A hybrid approach to decarbonisation: retrofitting our office buildings

Mitsubishi Electric’s Chris Newman explores the steps we can take to retrofit and decarbonise our buildings quicker.

The journey towards a net zero Carbon future is not a straight line and the circumstances of each individual building will dictate the speed, order and commercial viability of that building’s improvement measures. 

From the perspective of heating systems, it can be difficult to justify the removal of perfectly operational heating plant and its replacement with an alternative system such as a heat pump solution.  

Even though the heat pump will produce significantly lower operational carbon emissions, it is likely to cost more to install, may cost more to operate given respective electricity and gas prices, and may also require deep retrofit of the building’s M&E services and infrastructure (this was shown to be the case in section 7 of the UKGBC Retrofitting Office Buildings Report). 

As a result, a hybrid (or bivalent) approach may be appropriate. 

The concept of mixing technologies to achieve the best overall outcome in a particular circumstance is certainly not new. 

However, the urgency of climate change has often framed the question of heat decarbonisation in a binary way – continue to use fossil fuels or stop.

However, the urgency of climate change has often framed the question of heat decarbonisation in a binary way – continue to use fossil fuels or stop.  

Adopting a hybrid/bivalent approach may at least allow progress to be made until a trigger point is reached whereby deeper retrofit and therefore a longer term and ultimately lower carbon option can be adopted – this is discussed in more detail in section 16 of the UKGBC report.  

Helping lower emissions now 

This approach could also be particularly helpful when plant replacement is not due for several years, improvements are needed to the electrical infrastructure, sufficient plant space is simply not available, disruption levels must be kept to a minimum, or even if a targeted package of retrofit measures are planned at some point in the future where the full decarbonisation of heat can be incorporated.  

In the meantime, the hybrid/bivalent concept, if applied correctly, can reduce operational carbon emissions, improve energy usage intensity (kWhee/m2) and retain existing M&E infrastructure all while having a limited impact on disruption to the building and its operational costs. 

Balancing the temperature of heat delivered, the capacity of the heat pump delivering that heat, which ambient temperatures it will operate at, as well as how it is integrated into the existing heating system will all affect the outcome(*1) – see below example results of various hybrid/bivalent systems (with different design options as mentioned above) compared to an existing gas boiler solution along with comparisons against a high temperature cascade heat pump and a lower temperature heat pump solution  

For more information on hybrid/bivalent systems, please see the Mitsubishi Electric bivalent solutions overview document Bivalent Solutions Overview – Document Library – Mitsubishi Electric 

An intelligent combination  

We can see that optimising a heat pump solution to operate with lower flow temperatures can deliver operating cost parity (or even a small cost saving) but it is also possible to get close to the same overall outcomes with intelligent usage of the existing gas boiler alongside a heat pump solution. 

using low carbon technologies alongside existing fossil fuel systems is vitally important if we are to take steps forward today

The UK government is currently consulting on rebalancing of the energy markets(*2) and should the spark gap be narrowed and the 3:1 ratio of electricity to gas costs reduce, even more combinations of heat pump & gas boiler will achieve operational cost parity. 

In the meantime, though, we can be fairly certain that any kWh of heat delivered by a heat pump will have a lower operational carbon emission than delivering that kWh via a gas fired boiler 

So, in summary, considering all options for decarbonising heat, including using low carbon technologies alongside existing fossil fuel systems is vitally important if we are to take steps forward today while we wait for the economics of net zero on some individual buildings to catch up with the clear and obvious environmental benefits. 

Mitsubishi Electric are a UKGBC member and project partners on UKGBC’s recent report, ‘Building the case for Net Zero: Retrofitting Office Buildings.’ Learn more about the report here and more about UKGBC membership here. 

References  

*1 – assumptions for comparison as follows; weather profile based upon Manchester UK, gas boiler 93% efficient, grid emission factors of 0.210KgCO2/kWh gas and 0.136KgCO2/kWh electricity, cost ratio/kWh of 3:1 electricity to gas, bivalent heat pump option uses flow temps between 50-70oC and 25,50,75,100% of peak load sizing in various combinations, cascade solution based upon fixed 75oC flow temp, lower temp heat pump based upon 55oC fixed flow temp 

*2 https://researchbriefings.files.parliament.uk/documents/POST-PN-0694/POST-PN-0694.pdf 

Related