IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report, November 2009
Serial flow, "Low Flow"
∆M ges = 10 - 15 l/m 2 h related to
collector area
Parallel flow, "High Flow"
∆M ges= 50 l/m 2 h related to
collector area
∆M ges = ∆M col = 10 - 15 l/m 2 h ∆M ges = 4 * ∆M col = 50 l/m 2 h
∆p ges = 4 * ∆p col
∆p ges = ∆p col
∆T ges = 4 * ∆T col = 40 °C ∆T ges = ∆T col = 10 °C
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M col
∆Τ
∆p
∆M ges
∆M ges
Fig. 7: Hydraulics for the collector field for low flow serial (left) and high flow parallel (right)
systems. In all cases care has to be taken to maintain an equal resistance in all parallel
lines. This can be partly achieved by a “Tichelmann” arrangement where the added length of
the inlet and outlet pipes is the same for all parallel collectors (Streicher 2008).
Many existing solar combisystems (without cooling function) work with the low-flow principle
in the collector loop. The advantage is that the necessary temperature lift for domestic hot
water preparation and space heating can be reached in one step in the solar collector if the
solar radiation is high enough. Therefore, the auxiliary heater has to be operated less
frequently.
When connecting such a system to a thermally driven chiller, this can be a problem because
the temperature difference between flow and return of the hot side of the chiller is typically
small (~ 6 K). Therefore it would make more sense to operate the collector loop (at least for
the cooling mode) at this small temperature difference rather than the roughly 30-40 K that
are typical for low-flow solar combisystems due to the domestic hot water (DHW) demand.
During the summer high flow operation would be favourable for solar cooling but low flow
operation would be favourable for DHW production.
One option to overcome this dilemma is to switch the collector loop at least between low-flow
operation in winter to high-flow operation in summer. However, care has to be taken to
ensure proper turbulent flow and acceptable pressure drops in the collector circuit as well as
on the balanced flow through all parallel collector tubes.
There are in principle four options:
• The collector hydraulics stay the same in summer and winter but the mass flow
through the collector varies by switching the pump speed or the type of pump. In this
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