Solar energy collection and storage - Solar Energy Sciences and Engineering Applications - page 131

Figure 4.4.2 Distribution of continuous days of hot water production at 37 ◦ C is across Europe (Yohanis
et al., 2006a; 2006b).
4.4.3 Freeze protection methods
Freeze protection of a water collector in winter can be gained by either draining the col-
lector circuit using multiple glazings (Bishop, 1983) evacuation to eliminate convective
heat loss. (Mason and Davidson, 1993; Hongchuan and Guangming, 2001), internal
convection suppression (Smyth, 2001a) or heat transfer fluid choice. The heat transfer
fluid used in a solar collector during winter sign should not allow freeze expansion
damage. The most common heat carrier in solar collectors is an aqueous solution of
water and glycol together with appropriate additives that inhibit corrosion. Regulatory
compliance has meant that ethylene glycol has been superseded by less toxic propy-
lene glycol. Glycol-filled indirect system can produce warm water in winter unlike a
direct system which would still remain drained down (the latter being usually a bien-
nial operation). The advantage of a longer operating period is counteracted in summer
by the lower thermal efficiency of an indirect system with a, more viscous, propylene
glycol solution when compared with a direct system in which water is the heat transfer
fluid. For thermosyphon solar water heaters, the sources of an indirect system's relative