases. Once this difference is no longer sufficient for free cooling, the vapor-compression chiller takes over. It removes heat from the first hydraulic separator and raises the temperature level on the heat rejection side. The heat is transferred to the second hydraulic separator so that it can continue to be dissipated through the dry coolers. ![Cooling water supply temperature plot](/images/sites/green-city-library/pages/solutions/data-center-cooling/cooling.webp) *Figure 2: Cooling water supply temperature at the first hydraulic separator over time. Green areas indicate free cooling; red areas indicate operation of the vapor-compression chiller.* The simulation shows that the supply temperature does not remain constant at 17 °C, but fluctuates. These fluctuations result from the data center’s varying thermal load and the control hysteresis. When switching between operating modes, the supply temperature stays within the temperature range between approximately 16 °C and 19 °C, do to the start-up and shut-down times of the components. The model allows these control processes to be examined over time. Changes to switching conditions or control parameters can be evaluated directly for their effects on the cooling water temperature. ## Grid Outage and Island Operation In addition to normal cooling operation, the simulation model also represents a grid outage. This can be triggered at any point during the simulation. During a grid outage, the data center’s general electrical load is immediately reduced to 10% of its normal operating level. The electrical power consumption of the IT hardware, pumps and vapor-compression chillers that are connected to the emergency supply remain working, while the components of the general supply connection are dropped. The cooling concept remains in place during island operation, but operates at a correspondingly reduced capacity. A diesel generator takes over the electrical supply. ![Data center heat demand plot during a grid outage](/images/sites/green-city-library/pages/solutions/data-center-cooling/heat_demand.webp) *Figure 3: Heat demand of the data center over time. The dashed line marks the grid outage and the immediate drop in load.* The abrupt drop in power shows the transition from normal operation to the emergency supply. The reduction to about 10% load is a simplified assumption for this simulation example and is not a universal design requirement for data centers. The grid outage serves as a deliberate intervention in the simulation model. This allows exceptional operating conditions to be examined alongside normal system operation, and the interactions between the electrical supply and cooling equipment to be analyzed. ## Applications of Dynamic Simulation This application example shows how different operating conditions of a data center can be represented within a single simulation model. During normal operation, the focus is on the interaction between thermal load, outdoor temperature, and cooling controls. During a grid outage, the electrical and thermal loads change abruptly. Both scenarios can be investigated using the same model structure. Dynamic simulation therefore provides a basis for developing control strategies, comparing system concepts, and evaluating the behavior of technical systems under changing conditions as well as securing opperation redudency and system extentions. The [**GreenCity Simulation Library**](https://www.green-city-library.de/contact/) allows the required components to be connected into a complete system. This simplifies the creation of custom models and allows different operating strategies and scenarios to be tested without intervening in a real installation.