Energy use in buildings

Dr. Atila Novoselac Associate Professor

Department of Civil, Architectural and Environmental Engineering,

ECJ 5.422

Class Objectives

• Show where the energy goes in residential buildings– Energy modeling and basic concepts

• Analyze impact internal latent loads – What happens when we dry our clothes indoors?

You probably know this, but it is usful to remand ourselves

How to compare energy for heating and cooling?or more precisely, How to compare heating energy from gas and electric energy?

1) Convert all to end primary energy2) Convert end use energy from gas to electric energy you would get from using this gas

You will need:- Conversion factors: 1000 BTU = 0.293 KWh, 1,000,000J=0.278 KWh - Efficiency of electric generation systems (including transport): ≈33%

Energy PrinciplesPrimary, Secondary and Site energy

• Primary energy refers to the energy embodied in the natural resources that has not undergone any form of artificial conversion or transformation. Examples of primary energy sources are coal, crude oil, sunlight and uranium.

• Secondary energy refers to the energy obtained from the transformation of primary energy sources. Examples of secondary energy sources are electricity or energy from gas delivered to consumer.

• Site energy refers to the energy consumed by the final users.

Site Energy vs. Primary Energy

Site (End-use) energy is the energy directly consumed by end users.

Secondary energy is site energy plus the energy consumed in the production and delivery of energy products

Primary energy is site energy plus the energy consumed in the production and delivery of energy products.

Site energy (End use)

Secondary Energy

Primary Energy

SiteEnergy

PrimaryEnergy

HVAC System

HVAC – Heating, Ventilation and Air-Conditioning

Not so important

Electric Energy Generationin Power Plants

Reason for such a large energy conversion losses

or Boiler that burns fossil fuels

Temperature

Entropy

Tc

TH3

4

1

2

Rankine cycle

T condensation

K

Major loss is in power plant (thermodynamics)Can be improved with cogeneration systems

Basic calculations1kW EE 3 kW of thermal energy

Consequence #1: Cost of heating Cost of cooling, per unit of thermal energy

•1kWh of thermal energy from gas + 90% efficient furnace → 0.90 kWh of heating

•1kWh of thermal energy from gas → 0.33 kWh of electric energy → 0.9 kWh of cooling (with typical COP cooling = 2.7 [kWcooling/kWelectic energy ])

Consequence #2: Cost of gas heating Cost of heating with a heat pump, per unit of thermal energy

•1kWh of thermal energy from gas + 90% efficient furnace → 0.90 kWh of heating

•1kWh of thermal energy from gas → 0.33 kWh of electric energy + heat pump → 1 kWh of heating (with typical COP heating for Austin climate = 3 [kWcooling/kWelectic energy ])

NOTE: This calculation does not work always! For example

- when we have a large use of renewable energy sources, - with use of more or less efficient systems than assumed (COP>>3 or COP <<3),

- when politics impact the prices, - ….

Analysis of energy consumption in residential buildings

• We are going to use real data and model based on Austin Energy data

Model house:- Location in Austin-2300sf-R13 walls-R30 attic -4 occupants-Surface absorptivity to Solar rad.: 0.7-Typical (average) internal loads -Infiltration/Ventilation 0.5 ACH- Double glazed widows

- Glazing are 20% south, 25 north, 5% east and west- SHGC=0.54 (reflective – bronze - glass)

Energy consumption in Austin’s residential house (data from Austin Energy)

2000 (15,600 kWh)Including gas

CoolingMiscellaneous

Range

Dryer

Heating

Lighting

Hot water

Refrigerator

Washer

End use energy where energy from gas is converted to equivalent electric energy

Energy consumptionNew single family 2262 sf, 2-story home in Austin

(AE data)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

2000 2006 2010 target (2015)

Miscellaneous

Washers

Range

Refrigerator

Lighting

Dryer

Hot water

Heating

Cooling

15620 kWh

12862 kWh

11304 kWh

7086 kWh

Desired Value !

Energy consumption: kWh/year(to get approximate cost multiply by 0.1)

Units are in kW/h per year

Data from the model house(House built in 2010)

2000 (15,600 kWh)Including gas

CoolingMiscellaneous

Range

Dryer

Heating

Lighting

Hot water

Refrigerator

Washer

How to convert this data into this data?

Convert end use energy from gas to electric energy

Impact of outdoor temperature vs. impact of solar radiation

We need to understand impacts of solar radiation Through:- radiation- conduction - convection for roof, walls, and windows

Example: Direct calculation by eQUEST

energy simulation toolhttp://doe2.com/equest/

Class analysis:

• Impact of temperature of outdoor air vs. Impact of solar radiation

– Eliminate all impact of solar radiation and compare energy consumption results before and after solar radiation

– Consider the relevant (comparable) type of energy

– Be aware that that increase cooling energy demand could decrease heating demands

• For example internal loads

Analysis result:

CaseHeating [10e6 Btu of heat]

Heating [kWh of EE]

Cooling [kW of EE]

Base case 20.89 2020 4730Same like above with no internal loads (cooling an heating of empty house) 34.93 3377 3030Same like above with no solar radiation on windows (SHGC =0) 48.49 4688 1960Same like above with no solar radiation on external surfaces (surface absorbtivity=0) 53.89 5211 1450Same like above with no infiltration/ventilation (ACH=0) 48.88 4726 1110

Summary• For a typical home in Austin built in 2000 energy is used for:

– 30% cooling, 14% heating, 12% hot water, 44% light and appliances, and other internal electric devices

– Considering cooling only • internal loads 36% energy consumed for cooling• radiation through window 23% of cooling• effect of solar radiation to roof and walls 11% of cooling• infiltration 7% of cooling (2.2% of total energy consumed)• conduction: roof, walls, floor and windows 23% of cooling (7.0% of total energy

consumed)

We should consider combined effect on cooling and heating since removal of internal loads and solar radiation will increase demand for heating.

– Considering combined heating and cooling effects:• contribution of internal heating loads: 2.3% of total energy consumed• contribution of all solar radiation: ~19% of total energy consumed• contribution of infiltration: 5.5% of total energy consumed• contribution of conduction through roof, walls, floor and windows: ~17% of total

• For different climate condition, or non-typical house, or non-typical users these numbers will be different !– For other climate conditions, we would build this house differently

You can do further analyze specific impact of different building components

For the same internal load you can analyze:• Impact of infiltration rate and heat recovery • Impact of windows

– Area – Glass properties

• Impact of R value– Roof– Walls– Windows

• Impact of surface absorptivity to solar radiation– Roof– Walls– Floor

• Impact of different location • Efficiency of mechanical system • …

Boiling energy modeling tips: - Energy efficiency measures are NOT additive- Most of the time you have to consider the whole building - There is no single solution

What happens when we dry our clothes indoors?

• Example: