Book - School of Science and Technology

Energy targets 677Table 23.11 Heat loss from storage cylinders (insulated with 75 mm glass fibre sited in an ambienttemperature of 25 C)Capacityof cylinder(litre)WattHeat loss from cylinderMJ/annum150 40 1 300250 55 1 800300 60 2 000450 80 2 500650 100 3 3001000 135 4 3001500 180 5 6002500 250 7 6003000 280 8 8004500 360 11 300It is worthy of note that these two components of the total are often of the same orderof magnitude.A knowledge of the type of building will determine whether heat losses are continuousfor 24 hours per day, as in a hospital, or for some lesser period such as 8 hours per day in aschool. Similarly the days per annum will vary. For intermittent operation, the heat lossfrom storage vessels and pipework will continue after the use of the system has ceased,perhaps until the water temperature is in equilibrium with the surrounding air dependingupon the period of close-down.The heat consumption of the water actually drawn off, taken from cold at say 10 Ctohot at say 60 C, will be derived from data of water demand of comparable buildings.Table 20.4 provides data in this respect. The total of the two components will then formthe basis of the sum in which the calorific value of fuel and the seasonal efficiency aretaken into account as for heating (see Table 23.9(b)).Energy targetsThe CIBSE Building Energy Codes set down procedures to enable designers to comparecalculated energy demands for thermal and electrical consumption with energy targets,thereby enabling the most efficient systems to be selected. Code 1 is for heated andventilated buildings and Code 2 for air-conditioned buildings is in the drafting process.The procedures given in the Codes require calculations to be based on set formulae andon data given in the publications, thereby eliminating possible errors arising from engineeringjudgements. The calculation procedures are not intended to forecast energy consumption,but are a means of comparing a calculated demand with an equivalent energytarget.Code 1 requires that for a design to be deemed compliant, the total energy demandmust not exceed the total demand target and the thermal demand, for heating and hotwater, must not exceed the thermal demand target by more than 10%. For air-conditionedbuildings, Code 2 does not set-down energy targets due to the wide variety ofsystems that may be employed: however, it does provide a calculation procedure enablingcomparison of the energy demands of different buildings and system combinations.