1 ISAT Module III: Building Energy Efficiency Topic 7: Transient Heating and Air Conditioning Loads  Thermal Admittance  Intermittent Heating  Air Conditioning  Examples of Air Conditioning

2 z Under steady state conditions the heat loss from a building depends on the thermal transmittance and the ventilation rate. A true steady state is never attained since the outside climate varies on a 24-hour cycle as well as from day-to-day. z When a material is heated up or cooled down the energy change depends on the density and specific heat of the material as well as on the thermal conductivity.  Transient Building Loads

3 Comparison of Materials for Heat Transfer and Energy Storage z Heavyweight concrete needs a much greater thickness than expanded polystyrene for the same insulating effect but that it needs 3110 times as much energy to heat or cool that thickness.

4 z In the electrical circuit the impedance, Z, is introduced as the ratio of the voltage to the current; the reciprocal of the impedance, 1/Z is called the admittance. z The thermal admittance is given the symbol Y and since it is a reciprocal of a resistance-type term then it has the same units as thermal transmittance (i.e. W/m 2.K). z It can be defined as the rate of heat flow per unit surface area between the internal surface and a space for each degree of swing in the temperature of the space.  Thermal Admittance, Y

5 Thermal Admittance and Transmittance for some typical construction ConstructionY U (W/m 2.K) (W/m 2.K) External brick, 105 mm thick External brick, 335 mm thick Asbestos cement sheet, 5 mm thick external brick, 105 mm thick with plaster 13 mm thick on inside As above but with an air gap of 25 mm z With reference to the brickwork values, the thermal transmittance, U, of a structure reduces with thickness but the admittance, Y, tends to a constant value as thickness increases above a certain value. z Adding a second layer of brick with an air cavity, although substantially reducing the value of U, has a very small effect on the admittance, Y, which is largely determined by the layer of plaster on the inside surface.

6 z For the more common case of intermittent heating rather than steady-state cases, the inside temperature will vary throughout the 24-hour period. A simple on-off system with n hours on and (24-n) hours off would give the temperature and heat flow variations shown in Figures below.  Intermittent Heating

7 Fluctuating Heat Input z For intermittent heating the fluctuating heat input replace mean heat input, and the admittance, Y, is used instead of the thermal transmittance, U.

8 z The input which cause the need for air conditioning are as follows:  Air Conditioning Solar Radiation

9 Casual Gains Internally Total Load

10  Example of Air Conditioning A room 5 m by 4 m by 3 m high has one external wall ( 5m by 3 m) which faces south and which has a window, 3 m by 2 m. The window is single- glazed with normal exposure and white venetian blind fitted internally. There are two occupants who do light office work for 8 hours per day. For a typical sunny day in June using the data below calculate the required air conditioning load to maintain the inside dry resultant temperature at 18 o C, Assume that the rooms above, below, and at the sides of the room are at the same conditions as the room. Assume also that the room is well-sealed.

11 Data Mean total solar irradiance = 155 W/m 2 Peak total irradiance at solar noon = 535 W/m 2 24-hour mean external temperature = 16.5 o C Mean sol-air temperature = 20 o C External air temperature at 13:00 = 20.5 o C Decrement fatcor for external wall = 0.31 Time lag for glazing = 1 hour Time lag for external wall = 9 hours Sol-air temperatures at 04:00 = 10 o C Heat gains for light office work per person = 140 W Heat gains from office equipment = 2000 W Average period of use of equipment = 4 hours

12 Example of Air Conditioning (continued)

13 Example of Air Conditioning (continued)

14 Example of Air Conditioning (conclusion)