The
materials from which a trailer is constructed, as well as the
systems and appliances installed there, can dramatically affect
the amount of energy that the trailer will consume over its lifetime.
To help people compare the potential impact of one to another,
efficiency ratings have been devised for many trailer components
and energy systems.
A variety
of energy ratings now abound, which can be confusing to the
consumers these ratings were intended to help. We will try here
to end that confusion by explaining each of the ratings systems
listed below in as simple a way as possible.
Fabrication
Materials
The materials
from which a trailer is constructed can have a marked impact
on the structure's efficiency. Materials that allow a lot of
heat to pass through them lower the overall efficiency level
of the building. Conversely, materials that resist a significant
amount of heat transference can help ensure greater efficiency.
The degree to which a trailer component (such as a window or
wall system) transfers heat is referred to as its U-value. The
ability of an individual material (for instance, glass, wood,
metal) to resist heat transfer is called its R- value.
Appliances
and Equipment
When referring
to the efficiency of an appliance or energy system, we are actually
talking about how much energy that system must use to perform
a certain amount of work. The higher its energy consumption
per unit of output, the less efficient the system is. For example,
an air conditioner that requires 750 watts of electricity to
provide 6,000 Btu of cooling will be less efficient than one
that can provide the same amount of cooling for only 500 watts.
The most common ratings applied to energy systems are EER and
SEER for most central cooling systems; COP for some heat pumps
and chillers; HSPF for heat pumps in their heating modes; and
AFUE for gas furnaces and boilers.
EER
EER (energy
efficiency ratio) is a measure of how efficiently a cooling
system will operate when the outdoor temperature is at a specific
level (usually 95¡ F). A higher EER means the system is more
efficient. The term EER is most commonly used when referring
to window and unitary air conditioners and heat pumps, as well
as water-source and geothermal heat pumps. The formula for calculating
EER is:
Btu
of cooling at 95°
EER = ___________________
watts
used at 95°
For instance,
if you have a window air conditioner that uses 1500 watts of
electricity to produce 12,000 Btu of cooling, it would have
an EER of 8.0 (12,000 divided by 1500). A unit using 1200 watts
to produce the same amount of cooling would have an EER of 10
and would be more energy efficient.
Using this
same example, you can see how efficiency can affect a system's
operating economy. First, you=ll need to determine the total
amount of electricity B measured in kilowatt-hours B the unit
will consume over a period of time. (A kilowatt-hour is defined
as 1,000 watts used for one hour. This is the measure by which
your monthly utility bills are calculated.) To do this, let's
assume you operate your 8 SEER window air conditioner B drawing
1500 watts at any given moment B for an average of 12 hours
every day during the summer. At this rate, it will use 18,000
watt-hours, or 18 kilowatt-hours (kWh) each day, leading to
a total consumption of 540 kWh over the course of a 30-day month.
At a summer electric rate of 6.344 per kWh, it would cost you
$34.24 to operate that window air conditioner each month. At
the same time, a 1200-watt, 10-SEER system, consuming 14.4 kilowatt-hours
per day and 432 kWh per month, would cost you $27.39, a 20%
savings over the less efficient model.
SEER
SEER (seasonal
energy efficiency ratio) measures how efficiently a residential
central cooling system (air conditioner or heat pump) will operate
over an entire cooling season, as opposed to a single outdoor
temperature. As with EER, a higher SEER reflects a more efficient
cooling system. SEER is calculated based on the total amount
of cooling (in Btu) the system will provide over the entire
season divided by the total number of watt-hours it will consume:
seasonal
Btu of cooling
SEER
= _____________________
seasonal
watt-hours used
By federal
law, every central split cooling system manufactured or sold
in the U.S. today must have a seasonal energy efficiency ratio
of at least 10.0. To qualify for the CWLP Air Conditioner Rebate,
a new central air conditioner must have a SEER of at least 11.0.
Heat pumps with SEERs of 10.0 or higher can qualify for CWLP's
Heat Pump Rebate.
COP
COP (coefficient
of performance) is the measurement of how efficiently a heating
or cooling system (particularly a heat pump in its heating mode
and a chiller for cooling) will operate at a single outdoor
temperature condition. When applied to the heating modes of
heat pumps, that temperature condition is usually 47¡F. The
higher the COP, the more efficient the system. COP can be calculated
by two different methods. In the first, you divide the Btu of
heat produced by the heat pump by the Btu equivalent of electricity
that is required to produce that heat. This formula is stated:
Btu
of heat produced at 47¡F
COP
= _____________________________
Btu-worth
of electricity used at 47¡
For instance,
let's assume a heat pump uses 4000 watts of electricity to produce
42,000 Btu per hour (Btu/hr) of heat when it is 47¡F outside.
To determine its COP, you would first convert the 4000 watts
of electrical consumption into its Btu/hr equivalent by multiplying
4000 times 3.413 ( the number of Btu in one watt-hour of electricity).
Then you would divide your answer B 13,648 Btu/hr B into the
42,000 btu/hr heat output. This would show your heat pump to
have a 47¡F COP of 3.08. This means that, for every Btu of electricity
the system uses, it will produce a little more than three Btu
of heat when the outdoor temperature is 47¡F.
The second
formula is most frequently used to determine chiller efficiency.
Using this calculation method, you would divide 3.516 by the
number of kilowatts (kW) per ton used by the system. This formula
is stated:
3.516
COP
= _________
kW/ton
For example,
a chiller that consumes 0.8 kW per ton of capacity would have
a COP of 4.4 (3.516 divided by 0.8). On the other hand, the
COP of a new, more efficient chiller, using as little as 0.5
kW per ton, would be greater than 7 (3.516 divided by 0.5).
HSPF
HSPF (heating
seasonal performance factor) is the measurement of how efficiently
all residential and some commercial heat pumps will operate
in their heating mode over an entire normal heating season.
The higher the HSPF, the more efficient the system. HSPF is
determined by dividing the total number of Btu of heat produced
over the heating season by the total number of watt-hours of
electricity that is required to produce that heat. The formula
is written:
Btu
of heat produced over the heating season
HSPF
= ___________________________________________
watt-hours
of electricity used over the heating season
Most heat
pumps installed in Springfield today have HSPFs in the 7.0 to
8.0 range, meaning they operate with seasonal efficiencies of
anywhere from 205% to 234%. (To convert the HSPF number into
a percentage, you just divide the HSPF by 3.414, the number
of Btu in one watt-hour of electricity.) That means that, for
every Btu-worth of energy they use over the entire heating season,
these systems will put out anywhere from 2.05 to 2.34 Btu of
heat. Compare this to electric furnaces, which have nominal
efficiencies of 100% (for each Btu worth of electricity, they
put out one Btu of heat), or new gas furnaces, which have efficiency
ratings of about 80% to 97% (for each Btu worth of gas, they
put out 0.8 to 0.97 Btu of heat).
NOTE: When
comparing energy systems that use different primary fuel sources
with different costs per Btu, it is important that you understand
that higher operating efficiency is not necessarily equivalent
to better operating economy. Although an electric heat pump
might work with greater efficiency than a gas furnace, it won't
necessarily be more economical to run due to the pricing difference
between the two fuel sources.
AFUE
AFUE (annual
fuel utilization efficiency) is the measurement of how efficiently
a gas furnace or boiler will operate over an entire heating
season. The AFUE is expressed as a percentage of the amount
of energy consumed by the system that is actually converted
to useful heat. For instance, a 90% AFUE means that for every
Btu worth of gas used over the heating season, the system will
provide 0.9 Btu of heat. The higher the AFUE, the more efficient
the system.
When comparing
efficiencies of various gas furnaces, it is important to consider
the AFUE, not the steady state efficiency. Steady state refers
to the efficiency of the unit when the system is running continuously,
without cycling on and off. Since cycling is natural for the
system over the course of the heating season, steady state doesn't
give a true measurement of the system's seasonal efficiency.
For instance, gas furnaces with pilot lights have steady-state
efficiencies of 78% to 80%, but seasonal efficiencies B AFUEs
B closer to 65%.
Virtually
all gas forced-air furnaces installed in this area from the
1950s through the early 1980s had AFUEs of around 65%. Today,
federal law requires most gas furnaces manufactured and sold
in the U.S. to have minimum AFUEs of 78%. (Mobile home furnaces
and units with capacities under 45,000 Btu are permitted somewhat
lower AFUEs.) Gas furnaces and boilers now on the market have
AFUEs as high as 97%.
R-Value
R-value
is the measurement of how effectively a material resists the
transfer of heat via conduction. The higher the R-value, the
less heat transfer can take place.
Some materials
are more resistant to heat transfer than others, giving them
higher R-values. One of the best ways to enhance the product's
R-value is to increase the amount of gas (including air) inside
or immediately surrounding it. For instance, the glass of a
single-pane window has virtually no R-value, but the thin film
of air that normally exists on either side of the glass gives
the window an R-value of about 0.83. Adding a second pane of
glass and sealing the space between the panes will increase
the thickness of one of the insulating gas layers, thereby more
than doubling the window's R-value.
Another
example of how the presence of Adead-air@ spaces affect a product's
R-value can be seen with wood. Hard woods, like oak, typically
have an insulating value of R-1 per inch of thickness. However,
softer woods, such as pine, might have R-values twice as high
due to their greater number of air-filled pores.
Products
developed especially for the purpose of impeding unwanted heat
transfer are called insulation. Insulation can be made of a
variety of materials, including old newspapers and wood fibers,
glass fibers, and synthetic foams. It can also come in a variety
of configurations, including soft Ablankets,@ rigid boards,
or fluffy granular Aloose-fill,@ but what they all have in common,
is their abundance of air-filled pores or pockets.
The actual
R-value of insulation products can vary greatly, depending on
their composition and form. The least resistant and least common
are perlite and vermiculite loose-fills, at R-2.2 to R-2.7 per
inch of thickness; the most resistant are polyisocyanurate rigid
boards, at R-7 per inch of thickness. Fiberglass blankets and
cellulose loose-fills, two of the most common residential insulations
have R-values of 3.1 to 3.7 per inch.
U-Value
U-value
is the measurement of how much heat can be conducted through
a building component (such as a wall or window). As such, it
is the opposite of R-value, which measures the ability of material
to resist heat conduction. The higher the U-value, the more
heat the material(s) will allow to be transferred through it.
The lower a material's U-value, the higher its R-value will
be. U-values are always expressed in decimals (e.g., U-0.166).
To determine
the R-value of a product for which the U-value is given, you
first convert the U-value to its equivalent fraction and then
invert it. For instance, the equivalent fraction of U-0.166
would be 166/1000 or 1/6. This inverts to 6/1 or 6, giving you
an R-value of 6.
For most
consumers, U-value is likely to be of concern only when shopping
for new windows, where efficiency is frequently stated in terms
of U-value rather than R-value.
