INDUSTRIAL & COMMERCIAL HEATING HEAT PUMPS
Comparing CO2
0.6 0.5 0.4 0.3 0.2 0.1 0.0
1
Electric heating
2 3 4 Heat pump seasonal performance factor (average COP over the year) Figure 3: Carbon intensity of heat from various heat sources (CO2-equivalent figures from DECC)
considerably better than the break-even figures. For example, if we can achieve an SPF of 3.3, we would reduce CO2 emissions by 33% compared with mains gas, and almost 50% compared with oil. This level should be achievable for some properties if systems are installed and operated well. Unfortunately the performance levels
reported in the field trials suggest that some installations in some situations are not currently offering much of a green advantage. The pink line in Figure 3 indicates our possible future CO2 figure for electricity when the mains grid has been partly decarbonised. This leads to difficulties in assessing the merit of a system – should one consider the current CO2 figures for electricity, or should one allow for possible future decarbonisation? Should one meet halfway? And, of course, one needs to know
figures
Electricity now: 0.517kg CO2
/kWh Oil: 0.315 kg CO2
Electricity future: 0.3kg CO2
/kWh Gas: 0.23kg CO2
Heat pump Now Future
/kWh /kWh
the likely life expectancy of the installation. These estimations are made on the basis
of the standard Department for Energy and Climate Change (DECC) carbon factors for electricity generation in the UK. There is, however, an argument that we should really be looking at the seasonally-adjusted carbon factor, since heat pump systems require the greater part of their electricity in the winter months. The spike in power demand from heat pumps during cold snaps raises additional questions about peak loads on the grid. Is such an additional load likely to be acceptable, given that the UK may have serious problems keeping the lights on and supplying electricity for other essential purposes as things stand? The new generating and transmitting infrastructure is paid for mainly by increases in consumers’ electricity bills – a burden arguably falling disproportionately on lower- income households. How much will it cost us to beef up the electricity infrastructure to ensure it can cope and to provide the investment to develop the extra electricity generation capacity required, whilst existing ‘firm’ electricity generation capacity is falling? And what is, and will be, the carbon- intensity of electricity generated in cold periods – the electricity preferentially used by ASHPs? These, and other questions, still need to be answered.
l John Cantor is a heat pumps consultant. Kate de SelinCourt works for the AECB Sustainable Building Association. This article is based on their longer article, which gives practical advice on heat pump installation and which can be downloaded at
www.aecb.net/new_releases/detail/?nId=16 www.heatpumps.co.uk;
www.aecb.net www.energysavingtrust.org.uk;
www.decc.gov.uk
Temperature change ASHPs at the mercy of the weather
Simply regarding an air-source heat pump as a replacement boiler, and bolting it into a standard house, with a conventional radiator circuits designed for water temperatures of up to 70C, is going to be asking for trouble. To get good efficiencies with a heat pump, the working temperature needs to be much lower. This simple fact almost certainly lies behind some of the disappointing results with heat pump installations, particularly in retrofits in the UK. To make it more complicated,
56 CIBSE Journal October 2011
you cannot run a domestic hot water system (DHW) at 35C to 40C. So using a heat pump to provide hot water obliges it to work outside its ‘comfort zone’. Numerous other factors impact on the temperatures that the heat pump experiences – including the number of hours that it is ‘enabled’ per day, the control settings, and heating controls (as determined by both system designer and householder expectations). There is no getting around the fact that an air source unit will
operate at its lowest COP during the time when we need most heat – when the air is coldest. An ASHP might lose around one third of its efficiency when the temperature drops from 10C to zero. The lower the outside temperature (or the greater the ‘uplift’), the less heat is produced for the power drawn – in other words, the efficiency drops. Thus, while the COP of a heat pump is fixed in known conditions in the lab, the actual performance in real life depends on the weather. The colder it gets, the more heat
the building needs. Whilst the COP may only drop modestly on better machines in cold weather, the defrost downtime is another issue, and not ideal at a time when full capacity might be needed. Since the total heat output drops significantly as the outside air temperature drops, this, coupled with the reverse-defrost, shows why ASHPs are more likely to require some form of backup from an alternative conventional heat source. This is often provided by a conventional electric heater.
www.cibsejournal.com
Kg CO2/useful kWh
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