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Heat accounts for almost half of the energy usage in the UK. This briefing sheet looks at the ways that heat is generated and used as well as some of the political drivers for reducing the associated carbon emissions.
The total annual energy demand in the UK (after conversion and transmission losses) in 2010 was 6.66 EJ (1850 TWh), Figure 1.
Excluding non-energy uses, heat is the dominant requirement at 43.5%, followed by transport at 39.2%. Heat is supplied by a variety of fuels for different applications as shown in Table 1.
The dominant requirement for heat at 61.3% is for space heating, of which 76.2% is associated with the domestic sector. Low temperature industrial processes (at 12.3%) and hot water requirements ( at 12.2%) are the second and third most dominant requirements.
In all sectors and applications, the main fuel used for heating is natural gas. Yet this can vary from as low as 47% of the high temperature process heat requirement to over 82% for low temperature space heating in the domestic sector. As a consequence, the overall carbon emission factor for heat provision in different applications varies as shown in Table 2.
In most domestic and commercial buildings, both space and water heating are often provided from the same heat system, although this system may be provided by a variety of fuel sources.
Improvements to the insulation of the fabric can reduce the requirement for heating.Domestic buildings built 50 years ago will have up to twice the energy demand of comparable buildings built to the latest standards. In addition, in older buildings ventilation heat losses may reach 50%, but in modern buildings with high levels of fabric insulation ventilation losses may be as high as 80% or more, and effective recovery of the ventilation heat energy is essential for further reductions in demand. The most efficient heat recovery systems are of the regenerative heat exchange type such as employed in the low energy buildings at the University of East Anglia. (Tovey and Turner, 2006).
Space and hot water heating may be provided by directly fuelled dedicated boilers or furnaces fuelled by coal, oil, gas, electricity or renewable fuels such as biomass. The efficiency of modern condensing boilers can exceed 90% while old devices may have an efficiency as low as 60%. Heat pumps, both air- and ground-source are now becoming more popular. Seasonally averaged coefficients of performance range from 2.5 – 3.0 for some air-source configurations to as high as 3.5 - 4 in optimally designed ground or water source schemes. While heat pumps can be fueled by gas, the majority are powered by electricity. So the energy and carbon overheads associated with this energy source must be taken into account when assessing overall environmental benefits of such schemes.
The majority of central heating in buildings in the UK is provided by radiators. These may operate at temperatures as high as 60 oC, but heat pumps are most efficient if the supply temperature to the heating system is as low as possible and using underfloor heating temperatures in the range of 35-50 oC. Legionella bacteria survive and multiply in this temperature range. In cases where heat pumps are used for both hot water and space heating, there is a potential conflict between overall system efficiency and the requirement to run hot water systems at above 55oC to combat these health issues. Legionella cannot survive temperatures above 55 oC. So some systems have a “hot day” once every few days where the hot water temperature is raised above the critical level, either by temporary sub-optimal running of the heat pump or boosting the temperature by off peak electricity.
Space heating may also be provided by effective integration of energy uses. For instance, waste heat from cooling one part of a building might be employed to offset requirements in another. Equally, careful architectural design can minimize winter time space heat demand. However, care must be taken to ensure overheating in summer does not increase cooling energy demand.
Supply of heat for space heating and hot water may also be included as part of a district heating scheme which may or may not be integrated with combined heat and power (CHP). Such neighbourhood schemes are common in parts of Europe, particularly Denmark and Russia. This schemes can make more effective use of heat recovery from other processes such as the generation of electricity. However, allowance for heat loses from the distribution mains must be considered. In a simple centralized boiler systems as opposed to CHP systems, the network losses are likely to be at least 8-10% and may be as high as 25%. Losses are dependent on the flow rate, flow and return temperature and the level of insulation. These losses can be particularly high in summer when there is a low demand for heating and can reduce the net scheme benefit. Moreover, the additional costs need to be considered when determining whether district heating is a viable option for a given neighbourhood.
New developments in pipe technology have both flow and return pipes contained in the same insulation jacket. In Denmark, triple pipe combinations within the same configuration with one pipe much smaller than the other two are claimed to minimize heat losses from the distribution mains (IEA, 2008). One of the larger pipes is the common return pipe, but the smaller diameter flow pipe (thereby covered with thicker insulation) is used in summer while the third larger pipe is used in winter.
DECC (2012a). Energy Consumption in the UK
DUKES (2012) Digest of UK Energy Statistics 2011, Annexe A
DECC (2012b) Department of Energy and Climate Change: The Renewable Heat Incentive stand-by mechanism for budget management.
DECC (2012c) Department of Energy and Climate Change Consultation: Renewable Heat Incentive: providing certainty and improving performance.
IEA (2008) District Heating Distribution in Areas with low Heat Demand Density: Chapter 5 IEA Project Report 8DHC-08-03
Tovey NK and Turner CH (2006). Carbon Reduction Strategies at the University of East Anglia