Solar Water Heater Choices: Flat Plate or Evacuated Tube Collectors?

In our Guide to Choosing a Solar Water Heater, we talked about different types of solar water heaters such as active, passive, thermosyphon, and drainback systems. In this article, we discuss another key component of any solar water heating system: solar thermal collectors. These solar energy products are important as resellers often classify solar water heaters based on collectors. In this learning article, we compare and contrast the two main kinds of solar thermal collectors: flat plate heaters and evacuated tube heaters.

Related articles:

Guide to Choosing a Solar Water Heater

Solar Thermal Water Heaters: Highly Cost-Effective Renewable Energy

What is a solar collector?

Energy from the sun comes to Earth in the form of electromagnetic radiation as infrared, visible and ultraviolet light. A solar thermal collector, also just called a solar collector, is a device that collects heat by absorbing sunlight. It is one of the key devices in a solar water heating system. There are two main kinds of collectors, solar flat plate collectors and solar evacuated tube collectors.

Solar flat plate collectors are more commonly used. In these devices a glazed flat-plate collector is mounted on insulated, weatherproofed boxes fitted with a dark absorber plate under one or more plastic or glass covers (known as glazing). Solar evacuated-tube collectors, on the other hand, contain rows of tubes which are constructed by encasing metal absorber tubes inside transparent glass tubes. The air in the space between the two tubes is removed to create a vacuum, which prevents convective or conductive heat loss. In the rest of this learning article, we compare based on various criteria these two major solar thermal collectors for solar water heating.

Cost

Cost is obviously one of the most important factors when choosing a solar water heater. The advantage goes clearly to flat plate collectors. Evacuated tube collectors usually cost 20%-40% more than flat plate collectors.

Efficiency

As there is no heat loss through convection and conduction in evacuated tube collectors, these systems heat water more efficiently than flat plate absorbers. Pay close attention to the term "efficiency" here, which means heating efficiency per active absorber unit, not the total device efficiency or output. Absorber area ratio and collector's gross size will determine the active area together. Although evacuated tube collectors have a higher efficiency, especially in winter, flat plate collectors can match their efficiency during summer when there is a small difference between the temperature of the heated water and that of the ambient environment.

Absorber area ratio

When describing a collector, gross size is often used, like 4 x 7 sq ft. This is the size of the whole device. However, a more important factor is the size of the absorber in which the liquid is actually heated, which is the active absorbing area.

You should look at the absorber area ratio. This is defined as the absorber size divided by the gross size. The ratio for flat plate collectors is generally above 90% while the ratio for evacuated tube collectors usually stays between 60%-80%. Therefore two collectors with the same gross size may have very different overall efficiencies depending on whether they are flat plate or evacuated tube. The flat plat collector, though it is less efficient per absorbing unit, may perform just as well as its competitor due to a larger absorber area.

Overheat and freeze protection

In an evacuated solar water heater, there is no risk of freezing. Even if the device uses water, the system's design makes freezing impossible. However, overheating is a serious issue because you can't drainback the evacuated tubes the way you do with a flat plate collector. To prevent overheating in an evacuated solar water heater, you need to purchase a heat dump.

Sensitivity

Cylindrical evacuated tubes can absorb sunlight efficiently from a wide range of directions due to their shape. Throughout the day the incoming light passes through the collector fairly continuously at different angles. So evacuated tubes collectors work more efficiently in cloudy or rainy days. In contrast, flat plat collectors are more sensitive to the direction of incoming radiation. These systems need racking systems to optimize their tilt angle.

Performance in snow

Though low heat loss in evacuated tubes is usually seen as an advantage, it can actually harm the water heater's performance in snow. The high insolation of these systems prevents excess heat from melting snow or frost on the collector. Flat plate collectors on the other hand, are less efficient and therefore release more heat, melting snow quicker than evacuated tube systems. Since the efficiency of the water heater will be reduced by snow cover which prevents sunlight transmission, flat plate collectors have an edge in snowy areas.

Installation

Installation of flat plate collectors is difficult. The heavy collectors, which take the shape of panels, have to be hoisted onto the roof and mounted. Meanwhile evacuated tube collectors, which use lightweight individual tubes, are much easier to assemble.

Durability

Convection generates corrosion. Since there is no air in evacuated tube collectors, there can also be no convection in these devices. This makes them more durable than flat plate collectors. Nowadays most solar thermal collectors have a 10 year manufacture warranty and a 25 year 80% power warranty.

Maintenance

Evacuated tube collectors are practically maintenance free. If a tube were to lose its vacuum, you could simply replace the component. On the other hand, if a flat plate collector is damaged, the whole plate must be replaced. This could cost you significant time and money.

Making the choice between evacuated tube and flat plate collectors

Does your head spin after contemplating the choices? We can try to make your choice easier with the following two tables. The first concisely compares the two solar water heaters types.

Table 1: Comparison of the characteristics of flat plate and evacuated tube solar collectors