Smaller angles of inclina-
tion increase the yield in summer, whereas larger
angles increase yields in winter.
Angle of inclination
Angle to the vertical of solar radiation hitting col-
lectors and PV modules. The optimum angle of
inclination depends on the application. For solar
thermal transfer systems, it is approximately 45º,
and for photovoltaic systems about 30º.
Auxiliary heating
Hot water supply and auxiliary heating systems
are the two areas in which the use of solar col-
lectors is particularly effective and economical.
Both types of system can be efficiently supported
with solar combination storage tanks.
Azimuth
Collectors and PV modules are aligned to exploit
the solar radiation effectively for maximum yield.
The azimuth is the angular deviation of the sur-
faces of a collector or PV module from true south.
The angle of inclination is the deviation from the
horizontal.
C
Collector
This device uses an absorber to convert solar
radiation into heat, which it passes to a heat
transfer fluid. In addition to the storage tank and
control system, solar collectors are the most im-
portant components of a solar thermal transfer
system.
Collector circuit
Circuit of a solar thermal transfer system, in which
a heat exchanger in the storage tank is supplied
by the collector using a pump with safety devices
and valves.
Convection
Internal and external motion of air against the
collector. The efficiency is reduced by collector
convection losses.
D
Diffuse radiation
Component of solar radiation that reaches the
Earth's surface indirectly as a result of being
scattered from air molecules, dust and water va-
por.
Direct radiation
Radiation that strikes the Earth's surface without a
change in direction. This is the higher energy
component of solar radiation.
E
Efficiency
Ratio of the usable energy to the input energy.
Emission
Radiation of sunlight from a body. A collector ab-
sorbs 95% of the solar radiation and emits 5%.
Expansion tank
See membrane expansion tank.
F
Flat plate collector
The absorber of this type of collector is accom-
modated in a flat, thermally insulated housing with
a transparent glass cover. Seite 2
G
Global radiation
The solar radiation striking a horizontal surface on
the Earth's surface.It has direct and diffuse com-
ponents. The maximum value (20 year mean) in
Germany is more than 1320 kWh/m
2
.
H
Heat exchanger
The heat exchanger transfers the energy ab-
sorbed by the collector from the primary (or col-
lector) circuit to a secondary (water supply or
heating) circuit.
Heat transfer fluid
Fluid that absorbs the heat from the collector and
conveys it to the heat exchanger, where it is then
transferred to the water supply. Glycol antifreeze
is added to the water used as heat transfer fluid.
I
Integrated roof installation
This involves integrating collectors or PV modules
into roofing.
K
Kilowatt hour (kWh)
Unit of electrical work (energy consumption). It is
the product of the connected load and time.
1 kWh = 1000 watts (W) for one hour (h).
k value
Coefficient of heat loss of a collector in W/m²K.
The k value describes the influence of the design.
L
Level of solar coverage
The proportion of the energy demand covered by
the solar energy system.
Low flow
Lower flow rate of a heat transfer fluid per unit
area of collector when higher temperature differ-
ential reached across collector than with normal
flow.
M
Meander tubing
Continuous (without joints or branches) tube on
the back of the absorber.
Membrane expansion tank
This closed metal vessel contains two fluids flexi-
bly separated by an elastic diaphragm. One com-
partment contains nitrogen under an initial pres-
sure, and the other the solar fluid that flows into
the expansion tank when its pressure rises as it is
heated.
O
Optical efficiency
Indicates the proportion of the incident solar ra-
diation converted into heat by the collector.
Optical losses
The fraction of the solar radiation that cannot be
absorbed by the absorber. Losses are caused by
absorption in the glass and by reflection.
Output
This is the energy (electrical power) made avail-
able within a defined time. The unit is the watt
(W).
R
Regenerative energy
The energy sources or fuels that renew them-
selves naturally on a human timescale are called
regenerative or renewable energy. This concept is
in direct contrast with fossil (coal, petroleum and
natural gas) and nuclear fuels (uranium), which
have been formed by geological processes over
millions of years.
Renewable energy legislation (EEC)
Legislation governing the feeding of surplus, re-
generatively produced electricity back into the grid
and means of payment. Seite 3
Renewable energy sources
The sources of energy that are available indefi-
nitely on a human timescale (solar, geothermal,
tidal and biomass).
Roof integration
One method of installing solar energy systems is
to integrate them into the roof. The roof and solar
energy system then form a single unit. The col-
lectors or PV modules replace the normal roofing
(tiles, slates etc.).
S
Selective coating
Coating on the absorber that allows absorption of
a high proportion of the incident energy whilst
reducing emission.
Shading
Shading of solar energy systems by roof super-
structures or dirt leads to lower yields.
Solar
(Solar, L. solaris: pertaining to the sun).
Solar coverage
See level of solar coverage.
Solar energy installation
System for converting solar radiation into usable
energy in the form of heat or electricity.
Solar storage tank
See storage tank.
Solar thermal transfer
The conversion of solar radiation into directly us-
able heat (also covers cooling).
Solar thermal transfer system
System for converting solar radiation into usable
energy in the form of heat (also covers cooling).
Stagnation
Refers to the situation when the heat transfer fluid
does not flow in a collector system in the pres-
ence of solar radiation. The heat transfer fluid
evaporates and is forced into the expansion tank.
Standstill temperature
Temperature in the collector at the hottest point
on the absorber when no energy is being ex-
tracted.
Storage tank
Since energy is rarely available from the sun ex-
actly when heating is required, the heat produced
by a solar system has to be stored. Solar thermal
transfer systems use hot water storage tanks for
this purpose.
System utilization
Ratio of solar yield to total incident radiation. This
is a measure of the overall efficiency of the solar
energy system.
T
Temperature differential control
Controlling a solar thermal transfer system ac-
cording to the ratio of the collector temperature to
the temperature in the bottom section of the stor-
age tank.
Temperature stratification
Stratification of temperatures developed in the
solar storage tank, which gradually increase from
the bottom to the top.
Transparent thermal insulation
A translucent insulating material on a solid, black
storage wall with good thermal conductivity allows
efficient passive use of solar radiation. The sun-
light penetrates the transparent insulation, but is
absorbed by the black painted surface of the exte-
rior wall. The heat flows straight through the ma-
sonry and into the rooms to be heated.
Tubular collector
See vacuum tube collector.
V
Vacuum tube collector
The absorber of this design is in an evacuated
glass tube to achieve useable temperatures of up
to 150ºC that are useful in process heating appli-
cations. For hot water supply and auxiliary heating
systems, the characteristic values of flat plate and
tube collectors are virtually identical. Seite 4
W
W
Abbreviation for watt the unit of electrical
energy.