HEAT PUMPS SIMULTANEOUS HEATING AND COOLING
would appear to be virtually balanced in terms of monthly load, but just how much of this occurs at the same time? This can only be determined with an hourly load profi le. If we concentrate on simultaneous
heating and cooling in the true sense of the description – that is, at the exact same time – which heat pump confi gurations actually deliver it? And which deliver and provide carbon savings to shout about? Of course, one can enhance the design
of any system, including the fi rst of four examples outlined below, but all of these comparisons are based on a basic system confi guration, as outlined in the following schematics for comparison purposes.
Option 1: Single refrigerant reversible heat pump Let’s fi rst look at a single refrigerant reversible heat pump, which is a packaged unit with one or two compressors with a ‘ground side’ and a ‘load side’ fl uid fl ow path. Heating is provided by using the ground side heat exchanger in the heat pump as an evaporator and the load side as a condenser. When cooling is required, the reversing valve changes the fl ow direction of the refrigerant and the ground side then becomes the condenser and the load side becomes the evaporator. This system can only deliver either a
heating load or a cooling load in its simplest form. It cannot deliver both at the same time from the heat pump. This type of system is relatively simple and is suited to a load profi le that has totally separate heating and cooling loads, even if it is only hours apart (see fi gure 2 and 3). In short, this set up cannot provide simultaneous heating and cooling directly from the heat pump and, quite often, the heating and cooling emitters are the same system, keeping controls relatively simple. This system is well suited to domestic or small commercial systems where control is by thermostat or even simple manual ‘on demand’ switching.
Option 2: Slider header systems To use refrigerant reversible heat pumps – as many do – to provide simultaneous heating and cooling, multiple units are needed and a header confi guration that can direct fl ow to either the CHW or LTHW systems separately. These header systems are often referred to as a sliding header, as shown in fi gure 4. This system can provide 100% heating load and 100% cooling load by opening
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Non-simultaneous heating and cooling loads
60 50 40 30 20 10 0
Cooling kWh Heating kWh
Figure 2: Morning and afternoon separate loads can be delivered by a simple reversible heat pump system
and closing two port valves in the header to divert to either the LTHW or CHW systems. However, it can only provide 50% heating and 50% cooling simultaneously with two heat pumps, and 33%/66%, for example, with three heat pumps. Either way, it cannot provide full or greater than 50/50 simultaneous heating and cooling unless additional heat pumps are used to increase the overall capacity. For example, to provide 100% heating and cooling simultaneously, 200% heat pump capacity is required. This system will also provide the load to a controlled set point for both heating and cooling loads.
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
evaporator ‘cold side’ going to load
condenser ‘hot side’ going to ground
1.5 kW Heat pump Heat pump 1.5 kW 1.5 kW 25 kW in
Heat pumps change mode by way of a reversing valve, which changes the fl ow of the hot and cold refrigerant
1.5 kW
Cooling mode
Heat pump evaporator ‘Cold side’ going to load
25 kW in
Heating mode
Heat pump evaporator ‘cold side’ going to load
Heat pump condenser ‘hot side’ going to load
The analysis carried out in this paper can only be done with hourly loads. Anything else is futile and merely a guess
Simple refrigerant reversible system 100 kW
Only one load, either heating or coolong can be delivered
100 kW
Ground array Figure 3: A simple refrigerant reversible heat pump system in heating and cooling
Ground array
November 2012 CIBSE Journal 53
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