HEAT PUMPS SIMULTANEOUS HEATING AND COOLING Load shifting with thermal storage
160 140 120 100 80 60 40 20 0
Morning and afternoon loads to store
The thermal store option can still be used with this type of system to again load shift from one part of a day to another, to further enhance the effi ciency of the overall system. In true and simplistic effi ciency terms,
Cooling kWh Heating kWh
this may not quite reach the levels of a three- port valve system. However, the very simple pipe work – and controls – plus the ability to maintain two set points as opposed to one, provide compelling reasons to accept this slight drop in performance of the overall system over what is a very simple option.
Conclusions The analysis carried out in this paper can only be done with hourly loads. Anything else is futile and merely a guess. Hourly loads must be provided for even
Figure 7: A load profi le that warrants thermal storage to provide as much load simultaneously as possible
valves. Again, using a thermal store is a good way of using this uncontrolled load.
Option 4: Six-pipe heat pump with heat recovery The fi nal example is of a six-pipe heat pump with heat recovery (see fi gure 8). Importantly, internal to the heat pump,
the load is apportioned to either the CHW system, LTHW system or the ground array. The BMS merely calls for either the LTHW, CHW, or both loads and the heat pump delivers both loads to set points.
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
Pumps and compressors 28 kW
Load 200 kW System COP 7.14
Part simultaneous heating and cooling
Pumps and compressors 29.5 kW
Load 200 kW System COP 6.78
1.5 kW
Three internal refrigerant heat exchangers to CHW, LTHW and ground
25 kW in
Heat recovery heat pump. Delivers cooling and heating to set point and directs excess load to ground
Cooling mode
1.5 kW 1.5 kW Heat recovery system 100 kW 100 kW
the most simple types of systems and this must quickly become commonplace. Option one cannot provide the simultaneous loads. Option two can only provide the simultaneous loads ineffi ciently; this system is cheap to install but expensive to run.
Option three can provide truly simultaneous heating and cooling with the effi ciency of only using one compressor, or bank of compressors, to do it and thermal storage can further enhance the system effi ciency. Option four can do all of the same things
as system three, but is less complex in terms of controls and piping, and isolates the LTHW and CHW from the ground array fl uids. Options three and four present by far the most effi cient systems for simultaneous heating and cooling, on both lifecycle and carbon savings, beat option two hands down. It’s simple – the initial cost to the operator of these inferior systems appears low. However 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. The practice of installing cheap but
ineffi cient and inappropriate systems must stop. Production of hourly loads for even relatively simple systems must become the norm and be carried out very early in the overall building concept and design process. It is our job to help clients understand that the cheapest to install may actually prove to be a very expensive on-going mistake. CJ
Figure 8: A six pipe heat recovery system Ground array
Andy Howley is a certifi ed geo-exchange designer (CGD) and also is the technical director of Loopmaster Europe,
www.loopmastereurope.co.uk
58
CIBSE Journal November 2012
www.cibsejournal.com
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