Evacuated Tube Solar Collector vs. Flat Plate Solar Collector

Technology:

Evacuated tubes: 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 is 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 cost-effective designs, 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 water heater),

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:

Evacuated tubes: 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.

Flat Plates: 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 replaced.
 

Ease of Installation and Service:

Evacuated tubes: 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.

Flat Plates: Each plate is a heavy and bulky one piece and frequently needs a basketball team to lift and installed on the roof - see pictures below:

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; office@SolarPlusGreen.com