The next generation

Over the past few years consulting engineers have been working to reduce the amount of fossil fuels that are being used in buildings – using a combination of energy-saving measures in building fabric design and high-efficiency building services systems. These fossil fuel-saving measures can be assessed by reference to the amount of CO2 saved over a typical (or reference) set of data. Central to these carbon-saving measures is the understanding that energy supplied by renewable sources has a zero-carbon footprint, so that photovoltaic, wind, hydroelectric, wave, solar-thermal, biofuel and biomass all count as having a zero (or very low) carbon footprint.

For fossil fuel sources there are agreed CO2 emission ratings given in kg of CO2 per kW of energy supply. These emission ratings are published in the Building Regulations (Part L) and give a measure of the efficiency of conversion from fossil fuel to heat. From these figures it can be seen that, for instance, coal has a CO2 emission factor of 0.291 kgCO2/kW whereas natural gas has a CO2 emissions factor of 0.194 kgCO2/kW. This means that coal is 1.5 times more carbon intensive than natural gas as a primary fuel source.

Also included in the Building Regulations is the CO2 emissions factor for grid-supplied electricity which is given as 0.422 kgCO2/kW of electricity. There are a number of significant points about this figure which need to be understood:

• The emissions factor is given as kW of electricity delivered, which is not the same as kW of heat. It is important to understand this fundamental difference because although 1 kW of electricity can be readily converted into 1 kW of heat, by passing the electricity through a resistance heater, the ability to increase the heat output by driving a heat pump can significantly increase the amount of heat energy that 1 kW of electricity can produce.

• The emissions factor is an average factor for the electrical grid as a whole and takes into account the electrical generation carbon efficiencies of all fuel sources (including renewable energy sources) that are connected to the supply grid. The grid-supplied electricity factor takes into account the distribution losses, but is necessarily an average factor based on an average annual fuel usage. To make matters more complicated, the figure changes annually depending on the amount of electricity demanded and the mix of fuel sources used to satisfy that demand. Over the past 10 years, the carbon content of the grid-supplied electricity has generally been reducing, primarily as a result of the switch to natural gas as the primary fuel source and a reduction in the use of older inefficient coal and oil-fired power stations. In addition, there has been some rise in the use of renewable power sources.

• It should be noted that the grid-supplied electricity carbon content has not yet reached the target grid-supplied electricity carbon content of 0.422 kgCO2/kW given in the Building Regulations. In 2005 the grid-supplied electricity emissions factor was 0.455 kgCO2/kW as given by the DTI’s Digest of UK Energy Statistics 2006. Further reductions in carbon content are expected over the next few years as further efficiency improvements are made to the national grid primary power generators.

Figures given by the DTI for grid-supplied electricity power generation emissions factors for gas and coal in 2005 are:

• Electricity supply by gas 0.363 kgCO2/kW
• Electricity supply by coal 0.873 kgCO2/kW
• Average of all fuels (2005) 0.455 kgCO2/kW.

From these figures it can be seen that (in 2005) the grid-supplied electricity from gas was 20% less carbon intensive than the grid-supplied average, while electricity from coal was 92% more carbon intensive than the average.

As organisations strive to reduce the carbon content of their power usage, many are selectively misapplying these carbon content figures in their calculations to oversell the carbon savings that they are making. Their argument goes like this: provided the electricity is being generated at a carbon intensity that is less than the worst generators then carbon savings are being made. While these calculations could be seen as merely exuberant over-enthusiasm, the calculations adopted mean that in most instances the actual carbon emission savings for the schemes are considerably less than claimed. In some instances, the carbon usage is actually more than if conventional energy supply methods were adopted. Now that political will has changed, and the Climate Change Bill is in the draft stages, it is time to measure real carbon savings against real benchmarks. As a minimum, this benchmark should be taken as the average grid-supplied electricity factor, however, to be more rigorous, the benchmark should actually be based on the fossil fuel source being used. In this way, the generating efficiencies of all fuel sources will be increased.

As an example, a well-known manufacturer and installer of natural gas-fired combined heat and power plant (CHP) claims in its literature that for every kW of electricity that its system provides, 0.5 kg of CO2 is saved. Since the average grid-supplied power factor for natural gas is only 0.363 kgCO2/kWh, these claims are clearly absurd, however, these figures are regularly being used across the industry and in local government departments to promote the use of CHP installations.

A typical example of this type of claim can be seen graphically in figure 1, which is taken from a 2006 presentation by the London Climate Change Agency, a municipal company that is owned by the London Development Agency, working for the Mayor and the Greater London Authority (GLA). The analysis gives the overall carbon saving by changing from grid-supplied electricity to CHP plant as 54%.

In this instance, the values for the CHP plant using natural gas are realistic (albeit that they are more than 20% better than average figures given by the DTI), however the analysis compares this with a coal fired power station that generates electricity at an equivalent of 0.969 kgCO2/kWh (which is a massive 267% more than the figure given by the DTI for gas-fired centralised generation). Transmission and distribution losses are given in the above analysis as 23%, whereas the DTI’s Digest of UK Energy Statistics 2006 gives a total loss of 7.5%. With these calculations it is not surprising that very significant carbon savings are being claimed.

It is interesting to note that if this same logic was applied to electric cars then a G-Wiz would be in the same vehicle emissions band as a 2.0 litre X-Type Jaguar Diesel. Similarly, the drivers of a typical “gas-guzzling 4x4” could proudly claim that they are saving 50% carbon emissions because they consume less than half of the emissions of a Lamborghini Murcielago 147 Roadster (figures taken from the Vehicle Certification Agency).

A more realistic overall balance would be given in figure 2, over the page, where the grid power station is assumed to be a combined cycle gas turbine with an electrical conversion efficiency of 55% and the boilers are assumed to have an overall efficiency of 92%.

In this instance the overall saving for using a CHP plant is just 12%, which, while being a worthwhile saving, is 78% less than the saving given by the London Climate Change Agency for the same equipment.

With only 12% maximum savings possible in an ideal set up, it is essential that all of the heat generated by this system is being used, otherwise, despite the fact that financial profits can be made with this scheme, the carbon savings will be eroded. In particular, the community heat distribution systems (which are often quoted as having 10% losses) and the electrical distribution systems which themselves may have distribution losses of 2% (dependent on the size of the installation and the area served), will both need to be carefully analysed to ensure that actual carbon savings are being delivered.

Of course, by using the above figures, it can be shown that by using the grid electrical supply to drive a heat pump, the developments would use less carbon than a CHP system (provided that the average overall coefficient of performance of the heat pump was greater than 2.5). Similarly, using the grid electrical supply and providing heat energy by renewable sources – such as biomass or solar collector – will consume less fossil fuel than CHP.

Whilst the outlined analysis shows that modest carbon savings can be made with well-designed fossil fuel-driven CHP systems, the same cannot be said for gas-fired combined cooling and generation plant (sometimes called tri-generation plant). With these systems the waste heat from a gas turbine is used to drive an absorption chiller to provide chilled water for cooling.

Over the past 18 months these systems have been strongly promoted by the GLA as a way of reducing CO2 emissions. It is a requirement that tri-generation systems are considered in all submissions to the GLA as part of the energy statements required for all large new building developments under the London Plan. What most consulting engineers have realised is that these natural gas-fired systems consume considerably more carbon than conventional cooling systems using grid-supplied electricity and vapour-compression chillers, primarily because the coefficient of performance (COP) of the absorption chillers is so poor in comparison to the vapour-compression water-cooled machines. A typical comparison is outlined in figure 3, in which the following is assumed:

• The tri-generation fuel source is natural gas with a CO2 content of 0.194 kgCO2/kWh
• The tri-generation gas turbine has an electrical conversion efficiency of 35%
• The tri-generation absorption chiller has an average overall COP of 0.75
• The grid-supplied electricity is supplied by natural gas with a carbon dioxide content of 0.363 kgCO2/kWh
• The cooling is provided using water-cooled chillers with an average overall COP of 5.0.

From this analysis it can be seen that the CO2 emissions for the gas-fuelled tri-generation scheme are 39% more than gas-fuelled grid-supplied electricity. It should be noted, that in a similar way, it can be shown that gas-fuelled tri-generation schemes are 21% more carbon intensive than the grid-supplied electricity as defined in the Building Regulations. Clearly, this is not a “low-carbon cooling” system, as claimed by the London Climate Change Agency.

The way in which these systems are analysed by the GLA is by insisting that engineers use a carbon emissions factor for electricity in their calculations – taken from the Building Regulations – called the grid-displaced electricity factor. This factor is given as 0.568 kgCO2/kWh of electricity delivered, which is 25% higher than the current grid-supplied electricity and 56% higher than the current grid electricity supplied by natural gas. By using the grid-displaced electrical carbon emission factor, this analysis shows that the tri-generation scheme will save 12% carbon. As a result of these massive distortions in the carbon content of supplied electricity, engineers are able to show in their calculations that there are overall savings in carbon emissions while knowing the reverse to be true.

The thinking behind this logic is that it is assumed that local electrical generation will displace the most carbon-intensive grid electrical generating plant. Regrettably, this is not the case since grid supply is primarily based on cost and not carbon intensity. As a result, when the cost of one fuel source rises above the cost of another there will be a switch to the lower cost fuel. This is what has happened in the last two years with gas and coal where the gas cost (to major UK power producers) increased by 45%, and coal costs increased by 7%. The result has been a marked increase in the use of coal (up by 10.5% in 2006 compared with 2005) and a reduction in gas (down by 6.4% in 2006 compared with 2005) in the fuel mix (DTI figures).

Fortunately, despite the fact that energy supply companies can make a profit by running fossil fuel-powered tri-generation systems, there are currently few being proposed. However, recent moves by a number of local councils and, in particular, the GLA has meant that engineers are increasingly being obliged to promote these carbon-intensive schemes for political expediency, in order to obtain planning approval for their developments.

Now that there is the political will to drive for a real reduction in carbon emissions it is time to introduce some scientific rigour into the process of selecting and installing systems to ensure that actual carbon savings are being delivered. As proposed by the draft Climate Change Bill, it is important that an independent auditor check and verify that proposed carbon savings are real and achievable. It should no longer be acceptable for government or local authorities to take all their energy policy advice from the energy service companies without independent auditor advice.

In order to reduce fossil fuel consumption, it is essential that the UK uses fossil fuel-specific energy efficiency benchmarks. Failure to adopt a more scientifically rigorous carbon-saving analysis will mean that we will continue to deceive ourselves that we are reducing fossil fuel consumption while doing exactly the opposite.