Energy Efficient Buildings
Chillers
Vapor Compression Refrigeration
In a vapor compression cycle, the compressor raises the temperature of the refrigerant high
enough that it can reject heat to the atmosphere, then the expansion valve reduces the
temperature of the refrigerant low enough that it can absorb heat from the conditioned space.

QH
3
2
Condenser
Expansion
device

Wi

QL
Compressor
4
1
Evaporator
PH
T
Pressure drop effect
T

QH
3
2c
2
PL
3'

h
4
2
Condenser with
pressure drop
Wcomp

Throttling
1
Actual compression
3
Pressure drop effect
Subcooling
4 4'
Pressure drop
effect
QC
Isentropic compression
2'
1
1'
Evaporator with
pressure drop
S
S
a) Ideal cycle
Superheat at
evaporator exit
b) Real cycle
Ideal Cycle
Heat transfer to evaporator and from condenser at constant pressure
Isentropic compression
Adiabatic expansion
Real Cycle
Small pressure drop through condenser and evaporator
Sub-cooling to ensure 100% liquid into throttling value
Super-heating to ensure 100% vapor to compressor
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Irreversibilities during compression increase entropy, but heat loss from compressor reduces
entropy.
Compressors
Compressors are important since Win = Wcomp.
Compressing vapor requires more work than compressing liquid since:
dw = v dP and vliquid << vvapor
Reciprocating Compressors
Small (.1 to 50 tons)
Noisy
Turn on when compressing and completely off when idle
Often multiple reciprocating compressors staged on/off to meet loads.
Results in high part-load efficiency
Screw Compressors
Mid-sized (50-300 tons)
Quiet
Compress by squishing vapor between screws
At part-load, vapor flow reduced, but screw keeps rotating. Thus, poor part-load efficiency
unless motor slowed with VSD
Centrifugal Compressors
Large (300 tons +)
Vapor compressed as spun to outside
Like screw compressors, don’t stop impellor at part-load. Thus poor part-load efficiency unless
motor slowed by VSD.
Most centrifugal compressors are integrated into chiller with tube and shell heat exchangers for
condenser and evaporator.
Evaporators
An evaporator is a heat exchanger that absorbs heat from the conditioned space. While
absorbing heat, the refrigerant “evaporates” from liquid to vapor. An expansion value at exit of
evaporator pushes vapor to super heat region prior to entrance into the compressor. The two
primary types of evaporators are:
Refrigerant–to–Air
Finned tube HX, Fins on air side since hair<<href
Used in small to medium chillers
“Direct Expansion”, “DX” since refrigerant directly cools air as it expands
2
Refrigerant–to–Water
Tube-in-shell HX
Ref in shell, water in tubes
Used in large centrifugal chillers
“Chilled” water pumped from evap to air handling unit chilled water coils
Condensers
A condenser is a heat exchange that rejects heat from building and compressor to the
environment. While rejecting heat, the refrigerant “condenses” from vapor to liquid. The total
heat rejected by the condenser is the sum of the heat from the building and the work added to
the compressor.
Qcondenser = Qevaporator + Wcompressor
The two types of condensers are:
Refrigerant-to-Air
Finned tube HX, with fins on air side
Called “Air Cooled”
Tcondenser = Tair
Small to med chillers
Refrigerant-to-Water
Tube in shell with vapor on shell side
Condenser water cooled in cooling tower
Tcondenser water < Tair, due to evaporation
Colder condenser temperature increases efficiency
Increased efficiency is worth price of cooling towers in big chillers
Efficiency and Temperature
Chiller efficiency is a function of the temperatures that it absorbs heat from and rejects heat to.
An energy balance on a chiller shows that compressor work is the difference between the heat
rejected by the condenser and the heat absorbed by the evaporator.
Wcomp = Qcond – Qevap
Because dQ = T dS, on a TS diagram, the heat rejected by the condenser is the area under the
condenser line. The heat absorbed by the evaporator, Qevap, is the area under the evaporator
line. The compressor work, Wcomp, is the area below the condenser line and above the
evaporator line. The efficiency is the ratio of energy absorbed by the evaporator to the work
required by the compressor.
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PH
T
PH
T
PL
PH
PL
PL
S
a) base case
T
S
b) lower evap temperature
S
c)higher cond temperature
To remove heat from cold reservoirs, the evaporator temperature must be less than the
reservoir temperature. Thus, removing heat from cold reservoirs requires a lower evaporator
temperature. As the evaporator temperature declines, the cooling capacity (Qevap) declines
and the compressor work increases. Thus, the efficiency of a chiller declines when it must
absorb heat from a lower temperature reservoir.
To reject heat to the environment, the condenser temperature must be higher than the
environment. Thus, rejecting heat to a warmer environment requires a higher condenser
temperature. As the condenser temperature increases, the compressor work increases. Thus,
the efficiency of a chiller declines when it rejects heat to a higher temperature reservoir.
These concepts can be quantified by considering the following system. Heat naturally flows
from high to low temperature reservoirs. Chillers move heat from a low temperature reservoir
to a high temperature reservoir, requiring external work. Efficiency is the ratio of useful output
to required input.
TENV

Q cond


Wcomp
Q evap
TROOM
Actual Efficiency
Efficiency = Qevap / Wcomp
Efficiency = Qevap / (Qcond – Qevap)
Efficiency ~ 3.0
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Ideal (Carnot) Efficiency
Efficiency = Qevap / Wcomp
From an energy balance: Wcomp = Qcond - Qevap
Thus, Efficiency = Qevap / (Qcond – Qevap)
dQ = T dS
Thus,
Efficiency = (T dS)evap / ((T dS)cond – (T dS)evap)
In an ideal cycle, no turbulence or friction occurs during expansion and compression. Thus, no
entropy is produced and
(dS)evap = (dS)cond
Heat transfer from the condenser to the environment takes place at an infinitely small
temperature difference over a very long period of time. Thus
Tenv = Tcond and Troom = Tevap
And the ideal (Carnot) efficiency is:
Ideal Efficiency = Troom / (Tenv – Troom).
Example
Consider a chiller extracting heat from a room at 70 F and rejecting it to ambient at 90 F.
Compare the actual efficiency of a typical chiller of 3.0 to the ideal efficiency.
The ideal efficiency is:
Ideal Efficiency = (70 + 460)room / ((90+460)env – (70+460)room) = 26
The discrepancy between real and ideal efficiencies indicates lots of irreversibilities and room
for improvement. Irreversibilities include:
Fast compression
Fast expansion
Friction (pressure drops)
Heat transfer
Reducing these irreversibilities are the key design issues for improving chiller performance.
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Air-Cooled Chillers
Air cooled chillers reject heat to the atmosphere by blowing air over a fin-tube heat exchanger
through which flows the refrigerant. Air condensers are less expensive and easier to maintain
than the cooling towers required by water-cooled chillers. However, the evaporative effect of
cooling towers enables water cooled chillers to operate at lower condenser temperatures and
pressures, which increases chiller efficiency. Thus, air-cooled chillers are recommended for
applications in which low first cost and maintenance costs outweighs the reduced efficiency
compared to water cooled chillers.
The energy efficiency of air-cooled chillers is rated in terms of Energy Efficiency Ratio (EER).
EER is a dimensional measure of efficiency. It is the ratio of the rate of cooling to electrical
power consumption by the evaporator and the condenser fans.
EER (Btu/Wh) = Qevap (Btu/hr) / (Wcomp + W condfans) (W) @ ARI Std 590-92 conditions
The performance of a typical air cooled chiller is shown below. As predicted by a Carnot
analysis, efficiency improves when the temperature of air entering the condenser declines or
the leaving water temperature increases.
Air-Cooled Chiller Evaporator and Condenser Temperature Performance Map
LWT
°F
20
25
30
35
40
45
50
55
60
Percent
Glycol
28
24
19
14
0
0
0
0
0
Tons
11.4
12.9
14.5
16.1
17.9
19.5
21.3
23.1
24.9
75.0
Capacity System
KW
15.7
16.1
16.6
17.0
17.5
17.9
18.4
18.9
19.4
EER
8.7
9.6
10.5
11.4
12.3
13.1
13.9
14.7
15.4
Entering Condenser Air Temperature (°F)
95.0
115.0
Capacity System
Capacity System
Tons
KW
EER
Tons
KW
10.2
19.2
6.4
8.8
23.8
11.5
19.7
7.0
10.0
24.4
13.0
20.2
7.7
11.3
24.9
14.5
20.7
8.4
12.6
25.5
16.1
21.2
9.1
14.1
26.1
17.6
21.7
9.7
15.5
26.7
19.2
22.3
10.3
16.9
27.2
20.8
22.8
11.0
18.4
27.9
22.5
23.4
11.6
19.9
28.5
EER
4.4
4.9
5.4
5.9
6.5
7.0
7.5
7.9
8.4
Using the data in the preceding table, the following relation gives chiller EER as a function of
leaving water temperature when the condenser air temperature is 75 F. The R2 of the
regression is 0.9998.
EER = 4.688564 + 0.20999 * LWT -.000518143 * LWT ^ 2
The performance of another air cooled chiller, as functions of outdoor air temperature and
part-load ratio is shown below.
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Chiller Efficiency (kW/ton)
3
2
Outdoor
Temperature
35°C
25°C
15°C
1
0
0.2
0.4
0.6
0.8
1
Chiller Part-Load Ratio
Air-Cooled Chiller Part-Load and Condenser Temperature Performance Map. Source of data:
Chan, K. and Yu, F., 2004, “How Chillers React to Building Loads”, ASHRAE Journal, August, pp
52-58.
Water–cooled Chillers
The energy efficiency of water-cooled chillers is typically described in terms of specific power:
kW/ton = Wcomp / Qevap
The nominal kW/ton rating of a chiller is reported at 100% load and ARI standard conditions of
44°F leaving chilled water and 85°F inlet condenser water. Typical nominal kW/ton ratings are
shown below.
Chiller
Reciprocating
Screw
Centrifugal High
Moderate
New (kW/ton)
.78 to .85
.62 to .75
.50 to .62
.63 to .70
Older (kW/ton)
.90-1.2
.75-.85
.70-.80
Chiller efficiency is dependent on the temperature of chilled water leaving the chiller and the
temperature of condenser water entering the chiller. For example, chiller efficiency improves
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by about 1.5%, for every degree F increase in leaving chilled water temperature. This means
increasing chilled water supply temperature from 44°F to 54°F can cut chiller power use by
about 15%. Similarly, chiller efficiency improves by about 1.5%, for every degree F decrease in
entering condenser water. This means decreasing the temperature of entering condenser
water from the cooling tower by 10 F, can cut chiller power use by about 15%.
In addition, chillers seldom operate at full load since design conditions rarely occur. A
performance map for a standard-efficiency constant-speed chiller as a function of entering
condenser water temperature and part load is shown below.
Standard-Efficiency Water-Cooled Chiller Performance Map
A relation for the specific power of this standard-efficiency chiller, KWPT kW/ton, as a function
of part load, PL, and condensing water temperature CDWT can be derived from regressing data
from the curves shown above. The relation and regression coefficients are shown below. The
R2 for this relation is 0.98.
KWPT = a + b PL + c PL2 + d CDWT + e CDWT2 + f (CDWT) (PL)
a
b
c
d
e
f
0.57341
-1.2023
0.79481
0.0051964
2.2926 E-05
-0.000805732
A performance map for high-efficiency chillers showing constant speed (red) and variable speed
(blue) chiller performance as a function of entering condenser water temperature and is shown
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below. At equal condenser water temperature, the constant speed chiller uses less kW/ton at
full load than the variable speed chiller. However, variable-speed chiller performance improves
significantly at part load, and approaches 0.15 kW/ton at some operating conditions. The
declining specific power at low loads indicates excellent control efficiency.
High-Efficiency Water-Cooled Constant-Speed And Variable-Speed Chiller Performance Map.
Source: http://ateam.lbl.gov/cleanroom/doc/Applied_Final.pdf. Lawrence Berkeley National
Laboratory, Applications Team.
A relation for the specific power, KWPT kW/ton, as a function of part load, PL, and condensing
water temperature CDWT for the VSD chiller can be derived from regressing data from the
curves shown above. The relation and regression coefficients are shown below. The R2 for this
relation is 0.98.
KWPT = a + b PL + c PL2 + d PL3 + e CDWT + f CDWT2
a
b
c
d
e
f
-0.18972
-1.4381
2.5595
-1.2500
0.010935
-2.1739 E-05
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Thermal Storage
For most large buildings, the total charge for electricity includes a charge for both the total
energy used over a month (kWh) and the peak electrical power during the month (kW).
Depending on the particular rate structure, the peak electrical demand charge can be larger
than the total energy charge. Thus, electricity costs can be reduced by reducing peak power
use (kW) even if total energy use (kWh) remains the same or increases. Moreover, in many
large buildings the electrical power use during business hours is much greater than during
nighttime and weekend hours. Thus, operating the chiller only during nights and weekends
could shift chiller power from peak to off-peak periods and reduce electricity costs.
One strategy for shifting chiller use to off-peak periods is to over-cool the building at night and
use the buildings thermal mass to offset cooling loads during the day. This strategy requires
occupants to accept a temperature rise throughout the day as the building slowly warms up. A
second strategy is to use the chillers to create ice during off-peak periods and then use the ice
to cool the building during on peak periods.
Source: http://energy.gov
Absorption Chillers
Absorption chillers use heat instead of electricity to produce cooling. The cycle is similar to
vapor compression, except that the mechanical compressor is replaced by a generator and
absorber. The generator uses heat to raise the pressure and temperature of the refrigerant
before rejecting heat in the condenser.
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Qout
Condenser
Generator
Expansion
Valve
Thermal
Compressor
Qin
Win
Pump
Absorber
Qout
Evaporator
Qin
The cycle works by using two fluids, a refrigerant and an absorbate. Most cycles use water as
the refrigerant and lithium bromite as the absorbate. After leaving the evaporator, the
refrigerant vapor is absorbed into the liquid absorbate. The pump raises the pressure of the
combined refrigerant/absorbate liquid. The pump uses significantly less energy than a
compressor since the pump is raising the pressure of a liquid with small specific volue, while a
compressor is raising the pressure of a vapor with large specific volume.
w = v dP
Heat is added to the generator to evaporate the high pressure refrigerant out of the absorbate.
The high pressure refrigerant vapor flows through the condenser to reject heat, and the liquid
absorbate is returned to the absorber through a pressure reduction valve.
Absorption chillers can have one or two stages of generators and absorbers. The COP of double
effect absorption chillers is about 1.0. Absorption chillers may be especially appropriate in
applications with available heat or high electrical demand costs.
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