The UK Net Zero Carbon Buildings Standard - Part 3: On-Site Renewables and Energy Efficiency
The UK Net Zero Carbon Buildings Standard (NZCBS) is a critical step towards decarbonising the built environment. However, its current framework presents some challenges in appropriately recognising the role of on-site renewable energy.
This article explores these complexities, particularly the interplay between on-site renewables, operational energy limits, and the crucial distinction between consumed and exported renewable energy.
A key strength of the NZCBS is its emphasis on a "fabric first" approach. It prioritises energy efficiency measures in building design and retrofit. This is vital for reducing overall energy demand. However, the Standard also recognises that on-site renewable energy generation plays a crucial role in achieving net zero. Indeed, the Standard sets mandatory targets for on-site renewables (subject to "allowable constraints").
Consumed vs. exported renewable energy
A critical oversight in the current NZCBS framework is its failure to differentiate between renewable energy generated and consumed on-site versus that which is exported to the grid. The clear distinction is that on-site consumption directly reduces the building's reliance on grid electricity, while exported energy cannot sensibly be netted off against imported energy at different times.
The concept of differentiating consumed and exported on-site renewable energy is not new. The National Australian Built Environment Rating System (NABERS) has successfully incorporated this distinction for decades. It is concerning that the NZCBS did not adopt a similar approach, despite advocacy from industry experts. The influence of technically weaker methodologies, like CRREM, or potential misinterpretations of the International Energy Agency's net-zero analysis may have contributed to this omission.
The risk of unintended consequences
Without a clear distinction between consumed and exported renewable energy, the Standard risks creating scenarios where buildings that generate substantial renewable energy are penalised for not meeting stringent operational energy limits. This could happen even if they are achieving very low or even net-zero operational carbon through on-site consumption. It could lead to a number of potential issues:
- Suboptimal carbon outcomes: Pursuing deep retrofits solely to meet energy efficiency limits could result in higher overall carbon emissions (considering both operational and embodied carbon) than a strategy that incentivises on-site renewable energy generation consumed onsite.
- Missed opportunities: While it is not optimal for every building to maximise renewable energy generation in all circumstances, the Standard should encourage on-site generation that is consumed onsite as at least as valid for net zero as deep retrofit.
Addressing the Challenges
To ensure the NZCBS effectively drives decarbonisation and recognises the value of on-site renewables, several refinements should be considered:
Dual metric approach with Building Energy Use (BEU): As advocated by my colleague, Robert Cohen, introducing the concept of Building Energy Use (BEU) can help clarify this issue. BEU represents the total energy used by a building, regardless of source (grid or on-site). A more nuanced approach would be to establish two BEU limits:
1. A less stringent BEU limit achieved without renewables, ensuring a good level of energy efficiency.
2. A more stringent BEU limit that allows for a combination of further efficiency measures and on-site renewable energy consumption. This provides flexibility and recognises the lower embodied carbon of many renewable technologies. The value of this more stringent BEU should be informed by the EUI set in the Standard based on its top down - bottom up energy supply to demand reconciliation at a national level. It is important to reiterate that exported renewables should not be netted off against grid imports, as there is no guarantee they will contribute to the UK's overall net-zero goals and may even lead to increased curtailment of other renewable sources.
Refined verification processes: Verification processes should be updated to explicitly acknowledge the value of on-site renewable energy consumption in achieving net-zero operational carbon. This could involve developing clear guidelines for assessing the proportion of on-site generation that is directly consumed and factoring this into the verification process.
- Evolving metrics: The NZCBS is a living document and should continue to evolve. Future iterations should prioritise expanding the scope beyond upfront embodied carbon to encompass full life cycle carbon assessments. This would provide a more holistic understanding of the carbon impacts of various design and retrofit strategies, including the trade-offs between energy efficiency measures and on-site renewable energy.
Conclusion
The UK NZCBS is a landmark initiative, but its effectiveness hinges on its ability to adapt and address emerging challenges. By acknowledging the crucial distinction between consumed and exported on-site renewable energy, refining verification processes, and evolving its metrics, the Standard can better incentivise optimal decarbonisation pathways. A more nuanced approach that balances the "fabric first" principle with the strategic deployment of on-site renewables will be essential to realising the full potential of the NZCBS and supporting the advance towards a truly net-zero built environment.