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.
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 Apricus collectors will be installed in this format.
There are three main categories of plumbing format for solar thermal systems.
System Diagram Example
Below is an example of a direct flow system. This system has a mid mounted electric element, so the tank is accepting both solar input (bottom) while the top half of the tank is boosted as required (or with timer) to ensure hot water supply. This kind of system is suitable when there is not sufficient space for a dedicated solar pre-heat tank, or if the tank is large capacity.
The water is drawn directly from the bottom of the tank, through the collector and returned to a position about 1/4 the height of the tank.
System Diagram Example
Below are examples of two indirect systems, on the left with a coil heat exchanger, on the right using an external heat exchanger.
A coil heat exchanger is normally the best choice for domestic applications. An external heat exchanger may be chosen for a domestic application if the existing tank is to be used, thus avoiding the need to purchase a dedicated tank. Commercial applications also commonly use an external heat exchanger as they can be easily sized to meet the heat exchange demands.
A disadvantage of an external heat exchanger is the need for a pump on the hot water tank side to circulate water through the heat exchanger whenever the solar pump is operational. This adds costs to the system and also additional electricity usage.
Below is a simple drain back system diagram, showing the small drainback tank. The heat exchanger can be either a coil in the drainback tank, coil in the main tank or an external heat exchanger (for commercial projects). Note the simplicity of the "plumbing" equipment. All that is required is a circualation pump and check valve plus some isolation and drain valves.
Which is the best system? Which is the best for a specific application?
Each system format has its advantages and disadvantages. Each can work very effectively and reliably if designed correctly and installed by an experienced contractor. The decision of which format is suitable is a decision for an experienced solar engineer to make after assessing the specifics of the application. Some solar engineers or installers strongly prefer a certain system format over another, based on years of experience. The advice in such a case is to listen to them! If in doubt about the suitability of the system design you can always contact Apricus for expert advice.
Here is a summary table of the 3 system formats:
= Poor, Not Suitable, High Cost = Excellent, Very Suitable, Low Cost