Flow Rate In The Collector Array - Viessmann VITOSOL 200-FM SV2F Design Manual

Solar System Design Guide

Flow Rate in the Collector Array

All Vitosol collectors have a minimum required flow
rate (low flow) and maximum flow rate (high flow). The
collectors must operate within this range and the system
designer must choose a flow rate based on the specific
parameters of the system. At the same collector output,
a higher flow rate means a lower nt or temperature
spread across the collector array. Inversely a lower
flow rate will have a higher nt or temperature spread
across the collector array. When the nt or temperature
spread across the collector array becomes too large, the
efficiency of the collectors will also decrease.
For larger solar installations, high flow is usually not
recommended as this results in bigger pumps and larger
pipe sizes. Typically low flow would be used as the
decreased flow requirements results in smaller pumps
which would use less energy, and smaller pipe sizes,
reducing the overall installation and operating cost for the
system.
Operating modes:
Low flow operation
Operation with flow rates between:
0.010 - 0.012 USG/min/ft
High flow operation
Operation with flow rates between:
0.012 - 0.025 USG/min/ft
Matched flow operation
Operation with variable flow rates based on actual
temperature spread across panels. This mode is possible
with all SCU solar controls.
All operating modes are possible with Viessmann
collectors.
2 (25 - 30 L/h/m 2 )
2 (30 - 60 L/h/m 2 )
Which operating mode is the right one?
Since every project has it's own unique combination
of collector array size and piping length, the system
designer can choose the flow rate that best meets the
requirements of the project. The specific flow rate must
be high enough to ensure a reliable and even flow through
the entire array. High flow mode is usually best for
smaller collector arrays and low flow mode is often used
for larger scale projects.
The optimum flow rate (relative to the current tank
temperatures and the current solar insolation level) in
systems with a Viessmann solar control unit will adjust
itself automatically in matched flow operation. Single
array systems with Vitosol 200-FM can be operated
without problems down to approximately 50% of the
specific flow rate.
Example:
2x Vitosol 200-FM collectors
(collector absorber area = 25 ft
Target flow rate: 0.018 USG/min/ft
2 x 25 ft 2 x 0.018 USG/min/ft
(2 x 2.32 m 2 x 45 L/h/m
This value should be reached at 100% pump rate. An
adjustment can be made at the speed control switch of
the pump. The positive primary energetic effect is lost
if the required collector flow rate is achieved through a
higher pressure drop (equals higher power consumption).
Choose the pump speed that lies above the required
value. The flow rate is automatically reduced via the solar
control unit through a lower current supply to the solar
circuit pump.
Design and Operation
2 (2.32 m 2 )
2 (45 L/h/m 2 )
2 = 0.9 USG/min
2 ÷60 min = 3.5 L/min)
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