Transcript 1.44 MB - KFUPM Resources v3

Building Energy
Analysis
Main Features of Energy Efficient
Buildings
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Proper orientation of building
High levels of building insulation
Air infiltration control
Efficient windows
Reduced heat loss in water distribution system
Smaller windows on north side to allow natural ventilation
Energy efficient appliances
Waste heat recovery where possible
Critical Aspects of a Window Design
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Maximize solar gain
Reduce conduction losses
Control excessive ventilation losses
Control heat gains in summer
Annual heat loss comparison of window glazing
options (kWh)
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Building Orientation
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All major/more populated rooms should be southerly orientated to
maximise on solar gain to reduce heating and lighting load. Shading
techniques can be employed on south facing rooms to reduce cooling
loads. East and west window orientations result in more undesired heat
gain in the summer than winter. East and west sun glare is also more
difficult to control for occupant comfort because of low sun angles in
early morning and late afternoon.
Heat Supplied to a Typical Domestic
Building
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Heat supplied through emitters/radiators
Casual gain
Solar gain
Casual/Internal Heat Gain
Casual heat gain in a building is the heat gained from electrical equipment,
lighting, and/or occupants.
Apart from human occupancy a number of appliances and equipment also
contribute to internal gains, such as:
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Stove/Oven
Lights
Television
Washing Machine/Dishwasher
Computer
Shower
Heat gain from occupants
Activity level
Output (W)
Seated at desk
90
Walking in office
110
Light work
150
Heavy work
200
Heavy factory work
440
Solar Space Heating
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Active solar space heating
Passive solar space heating
Active Solar Space Heating
Active solar systems use solar collectors and additional
electricity to power pumps or fans to distribute the
sun's energy. The heart of a solar collector is a black
absorber which converts the sun's energy into heat. The
heat is then transferred to another location for
immediate heating or for storage for use later. The heat
is transferred by circulating water, antifreeze or
sometimes air.
Active solar heating systems can be designed to provide
the same levels of control of conditions in the heated
(or cooled) spaces as conventional systems.
Passive Solar Heating
Passive solar heating techniques generally fall into one of three categories:
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Direct gain
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Indirect gain
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Isolated gain
Direct Gain
Direct gain is solar radiation that directly penetrates and is stored in
the living space.
Indirect Gain
Indirect gain collects, stores, and distributes solar radiation using some
thermal storage material (e.g., Trombé wall). Conduction, radiation, or
convection then transfers the energy indoors.
Isolated Gain
Isolated gain systems (e.g., sunspace) collect solar radiation in an area
that can be selectively closed off or opened to the rest of the house
Heat losses (transmission and
ventilation losses) exit the building
through the envelope.
Heat gains enter the building
through the same envelope.
Using the law of energy conservation,
the sum of the gains equals the sum of
the losses as long as the Inner Energy
does not change.
ENERGY USED IN MAKING A LIGHT BULB
WORK
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Energy is consumed to produce and process oil
Oil burns to make heat
Heat boils water to produce steam
Steam pressure turns a turbine
Turbine turns an electric generator
Generator produces electricity
Transformer conditions electricity
Electricity powers light bulb
Light bulbs give off light and heat
WHAT ENERGY CONSERVATION
CAN DELIVER
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Financial gain
Human comfort
Environmental benefits
National security
FINANCIAL GAIN
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Domestic consumer may want to conserve energy to reduce energy bills.
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Industrial and commercial users may want to increase efficiency and thus
maximize profit.
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Energy conservation also reduces dependency of an individual or group on
national grid and thus promotes economic sustainability.
HUMAN COMFORT
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With low energy budget available, implying energy conservation
better human comfort levels can be attained.
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Energy conservation can help overcome fuel poverty.
ENVIRONMENTAL BENEFITS
Energy conservation reduces the amount of energy consumed to
undertake a certain task cutting down the associated
environmental impacts, it thus promote environmental
sustainability.
NATIONAL SECURITY
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On a larger scale, in modern world, energy conservation is being considered
as an integral part of national energy policy.
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Energy conservation reduces the energy consumption and energy demand per
capita, and thus offsets the growth in energy supply needed to keep up with
population growth.
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This reduces the rise in the need for new power plants, and energy imports.
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The reduced energy demand can provide more flexibility in choosing the
most preferred methods of energy production.
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Energy conservation is often the most economical solution to energy
shortages.
ENERGY CONSERVATION
APPROACHES
Less usage
Change attitudes/
Good housekeeping
Energy Conservation and
Management
Less usage
New technology
Less usage
Performance enhancement
ENERGY CONSERVATION
MEASURES IN FINANCIAL TERMS
Zero cost measures - change of attitude
Low cost measures
Medium to high cost measures
LIFE CYCLECOST OF ABUILDING ICEBERG ANALYSIS
ENERGY EFFICIENT DESIGN OF A
BUILDINGS
The process of energy efficient design should always include:
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identifying user requirements
designing to meet these requirements with minimal energy use
setting energy targets at an early stage, for each fuel and individual end-uses, and
designing within them
designing for manageability, maintainability, operability and flexibility
checking that the final design meets the targets and that the selected equipment conforms
with product performance benchmarks
Success depends on understanding the interactions between people, building fabric and
services
KEY FACTORS THAT INFLUENCE ENERGY
CONSUMPTION
External factor
Weather/climate
•Size
•Design
•Materials
•Ventilation
•Location
•Orientation
Building
Construction
/Envelope
Building
Services
Human Factor
•Fuels
•Type of appliances
•Type/Size of systems
•Plant efficiency
•Operating regime
•Comfort requirements
•Occupancy regimes
•Management and maintenance
•Activity
Net Heating Load - I
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During winter, the major sources of heat gain/loss into/from a
building are shown below.
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The net heat loss out is called the heating load. Based on an
energy balance, the net heat load out is:
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Qnet,out = (Qwalls + Qwindows + Qceiling + Qdoors + Qinfiltration +
Qground)out – (Qsolar + Qpeople + Qelectricity)in
Net Heating Load - II
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To maintain the interior of the building at a steady temperature, the furnace
must provide enough heat to the space, Qfurnace, to balance the net energy loss
from the space, Qnet,out. Because furnaces are not 100% efficient, some of the
energy supplied to the furnace, Qnatural gas, is lost in the exhaust, Qexhaust.
Qexhaust
Qnatural gas
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Furnace
Qfurnace
Qnet,out
Although energy balances can be written in a variety of forms, for example:
Net Heating Load - III
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In this case, an energy balance on the house gives:
Qfurnace – Qnet,out = (dE/dt)house = 0 (if the house temperature remains constant, i.e.
Steady State)
It follows that:
Qfurnace = Qnet,out
Efficiency is defined as:
In the case of a furnace, the useful output is Qfurnace and the required input is Qnatural gas.
Thus, the efficiency of a furnace is:
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η furnace = Qfurnace / Qnatural gas
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The efficiencies of furnaces have improved over the years from about 65% to 95%.
The efficiency equation can be rearranged to determine the natural gas energy use
consumed by the furnace.
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Qnatural gas = Qfurnace / η furnace
Example – Heating Load
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Consider a building has the following average loads in winter: Qpeople =
13,000 Btu/h, Qsolar = 3,000 Btu/h, Qelec = 3,000 Btu/h, Qwalls = 20,000
Btu/h, Qwindows = 15,000 Btu/hr, Qdoors=1,500 Btu/hr, Qceiling =
12,500 Btu/hr, Qinfiltration = 15,000 Btu/hr, Qground = 5,000 Btu/hr.
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Calculate the Qnet,out of the building.
Solution
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Qnet,out = (Qwalls + Qwindows + Qceiling + Qdoors + Qinfiltration + Qground)out
– (Qsolar + Qpeople + Qelectricity)in
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Qnet,out = (20,000+15,000+12,500+1,500+15,000+5,000)Btu/hr
– (3,000+13,000+3,000) Btu/hr
Qnet,out = 50,000 Btu/hr
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The furnace of this building operates 1,000 hours over the winter and is 80% efficient. Natural
gas costs $10 /mmBtu. Calculate the cost of fuel for the furnace over a winter.
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Qfurnace = Qnet,out = 50,000 Btu/hr
Qnatural gas = Qfurnace / η furnace = 50,000 Btu/hr / 80% = 62,500 Btu/hr
Qnatural gas, yr = Qnatural gas x HPY = 62,500 Btu/hr x 1,000 hr/yr = 625,500,000 Btu/yr
Cnatural gas, yr = Qnatural gas,yr x Cng = 625.5 mmBtu/yr x $10 /mmBtu = $6,255 /yr
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Net Cooling Load - I
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During summer, the major sources of gain into a building are shown below.
Note that ground losses/gains in the summer are typically small and are here
assumed to be negligible.
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The net heat load into a building is called the cooling load. Based on an
energy balance, the net heat load in is:
Qnet,in = (Qsolar + Qpeople + Qelectricity + Qwalls + Qwindows + Qceiling + Qdoors +
Qinfiltration)in
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Net Cooling Load - II
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To maintain the interior of the building at a steady temperature, the air
conditioner must remove enough heat from the space, Qac, to balance the net
energy gain to the space, Qnet,in.
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In this case, an energy balance on the house gives:
Qnet,in – Qac = dE = 0
It follows that:
Qac = Qnet,in
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Net Cooling Load - III
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To pump heat “uphill” from the cool space to the hot outdoors, electric air conditioners require
electrical energy Welec. As before, efficiency is defined as:
For an electric air conditioner, the useful output is Qac and the required input is Welec . Thus, the
efficiency of the air conditioner is:
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η ac = Qac / Welec
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The efficiency equation can be rearranged to determine the electricity consumed by an air
conditioner.
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Welec = Qac / η ac
Net Cooling Load - IV
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The efficiency of air conditioners is measured in several ways. The coefficient of performance
(COP) is a non-dimensional measure of steady state efficiency at a single set of operating
conditions. The COP of typical air conditioners is about 3, which means that an air conditioner
removes 3 units of heat from a space for every unit of electrical work it consumes. Air
conditioner efficiency varies with the temperature of the air returned to the air conditioner and
the temperature of the outdoor air to which heat is rejected. The average efficiency rating of air
conditioners over a season is reported as the Seasonal Energy Efficiency Rating (SEER) with
units Btu/Wh. The electricity consumed by an air conditioner can be calculated using either COP
or SEER with proper attention to units.
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Welec = Qac / η ac = Qnet,in / COPac = Qnet,in / SEER
Example I - Cooling Load
Consider a building has the following average loads in summer:
Qpeople = 3,500 Btu/h, Qsolar = 11,000 Btu/h, Qelec = 7,500 Btu/h, Qwalls =
2,500 Btu/h, Qwindows = 3,000 Btu/hr, Qdoors=750 Btu/hr, Qceiling = 750
Btu/hr, Qinfiltration = 3,000 Btu/hr. Calculate the Qnet,in of the building.
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Solution
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Qnet,in = (Qsolar + Qpeople + Qelectricity + Qwalls + Qwindows + Qceiling + Qdoors +
Qinfiltration)in
Qnet,in = (11,000 + 3,500 + 7,500 + 2,500 + 3,000 + 750 + 750 + 3,000) Btu/hr
Qnet,in = 32,000 Btu/hr
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The air conditioner for this building operates 1,000 hours over the summer, with a COP of 3.2.
Electricity costs $0.10 /kWh. Calculate the cost of electricity for the air conditioner over the
summer.
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Qac = Qnet,in = 32,000 Btu/hr
Welec = Qac / COP = (32,000 Btu/hr / 3.20) / 3,412 Btu/kWh= 2.93 kW
Welec, yr = Welec x HPY = 2.93 kW x 1,000 hr/yr = 2,930 kWh/yr
Celec, yr = Welec,yr x Celec = 2,930 kWh/yr x $0.10 /kWh = $293 /yr
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Example II - Cooling Load
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Calculate the cost of operating an air conditioner over a summer if the
average net cooling load is 36,000 Btu/hr, the SEER is 12 Btu/Wh, the air
conditioner operates 1,000 hours over the summer, and electricity costs $0.10
/kWh.
Solution - II
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Qac = Qnet,in = 36,000 Btu/hr
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Welec = Qac / SEER = (36,000 Btu/hr / 12 Btu/Wh) = 3.00 kW
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Welec, yr = Welec x HPY = 3.00 kW x 1,000 hr/yr = 3,000 kWh/yr
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Celec, yr = Welec,yr x Celec = 3,000 kWh/yr x $0.10 /kWh = $300 /yr