High-temperature heat pumps
While some heat pumps are can delivering high temperatures, this capability should not be routinely used if the potential of heat pumps to reduce carbon-dioxide emissions is not to compromised. Graham Hazell elaborates.
It has long been the ‘holy grail’ of electrically driven vapour-compression heat pumps to provide higher delivery temperatures at efficiencies that are acceptable from an environmental and economic point of view. This is not intended to facilitate heat pumps to be used with high-temperature heat emitters on space-heating systems, for instance in retrofit applications. Rather, high-temperature heat pumps are particularly desirable to produce sanitary hot water efficiently at a temperature that restricts legionella multiplication.
High-temperature heat pumps are now available using a variety of different approaches.
• Transcritical refrigerants such as carbon dioxide.
• Cascade systems with multiple compressors and different refrigerants.
Transcritical refrigerants require a very high difference between flow and return temperatures (e.g. return at 10°C, flow at 65°C) and hence are ideal for DHW as the mains inlet temperature is typically 10 to 12°C and a storage temperature above 60°C inhibits legionella bacteria.
Cascade systems use two compressors with two different refrigerants, each optimised for the particular delivery temperature and temperature lift. For instance the first unit may use R410A to produce heating water at 40°C and the second using R134a raises this to up to 80°C.
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| Fig. 1: The effect of weather compensation on flow temperature for existing high-temperature systems. 7°C is very close to the average temperature over the UK for the whole of the heating season. |
What exactly is ‘high temperature’ for heat pumps? BS EN 14511-2011 has the answer.
• Low temperature is 35°C
• Medium temperature is 45°C
• High temperature is 55°C
• Very high temperature is greater than 65°C
Just to confuse, when most people refer to ‘high-temperature’ heat pumps they are often actually referring to ‘very high temperature’ — i.e. greater than 65°C, although anything over 55°C is largely considered high temperature as this is a fairly standard flow capability.
To maximise efficiency, the overall guiding principle of using any heat pump is to operate it with the highest possible source temperature and the lowest possible delivery temperature. It is rarely possible to operate at the standard test delivery temperature of 35°C, so the Heat Pump Association recommends 45°C as a good compromise. However when looking at retrofitting to say a property that once used an oil boiler and does not have mains gas, it may be tempting to use the heat pump to produce much higher temperatures, even up to 80°C water flow at design ambient conditions (-2 to -5°C?) depending on location, as required by MIS 3005 V3.1a to provide sufficient heat output from the heat emitters.
Clearly there is a danger that high-temperature heat pumps will be used as straight replacements for existing fossil-fuel boilers with no optimisation of the system, for instance by changing radiators or updating controls. This would compromise the efficiency of the heat-pump system, reducing both carbon and fuel cost savings and is not encouraged by the Heat Pump Association. In the worst cases, savings could be severely reduced or, worse still, even reversed.
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| Fig. 2: CoP of a cascade high-temperature heat pump at various source and supply conditions (defrost cycles included). |
However, if it is not practical or feasible to change the heat emitters to low-temperature devices designed for use with heat pumps it is even more important to utilise weather compensation to ensure that only the flow temperature required to off-set heat loss at the prevailing ambient condition is produced. For much of the year, the heat pump will work at lower temperatures and, hence, much higher efficiencies than the design point, which is worst case.
Fig. 1 illustrates weather compensation for a retrofit situation where the heat emitters cannot be changed; of course where possible low-temperature heat emitters should be used. With weather compensation, an average flow temperature of 51°C could be expected during the heating season, giving an average seasonal CoP of around 3.4 to 3.5 (see Fig. 2). If the system were ran at 80°C all year round the ‘average’ seasonal efficiency of the unit would be about 2.5 (see Fig. 2).
It is more desirable to operate at much lower temperatures, so if the design point delivery temperature were reduced to 55°C (say at -5°C ambient) then the average heating season flow temperature would be much better at 39°C (Fig. 3) resulting in average unit CoPs of 3.7 to 3.8 (see Fig. 2 again).
So what level of CoP is acceptable? First, it would be better for this to be a seasonal CoP to allow for variation over the season based on the time-weighted ambient temperatures, as outlined in a revised standard BS EN 14825. However, it will take time for this standard to be widely adopted and understood, so we tend to use an ambient temperature of 7°C (one of the main EN 14511 test conditions).
Fortunately this temperature is very close to the average over the UK for the whole of the heating season and hence represents an average condition. It’s crude, but there are roughly equal numbers of hours below this temperature as there are above during the heating season.
Further, this is a 24-hour average and for most applications the systems are not operational during the night when it is consistently cooler, and hence this should represent a degree of conservatism.
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| Fig. 3: ‘Average’ flow temperatures for purpose designed low-temperature systems using weather compensation. |
Summary
Electrically driven high-temperature heat pumps are now available at efficiencies that can provide savings in energy, running costs and emissions.
For maximum benefit, systems should be purpose designed and optimised for use with low-temperature heat emitters and weather-compensated control of flow temperature.
In retrofit applications serious consideration should be given to replacing heat emitters to optimise for low-temperature operation rather than use high-temperature heat pumps as straight replacements.
Flow temperatures should be automatically adjusted according to need, especially when changing from space heating (low temperature) to DHW (high temperature) so that space-heating systems are not run at the temperatures required for hot-water generation. High-temperature heat pumps provide more choice and a wider range of applied solutions for renewable heat and energy efficiency.
Graham Hazell is a consultant to the Heat Pump Association.
Why we are looking for a seasonal performance factor of 2.9
An addendum to the European Energy Directive (RES) for calculating the renewable-energy contribution made by heat pumps (the Tumres formula) suggests we should be aiming for a seasonal performance factor (SPF) of 2.9 for the wider system, including pumps, fans etc.
The directive is based on the principle of a 15% improvement on heat-generating parity. With an EU-wide heat-generating efficiency of 40%, heat-generating parity of a heat pump would occur at a SPF of 2.5 and hence a 15% improvement would be an average SPF of 2.9.
Combining a local efficiency of 290% with a generating efficiency of 40% provides a combined efficiency of 116%. Compared with burning fossil fuel directly for heat at say 85% efficiency (EST SEDBUK results) this would represent an overall improvement of 31% and a commensurate reduction in CO2 emissions of 25% (slightly lower because not all electricity generation is via gas).
