Ecological energy balance for sandwich elements

The ecological valuation of products requires that they be evaluated in their entirety and over their entire lifecycles. It should begin at the use of raw materials and product manufacture and involve both the use of the product and its environmental-friendliness throughout its entire life. Once the useful life of the actual purpose has been concluded, the lifecycle of a product continues either by its being used for another purpose or else its being disposed of. Generally this also encompasses the question of environmental friendliness.

Special valuation criteria for sandwich elements

On the basis of their capacity to save energy, the ecological valuation criteria of insulating substance systems are subject to special conditions. Since sandwich elements represent integrated high quality core insulation, such special conditions also apply to sandwich elements.

Because of their potential for saving energy over an extended period of time, sandwich elements have a very important ecological and economic task. As is the case with regard to other insulating substances, sandwich elements can therefore not be valued in the same context as any other construction components. On the basis of their chiefly supplementary roles as heat insulating systems, they require a very special method of valuation in terms of their ecological and economic performance capacity.

Energy use for the production of sandwich elements

Production is associated with the consumption of energy, and every type of consumption of energy has its ecological effects for principally physical reasons [1]:

• Decrease in energy sources
• Changes in the landscape
• Emission into outside air and precipitation
• Emission into the atmosphere and/or geosphere and into the water system

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Every type of energy supply and energy use places a burden on the environment to a certain extent. These days everyone is aware of the connection between heating and ecological burdens; the same applies with regard to the connection between thermal insulation, energy savings and environmental protection. Thus the significant question with regard to ecological valuation of sandwich elements with integrated heat insulation is the following:

How much energy is needed to produce the heat insulation and how long is what amount of energy saved during the useful life of the building?

The table below shows the relative distribution of the production energy for a sandwich element with 1 m² surface, a polyurethane hard foam core insulation of 60 mm and aluminium cover layers that are 0.5 mm thick.

The longer insulating materials fulfil their function in a structural body and decrease the consumption of energy for heating, the greater is the advantage for the environment with regard to:

• saving energy resources
• reducing emissions

On the basis of the optimal heat insulation values of polyurethane hard foam and the thermal bridge free connection technique, the sandwich construction method is one of the most effective construction systems in this context as well.

Fig. 9.2.1 Distribution of oil consumption

The dual function of polyurethane hard foam

A large part of the heat and power that is generated serves to heat buildings. Due to this immediate connection between heat energy and emissions, between CO2 and the greenhouse effect, which on its own could lead to serious changes in the climate of the world, heat insulation became a major consideration in the 1970s. Due to its heat insulation using PUR hard foam, sandwich elements have a dual function. In the first usage phase they save energy for decades (20-50 years or longer), which represents a multiple of the production energy. In the second usage phase the high heating value of PUR hard foam with its incineration that involves minimal hazardous substances makes an additional contribution to power generation which returns approximately a third of the production energy.

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Energy savings in comparison to production energy

To produce polyurethane hard foam, approximately 834 kWh/m³ is needed [3]. In [4] this cubic metre of insulating substance is placed on a 10 m long sandwich element with a building width of 1.0 m and an insulating thickness of 100 mm and the heating energy consumption for this sandwich element with a k value of 0.25 W/m²K can be approximately calculated using the following rule of thumb:

k-Value x 10 = Litres of heating oil
(per m² of outer wall or roof surface)

The heating oil consumption thus equals approximately 2.5 litres per m². For the 10 m² large sandwich element according to the above example, some 25 litres of heating oil are required for one heating period. A step by step increase by 5 mm each time of insulating substance thickness leads to an exponential function in which the savings become greater as the two insulating substance thicknesses to be compared become smaller (see Fig. 9.2.2).

For the chosen example of a sandwich element with 100 mm thickness and a surface of 10 m², we arrive at a mean savings figure of some 100 litres of heating oil per heating period. The lifespan of a sandwich element can be, depending upon the product and use in question, between 20 and 50 years. If one considers a mean value of 35 years (approximately one generation), a cubic metre of PUR hard foam yields, for this usage period and at this mean value, a total energy saving of approximately 3,500 litres of heating oil.

The heating value H u of heating oil EL is 42,700 kJ/kg (at a density of 0.82 to 0.86 kg/dm³ in a temperature range of +20 degrees C). Here it should be noted that the extraction of oil, its transport, distillation, and storage require considerable amounts of energy. At a mean value of 0.84 kg/dm³, the heating value, on the basis of a litre of heating oil EL, is 36,000 kJ/litre (=36 MJ/litre).

Per heating period, 1 m³ of PUR hard foam can accordingly save an average energy mixture of 3.6 GJ and in total over a usage period of 35 years the amount saved is 126 GJ.

Another comparative study [2] leads to annual energy savings as high as 8 GJ/a. On the basis of a cubic metre of PUR hard foam, this study yields a possible total saving in 50 years of approximately 400 GJ. Calculated on the basis of the usage duration utilised here, this is 280 GJ in 35 years, in other words more than double the savings calculated here. This indicates that the basic criteria and figures in use in our example are very much on the “safe side”.

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Fig. 9.2.2 Chart showing savings in heating oil with thicknesses of PUR hard foam increasing in 5 mm increments

Energy recovery after usage period

To complete this energy balance, the energetic potential of the insulating substance after the usage time as core insulation must also be quantified. Of the three recycling processes that principally play an economic role with respect to PUR hard foam, the energetic valuation with energy recovery (thermal recycling) offers a good figure for the energy balance.

According to DIN 18230, PUR hard foam has a heating value between 24 and 27 MJ/kg, which is used here with an average heating value of 25.5 MJ/kg. The incineration of a cubic metre of PUR hard foam with raw thickness of 40 kg/m³ yields around 1,020 MJ or 1.02 GJ of energy. A comparison with the heating value of heating oil EL (36 MJ/litre) leads to an equivalent heating oil mixture of 28.3 litres. At an average energy price of 0.25 EUR / litre of heating oil, for each cubic metre of PUR hard foam about EUR 7.00 of the energetic valuation can be recovered.

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How sandwich elements pay for themselves

Experience allows us to conclude that buildings whose production price doubles for ecological reasons have relatively minimal market prospects. The market also dictates the limits of the financeability of ecological measures. A positive ecological energy balance, such as the legislator supports for example through the energy savings law and the heat insulation ordinance, will then be particularly effective if no grave economic disadvantages are involved with the ecological advantages. It becomes particularly advantageous when ecological advantages also go hand in hand with economic advantages. With regard to heat insulation this is always the case if the heating energy costs that are saved, associated with a long period of usage, at least equal or exceed the investment costs of the insulation measures. Here, too, with this interplay between ecological and economic advantages, the sandwich construction method stands out particularly on the basis of its minimal k-values and its long lifespan.

Another advantage is that sandwich elements as a supporting construction component combine a range of functions including structural engineering, static and optical functions. With the construction of a sandwich element, in addition to providing optimal heat insulation, several important functions for the outer shell of a building are installed at the same time. This must definitely be considered when reading the following investment costs.

Material and assembly costs of roof surfaces in the sandwich construction manner, including the costs of connections, can currently be estimated at an average value of EUR 52.80 / m². For material and assembly costs including the costs of connections of wall surfaces in the sandwich construction manner, the investment currently equals approximately EUR 43.50 per m² of wall surface (average value).

With today’s surface distribution, these figures work out, with approximately two-thirds walls and one third roof surfaces, at an average investment of EUR 46.60 / m² of sandwich surface.

For the raw materials costs of one cubic metre of PUR hard foam of rough thickness of 40 to 45 kg/m³, one should currently add some EUR 75 / m³. According to the example here (sandwich example with 10 m² of surface and 100 thickness), in view of the above-mentioned average value, some EUR 466 should be estimated for materials, installation and connections.

Fig. 9.2.3 Ecological energy balance for the core insulation with PUR hard foam in sandwich elements for a usage duration of 35 years. Source: [6]

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The raw materials share of PUR hard foam in these costs equals approximately one-sixth. In terms of proportionate consideration of the assembly costs (including connections) one arrives at a cautious estimate of approximately one quarter of these investment costs, that is to say to EUR 11.60 / m², which can be applied to the share of heat insulation costs in the sandwich element.

According to these values, the sandwich construction method facilitates average annual energy savings of EUR 25 / m³ of PUR hard foam. For a lifespan of 35 years, this results in total savings in heating costs of EUR 875 (this sum does not include any increase in utility rates and possible savings based on the installation of a more economical heating system). Calculated in terms of a square metre of sandwich surface, this example yields total energy savings of EUR 87.50 per m² and annual savings of EUR 2.50 per m².

Fig. 9.2.4 Amortisation of sandwich elements over a usage period of 35 years; Source: [6]

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In terms of cautious calculations, the investment share of the heat insulation system of a sandwich element pays for itself between the 4th and 5th year of use. The total investment of the sandwich elements including their assembly, that is to say all functions of the sandwich construction method, pays for itself after approximately 18 years. Thus the sandwich construction method is amortised after half of its expected usage period solely by way of savings in heating energy.

In the second half of the period of 35 years of use, the total savings in heating energy costs, at EUR 2.50 per m² annually, is added to a total of EUR 42.50 per m² of sandwich surface (at a constant energy price of EUR 0.25 per litre of heating oil). Basing calculations on the last increase in the costs of energy, which resulted in prices up to EUR 0.50 per litre of heating oil, savings of some EUR 80 per m² might be more realistic. In other words: the saved energy costs are at least two times, and in the medium to long term even up to three to four times as high as the investment costs of the sandwich elements. So the amortisation period obviously shortens proportionately.*)

With a cautious estimate of the energy savings potential, setting aside possible savings due to smaller dimensions of the heating systems and not considering the additional functions of the sandwich element, over the usage period of one generation at least 40 times the costs of the production energy for the core insulation with PUR hard foam in heating energy and at least twice the investment costs for the sandwich construction method is saved. These savings in resources and capital go hand in hand with a proportionately high reduction in emissions, be they the result of the combustion of heating oil or other organic fuels.

Autor: Rolf Koschade

*) These calculation savings values are utilised in the context of near-actual k-values of PUR hard foam. A constant energy price is assumed. In addition, savings for smaller heating systems are left out. A calculation of the total heating costs that includes investment and fixed costs in addition to energy costs and utility rate increases would yield shorter amortisation times.

Literature sources
[1] Gertis, K. A: Wärmeschutz Energieeinsparung Umweltschutz; University of Stuttgart, Chair of Constructional Building Physics, 1986
[2] BING: The environmental contribution of polyurethanes thermal insulation products - eco-profile; BING - Federation of European Rigid Polyurethane Foam Associations, Stuttgart, Nov 1998
[3] Isopa: Fact Sheet Recycling Polyurethanes - Polyurethanes in Perspective; Isopa - European Isocyanate Producers Association, Belgium, Brussels, April 1997
[4] IVPU: Eigenschaften von PUR-Hartschaum-Wärmedämmstoffen; 11th fully re-edited edition, June 1998, IVPU, Stuttgart
[5] IVPU-Nachrichten 54: Verwertung (Recycling) und Entsorgung von PUR-Hartschaum-Abfällen; IVPU, Stuttgart 1996
[6] Koschade, R.: Die Sandwichbauweise; Ernst & Sohn, Berlin, 2000; p. 312, 313

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