Comparison of Flat Plate And Vacuum Tube Collectors

A long-standing debate exists between these two technologies. Part of this can be attributed to the physical structure of evacuated tube collectors, which have discontinuous absorbance areas. Each collector tube is arranged on a rooftop with evacuated tubes, with two concentric glass tubes occupying the space between the collector tubes and two open (vacuum-filled) spaces. As a result, the square meters of roof area covered by evacuated tubes (total collector area) is greater than the area containing actual absorption (absorber plate area). If evacuated tubes are compared to flat-plate collectors based on the roof area they occupy, different conclusions may be reached than if absorber area were compared. Furthermore, the ISO 9806 standard[7] specifies how the efficiency of solar thermal collectors should be measured, but is ambiguous, since they can be measured either in terms of total area or absorber area. Unfortunately, output power is not given for thermal collectors, as it is for photovoltaic panels. This makes it difficult for buyers and engineers to make informed decisions. [Doubtful - Debate] [Doubtful - Debate]

Comparison of the energy output (kW.h/day) of a flat-plate collector (Heat S42-P [Doubtful - Debate], blue line, absorption 2.8 m2) and an evacuated tube collector (Green line, SunMaxx 20EVT [Doubtful - Debate], absorption 3.1 m2. Data obtained from an online signet ring cell document [Doubtful - Debate]. TM-Ta = temperature difference between the collector and water at ambient temperature. Q = solar radiation during measurement. First, as TM (Ta) increases, flat-plate collectors lose efficiency more rapidly than evacuated tube collectors. This means that flat-plate collectors become less efficient at producing water at temperatures higher than 25°C above ambient (i.e., to the right of the red mark on the graph). [Doubtful - Second, evacuated tube collectors generate significantly more energy under overcast conditions than flat-plate collectors, even though the output of both collectors drops off strongly under overcast conditions (low solar radiation). Many factors prevent extrapolation from the two. The basic relationship between the efficiency of the two different collector technologies remains valid (doubtful - discussed). Field testing demonstrated the flat-plate collectors exhibiting the differences outlined in the diagram at left. Similar sized evacuated tube collectors were installed adjacent to each other on the roof, with pumps, controllers, and storage tanks. Several variables were recorded during the day with intermittent rain and cloud cover. Green line = solar radiation. The top maroon line indicates the temperature of the evacuated tube collector. Therefore, the pump circulation slows significantly and is stopped for approximately 30 minutes during the cooler part of the day (low radiation), indicating a slower rate of heat recovery. The temperature of the flat-plate collectors dropped significantly during the day (low purple line), but as radiation increased, Cycling resumed later in the day. The temperature of the reservoir of the evacuated tube system (dark blue graph) remained constant, while the flat-plate system (blue graph) increased by 8 degrees Celsius during the day. Courtesy ITS-Solar [who?]

Flat-plate collectors typically lose more heat to the environment than evacuated tube collectors, and this loss increases with the temperature difference. Therefore, they are usually an inappropriate choice of solar collector for high-temperature commercial applications such as process steam production. Evacuated tube collectors have a lower absorption plate area to total area ratio (typically 60-80% of the total area) compared to flat plates. (In early designs, the absorber area accounted for only about 50% of the collector panel. However, this has changed as technology has advanced to maximize absorber area.) Based on absorber plate area, most evacuated tube systems are more efficient per square meter than comparable flat-plate systems. This makes them suitable when roof space is limited, for example, when the number of building occupants is greater than the number of square meters of suitable and available rooftop area. Generally, evacuated tubes provide slightly more energy per installed square meter when ambient temperatures are low (such as in winter) or when the sky is overcast for extended periods. However, even in areas without much sunshine or solar heat, some low-cost flat-plate collectors can be more cost-efficient than evacuated tube collectors. Although several European companies manufacture evacuated tube collectors, the evacuated tube market is dominated by East Asian manufacturers. Several Chinese companies have a long track record of 15 to 30 years. There is no clear evidence that the two collector technologies (flat plate and evacuated tube) differ in long-term reliability. However, evacuated tube technology (especially newer variants with sealed heat pipes) is still young and should demonstrate comparable equipment lifespans compared to flat plate collectors. Evacuated tube modules, for example, can be advantageous in terms of scalability and maintenance if the vacuum in a particular tube decreases.

This chart shows the normal operating range of a solar domestic hot water system, with evacuated tubes outperforming flat plate collectors by 120°F above ambient (shaded in gray). [8]

For a given light absorption area, evacuated tubes can therefore maintain efficiency over a wide range of ambient temperatures and heating requirements. In most climates, flat plate collectors are generally a more cost-effective solution than evacuated tubes. Instead, they are used in arrays measured per square meter. Efficient but expensive evacuated tube collectors have a net benefit in winter and can also offer real advantages in summer. They are well suited to cool ambient temperatures, perform well in consistently low sunshine conditions, and provide more consistent heat per square meter than flat-plate collectors. On the other hand, small-volume mediums (i.e., TM-TA) are more efficient at heating water than flat-plate collectors. Domestic hot water often falls into this medium category. Glazed or unglazed flat collectors are suitable for heating swimming pool water. [9] Unglazed collectors are suitable in tropical or subtropical environments if domestic hot water needs to be heated below 20°C. Contour maps can show which type is more effective (both in thermal efficiency and energy/cost) for which region. There are other differences in addition to efficiency. EHPTs' heat pipes act as thermal one-way valves. This also indicates their inherent maximum operating temperature, which may be considered a safety feature. They have less air resistance, allowing them to be placed on rooftops without tethering. They can also collect heat radiation from bottom to top. Tubes can be replaced individually without shutting down the entire system. There is no condensation or corrosion inside the tubes. One hurdle to widespread adoption of evacuated tube collectors in some markets is their inability to pass the internal thermal shock test required for durability certification in ISO 9806-2 Part 9, Class B. [10] If unprotected, evacuated tube collectors are exposed to the cold water tubes for too long before being filled with the tubes, which can cause damage due to sudden temperature changes in full sun. There is also the issue of vacuum leaks that affect their lifespan. Flat panels have been around for much longer and are less expensive. They may be easier to clean. Other properties, such as appearance and ease of installation, are more subjective.

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