This Presentation will help you tho know the theory of heat Transfer as well as Heat Energy Flows in the Buildings.

1. Heat Energy
Flows in
Buildings
BUILDING ENERGY FUNDAMENTALS
@RENETRIC ENERGY CO LTD
DAR ES SALAAM, TANZANIA.

2. Sensible vs. Latent Heat Flow
 Sensible Heat Flow – Results in a change in air temp
 Latent Heat Flow – Results in a change in moisture
content. Release or storage of heat associated with
change in phase of a substance, without a change in
substance temp.
 Total Heat Flow – sum of latent and sensible heat flows
 Heat Energy Flows in Buildings

3. Sensible vs. Latent Heat Flow
 Sensible vs. latent heat: it takes over 5x as much heat
to turn water into steam at the same temp than it does
to heat liquid water from freezing to boiling temps.

4. Sensible vs. Latent Heat
 Whenever an object is at a temp different from its
surrounds, heat flows from hot to cold
 In similar fashion moisture flows from areas of greater
concentration to areas of lower concentration
 Buildings lose sensible heat to the environment (or
gain) in 3 principle ways

5. Conduction, Convection,
Radiation
 Conduction: transfer of heat between substances which are in direct
contact with each other
 Convection: movement of gases/liquids caused be heat transfer. As
a gas or liquid is heated it warms, expands and rises because it is
less dense
 Radiation: electromagnetic waves travelling through space. When
these waves hit an object they transfer their heat to it

6. Heat Transfer

7.  Conduction takes placed through envelope
assemblies
 Convection is the result of wind or pressure
drive air movement
 Radiant heat is primarily from the sun

8. Thermal Effects
 Principals are the same, but heat flow under changing
conditions is more complex than under static
conditions
 Heat storage within materials is of greater concern
during dynamic conditions
 Under static conditions, heat flow is primarily a
function of temp difference and thermal resistance

9. Thermal Effects
 Under dynamic conditions, those two factors are still
important, but the heat storage in the building
envelope is a compounding issue
 Heat storage is a function of the density of the
material and specific heat; product of these two is the
thermal capacity (thermal mass)

10. Thermal Properties
 Every material used for the envelope has properties
that determine their energy performance

11. Thermal Conductivity (k)
 Material’s ability to conduct heat
 The faster heat flows through a material the more
conductive it is
 q=Resultant heat flow (watts)
 k=thermal conductivity (W/mK)
 A=surface area (m2)
 T=temp diff between warm and cold sides(K)
 L= thickness or length of material (m)

12. Thermal Conductance
 Conductivity per unit area
 In basic building materials heat flow is generally
measured by conductance (C), not conductivity
 Is an object property which relies on the materials and
the size

13. U-factor
 In layered assemblies, conductance is combined into a single
number called the U-factor or U-value
 Lower U factor means worse conduction, which means better
insulation
 Does not include latent heat (moisture related)
 Used only to describe air flow from the outside of the envelope to
airflow on the inside of the envelope (ie not for basement walls)

14. Thermal Resistance (R-value
=1/U)
 How effective a material is as an insulator
 R is measured in the hours needed for 1BTU to flow through 1ft2 of
a given thickness of material when the temp diff is 1f
 Object property, not material
 A 2x6 pine stud has three times the R-value as a 2x2 pine stud
 Higher R value indicates better insulating properties

15. Building Energy Loads
 How much energy your building needs
 Can be provided by electricity, fuel, or passive means
 Lots of terms that can get confusing, next slide has a
chart to help with these terms

17. Energy Loads
 Thermal Loads – heating and cooling energy needed
to keep people comfortable
 Heating loads – energy required to heat the building
when to cold
 Cooling loads – energy required to cool the building
when to hot
 Not just about temp, include moisture control (latent heat)

18. Loads
 Heating and cooling loads are met by the HVAC system
 Uses energy to add/remove heat and condition the space
 Equipment loads – HVAC etc, met by energy or fuel
 Plug loads – electricity for computers and appliances
 Lighting loads – electricity used for lighting

19. Thermal Loads
 Understanding the heating and cooling loads helps to
provide the right sized HVAC system for a space
 Reduce the loads as much as possible, and meet them
as efficiently as possible

20. External
 Heat transfer through the building envelope from the
sun and outside environment
 Building envelope includes the roof, walls, floors,
windows. Anything that separates inside from outside

21. External
 Common ways heat flows into or out of the building
 Heat conduction from the envelope to outside air or
ground
 Sunlight shining through windows to heat int.
 Sunlight warming up ext. of building
 Losing inside air to outside, or vice versa, through leaks

22.  How much energy from the sun’s radiation, outside air
temp, latent heat in the airs moisture that reaches the
inside to affect the comfort depends a lot on the
envelope
 Materials, design and how well it is sealed
 Understanding where heat energy is gained/lost is
important for successful passive design strategies

23. Internal Thermal Loads
 Come from heat generated from people, lighting, and equipment
(core loads, internal gains)
 Thermal loads from lighting and equipment is generally equal to
their use
 When a light fixture converts a watt-hour of electricity into photons,
those photons bounce around until they are absorbed which turns light
energy into heat energy
 All electrical energy not turned into photons is turned directly into heat
energy due to inefficiency

24. Internal Thermal Loads
 Similarly, electrical energy used to move mechanical parts is
transformed into heat via friction
 Energy used to power this equipment is turned into heat energy via
electrical resistance
 Thermal loads from people depend on the number of and what
activity they are doing
 Office buildings are generally dominated by internal loads
 Single family residences are typically dominated by external loads

25. Internal Thermal Loads

26. Heating and Cooling Loads
 How much energy you need to heat and cool the
building and control moisture within
 Gains that are more than envelope and ventilation
losses would cause a net cooling load (the building is
too hot)
 Losses that are more than the internal gains would
cause a net heating loads (the building is too cold).
 The heating setpoint is often different than the cooling
setpoint, so the distribution of heating and cooling
loads is climate dependent

27. Equipment and Lighting
Loads
 Lighting loads – energy used to power electric lights,
make up nearly 1/3 of commercial building energy use
(10-15% in residential)
 Look for more efficient lighting
 Reduce lighting loads
 Reduce cooling loads for the same visible brightness

28. Plug Loads
 Electricity used for other equipment
 20-30% of energy in commercial
 15-20% in residential (these numbers are growing)Equipment Rated Power (watts)
Desktop computer 120
Notebook computer 45
17” LCD Display 75
Desktop laser printer 120
Office laser printer 250
Office copier 750
Refrigerator 750
Dishwasher 1,200
Television 100
Commercial refrigerator 1,000
Commercial fryer 10,000
Clothes washer 350
Clothes dryer 2,000

29. Measuring Energy Use
 Energy Use Intensity - Energy intensiveness is simply
energy demand per unit area of the building's
floorplan, usually in square meters or square feet

30. EUI based on Building Type

31. EUI based on Floorspace

32. Site Energy vs. Source
Energy
 Energy intensiveness only considers the amount of
electricity and heat that is used on-site ("secondary" or
"site" energy)
 Does not consider the fuel consumed to generate that
heat or electricity. This "primary" or "source" energy
can be generated on-site or at a power plant far away.

33. Source Energy
 When measure energy used to provide thermal or visual comfort,
site energy is the most useful measurement
 When measuring total energy usage, source energy is more accurate
 Low on-site energy can cause more use upstream
 Ex, 2kW of natural gas burned on site is better than 1 kW of electricity
used on site. 1kW of site electricity from the average US electrical grid is
equal to 3.3kW of source energy due to inefficiencies
 Energy Efficiency

34. Source-Site Ratios

35. Energy End Use
 Commercial and residential use energy differently
 Commercial are dominated by internal thermal loads
(more people and equipment)
 Residential are dominated by external loads, larger
percentage of energy use is for heating and cooling to
meet those

36. Sankey Diagrams