13.2 Solar thermal system operating modes
Flow rate in the collector array
Collector systems can be operated with different specific flow rates.
The unit for this is the flow rate in l/(hm
2
). The reference variable is
the absorber area. At the same collector output, a higher flow rate
means a lower temperature spread in the collector circuit; a lower
flow rate means a higher temperature spread.
With a high temperature spread, the average collector temperature
increases, i.e. the efficiency of the collectors drops. On the other
hand, where the flow rates are lower, less energy is required to oper-
ate the pumps and the pipework sizing can be smaller.
Operating modes:
■
Low flow operation
Operation with flow rates up to approx. 30 l/(hm
2
)
■
High flow operation
Operation with flow rates greater than 30 l/(hm
2
)
■
Matched flow operation
Operation with variable flow rates
All operating modes are possible with Viessmann collectors.
Which operating mode is the right one?
The specific flow rate must be high enough to ensure a reliable and
even flow through the entire array. The optimum flow rate (relative to
the current cylinder temperatures and the current insolation level) in
systems with a Viessmann solar control unit will adjust itself auto-
matically in matched flow operation. Single array systems with
Vitosol-FM/-F or Vitosol-T can be operated without problems down
to approx. 50 % of the specific flow rate.
Example:
4.6 m
2
absorber area
Required flow rate: 25 l/(hm
2
)
This results in the following: 115 l/h, i.e. approx. 1.9 l/min
This value should be reached at 100 % pump rate. An adjustment
can be made via the output stages of the pump. The positive primary
energetic effect is lost if the required collector flow rate is achieved
through a higher pressure drop (= higher power consumption).
Choose the pump stage that lies above the required value. The flow
rate is automatically reduced via the control unit through a lower cur-
rent supply to the solar circuit pump.
13.3 Installation examples Vitosol-FM/-F, types SV and SH
Take ventilation into consideration when designing the collector
arrays (see chapter "Ventilation" on page 132).
High flow operation — single sided connection
≤
10
A
A
Collector temperature sensor in the flow
≤
10
≤
10
A
A
Collector temperature sensor in the flow
High flow operation — connection on alternate sides
≤
12
A
A
Collector temperature sensor in the flow
Information regarding design and operation
(cont.)
122
VIESMANN
VITOSOL
13
5822 440 GB
