Life cycle assessment of Irish district heating systems: a comparison of waste heat pump, biomass-based and conventional gas boiler

This paper presents a life cycle assessment of heat supply scenarios towards replacement of fossil-based energy systems through a case study focusing on an existing gas-fired boiler supplying heat for buildings located in Tallaght, Ireland. The three replacement systems considered are a waste heat-fed heat pump district heating system, a biomass-based district heating system, and an individual gas boiler. Current study found both the district heating systems have lower environmental impact than the conventional boiler system, with the biomass-based system being superior to heat pump. However, using 2030 electricity data showed almost similar impacts for both the district heating systems. Human toxicity potential was found highest among all impact categories studied due to the large additional infrastructure requirement across all three systems, whereas the other impacts, global warming, fossil fuel depletion and eutrophication, were due to involving usage of natural gas and electricity in use phase. The heating system employing biomass as resource showed reduced greenhouse gas (GHG) emissions by 45% and fossil fuel depletion by 73% compared to the conventional boiler. However, using 2030 electricity data, the heat pump system decreased GHG emissions by 42% and fossil fuel depletion by 47%. Further, replacing biomethane with the natural gas decreased global warming by at least 11.4%. The present study concludes that the environmental benefit of a district heating system is largely dependent on the carbon intensity of the electricity it uses, thus recommending the district heating systems for large-scale retrofitting schemes in Ireland to reach Europe’s 2030 GHG reduction targets.


Introduction
In the EU, building stock consumes 40% of energy and generates 36% of greenhouse gas (GHG) emissions (European Commission 2020). In Northwest (NW) Europe, space heating is the largest domestic energy user as it provides comfort in winter months. The EU has adopted targets to reduce GHG emissions by 40% and increase the renewable energy production share by 32% by 2030 relative to 1990 levels (European Commission 2018). For that, heat pumps are central to plans to decarbonise residential heating with many EU countries offering supports to incentivise uptake (David et al. 2017). However, NW Europe lags behind the EU average in uptake of district heating (DH), so there is little existing infrastructure (European Commission 2016). Most of the EU's housing stock was built before 1970 and so predates the introduction of the first thermal regulations (EEA 2015), and these buildings have inherent technical barriers to retrofit as many were designed to use a high temperature (+ 60 °C) wet radiator system leading to higher energy consumption, and associated emissions, to meet the heating system demand. In cool climates, an air source heat pump's efficiency and heat capacity will decrease significantly when attempting to raise the outlet water temperature above 60 °C (Wu et al. 2012). However, currently various technologies are in place to improve the residential buildings energy usage and environmental emissions, such as employing integrated energy systems, retrofitting energy systems to reduce energy consumption and usage of renewable energy (Bartolozzi et al. 2017).
DH is a popular heat generation system currently employed in European countries to enhance the energy efficiency and GHG mitigation potential of heating systems. The advantages of DH are greater heating efficiency and lower production-phase emissions owing to its larger-scale production compared to decentralised heating systems using fossil fuels (Gartland and Bruton 2016). DH can use different heating sources including combined heat and power (CHP) plants, distributing the co-product heat to a local heat demand. However, DH system can rely on fossil fuels, resulting in higher fuel costs and more GHG emissions compared to renewable energy technologies (Puettmann and Lippke 2013). Integrating DH with CHP has shown a wider application in domestic heating due to its contribution towards eco-efficient use of renewable energy resources such as geothermal heating system (GHS) and biomass (Nitkiewicz and Sekret 2014).
Currently, low-temperature GHS has been gaining popularity due to its suitability for large scale systems, wider availability, and surging market for heat pumps. Moreover, it has a significant potential to reduce the fuel consumption, costs and GHG emissions and increase energy savings by combining DH with waste heat recovery systems (David et al. 2017). However, it should be emphasised that GHS utilises fossils fuels for operation along with electricity for driving the compressor and gas for driving the generator (Kljajić et al. 2020). Utilisation of biomass as an alternative fuel for CHP has been also considered as an effective GHG mitigation strategy (Karlsson et al. 2018). Biomass CHP has a lower CO 2 contribution compared to equivalent hard coal or natural gas CHP (Bartolozzi et al. 2017). In addition, biogas can also be employed as a heating source in boilers or CHP as biogas produces 33% less CO 2 eq per unit of energy compared to natural gas (Whiting and Azapagic 2014).
Life cycle assessment (LCA) is an environmental accounting tool that measures a process or product environmental burden over the life cycle (Neirotti et al. 2020). Many authors have performed comparative LCA studies on DH with different energy generation systems such as non-renewable fossil fuels and renewable (biomass, geothermal, etc.)-based CHP and conventional and renewable energy-based boiler to identify the technologies with relatively higher sustainable viability and greater GHG mitigation potential. Puettmann and Lippke (2013) performed comparative LCA to assess the environmental impacts of a DH system in Seattle, USA, employing 56% biomass (wood) and 44% natural gas. The heating system with biomass as energy sources indicated a considerable decrease in GWP (104%) in comparison with all the natural gas boiler. This result indicates that a biomass boiler emits less carbon than sequestrated by the trees, leading to negative annual GWP. Koroneos and Nanaki (2017) conducted LCA to evaluate the environmental performance of ground source heat pump installed at Town Hall of Pylaia in Greece. The study found acidification potential as a dominating impact (74%), attributing mainly to the release of emissions like sulphur dioxide (SO 2 ) and nitrogen oxides (NOx) during the production of raw materials and operation of system. Neirotti et al. (2020) estimated the impacts associated with thermal energy supplied using fossil-based CHP system to Turin DH system and compared with energy supplied from natural gas fired boiler. The LCA results revealed the combustion stage was the main contributor (74%) towards the GHGs emissions in both the heating systems. However, the authors observed the 25% share of emissions were due to gas infrastructure, which has a significant influence on GHGs that could increase the impacts by 31%. Most studies are related to production and distribution phases involved in DH systems, with co-generation system, i.e. CHP, focussing on energy consumption and evaluation of environmental performance in terms of GHG emissions. However, up to the authors knowledge, only a few studies have considered the real-time data and studied a broader assessment perspective towards accounting for the raw material extraction for equipment and infrastructure impacts of the DH systems with a lack of literature based on Ireland's district heating system. The novelty of the study lies in the fact that this study represents the first life cycle study of an Irish DH system, i.e. for Irelands' biggest planned DH system situated in Dublin in a broader perspective and identifying its potential role in decarbonising by comparing three different heating systems, i.e. heat pump-based CHP, biomassbased CHP and conventional gas boiler.
The present work was based on the EU Interreg Heat Net project, which aims to address the challenges of reducing CO 2 emissions in NW Europe by creating an integrated transnational NWE approach to the supply of renewable and low carbon heat (incl. waste heat) to residential and commercial buildings (HeatNet NWE 2019). In Ireland, South Dublin County Council (SDCC) plans to develop Dublin's first large-scale DH system, harnessing the waste heat of a data centre via a heat pump.
Therefore, the aim of this paper was to evaluate and compare the environmental impacts of Dublin's first large-scale DH employing three different heat supply systems, i.e. heat pump-based CHP (WHP-DH), biomass-based CHP (BCHP-DH) and conventional gas fired boiler (GB) system. Through this study, the authors aimed to identify the best combination of heating systems along with the stages of life cycle of heating systems that contribute significantly towards environmental impacts as the infrastructure impacts of district heating are exacerbated in NW Europe due to lack of existing infrastructure and heat planning. This work also determines whether the impact of the infrastructure necessary to match waste heat supply demand outweighs the use of fossil fuel combustion for heating, and subsequently ascertain whether Heat pump WHP-DH has a potential role in decarbonising NW Europe's heating. Since EU HeatNet is a demonstration project, the learnings will be applied to other similar projects throughout NW Europe.

Life cycle assessment (LCA)
This study presents an LCA of heat supply scenarios for the replacement of an existing gas-fired boiler supplying heat for three buildings located in Dublin, Ireland. The three replacement systems considered are a waste-heat-fed heat pump district heating system, a biomass CHP plant district heating system, and an individual gas boiler system. The LCA was carried out in accordance with ISO14040 standards in four steps, i.e. goal and scope definition, inventory analysis, impact assessment and interpretation. The LCA was performed using the 8.2.0.55 version of Gabi which is an LCA accounting tool developed by Thinkstep (Thinkstep 2018).

Goal and scope
The goal of the current study was to evaluate the life cycle environmental impacts of waste heat-fed heat pump district heating system (WHP-DH) and biomass CHP plant district heating system (BCHP-DH) and compare with an individual gas boiler system (GB). The system boundary was considered as 'cradle to gate' that includes infrastructure facility (equipment, construction work and buildings related to each system), equipment raw material extraction, fuel extraction, electricity generation, and distribution and transportation of raw materials.
As the replacement heating system in the SDCC buildings was only in the planning stage, this LCA is prospective using data provided by Dublin's Energy Agency (Codema 2018) and published LCA studies. Moreover, the study area has heat densities two to three times above 150 TJ/km 2 which is considered highly feasible for district heating systems from a Danish perspective (Euroheat and Power 2013). As stated previously, the boundary "cradle to gate' would follow the study carried out by Nitkiewicz and Sekret (2014) which states a 20-y lifecycle and excludes disposal due to probable advances within the timespan considered in the LCA. 20 y was chosen as it is the lifespan of most of the equipment, but it should be noted that the pipeline's lifespan is much longer. Materials to make the equipment specifically for the system, such as the heat pump, were included. Avoided use of other fuels due to the energy generated within the system were also not included. A system diagram showing the processes within the cradle-to-gate the system boundary is shown in Fig. 1. In the heat pump DH system (WHP-DH), the boundary starts at the collection of the waste heat and includes all necessary steps, along with equipment, needed to process this heat. The product is the heat produced. The BCHP-DH system includes the generation of the fuel from the forest nursery phase comprising all the infrastructure necessary to deliver the heat to the required buildings. Waste disposal and the machinery needed for generating fuel or construction in all phases were excluded.

Function unit (FU) and allocation
The main function of the studied system was to produce heat for the SDCC buildings, so the functional unit is 1 MWh of heat produced. In a CHP plant, both electricity and heat are useful outputs, so electricity was allocated a proportion to the environmental impact. In the absence of Irish case study data, this allocation was based on an average Irish CHP efficiency of 86% (54% heat and 32% electricity production) (SEAI 2018).

System description
Data centres are computer warehouses that store data, with high energy demands and large amounts of heat generated. In 2016, total data centre energy use made up 4.8% of Ireland's total electricity demand (Bitpower 2017). Lu et al. (2011) used real production data from Finland to calculate that waste heat could be captured from 97% of the total power consumed in the data centre creating large sources of potential heat. The use of waste heat from data centres to power a district heating system via a heat pump has been demonstrated in Mäntsälä, Finland (EHPA 2019).
In the first system (WHP-DH), waste heat from the data centre was harnessed through a collector coil with exhaust air heating water of 20-35 °C. This warm water gets heated in the energy centre by an ammonia heat pump with a seasonal performance factor of 3.6, to 70 °C on the primary side (Nitkievwicz and Sekret 2014). Average Irish grid electricity was assumed with a carbon intensity of 428 kg CO 2 / MWh (SEAI 2018). The hot water was transported 701 m through a heat exchanger and distributed to the customer. The end user buildings had the existing gas boiler replaced by a pump, control valves and a meter situated in the heat exchanger substation. The water was pumped to the council building with the existing radiator system being maintained. The total installed pipeline was 2,142 m long, consisting of a 5 MW gas boiler to top up the temperature of the DH system in winter and act as a back-up. The gas boiler provides 20% of the heat with the heat pump providing the remainder.
The second system consisting of a 5 MW Biomass CHP district heating system (BCHP-DH) was built on the Council brownfield site to meet the base load, while the existing gas boilers were replaced to meet the peak demand. A feasibility study on the potential DH system in Tallaght was found to have a payback period of 15 y (Gartland 2014). The CHP produces temperatures of 75 °C and has an efficiency of 86% (54% heat and 32% electricity production) (SEAI 2018). A pipeline of 137 m connects the CHP to the Council building. The CHP plant transports hot water to a heat exchanger substation after which it is distributed to the end user. The impacts of biomass production and processing were based on the biomass supply chains in Murphy et al. (2014) and Murphy et al. (2016). Biomass consisted of wood chips sourced from the Laois, Ireland, area.
The third system consists of maintaining the existing individual gas boilers in each of the Council buildings. These boilers were installed 18 y ago and so require replacement. The LCA examines the replacement of these boilers with a new efficient gas boiler while maintaining the rest of the heating system.

Impact categories
The impact categories examined in the current study were global warming potential (GWP), eutrophication potential (EP), fossil fuel depletion (FFD) and human toxicity potential (HTP). Environmental impact assessment was performed according to CML 2001 methodology (Neirotti et al. 2020). The above considered impact categories were chosen over more policy relevant indicators like particulate matter due to the representativeness of the impact of mining in infrastructure, so better meeting the goal of the study. However, EP was included as it is important to measure the impact on flora and fauna as NW Europe has large land water bodies (Bartolozzi et al. 2017).

Assumptions
The efficiency of the large boiler in the district heating system was assumed to an average of 90% over the lifetime, based on the GaBi database with the smaller boilers in the SDCC buildings assumed to be 85% over their lifetime including degradation. It was assumed that all raw materials were produced in the EU-28 apart from cast iron. An assumption was made that the material is transported from Rotterdam port to Dublin Port to the system site. The only exception to this is the wood chips which would be sourced from Laois sawmills.
As there was limited information on waste heat being taken from a data centre, an assumption is made that the collector coil for the waste heat within the data centre exhaust is of a similar size to the heat exchanger. This assumption is based on both pieces of equipment performing the same function. The thermal store for the district heating systems was assumed to be 1.5 cm thick with insulation made up of high-density polyurethane (HDPU). Both systems were assumed to use gas boilers to provide 20% of the required heat. The transmission heat losses for the district heating systems were 3% for the Heat pump and 2% for the Biomass CHP, which was adopted from analysis carried out by Codema (2018). The decreased losses in the analysis were attributed to the superior system characteristics such as better insulation and capability to generate higher heat density.
The data for the biomass boiler were taken from the gas boiler information for the district heating system as both were assumed to have heating capacity of 5 MW (Table 1). The wood was assumed to come from a large, forested area in Laois which is 80 km away from the CHP plant site. Loss in wood during chipping is assumed to be 5%.
For most of the equipment, dimensions and material type were sourced from the equipment manufacturers. Therefore, the assumed density of the materials was needed to calculate the weight of equipment. The density of steel, high-density polyurethane (HDPU) and high-density polyethylene (HDPE) was assumed to be 7850 kg/m 3 , 950 kg/ m 3 and 100 kg/m 3 , respectively. Ammonia refrigerant in the heat pump was assumed to have no GWP impact (ASHRAE 2017).

Life cycle inventory
The life cycle inventory stage requires collection of input and output data for the studied system. To analyses the different DH systems, two types of data were used. Foreground data describe the data related to the inputs (electricity, fuel usage, etc.) for the heat pumps and gas boiler, along with the material used for manufacturing these equipment's and building infrastructure. Background data refer to data that represent the generic materials, energy and transport involved in production processes and delivered to the foreground system as aggregated datasets. These datasets are generally taken from databases and literature. In this study, a 2016 average Irish grid electricity data were adopted from SEAI report (2018). All other fuel types were taken from GaBi's professional database of Irish fuel emissions, as extraction and transport were included in these datasets. Data for the heat pump DH (WHP-DH) and gas boiler systems were both provided by Codema (2018) with the Biomass CHP district heating system (BCHP-DH) taken from Gartland (2014), Murphy et al. (2014) and Murphy et al. (2016). These data sources were used to define the three system's equipment needs (Table 1) with fuel needed to produce 1 MWh of heat calculated from efficiency data. The piping systems employed in the study were adopted from Logstor et al. (2018) and Codema (2018). Further, inventory related to pipes in terms of construction materials and dimensions considered in the model are shown in Table 2. When the system processes were defined, the background data for these heating systems were taken from Ecoinvent 3.7, GaBi professional database (Thinkstep 2018) and other published LCA studies (Table 1).
The data and sources for the waste heat fed heat pump system are outlined in Table 1. The seasonal performance factor for the heat pump was 3.6 with average 2016 Irish electricity used to power it (SEAI 2018). Heat losses in transmission were 3% of total heat (Codema, 2018). The refrigerant used in the heat pump was ammonia.
The woodchips for the biomass-based DH system were harvested according to Scenario 3 proposed by Murphy et al. (2014). The energy of the woodchips at moisture content n/a n/a 35% was 11 GJ/t which is adapted from Murphy et al. (2016). The wood at the sawmill has a moisture content of 20% and has an energy content of 12.65 GJ/t. The building area which houses the biomass boiler was 100 m 2 , and the pipeline to the end users was 137 m with a heat exchanger substation. Allocation of burden to heat in the CHP was done according to the average Irish CHP efficiency, with heating allocated 62.8% of the burden and electricity by 37.2%. For the gas boiler system (GB), the only infrastructure change was the gas boiler, with the quantity and size provided from Dublin's Energy Agency (Codema) with the efficiency of 85% through a connection to gas already established.

Results
The results section shows the studied systems contribution to each impact category, categorised into three groups: use phase, infrastructure and other. The use phase relates to emissions and energy used in the heat generation stage. Infrastructure includes all equipment, construction work and buildings related to each system. The other category includes the transport impact across all materials in the system and the production of woodchips in the biomass-based DH system.

Global warming potential (GWP)
The LCA results showed that the individual gas boiler (GB) system has a significant GWP (268 kg CO 2 eq/MWh), which is 34% and 44% higher than WHP-DH and BCHP, respectively (Fig. 2a). The higher emissions from the GB system were mainly due to the higher usage of gas as fuel (i.e. around 95%) in comparison with electricity which accounts for around 4.5% (Table 3). Figure 2a shows that across all the heating systems, the use phase contributed considerably to the GWP, owing to the fuel usage which accounts for at least 84.7% of the GWP in each system. The overall impact of the use phase along with percentage contribution of certain energy sources within the full system results is shown in Table 3. The heat pump DH system needed gas to meet peak load to maintain the efficiency of the heat pump. However, it was electricity that contributed 56% of the GWP as electricity provides 80% of the heating output. In the BCHP-DH system, biomass makes up 80% of the heat generation, but it was the natural gas which made the largest contribution.
In total, natural gas was responsible for largest proportion of the GWP in conventional boiler (GB) and biomass-based DH systems, whereas electricity was dominating in the heat pump DH system. However, data on electricity production can change significantly depending on location and time which can lead to uncertainty in comparison. And importantly the difference in the results in GWP for non-fossil fuel system is due to the related infrastructure, as the biomass considers CO 2 sequestration from atmosphere resulting in lower GWP values in comparison with fossil-based heating systems.

Fossil fuel depletion (FFD)
The individual gas boiler (GB) system uses twice as much fossil fuel than either of the DH systems (Fig. 2b). The DH system with heat pump uses close to twice the fossil fuel than the biomass DH system. The lower fossil fuel consumption can be attributed to the reduced usage of electricity for heat generation, whereas the other two systems (heat pump DH and GB) rely on both electricity and gas for heat generation. Figure 2b shows the main contributor to resource depletion is again the use phase which makes up at least 69.7% of the total contribution in each system. Table 3 shows that gas makes up higher proportion of FFD than its contribution to heat output as expected. However, it also highlights the contribution of electricity to FFD with it being the largest contributor in the heat pump DH system. In the biomass DH system, there is also a large contribution from the production of the wood chips which is included in the other category (20.8% of total system FFD).

Human toxicity potential (HTP)
The individual gas boiler (GB) system (3.68 kg DCBeq/ MWh) has the lowest associated HTP which is five times less than the biomass-based DH (17.2 kg DCBeq/MWh) system and eight times less than the heat pump-based DH system (26.4 kg DCBeq/MWh). The results for HTP show the largest contribution came from infrastructure (Fig. 2c). This is highlighted by the low human toxicity towards the gas boiler (GB) as much of the infrastructure for this system is already in place in comparison with the other systems. The results were further categorised into pipeline infrastructure and other infrastructure. Pipeline relates to all materials needed and the trenching works to build the pipeline to connect to the end user. While the other infrastructure relates to any other equipment needed such as the heat pump or the energy building. The WHP-DH and BCHP-DH systems have similar HTP values in terms of infrastructure other than the pipeline. For the pipeline, however, the values of the two systems diverge significantly with the heat pump pipeline having close to four times the attached HTP for its pipeline. This is due to the total length of the heat pump pipeline being 2,187 m compared to the BCHP-DH pipeline of 137 m.

Eutrophication potential (EP)
The waste heat pump system (WHP-DH) (0.0317 kgPO 4 eq/MWh) has the largest eutrophication potential, with the biomass-based CHP system (BCHP-DH) (0.022 kg PO 4 eq/MWh) having the least with the individual gas boiler (GB) system having an EP of (0.023  kgPO 4 eq/MWh). However, these results show the smallest difference between systems of any of the impact categories (Fig. 2d). EP also has a more even contribution from the different phases in each system apart from the GB system where the use phase accounts for over 99% of total EP.

Sensitivity analysis
To identify the most relevant source of uncertainty in the study, sensitivity analysis was performed. It estimates the consequences of changes in uncertainty factors on the global warming potential (GWP) of the heating system (Hammar and Levihn 2020). The uncertainty factors considered in the current study are transport distance, time range of electricity, replacement of renewable energy source with fossil-based fuels and changes in geographic location of system.

Transport
As this LCA study is prospective, an assumption of where the material was transported from was needed for the infrastructure with Rotterdam port being used. The sensitivity analysis explored the effect of transport distance on the GWP. Materials such as concrete, sand and cement can be made in Ireland, so the transport distances may even decrease. The data for woodchips were specific to Ireland, so no changes were made. For the increased distance scenario, all materials excluding woodchips were modelled to come from Stuttgart, Germany, which is one of the most industrialised areas of the EU. In the decreased distance scenario, cement, sand and concrete were modelled from Westmeath, Ireland. Details of the transport distances are in Table 4. Figure 3a indicates that changing the transport distance by well over 10% has a low relative impact on the overall GWP results. With the heat pump system decreasing by 1 kg of CO 2 eq/MWh and increasing by 3 kg of CO 2 eq/MWh  when the respective changes are made. This confirms transport distances are not the main driver of overall impact if sourced from within Europe.

Electricity
As stated earlier in Results section (Table 3) electricity used in the use phase has a large impact on both the individual GB system and WHP-DH environmental impact. However, as there can be large differences in the electricity data depending on the geographical coverage it is important to study the impact using different data may have. As the lifetime of the study is 20 y, then taking projective electricity data at the halfway point of the project, 2030, may create a more accurate result. Irish EPA (2019) predicts that there will be a 27% drop in CO 2 eq per unit of grid electricity if additional measures are implemented due to the replacement of coal and peat which also have high human toxicity and eutrophication impacts.
Adopting the 2030 electricity data reduced the GWP in each system with the difference being most pronounced in the WHP-DH system, as shown in Fig. 3b. If the 2030 electricity data were adopted into the model, then the difference in the GWP between the heat pump system and the BCHP-DH plant was much smaller. However, it is important to note that using predictive data introduces more uncertainty into the results but could increase accuracy (Hammar and Levihn 2020).

Biogas
The use of gas is a large hotspot in the use phase, but natural gas could be displaced by biomethane. The data for the biogas production were taken from GaBi professional database. The data were adjusted to consider the 43 kg CO 2 eq/ MWh due to the upgrade from biogas to biomethane, injection and delivery in the gas grid (DBFZ 2016). Data have a geographical coverage of Germany due to data availability. Figure 3c clearly shows that the adoption of biomethane over natural gas would lower GWP by 11.4% in the WHP-DH system, 14.2% in the BCHP-DH system and 38.2% in the individual gas boiler (GB) system.

Pipe length change
Human toxicity impacts in previous studies have been due to infrastructure, so to reduce the quantity of material in the pipeline, analysis was carried out on relocating the energy centre from the data centre to the proposed BCHP-DH. This would lead to the heated water of 20 to 35 °C degrees leaving the data centre and being transported 700 m to the new energy centre. The lower temperature water would not require the steel pipes used to carry the 70 °C water.
These steel pipes could instead be replaced by high density polyethene pipes which would be lighter and easier to install. However, using polyethylene pipes instead of steel pipes at 20 to 35 °C could result in a significant increase in biofilm growth and Legionella sps. (Van Der Kooij et al. 2005). The relocation would see 700 m of the 200 mm steel pipe replaced by 720 m polyethene pipe. As shown in Fig. 4, there would be a small decrease in FFD and HTP if the energy centre was relocated; however, it is not significant enough to carry out due possible impact on future expansion of the system. Figure 2c and Fig. 4 indicate that it is the total length of the pipeline and sequential trenching work that had the biggest impact on the human toxicity. This suggests minimising infrastructure health impacts by better matching of heating demand, and waste heat supply is needed. Table 3 and Fig. 3b show the impact that electricity consumption has in each system. The set of electricity data used makes a large difference in the impact of the heat pump system. Most LCAs are static in terms of time with a steady state assumed over the lifecycle (Miralles et al. 2020). However, Hammar and Levihn 2020 argue that a more accurate study of future systems would involve predicative modelling.

or 2030 electricity data?
In this study, electricity production during 2016 and 2030 has been evaluated to predict the future consequences using the developed models. If the 2016 electricity data are used, then the BCHP-DH is most beneficial. Electricity generation is due to change dramatically across Europe over the next decade so the use of 2030 prediction would be more accurate in predicting lifecycle impacts. Therefore, adopting 2030 electricity data into the model not only had a significant impact in decreasing the GWP of the WHP-DH system (Fig. 3a), but also a resulted in a significant decrease across all indicators. While BCHP-DH may still have lowest environmental impact, its GWP would be only 5% less than the heat pump DH system when using the predicted 2030 electricity grid mix. The BCHP-DH system model has greater uncertainty than the WHP-DH system due to positioning of the plant being assumed and the exclusion of the pelleting stage by allocation, the most energy intensive step of wood processing. This uncertainty makes it difficult to give a definite assessment on which system would have the least environmental impact and would depend on the specifications of the systems (Murphy et al. 2016). Moreover, a further analysis is required to thoroughly understand the ecological significance of the studied system based on embedded uncertainties within the study.
In Ireland, marginal electricity generation is usually generated by carbon intensive coal or gas power plants (SEAI 2019). 82% of renewable electricity in Ireland is from wind power which despite output peaking in the evening, produces a large output throughout the night when demand for electricity drops. When demand drops, the total contribution of renewable electricity to the total electricity consumption greatly increases as it displaces carbon intense marginal electricity generation (SEAI 2019). Heat pumps with thermal storage could be run mostly at night which can then be stored and used throughout the day levelling electric demand which can relieve pressure on electric networks from high penetration of renewables (Li et al. 2021). Further research is needed on the environmental benefits of additional thermal storage in load levelling.

Is gas the right peak energy source?
Both district heating systems require the use of a peak energy source due to feasibility or to work as a back-up source during plant downtime. In both the systems the natural gas boiler functions as this peak energy source. However, Fig. 3c shows this was the least environmentally friendly option.
As mentioned earlier in the results section, the individual GB system represents the largest GWP per MWh heat produced. In addition, Table 3 shows that gas contributes 53.30% of the GWP related to the use phase in the BCHP-DH system and 28.7% in the WHP-DH system. An alternative that would require little adaptation would be the integration of biogas to supply some or all the of gas demand (Karlsson et al. 2018). However, a biogas boiler cannot compete on price with a natural gas boiler in Ireland, so a full replacement in the individual GB system is not feasible (REE 2016). With the odour concerns of anaerobic digesters, it is unlikely that a large-scale plant would get planning permission in such a built-up area (IRA 2017).
An alternative is to buy credits for biomethane injected into the national grid, which would require no change to current systems. This supports the creation of biogas, but the price is usually higher due to processing costs (REE 2016). In the case of both the district heating systems, the use of biomethane in place of natural gas would decrease CO 2 eq/MWh of heat by 11.4% in the heat pump system and 14.2% in the biomass CHP DH system. These savings are significant enough that they should be considered if biomethane prices are at feasible levels by the time of project commencement.
BCHP-DH will decrease the carbon intensity of the electricity used in the heat pump, so deployment of both district heating systems together would create a mutually beneficial relationship (David et al. 2017). Whiting and Azapagic (2014) concluded that biogas produces at least twice the CO 2 eq per unit than a woodchip CHP.

Comparison to other studies
Prospective LCA studies are used in a choice making LCA at the start of a planning stage between different systems, they are less detailed but they are a useful tool for comparison. Nitkiewicz and Sekret (2014) performed a prospective LCA with a 20-y lifecycle and found that the heat pump had a lower overall environmental impact; however, its impact on human health was higher than a gas boiler. Further, Neirotti et al. (2020) concluded that waste heat systems associated with Turin district heating system has much lower GWP and FFD rates than conventional gas boilers, but they are not significantly different in other environmental indicators. Miralles et al. (2020) identified that the damage categories such as climate change and human health related impacts were greater in case of heat pump in comparison to biomass (wood pellets)-based boiler employed at University of Jean building in Spain. Similar results were found in this study where the heat pump did have a lower environmental impact, especially if 2030 electricity data were adopted, but showed higher impact in terms of HTP in comparison with the gas boiler. This is mainly due to the electricity grid mix and infrastructure having a large impact on renewable systems emissions, especially on HTP.
Electricity grid mix and infrastructure play a major role in influencing the environmental impacts of the heating systems. For instance, in contrast to the current study results, Greening and Azapagic (2012) found that a domestic heat pump was less environmentally beneficial than a gas boiler, noting that most of the environmental impact was due to the operation of pump. The higher impacts were attributed to the electricity grid mix, which derives a major part of energy from coal and natural gas. LCA study conducted at Republic of Serbia by Kljajić et al. (2020) towards integration of local geothermal potential (heat pump system) with DH system suggested that the geothermal heat pump system can bring energy and economic benefits but with lower environmental impacts than existing gas boiler. This was mainly due to the unfavourable electricity generation mix in the Republic of Serbia, which is predominantly coal based, i.e. around 71%. Koroneos and Nanaki (2017) studied absorption heat pumps in Greece finding that the largest environmental impacts were GWP and acidification. The authors ascertained that the impacts were linked to the extraction of the raw materials for the pump and the use of electricity which uses a large amount of coal power. However, an assessment into industrial waste heat use in Sweden found that a district heating system had a marginal environmental benefit at average heat source mix but with the use of waste heat the system had a clear environment benefit compared to the system in place (Ekvall and Ljungkvist 2014). Puettmann and Lippke (2013) performed comparative LCA to investigate the environmental impacts of a DH system in Seattle, USA, employing 56% biomass (wood) and 44% natural gas. The system with renewable fuel (biomass) showed a significant reduction in GWP (104%) in comparison with all the natural gas boiler. This result indicates that a biomass boiler emits less carbon than sequestrated by the trees, leading to negative annual GWP. The authors also argued that the major contribution to GWP comes from feedstock combustion, but transportation contributes to less than 10%. On the other hand, Murphy et al. (2016) argue that in the Irish biomass supply chain, transport is the most energy intensive stage and has the largest contribution to global warming potential. Rinne and Syri (2013), using a consequential LCA, found if the electricity mix in the area mostly comes from condensing coal power, then CHP is a more environmentally friendly system than a large-scale heat pump as it displaces coal powered electricity generation. Similarly, a consequential life cycle analysis of Nordic region's district heating system resulted in greater environmental benefits towards CHP-based electricity, as the marginal electricity in Nordic region is considered as coal condensing (GWP is more than 500 kg CO 2 eq per MWh) (Karlsson et al. 2018). However, authors pointed that heat supplied through heat pumps using the Swedish electricity mix has a smaller carbon footprint than supplied through CHP.
The results of the current study were in line with the above-discussed works as the results show that employing waste heat utilisation (CHP) and renewable energy-based heating systems in a combined strategy could facilitate in an environmentally friendly DH system provided fossil-based energy sources are replaced with renewables in the background processes (electricity grid mix) (Bartolozzi et al. 2017). Therefore, to achieve sustainable energy systems, local DH systems development must comprise improved integration with renewable energy systems, application of blended-fuel technology and more flexible energy systems (Karlsson et al. 2018).

Policy
The EU has agreed a 40% reduction of GHG emissions on 1990 levels by 2030. Figure 2a shows the clear impact that either DH system, i.e. heat pump or biomass, could have in reducing GHG emissions in urban areas. The BCHP-DH system produces 45% less CO 2 eq/MWh than the individual GB system which would equate to an avoided GHG release of 3,696 t of CO 2 eq over the lifetime of the heating system. The WHP-DH system produces 35% less CO 2 emissions; however, using 2030 electricity mix results in 42% less CO 2 emissions. One third of houses in Ireland are powered by gas with the majority in urban areas which can support largescale district heating systems (Gartland and Bruton 2016). The replacement of gas boilers with district heating could contribute to the 2030 GHG targets in large-scale building retrofit schemes.
The EU Renewable Energy Directive committed Ireland to produce 16% of all energy consumed from renewable sources by 2020 (SEAI 2019). The 2010 National renewable energy action plan targeted 12% of heating to come from renewable sources by 2020. However, by 2020 only 6.3% of heating and cooling came from renewable sources with very little growth from the 2010 figure of 4.3% (SEAI 2020a). As this target will likely be missed, the 2030 target of a 32% renewable energy penetration is vital. The systems studied are possibly only the first phase of a district heating which could likely lead to much of localities heat demand being met.
Energy security is a large concern throughout the EU (2014) with gas supply vulnerable to geopolitical events; however, the expansion of renewable energy may decrease this vulnerability. Ireland has had an above average import dependency since 1990 relative to the rest of the EU (SEAI 2020b). This import dependency is due to a lack of natural resources and a reliant on using fossil fuels to deal with energy needs. Renewable energy makes up l.6% of imported fuel by energy, but it produces 11% of Ireland total primary energy requirements (SEAI 2020a). A move from a fossil fuel system to renewable power systems could be a potential solution to decreasing energy dependency and could provide potential economic benefit (Kljajić et al. 2020).
To optimise the adoption of district heating in NW Europe current policies require review. Sweden's introduction of a carbon tax in 1991 in conjunction with grant programmes is seen as a driving force in uptake of district heating (David et al. 2017). Increasing carbon taxes would likely see a rise in the uptake of district heating system. However, increasing carbon taxes is controversial in NW Europe and without a policy framework in the first place, uptake will not satisfy the pace needed to decarbonise heating. Interactive effects of policies for other technologies can impact adoption such as amended Gas Act of Netherlands (IEA 2020) where policy is not representative of its low uptake. Table 5 demonstrates countries with a well-rounded framework of measures supporting district heating with a higher uptake. But, for countries with a low uptake of district heating, developing a comprehensive regulatory framework and engagement with local stakeholders should be the initial focus.

Conclusions
The LCA results indicate that the district heating systems have a lower environmental impact in terms of fossil fuel depletion and global warming potential than individual gas boilers. Whereas BCHP-DH system showed lower environmental impact than the WHP-DH system across all the impact categories considered. However, if estimates for the electricity carbon intensity are adopted for the mid-way point of the project (2030), GWP of the WHP-DH is within 5% of the BCHP-DH. Further, BCHP-DH and CHP-DH has the potential to reduce GHG compared to the individual gas boiler system by 3,696 and 3,449 t of CO 2 eq, respectively. However, both district heating systems are highest in terms of human toxicity potential because of the effect of large amounts of infrastructure being built. Infrastructure impact was linked to the length of pipeline so could be minimised with improved co-ordinating of heat demand with waste heat supply. The main hotspots for the heating systems in terms of EU, FFD and GWP are due to the use phase of the lifecycle involving prominent usage of natural gas and electricity for heat generation. Moreover, the use of biomethane as a peak energy source instead of natural gas was likely to reduce GHG emissions by at least 11.4% in both district heating systems.
So, a conclusive decision on which system has the lowest environmental impact if 2030 electricity data are used will depend on the specification of the system. This study concludes that district heating, either through biomass CHP or heat pump, provides the lowest overall environmental impact for heating South Dublin County Council buildings and gives flexibility for future expansion. District heating should be considered when planning heating systems for large-scale retrofit schemes. Both district heating systems can help contribute renewable heat and could be replicated across NW Europe. BCHP-DH system could reduce fossil fuel use by 73% with the WHP-DH reducing it by at least 47% compared to the existing gas boiler system; however, the impact of increased biomass imports must be studied. To better deploy district heating NW European countries should focus on developing legislation on market competition and mandatory standards as well as greater engagement with local stakeholders with support given where required. Future study of policy-oriented indicator like particulate matter and the interaction of biomass combustion in an urban environment may be useful.
Acknowledgements The authors would like to thank the staff of Energy Efficiency Agency Dublin (Codema), especially John O'Shea for sharing their technical knowledge and advice on the project.
Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.

Data availability
The authors confirm that the data supporting the findings of this study are available within the article.
Code availability Not applicable.