Book - School of Science and Technology

Capacity of the heat store 121Warmed floors or walls where heat is retained in the building structure and emittedcontinuously with no control of output.Central storesWarm air units which are, in effect, large-scale storage fan heaters arranged as a centralheat source for indirect warm air systems.Dry-core boilers in which heat is retained as for storage fan heaters but arranged so thatoutput is used to supply an air/water heat exchanger and, thence, an indirect system ofpiping, radiators, etc.Wet-core boilers in which heat is retained in a hot water vessel, at atmospheric pressure,arranged so that the contents are used for circulation through an indirect system ofpiping, radiators, etc.Thermal storage cylinders in which heat is retained in water, at elevated temperature andpressure, to serve indirect systems of all types in large commercial buildings. Electrodeboilers, which are commonly used to charge such cylinders, are described in Chapter 10.Capacity of the heat storeThe amount of energy which must be stored in order to provide a satisfactory service willdepend upon which particular system is used. In all instances, however, since heat will bedischarged from the store over the whole 24 hour cycle, albeit perhaps at a low levelduring the period when output is not required, adequate capacity must be provided toinclude for the static heat output during the charging period. Some proportion of thisoutput will, where the store is within the space to be heated, be absorbed in the surroundingbuilding structure and thus not be wholly wasted.Where heat output is uncontrolled, as in the case of all floor- or wall-warming systemsand many input-controlled storage radiators, the night-time loss may be between 15 and20% of the total input to storage. Where equipment such as some more modern storageradiators and most storage fan heaters is output-controlled, the situation is better in thatnight-time loss is a smaller proportion, about 10±12% of the total input. It must be added,nevertheless, that less than half of the energy which then remains in the store will beavailable for controlled output, the balance being a day-time static discharge. Those typesof equipment which are designed to serve indirect systems are not subject to the samedimensional constraints as apply to room-sited units and thus can be not only betterinsulated but also better shaped as to their surface/volume ratio: static losses may thus beof the order of 5% or less of total input.The foregoing comments are intended to emphasise that conventional methods ofcalculation for heat requirements must be modified to take account of the abnormalcharacteristics of storage systems. Research organisations in the electrical industry haveproduced a variety of application routines designed to provide the necessary adjustments:the notes which follow are a digest of these.CriteriaIn its simplest form, determination of the capacity of storage heaters would takeaccount of the total design heat loss over 24 hours, calculated as described in