Natrual Convection Heat Exchangers (NCHE)

Effects of Heat Exchanger Configuration on Solar DHW Performance

The Features and Benefits of NCHE, Stratification, and Low Flow Design in Solar Thermal Systems

Above is a video showing a side arm heat exchanger charging a standard North American style hot water heater using a low flow system.

Benefits of External Brazed Plate Heat Exchangers in Natural Convection Flow Design 

Wang, L. et all, 2007, discusses the advantages of using a side arm plate heat exchanger versus other designs:

"First the system performance is enhanced.  External heat exchangers can be configured so that the potable water circulates by natural convection or thermo-siphon action that allows excellent temperature stratification to be achieved in the storage tank.  With the hot water remaining in the top of the tank, its usable supply then becomes readily and more rapidly available.  Second, the thermodynamic efficiency of the system is improved with the external heat exchanger configuration.  The rate of the Heat transfer is directly proportional to the temperature difference between the potable water being heated and the antifreeze from the solar collectors... Third the overall cost of the system is reduced owing to the long lifetime of the external heat exchanger in the comparison with that of the solar tank...  Plate heat exchangers, with their additional advantages of relative compactness, easy maintenance, and low cost are thus a natural and attractive choice for use in solar thermal applications." 

Exert taken from: "Plate Heat Exchangers: Design, Applications and Performance." Wang, L. Sunden, B. Manglik, R.M., WITpress, Southampton, Boston. 2007. 

Natural Convection Heat Exchangers in Solar Water Heating Systems; Theory and Experiment 

"In the immersed-coil arrangement, the freeze-protected collector fluid passes through a coil immersed at the bottom of the tank, rather than through the external exchanger as shown in Fig. 1. There are certain practical advantages to each arrangement. The practice in North America is to use an inexpensive mass-produced pressurized hot water tank for the solar preheat tank, and it is not generally possible to interest the manufacturers of these tanks in inserting coils in the bottom of a few units in order to serve the (presently small) solar market. And once manufactured, the tanks cannot be retrofitted with a coil. But there is a thermodynamic reason that, in this writer's opinion, favours the external exchanger arrangement. Water heated by the immersed coil rises to the tank top as a plume, and en route it entrains other water at intermediate height, producing a mixing action. Water passing through the external exchanger, on the other hand, will rise to the top in the external loop, and hence it avoids this mixing. When high collector flow rates are used, this extra mixing in the immersed coil arrangement is probably not very damaging to system performance. But in the now-favoured "low-flow stratified-tank" approach (see Ref. [IO] for a summary of this concept), the performance is severely compromised by mixing, and so the immersed coil approach must be expected to yield inferior performance as measured, say, by the yearly solar energy delivered to the load."

Exert taken from: "Natural Convection Heat Exchangers in solar Water Heating Systems; Theory and Experiment”
M.G. Parent, TH. H. Van Der Meer, and K.G.T. Hollands
Centre for Solar Thermal Engineering, Department of Mechanical Engineering, University of Waterloo
Solar Energy Vol 45, No 1, pp 34-52, 1990