Figure 4: Complex refrigerant reversible system with sliding header Simple COP analysis
Full heating or full cooling only Pumps and compressors 28 kW
Load 100 kW System COP 3.57
Full simultaneous heating and cooling N/A
Simple COP analysis
Full heating or full cooling only Pumps and compressors 28 kW
Load 100 kW System COP 3.57
Half simultaneous heating and cooling Cooling load 50 kW
100 kW Nil 50 kW
Pumps and compressors 14 kW System COP 3.57 Heating load 50 kW
Pumps and compressors 14 kW System COP 3.57
50 kW Nil Simple COP analysis
Full heating or full cooling only Pumps and compressors 28 kW
Load 100 kW System COP 3.57
Full simultaneous heating and cooling N/A
100 kW
50 kW 0.75 kW
Both heat pump evaporators ‘cold side’ going to load
Both heat pump condensers ‘hot side’ going to ground
0.75 kW 0.75 kW
Cooling mode 50 kW
0.75 kW
Cooling mode 50 kW
Sliding header with valves to open or close fl ow paths
0.75 kW
‘cold side’ going to load
0.75 kW
Heat pump evaporator
Cooling mode 50 kW
0.75 kW
Heating mode 50 kW
Heat pump condenser ‘hot side’ going to load
0.75 kW 50 kW in
Heating mode
0.75 kW 50 kW in 0.75 kW 0.75 kW 0.75 kW
Both heat pump evaporators ‘cold side’ going to ground
Heating mode
50 kW
Both heat pump condensers ‘hot side’ going to load
Ground array
Ground array The main drawback with this system is
These inappropriate systems are not ‘value engineered’ for the operator. They are, however, a much easier and cheaper option for an inexperienced and unqualifi ed installer
that it must use one compressor to provide the heating and one to provide the cooling. As a result it is a comparably ineffi cient means of providing simultaneous heating and cooling. It also requires pumping to deliver fl uid to both sides of all the heat pumps in operation, creating an increased ‘parasitic’ input pumping power. We have seen claims that the coeffi cient of performance (COP) of the heat pumps operating together can be added together for an overall system COP, but this is an incorrect methodology for calculating COPs, as can be seen in the COP calculation in Figure 4. So, this system can provide simultaneous
heating and cooling, but not effi ciently in terms of input energy or pumping power. It is best suited to providing separate heating and cooling loads that do not occur simultaneously or, if there are some simultaneous loads, these are not regular throughout the year (as circled in fi gure 5) and the lower system effi ciency is tolerable for these short durations in return for a relatively simple system. Heat pumps – regardless of type and/
54 CIBSE Journal November 2012
Ground array
or operation – have a hot and a cold side by virtue of the evaporator and condenser operation. An effi cient means of providing simultaneous heating and cooling is by the use of both sides of a heat pump, or multiple heat pumps.
Option 3: Simple refrigerant reversible system Figure 6 is a simple schematic using three port valves to switch between the ground array and the load on both the evaporator and the condenser. This does truly provide simultaneous heating and cooling at 100% of the units’ capacity. A further advantage of this system is the
ability to use thermal storage to increase the amount of load that can be generated simultaneously. With previous example systems, there is no point in providing thermal storage because there is no real increase in effi ciency. However, with the evaporator/condenser both providing load, a morning heating load can be used to charge a thermal store with chilled water ready for the afternoon cooling, and the store is depleted or used as needed prior to, or in conjunction with, bringing on the
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