Heat transfer options

The table below gives an overview of the different heat transfer options we have tested

	Thermal conduction Energy transfer through conducting fins can be an option for the cases when a cooker or a fryer is in the form of a hot plate. The hot plate can receive energy from the source directly or indirectly and transfer the energy to and from the storage through conducting fins: A hot plate can be solar illuminated directly (see SK14 , direct fryer and double reflector) Heating elements can be attached to the bottom surface of the hot plate (see beans cooker and injera fryer ) Heat can be supplied to the top surface having channels with a circulating heat transfer fluid (see injera fryer steam )
	Forced convection with a heat transfer fluid A pump or a fan can be used for circulation of the heat transfer fluid. We try to avoid pumps and fans, as it reduces the robustness of the system and requires power (battery for off-sun periods). However, we have experience with some cases: A pump was used in tests with a piston system to separate hot and cold fluid A fan was used in air-rock bed tests A manual pump was used in a three tank system , with gravity driven flows and manual pumping when needed
	Natural circulation with a heat transfer fluid Hot oil has less density than cold and the density difference can induce natural circulation between the heating part of the system and the heat extraction part (the cooker). Natural circulation of oil heated in a trough concentrator Natural circulation in an air based rock bed hot air for drying Natural circulation in oil based cooker
	Liquid-vapor thermosyphon A heat transfer fluid can evaporate at the hot side and condense at the cold side (the cooker). The condensate drains by gravity back to the hot side. This gives efficient heat transfer as the temperatures on the hot and the cold side in the ideal case are equal. Heat transfer from the focal point of a dish concentrator to a PCM based heat storage (Injera fryer steam
	Heat pipes A thermosyphon type of heat transfer can be obtained using a single pipe with counter-current upwards vapor flow and downwards gravity driven condensate flow. The term "heat pipe" is, however, mostly used for the case where the liquid transport is in a wick, in which case it can also work for horizontal pipes. Heat transfer from a heat storage to a frying pan (Oil/PCM frying) A test with pulsating heat pipe (PHP) for heat transfer from the focal point can indicate that it is difficult to scale up PHP from micro systems to larger diameter heat transfer pipes..
	Light guides Internal reflecting tubes can guide incoming rays to the tube outlet. The inlet can be positioned in the focal point of a dish concentrator and an absorber can be positioned at the outlet. A test with a square shaped Light Guide, was made without much success. Ray tracing also shows about half of the rays returning to the inlet of the guide with this particular geometry. Pipe guides perform better and can be feasible for short pipes (a few meters) A Ray Tracer was extended for torus geometries, and has been useful for the assessment of light guides for direct illumination of a heat storage