Optical Efficiency (Zero-loss coefficient)
Optical efficiency or the zero loss coefficient refers to the amount of
radiation absorbed by the panel if the panel is at the same temperature as the ambient surroundings.
i.e. it is not losing or gaining any HEAT through the walls/back of the collector.
Simply speaking it percentage of light (over the whole sprectrum) emitted through the glass face.
Solar radiation when it arrives on earth is made up of different wavelenghts,
from Ultra Violtet to Infra-Red. Plate (standard Window) glass is very good at
letting through visible radiation, but reflects Infra-Red and Ultra-Violet.
Because of this only about 60% the energy in the spectrum is transmitted through window glass.
Solar panel glass is normally made up with a low-iron content glass,
this makes the glass far more transparent to Infra-red light as well as the visible
radiation, this can increase the amount of energy transmitted through the glass up to
about 80%. The energy passing through the panel glazing as a percentage of the
overall energy striking the panel is known as the zero-loss coefficient or the optical efficiency.
Flat Plate panels normally have a higher optical efficiency figure
than flat plates, but poorer thermal performance as the temperature of the collector rises
significantly above ambient. Recent advances with new coatings on both sides of the cover
glass has raised flat-plate optical efficiencies to around 82+%. This also improves the
Incident Angle Modifier (IAM)
characteristic by reducing the light reflected off the panel when the panel is not directly facing the sun.
An open swimming pool type collector without glazing would typically
have an optical efficiency of greater than 90%. (less than 10% of the incident radiation
is reflected). However heat is poorly retained in this type of collector and it is only
suitable for low temperature applicaptions.
As the panel heats up it begins to loose heat from the walls and the
back as well as through the front face of the panel. The hotter the panel gets in comparsion
to the ambient temperature the more heat it will lose. This reduces the efficiency of the
panel. The amount of heat each meter sqaured loses per degree above the ambient
temperature is known as the thermal efficiency. However this effect is not quite
linear because as the panel heats up, the wavelenght of emitted heat changes, this
changes the thermal characteristics of the panel. To cater for this mathematically,
two loss coefficients are used. The first a1, is by far the most important.
Loss Coefficient a1
Loss Coefficient a2
This is normally incorporated into the following formula which can be used to calculate the panel output.
The rate of heat loss by a surface depends on the wind speed above that surface:
the faster the wind speed, the more readily the surface cools. The effect of
wind is to reduce any warmer objects to the ambient temperature more quickly.
Due to the accelerated heat loss, the equivalent outside temperature is colder
and the collector will perform below its predicted level. This has implications
for siting collectors in exposed locations, and in this situation a collector
with a lower heat loss coefficient will suffer substantially less fall off in output.
Efficiency Curves of different types of panels
A sunny spring day with and output graph might look like the following,
the panel will start off in the morning at about 30C and get hotter as the
day progresses, reaching a useful maximum of 75C as the thermal store becomes fully charged.
From the graph below, the Non-Selective Flat plate, the Selective Flat Plate,
Sydney Vacuum Tube and the High Efficiency Vacuum Tube are all suitable for heating
water. The unglazed collector will not be effective in heating water beyond 30C,
which by lucky concidence is the upper temperature of a swimming pool!