Evacuated Tube Solar
Collector vs. Flat Plate Solar Collector
Newer technology, more hi-tech, rapidly improved in recent years,
already dominant in most European and Asian markets.
Flat Plates: Older technology,
moderately improved in the past 40 years, largely phased out in most
European and Asian markets but still dominant in lagging markets, including the U.S.
Heat Loss Protection:
Evacuated tubes: Heat absorber
sealed within a vacuum glass tube, like a Thermos bottle,
eliminating convection and conduction heat losses. This not only
increases system efficiency, but also allows much more reliable and
such as PV controlled solar heating systems (elimination of the
costly and most troublesome thermal sensors and controller
components) and integrated collector-tank systems in freezing
climate. It also allows the use of an existing electric water heater
to also serve as a solar water tank with the addition of an
inexpensive external heat exchanger. Evacuated tube collectors can
heat water to much higher temperature and when used with an
automatic tempering valve (to mix hotter water with cold water to
achieve a fixed temperature water [commonly 120 degrees]), one can get more hot water from
the same water tank. This saves money by not needing to purchase a
separate solar water tank, and also saves valuable space (quite
often there is no space to add another water tank near the existing
Flat Plates: No vacuum or real
thermal insulation and the
heat absorber breaths air. Large heat losses due to conduction
and convection, especially in winter when heat is most needed. Half
or more heat could be lost from the large collector surface area
(~30 SF per panel) when the collector is hot and the air is very
cold and windy.
Because of the large heat loss, it is not possible to use some
more elegant, reliable, efficient, and cost effective
designs, e.g, a PV controlled solar heating system will not work
well because when the plate is not hot enough in partially sunny
winter time and the PV produces electricity to drive the pump to
circulate fluid, heat from a hot water tank will be transferred
to the collector and lost to the cold air. An integrated
collector-tank system will not be possible in freezing climate
because the heat lost in the flat plate collector will lead to
freezing and collector damage.
Because of large heat loss, it is generally not a
good idea to connect flat plate collectors to an existing water
heater because flat plate collectors cannot heat water to higher
temperature (the higher the temperature, the more heat loss from
flat plate collectors to the air). Whenever hot water is
used and the tank water temperature drops below a set temperature
(~120 degrees), the electric water heater will turn on to heat
the water up to the set temperature. The already hot water tank
will greatly reduce the heating efficiency of the flat plate
collectors. When the tank water is warmer than the flat plate
collectors, the system will also be shut down and no more heat
can be transferred from the collectors to the tank.
Durability and Reliability:
The vacuum protects the heat
absorber surface from moisture, condensation, corrosion, and
degradation of the selective coating.
Heat pipe is used for super-efficient heat conduction and transfer.
No fluid ever touches the solar collectors, eliminating corrosion
and efficiency losses. If
one or more tubes are damaged, the system still works and the
broken tubes can be easily replaced without shutting down the
system. It's similar to replace a fluorescent light tube.
Heat absorber breaths air and
the selective coating is subject to moisture, condensation,
corrosion, and degradation. Fluid is
circulated throughout the collector. This makes them more prone
to leakage, corrosion, and restricted flow
When a small portion of a collector panel fails, the entire
system must be shutdown and the whole collector must be
Ease of Installation and Service:
One person can easily install
evacuated tube collectors without a hoist or special tools
because the tubes, manifold, and frame can be brought onto the
roof separately and assembled easily.
The cylindrical glass tubes absorb the sun's radiant heat energy
at a perpendicular angle no mater what position the sun is at.
Thus they are relatively adept at accepting most installation
angles, allowing for more freedom and aesthetics in the system
design and installation. Also the heat produced by the evacuated
tubes is more uniform throughout the day.
Since the evacuated tubes are
installed in the manifold via dry connection, any tube can be
removed without shutting the system down.
Each plate is a heavy and bulky one piece and frequently needs a
basketball team to lift and installed on the roof - see pictures
Flat plate collectors need to be placed directly facing the
south, and at roughly the same angle as the location's latitude.
This means there are far fewer options for the installation, and
aesthetics must often be sacrificed in order to protect
performance. Also the heat produced by flat plates varies greatly throughout the day - substantially lower in the
morning and afternoon and peaks at noon when heat demand is
generally the lowest.
Sometimes those who are used to the old technology and are reluctant to learn a new one or don't wish to make more effort in finding and ordering evacuated tube collectors
will try to persuade customers to go with flat-plate collectors by saying that flat-plate collectors have higher efficiency for
most water heating based on a well-known textbook graph showing that
heating efficiency is higher for flat plat collectors when the solar
heating fluid is no more than 70 degrees above air temperature.
First of all, the graph is no longer valid as the data are outdated by many years.
Evacuated tube technology has rapidly improved in recent years and heating efficiencies have increased ~50% than those depicted in the textbook graph whereas flat plate efficiencies have increased just slighly. Secondly, to heat water to 120 degrees for general domestic hot water use, the solar heating fluid
needs to be above 140 degrees because there is efficiency loss in the heat exchanger
plus heat loss from the long pipe. Winter air temperature is often 30-40 degrees
or lower. That means the solar heating fluid temperature generally needs to be 90 degrees above the air temperature, and this leads to great heat loss and makes flat plate collectors far less efficient than evacuated tube collectors even based on the textbook graph. It is also common sense that a large un-insulated surface area (~30 SF for
a single flat plate panel, larger area for multiple panels) will lose large amount of heat in the winter and its heating efficiency
is obviously substantially lowered when heat is needed the most. It takes time to phase out flat plate collectors just as it takes time to phase out incandescent light bulbs and replace them with the much more efficient compact fluorescent or LED light bulbs.
SolarPlusGreen LLC, 1104 Hendersonville Rd,
Asheville, NC 28803;
see location map.
Ph: 828-301-2021, Fax: 828-277-1240;