Building Services and Sustainability

Building thermal and solar performance

The thermal and solar performance of a building envelope describes how much heat passes through walls, roofs and openings, and how much solar energy enters the interior. Three quantities govern the opaque envelope: the U-value, which gives the steady heat flow per unit area per unit temperature difference, the surface conductance at each face, and the solar absorptance of the outer skin.

The solar gain factor (SGF) of an opaque wall combines the absorptance and the U-value, scaled by the external surface conductance. It is dimensionless and is given by \text{SGF} = \dfrac{\alpha U}{f_o}, where \alpha is the absorptance, U the U-value and f_o the external surface conductance. A dark, high-absorptance wall therefore has a larger SGF than a light-coloured wall of the same construction. For example, a wall with \alpha = 0.5, U = 1.5 W/m^2°C and f_o = 17 W/m^2°C gives an SGF of about 0.04.

A sun-path diagram is a polar plot of the sun's position for a given latitude. The observer stands at the centre, which represents the zenith. The concentric circles are contours of constant altitude angle, so the outer boundary is the horizon at zero altitude. The radial lines give the azimuth angle, and the curved lines connect sun positions at the same clock hour or on the same date.

The zenith angle is the complement of the altitude angle, \theta_z = 90^\circ - \alpha, and it does not depend on the azimuth. An altitude of 25^\circ, for example, corresponds to a zenith angle of 65^\circ.

The shape of a sun-path pattern depends mainly on latitude. Near the equator the sun passes close to the zenith, so the paths form a band that crosses the centre of the diagram. In the southern hemisphere the sun arcs lie toward the north side of the diagram, and at high latitudes the sun stays low, producing flatter and more compressed arcs. Matching a city to its diagram therefore starts with its latitude and hemisphere.

Direct solar radiation is kept out of a building mainly by shading devices that project from the openings. A chajja, a horizontal projecting slab above a window, intercepts high-angle sun. A mashrabiya, a lattice screen, shades the opening while still allowing air to pass. The badgir and the malqaf are wind-catching elements that channel air into the interior and do not reduce solar gain.

Thermal damping describes how much the indoor temperature swing is reduced relative to the outdoor swing. The decrement factor is \text{DF} = \dfrac{\text{indoor temperature swing}}{\text{outdoor temperature swing}}, and the thermal damping is (1 - \text{DF}) \times 100\%. A room whose indoor swing is one quarter of the outdoor swing has \text{DF} = 0.25 and a damping of 75 per cent.

A blackbody emits radiant energy at a rate given by the Stefan-Boltzmann law, E = \sigma A T^4, where \sigma = 5.67 \times 10^{-8} W m^{-2}K^{-4} and T is the absolute temperature in kelvin. Surface temperatures in °C must be converted by adding 273.15 before the fourth power is taken. Real surfaces emit a fraction of this value set by their emissivity.

Daylight autonomy (DA) is the fraction of annual occupied hours during which a specified illuminance threshold is met by daylight alone, without supplementary electric lighting. Its value depends on how much daylight reaches the space, so building orientation, the size of the fenestration and the latitude and climate of the site all influence it. Glare is not part of the DA calculation. It is a separate visual-comfort criterion assessed with metrics such as Daylight Glare Probability.

The Tropical Summer Index (TSI) is an empirical regression index that converts globe temperature, wet-bulb temperature and air velocity into a single equivalent temperature in °C. Air velocity acts through convective cooling of the body, so in warm conditions an increase in air speed lowers the index. Because TSI is a fitted equation, its values are approximate and it is applied within its range of validity.

Thermal diffusivity measures how quickly a temperature change travels through a material: \alpha = \dfrac{k}{\rho c_p}, where k is thermal conductivity, \rho is density and c_p is specific heat capacity. Diffusivity is directly proportional to k and inversely proportional to both \rho and c_p. A material of low diffusivity transmits temperature changes slowly, so periodic temperature variations at the surface are strongly attenuated through the wall. This amplitude damping is why dense, heavy walls perform well against daily temperature swings.

Albedo is the ratio of reflected radiation to incident insolation. It is a dimensionless fraction between 0 and 1 that depends on the colour and material of the surface. Light-coloured paving and roofing have high albedo and absorb less solar energy.

In a steady-state room, the heat removed by ventilation air equals the internal heat gain. The ventilation heat flow is Q = \rho c \, V \, \Delta T, where \rho c is the volumetric heat capacity of air in J/m^3·K, V is the ventilation rate in m^3/s and \Delta T is the difference between indoor and outdoor temperature. Rearranging gives V = Q/(\rho c \, \Delta T). For instance, a 3 kW gain with a 5 K difference and \rho c = 1200 J/m^3·K requires V = 0.5 m^3/s.

Heat gain through glazing has two parts. Conduction is Q_c = UA\Delta T. Solar radiation admitted through the glass is Q_r = \text{SHGC} \times A \times I, where SHGC is the solar heat gain coefficient. The SHGC can be obtained from the visible light transmittance (VLT) and the light-to-solar-gain ratio (LSG) as \text{SHGC} = \text{VLT}/\text{LSG}, because \text{LSG} = \text{VLT}/\text{SHGC}. The total gain is the sum of the conduction and radiation parts.

Lighting power density is the installed lighting load per unit floor area, in W/m^2. This is distinct from luminance, which is measured in cd/m^2. Sound power is the rate at which a source emits acoustic energy, with the SI unit of the watt.

Sustainability indices and acronyms

Sustainability assessment uses a large set of indices and acronyms, and each has a fixed meaning. Recognising the domain of each acronym is often enough to answer a matching question.

PMV, the Predicted Mean Vote, was developed by Fanger. It combines air temperature, mean radiant temperature, air velocity, humidity, metabolic rate and clothing insulation to predict the average thermal sensation of a large group of occupants on the ASHRAE seven-point scale, from cold (-3) to hot (+3). PMV is therefore a thermal comfort index, whereas NDVI (a vegetation index from remote sensing), DEM (a terrain model) and PCA (a statistical method) belong to other fields.

The coefficient of performance (COP) of a heat pump is the ratio of useful heat delivered or removed to the work input, \text{COP} = Q/W, and it is dimensionless. The energy efficiency ratio (EER) is the cooling-mode rating expressed as cooling output in Btu/h divided by electrical input in watts. Because both describe the same energy conversion, EER is obtained from COP by unit conversion, with \text{EER} \approx 3.412 \times \text{COP}.

• LCA — Life Cycle Assessment, which evaluates environmental impacts from cradle to grave, covering extraction, manufacture, use and disposal.
• IPCC — Intergovernmental Panel on Climate Change, the body that assesses climate change science.
• Mtoe — million tonnes of oil equivalent, a unit that expresses an energy quantity as the amount of oil releasing the same energy.
• LEED — Leadership in Energy and Environmental Design, a green building certification system.

Room acoustics absorption and reverberation

Reverberation time (RT60) is the time required for the sound level in a room to decay by 60 dB after the source is switched off. Sabine's formula relates it to the volume and the total absorption: T = \dfrac{0.161\,V}{A}, where V is the volume in m^3 and A is the total absorption in metric sabins (m^2). The total absorption is the sum of \alpha S over all surfaces, where \alpha is the absorption coefficient and S the area of each surface.

An opening must be handled separately. An open window or doorway is treated as a perfect absorber with a coefficient of 1.0 because sound passes through it completely. Its area is subtracted from the surface it occupies and counted at its own coefficient. For a closed opening, its own coefficient applies. Care with the surface area is essential, because the total area of a rectangular hall is the sum of the areas of all six faces.

The Noise Reduction Coefficient (NRC) is the arithmetic mean of the sound absorption coefficients at the four mid-frequency octave bands, 250 Hz, 500 Hz, 1000 Hz and 2000 Hz, rounded to two decimal places. The NRC is used to rate panels and finishes and does not include the low and high frequency bands.

A decay curve plots sound level in dB against time after the source stops. The reverberation time is found from its slope: divide 60 dB by the decay rate in dB per second. Once RT60 is known, the total absorption follows from Sabine's formula as A = 0.161\,V / \text{RT}_{60}. For example, a hall whose level falls by 30 dB in 0.6 s has a decay rate of 50 dB/s and an RT60 of 1.2 s.

Building energy audit

A building energy audit quantifies how much energy a building uses and expresses it in a normalised form so that buildings of different sizes can be compared. The Building Energy Index (BEI) is the annual energy consumption divided by the built-up area, in kWh/m^2/year. Energy performance is also reported through an energy performance index with the same unit, kWh/m^2/year, which is the annual energy use per unit built-up area.

When the audit starts from the electricity bill, the monthly bill is divided by the unit tariff to obtain kWh, and the monthly figure is multiplied by 12 to get the annual energy. The built-up area is found from the floor area ratio, \text{FAR} = \dfrac{\text{built-up area}}{\text{plot area}}, so the built-up area equals FAR multiplied by the plot area.

When part of the energy is supplied by on-site solar generation, the BEI is computed from conventional sources only. If solar supplies a known percentage of total annual consumption, the total is found by dividing the solar energy by that percentage. The conventional energy is the total minus the solar share, and this value is divided by the built-up area.

Air-conditioning system components

A unitary air conditioner is a vapour-compression system with four main components: the compressor, the condenser, the expansion valve and the evaporator. The role of each component depends on the operating mode. In cooling mode, the indoor coil absorbs heat from the room air, the refrigerant evaporates in it, and the indoor coil therefore acts as the evaporator. The outdoor coil rejects heat to the surroundings and acts as the condenser. The compressor raises the refrigerant pressure, and the expansion valve throttles it between the high- and low-pressure sides.

HVAC dampers are movable plates or blades installed in ducts and plenums. They open, close or partially obstruct the airflow so that each air-conditioned zone receives the air quantity it needs. Dampers are therefore airflow control devices. Refrigerant flow is controlled by expansion devices and valves in the refrigerant circuit, and dampers do not regulate it.

Lift safety under NBC 2016

The National Building Code of India 2016, in its fire and life safety provisions, requires lifts to incorporate an Automatic Rescue Device (ARD). When the power fails or a fault stops the car between floors, the ARD moves the car to the nearest landing so that the passengers can leave safely. The function is confined to the lift installation. Fire control at plenum level is achieved through fire-rated separation and dedicated protection, escape route lighting is supplied by the emergency lighting system, and sprinkler heads or the fire alarm system trigger sprinkler operation. None of these is the role of the rescue device.