HOT&COLD BLOWING
The installation of cheap but ineffi cient and inappropriate ground source heat pumps to achieve ‘simultaneous’ heating and cooling must stop, declares Andy Howley. Here he analyses four systems to see which works best
W District system monthly load profi le
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hat is simultaneous heating and cooling? Is it cooling and heating at the same time? Is it cooling and heating at the
same time with the same equipment? Or, is it heating and cooling loads in the same day, week or month? Loopmaster Europe sees all manner of systems asking for simultaneous heating and cooling and there is often very little understanding of the energy fl ow or the associated timings of the different loads. If we consider a conventional heating and
cooling system using boilers and chillers, simultaneous heating and cooling, is quite clearly possible with two separate systems running at the same time. The boiler can provide low temperature hot water (LTHW) on demand and the chiller system can provide chilled water (CHW), again on demand, assuming the distribution system is capable. The two function independently of each other and can provide heating and
cooling up to the maximum plant capacity if required, at the exact same minute of the day. As regards heat pumps, what would a simultaneous heating and cooling system look like? Firstly, we must defi ne just how ‘simultaneous’ the load actually is, to decide which system best suits the loads. Is it simultaneous on a ‘second by second’ basis, or on an hourly basis? Is there a heating load in the morning and a cooling load in the afternoon? Is there a distinct heating season and a separate cooling season? All of the above have, at some time, been described as ‘simultaneous’ in specifi cations and tender documents. However, only the fi rst – on a ‘second
Monthly cooling kWh Monthly heating kWh
Figure 1: Annual district heating system load profi le
by second’ basis – is truly a simultaneous heating and cooling load. Hourly and daily loads can be shifted to a certain degree to provide them simultaneously to a store to be used when required. Of course, using ground source systems for heating and cooling provides energy storage in the ground; the summer’s heat rejection can be stored in favourable geology and used to enhance the heating performance during the winter, and vice versa in the summer. So, in order to determine how simultaneous a load is, we need load data. Figure 1 shows an annual profi le by month for a ground source heating and cooling system in Yorkshire. Although this is probably suffi cient to design a simple system, does this provide suffi cient information to decide which type of heat pump combination is the most carbon and operationally cost effective? Looking at the month of May, this
52 CIBSE Journal November 2012
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SLAVO VALIGURSKY / SHUTTERSTOCK
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