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Solar Geysers / Solar Water Heating Systems – Part 1


Learning about the different types of solar geysers / solar water heaters, from direct passive ICS’s to indirect active flat plates and everything in between, along with the pros and cons of each system.

Intro: Solar Water Heating – Part 1

I’ve been looking into a bunch of different options when it comes to water heating solutions, including gas geysers, rocket mass water heaters and solar geysers.


You think you know something until you decide to google.

Hopefully this research post will save you some time if you’re thinking about installing a Solar Geyser. In Part one, I’ll be discussing the different types of systems I learned about, and what their pros and cons are (as I understand them – note that I am in no way whatsoever an expert at any of this).

I’m fairly certain at this point that people who sell solar geysers /solar water heating systems have intentionally made things confusing.

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Types of Solar Water Heating Systems

To begin to evaluate the different options out there, you first need to understand the three primary factors to any solar water heating system. These are:

  1. The manner in which the water is heated – either Direct (open loop) or Indirect (closed loop).
  2. The way in which the water or heat transfer fluid is circulated – either Active or Passive.
  3. The solar collector used to heat the water – namely Batch (Integrated Collector Storage or ICS for short), Flat Plate or Evacuated Tube.

1. How the Water is Heated: Direct or Indirect Systems

Direct Solar Energy is classified as involving one energy transfer, while Indirect means that more than one energy transfer occurs.

In the context of solar geysers, a Direct system (also called an Open Loop) takes the heat energy from the sun and directly heats up the water that will come out your tap by moving it through the collector to absorb the sun’s heat and then moving it back into the storage tank until you need it. There is one energy (heat) transfer – from sun to water.

An Indirect system (also called a Closed Loop) takes the heat energy from the sun and heats up tubes filled with fluid in the collector. Water from the storage tank goes into the collector, circulating around the tubes while absorbing their heat, and then moving back to the storage tank until you need it. There are two energy (heat) transfers – from sun to fluid, from fluid to water.

Direct / Open Loop System:

In the Direct system model the water is exposed to low temperatures.

This makes it most suitable for areas that don’t experience freezing temperatures or frost. Frozen water expands which would damage the collector piping.

If your area experiences temperatures lower than
, this system is not suitable for you.

Since your storage water runs directly through the collector pipes, these systems require maintenance to keep the pipes clear of mineral deposits (scaling).

If your area has high levels of minerals in the water (for example if you regularly have to descale your kettle), this system is not suitable for you.

The benefit of a Direct solar water heater system is that it’s reliable and efficient thanks to its relative simplicity.

Direct  Open Loop solar water heating system

Indirect / Closed Loop System:

The heat transfer fluid in the collector tubes is usually glycol, freon or distilled water.

These tubes make for a more robust, freeze resistant system.

Maintenance wise, the fluid in the collector tubes may evaporate over time so it needs to be checked on regularly and topped up when needed.

They also tend to cost more than indirect systems.

The benefit of an Indirect solar water heater system is that they work well in areas that experience frost and freezing temperatures.

Diagram of an indirect, closed loop system

2. How the Water Is Circulated: Active vs Passive Systems

A passive system uses natural processes to move the water and is not reliant on pumps and controllers. An active system makes use of a mechanical pump and controller to move the water and requires power to run.

Solar Circulation: Passive VS Active Systems

Active System:

An active system uses pumps and controllers to circulate the water or heat transfer fluid.

They require mains or photovoltaic electricity to power the circulation pump and controller.

Active systems by default need more equipment and maintenance. More equipment means they’re more expensive, and there are more moving parts vulnerable to failure.

On the upside, they tend to be more effective than passive systems and work well in cold climates.

Diagram of a solar healing active system
Diagram of an active system. Image from Appropedia

Passive System:

A passive system doesn’t use a pump to circulate the heating fluid or water. Instead it makes use of natural processes – gravity and the thermosiphon effect (the natural circulation of water that occurs when it heats up).

Cheaper than their active counterparts, they also tend to be more lightweight, reliable and last longer. They are well suited for off grid areas with no municipal water supply.

On the downside, they don’t work on every roof design, and the distance between the taps and the system will affect the end pressure.

They aren’t always a great option for an existing building, but would work well for a new build where these factors can be taken into consideration for maximum effectiveness.

Example diagram of a passive system.
Example diagram of a passive system. Image from Simple Solar Info

If a tube fails, the whole system needs to be replaced.

Mixing hot and cold water can be a problem if the cold water is more pressurized than the hot water supply, as the cold water will overwhelm the hot water coming out the tap. This is particularly a problem in winter and in colder climates.

Freezing weather can prevent the circulation effect.

3. The Type of Solar Collector Used

There are three main types of solar collectors that capture the sun’s heat energy to transfer it to your water in different ways. Batch / ICS collectors, Flat Plate collectors and Evacuated Tube collectors.

Batch / ICS Collectors:

In a Batch or Integrated Collector Storage system, the tank acts as both the collector and the water storage – it is all one unit.

Pros and Cons:

  • Quick and easy to install
  • Newer models are more reliable and less expensive
  • Low maintenance
  • Long lasting
  • Heavier than flat plate collectors
  • Not the most efficient system
  • Struggle at night and in very cold conditions
integrated collector storage system
Diagram of an Integrated Collector Storage system. Image from Alternative Energy Tutorials

Flat Plate Solar Collectors:

A Flat Plate Collector system consists of a dark, flat box with a transparent top made of tough material to withstand the elements. Inside the box there are pipes laid out against a large heat absorbing plate.

Pros and Cons:

  • Very cost effective
  • Durable
  • You might still need a back up water heating system
Example of a flat plate solar collector
Example of a flat plate collector

Evacuated Tube Collectors:

An Evacuated Tube Collector has rows of individual clear glass tubes containing a vacuum that helps prevent heat from being lost back into the environment.

Pros and Cons:

  • Can endure freezing conditions since there is very little water in the system
  • Has a wider angle, so spends more time absorbing heat
  • Better heat retention, better performance on overcast days and at night
  • If one tube breaks, you can replace it individually (you don’t have to replace the whole collector)
  • Better handling in strong winds due to air being able to move between the tubes
  • More expensive than other collector systems
Diagram of an Evacuated Tube Collector.
Diagram of an Evacuated Tube Collector. Image from EPA