Heat captured by solar collectors is leaded to the power block using an
organic heat transfer fluid. This fluid is a eutectic mixture of
biphenyl and diphenyl oxide, both derived from benzene molecules. It is
a eutectic mixture because its freezing point is lower that each
components separately: It is 12ºC.
Thermal fluid undergoes degradation with use, it has two important impacts:
- The economic impact is negligible and usually ill-considered in the budget of O & M
- The impact on safety, as some degradation products are potentially dangerous.
Fluid thermal degradation mechanisms are three:
- Pollution, with waste remains and water coming from water steam cycle
- Oxidation, by oil reaction with the ambient oxygen
- Cracking, It happens inside the absorber tubes and auxiliary boiler by increasing
(in the oil layer in contact with the metal absorber tube of the solar
collector) thermal fluid temperature. This increment helps cracking
reaction or the rupture of the original molecules into smaller ones.
The products of these degradation mechanisms are three:
Solids, usually carboxylic acids, coal and cinders. These products burn
easily, erode pumps and rotating equipment and are deposited on valves
seats avoiding sealing. By their acidic properties, also cause
corrosion.
Short-chain hydrocarbons come from biphenyl and diphenyl oxide
molecules break. These short-chain hydrocarbons have low boiling
points; also change the viscosity and ignition point.
Long-chain hydrocarbons coming from the union of many short-chain
residues. These hydrocarbons change the viscosity and thermal
properties.
To remove them, the installation is equipped with 3 systems:
Main filter
Waste disposal system
Reclamation system
The good condition of this system ensures a thermal fluid free of degradation products.
Concerning the amount of oil that is necessary to be replaced, it
depends greatly on the operation way. An unstable operation and not
very well carried out produces lost of degradation wastes, while a
careful operation ensures a long life. Only as estimation purposes, may
be considered:
In a plant with 2000 tons of thermal fluid is necessary to replace approximately 500 kg per day.
Every ten years will be necessary to replace all the fluid thermal.
Taking into account that the average price is around 3€/kg. It is
understood that the budget for thermal fluid replacement is quite high.
