Solar Thermal Heat Utilisation - Renewable Energy: Technology, Economics and Environment - page 145

resistance of up to 290 °C in combination with alkylen-glycol and completely
desalinated water have been marketed especially for solar systems with standstill
operation /4-10/.
Pipes. The collector and the storage are connected by pipes. The size of the sys-
tem and the absorber material determine the material chosen for these pipes.
Mostly hard or soft copper pipes or corrugated stainless steel pipes, and addition-
ally, steel and polyethylene pipes are used. However, if the absorber is made of
aluminium, the use of copper piping is not advisable due to the associated danger
of corrosion. However if it is the case, at least galvanic isolation has to be applied.
During operation of solar thermal systems providing domestic hot water, flows
of 30 to 50 l/h are common per square metre of collector area. For a number of
years now, systems with lower flows (10 to 15 l/h per square metre of collector
area), so-called Low-Flow concepts, have been used /4-2/, /4-6/. Even one single
flow through the collector circuit can heat up the heat transfer medium by the
required temperature difference. The advantages are lower pressure losses within
the pipes and the quicker supply with hot water from the collector circuit.
Disadvantages are higher thermal losses in the collector and thus lower specific
energy yield. Furthermore, these systems require longer series connection of the
collectors in order to achieve turbulent flows and thus obtain a good heat transfer
within the collector. Such Low-Flow concepts are only superior to high flow con-
cepts if external heat exchangers in connection with several charging levels into
the storage or stratifying charging units via external or especially built internal
heat exchangers are used (see Fig. 4.14, right).
Cross-section and the hydraulic flow scheme determine the pressure loss to be
overcome and the mass of the heat transfer medium inherent in the pipes. Large
cross-sections reduce the pressure loss but make control more difficult, as the
thermal mass of the pipes increases with larger cross-sections. Furthermore, the
area of the pipe also increases and the thermal losses increase proportionally.
In order to reduce thermal losses, the pipes of the collector circuit have to be
insulated. Materials that can be used are mineral wool, polyurethane-shells and
foam rubber. Increasingly, pre-insulated double pipes made of high-grade corru-
gated steel pipes with an integrated duct for the collector sensor cabling are used.
In standard solar thermal systems for the domestic hot water supply, the ther-
mal losses that still occur in the pipes, in spite of insulation, are 10 to 15 % of the
energy released by the collector /4-1/.
Heat exchanger. Heat exchangers serve to transfer heat from one medium to
another while separating the media physically. They have to be used if the storage
is charged or discharged indirectly. The transferred heat depends on
− the temperature difference between the two media,
− the area of the heat exchanger and
− the heat transfer medium and the flow speed on both sides of the heat ex-
changer (heat transfer coefficient).