Why opt for electric heating?
So why choose electric heating over a wet system?Developers and contractors tend to prefer electric heating in multiple occupation buildings, such as flats and students halls of residence, over piped systems. The capital cost is lower than gas-fired water installations. Installation times are shorter, metering arrangements easier. There is no pipework (gas or hot water from a community heating scheme) or flues to be distributed through the building, minimising riser space and optimising floor area. Some occupants state a preference for electric heating, perceiving it as cleaner than gas-fired systems, and with fewer maintenance issues.
Of the electrical power delivered to the dwelling, 100% is used to provide heat, unlike gas which suffers from losses arising say from flue gases or distribution. While these are valid points, can they displace the associated carbon emissions? Electric heating cannot pass using the elemental method. Instead we use the more flexible target U-value method. This method basically averages the U-value of the dwelling, makes allowances for window material, solar gain and fuel efficiency, and compares this figure to a calculated target.
UT = [0·35 – 0·19 (AR/AT) – 0·10 (AGF/AT) + 0·413 (AF /AT)] A typical new-build three bedroom apartment is used as the basis for all of the comparisons in this article (see table 1). Initially two options are explored: gas-fired condensing combi boiler serving radiators and electric storage heaters.
For the gas system the UAVG is 0·82 W/m2K with a target UT of 1·18 W/m2K . As the average U-value is less than the target it passes. The electric heating has a more stringent target UT value in order to allow for the greater carbon emissions. A factor of 1·15 is applied, making our target UT now 1·02. As UAVG is still 0·82, the system passes. On closer inspection this factor seems lenient. Gas has at a system efficiency of 78% and 54 kg CO2/GJ, a total load of 72 kg CO2/GJ. For electricity at 115 kg CO2/GJ and a system efficiency of 100%, the factor applied to make it equivalent to gas should be at least 1·66. ((115/54) x 0·78 = 1·66). This would give a target UT of 0·71, and the system would fail.
The reasoning behind the 1·15 factor is that it averages out all other alternatives to gas, but by doing so, it allows a loophole for electric heating. Interestingly, this applies almost exclusively to flats, the main market for electric heating. Part L1 recommends that dwellings should in general have a minimum of 17% glazing to floor ratio, up to a maximum of 25%. However for flats, this ratio is likely to be in the region of 10% as there are fewer external walls. This is not the glazing to wall ratio, which at 25% in this case, is average for most dwellings. Using the typical new-build three bedroom apartment again, knowing we need to meet a target U-value of 1·02 W/m2K, it is possible to calculate the maximum area of glazing for a particular window U-value, keeping all other variables constant. To keep the comparison simple, the apartment is mid-terrace and mid-floor so there are no floor or roof values to take account of (see table 2). Figure 1 shows that even for relatively poor U-values, a 10% glazing to floor ratio passes. Above the line of the graph, the building will fail. It is unlikely that a house would pass, as the glazing to floor ratio is likely to be around 20%. It could be argued that because of this relationship between floor area and window area, the heat losses are relatively low, and so the higher carbon emissions can be forgiven.
Exploring the facts and figures
In order to compare the emissions of electric heating to gas based systems, our typical new-build three bedroom apartment is analysed using the goverments SAP 2001 method to produce figures for heating and hot water consumption. This forms the basis for the Carbon Index method of compliance. Three cases are investigated. The first is a gas-fired condensing combi boiler serving radiators with a SEDBUK rating of 83%. The second is modern electric storage heating, with controls, and electric water heater with 140 litre storage. The third is a gas-fired boiler based community heating scheme, with heat meters installed, see table 3.
It is clear from the above that the electric heating poses a much greater carbon burden than the gas system. As the carbon index is less than 8·0, the dwelling cannot comply with Part L1 using the Carbon Index method. The CO2 produced is 80% greater than individual gas-fired condensing combi boilers installed in each apartment. This approach is not favoured by developers due to the extra space requirements for flues, the increased installation time for pipework, and the additional perceived risk of running gas pipework throughout the building. To this end community heating has also been analysed. A community boiler can distribute hot water to each dwelling, and occupants charged using a heat meter. The SAP rating and cost is very similar to a boiler in each dwelling, but there is a carbon penalty, mainly due to the losses incurred in the distribution system. The figures show a 52% improvement on carbon emissions than from the electric system.
There is, of course, scope for providing heating in a similar manner, but coupled to waste heat rejected from power stations or chp plant, if there is a suitable year round heat requirement nearby – such as a swimming pool. Efficiencies for these can be greater than for the example shown above, depending on the system.
Should the days of electric storage heating be numbered? Manufacturer's of electric storage heaters will argue that their products are extremely efficient, turning 100% of electricity into heat, and they are entirely correct. The storage heating units themselves are now very effective and a far cry from the units of yesteryear which were unresponsive and often cold by evening when heat was required. They are now available with sophisticated controls, delivering heat when and where required. In fact, if a zero carbon source, such as wind or solar power, could be coupled directly to a block of flats, without relying on the national grid, it would beat the equivalent gas system hands down.
But even with the government's commitment to increasing power supplied from renewables, electric heating from the national grid does not look to be an option in the long-term. Despite the fall in coal generated power stations, the accompanying reduction in nuclear generated power is outstripping the increase in renewables. This shortfall is being made up by combined cycle gas turbines, resulting in a generation mix very similar to 10 years ago. At today's prices, the cost of running the three bedroom apartment discussed is 45% higher for the electric heating than for the gas system. If electricity prices increase in line with the carbon emissions, this could rise to 80% more expensive.
So, why the loophole? It is perfectly feasible to introduce a more realistic factor into the target U-value specifically for electric heating. However, this would have a negative impact on the manufacturers of electric storage heaters (who do have a very efficient product) and developers (who want to keep capital costs low and installation simple). A major shift would be required in the generation of Britain's electricity before it could properly compete with gas, but until that day this loophole is protecting all interested parties, except of course the occupier and our environment.
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Building Sustainable Design
Melanie Starrs is an engineer with the Halcrow Group. The author would like to thank Brian Anderson at BRE and David Wilson at DTi.