A typical solar water heating system incorporates a solar collector, circulation pump, storage tank, controller, piping and various valves. There are many different configurations possible, the choice of design will depend on the purpose of the system, climate, space limitations and many other factors. Lets start by having a look at the basic kinds of systems configurations.

Active vs Passive Flow

A solar thermal system can be active or passive. This term relates to the movement of the heat transfer liquid through the solar collector. Passive systems rely on thermosiphoning action to slowly move the water through the collector. Active systems utilise a circulation pump to move the liquid. This page focuses on active systems as most He Jia collectors will be installed in this format.

Direct, Closed Loop & Drainback

There are three main categories of plumbing format for solar thermal systems.

• Direct

• Closed Loop (aka Indirect)

• Drainback

Let's review each one by one, with a summary overview at the end of this page:

Direct System

Suitable For

• Moderate climates (winter night temperatures > -10^oC / 50^oF)

• Low water hardness (mineral content), otherwise scale will form in the collector

• Domestic or commercial applications

Advantages

• Efficient heat transfer as no heat exchangers are required.

• Allows simple retrofitting of existing hot water tank.

Disadvantages

• Not normally suitable for very cold areas. (certain system designs may allow direct flow to work in cold climates)

• Not suitable for areas with particularly hard water as limescale will form in the solar collector.

Closed Loop

Suitable For

• Cold climates with winter night temperatures < -10^oC / 50^oF

• Poor water quality that would cause corrosion of the copper, or scale formation in the collector.

• Domestic or commercial applications

• Commercial applications when final liquid to be heated is not potable water.

Advantages

• Design to withstand freezing temperatures

• Can protect solar collect from scale in areas with poor water quality

• Suitable for commercial style applications such as spas, pool heating, industrial heating etc.

Disadvantages

• Not as efficient as direct flow system, due to the need to use a heat exchanger

• Glycol based heat transfer fluids must be inspected and changed periodically (every 2-5 years)

• System cost is higher due to need for:

  • Heat transfer fluid (normally propylene glycol based anti-freeze)

  • More complex pump station (fill & drain valves, pressure relief valve, air separator etc)

  • Expansion tank required

  
  

  
  

Drain back

Suitable For

• Cold climates with winter night temperatures < -10^oC / 50^oF

• Poor water quality that would cause corrosion of the copper, or scale formation in the collector.

• Commercial applications when final liquid to be heated is not potable water.

• System where overheating is a potential issue

• Domestic or commercial applications

Advantages

• Prevents freezing issues, even during power outages.

• Prevents excessive heat related issues.

• Avoids the need for glycol-based heat transfer fluid.

Disadvantages

• Not as efficient as direct flow system, due to the need to use a heat exchanger.

• Piping collectors must be slightly sloped to facilitate complete drainage, this increases the requirement for the installation quality, and required straight hard metal pipe rather that flexible line sets or soft copper.

• A more expensive higher head pump is required which may use 2-4 times as much electricity as a direct or indirect circulation pump for equivalent number of collectors and pipe run length.