Pushing down stagnation temperature

Mon, 30/01/2017 - 12:37


Jens-Peter Meyer

SUN & WIND ENERGY
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The idea of having temperatures in excess of 200°C on their roof scares off a lot of potential solar-thermal customers. But the industry has now found ways to solve stagnation problems and limit high stagnation temperatures.

It is a fact that every solar thermal system can go into stagnation at any time. The culprit could be just a broken pump. But with properly designed safety measures and system pressure stagnation is not a problem. Nevertheless, the high temperatures reached in the newest collectors can put hi stresses on components such as the heat transfer fluid, insulation, and pipes. which is why the industry has continued to look for new, elegant concepts for controlling stagnation. A proven process is the drain-back method, which Vaillant introduced this year in Germany under the name "return-line-controlled solar system". At night the heat transfer fluid drains into the return-line tank and the collectors contain nothing but air. "As soon as the sun comes up, the pump switches on and fills the collectors with the solar medium," explains Vaillant product manager Arne Finger. "Once the tank is full, the pump simply shuts itself off, and the solar liquid  flows back into the return-line tank."

For companies such as Vaillant that also supply their products to HVAC installers that do not deal with solar technology on a daily basis, installing the systems cannot be difficult, which the company believes it has achieved with  its return-line-controlled solar systems. Nevertheless, a new mindset is needed. Up to now installing perfectly level pipes was seen as a must in heating installation. However, the pipes in return-line-controlled solar-thermal systems require a gradient of at least 4%.

Paradigma has found another way to solve the stagnation problem. The German company has long used only pure water as a heat transfer fluid in its vacuum-tube collectors because it is not damaged by turning into steam. "All of the active elements, such as pumps and valves, are in the solar return line. This part of the system also contains a zone valve that closes completely during stagnation because when that occurs the system should drain through the solar supply line. This protects all of the active components from overheating," says Wilfried Griesshaber, the head of the project management department at Paradigma. In addition, this prevents the forced flow of the fluid through an overheated collector. Paradigma generally positions the diaphragm expansion tank on the heating side, down-stream from the heat exchanger in the hot water tank. "The only component approved for installation in the solar supply line is a heat and steam-resistant drain and filling cock between the collector and the hot water tank," says Griesshaber.

Temperature limiting absorber coating

This year, Viessmann of Germany made another concept ready for the market. The idea is simple: what would happen if during stagnation phases the collector never got hot enough to create steam? Temperature loads would be lower and safety measures and pressurisation systems would be much simpler. Implementing the idea, which was done jointly with the solar research institute ISFH surely cost a good deal of effort, but now it works. After all, says Viessmann product manager Michael Beckmann, "The Vitosol 100-FM and Vitosol 200-FM flat-plate collectors have a special absorber coated with the patented Thermprotect selective layer, which changes its physical characteristics at high temperatures." From approximately 80°C, emissivity slowly increases, limiting the stagnation temperature to about 145 °C. At an operating pressure of 3 bar, this eliminates the possibility of steam development in the system. That protects the expansion tank and reduces its expansion volume. Auxiliary vessels and stagnation coolers are also unnecessary. "Of course this reduces stress on all of the other components as well," says Beckmann. One possible disadvantage, he says, is that the higher operating pressure reduces the working volume of the expansion tank. "But in reality, that is not a problem because for systems with Vitosol flat-plate collectors that have safety valves with an actuation pressure of 6 bar, the valve can be swapped out for one that actuates at 8 bar."

Solar thermal systems that are also used for space heating go into stagnation more frequently when the hot water tank is too small in relation to the collector area. With the new absorber coating, Viessmann now allows for generously designed collector fields.

Two circuits in one collector

Tigi, an Israeli company, has also developed a new method of limiting the stagnation temperature to about 150°C. In its transparent-insulated honeycomb collectors, the company installs not only the primary circuit, which draws heat from the collector but also a second one in the form of a closed heat-pipe circuit. As soon as the temperature of the absorber rises above 105 °C, the heat transfer fluid in the heat pipe produces steam which travels upward to the condenser integrated into the collector. The condenser then dissipates the thermal energy into the surrounding air, preventing the collector from heating up over 150°C. Like the Viessmann solution, the temperature limitation always works and, more importantly, so does the restarting of the system. After all, a defective sensor or improperly adjusted regulator can have no influence on plant shut-down. The disadvantage is that the honeycomb collector with its costly materials does not come cheap.

Bimetallic plates prevent heat transfer

The heat pipe vacuum-tube collectors made by Kingspan have long been equipped with a temperature limitation system. The Northern Irish  vacuum-tube specialists use a stack of bimetallic plates inside the heat pipe. As soon as a certain temperature is reached, the plates flex and close off a valve. That interrupts heat transfer to the condenser head. Kingspan sets the limit temperature to 90°C for its HP400 model and 135°C for its HP450. When the temperature falls to 80°C in the HP 400 or 121°C in the HP 450,  the plates snap back to their original position, reopening the valve. Although these collectors can indeed get hotter than the limit temperatures, they never exceed 170°C. That is important, says the company, because the glycol in the heat transfer medium begins to degrade at such high temperatures. Despite its temperature limitation system, Kingspan recommends the opposite of Viessmann: the size of the collector field should be designed conservatively to avoid stagnation problems.

Temperature limitation in heat pipe vacuum-tube collectors is also possible without any moving parts by using bimetallic plates and valves. Narva limits the stagnation temperature to 160°C, and Viessmann to 145°C. Both companies take advantage of the fact that above a certain temperature condensation is interrupted within the heat pipe, which interrupts heat transfer to the condenser head.

Narva, a German company, has worked in the past with the flat-plate collector manufacturer KBB and the ISFH solar research institute on projects aimed at developing new, efficient aluminium heat pipes. They are designed so that maximum temperatures can be set according to the application. Now, companies such as AkoTec, a manufacturer of vacuum-tube collectors that uses Narva tubes, have introduced new heat pipe collectors. The company says that the new collectors have overheating protection  which limits the temperature to 100°C.

The flat-plate collector with gravity heat pipes

Another innovation are flat-plate collectors that use so-called gravity heat pipes. "The essential difference to typical flat-plate collectors is that the solar circuit fluid no longer flows through the entire absorber, but rather only along the top of the absorber through a collector pipe. In the normal operating temperature range, heat is transferred through the gravity heat pipes from the absorber to the solar-circuit fluid in the collector pipe," explains Steffen Jack, a product developer at the German company KBB. The goal of the development is to ensure that the collector never gets hotter than 120°C. "The collector's absorber reaches typical stagnation temperatures," says Jack. "We don't see this as a problem because for many years, even decades, we have known that stagnation temperatures are not a problem for the collectors themselves." Jack assumes that in a properly designed system, temperatures at connections should not exceed 100 °C.

Low operating pressures also prevent the development of steam. That opens up huge potential savings in the whole system. As with the Viessmann system, a small expansion tank is all that is needed. Also, the system could conceivably use plastic pipes. Its pumps last longer, and cheaper pumps can be used. Maintenance intervals are longer because the heat transfer fluid cannot degrade. The internal hydraulic connections make connecting the collector field easier and less error-prone, Jack emphasises. Added to that are the savings in the collector itself because aluminium gravity heat pipes replace expensive copper pipes. However, the efficiency of the collectors is lower than that of conventional models by a few percentage points because the heat pipes create additional thermal resistance. "Nevertheless, the additional cost of the somewhat larger collector area is marginal when compared to the high overall cost reduction potential of the system," says Jack.

In principle, all of these approaches to temperature limitation can be used in both small and large solar-thermal systems. But in small systems the proportion of overall cost for stagnation safety measures is particularly high. In solar heating plants in the megawatt range which supply district heating networks, the heat sink is so large that stagnation is never an issue in normal operation. For the rare exceptions, planners incorporate redundant stagnation prevention measures. "Sometimes planners consciously integrate a feature into the plant, whereby a safety valve is actuated and then the system is refilled," says Michael Beckmann of Viessmann. Such systems often have to provide high temperatures in excess of 100°C. To limit the temperature in such systems, designers have to adapt the limitation to the required temperature level.