How can the energy consumption of an entire city be reduced as inexpensively as possible and without loss of comfort? This question was tackled by the "Energy and Systems Studies” group at Max Planck Institute for Plasma Physics in Garching near Munich as a participant in the development of a municipal energy efficiency programme launched by the city of Vienna. The recently adopted programme is to reduce Vienna's final energy consumption annually by 180 gigawatt-hours – the heating energy for 30,000 inhabitants. A total reduction of 1800 gigawatt-hours is envisaged by 2015. Taken alone the area of the private households investigated by IPP it should be possible to save 90 gigawatt-hours annually, i.e. 900 gigawatt-hours by 2015. All in all, the saving programme presents ways of slowing from twelve to seven per cent the increase of Vienna's final energy consumption expected to occur by 2015.
From 1993 till 2003 the final energy consumption in Vienna's households, traffic network, industry and commerce, and public and private services rose by 24 per cent to about 38,000 gigawatt-hours. Business as usual being assumed, by the year 2015 – according to calculations of the Technical University of Vienna – consumption would rise a further 12 per cent to 400,000 gigawatt-hours. This undesirable development is now to be counteracted by Vienna's municipal authorities with an energy-saving programme. Besides municipal energy experts, the project group formed at the end of 2004 included the "Energy and Systems Studies" group of Max Planck Institute for Plasma Physics in Garching, the Technical University of Vienna, the Austrian Energy Agency and the IRM AG company. The objective was to indicate ways in which Vienna's consumers can save as much energy as possible by increasing energy efficiency at the least possible economic expense.
A third of Vienna's total energy consumption, and hence the largest single item, is needed by private households. In devising the energy-saving programme, households were therefore analysed the most exactly of all the city's consumption sectors, this task being tackled by IPP scientist Stefan Winkelmüller. It should in fact be possible to save a great deal of energy in households, as the physicist was able to show in his PhD thesis: "The remedies proposed by us could reduce the total consumption of private households by 2015 in relation to 2005 by six per cent or about 900 gigawatt-hours."
Stefan Winkelmüller's analysis starts with a comprehensive description of possible technical saving measures in households, e.g. redevelopment of buildings or a change to more economical electrical appliances. From the very outset it was checked whether these measures can actually be implemented, i.e. supported by legislation or promotional programmes. In order then to map the energy consumption of Vienna's households in a computation model, the second step was to structure the detailed municipal data material, which provides information on the age, condition and heating of Vienna’s 168,000 buildings and the equipping of the 800,000 households with electrical appliances.
Single and multiple dwellings were each sorted in great detail into seven age categories. For each building category there was a choice of five versions of building redevelopment and fifteen different heating systems – such as remote heating, oil or gas-fuelled central or individual heating, and electrical, biomass or heat pump heating. In addition, with new buildings different thermal-insulation standards were taken into account. The most important electrical appliances – for lighting, cooking, washing and cooling – were divided into different efficiency classes. For all applications the efficiency and specific cost were taken into account, and also in each case the stock of appliances and their age structure in the first year of the model computations. In time series from 2002 to 2015 the "perpetrators" of the energy consumption were specified: How will demand for accommodation develop, how well will households be equipped with electrical appliances?
All of these determining factors were incorporated in a computer programme developed by the IRM AG company that can map complex energy systems and compute their possible development under given boundary conditions at optimum cost. By means of this program Stefan Winkelmüller – on the basis of the final energy consumption measured for the years 1993 to 2002 – computed first the actual state and its anticipated further development in the future. This business-as-usual scenario describes how the energy consumption of Vienna's households would probably develop by 2015 if none of the measures surpassing previous efforts at saving were adopted.
The model was now subjected to different amounts of saving – 900, 1,500 and 2,000 gigawatt-hours in the year 2015. The data on Vienna’s households and the cost of energy supply, redevelopment and replacement of equipment were now utilised by the model to determine the least overall expensive way to cover the given energy requirements. For this purpose the model decides how much of what technology when needs to be added in order to achieve the desired energy saving at least expense. The model can also take finer aspects into account: listed buildings, for example, cannot be arbitrarily redeveloped. Furthermore, the users involved will most likely scarcely all able to decide just according to economic aspects. Boundary conditions were therefore introduced that describe the probable course of development and thus lead to more realistic scenarios. These are, for example, characteristic quantities that give the maximum increase in low-energy appliances to be expected.
This yields for each of the three saving scenarios the economically optimum mixture of measures and the respective increases in cost as compared with the business-as-usual scenario. Stefan Winkelmüller explains the result: “A saving of 900 gigawatt-hours by the year 2015 in relation to business as usual does not incur additional cost but, on the contrary, saves cost as well as energy. These efforts to save energy are thus worthwhile not only for the sake of the environment but also for financial reasons.” The greatest saving accrues from redevelopment of buildings from the fifties to the seventies and a change to more efficient heating systems. The mixture of measures determined – according to a sensitivity analysis – even stands up in the face of further rising energy prices. However: The greater the saving of energy envisaged, the smaller are the savings in cost, as is to be expected. Ultimately, very big savings in cost will make the scenarios more expensive than business as usual.
The overall report of the project team also contains, apart from the household sector, saving strategies for the services, industry and commerce sectors. Now that the final report was recently approved by Vienna Municipal Council, implementation can go ahead next year. Stefan Winkelmüller states: "The catalogue of measures approved by the city can only be implemented with a great deal of effort from all concerned. But this should ensure that the increase in energy consumption is slowed down by 2015 from the predicted 12 to 7 per cent. This is equivalent to a huge annual reduction of 180 gigawatt-hours – or the entire annual heating energy consumption of 30,000 inhabitants."