Balancing fluid flow in collector field

A large solar hot water system will flow through many solar collectors. The flow through each solar collector should have basically the same pressure drop. This will ensure that the system is balanced such that each collector is receiving the same flow rate of heat transfer fluid. Thus the fluid temperature increase of each collector will be equal to the others. With several collectors, a reversed return piping system is used to achieve the equal pressure drop. This assumes each collector is arranged to have parallel fluid flow. In some systems, the collectors are arranged for flow in series. Here the fluid goes from one collector to the next collector picking up heat along the way. Figure 3.27 shows the parallel and series flow arrangements. There is a limit to the number of collectors that can be arranged in series due to the pressure drop of the flow through so many collectors.

In large collector systems a combination of parallel and series flow is used. An arrangement of collectors that have series flow are placed in a group or zone. Each group is then arranged to have parallel flow with the other collector groups. Figure 3.27 shows such arrangement. The collector layout could also have a group collectors with parallel flow placed in series with another collector group having parallel flow.

The design objective is for the pressure drop of each group to be equal. This is accomplished using a reverse return piping layout (Figure 3.27). The amount of piping should be kept at a minimum to keep installation costs low and to minimize the system resistance to flow, which helps keep the required pump pressures at acceptable levels. A rule of thumb is for the pressure drop per pipe length in the collector to be slightly more than three times the pressure drop per pipe length in the general piping system.

With these large solar thermal systems it is advised to design the collector loop system as a low flow system. That means that the flow rate is ~0.37 gal/sq ft*h (15 l/m2). By contrast, in the so called high flow type the flow rate is in the range of 1.23–1.72 gal/sq ft*hr (50–70 l/m2*hr). This type system is well suited for small (less than 161sq ft [15 m2]) solar DHW systems and for applications having a small temperature rise such as solar cooling applications. It is possible with a low flow system to connect more collectors that are piped for series flow. The flow through an individual collector is then less compared to a high flow operation mode and a higher  temperature rise is possible. The benefits of a low flow system are its:

• lower investment costs due to smaller pipe sizes required
• lower piping lengths, as more collectors are connected in series (lower investment) and reduced heat losses
• smaller pump requirements (lower investment), which use less pump energy (lower operation costs) due to lower volume flow
• requirement for less fluid in the solar loop, and consequently less glycol (lower investment)
• quicker response to achieving the target temperature in heat storage tanks including those using stratified charging
• ability to achieve a useful temperature in a single flow cycle a greater percentage of the time.

Table 3.10 lists the range of specific mass flow rates of the various operating modes of the solar installations and the differences in the total mass flow rate in the feed and return piping of an assumed collector area of 10,760 sq ft (1000 m2). This example shows that larger manifold pipe sizes and higher electric pump power are required when the high-flow operating mode is used in large-scale solar thermal systems. The costs of installing and operating this type of system would therefore soon exceed acceptable limits.