Heat Gain and Heat Loss Estimation

HEAT GAIN AND HEAT LOSS ESTIMATION

Heat gain estimation

Heat gain: A heat gain is the rate at which heat enters inside the building or generated within a space at a given instant.

The heat gain in a building is due to solar radiation through infiltration of heat from equipment and people.

The heat gain by transmission through any surface is given by the following expression.

where,

Q_g -> Heat gain through transmission by the wall, roof, ceiling, floor or glazing.

A -> Area of the wall, roof, ceiling, floor or glazing.

U -> Thermal transmittance, air to air.

T_i -> Inside air temperature.

T_o -> Outside air temperature.

Heat loss estimation

Heat loss: A heat loss is the rate at which heat flows out from a building to the surrounding environment.

Heat losses is due to heat conduction through the envelop and due to the exchange of warm and cold air through ventilation and infiltration.

The overall heat loss from a building is calculted as

Q_L = Q_t + Q_v + Q_{i..............................(2)}

where,

Qt -> Heat loss due to transmission through walls, windows, doors, floors, etc.

Q_v -> Heat loss caused by ventilation.

Q_i ->  Heat loss caused by infiltration.

Heat loss by Transmission

The heat loss through walls, s, windows, doors, ceilings, floors, etc. is calculated as

Q_t =AU [T_i - T_o]............................(3)

where,

Q_t  -> Tranmission heat loss.

A  -> Area of exposed surface.lg

U -> Overall heat transmission coefficient.

T_o -> Outside air temperature

Heat loss by ventilation

The heat loss due to ventilation without heat recovery is expressed as

Q_v = C_p P q_v [T_i - T_o]................................(4)

where,

Q_v  -> Ventilation heat loss egradoxe

C_p → Specific heat capacity of air

P -> Density of air

q_v  -> Volume of air flow

Heat loss by infiltration

The heat lose caused by infiltration is calculated as

Where,

n → Number of air shifts i.e., Number of times that the air is replaced in the room per second.

V→ Volume of the room.

Therefore, the overall heat loss from a building shall be obtained by substituting equations (3), (4), and (5) in equation (2)

Thermal Performance of Buildings

Definition

It refers to the process of modelling the energy transfer between a building and its surroundings.

• For a conditioned building, it estimates the heating and cooling load. Hence, the capacity and selection of Heating, Ventilation and Air conditioning (HVAC) equipment can be correctly made.

• For a non-conditioned building, it calculates temperature variation inside the building over a specified time. It helps us to estimate the duration of uncomfortable periods.

• These quantifications enable us to determine the effectiveness of the design of a building. They help in evolving improved designs for realizing energy efficient buildings with comfortable indoor conditions.

• The various heat exchange processes between a building and the external environment are shown in fig. 1.15.

Heat Exchange processes between a building and the external environment

• Heat flows by conduction through various building elements such as walls, roof, ceiling, floor, etc. Heat transfer also takes place from different surfaces by convection and radiation.

• Besides, solar radiation is transmitted through transparent windows and it is absorbed by the internal ao surfaces of the building.

• There may be evaporation of water resulting in a cooling effect. Heat is also added to the space due to the presence of human occupants and the use of lights and equipments.

The interaction between a human body and the indoor environment is shown in fig. 1.16.