To calculate the changes in state or temperature during a simulation interval, a balance of thermal energy is formed. Mathematically, this involves the numerical solution of a system of differential equations.

Temperature Change = Sum of all supplied and dissipated energies / Sum of all heat capacities

Balancing implies that the sum of all supplied energies, dissipated energies, and the storage of energy through the heat capacity of system components must be equal to zero. This balancing is not done collectively for the entire system but for each individual system component:

• Collector
• Collector loop
• Heat exchanger
• Storage tank

For each of these components, the temperature change is calculated using the above formula based on the supplied and dissipated energies and the heat capacities of the respective component.

Supplied energies can include (depending on the component):

• Irradiation
• Heat supply to the heat exchanger
• Heat transport by mass flow due to consumption or circulation
• Mixing of layers in a storage tank

Dissipated energies can include:

• Heat losses by radiation from the collector (quadratic passage coefficient)
• Heat losses at the insulation of the collector, the piping (collector loop or circulation), the fittings, or the storage tanks
• Heat transfer at the heat exchanger
• Heat transport by mass flow due to consumption or circulation
• Mixing of layers in a storage tank

The heat capacities of the following components are considered:

• Collector
• Piping of the collector loop
• Storage tank contents