Expansion Vessel And Heat Sink For The Solar Circuit - Viessmann VITODENS 222-F Technical Manual

Design information
Pre-charge pressure: 0.75 bar (0.075 MPa)
Determine the required size of the expansion vessel to be installed
in accordance with EN 12828.

Expansion vessel and heat sink for the solar circuit

Stagnation in solar thermal systems
All safety equipment in a solar thermal system must be designed for
stagnation. If, during insolation on the solar collector array, heat can
no longer be transferred within the system, the solar circuit pump
stops and the solar thermal system goes into stagnation. Longer
system idle times, e.g. due to faults or incorrect operation, can never
be completely ruled out. This results in a rise in temperature up to
the maximum solar collector temperature. Energy yield and loss are
then the same. In the solar collectors, temperatures are reached that
exceed the boiling point of the heat transfer medium. For this rea-
son, solar thermal systems must be designed to be fail-safe in
accordance with the relevant regulations.
Being fail-safe means the following:
■ The solar thermal system must not be damaged by stagnation.
■ The solar thermal system must not pose any risk during stagna-
tion.
■ Following stagnation, the solar thermal system must automatically
return to operation.
■ Solar collectors and pipework must be engineered for the tempera-
tures expected during stagnation.
A lower system pressure is beneficial where stagnation characteris-
tics are concerned: 1 bar (0.1 MPa) positive pressure during filling
(temperature of heat transfer medium approx. 20 °C) at the solar col-
lector is adequate.
A definitive parameter when designing pressure maintenance and
safety equipment is the steam-producing power. This indicates the
power of the solar collector array, which during stagnation is trans-
ferred to the pipework in the form of steam. The maximum steam-
producing power is influenced by the draining characteristics of the
solar collectors and the array. Subject to solar collector type and
hydraulic connection, different steam-producing power levels can
occur (see diagram below).
A B
A Flat-plate collector without liquid pocket
Steam-producing power = 60 W/m
B Flat-plate collector with liquid pocket
Steam-producing power = 100 W/m
VITODENS
(cont.)
C D
2
2
If the integral expansion vessel is insufficient, install a suitably sized
expansion vessel on site.
C Vacuum tube collector, header casing at the side
Steam-producing power = 100 W/m
D Vacuum tube collector, header casing at the top
Steam-producing power = 200 W/m
The length of pipe that holds steam during stagnation (steam
spread) is calculated from the balance between the steam-producing
power of the solar collector array and the heat loss from the pipe-
work. The actual values assumed for the loss from a solar circuit
pipe made from copper and 100 % insulated with commercially avail-
able material are as follows:
Dimensions Heat loss in W/m
12 x 1/15 x 1/18 x 1
22 x 1/28 x 1.5
■ Steam spread is less than the pipe run in the solar circuit (flow
and return) between solar collector and expansion vessel:
The steam cannot reach the expansion vessel in the event of stag-
nation. The displaced volume (solar collector array and pipework
filled with steam) must be taken into account when sizing the
expansion vessel.
■ Steam spread is greater than the pipe run in the solar circuit (flow
and return) between solar collector and expansion vessel:
Include a cooling line (heat sink) in the design, to protect the
expansion vessel diaphragms against thermal overload (see dia-
grams below). Steam condenses in this cooling line and reduces
the temperature of the liquefied heat transfer medium to below
70 °C.
2
2
25
30
VIESMANN
61
5