Energy Loss in buildings Flashcards
What is heat flux
Heat transfer per unit area, driven by temperature difference
What is a heat pump?
Moves heat from low temperature regions to high temperature regions, against the natural direction
What is conduction?
Heat transfer through a stationary medium, with a temperature gradient
What is ficks law?
q’’ = -kDT/dx
What is convection?
Heat transfer between a moving fluid and stationary object at different T
Free Convection
Benefits/drawbacks
Entirely due to density gradients. Low heat flux, plenty of space and low noise
What is radiation
Heat transfer between two or more surfaces at different temperatures with the absence of an intervening medium
How to find total U for a non homogeneuous wall?
1/Ut = Rtot = R1+R2+…
What is U and R in terms of ficks law
U = k/d
R = 1/U
What is the equation for heat flux in radiation
q’’ = sigma T^4
Equation for convection heat flux
q’’ = UdTm
What is the logarithmic mean temperature
(dTa-dTb)/ln(dtA/dTB)
Sources of heat loss and gain in buildings
Heat loss - conduction and convection through walls and openings.
Heat gain - people, equipments, sun radiation.
Impact of sunlight on heat gain. Factors influencing
Raises surafce temperature and therefore DT
Depends on angle of rays, colour, roughness, reflectivity
Ventilation heat flow equation
Q = MCN (Ti-To)/3600
What is the ventilation factor
Heat loss due to room air changes - 0.33
What is the building design heat loss?
H = Qloss/dT
How to reduce error in building heat loss calculations. Why is this needed?
The surface temperature is rarely equal to internal temperature due to radiative and convective heat transfer.
Use the environmental temperature.
Environmental temperature calculation
Te = 0.33Ti + 0.66Tr where Tr is the mean temp of surfaces in the room
What is comfortable temperature?
Subjective and affected by air temp, velocity, humidity etc
Tc = 0.75Te + 0.25Ti
Procedure for energy loss in buildings
1.) select recommended internal and external temp
2.) Determine factors to account for Te and Ti
3.) Determine Q for each zone and hence total heat loss
4.) Note design heat loss
What are degree days used for?
Estimating heating costs over a long time period, allowing for seasonal variations
dT x time
How are degree days calculated?
average of max and min temperatures, ignoring above base.
What are Y values?
Measure of response of a material to temperature change and depends on thermal inertia of material
If
