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Heat pumps 1 CHP Heat pump efficiencies Heating 1.0 Text - 0°C


Electrical energy


required per unit of heating


0.5


0


0


50


100


Tin (°C)


7


5 6


Ideal


coefficient of performance


4


1 2 3


0 0 50 100 Tin (°C)


Figure 1: Top graph shows ideal electrical energy required to pump heat into a place at temperature Tin when the heat is being pumped from a place at temperature Tout = 0C. Bottom graph: the efficiency is conventionally expressed as a coefficient of performance – the heat pumped per unit of electrical energy Source: Sustainable Energy – Without the Hot Air. ©David MacKay


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this technology does form part of a building services design, and why gas-fired combined heat and power (CHP) should still be considered in dense urban sites.


Heat pumps A heat pump undoubtedly can be a useful heat source. It increases the efficiency of electrical heating by transferring heat from one source to another using a refrigerant cycle. Typically this is from outside, the ground or the air, to inside. MacKay’s book considers the theoretical limits of coefficient of performance (COP) of heat pumps and analyses some of the implications of these. There are also some practical considerations that should be taken into account when using this equipment. CIBSE Guide F: Energy Efficiency in Buildings states


that: ‘A drop in condensing temperature [heat supply] of 1C reduces energy use by around 3%... A rise in the evaporator temperature [heat source] of 1C reduces energy use by approximately 3%.’ (See section 8.3.1.) This principle is demonstrated in two graphs


in MacKay’s book (see Figure 1) detailing the ideal efficiency of a refrigeration cycle. It can be summarised succinctly as: the closer the inside temperature is to the


34 CIBSE Journal August 2010


outside temperature, the better for refrigeration cycle efficiency (see page 301 of the book for the details). This means that the further the outside conditions


are from the inside requirements, the less efficient the heat pump. Therefore good engineering practice is to incorporate low-temperature heating systems within a building (such as underfloor heating systems) and warm temperature heat sources (the ground) to maximise efficiency. However, the situation changes when domestic hot


water (DHW) loads are considered. As the building fabric insulation efficiency of new (in particular) or existing residential building stock is increased, the dominating thermal load becomes DHW. This is required to be heated and stored at 55C to 60C in the UK to prevent bacterial growth. The alternative is to instantaneously heat water, though this is only suitable for low flow rates and a heat pump is not normally able to provide this. Hot water demand can be reduced using techniques


such as aeration and flow limiters, but there is still a requirement to raise water temperature from 10C to approximately 60C, whichever system is selected. Figure 1 shows that heating something to 60C using an external source at 0C has an ideal efficiency (COP)


www.cibsejournal.com Text - 0°C


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