Energy, Carbon and the Passivhaus






In a 2015 conference paper [1], a comparison is made between both the embodied energy and the lifetime operational energy of two houses; one real, built to Passivihaus (PH) standards and the other notional, and conforming to UK building regulations Part L. Section 3.3 of the paper gives energy figures for both buildings: the PH embodied 1795 GJ of energy, and the Part L house1460 GJ; operational energy over fifty years was 6004 GJ for the PH, and 7451 GJ for the Part L building. While the PH embodied more energy, this was more than offset over time by the saving in operational energy. The paper does not quantify the associated equivalent CO2 emissions of the energy, and in view of the progressive decarbonisation of the UK’s energy supply, it seems likely that in terms of CO2e, the net advantage of the PH may be less that would appear from the energy figures alone.

In order to compare the embodied and the operational CO2e emissions of the Passivhaus, projected emission factors are needed for UK energy supplies over a period of fifty years as used in the above conference paper. Estimates of UK emission factors for the years 2020 to 2050 are available in a study on transitions to a low-carbon energy system for the UK [2]. If we base calculations on a 2020 construction date, the data will cover that part of the fifty year period during which the highest emissions factors are anticipated, and we will need to make a further projection for the years from 2050 to 2070.



The authors of the UK energy system study consider three pathways to low carbon energy, the first termed Market Rules (MR), the second Central Co-ordination (CC) and the third Thousand Flowers (TF), after a misquotation from Chairman Mao Zedong.  Each pathway reflects “the dominant logic of particular governance arrangements, i.e. those of the market, government and civil society … for the evolution of the UK power sector to 2050.” The energy sources considered include coal, gas, oil, nuclear, wind, hydro, biomass, wave, tidal, solar, imports, pumped storage, and combined heat and power.



Greenhouse gas emissions under the three UK transition pathways are shown graphically in Figure 16 on p.468, and from them average emission factors for each pathway can be calculated for the years 2020 to 2050. The decline is greatest between 2020 and 2030, and relatively slow thereafter, and to estimate the figures for the period from 2050 to 2070, we have assumed that the percentage decline will be the same as that from 2030 to 2050. The average emissions factors calculated in this way for the years 2020 to 2070 in the three transition pathways, expresses  in units of kgCO2e/kWh are 0.153 (MR), 0.120 (CC) and 0.088 (TF). From the above graph, the emission factors for each path in 2020, again in kgCO2e/kWh, are 0.46 (MR), 0.43 (CC) and 0.39 (TF), and these figures will be used to estimate embodied CO2e.



The excess of embodied energy in the Passivhaus compared with the Part L building is 335 GJ or 93,100 kWh [1]. Using the emissions factors for 2020 noted above, this corresponds to excess CO2e of 42,780 kg CO2e (MR), 40,010 kg CO2e (CC) and 36,270kg CO2e (TF). The saving of operational energy in the Passivhaus is 1447 GJ, or 402,000 kWh compared to the Part L building [1]. This saving corresponds to 61,506, 48,240 and 35,376 kg CO2e, using the average emissions factors derived for the years 2020 to 2070 for the MR, CC, and TF pathways respectively. The net savings under the three pathways (the operational saving less the embodied excess) are 18,726 and 8,230 kg CO2e for the MR and CC pathways, but a negative value of 894 kg CO2e for the TF pathway. Under this carbon reduction pathway, the Passivhaus does not justify its extra embodied carbon.



Bearing in mind that the comparison is based on a sample of only two buildings, that embodied CO2e has been based on overall figures for the UK power system rather than on an analysis of building materials, and that there are inevitable uncertainties involved in predicting future emission factors, no conclusion should be drawn except that the assumption that Passivhaus design guarantees an overall CO2e advantage in every regime of carbon reduction may not be entirely secure.



References



[1] Pelsmakers, Andreou et. al.,, Should the Passivhaus standard include the environmental impact of materials in its standard?

In: Tagungsband 19 Internationale Passivhaustagung 2015. 2015 International PassivHaus Conference, 17-18 April 2015, Leipzig. TIB , Leibniz . ISBN 9783000486036

http://eprints.whiterose.ac.uk/97135/



[2] J Chilvers et al.,

Realising transition pathways for a more electric, low-carbon energy system in the United Kingdom: Challenges, insights and opportunities

Proc IMechE Part A: J Power and Energy 2017, Vol. 231(6) 440–477

https://journals.sagepub.com/doi/pdf/10.1177/0957650917695448


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