In the last Solar Design Notebook (August 2008, page 14), we compared the thermal efficiency of flat plate and evacuated tube collectors. To recap: Either collector can have the higher efficiency depending on the instantaneous operating conditions. At lower water temperatures, flat plate collectors usually have the higher efficiency. At elevated temperatures, evacuated tubes hold the performance edge.
Ultimately, the question is not which collector has the better efficiency, but rather, which collector yields the greatest harvest of solar energy (all other conditions being equal)? Answering this question requires analysis that runs a specific solar thermal system through an entire operating season, and subjects it to the full range of ambient temperatures, incident solar radiation, and system fluid temperature variations. This will be the subject of the December Solar Design column.
While performance numbers are interesting, and certainly play a part in collector selection, they don’t tell the whole story. This month we’re going to look at several qualitative factors that may just tip the scales in the decision process.
WeightOne potential advantage of evacuated tube collectors is that they don’t have to be hoisted on a roof as a single assembly. The header/frame assembly is first installed, and each tube is then inserted and clamped into the header. This reduces the weight that must be lifted to the mounting surface at a given time. In contrast, a large (4 by 10 foot) flat plate collector can weigh more than 150 pounds and is usually hoisted into position without any disassembly. While two installers can usually manage to lift such a collector off a truck and carry it around a jobsite, getting it safely on a roof, or delicately maneuvering it onto brackets fastened to a steeply pitched roof can be a challenge without a crane. Be sure you plan the installation logistics carefully.
Snow SheddingImagine a high-quality Thermos® bottle filled with hot coffee laying on a roof during a snowfall. Due to its very low rate of heat loss, it’s more than likely that bottle will be covered with snow, and may stay covered until thawing weather conditions eventually melt the snow around it.
Evacuated tubes are very similar to Thermos bottles with regard to heat loss. They’re specifically designed to minimize heat loss through the outer glass tube, and thus, their ability to shed snow is not good, as can be seen in Figure 1.
While this characteristic is hardly a concern in areas with minimal snow, it could be a deal-breaker in areas like upstate New York where the white stuff comes down from November through April. This is an especially undesirable characteristic in systems design for solar space heating. If you think snow rakes are the answer, make sure you sell the owner a helmet and face shield in case one of those tubes happens to wiggle out of the manifold and launch itself toward the operator end of the rake.
While flat plate collectors can also get covered with falling snow, their higher heat loss characteristic allows for faster melting when the sun comes out. However, for an efficient avalanche to occur, the snow sheet cannot be impeded by any structure or brackets below the collectors (see Figure 2).
LongevityI have flat plate collectors on my roof that we installed in 1981. They’ve been working for the last 27 years with hardly a hiccup. I doubt they would currently test out to the same thermal efficiency as when first installed, but they continue to deliver a high percentage of our domestic hot water needs on an annual basis.
Evacuated tube collectors minus their vacuum are not going to perform very well, and while it’s impossible to know exactly when the vacuum seal of any given tube will fail, I’ve not found any manufacturer willing to suggest life expectancy of more than 15 years.
Freeze Protection OptionsTo date, nearly all evacuated tube collectors are used in closed-loop antifreeze-based systems. Although there are rumors of impending designs that would allow evacuated tube collectors to be combined with drainback freeze protection, I’m not aware of any products of this type currently on the North American market.
Flat plate collectors with “harp” style absorber plates can be used with either closed-loop or drainback-type systems. For the latter, I highly recommend designers verify any special installation constraints such as sideways sloping or special couplings that may be necessary when the collectors are used in drainback installations.
Figure 3 shows collectors that are part of a drainback system and have been installed with a slight sideways slope (minimum of 1/4" per foot pitch). Each bank of collectors slopes to a common supply point at the center. Each bank has a return pipe at the upper/outer corners. These returns are connected together inside the building.
Stagnation ProtectionAll solar collectors tested to the SRCC (Solar Rating and Certification Corporation) OG-100 standard undergo a 30-day stagnation test to determine their ability to withstand very high temperatures without a significant degradation. Although this is not a guarantee that such collectors will never be damaged due to a combination of high solar intensity and no flow, it certainly recognizes that such conditions are inevitable during the design life of the system.
Given that evacuated tube collectors are used with antifreeze-based fluid, it’s vital to provide a means of protecting that fluid from thermal degradation due to high collector stagnation temperatures. The excess heat might be transferred to a pool through a heat exchanger, or dissipated through some type of hydronic heat emitter. In either case, the means of heat dissipation should be able to operate without AC power - given the loss of such power may be the reason that the collectors are stagnating.
Stagnation protection is also needed for flat plate collectors used in antifreeze-based systems. However, when flat plate collectors are used in drainback systems they should immediately empty upon a power failure, or when the system’s operating controller determines that the storage tank has reached its upper limit. In this “dry” state they should be able to withstand stagnation conditions, at least to the extent represented by the OG-100 testing standard.