Operational and embodied carbon of building services

Decarbonisation of the UK grid and increase of passive design results in the operational carbon emissions getting smaller. While building services account for an average of 11% of embodied carbon, their embodied carbon or whole life cost are not currently being considered when selecting equipment.

Capacity of heat generating equipment (kW) affects the sizes of the equipment, distribution system and heat emitters. The greater the capacity the larger the components of the whole system. Thus, the reduction in the capacity of the heating system will reduce its embodied carbon.

When it comes to operational carbon, for heating systems this depends on the amount of time the building is required to be heated to the desired temperature and the efficiency of the system in moderating its environments passively. Operational carbon can be reduced with efficient fabric and selection of systems with high operational efficiency.

The focus of the industry to date has been operational carbon, however as this has been improving over time, the embodied carbon is becoming a bigger contributor, especially for highly efficient buildings designed to standards such as Passivhaus.

Refrigerants

Refrigerants are fluids used in heat pumps in the process of extracting heat – the refrigeration cycle. Fluids used must be suitable with evaporation and condensation points at suitable temperatures. Their performance will vary, and some will require more refrigerant fluid charge which affects the system efficiency.

The Global Warming Potential (GWP) of refrigerants varies greatly, from GWP of 2088 for R410 down to GWP of 1 for R744. Refrigerant leakage occurs at three stages: when the equipment is being manufactured, during equipment operation and at the point of decommissioning. Whole Life Carbon of heat pump equipment can be low as long as refrigerant leakage and refrigerant GWP are both low. The Whole Life Cycle (WLC) calculation is very sensitive to the energy demand and as the energy demand decreases the impact of refrigerant leakage becomes higher. In case of efficient buildings, the carbon emissions associated with refrigerant leakage form significant portion of WLC and can account for more carbon emission than operational energy.

Considerations for using refrigerants with low GWP:

  • Most are toxic and might require leak detection
  • They tend to need high operating pressures and therefore have higher leakage
  • Lower COP means that the equipment needs to be larger and will need more operational energy
  • From 2022 refrigerants with GWP higher than 150 will be banned in the heat pump equipment in the EU

Comparative Study

We have selected four units with different refrigerant types for comparison:

  • Option A – Monoblock ASHP using R32 Refrigerant
  • Option B – DHW Cylinder with integrated Heat Pump using R290 Refrigerant
  • Option C – Monoblock ASHP using R744 Refrigerant
  • Option D – Monoblock ASHP using R290 Refrigerant

The size of the units is adequate for domestic use. Option B considers DHW cylinder with integrated Heat Pump which is used solely for meeting hot water demand.

Refrigerant Options Summary

Option A – Monoblock ASHP using R32 (HFC) Refrigerant:

  • High GWP
  • Medium to High operating pressures
  • Nontoxic
  • Flammable. Mild flammability limits the refrigerant charge size (should not be limitation for dwellings)
  • Medium leakage potential
  • lower refrigerant charge than Ecodan
  • 541.6 kg CO2 equivalent at refrigerant full charge for selected unit
  • 32.5 kg CO2 equivalent leakage per year (assumed leakage 6% / year)
  • 487 kg CO2 equivalent leakage over 15 years lifetime

Option B – DHW Cylinder with integrated Heat Pump using R290 (HC) Refrigerant:

  • Low GWP
  • Medium to Low operating pressures
  • Toxic
  • Highly flammable, units using this refrigerant have limited charge size, used in units with low charge
  • A3 safety group classification for refrigerants (high flammability and lower toxicity)
  • Low leak potential due to low operating pressures
  • Selected unit only does DHW but is very small in size and has very small refrigerant charge
  • 0.6 kg CO2 equivalent at refrigerant full charge for selected unit
  • 0.036 kg CO2 equivalent leakage per year (assumed leakage 6% / year)
  • 0.54 kg CO2 equivalent leakage over 15 years lifetime

Option C – Monoblock ASHP using R744 (CO2) Refrigerant:

  • CO2 refrigerant GWP of 1
  • High operating pressures
  • Nontoxic
  • Non flammable
  • High leak potential due to high operating pressures
  • High pressure systems are more complicated and require additional skills for service technicians
  • Vented during service
  • Good SCoP at 55°C
  • 1.2 kg CO2 equivalent at refrigerant full charge for selected unit
  • 0.069 kg CO2 equivalent leakage per year (assumed leakage 6% / year)
  • 1.04 kg CO2 equivalent leakage over 15 years lifetime 

Option D – Monoblock ASHP using R290 (HC) Refrigerant:

  • Low GWP
  • Medium to Low operating pressures
  • Toxic
  • Highly flammable, units using this refrigerant have limited charge size, used in units with low charge
  • A3 safety group classification for refrigerants (high flammability and lower toxicity)
  • Low leak potential due to low operating pressures
  • Selected unit only does DHW but is very small in size and has very small refrigerant charge
  • 1.8 kg CO2 equivalent at refrigerant full charge for selected unit
  • 0.108 kg CO2 equivalent leakage per year (assumed leakage 6% / year)
  • 162 kg CO2 equivalent leakage over 15 years lifetime

Conclusions

Having looked at four different Heat Pump units which use three different types of refrigerants: the R32 hydro fluoro carbon based refrigerant with GWP of 677, the R290 propane hydrocarbon based refrigerant with GWP of 4, and the R744 carbon dioxide based refrigerant with GWP of 1 our conclusions are as follows:

  • In terms of CO2 we strongly advise against using the R32 based refrigerant (Option A) because of its high Global Warming Potential of 677.
  • Option C – R744 CO2 refrigerant is well suited for the application where heat pump is connected to radiators and not to an UFH system. CO2 refrigerant has good efficiency at higher temperatures (SCoP at 55°C). Selecting heat pump with other type of refrigerant might have detrimental effect on the efficiency as typically heat pumps have good SCoP at 35°C which makes them better choice for UFH system but not for radiators.
  • Option B – R290 refrigerant in the ASHP DHW cylinder unit could also work well in the above application as the unit is the right size to provide DHW. Small equipment size means less refrigerant charge and although Option C has excellent GWP of 1, Option 2 has less CO2 equivalent charge because of the smaller size of the unit. This also applies to annual leakage of CO2 equivalent.
  • Option D – R290 refrigerant in the monobloc ASHP also has good efficiency at higher temperatures which works well in the above application where heating is delivered via radiators. For the 3.5kW unit the refrigerant charge is 0.6kg and estimated refrigerant leakage over 15 years is 162kg of CO2 equivalent which is higher than Option B and Option C but still very marginal compared to Option A.

Written by Kasper Maciej

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