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Home » Solar Basics » Solar Glossary

Solar Glossary

Here are a list of some terms you may encounter when reading through our web site.
 We have tried to make explanations as easy to understand as possible, but if you are still unclear please feel free to contact us.

A - B - C - D - E - F - G - H - I - PR 



Aperture: The part of the collector through which light enters. For evacuated tubes this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter.
 (Eg. 0.0548m x 1.72m = 0.094m2). 1.72m is the exposed length of the evacuated tube.  Aperture is the most widely used surface area size used when quoting solar collector efficiency values. 


Absorber: The part of the collector that actively absorbs the light rays.  For evacuated tubes this is defined as the cross-sectional area of the inner tube (selective coated) measured using the outside diameter. (Eg. 0.047 x 1.72m = 0.08m2).    For evacuated tube collectors with reflective panels, the entire circumferential surface area of the inner tube is often used when calculating absorber area, as the reflective panel is designed to reflect light onto underside of the evacuated tube. The Apricus AP solar collector does not use reflective panels, as they can become dirty over time reducing system performance, add significantly to wind and snow loading and can trap leaves and other debris. 



BTU: Stands for British Thermal Units. This is an imperial unit of measurement for heat widely used in the US and also in the UK.  The conversion to the metric unit kWh is: 1 kWh = 3412Btu, and for surface area values, 1kWh/m2/day = 314Btu/ft2/day.



Collector: A solar collector is just one component of a solar water heating system which may include a tank, pump, controller and solar collector panel.  A solar collector is that part of the system which absorbs the sun's energy and converts it into heat.


Celsius: The metric unit for temperature measurement.  

Convert as follows: 
Fahrenheit = (oC x 1.8) + 32          
Celsius = (oF - 32)/1.8

For Delta-T measurements the relative temperature difference is needed.  

eg. Delta-T = 7oC turn pump on, Delta-T 2oC turn pump off.  How much is that in oF?  

Convert as follows:  
Fahrenheit = oC x 1.8
      Celsius = oF / 1.8



Delta-T: Refers to the difference in two temperatures, also represented with a triangle symbol :  ∆T.  This term is often use in relation to a solar controller.  In such case the Delta-T is the difference between two temperatures, used to control a certain function such as the circulation pump turning ON and OFF to push liquid through the solar collector.   For more information about the Apricus MFC-1 solar controller, click here



Efficiency: Solar collector efficiency is usually expressed as a percentage value, or in a performance graph.  When assessing a collector's performance make sure it is based on the correct surface area values, namely gross, aperture or absorber, with aperture the most common.  

For more information about solar collector efficiency, click here.



Flow Rate: The volume of water flowing through plumbing in a given period of time. Usually measured in volume/minute or volume/hour.  In a solar water heating system, the flow rate is chosen to optimise solar collector efficiency of the optimise heat transfer.  


1 US Gallon = 3.785 Litres

1 Litre = 0.264 US Gallons

1m3 = 1000 Litres



Gross Area: The total surface area of the collector including the frame, manifold and absorber.  This area is sometimes used when comparing collectors, but aperture is more widely accepted.  It is important to ensure that when comparing two collectors you are using the same surface area measurement. 



Heat Pipe: An evacuated rod or pipe used for heat transfer. Click here for more information.



Insolation: Don't confuse this with insulation - the one letter change makes a big difference.  Insolation refers to the amount of sunlight falling on the earth. Click here to learn more


Insulation: The ability to protect against transfer of heat/cold.  Apricus AP evacuated tube solar collectors use compressed glass wool to insulate the header from heat loss.  Glass wool has excellent insulation properties, is lightweight and can withstand high temperatures, making it an ideal choice for a solar collector.   It is made from a least 80% old glass bottles and can be recycled so is very environmentally friendly.


Irriadaince, Irradiation: Basically the same as Insolation - explained above.


Incidence Angle Modifier (IAM):  Refers to the change in performance as the sun's angle in relation to the collector surface changes.  Perpendicular to the collector (usually midday) is expressed as 0o, with negative and positive angles in the morning and afternoon respectively.  Collectors with a flat absorber surface, which includes some types of evacuated tubes, only have 100% efficiency at midday (0o), whereas Apricus evacuated tubes provide peak efficiency mid morning and mid afternoon, at around 40o from perpendicular.  This results in good stable heat output for most of the day.  For more information about IAM, click here.



Pressure:  Consideration of the system operation pressure, circulation pump pressure and pressure drops are extremely important considerations in a solar hot water system design.  The conversions for the most commonly used units are: 1 bar = 1.02kg/cm2 = 14.5psi = 100kPa = 0.1Mpa = 10m water head.   Please note that atmospheric pressure at sea level is already 1bar, so a 4 bar pressure is only 3 bar above atmospheric pressure.



Reverse Thermosiphoning: This can happen in direct flow systems at night if the bottom of the storage tank is warmer than the solar collector and/or solar loop piping.  Warm water in the flow or return line close to the tank may rise up the piping, cool and flow back down.  The result is a cooling of the bottom of the storage tank.  This can be prevented by installing a check valve on the hot (from solar to tank) line in addition to the one already installed after the pump on the cold (from tank to collector) line.  A swing check valve may still allow creep flow, so ideally use a sprung check valve.  Another option is to form a U shaped heat trap in the piping close to the tank. 


If there are any words that you think we should add to this glossary, please let us know.


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