Solar Heat Exchangers
Heat exchangers are a key component in closed-loop solar heating systems. And, properly sizing your heat exchanger is critical to the performance, efficiency and cost-effectivness of your solar thermal system.
Many in the solar thermal industry believe that closed-loop systems are far less efficient (and therefore inferior) to open-loop systems. However, over the years, we have determined that much of this is due to improperly sized heat exchangers. Most likely, these heat exchangers were sized for traditional applications instead of solar.
The difference in temperature between the hot and cold fluid in the separated loops of a hot water system is known as the approach. In traditional boiler systems, the approach is typically 80F (180F – 100F). In a solar system, however, we want the collectors and the storage tank temperatures to be as close as possible (this will significantly increase collector efficiency). Typically, we try for an approach of 10-20F in solar systems.
To accomplish, we need to skew the output numbers of heat exchangers that have been rated for traditional boiler systems. The guidelines we use are based on these “low approach” solar temperatures (the maximum heat load is determined from the typical operating temperature under full sun).
Using these principles, closed loop systems with a brazed plate heat exchanger will operate within 5% of an open-loop system.
Sizing Brazed Plate External Heat Exchangers
Brazed plate heat exchangers have high efficiencies and compact sizes, which allows them to be sized for low approach temperatures (down to 10F) and still remain cost-effective. Using these sizing methods, these systems can parallel open-loop system efficiency.
Pressure drops at design flow rates will be around 1.5psi
Sizing Internal Heat Exchangers
Typically, the efficiency of these heat exchangers is lower than brazed plate external heat exchangers. This is mostly due to the fact that they rely on natural convection for heat transfer away from the exchanger. For this reason, they can be economically sized for approaches down to about 20F.
The actual heat transfer from your heat exchanger will depend on the configuration of your heat exchanger, there is a simple rule of thumb that will help your heat exchanger with little difficulty. Use 1 square foot of tube for every 14 square feet of collector area. This should give you an approach around 20F and a pressure drop at the design flow rate of about 1psi.