Solar Water Heating Systems
All You Need to Know
Solar water heaters are a common solution for generating hot water for domestic and small commercial applications. They are a cost-effective solution because they only rely on the sunshine, which is freely available everywhere.
They come in different designs depending on various factors such as your geographic location, weather, roof design, water quality and budget, all of which determine which type is best for a specific situation.
Types of Solar Water Heaters
All the designs consist of two main components; a collector (panel) to harness the sun’s thermal energy (heat), and a storage tank to store the hot water.
Solar water heaters are defined according to three main criteria: the manner in which water is heated, the collector type, and the water circulation method. Therefore, to fully describe the required solution, one must state where it falls in each of the three categories.
1. The manner in which water is heated
- Direct (Open Loop) Systems – Water is heated directly as it moves through the collector tubes and back into the storage tank. Due to their relative simplicity, this type of water heater is reliable and efficient when used with soft water, and not suitable for salty water because mineral deposits (scaling) may gradually lead to blockage of the collector pipes. They are also not suitable for areas that experience freezing temperatures because frozen water expands and may damage the pipes.
- Indirect (Closed Loop) Systems – These make use of a heat transfer fluid such as glycol, which is heated as it moves through the collector pipes. This fluid then flows through a heat exchanger located in the storage tank, and indirectly transfers the heat to the water as it gets in contact with the surface of the manifold that contains the hot fluid. The advantage of using a heat transfer fluid is that it is freeze resistant so indirect solar thermal systems operate well in areas prone to freezing. One disadvantage is that the fluid may evaporate over time, so it needs to be topped up.
2. The Type of Collector
a) Flat Plate Collectors – A flat-plate collector system consists of solar heat-absorber plates fitted with a network of copper flow tubes.
The tubes are in a glass-covered (glazed) insulated box. The sun passes through the glass and heats up the fluid in the copper tubes. The glazing reduces the amount of heat that escapes and protects the panels from moisture and other contaminants. Most household systems consist of 1 or 2 collectors.
- Typically performs better on warm, clear days
- Are less expensive
- The direction and angle of installation is less critical, hence providing more installation options
- The whole collector needs to be replaced if a portion of it fails
b) Evacuated (vacuum) Tube Collectors
An evacuated tube collector system consists of a series of insulated glass tubes that are arranged in parallel rows, each containing a small absorber pipe. The inner pipe absorbs solar energy and transfers it to water or glycol solution. The air between the pipe and the glass is evacuated to trap heat in (like a thermos), maximizing the amount of heat energy transferred to the fluid.
- They take up less space than equivalent capacity of flat-plate collector systems
- They are the most efficient collectors available, and can produce higher output temperatures even during cold weather
- They are more expensive
- The direction and angle of installation are more critical, providing fewer installation options
- Individual tubes can be easily replaced with little effect on the whole system.
3. The Water Circulation Method
- Active (pressurized) Systems – These are characterized by the fact that they use pumps to circulate the water from the cold-water source to the hot water storage tank and into the house, thereby eliminating the problem of low water pressure at the points of use (showers etc.)They are more efficient than the passive ones but need more equipment and maintenance. They tend to be more expensive but can work in cold weather. Power from the mains or solar photovoltaic electricity can be used to power the circulation pump.
- Passive (non-pressurized) Systems – These water heaters use of natural circulation processes such as gravity and the natural circulation of water as it heats up (thermosiphon).Thermosiphon is the process by which water circulates due to temperature difference. When the sun heats the water in the collector it becomes lighter than the cooler water in the elevated storage tank. Gravity then pulls the heavier, colder water down into the collector inlet and forces the warmer water back into the storage tank. This process repeats itself until the sun sets.Passive water heaters are generally cheaper than their active counterparts and tend to be more reliable and last longer. However, they don’t work on every roof design e.g., there must be a minimum height between the storage tank and the point of use (shower) for water to flow at a good pressure.
Sizing, Installation & Maintenance of Solar Water Heaters
A properly sized water heater will meet your household’s hot water needs while operating efficiently. Therefore, before purchasing a water heater, make sure it’s the correct size.
Sizing your solar water heating system basically involves determining the total hot water needs of your household. A general approach is to use your house occupancy, whereby it is estimated that one person uses 10 – 20 liters of water per 10 minutes bath.
The proper installation of solar water heaters depends on many factors. These include solar resource, climate, local building code requirements, and safety issues; therefore, it’s best to have a qualified solar thermal systems contractor install your system.
After installation, properly maintaining your system will keep it running smoothly. Passive systems don’t require much maintenance. For active systems, discuss the maintenance requirements with your system provider, and consult the system’s owner’s manual. The glazing may need to be cleaned in dry climates where rainwater doesn’t provide a natural rinse.
Regular maintenance on simple systems can be as infrequent as every 3–5 years, preferably by a solar contractor. Systems with electrical components usually require a replacement part or two after 10 years
Cost of Solar Water Heaters
The table below shows the prices of the common sizes of vacuum tube water heaters in Kenya. These prices exclude installation costs and are only for budgeting purposes. Actual prices need to be confirmed by contacting the supplier.
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