ASHRAE Journal - April 2021 - 30

TECHNICAL FEATURE

COP

Boiler or CT Loads Btu/h (Thousands)

COPc

COPh

main common supply, it may be difFIGURE 9 Percentage changes in COPc (left) and COPh (right) when 4PC is installed instead of 2PC.
ficult to use different design water
Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
flows for heating and cooling modes.
1.4
8
Thus, the simulations have been
WSHP1
WSHP1
1.2
7
done for various options of water
WSHP2
WSHP2
1
WSHP3
flow rates and tank sizes as shown in
WSHP3
6
0.8
Table 1.
5
0.6
The tank size ranges from 0 gal4
0.4
lons when no tank is installed,
3
to up 30,000 gallons (113 562 L).
0.2
2
The 10,000 gallon (37 854 L) tank can
0
deliver water to WSHPs for up to one
1
-0.2
hour. For a fair comparison, the size of
-0.4
0
0
4
8
12
16
20
24
0
4
8
12
16
20
24
the common tank used for the 2PC is
Hours
Hours
equal to the total size of the 4PC tanks.
Figure 12 shows the annual energy savFIGURE 10 Heat added by boiler and heat removed by the cooling tower (CT) for 2PC and 4PC.
ings and cooling tower and boiler load reducCase 1
Case 2
Case 3
Case 4
Case 5
Case 6
tions due to the 4PC. The resulting WSHP
80
energy savings due to the 4PC could range
CT 2PC
CT 4PC
Boiler 4PC
Boiler 2PC
from 1% up to 5% with larger tank sizes.
60
As an example, when no tanks are
40
installed for both configurations, the WSHP
20
energy savings ranges from 0.5% to 1.5%,
depending on design water flow rates, and
0
it ranges from 1.5% to 5% when the tank size
-20
is 30,000 gallons (113 562 L). Reducing the
water flow rate or increasing the tempera-40
0
4
8
12
16
20
24
ture difference across the heat pump in the
Hours
4PC increases WSHP energy savings due to
the higher temperature difference between
FIGURE 11 COPc and COPh variations with supply water temperature.
cold and warm water. This also leads to lower
required heat to be rejected by the cooling
9
8
tower or added by the boiler. If the amount
y=-0.0002x2 + 0.0563x + 1.5627
7
of heat to be added to or removed from the
R2 = 0.9995
6
water loop is kept the same for both con5
4
figurations, the energy savings in the WSHP
3
operation will be significantly higher than
y=0.0003x2 - 0.114x + 10.816
2
R2 = 1
shown in Figure 12.
1
00
In the 4PC, only the water supplied to the
20
40
60
80
100
120
Supply Water Temperature (°F)
WSHPs operating in cooling mode flows
COP
COP
Fit
Fit
through the cooling tower, reducing its
Cooling
Heating
(COP Heating)
(COP Cooling)
required pumping power and making the
cooling tower more efficient as it operates
with the higher entering temperature from
mode flows through the boiler, reducing pumping
the warm water tank (instead of the loop-average tempower and improving the boiler efficiency as colder
perature) and reduced water flow rate. Similarly, just
water from the cold tank enters the boiler. The potenthe cold water supplied to WSHPs operating in heating
tial additional savings in the 4PC from water pumps
30

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ASHRAE Journal - April 2021

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