The present invention relates to the use of a double salt of Ca(NO3)2 and KNO3 for the manufacture of a melt, in particular a nitrate-based heat transfer fluid (HTF) and/or thermal energy storage fluid, for example in solar energy applications, such as in solar electrical power plant systems using a parabolic through, a central receiver or a linear Fresnel, which have both a low melting temperature and a high decomposition temperature, as well as to methods for the manufacture thereof. Using said double salt, a melt could be manufactured comprising at least NaNO3, KNO3 and Ca(NO3)2, preferably, an eutectic ternary melt comprising Ca(NO3)2, NaNO3 and KNO3 in a weight ratio of about 42:15:43 with an operating temperature of about 131° C. to about 560° C.