Digital Twin technologies are rapidly gaining popularity in the fields of building construction and operation^1-4. Both government and industries are supporting Digital Twin technologies such as building information modelling (BIM)^5-10, virtual reality (VR)¹¹, robotic technologies^12-13, and city landscaping¹⁴ to transform Hong Kong into a smart city and achieve carbon neutrality by 2050¹⁵. With BIM, you can obtain energy performance estimates of your building designs, ensuring building energy efficiency compliance like never before. You can also manage your buildings using VR systems with colleagues in your office or remotely, avoiding the need to be in noisy plant rooms. In contrast, building energy audit practices have changed little since their debut in the 1970s with procedures developed in the past remaining quite similar to those used today¹⁶. Despite the HKSAR Government’s recent push for more frequent building energy audits for carbon neutrality by 2050¹⁷, is it possible to integrate these practices into our Digital Twins and smart cities? We believe it is.

Global technological development for building energy audit technologies

Building energy audits involve surveying existing buildings to identify energy management opportunities by collecting building energy system data, including specifications, usage patterns, and energy consumption¹⁸. While these data are organised and analysed with reference to conventional energy audit procedures, they can also be easily integrated into the data schema of BIM and serve as a foundation for Digital Twin technologies^19-20. The data collected through building energy audits may not be comprehensive enough to create a BIM model like a new building BIM model during its construction process²⁸, but it can still be considered as an enabler of Digital Twin and other smart building technologies. For example, it can be used to support the creation, in existing buildings, of building management system based on Internet-of-Things (IoT)³¹. With increased accessibility to building performance analysis (BPA) for BIM users⁴⁴, BPA practices used for BIM models of new buildings can also be extended to building operation phases to support regular operation and maintenance practices such as building energy audits.

Example of building energy data in a digital data schema

One example of utilising Digital Twin technologies in existing buildings for building energy audits involves the use of online portals provided by governments or other entities. These portals simplify energy audit procedures and enable the digitalisation of energy audit data entries^{21-23, 38, 40}. On the one hand, they speed up the process by generating energy audit statuary reports and other relevant deliverables digitally, reducing the need to double-check typos in statutory submissions. These platforms streamline data input, eliminating duplicate data entry requirements and facilitating the transformation of raw site survey data into analysed data. This minimises the need to check copy-andpaste errors and duplicated analysis work simultaneously. On the other hand, some platforms support auto-recommendation of energy management opportunities (EMOs) through rule-based methods or building simulations by digitising energy audit data with the aforementioned data schema. Rule-based methods include comparing regulation requirements with the current installations (namely, maximum lighting power density in regulations relative to the current installation density), benchmarking energy utilisation indexes, or conducting regression analysis comparisons with benchmark models^49-50. Any discrepancies between the energy efficiency level benchmarks and the current situation can be automatically identified in the data, suggesting EMOs with automatic calculations of energy-saving levels and paybacks. The BPA models created from these digital data can also be used to design EMOs by simulating the effects of improvements that may be difficult to quantify otherwise, such as building envelope improvement for less building energy consumption^45-46. Emerging research on laser scans of existing buildings for automating building energy audits^{2, 24-25} is paving the way for even faster data digitisation and analysis in this field.

User interface showing automatic lighting retrofit calculation on an energy audit digital platform

This rapid identification of energy management opportunities enables building owners not only to reduce energy cost and associated carbon emission, but also to create their own BIM models, which can further explore the aforementioned opportunities, such as IoT-based building management systems. This ultimately transforms their buildings into intelligent and smart buildings^{26, 31}. The data also facilitate more cost-effective creation of baseline building models for measurement and verification (M&V) in energy performance contracting⁴⁷ or for quantifying potential carbon credits⁴⁸, thus directly linking financial incentives with energy reduction effort. Building energy audit data also support wider Digital Twin applications such as building risk and resilience management²⁷ and building and city asset management and benchmarking^{4, 41}. As a result, digitisation-based energy audit technologies are emerging worldwide, driving advancements in energy efficiency and smart building solutions.

Potential of energy audit technologies for Digital Twins in Hong Kong

But are Digital Twin technologies applicable to Hong Kong? Stakeholders in the region are promoting digital technologies in both new and existing buildings for cost saving, occupant well-being, and environmental impact reduction²⁸. The rise of Digital Twins for existing buildings is also evident in the adoption of IoT systems for building operations such as remote monitoring³¹ and fault detection and diagnostics, which can bring an energy saving of 3% to 9% to buildings⁵¹. Multi-building and city Digital Twin management applications are also being developed, including those related to city-wide energy efficiency for developing practices and policies to reduce city energy consumption^{32, 39}. On the demand side, it is common for new construction buildings to use BIM due to statuary requirements^29-30. The HKSAR Government is proposing more frequent building energy audits, shortening the intervals from ten to five years and expanding the building types that require building energy audits¹⁷, potentially demanding faster building energy audit practices. Sustainability reporting frameworks are increasingly requiring companies to report not only carbon emissions of their value chains, but also their energy consumption, raising incentives for businesses in Hong Kong to further manage their energy use and potentially disclose their energy data to their value chains⁵⁵. The enabling technologies, along with enhancing incentives, are driving the need for faster and better energy audits. Yet, most deliverables related to city-wide planning or policymaking remain in the research stage^{33, 42-43, 56} due to the high cost of digitising and modelling existing buildings²⁸ and the lack of open data for non-commercial buildings³⁴.

To exemplify the case, our team has utilised an energy audit platform to support the performance of building energy audits for hundreds of premises in Hong Kong. A subset of them was used for a case study, and the case study demonstrates that the digital platform’s capability to not only generate energy audit reports for clients automatically but also to characterise the energy-consuming equipment and identify energy-saving potential in both individual buildings and the city as a whole^{39, 52}. For instance, the study reveals that most offices inspected still retain a high lighting power density relative to the regulatory requirements, resulting in an unsatisfactorily high lighting energy utilisation index (EUI) relative to the 500 MJ/m2 benchmark. This issue can be attributed to the continuously widespread use of tubular fluorescent lamps in these offices.

Distribution of Lighting Power Density (LPD), lighting EUI and types of luminaires in offices among our digitised energy audit cases³⁹

Another notable discovery is that the air-conditioning energy use is much more dependent on the installed capacity per floor area than the coefficient of performance (COP) of the installed equipment. This finding suggests that limiting the air-conditioning capacity in buildings is as important as raising the COP of the equipment in reducing energy consumption. In individual energy audit scenarios, we can explore the potential to decrease the number of operating air-conditioning equipment in cases in which the rated cooling capacity per unit area is relatively high compared to buildings with similar functions. This approach is similar to how we address buildings with significantly higher EUIs than their benchmark EUI⁵³. For city-wide applications, we can use the data to further refine our regulations for building energy efficiency without relying on manual building energy data surveys for detailed analyses⁵⁴. These findings based on energy audit digital data highlight the benefits of energy audit data digitisation in both individual energy audit cases and city-wide Digital Twin practices simultaneously.

How do we start?

One way to accelerate Digital Twin development for existing buildings is to follow professional suggestions and start with small steps³⁵. This can be achieved by integrating digitisation into regular building inspection practices for data digitisation. For instance, building energy audits, which suggest energy management opportunities based on full sets of energy equipment lists, can be integrated with energy system data digitisation to create buildings’ first digital data sets using appropriate digitisation tools^21-22.

Screenshot of an energy audit data digitisation platform

Data collected for green building certifications, particularly data supporting building energy modelling, can also be used for similar purposes^36-37. Data for the development of smart building systems already existing inside the building, as well as the collected data, can also be integrated to generate a BIM model of the building. Digitisation platforms not only speed up original building operation practices and reduce costs, but they also yield data for the potential development of Digital Twin technologies and contribute to managing building energy consumption, in line with Hong Kong’s carbon neutrality target²⁸. Imagine putting your BIM model energy equipment data into these platforms and continuously recording their changes (namely, from your Form of Compliance records during your major retrofitting work): you can expect receiving drafts of energy audit reports and statuary forms from these platforms annually in no time. You can also effortlessly satisfy your clients’ or tenants’ requests for your building energy records when asked to fulfill their sustainability (namely, environmental, social and governance (ESG)) requirements. Through new data digitisation platforms, building energy audits and other regular building inspection practices can create your first building Digital Twin, retrofitting existing buildings into smarter and more energy efficient buildings.

Correlation of building air-conditioning EUI with COP and cooling capacity of the equipment in offices and banks³⁹

About the authors

Dr Howard Cheung (PhD) is an Assistant Director of Carbon Exchange (Hong Kong) Ltd and Building Energy Analytics Company Ltd; Ir Tony Ho (REA, RPE, MHKIE, BEAM Pro, CAP, CSDP) is the Director and the Founder of Carbon Exchange (Hong Kong) Ltd and Building Energy Analytics Co Ltd.

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