The research paper “Zero-carbon Balance: The Case of HouseZero” provides readers with a case study of the data, methods, and scenarios used to calculate HouseZero’s carbon balance over a 100-year estimated building life (the carbon balance for a 60-year estimated building life is also calculated in the full paper). To do this, the team calculated the total embodied carbon emissions from as-built data, simulated operational carbon emissions, and projected carbon offsetting from onsite renewable energy over the entire life of the building. To make the findings comparable to other studies, the paper reports the embodied carbon emissions per square meter of occupied space, which results in 488 kg CO2e/m2 for the 100-year estimated building life. This calculation includes carbon emissions generated from the extraction, manufacturing, construction, and replacement of all elements considered to be within the building’s physical system boundary. This incorporates the building’s material systems (e.g., building assemblies including the foundation, structure, enclosure, and interior partitions and finishes) as well as technical systems that are not as typical in most studies (e.g., energy well, heat pump, photovoltaic panel and balance-of-system, and elevator). When temporal and physical system boundaries are aligned to current benchmarking studies, the normalized embodied CO2e emissions of HouseZero are much lower – less than half – at 233 kg CO2e/ m2.
Researchers also found that achieving zero carbon balance is dependent on how much carbon is emitted by the current and future energy supply from the electrical grid. The carbon balance results showed that HouseZero can achieve net-zero carbon balance with its simulated energy consumption and onsite power generation under current energy grid carbon-intensity scenarios when a zero-carbon emission grid is not achieved by 2050. However, if zero-carbon grid emissions are achieved by 2050, the project cannot achieve a net-zero carbon balance. These results highlight the sensitivity of the carbon balance analysis when designing a zero-carbon emissions building, the need for transparent CO2e accounting, more uniform systems boundary definitions, and the importance of data related to the future building and energy grid scenarios prescribed in the LCA study.
Other highlights from the paper:
The whole building LCA showed that around 42% of the building’s embodied CO2e emissions were generated by technical systems beyond what is typically defined as a building’s LCA system boundary (33% when batteries are not included in the LCA calculation).
For the technical systems, the majority of emissions (61-73%) are associated with renewable energy products, such as photovoltaic panels and batteries.
For the technical systems, 54% of emissions are associated with future product replacement, compared to 24% for material systems. These future-use stage scenarios have a high level of uncertainty.
The project’s low energy use intensity (EUI) indicates that the building integration and coupling of passive-active systems intended for low operational energy use and optimal thermal comfort produces a meaningful reduction of operational energy use, and therefore positively impacts the building’s net-carbon balance.
And, in addition to potential operational CO2e emissions reductions, these building-integrated designs have a co-benefit of reducing CO2e emissions that are often disregarded and unaccounted for, associated with HVAC equipment such as ducts and fans, which also have a high occurring replacement.
Early lifecycle assessment was used to identify materials with high embodied emissions, such as concrete. Low-carbon solutions were carefully considered as some low-carbon concrete products may pose adverse health effects. Human health concerns led to selecting high slag concrete over other waste-stream aggregates. The use of high-slag-content concrete reduced the embodied CO2e emissions per unit by 44-58% compared to relevant benchmarks.
Similarly, thermal insulation was also identified as a significant contributor to the total embodied CO2e emissions. Wood fiber insulation, which had a lower embodied CO2e per unit, was used as an alternative to mineral wool and polyisocyanurate insulating materials.
Read the full paper, "Zero-carbon balance: The case of HouseZero" in Building and Environment.
The first year of actual operation data included energy consumption and onsite energy production. Most building systems in HouseZero were commissioned in May 2020. The first full year of data (June 2020- May 2021) showed good results, although the building was not occupied during that time frame due to COVID-19. The final test for HouseZero’s performance will be completed after 24 months of uninterrupted, normal operation.
The ambition of the building is to reduce the need for delivered energy (electricity that leads to a high carbon footprint) to a minimum by utilizing thermal energy sources (e.g., a ground source heat pump and solar thermal panels) in combination with thermal mass and a low-energy system for heating and cooling.
The results from the first-year data: Total Energy Use Intensity (EUI) is approximately 54.1 kWh/m2 total energy. The EUI is 26.6 kWh/m2 without IT and plug load. As a lab operating 24/7, the building has a large IT load, which consumes 17.8 of the 54.1 kWh/m2 total energy (33%).
Advanced Energy Design Guides, led by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, proposed a target EUI of 73.2 kWh/m2/year for small to medium offices at 5A climate zone. Net Zero Annex (U.K.) and New Buildings Institute also provided two more aggressive targets, i.e., 55 kWh/m2/year and 56.8 kWh/m2/year, respectively. HouseZero currently meets these targets.
In terms of onsite energy production, the project aims to balance the carbon emissions relating to imported energy for operation (electricity) and the embodied emissions relating to materials, with the production of new renewable electricity exported to the grid. The production considered constraints, including:
- Available surface on the building with sufficient solar exposure, taking into consideration neighboring buildings.
- Positioning of PV panels in accordance with the original architectural expression in the area, which is protected by historical/preservation regulation.
The measured PV production for the first year is 38.7 kWh/m2 of the total area of the building, which is approximately 13% lower than anticipated as several disruptions due to testing happened during the first year. Utility meters versus building electrical energy consumption is used to estimate PV production. The PV system was partially functioning from September 1 to 24, 2020; January 20 to February 12, 2021; and May 7 to May 29, 2021, due to setting experimentations on the batteries side. In addition, it was found that there have been uncertainties in PV harvest measurements with the existing PV metering systems and possible system efficiency loss. Inverters are identified as a possible source of inefficiency. At the end of August 2021, a new meter was installed to record the PV harvest that can be utilized by the building or exported to the grid, which will allow for cross-validations and a higher degree of PV harvest reporting.
Based on data from the first operational year (June 2020-May 2021), HouseZero demonstrates that surplus onsite renewable energy generation can offset emissions associated with the primary building structure (which does not include emissions from technical equipment), provided that the photovoltaic (PV) production can achieve optimal efficiency as designed, and the building performance is maintained. Using existing standards, calculations were conducted by the CGBC based on total load without IT (e.g., data servers) and plug loads and the interpolation of some unavailable data.
Read the full publication, "Comprehensive assessment of operational performance of coupled natural ventilation and thermally active building system via an extensive sensor network" in Energy and Buildings.