of approximately 5.0. This is an ideal efficiency – it can be approached, but never achieved because of losses in the refrigeration cycle. If the temperature source reduces further below
0C, the COP reduces further, by roughly 3% per each 1C reduction in heat source temperature in line with the principle quoted earlier. The decrease in efficiency for an air-source heat pump is actually worse at these air temperatures. This is because as the receiving coil temperature reduces below 0C, ice forms on the coil from water in the air, preventing thermal transfer to the refrigerant. This then requires a reversal in the cycle or an electric heater to defrost using more energy, and further reducing efficiency and therefore moving away from the simple 3% principle quoted. This is a particular problem in the UK, which is a maritime climate, where the relative humidity of the air is close to saturation and water, and therefore ice can form with relative small changes in temperature. Together, these effects can reduce actual air-source
COPs substantially. In some cases, equipment at these higher temperatures operates at actual COPs of 2 to 2.5. This means we are not even approaching the lines shown in Figure 1. An engineer can manage this by moving to a source
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with a more constant temperature: the ground, or possibly a water supply (sea/lake/river). This is why these systems tend to have a higher COP in comparison with air systems. However, there is also a greater capital cost for the ground loop. To achieve the best heat pump efficiencies, a hybrid
heat pump system is required. This draws energy from the ground in winter and from the air in the summer, switching systems from the higher temperature source as the season changes to minimise the temperature difference and maximise efficiency. These systems also provide an opportunity for ground temperatures to recover over the summer season.
Combined heat and power Professor MacKay uses a graph (see Figure 2) to show that current combined cycle gas turbine generation with heat pumps operating at COPs of 3 and 4 are better than CHP. In this graph, CHP is better than a heat pump if the CHP points are to the right of the green line which represents the heat pump efficiency. MacKay states: ‘The main thing to notice in this
diagram is that the electrical efficiencies of the CHP systems are significantly smaller than the 49% efficiency delivered by single-minded electricity-only >
August 2010 CIBSE Journal 35
A heat pump increases the efficiency of electrical heating by transferring heat from one source to another using a refrigerant cycle
Simon Weir
www.simonweir.com
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