Solar water heating systems (or solar thermal systems) use sunlight to heat water. You can heat water with solar energy in two ways. One, you can install a solar panel system, and the electricity generated by the system helps heat the water—this is like standard electric heaters. Two, you could directly heat the water, without any electricity and only the sunlight—this is done through solar water heaters.
But what are solar water heaters? What are the different types of water heaters? And lastly, how do solar water heaters work?
This is today’s agenda—discussing all you need to know about solar water heaters.
What are solar water heaters?
Solar water heaters are simple devices that capture solar radiation and convert it into heat. The sunlight is trapped by solar thermal collectors, which heat water or a working fluid that can transfer heat. No power input is required for solar water heating systems, though some systems may use pumps that require electricity.
Solar water heating is a cost-effective alternative to traditional electricity-based water heating systems. You have to pay only for the upfront installation and routine maintenance. A right-sized solar water heater could cater to your family’s needs in summer when the efficiency of the solar collectors is at its highest (around 80%). But in winter, the efficiency drops to 30-35%, and you might need a traditional backup heating system to avoid any inconvenience.
Components of solar water heaters
A basic solar water heater has two primary components other than auxiliary elements (pipes, valves, etc.): a water storage tank and a solar collector. The water tank works as the reservoir for both hot and cold water, while the solar collector is the heating device.
Solar heaters can be constructed in different ways. For some heaters, only a storage tank and solar collector would suffice. But the type of solar water heater you should use depends on your location and preferences. For example, if you live in a colder region where the water freezes in winter, you would need to have freeze protection in your system. That would require the system to have a non-toxic anti-freeze fluid and heat exchangers for circulation and heat transfer across the system.
Some systems might need two storage tanks, especially in winter and colder regions. Since the efficiency of systems drops, the solar water storage tank may only be used to preheat the water a bit before the traditional heating system does its job.
Types of solar water heaters?
There are two types of solar water heating systems:
- Passive solar water heating systems—which has natural water circulation mechanism.
- Active solar water heating systems—which use pumps and other controls for moving water through the system.
Each system is further categorized into two, based on whether the potable water is heated directly in the solar collector or indirectly through a combination of fluid and heat exchangers.
- Direct or open loop systems, where the water is heated directly in the solar collector and passed on to the tank.
- Indirect or closed loop systems, where the sunlight heats an anti-freeze fluid, from where the heat is transferred through heat exchangers to water.
Let’s break down each system further.
- Active solar water heaters
Active systems use pumps for circulating water (or heating fluid) across the system. These systems are more expensive than passive ones but are also more effective in colder climates. One of the advantages of installing an active system is the flexibility of tank placement. You could keep it above the roof or somewhere else on the ground at a strategic location close to where the water is to be used. This helps prevent heat loss, especially in winter.
These systems are split into two categories—direct and indirect circulation systems.
- Direct circulation system
Image Credit: https://www.energypurse.com/solar-water-heaters/
Direct circulation heats the usable water directly. The water is pumped to solar collectors, where it is heated directly when the sun shines. Once it is heated, it moves to the storage tank and is ready for use at home. While this is a simple system and doesn’t cost much, it’s not suitable for climates where water freezes. They are suitable more for relatively warmer climates where it rarely freezes in winter.
- Indirect circulation system
Image Credit: https://vesr.nrs.ucsb.edu/about/sustainability/indirect-solar-hot-water-system
In direct systems, the water is prone to freezing. What is the solution? Indirect systems.
Indirect circulation systems do not directly heat water. Instead, a non-freezing heat-transfer fluid (usually low-toxic propylene glycol) is used. An additional component in the indirect active heating system is the heat exchanger.
The fluid goes to solar collectors, where it is heated. From there, it flows back to the tank, but before it is injected into the tank, it passes through heat exchangers. The heated fluid transfers heat to water—the water is now heated and available for use.
Indirect heating systems cost more than their direct counterparts, but they work best in colder climates where temperatures drop below the freezing point (0°C).
- Passive solar water heaters
Rather than using pumps, passive solar water heaters rely on gravity to let the water or fluid circulate the system. These systems are some of the most reliable and affordable heating systems, though their efficiency doesn’t match that of active systems.
Passive systems also come in two varieties.
- Integral collector-storage (ICS) passive system
Image Credit(see page 9): https://www.icrc.org/en/doc/assets/files/2012/icrc-solar-water-heating-systems.pdf
Also called batch or bread box systems, the ICS passive systems combine the solar collector and tank. Both act as one unit, unlike active systems where the storage tank is separate from the solar collector.
In these systems, an ICS box, insulated from all three sides except the one facing the sun—this side is glazed—is installed on the roof. The storage tank also sits in this box. The sunlight falls on the box that directly heats the water. The storage tank is usually painted black to maximize energy absorption.
On the negative side, integral collector-storage passive systems have two flaws. One, since the whole unit is outdoors, the water is susceptible to freezing, especially when the ambient temperature is low in the surrounding. Two, since both the tank and collector are installed as one unit, the system becomes heavy for some properties. You will have to install it on the ground, or proper arrangements should be made for installation on the roof.
- Thermosiphon systems
Image Credit: https://www.appropedia.org/Thermosiphon#/media/File:Thermosyphon.gif
Thermosiphon is another technology used for harvesting solar radiation for water heating. The principle behind the working of these systems is the basic law of thermodynamics called the thermosiphoning effect. That is, when the water is heated, it becomes less dense, expands, and tends to rise. But when it is cooled, the water becomes denser and tends to sink.
The solar tank is installed above the solar collector. When solar radiation falls on the solar collector, it heats an anti-freeze fluid in the collector. The non-freezing liquid is only used if you live in a colder climate. Thanks to the effect mentioned above, the heated fluid then rises to the tank and passes through a heat exchanger, where the heat is transferred to the water. There you have it—the hot water.
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