Environmental Engineering Reference
In-Depth Information
External and internal heat exchangers are used for solar thermal systems.
One advantage of internal heat exchangers is that they do not require a lot of
space. A disadvantage is the relatively low heat output, the required larger tem-
perature difference and the limited size. Straight-tube and ribbed-tube heat ex-
changers are used. Sometimes double-mantle heat exchangers are used.
External heat exchangers are mostly designed with counter current mass flows
(counter current heat exchangers). Common designs are shell and tube (for large
systems), plate and coaxial heat exchangers. The advantages of using external
heat exchangers are the higher heat transfer output at a lower temperature differ-
ence plus the possibility to charge the top of the storage with the heated-up water.
For that reason, they are preferably used for collector areas larger than 15 to
20 m
2
. Within the storage, a better temperature layering than with internal heat
exchangers can be achieved. The higher thermal losses, the requirement for more
space, and an additional pump required within the secondary circuit are disadvan-
tageous.
Using a rough average, a ribbed exchange area of approximately 0.4 m
2
for
ribbed tubes, and of around 0.2 m
2
for bare exchange tubes is required per square
metre collector area in hot water systems with an internal heat exchanger /4-11/.
Due to a better heat transfer, this exchange area can be reduced to 0.05 up to
0.08 m
2
/4-6/ for external heat exchangers.
Pumps.
In solar thermal systems with a forced circulation a pump is required to
operate the collector circuit. For standard domestic solar thermal water systems,
volume flow amounts of 30 to 50 l/(h m
2
collector area
) are common (High-Flow)
/4-1/. For Low-Flow systems the volume flows are between 10 and
15 l//(h m
2
collector area
). The layout of the collector circuit pumps also depends on
this volume flow rate.
High-Flow systems always have simple centrifugal pumps, mostly equipped
with a manual adjustable speed control. For Low-Flow or Drain-Back systems,
however, vane or gear pumps are used that still show good efficiencies at a higher
pressure rise and a lower volume flow rate.
The pumps are normally electricity-driven and are generally directly plugged
into the public grid. However, they can also be connected with a photovoltaic
module of the required power. They then operate as direct current pumps. For this
more costly way of pump electricity supply it is advantageous that electrical en-
ergy is mainly used for the pumps if the corresponding solar energy supplies are
available. Thus, radiation supply and energy demand correlate. Electrical energy
storage is not necessary although the pump is driven independently of the grid.
The electrical energy required to drive the pump is between approximately 1
and 2 % for the standard solar thermal systems for domestic hot water supply.
This is related to the heat available at the outlet of the solar installation. For larger
systems the required electrical energy is even lower due to better pump efficien-
cies.
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