Solar Thermal


Other Parts

Assembly Guide

Installation Guide




Other Parts

Installation Guide

LED Lighting

Heat Pumps

Rainwater Harvesting

Solar Overview












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.

Alternating current.  An electricity supply where the voltage and polarity varies regularly, in the case of a domestic supply, which is AC, 50 times per second.  See frequency.

Antifreeze solution
See Propylene Glycol

Aperture Area
The projected area of the outer tube of the evacuated tube assembly.

Group of pv panels connected together a a single electrical and/or mechanical entity.

Building Control
Local authority department which regulates and certifies building work.  Generally do not need to be consulted for solar thermal, but probably do for PV.  See your local authority website for contact details.  Note that they are separate from the planning department, which decides whether a particular development is a good idea.

Fundamental component of a pv module.  It is a single slice of silicon with wiring attached to allow a current to flow.

Direct current.  An electricity supply where the polarity is fixed.  PV panels produce DC

Acronym for domestic hot water, what comes out of the hot tap.

Acronym for Distribution Network Operator, which is officialese for the operator of your local electricity distribution network..

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.

Feed in Tariff
Procedure used by power companies and governments to incentivise pv installation using enhanced payments to the householder for energy produced.

The rate at which AC electricity switches polarity (reverses direction).  Measured in Hertz (Hz), which are cycles per second.  European domestic supplies are 50Hz

This is the designation given to a set of rules and procedures covering SSEG's.  Generally limited to 16A/phase (about 3.5kW in most domestic situations), DNO's have discretion to use G83/1 for slightly larger installations, rather than the more onerous G59/1.  In other words if you want a larger installation it is worth asking your DNO to consider it under G83.

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.

Gross Generation Meter
Fitted to the output of a pv array to record the total amount of energy generated.  Additional to the utility meter, and required for participation in most tariff schemes. 

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.

Used in photovoltaic installations, an abbreviation for Short Circuit Current, Isc is the maximum current produced by a pv array (or the part of the array in question) under standard test conditions (sometimes Isc(stc) is used).  It is a figure used as the basis of calculations to specify cables, switchgear etc.  See also Voc.

Synonym for PV panel.  Consists of a number of cells connected together, generally laminated under glass, mounted in a frame, with connecting leads attached.

Acronym for Maximum Power Point Tracking.  Used by inverters and sometimes some other devices in pv systems to manipulate current and voltage to maximise power output.

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  The same unit can be used to describe any type of energy, for instance in the form of hot water from solar thermal installations or electricity produced by pv systems.  See here for more about this.

Propylene Glycol
In UK solar thermal 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 thermal circuit, including flow setter, pump, non return valve, pressure relief valve, pressure gauge, fill and drain valve and differential temperature controller.

Abbreviation for photovoltaic.  PV panels convert light (photons) into electricity (volts) using a clever process we won't go into here.

Feed in Tariff
This is a scheme whereby electricity companies pay householders a premium rate for electricity they produce.

Selective Coating
This is the clever part of the solar thermal 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 in a solar thermal system 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 a solar thermal system 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.

Acronym for Small Scale Embedded Generators, used in officialese to mean pv array, wind turbine etc.  Small, in this case, means up to 16A per phase, which limits array size to around 3.5kW

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

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.

Device to enable a solar collector to always present the optimum orientation to the sun.  Generally not financially viable for solar thermal, but can be beneficial for pv.  Can increase yield over a fixed collector by 20-60%

Used in photovoltaic installations, an abbreviation for Open Circuit Voltage.  Voc is the maximum voltage of a pv array (or the part of the array in question) under standard test conditions (sometimes Voc(stc) is used).  It is a figure used as the basis of calculations to specify cables, switchgear etc.  See also Isc.



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Last modified: 30-06-10