Heat Transport

Efficient heat transport, or heat transfer, is a focus of numerous projects across a wide range of our research activities at Fraunhofer ISE. The applications are analyzed both experimentally and simulatively.

Heat transport processes are simulated differently depending on the type of transfer. The calculations are based in each case on spatially resolved partial differential equations. Pure heat conduction, e.g. in solids, is solved by a heat conduction equation (in simplified form Fourier's law); free or forced convection is calculated coupled with a flow simulation; heat radiation is calculated on the basis of surface properties and temperatures (e.g. Stefan-Boltzmann law). The investigated media exist in all classical aggregate states (solid, liquid, gaseous) and also undergo phase changes during the simulations due to the process.

The software Comsol Multiphysics®, OpenFOAM and Ansys Fluent / Ansys CFX are available for Fraunhofer ISE for the simulations.

© Fraunhofer ISE

R&D Services for Heat Transport

In our business areas we offer the following services in the field of heat transport:

Application Examples

Coaxial Heat Exchanger for Decentralized Ventilation

A coaxial heat exchanger is a tube-in-tube heat exchanger. For use in a ventilation unit, the concept of the heat exchanger was rethought and the surface area between the two air streams (exhaust and supply air) was significantly increased by geometric modification in a center section of the tubes (see figure). In winter, the supply air flowing in from outside is heated on this large surface by the warm exhaust air and fed into the room. The essential task in the heat exchanger development was to design the geometry in such a way that an energy-efficient, compact and low-noise decentralized ventilation unit is possible. However, the geometric variations are innumerable (number of channels, length of transition areas, wall thicknesses, ...), so an iterative process of simulation of flow and heat transfer using FEM together with experimental work on the heat exchanger was performed. For this, it was necessary to vary the geometry under boundary conditions that allowed additive manufacturing of the component and easy installation of the device in the wall of the house.

Further information on this topic

© Fraunhofer ISE
Coaxial heat exchanger for decentralized air conditioning of buildings. Left: Visualization of the geometry and a flow section. Right: Additively manufactured sample for measurement and validation of the simulation.

Brief Description:

  • Goal
    • Development of an additively manufactured heat exchanger geometry that enables thermal and hydraulic performance increases
    • Increase of heat recovery efficiency in decentralized ventilation units
    • Reduction of noise emissions through quiet, slow and continuously operating fans
  • Procedure
    • Conversion of the basic geometry in a specific file format for additive manufacturing and CFD simulation with geometrically free parameters
    • Simulation of the bypass condition
    • Import of the file into the CFD software Comsol Multiphysics®
    • Determination of air flows (exhaust and supply air), pressure losses and heat flows in the ventilation unit using CFD
    • Determination of key figures for the evaluation of the geometry
    • Modification of the geometry in step 1 (based on simulations and measurements) and repeating the process until the desired performance improvement is achieved
  • Result
    • Geometry and pattern of a coaxial air handling unit with high coefficients of performance

Optimization of the Thermal Resilience of a Junction Box

Bypass diodes are located in the junction box of a PV module. When a bypass condition occurs, the high currents in the bypass diodes can cause severe heat generation. If the heat is not dissipated well, this leads to overheating of the junction boxes and thus to failure of the module.

FEM Simulation of the thermal resistance of a junction box
© Fraunhofer ISE
FEM Simulation of the thermal resistance of a junction box.

Brief Description:

  • Goal
    • Optimized heat dissipation from the bypass diodes in a PV module junction box
  • Procedure
    • 3D model of the junction box based on a CAD model
    • Simulation of the bypass condition
    • Validation by temperature measurement with an IR camera
    • Simulation of different fillers
  • Results
    • Reduction of junction box temperature
    • Identification of suitable filler materials

Further Information on this Research Topic:

R&D Infrastructure

Center for Heating and Cooling Technologies

Research Project


Development and Optimization of Filler Materials for Thermal Storage

Research Project

SubSie Platform

Sorption Steamers for Vessel Temperatures Less 0°C

Research Project


Development and Evaluation of an Innovative Safety Feature for Heat Pumps with Natural Refrigerants Based on Sorption Elements and Novel Sensor Technology