Solar Collector 101
If you want to heat water using the sun then a solar collector is one of the major parts of the system you will need to get the job done.
It does exactly as the name says, collecting thermal energy (heat) from the sun as efficiently as possible and then exposing it to water or another heat transfer fluid to enable the heating process.
And just to address some confusion about the term that sometimes creeps in, domestic solar collectors don't generate electricity, they only collect heat. A photovoltaic solar panel is the device that harvests solar energy to be converted into current for household use, but that isn't used to heat water, instead it powers lights, appliances etc.
Domestic Solar Collector Types
There are four different types of solar water heater collectors that are used to harvest thermal energy from the sun and supply hot water in households...
Flat PlateA more descriptive name would probably be the box plate because that's exactly what these type of solar thermal collectors are. A rectangular box with a cover that contains the pipes through which the water flows to be heated by the sun.
As you can see in the picture to the left, the flat plate solar collector is dark in colour to absorb as much heat as possible which is then transferred to the water flowing through the pipes inside the box.
The glazed / unglazed cover, made of glass or plastic, is permeable to light and heat but it also needs to prevent heat from radiating outwards again and be very durable to withstand the elements for many years.
The flat plate design is well suited to warmer climates as it's less efficient at low temperatures and doesn't easily overheat. It also, on average, costs less to install than evacuated tubes.
This solar hot water collector typically consists of a number of glass tubes that contain a vacuum and a heat pipe.
The vacuum reduces convection and conduction heat loss to the outside and the pipe contains a liquid which causes the top end to heat up when the sun shines on it.
The heated tip is then immersed in water or a heat transfer fluid (glycol) to generate hot water.
This design is well suited to colder climates as the evacuated tubes operate more efficiently in lower temperatures and cloud cover than flat plates, but they are on average more expensive to install.
Parabolic Dish and Trough
A dish shaped parabolic solar collector for domestic hot water generation is a reasonably new innovation in the water heater industry.
The reflective surface of the dish concentrates the sunlight onto a block of aluminium which maximises the thermal transfer to the poly-glycol fluid, which in turn generates hot water in the tank through a heat exchanger.
The sun tracking ability and the small surface area of the dish makes this the most efficient thermal collection system and it can heat large amounts of water in a comparatively short time.
The parabolic trough design has up to this point been used in a domestic capacity only to heat swimming pool water.
Batch - Integrated Collector Storage (ICS)
In a batch passive solar collector, the storage and heating of the water takes place in the same container, typically a large black tank housed in an insulated box with glazing over it to let the sun's heat in.
The tank is unpressurised and feeds the solar generated hot water into a conventional water heater using gravity where the backup element does its job when the water temperature drops below a certain level.
The smaller surface area of the tank exposed to the sun and the fact that it's not insulated causes the batch solar water heater to be less efficient than the others. It should only be utilised in mild climates due the risk of freezing in cold temperatures.
Advantages are that it's low cost and a very simple design which increases reliability. This is also one the easiest DIY solar collector installations due to its simplicity. Diagram courtesy of EERE.
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