Infos Utiles
1. Solar Glossary
Here are a list of some terms you may encounter when reading through our website.We have tried to make explanations as easy to understand as possible, but if you are still un-clear please feel free to Contactez-nous.
A-B-C-D-E-F-G-H-I-M-P-S-V-W
A
Absorber
The part of the collector that actively absorbs the light rays. For solar tubes this is defined as the cross-sectional area of the inner tube (selective coated) measured using the outside diameter. (E.g. 0.047 x 1.72m = 0.08m2) This value is used when calculating efficiency values. For solar 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 supposed to reflect light onto underside of the vacuum tube.
Aperture
The part of the collector through which light enters. For vacuum tubes this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter. (E.g. 0.0548m x 1.72m = 0.094m2). 1.72m is the exposed length of the vacuum tube.
Antifreeze solution
Refer to Propylene Glycol
B
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
C
Collector
A solar collector is not really a solar water heater. A solar water heater is a 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 = (℃x 1.8) + 32
Celsius = (℉ - 32)/1.8
D
Delta-T Controller
Delta-T refers to the difference in two temperatures. This term is often use in relation to a solar controller. In such case the Delta-T is the difference between the solar collector temperature and the temperature of the water in the solar storage tank. A Delta-T controller can be configured to turn on the pump when the Delta-T difference exceeds a certain level (Eg.7oC / 12.7oF) and off again when the temperature difference drops below another setting (E.g. 2oC / 3.6oF). The controller turns on the pump when there is heat potential in the manifold. A Delta-T controller can also be used to provide freeze protection by circulating warm water from the tank through the manifold when the manifold temperature drops below 5oC.
DHW
Acronym for domestic hot water, what comes out of the hot tap.
E
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. E.g. If performance values are based on gross area, then the gross area must be used when determining total heat output. IAM values have a significant influence on actual heat output throughout the day, and should be considered. Looking at just the percentage efficiency value will not give a true indication of daily heat output.
Efficiency testing is usually completed by testing bodies such as SPF, SRCC and other government approved testing bodies.
Tm* is the x axis value on performance graphs for solar collectors.
Tm* is calculated as:
(water temp - ambient temp)/Insolation
E.g. (44oC - 20oC)/800Watts = 0.03
Expansion Vessel
A vessel designed to allow for thermal expansion of the solar fluid, and also to accommodate the solar fluid in a stagnation situation. It comprises a container divided by a flexible membrane, with pressurized gas on one side, and open to the solar fluid on the other. As the volume of the solar fluid changes due to thermal expansion, the membrane moves to allow for this.
F
Flow Rate
The volume of water flowing through plumbing in a given period of time. Usually measured in volume/minute or volume/hour. 1 Liter/min = 0.264 US Gallon/min
G
Getter
A coating of a barium compound used inside the vacuum tube to soak up gas molecules present during manufacture and thus maintain the integrity of the vacuum. It has the secondary function of visually indicating that the tube is functional, since the coating changes from shiny silver to a powdery white color should the vacuum be lost due to breakage.
Gross Area
The total surface area of the collector including the frame, manifold and absorber. This area is often used when comparing collectors, but a better comparison to use is value for money. Roof size is not usually a limiting factor for domestic solar water heating installations, so the size of the collector is not really that important.
H
Heat Pipe
An vacuum copper rod or pipe filled with a bit of non-toxic medium liquid used for heat transfer.
Heat Dump
A heat dump circuit is used to waste excess heat, for example when a solar equipped house is left vacant in summer. It usually takes the form of a branch of the solar circuit which bypasses the heat exchange coil in the hot water cylinder, and passes the solar fluid instead through a small radiator, typically located in an attic or garage. It may be operated by a motorized three port valve actuated by the differential temperature controller when the temperature of the water in the hot water cylinder reaches a preset maximum.
I
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 vacuum tubes, only have 100% efficiency at midday (0o), whereas INLIGHT solar 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.
Indirect System
The term indirect refers to the use of a heat exchanger. Instead of the actual water to be used (DHW, swimming pool water etc) being heated by the solar collector, a secondary circuit of solar fluid is heated and this fluid transfers heat to the water to be used. This has many advantages including reduced scale build up, frost protection and better control.
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.
Insulation
The ability to protect against transfer of heat/cold. Solar water tank and vacuum tube manifold use integrated polyurethane foam to prevent heat loss. Heatpipe Collecteur solaire use compressed glass wool (or aluminum silicate) together with polyurethane foam to insulate the flow passage from heat loss. Glass wool has excellent insulation properties, is very light 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.
Irradiance / Irradiation
Basically the same as Insolation - explained above.
M
Mixing Valve
Also known as a tempering valve, it can be fitted to the outlet of a hot water cylinder to mix the hot water with cold water to automatically maintain a suitable temperature at the hot tap, removing the risk of scalding. It allows the hot water to be maintained at a higher temperature than would otherwise be safe, effectively increasing the amount of heat stored.
P
Power
The power output of the collectors is given in Watts. Note that this figure (E.g. 900W) is an instantaneous figure. 900W fo\R5]; JN Nr 7.5 hours would provide 6.75kWh. 6.75kWh is an amount of energy, sufficient, for example to heat 100L of water by 58°C.
Pressure
Refers to the water pressure in the system. The conversions for the most commonly used units are: 1 bar = 1.02kg/cm2 = 14.5psi = 100kPa = 0.1Mpa = 10m water head
Propylene Glycol
In European installations the solar fluid usually contains between 25 and 50% propylene glycol as an antifreeze additive. Propylene glycol is related to the commonly used vehicle antifreeze Ethylene glycol, but it differs in that it is non-toxic, which is a requirement to guard against risk should a heat exchange coil in a hot water cylinder fail.
S
Selective Coating
This is the clever part of the solar collector, applied to the inside of the inner vacuum tube. Its function is to absorb the solar radiation while minimizing radiation emitted. Most materials good at absorbing radiation are equally good at emitting it, and vice versa. The selective coated applied to INLIGHT vacuum tubes absorbs around 94% of incident radiation and emits approximately 6%.
Solar Fluid
The function of the solar fluid is to transfer heat from the collector to the water in the hot water cylinder. It is usually a mixture of water, propylene glycol and corrosion inhibitors. The glycol allows temperatures of over 100°C without boiling of the fluid.
Solar Fraction
The solar fraction is the proportion of hot water heated by solar compared with total hot water use, expressed as a percentage. Most installations aim for around 60-70%. Installing more panels will raise the solar fraction, but with diminishing returns, and also create more waste heat in the summer months which must be dissipated.
Stagnation Temperature
This is the temperature that the collector can reach if no solar fluid is removing heat from the collector. It is governed by the balance of heat input from the sun and heat lost to the environment. In the case of INLIGHT collectors it is in excess of 200°C.
SWH
Acronym for solar water heater.
SWHS
Acronym for solar water heating system.
V
Vacuum Tube / Vacuum Tube
The insulating element of the INLIGHT solar collector. It consists of two glass tubes, one inside the other, made from a single piece of glass. A vacuum between the tubes effectively eliminates heat transfer by conduction and convection. See here for more information.
W
Workstation
A workstation is a convenient package of commonly used components required for a solar circuit, including flow setter, pump, non return valve, pressure relief valve, pressure gauge, fill and drain valve and differential temperature controller.