ASHRAE Journal - April 2021 - 28

TECHNICAL FEATURE

As two separate tanks are used in the 4PC,
FIGURE 5 Example illustrating the steady-state conditions for two cases of 4PC WSHP operation;
two pumps operate in heating mode (top) or in cooling mode (bottom).
each tank contains the same amount of water
but at different temperatures. The cold tank
40°F
50°F
Cold Return Water
receives relatively cold water from the WSHPs
Warm Return Water
operating in heating mode, and the warm
40°F
40°F
50°F
ON
ON
ON
tank receives relatively warm water from the
2.4 gpm
Heat Added
Absorb:
Absorb:
Reject:
WSHPs operating in cooling mode, as shown
Cold
12 kBtu/h
Warm
12 kBtu/h 12 kBtu/h 12 kBtu/h
Water
Water
Warm
Boiler
in Figure 8. The 2PC tank temperatures are
50°F
Tank
Tank
Supply Water 4.8 gpm
ON
mostly (but not always) between the 4PC cold
40°F 2.4 gpm 45°F
and warm tank temperatures. This means
Heat Rejected OFF Cold Supply Water
CT
0 kBtu/h
that the temperature of the water supplied to
the 4PC WSHPs operating in cooling mode is
90°F
lower than the water temperature for the 2PC.
100°F
Cold Return Water
Warm Return Water
Similarly, the temperature of the water sup90°F
100°F 100°F
ON
plied to the 4PC WSHPs operating in heating
ON
ON
2.4 gpm
Heat Added
Absorb:
Reject:
Reject:
mode is higher than the water temperature
0 kBtu/h
Cold
Warm
12 kBtu/h 12 kBtu/h 12 kBtu/h
Water
for the 2PC. This improves the efficiency of
Water
Warm
Boiler
100°F
Tank
Tank
Supply Water 2.4 gpm
the WSHPs because they operate at reduced
100°F OFF
95°F
4.8 gpm
90°F
lifts.
ON
Heat Rejected
Cold Supply Water
CT
Let us look at each case separately. In Case
12 kBtu/h
1, as shown in Figure 8, right, when all WSHPs
are in heating mode, the tank temperature
FIGURE 6 Example illustrating the transient periods for two cases of 4PC WSHP operation; two
pumps
operate in heating mode (top) or in cooling mode (bottom).
drops gradually from the initial value of 70°F
(21°C) to below 50°F (10°C)-when the boiler
50°F
60°F
Cold
Return
Water
should turn on to maintain the water supWarm Return Water
50°F
60°F
50°F
ply temperature at 50°F (10°C), as shown
ON
ON
ON
2.4 gpm
in Figure 10. In the 4PC, a large temperature
Heat Added
Absorb:
Absorb:
Reject:
Cold
0 kBtu/h
difference between the cold and warm tanks
Warm
12 kBtu/h 12 kBtu/h 12 kBtu/h
Water
Water
Warm
Boiler
60°F
occurs, delaying when the 4PC boiler starts to
Tank
Tank
Supply Water 4.8 gpm
*60°F OFF
50°F 2.4 gpm
operate as opposed to the case in the 2PC. Due
*Previous Time Step. Heat Rejected OFF Cold Supply Water
to the higher water supply temperature in the
CT
Water
Temperature Leaving 0 kBtu/h
4PC, as shown in Figure 8, left, the 4PC COPh
the Tank Drops Over Time.
for all three WSHPs is greater than for the 2PC
80°F
COPh (an increase of around 1.2%), as shown
90°F
Cold Return Water
Warm
Return
Water
in Figure 9. Case 1 represents the transient
80°F
90°F
90°F
ON
ON
ON
period, and if the steady-state condition is
2.4 gpm
Heat Added
Reject:
Reject:
Absorb:
reached, the boiler will add the same amount
Cold
0 kBtu/h
Warm
12 kBtu/h 12 kBtu/h 12 kBtu/h
Water
Water
of heat, and the COPh will be the same for both
Warm
Boiler
90°F
Tank
Tank
Supply Water 2.4 gpm
configurations.
*80°F 4.8 gpm 100°F OFF
80°F
In Case 2, when lower heating loads are
*Previous Time Step. Heat Rejected OFF Cold Supply Water
CT
assumed, the two WSHPs in heating mode
Water Temperature Leaving 0 kBtu/h
receive water at the same temperature of 55°F the Tank Drops Over Time.
(13°C) and operate at the same COPh in both
7% higher than the 2PC's COPc. Because the 4PC sepaconfigurations. However, because the cold tank in the
rates the water between two tanks, one contains more
4PC contains water at a temperature lower than 50°F
heat than the other, and the 4PC boiler adds less heat to
(10°C), the WSHP in cooling mode receives colder water
as opposed to the case in the 2PC, making the 4PC's COPc the water loop as compared to the 2PC boiler (Case 1 and
28

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