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Absorber Area
The projected area of the inner tubes of the evacuated tube assembly.
This area is covered by the selective coating, and actively absorbs the incoming
solar radiation. Note that the quoted area in the Eco-nomical panel
specification ignores the effective increase in this figure created by the
reflector.
Antifreeze solution
See Propylene Glycol
Aperture Area
The projected area of the outer tube of the evacuated tube assembly.
DHW
Acronym for domestic hot water, what comes out of the hot tap.
Evacuated Tube
The insulating element of the Eco-nomical 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.
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 pressurised 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.
Getter
A coating of a barium compound used inside the evacuated 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 a shiny silver to a powdery white
colour should the vacuum be lost due to breakage.
Gross Area
The overall size of a collector, ie how much space it takes up on your roof.
Note that there is much confusion over the way collector areas are stated (see
aperture and absorber areas), be sure to understand which is being used when
making comparisons.
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 motorised
three port valve actuated by the differential temperature controller when the
temperature of the water in the hot water cylinder reaches a preset maximum.
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.
Power
The power output of the collectors is given in Watts. Note that this
figure (eg 900W) is an instantaneous figure. 900W for 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
See here for more about this.
Propylene Glycol
In UK 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.
Pumping Station
A pump station 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.
Selective Coating
This is the clever part of the solar collector, applied to the inside of the inner evacuated tube. Its
function is to absorb the solar radiation while minimising radiation emitted.
Most materials good at absorbing radiation are equally good at emitting it, and
vice versa. The selective coated applied to Eco-nomical evacuated 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 Eco-nomical
collectors it is in excess of 200°C
Thermostatic 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.
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