I recently completed a research project as part of my doctorate, during which I looked to determine whether the commercial building stock in New Zealand can be retrofitted to be net zero energy. New Zealand has committed to reducing Greenhouse Gas emissions to 30% below 2005 levels by 2030, and one way to help achieve this reduction is to retrofit the New Zealand commercial building stock.
During my undergraduate and honours degrees I became passionate about the impact buildings have on sustainability and climate change. This led me to focus my research on net zero energy buildings. I was fortunate enough to get funding from the Building Energy End-use Study (BEES) with whom I collaborated to further my research during a doctorate degree. BEES was a project that monitored and analysed the energy and water consumption of non-residential buildings around New Zealand.
What is net zero energy?
The concept of net zero energy buildings are a response to buildings’ significant contribution to climate change inducing Greenhouse Gas emissions.
A net zero energy building is low energy and offsets any energy that is generated from greenhouse gas emitting fuels with renewable energy generation such as hydro, solar and wind.
When extrapolated to retrofitting a whole building stock, the net zero energy ideal is defined as: A community of buildings which have a greatly reduced demand for energy and:
- the building stock only consumes energy from the country’s existing Greenhouse Gas free energy infrastructure (hydro, wind and solar); or
- if it cannot meet the above, it needs to generate onsite at least as much Greenhouse Gas free energy as consumed from Greenhouse Gas producing (coal, gas or oil) energy sources.
45% of the New Zealand commercial building stock’s energy consumption is generated from Greenhouse Gas producing energy sources. Therefore, to be net zero energy the commercial building stock must reduce their total energy consumption by 45% and move away from greenhouse gas emitting energy sources such as gas, coal, oil and non-renewable electricity generation.
The commercial building stock
In my research I defined the commercial building stock as buildings which housed office, retail or mixed office and retail spaces.
Figure 1 shows 23% of the commercial building stock’s floor area is in buildings which are 5-649m2 in size. This equates to approximately 20,000 commercial buildings, and also has the same percentage of commercial building stock floor area as the 342 buildings that are larger than 9000m2.
Figure 1 – Distribution of commercial buildings and their building floor area split into five building floor area size groups.
Representing the real commercial building stock
Previous studies researching energy lowering upgrades to a large group of buildings used Prototypical energy models. Prototypical energy models are a theoretical average, and do not represent any real building. They are constructed using approximations, assumptions and simplifications of what is deemed average.
I represented the existing New Zealand commercial building stock using a statistical sample of real commercial buildings. Calibrated energy models were constructed to match the real buildings’ physical attributes and behaviour and the simulated energy consumption was matched to the real buildings’ energy consumption.
The results from the statistical sample of calibrated energy models were aggregated to represent the whole commercial building stock. Using this method of representing the commercial building stock, meant the results were based on real building energy performance. For example, the research used real buildings that represented real urban environments with varying street widths and surrounding buildings, as well as number of occupants and heating system types etc.
Retrofitting towards net zero energy
To establish a set of Energy Conservation Measures (ECM) that could be adopted for the retrofit process, I undertook a survey of real net zero energy buildings from around the world. The final set of ECMs were:
- Optimum envelope insulation
- Optimum glazing type (U-value, SHGC, t-vis)
- Passive solar heat gain
- Solar shading
- Natural ventilation/passive cooling
- Skylights/solar tubes
- Energy Efficient lighting
- Energy Efficient equipment
- Advanced lighting controls
- Efficient air sourced heat pumps
- Photovoltaics (if required)
These ECMs were retrofitted to each calibrated building energy model and optimised to maximise energy savings. The resulting retrofitted buildings included ECMs not too dissimilar to Aorangi House in Wellington.
Can net zero energy be achieved?
Figure 2 displays the retrofitted commercial building stock’s energy balance (without Photovoltaics). The energy balance is a comparison of existing Greenhouse Gas free energy generation supplied to the commercial building stock and the retrofitted commercial building stock’s energy consumption. If the resulting energy balance (circled) is below the net zero energy line, it is considered to be net zero energy. While Figure 3 displays the energy reduction which is attributed to each of the five building floor area size groups.
Figure 2 (above) - Commercial building stock net zero energy balance.
Figure 3 (above)– Distribution of commercial building stock energy reduction for five building floor area size groups.
Figure 2 shows the retrofit of the commercial building stock could achieve net zero energy, even without Photovoltaics. Furthermore, Figure 3 shows that half of the energy reductions can be achieved just by retrofitting the 1,200 buildings over 3,500m2 in size.
My research demonstrated that net zero energy can be achieved by retrofitting the commercial building stock to consume less energy, and additional Greenhouse Gas free energy generation is not required. The energy reduction from retrofitting to net zero energy is equivalent to 330,000 typical New Zealand homes. The Greenhouse gas reduction equates to the methane emissions from 200,000 dairy cows.
Half of the net zero energy target could be achieved by retrofitting just 1,200 of New Zealand’s largest commercial buildings (sized over 3,500m2).
The savings from these large buildings would be equal to the annual electricity generated by all wind turbines in New Zealand.
If the retrofit were undertaken, the reduced energy consumption from the net zero energy commercial building stock would generate a surplus of energy. This new surplus energy can either be used elsewhere in New Zealand to power other energy consumers (for example electric cars) or not used in order to reduce climate change related Greenhouse Gas emissions.
I’d like to clarify what I mean by this statement:
“45% of the New Zealand commercial building stock’s energy consumption is generated from Greenhouse Gas producing energy sources. Therefore, to be net zero energy, the commercial building stock must reduce their total energy consumption by 45% and move away from greenhouse gas emitting energy sources such as gas, coal, oil and non-renewable electricity generation”.
Firstly, the figures represent an extreme year, or a dry year, from 2006 to 2012. When it is dry, there is less hydro-electric generation and there is more need for non-renewable generation. A dry year was chosen because if net zero energy were to be an aim it would need to account for the lowest renewable energy generation scenario, to account for when there is less renewable generation which would need to be replaced with more non-renewable generation.
Secondly, electricity generation in New Zealand is approx. 85% renewable. However, commercial buildings also consume natural gas, coal and oil directly at the site (for space heating and hot water heating). The 45% accounts for all non-renewable GHG emitting energy sources consumed by the commercial building stock, not just electricity.
The 85% renewable figure is final energy. My 45% figure is primary energy. Primary energy takes into account the efficiency losses by converting the coal, gas and oil into electricity. For example, it takes approximately three times the amount of gas to generate one unit of electricity. Hence, when looking at primary energy, the gas portion of the electricity generation is roughly three times higher than the final energy.
So, in saying this, I could change my statement to:
“In a dry year, 45% of the New Zealand commercial building stock’s primary energy consumption is generated from Greenhouse Gas producing energy sources. A dry year is when there is less hydro-electric generation and non-renewable GHG emitting energy sources needed to replace the lower renewable energy generation. Primary energy takes into account the efficiency losses of converting fossil fuel sources to electricity (e.g. it takes three units of gas to generate one unit of electricity). Therefore, to be net zero energy, the commercial building stock must reduce their total energy consumption by 45% and move away from greenhouse gas emitting energy sources such as gas, coal, oil and non-renewable electricity generation.”