Using PassivHaus to get to Net Zero Carbon

The UK’s 2030 and 2050 Climate Change targets now look unlikely to be achieved with predictions showing that Buildings emissions will not reduce significantly given current use patterns and trajectories. To combat this, Scotland has indicated its intent to move towards a Zero Carbon building standard for all new buildings, using the Building Regulations as the primary mechanism. But what is a Zero Carbon building? In this article we explore this definition and present the Passivhaus Standard as a viable strategy in helping us achieve net zero carbon.

Embodied Carbon or Operational Carbon

The total carbon emissions of a building are made up of two types. Embodied Carbon describes the carbon emissions inherent in the creation of our construction materials. Operational Carbon, describes the carbon emissions generated in the use of our buildings through heating/ cooling, hot water, power and light. Our understanding of and ability to accurately calculate Embodied Carbon, whilst improving, is highly complex. Furthermore, the relationship between trading more Embodied Carbon to reduce Operational Carbon (for example, more insulation to reduce heating demand) is not yet fully understood or well established empirically. What is undeniable, is that we continue to build homes that are inefficient in operation, and unless this is addressed, high levels of Operational Carbon will continue to erode our ability to meet our Climate Change targets.

Carbon or Energy

In considering how best to reduce carbon emissions in the context of buildings, if we focus solely on carbon, it can be quite difficult to manage and propose strategies to reduce emissions. This is due to the complexities introduced by changing carbon factors associated with the energy mix of the grid over the life of the building. Furthermore, upgrading and replacement of different services over the life of the building can have significant carbon impacts that are difficult to estimate. It is far better to focus on the energy demand of the building. This is relatively consistent throughout the life of the building and is something that can be controlled and modelled at design stage. If the energy demand is minimised in the first place, then it follows that carbon emissions will be reduced. Design strategies such as Passivhaus, follow fabric first approaches to minimise the energy demand of the building. This energy use is focused on 4 principle areas; Heating & Cooling, Domestic Hot Water, Lighting and Auxiliary Electrical and finally, Appliances. To achieve net zero carbon emissions we need to offset actual energy use with energy derived from renewable energy sources.

How to Achieve Zero Carbon

The amount of carbon that we use is directly related to how we generate our energy. In the UK the aspiration is to 'decarbonise' our grid electricity by introducing renewable technologies into the energy mix. This has had a significant effect already, reducing the carbon emission factor from 0.519KgCO2/kWh down to 0.233KgCO2/kWh. Simply switching from gas to heat pumps to generate our heat and domestic hot water, would reduce this again by half. So, is it a simple case of installing heat pumps in all houses alongside solar pv to offset the electricity used? The chart below breaks down the annual energy demand for a typical new build UK home of 68m2 , firstly with a gas boiler, then with an air source heat pump. The chart indicates that 16 pv panels would be enough to offset the energy demand of the home with an air source heat pump. This, on the face of it, would appear to be easy! There are however, problems…….

The Performance Gap

The Performance Gap is a term that describes the difference between modelled energy use and actual energy use in our homes. There is increasing evidence that the energy performance of UK homes in use is actually 40% higher than predicted. This is due mainly to poor construction quality. Including the Performance Gap in our calculations pushes the energy demand of our average home from 4300 kWh/yr to 5400 kWh/yr. To offset this would increase your pv panels from 16 to 20. Introducing stringent construction site standards is imperative to give us a fighting chance of reducing the Performance Gap of our buildings and achieving Net Zero Carbon. In contrast the construction detailing and quality assurance procedures of the Passivhaus Standard ensure that there is little or no Performance Gap between the modelled and actual energy use of the building.

Seasonality of Renewables

When it is dark we need light. When it is cold days are shorter and we require more heat. When it is sunny with long days, we require less heat and less light. This is the classic conundrum of renewable energy sources - they are at their highest availability when we do not need them. This is known as Seasonality. In short, our energy demand is at its highest when the availability of renewable energy sources is at its lowest. We can store the energy, but there is significant storage losses incurred when doing this. The result is that ultimately, the effective energy demand of the standard home increases, as does the amount of renewables required to offset it (7700 kWh/yr and 28 pv panels!). This is clearly unrealistic as a viable strategy. The key is to reduce energy demand and close the Performance Gap.

Reducing Demand and Closing the Performance Gap

Emissions associated with heating and cooling a building can be significantly reduced by employing a fabric first design strategy and improving quality of construction. This is the primary focus of the Passivhaus standard, reducing thermal losses by improving the building thermal envelope and aligning this with ventilation heat recovery. Undertaken alongside efficient domestic hot water design and educating occupants in energy efficiency behaviour can drastically reduce the energy demand of the home to a level where energy use in the average 68m2 home with an ASHP could be held as low as 3700 kWh/yr, offset by just 14 pv panels.

Comparing Zero Carbon Strategies

The Passivhaus Trust, in their publication Passivhaus: the route to zero carbon?, presents data of net emissions from 5 Zero Carbon scenarios along with the required area of pv panel required to offset regulated energy (heating/ cooling, hot water, lighting + aux elec) and achieve net zero carbon. It demonstrates that a 68m2 Passivhaus fitted with an ASHP and sufficient pv generation capacity to offset energy used has significantly lower net emissions (8kgCO2/m2.yr) than any of the approved building regulations scenarios, in some cases in excess of a 50% improvement. However, none of these scenarios achieve true net zero carbon as unregulated energy use (appliances, etc) and storage losses have not been added when calculating the required renewables to offset. This time, the results show that the proposed move to a notional net zero building (using an ASHP and reducing the TER by 19%) will still have emissions of 3 kgCO2/m2 .year, which would require 32m2 of solar panels to offset. In contrast, the Passivhaus fitted with 22m2 of solar panels lowers emissions to actual zero. Of all the scenarios modelled, this is the only actual zero carbon building.

Conclusion

The Passivhaus Trust's study demonstrates that the current Zero Carbon targets won't necessarily realise zero emission buildings. The notional zero carbon buildings that the building regulations demand will in fact generate around 18kgCO2/m2.yr, requiring massive renewables expansion that will not be achievable on every home or flat as there is simply not enough area to accommodate it. Initiating design and build strategies aligned with Passivhaus principles is really our only way to achieve a truly zero carbon building with a manageable level of renewables to offset the energy used. Furthermore, consideration will have to be given to offsite implementation of renewables as it will not be possible on every site. Governments and Developers will have to consider something akin to a Developer's Renewables Contribution scheme to fund offsite renewable generation where onsite capacity cannot be achieved in order to ensure that net zero carbon emissions targets can be met.

In closing we would encourage readers to refer to the detailed report issued by the Passivhaus Trust Passivhaus: the route to zero carbon?

Contact Us