Heat Energy Flows in Buildings

HEAT ENERGY FLOWS IN BUILDINGSBuilding energy fundamentals

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

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.

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

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

Conduction takes placed through envelope assemblies

Convection is the result of wind or pressure drive air movement

Radiant heat is primarily from the sun

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

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)

THERMAL PROPERTIES Every material used for the envelope has

properties that determine their energy performance

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)

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

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)

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 1f

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

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

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)

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

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

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

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

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

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

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

INTERNAL THERMAL LOADS

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

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

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

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

EUI BASED ON BUILDING TYPE

EUI BASED ON FLOORSPACE

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.

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

SOURCE-SITE RATIOS

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

SANKEY DIAGRAMS