Flat Plate Vrs. Evacuated Tube
Solar Collector Performance
When designing active solar energy systems, it’s important to be able to predict the thermal performance
of solar collectors over a wide range of operating conditions. One method of expressing the thermal
performance of a collector is a numerical value for thermal efficiency, which is the ratio of the
instantaneous heat output from the collector divided by the rate solar radiation strikes the panel. It is similar
to the thermal efficiency of a boiler in that it states the desired output quantity (collected heat) as a
percentage of the required input quantity (solar “fuel”).
The thermal efficiency of a collector changes whenever the fluid inlet temperature, the ambient air
temperature or the intensity of solar radiation striking it varies. To account for these factors, the thermal
efficiency of a solar collector is typically expressed graphically, as shown in figure 15.
Comparing Solar Collector Performance
Given the different construction of flat plate and evacuated tube solar collectors, it’s reasonable to ask
which type is better. There is no simple answer to this question. The collector with the greatest heat
collection potential depends strongly on the specific application in which the collector will be used. Beyond
thermal performance, the designer must also weigh factors such as differences in roof area requirements,
maintenance requirements, ability to shed snow and the type of freeze protection options available for each
type of collector.
From the standpoint of thermal performance only, the collector with the best performance depends on the
temperature required by the load the system supplies. This is demonstrated by comparing the three
collector efficiency lines shown in figure 16.
This graph is based on a sampling of performance ratings for different types of collectors as determined by
the SRCC (Solar Rating and Certification Corporation). It shows that the collector with the highest thermal
efficiency depends on the value of the inlet fluid parameter, which itself depends on collector inlet fluid
temperature, outdoor air temperature and solar radiation intensity.
If the load is a swimming pool where water temperature is at or just above ambient air temperature, an
unglazed flat plate collector will provide the highest thermal efficiency. This is the result of two factors:
First, the incoming solar radiation is not attenuated by passing through a glazing; secondly, there is very
little if any heat loss from an absorber plate operating close to ambient air temperature.
However, as the load temperature increases, an unglazed collector rapidly loses efficiency relative to a
glazed/ insulated flat plate collector. At even higher inlet fluid temperatures, an evacuated tube collector,
with its very low heat loss characteristics, retains higher thermal efficiency than a glazed flat plate collector.
The only accurate way to compare seasonal performance of flat plate versus evacuated tube collectors is
through computer simulation based on a specified load in a specified climate. Other issues differentiate flat
plate and evacuated tube collectors.
Flat plate collectors typically have a higher ratio of absorber plate area per square foot of collector
enclosure compared to evacuated tube collectors. This means evacuated tube collectors usually require
more roof area to accommodate a given amount of absorber plate surface.
Because of their manifold design, most current generation evacuated tube collectors must operate with
an antifreeze solution and are not suitable for other types of systems.• Evacuated tube collectors tend to
be more expensive than flat plate collectors on a dollar per square foot of absorber plate area basis.
Flat plate collectors cannot produce water temperatures as high as those possible with evacuated tube
collectors. This may or may not be an issue, depending on intended application.
Some flat plate collectors can be architecturally integrated into roofs to minimize their visible profile.
Flat plate collectors sloped at 40º or more shed snow sooner than evacuated tube collectors mounted at
the same angle. The low heat loss of evacuated tube collectors increases the time needed to warm snow
to the point where it will slide from the tubes.
Additional Information: Sunny Days - Demystifying Solar Heating
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