Solar water heater collector

Solar-water-heater-collector is one of the vital components required to generate hot water for hygienic or heating system. Its application could be domestic or industrial use.

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Materials used on solar water heater collector

1. Copper :having a specific heat of 0.093 (Btu/Lb./°F), sp. gravity of 8.79

2. Aluminium: having a specific heat of 0.22(Btu/Lb./°F), sp. gravity of 2.72

3. Glass: having a specific heat of 0.20(Btu/Lb./°F), sp. gravity of 2.9

Types of solar-water-heater-collector

1. Copper tube on copper plate.

2. Copper tube on aluminium plate.

3. Hot-rolled aluminium plate absorber.

4. Glass tubes absorbers.

It is strong like copper and heat resistant due to its double wall construction. The main tube absorber is fitted inside another glass tubes or outer-core to provide protection from harsh weather hail storm. The outer surface of the inner tube which is the main absorber is applied with a dark coating to enhance heat absorption while the interior wall with a reflective coating to retain the heat being collected by the circulating liquid.

All types of solar water heater system uses heat-transfer fluid usually an inhibited glycol which is non-corrosive in nature.

Most of the solar-water-heater-collector from early stage were copper tubes on copper plate. This is due to the fact that copper has a lower specific heat value 0.093 (Btu/Lb./°F, strong and durable non- corroding material.

As time goes by, the cost of copper increases along with other metals so, collector manufacturers changed the copper plate to aluminium plate while others shifted to a fusion bonded hot-rolled absorbers.

Process of aluminium plate solar-water-heater-collector

Aluminium plate absorbers are produced by pre-heating the top and bottom plates and being bonded by hot-rolling mills. The aluminium grade involved is 6000 or 8000 series depending on the design specifications or combinations. Prior to pre-heating the plates, the process started from decoiler units then straightening and levelling to attained a smooth flat surface of the coils. The bottom plate is thoroughly degreased by a brushing machine or an equivalent degreasing solution. Then the degreased plate is introduced to a silk-screen printing machine where the circuit or channel of the collector is being printed using graphite ink. The graphite ink serves as the barrier and thus preventing that area in joining together. Right after leaving the rolling mills, the bonded plates where annealed and allow it to cool. At the end of the process , a high air pressure is being injected inside the bonded plate at the inlet side to inflate along the printed channel to a controlled height. A dark paint is applied on the collector in order to enhance the heat being absorbed, a white or silver colours will likely to bounce back some of the heat landed on the surface of the collector.

This process is preferred for mass production at 24/7, to compensate the overhead involved. For small quantity, the draw back of this process is the cost in energy, manpower and maintenance.

Tube on plate collector:

This type of collector is cost effective and durable compared to other absorber.

Usual type of tube on plate collector:

1. Straight riser pipes were both ends are welded to a main collector pipe.

2. Serpentine form tubes.

Types of solar-water-heater-collector system:

1. Thermo-siphon, Closed-loop, Indirect type:The heat-transfer liquid rises and circulates by itself due to change in temperature and pressure.
2. Dynamic, Active, forced-circulation type: Pumps, valves and system controllers are introduced in the system to circulate the heat-transfer liquid in the storage tank.
3. Direct type: Water is directly circulating inside the collector and stored directly into a tank ready for domestic use. Disadvantage of this type is that when temperature is dropping to freezing point then you have to shut the inlet and outlet valves from the collector and back drain all the remaining water inside the collector and piping exposed to freezing temperature. It would be disastrous for this type if water freezing occurs in the collector.

Today manufacturers of solar-water-heater-collector have considered using aluminium tube instead of copper. It’s undeniable that copper has a higher specific heat characteristic compared to aluminium.

In the design of solar-water-heater-collector , the main factor being considered or calculated is the heat transfer coefficient of the material (Cp). The heat transfer coefficient refers to the amount of heat absorbed per unit area therefore, the specific heat of the tube is of little effect on the collector efficiency.

As a remedy, the solar-water-heater-collector manufactures increased the thickness of the aluminium plate to match the heat transfer coefficient equivalent to copper tube collectors.

Advantages of aluminium on solar water heater collector:

1. Cost effective since aluminum weighs 1/3 compared to copper. Thus becomes more affordable for end users.

2. Aluminum is considerably lightweight compared to copper which is 1/3 of copper.

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