1 ISAT Module III: Building Energy Efficiency Topic 6:Stead-State Building Loads z Fabric Loss z Ventilation Loss z Environmental Temperature z Steady-State Network z Indoor Comfort with Dry Resultant Temperature

2 z The heat loss through the fabric depends on the thermal resistances of the various elements making up the walls, roof, floors, etc. and the thermal resistances of the inside and outside surfaces of the building due to convection through the fluid film on the surface and thermal radiation from the surface to the surroundings. z The overall heat transfer coefficient, or thermal transmittance, U, in the building walls case is given by Fabric Loss

3 z The thermal transmittance depends on the weather conditions outside since the value of h o will vary, mainly with air speed; it is necessary therefore to modify the thermal resistance for a particular wall construction depending on whether it is under exposed or sheltered conditions. z The thermal transmittance is sometimes known as the U-value in building services design. Thermal Transmittance, U

4 Thermal Transmittance of some Typical Constructions U-value

5 Ventilation Loss z In all cases the building should be constructed to be as air tight as possible; air which leaks into and out of a building through fortuitous cracks, inadequate sealing etc. is known as infiltration. z Ventilation Heat Loss rate is given by

6 z Environmental temperatures are equivalent temperatures introduced to allow for radiation effects, either externally from solar radiation on the outside surfaces or internally from radiant interchanging between internal surfaces. z Outside environmental temperature is taken to be equal to the mean outside air temperature, I.e., t eo = t ao. z The approximate expression for inside environmental temperature, t ei is Environmental Temperature

7 Environmental Temperature (continued) z The mean radiant temperature, t mr, is the temperature of a small sphere completely surrounded by a number of surfaces at different temperatures in the absence of convection. For a cubic enclosure at the temperatures normally encountered in buildings t mr is approximately equal to the mean surface temperature, t ms, given by: where A s is the area of any surface at temperature t s. The fabric loss for the building is then given by: where A o is the area of any element of the fabric which transmits heat to the outside air.

8 Example (Building Load) An office has one external wall, 5 m long, containing a window of area 4 m 2 ; the width of the office from the window to the wall adjacent to the corridor is 4 m, and the ceiling height is 3 m. There are similar offices above, below, and on either side. The environmental temperature in the corridor is 16 o C, the environmental temperature in the room is 20 o C, the internal air temperature in the room is 19 o C, and the external design temperature is -1 o C. Taking thermal transmittance for the external wall, window, and internal wall as 1.0, 5.6, and 2.7 W/m 2.K, respectively, and an air change rate of one per hour, calculate the required rate of heat input to the room.

9 Example (Building Load) – continued A tabular method of laying out the data is useful (especially for more complex cases).

10 Steady State Building Loads Network z A model (developed by CIBSE) of the heat transfer process using an electrical resistance analogy can be used as shown in the Figure at right. Part of the heat input, Q a say, is taken to be at the air temperature, t ai, and part, Q e say, at the environmental temperature, t ei. It is assumed that convective heating is input at the air point, and that at the environmental point the heat input is partly radiant and partly convective.

11 z Many researchers have attempted to define comfort criteria and numerous comfort indices have been proposed. Comfort is a subjective state but it is generally agreed that air temperature, mean radiant temperature, air velocity, and relative humidity are the main factors governing thermal comfort. The dry resultant temperature, t c is defined as the temperature recorded at the center of a blackended globe of 100 mm diameter in still air. It can be taken to be given by Dry Resultant Temperature

12 Example (Steady State Network) Using the data given below calculate the fabric loss, ventilation loss, and total heat load for a small workshop when it is heated by ducted warm air (i.e., Q e = 0). Data Size of workshop = 15 m  10 m  4.5 m Thermal transmittances: walls (all external), 0.9 W/m 2.K; windows (area 50 m 2 ), 5.6 W/m 2.K; door (area 5 m 2 ), 3.0 W/m 2.K; floor, 0.7 W/m 2.K; flat roof, 0.9 W/m 2.K Air change rate = 2 per hour Dry resultant temperature required = 16 o C Outside design temperature = -1 o C

13 Example (Steady State Network) – continued Using a tabular method one can find the thermal resistance of the fabric heat loss link from internal environmental temperature to the outside temperature is 1/  (UA)=1/688 (K/W).

14 Example (Steady State Network) – continued The equivalent heat transfer coefficient between the inside air temperature point and the dry resultant temperature point can be taken as h ac = 6 W/m 2.K, and the equivalent heat transfer coefficient between the dry resultant temperature point and the equivalent the equivalent temperature point can be taken as h ec = 18 W/m 2.K. The thermal resistance of these two links are then given

15 Example (Steady State Network) – continued For the ventilation heat loss the thermal resistance between the inside and outside air points is given by

16 Example (Steady State Network) – continued The heat input, Q i, is equal to Q a, and is also equal to (Q F + Q v ) in the ducted warm air case, the fabric heat loss, Q F, also flows along the top arm of the network and hence, referring to the Figure shown on slide #13.

17 Internal Gains Modern buildings have a wide range of internal thermal gains the most common of which are from lighting and people. Offices nowadays have equipment with considerable thermal energy output, e.g. computer terminals, fax machines etc., and factories have an even wide range of process equipment and machinery.