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Building Physics: Thermal
Comfort and Energy Efficiency
Building Physics
Steady-flow Process
A steady-flow process is a process during which a fluid flows through a control volume steadily, with no change over time.
Energy Change in a Room
An electric heater provides 2 kWh of energy to an office room (considered a closed system). The room experiences 1.5 kWh of thermal loss through the external wall and window. People inside the room release 200 Wh of heat, and the electric lights and equipment release 300 Wh. The energy change, ΔE, of the room is calculated as:
ΔE = 2 + 0.2 + 0.3 - 1.5 = 1 kWh
Heat Pump Coefficient of Performance (COP)
A heat pump absorbs 250 W of electricity and provides 10800 kJ of heat while working steadily for 4 hours. The heat pump's COP is calculated as:
COP = Qh / W = [(1800 / 3.6 × 4)] / 250 = 3
or
COP = Qh / W = 1800 / (250 × 3.6 × 4) = 3
Direct Solar Radiation on a Vertical North-Facing Wall
At the given latitudes, a vertical wall facing north receives direct solar radiation during the following months:
April, May, June, July, August March and September (during the last 10 days and the first 20 days, respectively)
Radiation Heat Transfer between a Person and
Surroundings
A person is enclosed in a room with a surface temperature of 20°C. The person's body surface area is 1.5 m², with a temperature of 35°C and an average emissivity of ε = 0.6. The rate of radiation heat transfer between the person and the surroundings is calculated as:
Q_rad = ε × σ × A_b × (T_b⁴ - T_s⁴) where σ = 5.67 × 10⁻⁸ W/m²K⁴.
Heat Flow through a Glazing
A glazing with a conductance of 4 W/(m²K) separates external air at 5°C from an internal room at 20°C. The inside and outside heat transfer coefficients are 8 W/(m²K) and 25 W/(m²K), respectively. The heat flow rate density through the glazing is calculated as:
U_win = 1 / (1/h_i + 1/U_g + 1/h_o) = 1 / (1/8 + 1/4 + 1/25) = 2.41 W/m²K Q = U_win × ΔT = 2.41 × (20 - 5) = 36.14 W
The internal surface temperature is calculated as:
Q = h_i × (T_i - T_si) 36.14 W = 8 W/m²K × (20°C - T_si) T_si = 15.5°C
Since the internal surface temperature (15.5°C) is higher than the dew point temperature (12°C), condensation does not occur.
Environmental Parameters Affecting Human Thermal
Comfort
The four environmental parameters that affect human body heat exchanges and thermal comfort are:
Air temperature Air movement (or velocity) Radiation (or radiant temperature) Air humidity
Types of Mechanical Ventilation Systems
The three types of mechanical ventilation systems are:
Extract Supply Balanced
Heat Flux Density Comparison
Two rooms have the same external wall-type with a U-value of 0.5 W/m²K. The inside air temperature is 20°C, and the outside air temperature is 16°C. One wall (A) is perfectly shaded, while the other wall (B) is under the effect of solar radiation (irradiance G = 600 W/m², absorptance α = 0.5, outside heat transfer coefficient h = 25 W/(m²K)).
The heat flux densities are calculated as:
Q_A = 0.5 W/m²K × (20°C - 16°C) = 2 W/m² In → Out
T_sa = T_a + [(G × α) / h] = 16°C + [(600 W/m² × 0.5) / 25 W/m²°C] = 28°C Q_B = 0.5 W/m²K × (20°C - 28°C) = -4 W/m² Out → In
