Solar thermal energy

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Research

Solar cookers, in various shapes and sizes, have been available for a long time. The SK14 is an example of a dish concentrator where the cooking pot is positioned in the focus point and the solar tracking is made manually.

Our research direction has been on including a heat storage, such that heat can be available at times without direct sunshine. The concepts are illustrated schematically below.

Established concept for direct solar cookers



Project concept 1: heat transfer to a storage. This system can be upscaled


                       

Project concept 2: direct illumination of a storage (Scheffler reflector) and direct charging of moduar storage units (portable heat batteries)


Technical challenges

The technical challenges with solar thermal systems are illustrated in the figure below

Technical challenges with a solar systems

Different concepts have been considered for heat collection and storage (see some slides for some pictures and the NUFU report for more details)

Concepts

  • Air based dish system with a rock bed heat storage
  • Oil based trough system with oil or solar salt for heat storage. Pumped or self-circulating
  • Self-circulating water-steam system with cooking plate and solar salt heat storage
  • Pumped oil system for injera baking applications , including a new ceramic injera pan (Addis Abeba)
  • A double reflector system for direct heating of a latent heat storage (heat battery)
  • Heat storage

    The heat storage is the key element in our work. Two methods have typically been tested:
  • Heat storage in the form of sensible heat and
  • Heat storage in the form of latent heat (Phase Change Material, PCM)
  • There are pros and cons woith both. When heat is demanded at a given temperature (e.g. cooking or frying), then a PCM system can be beneficial. Constant temperature heat can also to some degree be achieved with thermal stratificaton in a sensible heat storage.

    Computational work

    The work has typically included experiments (outside in the sun and inside in the laboratory) and numerical modeling, including programming of a Ray Tracer. The modeling has involved:

  • 1D dynamic system models, based on integration of mass, momentum and energy concervation equations - for both compressible and incompressible flow. Matlab implementations.
  • Storage evaluations using heat conduction modeling in COMSOL Multiphysics
  • Programming of a ray tracer, first in Matlab, then in C++
  • Socio economic aspects

    We have had an interdisciplinary research team, with participants also from the social sciences (at NTNU, UKZN Durban and UEM Maputo). See a presentation at a final seminar of a NORGLOBAL project: Slides