Advantages disadvantages of evacuated tube collectors

Advantages & Disadvantages of Evacuated Tube Collectors:
Advantages ;
 Achieves a high efficiency with large temperatures differences between absorber and
surroundings
 Supports space heating
 Achieves higher temperatures

Low weight system
Disadvantages;
 More expensive system
 More susceptible to breaking in domestic use
 Cannot be mounted horizontally, must have a slope of 25°C
6. Heat Transfer Medium
Heat transfer mediums are used to transfer the heat generated in the collector plates to the
water storage. There are a number of properties necessary for a fluid to be a good heat
transfer medium, such as the fluid having a high thermal capacity. Thermal capacity can be
defined as the amount of heat energy required to raise the temperature of a fluid by one
degree (DGS, DLR, Ecofys, 2009). Components of solar thermal systems).
The fluid must also have a high thermal conductivity. Thermal conductivity is the ability to
pass a unit of heat through a unit area of a material (DGS, DLR, Ecofys, 2009).

Finally the fluid must have a low viscosity. This means the fluid used as the heat transfer
medium must provide little resistance to gradual deformation by shear stress or tensile stress
(DGS, DLR, Ecofys, 2009).
6.1 Types of Heat Transfer Medium
There are many common heat transfer mediums used in solar thermal systems today such as
water, air, Hydrocarbon oils, refrigerants and a water/ glycol mixture. The most common heat
transfer medium in use is the water/ glycol mixture. The mixture has a high thermal capacity,
a high thermal conductivity and a low viscosity rating. The mixture is also non-combustible
so the pipe system can be pressurised. Water is non-toxic to the environment meaning the
environmental impact of a leak is less severe, it causes less damage to our O-Zone layer and
is easy to dispose of after use, as it is biologically degradable. Water is cheap and widely
available making it the more economical choice. Glycol is added to the mixture for two main
reasons. Firstly, it reduces the freezing point and secondly, it increases the boiling point of
water. Adding 40% propylene glycol provides frost protection by reducing the freezing point
of water from 0° C to -23°C. The main operating temperatures of collectors can be as low as
-15°C in the winter, meaning it is necessary for the heat transfer medium not to freeze at
these temperatures. If water was used and it operated at these temperatures the water would
freeze, causing damage to the entire solar thermal system and the house and may cause the
pipes to burst causing severe damage to the house itself. Adding the glycol also increases the
boiling temperature from 100°C to 150°C depending on the pressure in the system. This
means the water can reach much higher temperatures before vaporising thus carrying more
heat energy (DGS, DLR, Ecofys, 2009).