ASHRAE Journal - April 2021 - 26

TECHNICAL FEATURE

modes, whereas in the 4PC, the relatively cold FIGURE 3 Example illustrating the transient periods for two cases of 2PC WSHP operation; two
pumps operate in heating mode (top) or in cooling mode (bottom).
water at 65°F (18°C) is supplied to the WSHP
operating in cooling mode, and relatively
56.6°F
warm water at 75°F (24°C) is supplied to the
50°F
70°F
50°F
one operating in heating mode. This scenario
ON
ON
ON
improves the efficiency of both heat pumps
Absorb:
Reject:
Absorb:
due to the reduction of the heat pump lift.
12 kBtu/h
12 kBtu/h
Heat Added
Heat Rejected 12 kBtu/h
Let us examine the cases in which excess
Tank
0 kBtu/h
0 kBtu/h
2.4 gpm
2.4 gpm
2.4 gpm
or deficient heat occurs in the water loop
CT
Boiler
60°F
60°F
7.2 gpm
OFF
OFF
60°F
as shown in Figure 5 for steady-state conditions and in Figure 6 for transition conditions.
Compare the steady-state condition example
83.3°F
in Figure 2 with the one in Figure 5. The tem90°F
90°F
70°F
perature of the water delivered to all WSHPs
ON
ON
ON
in the 2PC is 90°F (32°C) for the case of excess
Absorb:
Reject:
Reject:
Heat Added
Heat Rejected 12 kBtu/h
12 kBtu/h
12 kBtu/h
heat or 50°F (10°C) for the case of deficient
Tank
0 kBtu/h
0 kBtu/h
heat.
2.4 gpm
2.4 gpm
2.4 gpm
CT
Boiler
However, in the 4PC, the temperature of the
80°F
80°F
7.2 gpm
OFF
80°F
OFF
water is 100°F (38°C) for the WSHPs operating in heating mode instead of 90°F (32°C) in
the case of excess heat, and it is 40°F (4°C) for
FIGURE 4 Four-pipe configuration (4PC) WSHP. (Tanks are not installed.)
the WSHP operating in cooling mode instead
65°F
Cold Return Water
75°F
of 50°F (10°C) for the case of deficient heat. In
Warm Return Water
Isolation Valve
both cases, it is 10°F (5.6°C) colder or warmer,
75°F
65°F
ON
OFF
ON
making those heat pumps more efficient as
Heat Added
0 gpm
Absorb:
Reject:
they operate with lower lifts.
Warm
0 kBtu/h
12 kBtu/h 0 kBtu/h 12 kBtu/h
Warm
Water
Boiler
A similar scenario can be found during the
2.4 gpm 75°F
Supply Water
75°F
Tank
OFF
65°F
transient periods (compare Figure 3 with Figure
2.4 gpm
OFF
6). In the transition period, the supply water
Heat Rejected
Cold Supply Water
CT
0 kBtu/h
temperature varies over time until the steadystate conditions are obtained.
To discuss the performance of 4PC vs. 2PC WSHPs and
(Figure 8), the WSHP coefficient of performance in coolthe energy savings obtained by the 4PC, an example
ing mode (COPc) and in heating mode (COPh) (Figure 9),
and the amount of heat added by the boiler or removed
with three 2 ton (7 kW) WSHPs is used. The WSHP coolby the cooling tower (Figure 10). The COP of each 2 ton
ing and heating loads are assumed as shown in Figure
(7 kW) WSHP at a constant flow rate of 4.5 gpm (0.3 L/s)
7, representing six different cases: all three WSHPs in
is obtained from manufacturers' data as a function of
heating mode (Case 1), most WSHPs in heating mode
supply water temperature as shown in Figure 11.
(Case 2), all WSHPs off (Case 3), most WSHPS in coolOne common tank of 810 gallons (3066 L) is used for
ing mode (Case 4), two WSHPs in cooling mode and the
the 2PC, and two tanks of 405 gallons (1533 L) each are
other off (Case 5) and all WSHPs in cooling mode (Case
used for the 4PC. The 810 gallon (3066 L) tank for the
6). Each case covers a time of four hours with constant
2PC is sized to supply water for about one hour (i.e.,
loads to demonstrate both transient and near steady4.5 × 3 × 60 = 810 gallons). The percentage changes in
state conditions.
COPc and COPh when the 4PC system is used instead of
Figures 8 through 10 show the 2PC and 4PC results,*
the 2PC are illustrated in Figure 9.
including the tank and water supply temperatures
*The water temperature of the tank is found by solving numerically first order differential equations with a time step of one minute. The
equations are obtained from applying energy and mass conservations for each tank.
26

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