Book - School of Science and Technology

128 Electrical storage heatingWet core boilersAs for dry core units, the characteristics of both store and system lead to specialconsideration of their application. The routine of calculation to determine the requiredrating is similar to that set out above except that the charging period is commonly reducedto 4 1 ¤ 2 hours in this case and rather more allowance is made for cold weather and eveningrequirements for heat to be dealt with by direct methods. Each of these approachesreduces the volume necessary for storage.Thermal storage cylindersIn this case, where purpose designed cylinders are used to supply indirect systems in a largebuilding, the method of calculation is a little different. A careful assessment of the true 24hour load must be made, system by system and hour by hour. To the total thus established,it is necessary to add an allowance to take account of the 24 hour static heat losses from thevessels. The rating of the energy supply required is a simple application of the expressiongiven above for dry core boilers, save that the factor `Y ' is not usually applied.Determination of the net storage capacity required is based upon the assumption thatstratification will exist within the vessels, temperature `layers' remaining relatively undisturbedas discharge takes place. Hence, reference is made to the temperature at which thestore is to be maintained and the volume-weighted average temperature of that returningthereto from the indirect system, thus:S ˆ 3600(HQ ‡ L)/c(t s t r )whereS ˆ water storage capacity (kg)c ˆ specific heat capacity (Table 5:13) (kJ/kg K)t s ˆ temperature of stored water ( C)t r ˆ temperature of return water ( C)This capacity may then be converted to volume using the specific mass of water, again asread from Table 5.13. Additions are necessary to cater for the expansion of water whenheated, as discussed later.Equipment for room storesThere have been very considerable advances in the development of equipment forelectrical thermal storage. When first introduced storage radiators were clumsy and tookup a disproportionate amount of room floor space. Figure 5.2 and Table 5.6, reproducedfrom the 1979 edition of this book, illustrate the equipment then available.Two particular changes in practice were adopted by most manufacturers of dry corestorage equipment; the first relating to the material from which the pre-cast storage blockswere made, either a high density refractory or an iron oxide (Feolite). Table 5.7 lists theproperties of these and, for comparison, those of water and cast iron. The second is theuse of opacified microsporous silica panels (Microtherm) for insulation: this material hasa thermal conductivity of only 0.030 W/m K at 800 C, and is thus about three times aseffective as any material previously used.