Modern solar collectors are more efficient than ever. Furthermore, the possibility of using absorptive surfaces in colors other than black has helped architects to identify new creative opportunities for solar collectors.
The solar heating market is one of the most expansive in the European energy sector. In Austria, for example, the market grew 18% annually between 1994 and 1999. This case story highlights the role of a compact brazed plate heat exchanger (BPHE) in an efficient solar heating system.
One of SWEP’s distribution channels in Austria is AEE, an independent association focusing on renewable forms of energy. AEE acts as a consultant in numerous energy projects.
Heat transfer efficiency in conventional single-circuit systems is sometimes low. The ethylene glycol heated by the sun flows through a helical pipe inside the water tank. Energy is transferred from the ethylene glycol to the water in the tank. However, contaminants such as algae or rust on the pipes eventually lead to impaired heat transfer and poor hygiene. The tank must therefore be opened and cleaned periodically, which is both undesirable and time-consuming.
There are two main reasons for insufficient heat transfer in a single-circuit system. First, low turbulence in the pipes inside the tank, and second, the formation of deposits on the pipes due to stationary water in the tank.
Using an intermediate SWEP BPHE connecting a double-circuit system avoids these problems. The BPHE creates a fully turbulent flow, which improves heat transfer between the two circuits, making it considerably more efficient than the single-circuit solution. Although a single-circuit system has the advantage of being able to work without an additional pump, the internal piping occupies a lot of space and heat transfer is less efficient than in a BPHE.
There is no stationary water surrounding the pipes, so the water tank does not need to be cleaned. The double-circuit system thus provides a maintenance-free heat transfer solution.
Furthermore, the double-circuit system has control advantages. It is easier to achieve more precise control of heat extraction when the whole piping system is outside the tank.
A 40% ethylene glycol solution is heated by solar radiation when it passes through the solar collectors. In a single-circuit system, the warm glycol then heats the water when passing through a helical pipe arrangement inside the water tank. In a double-circuit system, the pipe inside the tank is not needed because all the solar heat is transferred to the water through the BPHE. When using the low-flow variant of the double-circuit system (see table below), the water can be used both for radiators and for sanitary water. In such systems, the water temperatures inside the tank are controlled so that different temperature layers are formed. The radiator water and the tank water for heating the sanitary water are then taken from different temperature layers. As can be seen in Fig. 2, there are at least two BPHE applications in the double-circuit system.