| The
STL is composed of a tank(s) filled with nodules (balls) and heat transfer
fluid. Approximately 60% of the volume of the tank is occupied by the nodules
and the remaining 40% is fluid. The heat transfer fluid is usually mono-ethylene
glycol. Other brines such as mono-propylene glycol or calcium chloride may also
be used. The number of nodules in a system determines both the heat exchange rate
between the nodules and the heat transfer fluid and also the total energy stored
in the STL. |
The
Nodules |
The
spherical nodules (balls) are blow moulded from a proprietary blend of
polyolefins and filled with PCM (Phase Change
Material). A range of PCMs allow thermal energy to be stored at temperatures between
-33°C and +27°C (see table).
The
nodules are manufactured in three diameter sizes : 77 mm, 78 mm and 98 mm for
use respectively at low phase change temperatures (-18 to –33°C), intermediate
phase change temperatures (-3 to –15°C) and conventional building air conditioning
ice storage temperature (0°C). A special heat storage nodule with phase change
capability at +27°C for use as a heat recovery media is also available.
The
mechanical and chemical characteristics of the nodule shell are fully adapted
to the nodule life conditions. The nodule lifetime is higher than “the equivalent
of 20 years of normal utilization”. Indeed, the nodules withstand 10,000 cycles
without any breakage. In addition, the polyolefine materials used are completely
neutral towards phase change materials (eutectic salts) and heat transfer fluids.
Once filled with a phase change material the nodule plug is sealed by an ultrasonic
welding machine to ensure a perfect watertight seal. |
Characteristics
of the nodule |
Blend
of polyolefins
Chemically neutral towards eutectics and heat transfer fluid
Thickness
10/10 : no migration of the heat transfer fluid
Sphere obtained by blowing
: no leakage
Welding of the cap by a ultrasonic welding machine
Air pocket
for PCM expansion : very little stress on the shell | |
|
The
Cristopia Nodule |
The
nodule is the result of a long product development work. Several tests were used
to determine the nodule characteristics (diameter, shell thickness, shell thickness
regularity, shell materials, etc...) : Tensile
test
Accelerated life test
Measurement of failure pressure
Determination
of viscoelastic behavior (creep test and relaxation test)
Chemical aging in
commercial glycols and PCMs
Measurement of internal pressure during freezing
Polarized
optical microscopy
Impact test The
result is an outstanding lifetime, higher than 10000 cycles (more than 27 years). |
|
...In
a made to measure tank |
A
purpose manufactured tank is filled with nodules around which heat transfer fluid
is circulating to ensure heat exchange.
These tanks may be cylindrical, with
design service pressures up to 10 bars or rectangular for atmospheric systems.
Cylindrical tanks can be designed horizontally, vertically, or for burial underground.
Tanks
are fitted with upper manholes for installing the nodules, lower manholes for
emptying the nodules and internal baffle pipework designed to generate maximum
heat transfer efficiency.
Each tank is specifically designed to accommodate
the energy storage requirement for a particular system. |
