Energy Efficient Buildings
Air Conditioners and Heat Pumps
Types of Air Conditioners
Three principle types of air conditioners are used in residential and small commercial
buildings.
Window units typically sit in windows. One fan draws room air through the evaporator
and another fan draws outdoor air through a condenser. Cooling capacity ranges from
5,000 to 24,000 Btu/hr and the seasonal energy efficiency rating (SEER) is about 10
Btu/W-hr.
In split systems, the evaporator is located indoors in the air handling unit and the
compressor, condenser and condenser fan are located outdoors on a concrete pad.
Cooling capacities range from 20,000 to 60,000 Btu/hr with seasonal energy efficiency
ratings (SEER) between 13 and 22 Btu/W-hr.
In rooftop or package units, the evaporator, supply air fan, compressor, condenser and
condenser fan are packaged together. Cooling capacities range from 2 to 50 tons with
seasonal energy efficiency ratings (SEER) between 10 and 22 Btu/W-hr.
1a) Window unit
1b) Outdoor part of split unit
1c) Package unit
1
The Vapor Compression Air Conditioning Cycle
Ideal and actual vapor compression air conditioning cycles are shown below.
Ideal
Actual
An energy balance on the system yields:
 comp  Q cond  0
Q evap  W
SS
Thus, condenser must reject heat from space plus compressor electricity
 comp
Q cond  Q evap  W
Air Conditioner Components
Air conditioners have five primary components.
1. Condenser Heat Exchanger and Fan
o Fin/tube HX to transfer heat from refrigerant to outdoor air
o From energy balance, Qcond = Wcomp + Qevap
o Condenser fan for a typical 3-ton unit draws about 3,000 cfm of air through the
condenser at a very low pressure drop and uses about 220 W.
2
2. Evaporator Heat Exchanger and Fan
o Fin/tube HX to transfer heat from indoor air to refrigerant
o Evaporator is sized according to peak cooling load.
 cfm 
o Evaporator (supply air) fan ~ 400 
 cooling @ ΔPstatic  .5in  H2O
 ton 
o Evaporator supply-air fan for a typical 3-ton unit moves about 1,200 cfm across
the evaporator at a pressure drop of about 0.5 in H20 and uses about 325 W.
3. Compressor
o Compressors, which raise the temperature of the refrigerant to a high enough
temperature that it can reject heat to the environment, are the biggest energy
user in an air conditioning system. The work transferred to the refrigerant vapor
by a compressor is:
 rev  v dP
W

Compressors use much more power than pumps to deliver an equivalent
pressure rise, because the specific volume of a vapor is much larger than for a
liquid. For example, a compressor compressing water vapor would use about
1,600 times more power than a pump pumping water liquid to increase the
pressure by an equivalent amount, since the specific volume of water vapor is
about 1,600 times greater than liquid water.
 vg 
 vap  1600  W
 liq for same ΔP!
 
 1600 ! Thus, W
 v f water
o For 3 ton AC, Wcomp ~2,700 W
o Most residential air conditioners use scroll compressors.
4. Pressure Reduction Valve
o Hard to see
o Located at inlet to evaporator to minimize heat gain from environment
5. Refrigerant
o The main design criterion is to find a substance that changes from liquid to vapor
at proper temperatures and pressures.
o Also want non-toxic, non-corrosive, low cost, etc.
o The first refrigerant was ammonia.
o Low cost, good thermal properties, but toxic
o 1930s Dupont & GM develop CFCs = Freon = R11& R12
o “Answer” to all problems
o 1970s – Discover that CFCs destroy upper atmospheric ozone and increase UV
3
o 1980s – Discover that CFCs are potent green house gasses
o Response is:
o Montreal Protocol to phase out CFCs
o Revised Montreal Protocol (1987) says no CFCs after 1995 and no HCFCs after
2010
o CFC = Freon (bad)  HCFC R123, R22 (better)  HFC = Puron = R410a (best)
o For a 3-ton AC
o Diameterliquid line  5 " , Diametersuction line  3 "
15
4
o Charge R22 ≈ 116oz.
Power Use Breakdown
Electrical energy use by component for a typical 3-ton air conditioner is shown below.
 tot  W
 cond fan  W
 evap fan  W
 comp  220 W  325 W  2,700 W  3,245 W
W
The fractional breakdown of energy use by component is:
 evap fan
W
325

 9%

Wtot
3245
 cond fan
W
220

 7%

Wtot
3245
 comp 2700
W

 84%
 tot
W
3245
Efficiency and Coefficient of Performance (COP)
Efficiency is the ratio of useful output to required input. For an air conditioner, the
useful ‘output’ is the heat removed from the space and the required input is the
electrical energy. The efficiency ( of devices such as air conditioners and refrigerators
that extract more energy from a space than electrical energy consumed is also called
Coefficient of Performance (COP):
η
Q
Q evap
Euseful
 evap 
 COP
 comp Q cond  Q evap
Erequired W
Efficiencies of Ideal and Actual Systems
In an ideal system, the compressor and pressure reduction valve operate isentropically
with no entropy generation, and all heat transfer to and from the evaporator and
condenser occurs at constant temperature as shown in the figure below. Heat transfer
by the evaporator and condenser is the product of the temperatures and entropy
changes.
Q
 TcondΔS
Q
  TdS  cond
T
and Q
Q evap  Tevap ΔS

cond
4
1
2

3
Q evap
4
S
Thus, the efficiency of an ideal Carnot air conditioner is given by:
ηcarnot 
Q evap
Tevap Δs
Tevap


Q cond  Q evap TcondΔs  Tevap Δs Tcond  Tevap
Moreover, in an ideal system, the evaporator and condenser are infinitely large so heat
can be transferred into and out of the refrigerant with no temperature difference
between the refrigerant and the air. Thus, the evaporator temperature is equal to the
temperature of the space being cooled and the condenser temperature is equal to the
outside air where heat is being rejected.
Tcond  Toutside air

Tevap  Tinside air
Hence, the efficiency of an ideal air conditioner operating between an indoor air
temperature of 70 F and an outside air temperature of 90 F is:
ηcarnot 
Tia
70  460

 26.5
Toa  Tia 90  460   70  460 
This indicates that an ideal air conditioner could transport 26.5 times more cooling
energy than work input energy! Unfortunately, actual air conditioners have lots of
irreversibilities and the efficiency of actual air conditioners operating between these
temperatures is about 3.5. This demonstrates the tremendous potential for improving
the performance of air conditioners by reducing friction, turbulence, temperature
differences and motor inefficiencies.
Energy Efficiency Ratio (EER)
Another measure of air conditioner efficiency is energy efficiency ratio (EER). EER is
dimensional measure of efficiency at conditions specified by Air-Conditioning &
Refrigeration Institute (ARI).
EER 

Q evap  Btu 
@ ARI
 elec  W  h 
W
Q evap
 comp  W
 cond fan  W
 evap fan
W
 Btu 
 W  h @ ARI


ARI conditions for determining the EER of air conditioners are:
5
Tia  80F, Tia, wb  67F, Toa  95F,
Seasonal Energy Efficiency Ratio (SEER)
Another measure of efficiency is seasonal energy efficiency ratio (SEER). SEER attempts
to characterize the average efficiency over an entire cooling season. SEER is determined
by measuring air conditioner performance at more moderate conditions than the
standard used to measure EER.
SEER  Seasonal Energy Efficiency Ratio

  Btu 
Q evap


@ SEERconditions
W
 comp  W
 cond fan  W
 evap fan   W  h 


ARI conditions for measuring the SEER of air conditioners are:
Tia  80F, Tia, wb  67F, Toa  80F,
Thus, SEER is greater than EER since Toa is lower. For example an air conditioner with an
EER of 10.75 may have an SEER of 12.
Minimum Efficiency Standards
On August 17, 2004, the U.S. Department of Energy published a technical amendment
on minimum efficiency standards for residential air conditioners and heat pumps (DOE,
2008). The amendment stated that all residential air conditioners and heat pumps
manufactured after January 23, 2006 would have to meet the following standards.
On January 12, 2001, the U.S. Department of Energy published a final rule on energy
efficiency standards for commercial package air conditioners and heat pumps (DOE,
2008). The rule raised the previous EPCA standards to those adopted by ASHRAE 90.1
6
1999 for all equipment manufactured after October 29, 2003 or October 29, 2004 for
large packaged units. The previous and new standards are shown below.
Efficiency Standards for Commercial Package Air Conditioners and Heat Pumps
On Novermber 11, 2004, the U.S. Department of Energy recommended new standards
for commercial package air conditioners and heat pumps pumps (DOE, 2008). The
recommendation was that commercial package air conditioners and heat pumps
manufactured after January 1, 2010 would have to meet the following standards.
Where two EERs are listed, the first refers to systems with electric resistance heat or no
7
heating, and the second refers to systems with all other heating-system types that are
integrated into the unitary equipment.
Air-Cooled Products
>65,000 - <135,000 Btu/h
>135,000 - <240,000 Btu/h
Efficiency Standards
11.2/11.0 EER for Air Conditioners
11.0/10.8 EER for Heat Pumps
3.3 COP @ 47ºF for Heat Pumps
11.0/10.8 EER for Air Conditioners
10.6/10.4 EER for Heat Pumps
3.2 COP @ 47ºF for Heat Pumps
Efficiency Standards for Residential Air Conditioners and Heat Pumps
Energy Star
Energy Star rating for products in top 15% of energy efficiency.

Q cond
Energy Use Example
Calculate daily cost of running AC if cooling load on house is:


Wcomp

Q evap
Q CL
AC
 Btu 
 Btu 
12,000 
, SEER  10 

 , and electricity costs $0.10 per kWh.
 hr 
W  h
 CL  Q
 evap  0, Q
 evap  Q
 CL , SEER 
E-balance on house: Q
Q evap
Q
 elec  evap ,
, W
 elec
W
SEER

 elec  Δt  UC elec  Q CL  Δt  UC elec
Celec  W
SEER
 hr 
1  kW 
 Btu  1  W hr 
 $  $2.88
 12,000 
 

 24 
 0.10 






day
 hr  10  Btu  1000  W 
 kWh 
 day 
The Effect of Outdoor Air Temperature on Capacity and Efficiency
PH

dQ=TdS
Q evap  Area under 2  3 ////

Q cond  Area under 4  1 \ \ \ \

Wcomp  Net area under 4  1 and above 2  3 
Q cond
T
Tcond
Toa
4
1
PL

Wcomp
3
2

Q evap
S
8
This means that when Toa increases,
o ΔT  Tcond  Toa decreases and Q cond decreases
o With less heat removed by the condenser, Tcond increases
 comp (area in the box) increases and η decreases
o When Tcond increases, W
o Thus, as Toa ↑, η ↓
 comp increases…
o Also, when Q cond decreases, and W
 evap  Q cond  W
 comp   Q evap decreases
o Q
o Thus, as Toa ↑, Q evap ↓
In summary:
COP
Q evap  cooling capacity
Toa

Thus, lowering outside air temperature increases both efficiency and cooling capacity.
Air Conditioning Psychrometrics
The properties of moist air as it passes over an evaporator coil are shown on the
psychrometric chart below. When air traveling over the evaporator is cooled below its
dew-point temperature, water condenses out of the air and it becomes dryer. The total
cooling is:
V
 a h1  h2   1 h1  h2 
Q tot  m
v1
The total cooling can be decomposed into latent and sensible cooling
Q tot  Q lat  Q sen
 a h1  h2   m
 a h2  h2 
Q tot  m
The amount of water removed is:
9
w m
 a w1  w2  
m
V1
w1  w2 
v1
V1 (ft3/lba)
h1
h2'
1
h2 (ft3/lba)
ω1 (lbw/lba)
Qlat
2
T2(F)
Qsen
2'
ω2
T1
Air Conditioning Sizing and Duct Systems



The Air Conditioning Contractors of America (ACCA) reports that 70% of air
conditioners are oversized. Oversizing, especially in humid climates, decreases
the ability of the air conditioner to remove moisture from the air and may
increase humidity control problems.
Manufacturers expect air conditioners to be coupled to an efficient duct system
with 0.5" of static pressure drop across the fan. The static pressure drop created
by the duct system in the average home is 1.8" water column, which is over 3
times greater than the design pressure drop. A major contributor to this
pressure drop is undersized return air systems. Up to 90% of houses have return
air filter grilles and ducting that are too small. At these pressure drops, air flow
to the house is dramatically reduced, which compromises system performance.
According to the Air Conditioning Contractors of America (ACCA), an acceptable
duct leak rate is 3%; however, the average house has a 35% to 44% leak rate.
This causes ducting to lose about 1/3 of the air that should be delivered to the
space. If this air is lost to unconditioned space such as the attic, it will cause
under-pressurization of the conditioned space and drive infiltration. Infiltration
air often enters from the attic at temperatures as high as 130 F. Thus, poor duct
systems compromise the performance of even the highest efficiency air
conditioners (Galveston Air Conditioning, 2008).
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Heat Pumps
An air conditioner can be called a “heat pump”, since it “pumps heat uphill” from Tia (~
70°F) to Toa (~90°F).
Toa

Q OA
WE
Tia


Q HP

QH
AC
This suggests that the same technology could be used during winter to heat a house by
“pumping heat uphill” from the outside air at a low temperature to the inside air at a
higher temperature. Heat pumps are air conditioners that have been modified to pump
heat into a house during winter and out of a house during summer.
Toa

Q OA


WE
Q HP
Tia

QH
HP heat in winter
and cool in summer
HP
Heat pumps are a good choice for climates with moderate heating requirements since
the a heat pump eliminates the need for natural gas service and a separate furnace, and
since heat pump performance is better at moderate temperatures than extreme
temperatures.
Heat Pump Components
Heat pumps are made of the same components as air conditioners. However, the
diagrams below show how heat pumps reverse the flow of the refrigerant during cooling
and heating modes. This is accomplished using four valves and bypass piping around the
compressor. It enables the interior heat exchanger to act as a condenser when the heat
pump is in heating mode and an evaporator when in cooling mode. Similarly, the
exterior heat exchanger acts as an evaporator when the heat pump is in heating mode
and a condenser when in cooling mode.
11


Q HP (From Tia)
Q HP ( To Tia )
1
4
2
1
COND
Comp Piping/Value Detail
EVAP


WE
(HEATING)
WE
(COOLING)
COND
EVAP
2
B
3
4

3
A
A
B
Cool
A Open
B Closed
Heat
A Closed
B Open

Q A ( To Toa )
Q A (From Toa)
Ideal and Actual Heating Cycles
As in cooling mode, an ideal heating cycle has isentropic compression and expansion,
and negligible temperature differences between the air and refrigerant in the
evaporator and condenser. When operating between 30 F outside and 70 F inside, the
ideal Carnot efficiency is about 13.25, as shown below.
η


Euseful
 COP  Q H   Q HP 
Erequired
WE Q HP  Q A
η
THds 14
THds 14  TAds 23
for Carnot : ds 14  ds 23 , TA  Toa , TH  Tia
ηc 
Tia
Tia  Toa
For Tia  70F and Toa  30F
ηc 
70  460
 13.25
70  460   30  460 
In an actual cycle the temperature differences between the air and refrigerant in the
evaporator and condenser is at least 10 F. These temperature differences alone reduce
the efficiency to 9.0. Friction, turbulence, heat gain and motor inefficiency reduce the
actual efficiency during heating mode to about 1.5 to 3.0. This indicates the
tremendous potential for improved energy efficiency with improved design.
PH
4adb
4s
T
ΔTHX  10F
4act
PL
1
2
η
70  10  460
 9.0
70  10  460   30  10  460 
3
S
12
The Effect of Outdoor Air Temperature on Capacity and Efficiency
As shown in the figures below, heat pump capacity and efficiency decline as outdoor air
temperature decreases.
-
T
4
1
TA
-
As Toa decreases, ΔTevap = Toa – TA
decreases, so Q A decreases
A W
 e  As Q A decreases, so
Q HP  Q
does Q HP
-
3
2
Thus, as Toa ↓, Q HP (heating capacity) ↓
QHP
S

Q evap
Toa
-
T
1
4
-
E
As Toa ↓, TA ↓ from TA1 to TA2, thus W
↑
Thus, as Toa ↓, η (= COP) ↓

2
TA1
TA2
3
WE
COP
Toa
S
Heat Pump Efficiency Performance Metrics
As for air conditioners, the average cooling efficiency of heat pumps is characterized
using Seasonal Energy Efficiency Rating (SEER).
SEER 
Q HP  Btu 
 e  W  h 
W
Average heating efficiency is characterized using Heating Season Performance Factor
(HSPF).
HSPF 
Q HP  Btu 
 e  W  h 
W
13
The corresponding Coefficient of Performance (COP) is.
 W  h
COP 

3.412Btu
W h
SEER Btu
COP 
 W  h

3.412Btu
W h
HSPF Btu
Testing procedures for SEER and HSPF are specified in ANSI/AHRI Standard 210/240 2008 (formally ARI 210/240). As of 2011, minimum SEER is 13.0 and the minimum HSPF
is 8.0. Manufacturers typically offer models with efficiencies ranging between about 13
to 18 SEER and 8.0 to 9.5 HSPF.
Heat Pump Sizing and Balance Temperature
The size of an air conditioner or furnace is typically selected so that the equipment can
deliver the required cooling or heating at the most extreme conditions expected in a
given climate. However, heat pumps supply both heating and cooling, and peak heating
and cooling loads are rarely identical. Thus, a heat pump could be sized to meet the
peak cooling load or the peak heating load, but rarely can it be sized to meet both peak
loads.
In most cases, the size of the heat pump is determined to meet the peak cooling load.
This eliminates the problem of over sizing the unit and creating humidity control
problems, or under sizing the unit and not being able to deliver enough cooling. If sizing
for the peak cooling load results in extra heating capacity, there is no significant heating
performance penalty. If sizing for the peak cooling load results in insufficient heating
capacity, the additional heat is typically supplied using electric resistance heating.
The quantity of electrical resistance heating required can be determined using the
concept of a heat pump balance temperature. To understand heat pump balance
temperature consider the following graph. Heating capacity of a heat pump decreases
and the heating load of a house increases at colder outdoor air temperatures. The
outdoor air temperature where the heat pump capacity equals the heating load is called
the heat pump balance temperature. Below this temperature, additional heat is
supplied to the house by electrical resistance heating.
14
Q
Toa

QA

WE
QHP,bigger QHP
QH = UA(Tia-Toa)
Under Capacity
Auxiliary Heat
Tia

Over
Capacity

QHP
QH
HP
(In Heating Mode)
TBal,HP
TBal,HP,bigger
TC
Ground-Source Heat Pumps
Air-source air conditioner efficiency and capacity decline at high ambient air
temperatures when cooling loads are typically largest. Similarly, air-source heat pump
heating efficiency and capacity decline at low ambient air temperatures when heating
loads are typically largest. Ground-source heat pumps reject and extract heat from the
ground rather than from the air. The temperature of the ground a few feet below the
surface is close to the annual average temperature. Thus, the ground is typically cooler
than the air during summer and warmer than the air during winter. This energy storage
capacity results in a cooler sink for heat extracted from buildings during the summer
and a warmer source of heat for buildings during winter. Thus, using the ground as a
heat sink and source increases the efficiency and capacity of a heat pump in both
cooling and heating modes. This principle is demonstrated by the figures below.

Q HP
1
Winter
Heating
Mode
T
90F
TG  TOA
TOA
4
2
50F

WE
3
TG
Ground Loop

QG

WE , pump
10F
QG for TG
In winter, source of heat @ TG > Toa. Increases η & Q HP
In summer, sink for heat @ TG < Toa. Increases η & Q
HP
15
Water-to-air systems use water in the ground-loop and produce warm or cool air for the
house. Water-to-water systems use water in the ground loop and produce warm or
cool water for domestic hot water and/or hydronic heating and cooling. As of 2008,
about 1 million ground-source heat pump systems have been installed in the U.S. and
about 50,000 new systems are installed each year (Masia, 2008).
Typical Ground-Source Heat Exchanger Configurations
Ground-source heat pumps exchange heat with the ground by pumping fluid through
polyethylene or plastic tubing. The tubing can be configured in buried coils or vertical
shafts. Coils of tubing can also be submerged in water. In open systems, well water is
pumped to the heat pump and then discharged. A 3-ton system requires about 1,500
linear feet of looped buried polyethylene coils.
Source: www.energysavers.gov/your_home/space_heating_cooling/
16
In both types of systems, pumping energy is a parasitic load which reduces the overall
energy efficiency of the system and is not included in heat-pump performance metrics.
Pumping energy can be reduced by control systems that employ variable speed pumps
and reduce water flow during periods of low loads.
Efficiency Ratings for Ground-Source Heat Pumps
ARI has three standards for rating ground-source heat pumps, but none of these
standards actually represent average seasonal performance
H/C Enter Water Temp
H/C Entering Air Τemp
Pump Work
ARI 320
Water Source
70/85
70/80
ARI 325 Ground
ARI 330
Water Source
50/70
32/77
70/80
70/80
Gpm*[(5*ΔPpsi)+65] .8*gpm*(ΔPftH2O+17)
Performance specifications for a typical nominal 2-ton ground-source heat pump are
shown below.
ISO 13256-1PERFORMANCE DATA Rated at 800 CFM and 5.0 GPM
Water Loop
Ground Water
Ground Loop
Cooling
Heating
Cooling
Heating
Cooling
Heating
Capacity EER Capacity COP Capacity EER Capacity COP Capacity EER Capacity COP
25,500 15.2
29,000
5.1
29,500 24.3
22,600
4.3
27,400
18
17,500
3.6
COOLING EFT Range (Standard), 45°F to 110°F. All performance at 350 CFM and 3.0 GPM
Entering
Fluid
Temp.
50°
70°
100°
Ent. Air
Total
Wet Bulb Capacity
Temp.
BTUH
61°
28028
64°
29387
67°
30769
70°
32174
73°
33601
61°
25605
64°
26847
67°
28109
70°
29392
73°
30696
61°
21971
64°
23036
67°
24119
70°
25220
73°
26339
Watts
Input
1158
1183
1209
1235
1261
1502
1535
1569
1603
1637
2019
2084
2109
2154
2200
Heat
Sensible Capacity BTUH
Rejection
Ent. Air Dry Bulb Temp.
BTUH 75°
80°
85°
31978
21330
25580
28025
33424
19954
25332
28094
34894
18368
24000
27692
36387
15316
21170
27541
37904
18044
24730
30731
19488
23378
25605
32086
18229
23142
25685
33462
16781
21925
25298
34861
13891
19340
25160
36281
16484
22592
28860
16720
20059
21971
30078
15841
19357
22022
31315
14399
18513
21707
32571
12005
16595
21558
33845 14144
19385
EER
24.2
24.5
25.4
26.1
26.6
17.0
17.5
17.9
18.3
18.8
10.9
11.2
11.4
11.7
12.0
17
HEATING EFT Range (Standard), 25°F to 80°F. All performance at 350 CFM and 3.0 GPM
Entering
Fluid
Temp.
50°
80°
Heating Heat of Power
Dry
Capacity Abs.
Input COP
Bulb
BTUH BTUH Watts
60°
25433 19818 1546 4.6
70°
24124 18402 1677 4.2
80°
22585 16743 1712 3.9
60°
35523 29294 1826 5.7
70°
33694 27348 1860 5.3
80°
31544 25065 1899 4.9
CONDENSER
Water Flow (GPM) Press. Drop (FOH)
2.0
1.0
3.0
2.3
4.0
4.1
5.0
6.4
6.0
9.2
8.0
16.4
BLOWER
Power (hp)
Static Pressure (inches water)
Speed (RPM)
0.5
0.6
800
The U.S. Department of Energy estimates that residential scale ground-source heat
pump systems have an installed cost of about $2,500 per ton and payback in about 5years (Masia, 2008).
References
Figure 1a: http://www.energystar.gov/ia/news/images/roomac-sm.jpg
Figure 1b: http://cstx.gov/images/1630102812004energy_16.jpg
Figure 1c:http://upload.wikimedia.org/wikipedia/commons/9/90/
Department of Energy, 2008,
http://www1.eere.energy.gov/buildings/appliance_standards/
Galveston Air Conditioning, 2008, http://www.galvestonairconditioning.com.
Masia, S., 2008, “Ground-source heating”, Solar Today, Vol. 22. No. 6.
18