Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 132

inefficiency are, in order of significance (Norton and Edmonds, 1991; Norton et al.,
1992):

(1) the higher viscosity of aqueous glycol solutions (compared with water) reducing

the natural-circulation flow rate,

(2) the additional (compared with a direct system) fluid frictional flow resistance

caused by the heat exchanger,

(3) heat transfer resistance associated with the, frequently double-walled, in the heat

exchanger used, and

(4) the lower specific heat capacity of aqueous propylene glycol compared with

water.

A major concern in the design of indirect systems is the choice of antifreeze heat
transfer fluid. Being both readily available and cheap, glycol solutions are used usu-
ally. However, even the use of low-toxic propylene glycol is only permissible in many
building codes, standards and regulations with a double-walled heat exchanger. The
latter, however, further reduces system performance. Nonaqueous liquids, such as sil-
icones and some hydrocarbons, are low to moderately toxic; but as they have higher
viscosities (compared with aqueous solutions) together with lower specific heats and
thermal conductivities, their use impairs performance. An aqueous-glycol solution may
be used with a single-walled heat exchanger in some jurisdictions if it is both effectively
nontoxic and stable chemically over the full range of the water heater's operating con-
ditions, propylene glycol solutions containing the appropriate inhibitors satisfy these
requirements.
Many solar energy water heaters act as a preheater in series with an auxiliary heater
(DGS, 2005). In a preheat system, the mass of water withdrawn from the solar system
forms the hot water consumption in the building, so energy is always gained from the
solar energy system even when it raises the water temperature only slightly above main
supply temperature. An auxiliary heater raises this water to a temperature above 55 ◦ C
sufficient to eradicate any Legionnella bacteria. The auxiliary outlet water temperature
is thus above usual bathing temperatures and is normally mixed with cold at the faucet,
shower or bath to achieve the temperature desired. If water is drawn off at temperatures
above the auxiliary heater set point, then it can be argued that the draw-off volume
should be reduced because a greater proportion of cold water will be used to give
the desired temperature at the point of use. In practice this will depend on whether a
two-tank system is used and on heat losses from the auxiliary heater tank.
The freezing point of commercially available propylene-glycol solutions varies with
concentration. The ambient temperature distributions for each of the months when
frost is recorded also changes with location. Ensuant critical concentrations for system
survival are thus location dependent . It is usually worthwhile to continue operating a
solar energy water heater in winter if produces a net heat output. In London, UK, for
example there is no month in which a solar energy water heater if designed appropri-
ately containing propylene glycol at the critical concentration necessary for survival,
would not produce heat output.
It could be preferable to use a direct solar energy hot water system, drain the
collector in winter, and forego the energy that is available during the winter months
in order to benefit from greater hot water production efficiency during the summer.