A solar water heater utilizes energy from the sun to heat water for domestic use. Heat is collected from the sun’s radiation by solar collector mounted either on the roof or on the ground surface. In most cases, water in circulation is heated and then flows into a storage tank.
A solar water heater is an ideal method to make use of the ‘free’ sun’s energy, which in turn helps saving the conventional electricity energy costs, as well as saving greenhouse gas emissions. However, the efficiency of your solar water heater depends on your location. The higher the light intensity, the more energy will be collected thus providing more than 70% of your domestic hot water needs. The solar water heaters come with gas or electric boosters to offer additional hot water needs whenever sunshine intensity is low.
How Does Solar Water Heater Work
Basically, a solar water heater consists of a solar collector and a storage tank. Solar collector is a glassy, dark insulated box, and series of tubes in which water passes through. The insulated box is darkened inside to absorb sun’s heat, and transform it into usable energy. A storage tank holds water, which is to be heated.
The thermally insulated box has a transparent cover, usually glass material, which helps in minimizing heat loss through conduction. For a typical system, the absorber contains a fluid (anti-freeze) that flows through the collector and storage tank. The fluid is usually heated while in the absorber, and then transfers heat to the water storage tank.
The operation of the solar water heater is aided by an automatic controller (regulator) that detects difference in temperature between the collector and the storage tank. Whenever temperature in the collector is higher than that of the storage tank, the regulator turns on the solar circulation pump. Then the fluid, in the absorber, transports heat to the storage tank.
Advantages of a Solar Water Heater
1. Adequate hot water supply
Solar water heater is a reliable system that works throughout the year. Once you have installed the system, you will have sufficient hot water supply for your home. During the winter seasons, you may be required to use immersion heater or boiler to heat the water further. However, it is advisable to consider the level of water usage and demand before installing a solar water heater system.
2. Reduced energy cost
Utilizing solar energy to heat water cuts the use of electricity and fossil fuels by about 80%. Use of electricity or fossil fuels is relatively more expensive in the long run compared to solar energy which is free to harness. You only require the initial investment of installation, and cut down your hot water bills.
3. Environmentally friendly energy
Apart from being a renewal source of energy, solar energy is quite green. There are no gas emissions, which may cause greenhouse effects to the environment. The system becomes very reliable when matched appropriately with the climate and home’s energy demand.
Different Types of Solar Water Heaters
Solar water heaters are of two categories; active or passive. Active heaters use electrical pumps and controls to circulate water through the system, while passive heaters are the simplest systems that use gravitational force to move water through the system.
(A) Active Solar Water Heater Systems
They are of three types;
1. Active Open Loop System (Direct Water Heater System)
This is the least expensive and simplest to install. With no heat exchanger, the heat is transferred directly to the water. A controller compares difference between temperatures of two sensors; one located at the solar collector and the other at the storage tank. If the collector temperature is higher than the storage tank by a few degrees, the controller turns on the circulating pump that draws cold water from the storage tank and circulates it through the collector. The solar heated water returns to the storage tank.
Another version of this system uses solar electricity to operate the circulating pump using direct current (DC). Direct systems are commonly used in moderate climates where freezing is least.
2. Active Closed Loop System (Indirect Water Heater System)
The system consists of a heat transfer fluid (glyco antifreeze) through the solar collector and heat exchanger. Once the fluid is heated at the collector, the heat exchanger transfers the heat from the fluid to the water in the storage tank. This system is different from the direct system due to availability of heat exchanger coil inside or outside the storage tank. This system is suitable for climates prone to freezing temperatures. The antifreeze is changed after a specified period, usually 3 to 10 years.
Note: The antifreeze doesn’t mix with water in the storage tank.
3. Drainback Water Heater System
This system utilizes pure (distilled) water as the heat transfer liquid in the solar collector. A pump is used to move distilled water through the collector, but water from the storage tank moves to the heat exchanger and back to the storage tank through gravitational force or convectional forces. When the pump is off the solar collector is empty, hence no freezing. Also, this helps the system to turn off when water in the tank is too hot. It suitable for colder climates since the heat transfer fluid is not exposed to extreme cold weather.
Note: This system differs from indirect system due to availability of a separate drainback tank for distilled water, and also uses distilled water as the heat transfer fluid.
(B) Passive Water Heater Systems
These are the most popular due to their structural simplicity, as well as low initial cost. There are no pumps or controllers required since cold water flows directly to the solar collector mounted on the roof. The water is heated, and then flows to the storage cylinder/tank sited on the ground surface. The stored hot water is available for use either during the night or in the morning. For those who use large volumes of hot water can save electricity energy significantly, hence reducing energy bills.
Batch and thermosiphon are types of passive solar water heater systems. Batch has both collector and water tank combined, while thermosiphon’s solar collector and storage tank are separate.