Solar Hot Water Systems Design

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Collector piping alternatives

Solar Hot Water Systems Design Collector piping alternatives

It is advised to choose collectors that feature a good emptying behavior during stagnation. This will reduce the strain on the heat transfer fluid and reduce the steam production of the collector field. During periods of stagnation, it is likely that steam will develop in the collectors. The steam will push the fluid out of the collectors. The fluid will later be sent back into the collector field when the collector field is cooled down again.

Tables 3.1 to 3.9 list a selection of collector designs. The pipe manifolds are shown in relation to the arrangement of the tubes attached to the collector absorbers. Both flat plate and evacuated tube collectors are addressed. An attempt is made to assess the different absorber piping designs with respect to:

  • steam producing power (SPP) (a low SSP is a positive property with respect to a smaller expansion vessel needed)
  • glycol strain (a low glycol strain is a positive property)
  • air vent possibility (a good air vent behavior is a positive property).

For the designs listed in Tables 3.1 to 3.9, real measurements of the SPP were carried out on sample collectors. For the other designs, estimations are provided that are deduced from the experience made in the investigations. In these tables, note that:

  • A question mark “?” in the tables means that even no rough estimation on the expected behavior can be given. The assessment is very rough and the given collection of piping designs is not complete.
  • ETC = Evacuated tube collector
  • FPC = Flat plate collector.
Table 3.1. Evacuated tube collector 1 (ETC), SPP 14,130 Btu/hr/sq ft (385 W/m2).
Table 3.2. Flat plate collector 1 (FPC), SPP 5,505 Btu/hr/sq ft (150 W/m2).
Table 3.3. ETC 3, SPP 2, 202 Btu/hr/sq ft (60 W/m2), The collector does produce steam. Whether it is too much or not depends on the system.
Table 3.4. FPC 2, SPP 551 Btu/hr/sq ft (15 W/m2).
Table 3.5. ETC 1 upside down.
Table 3.6. FPC.
Table 3.7. FPC, horizontal.
Table 3.8. FPC.
Table 3.9. FPC, horizontal.

There are two methods connecting a row of collectors together – with an external manifold or an internal manifold. The external manifold uses a supply and return header and all the collectors in the row have supply connections to the supply header and also have a flow outlet connected to the return header. The internal manifold type connection has the supply pipes of each collector connected to the next collector. The same applies to the return side of the collector piping. The piping of the collector is similar to that listed in Table 3.9. Internal manifolds offer the following advantages:

  • less piping reducing costs and heat loss
  • more attractive installation.

Disadvantages of internal manifolds are that they:

  • make it more difficult to achieve a drainable system
  • make it difficult to balance collector flow for a long row of collectors
  • make it hard to remove a single collector
  • create more significant expansion and contraction issues for a long row.
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