Low carbon heating in domestic buildings - technical feasibility: report

Executive Summary

Motivation and methodology

Scotland's Climate Change Plan set an ambition for emissions from buildings to be near zero by 2050, and targets 35% of domestic and 70% of non-domestic buildings' heat to be supplied using low carbon technologies by 2032. The Climate Change (Emissions Reduction Targets) (Scotland) Act 2019 set a new target for emissions to be net zero by 2045, with interim targets of 75% by 2030 and 90% by 2040. The update to the Climate Change Plan will be published at the end of 2020 to reflect these new targets. The Energy Efficient Scotland programme, launched in May 2018, sets out a wide range of measures to promote low carbon heating alongside energy efficiency improvements in Scotland's buildings. Meeting these targets will require almost all households in Scotland to change the way they heat their homes. It is therefore imperative to advance our understanding of the suitability of the available low carbon heating options across Scotland's building stock.

The aim of this work is to assess the suitability of low carbon heating technologies in residential buildings in Scotland. The outputs generated through this work will form a key part of the evidence base on low carbon heat which the Scottish Government will use to further develop and strengthen Scotland's low carbon heat policy, in line with the increased level of ambition of achieving Net Zero by 2045.

The study was carried out around three key stages, represented in the diagram below.

Review of technical feasibility of low carbon heating in homes → Creation of a model representing the Scottish domestic building stock in 2017 and 2040 → Analysis of the suitability of low carbon heating in current and future Scottish homes

The first stage focussed on the review of technical feasibility of low carbon heating in homes. A comprehensive literature review of the factors influencing suitability and of the barriers to deployment of low carbon heating in domestic buildings was performed, including any features that could be relevant in the Scottish context. The information thus collected was utilised for the identification of the most relevant attributes that should be considered in the following stage for the creation of a stock model.

The second stage aimed at the creation of a model representing the Scottish domestic building stock. A set of useful attributes influencing the suitability of low-carbon heating technologies in Scottish homes was identified, based on the available information on the buildings stock offered by Home Analytics. The existing domestic building stock in 2017 was then mapped to the produced list of archetypes. Finally, a scenario for the deployment of energy efficiency measures was developed according to the targets of the Energy Efficient Scotland Route Map,^[1] in order to identify the characteristics of the likely building stock in 2040.

In the last stage an analysis of the suitability of each of the archetypes representing the Scottish domestic building stock for a set of 26 low-carbon heating systems was performed. The suitability of the current and future Scottish domestic building stock for low-carbon heating could therefore be tested and the characteristics and number of homes that are suitable only for a restricted range of low-carbon heating technologies was more closely examined.

Creation of a model representative of Scotland's domestic building stock

The archetypes utilised in the stock model are determined by a list of attributes that can assume a range of values. While the set of attributes is fixed for all archetypes, each archetype is uniquely identified by a different combination of attribute values.

The choice of attributes to be included in the archetype definition was based on relevant attributes influencing the suitability of low-carbon heating and on information provided by Home Analytics on the characteristics of the Scottish housing stock.

Attributes selected for the creation of dwelling archetypes

Attributes / Values

Age

• Pre-1919
• 1919-1991
• Post-1991

Property type

• Detached
• Semi-detached
• Terraced
• Flat (block)
• Flat (other)

Size

• Small (< 66 m²)
• Medium (66 - 108 m²)
• Large (> 108 m²)

Wall insulation

• SWI - Solid wall insulated
• SWU - Solid wall uninsulated
• CWI - Cavity wall insulated
• CWU - Cavity wall uninsulated with low exposure
• CWU exposed - Cavity wall uninsulated with high exposure

Roof insulation

• <100 mm
• 100-250 mm
• >250 mm
• Room in roof
• No loft

Existing heating system

• Gas boiler
• Oil boiler
• Electric
• Other

Orientation suitable for solar thermal

• Yes
• No

Location

• Urban
• Rural

Coastal location

• Yes
• No

Gas network location

• On gas grid
• Off gas grid

District heating potential

• Yes
• No

From this list, a set of ~140,000 different dwelling archetypes was produced, each representing a unique combination of all chosen attribute values. Each archetype constitutes therefore a simplified representation of a dwelling, equipped with a unique set of physical and geographical characteristics that are useful in the assessment of low-carbon heating suitability.

In order to calculate the stock of each dwelling archetype in the Scottish landscape, all existing homes in Scotland entered in the Energy Saving Trust (EST) Home Analytics database were aggregated based on the chosen attributes. The number of existing homes represented by each archetype was therefore included in the model, providing information on the number of real homes that may be associated with the same suitability constraints of a particular archetype.

Scotland's housing stock in 2017 and 2040

An initial useful output of the stock model is a representation of the characteristics of the current Scottish housing stock, based on a set of interesting attributes. The assessment of the current state of the Scottish housing stock is based on information available for the year 2017.

Figure description:

Vertical bar chart showing that Scotland’s housing stock in 2017 consisted of approximately 2.46 million homes, which are broken down: by age (479,000 were built pre-1919, 1,521,000 between 1919 to 1991 and 460,000 post 1991); by property type (151,000 are flats (blocks), 845,000 flats (other), 246,000 terraced, 697,000 semi-detached and 521,000 detached); by size (625,000 are small, 1,225,000 medium and 609,000 large-sized); by amount of wall insulation, by amount of roof insulation, by heating system (1,843,000 have a gas boiler, 184,000 oil boiler, 340,000 electric storage heating and 93,000 other heating systems); and by whether they have designated heritage status.

Energy efficiency measures will need to be implemented between 2017 and 2040 to comply with the requirements imposed by the Energy Efficient Scotland Route Map, including fabric measures influencing the type of insulation and annual space heating demand.

In order to assess the influence of the implementation of expected energy upgrade measures on the characteristics of the existing building stock in 2040, EST's Portfolio Energy Assessment Tool (PEAT) was used to forecast future changes. The influence of the implementation of energy efficiency measures considered by PEAT on the characterisation of the housing stock primarily affect two of the modelled attributes: wall insulation and roof insulation, the modification of which is reported in the figure below.

Figure description:

A horizontal bar chart shows the expected upgrade in wall and roof insulation in Scotland’s 2.46 million homes between the years 2017 and 2040. The bar chart to the left breaks down wall insulation and shows that SWI homes will increase from 494,000 in 2017 to 1,124,000 in 2040, 714,000 SWU will decrease to 83,000, 175,000 CWU will decrease to 27,000, 232,000 CWU exposed will decrease to 36,000 and 846,000 CWI will increase to 1,190. The bar chart to the left breaks down roof insulation and shows that 98,000 homes with less than 100 mm in 2017 will decrease to 2,000 in 2040, 536,000 between 100 to 250 mm will decrease to 53,000, 874,000 with more than 250 mm will increase to 1,698,000, 295,000 with room in roof will decrease to 49,000 and 658,000 homes with no loft will remain the same.

The number of homes with wall insulation (Solid Wall Insulated or Cavity Wall Insulated) is expected to increase from 54% of the stock in 2017 to 94% in 2040, with nearly two thirds of wall insulation upgrade interventions performed on solid wall homes.

While 27% of dwellings in Scotland are not connected with their building's roof and therefore require no roof insulation (value "no loft"), the number of homes with roof insulation of less than 250 mm thickness or with poorly insulated room-in-roof is expected to reduce from 38% of the stock in 2017 to 4% in 2040.

Barriers to technical suitability of selected low-carbon heating technologies

Low-carbon heating options investigated in this study include a comprehensive selection of 26 technologies reported in the table below.

Heat pumps

1 Air Source Heat Pump (ASHP)

2 Ground Source Heat Pump (GSHP)

3 High-temperature ASHP

4 High-temperature GSHP

5 Communal ASHP

Electric resistive heating

6 Electric storage heating

7 Direct electric heating

8 Electric boiler

Bioenergy boilers

9 Solid biomass boiler

10 BioLPG boiler

11 Bioliquid boiler (B100)

Low carbon gas

12 Hydrogen boiler

13 Biomethane grid injection

Hybrid heat pumps

14 Hybrid ASHP + gas boiler (no hot water cylinder)

15 Hybrid ASHP + gas boiler (with hot water cylinder)

16 Hybrid ASHP + bio-liquid boiler (no hot water cylinder)

17 Hybrid ASHP + bio-liquid boiler (with hot water cylinder)

18 Hybrid ASHP + hydrogen boiler (no hot water cylinder)

19 Hybrid ASHP + hydrogen boiler (with hot water cylinder)

20 Hybrid ASHP + direct electric heating (no hot water cylinder)

21 Hybrid ASHP + direct electric heating (with hot water cylinder)

Heat networks

22 District heating

Combinations with solar thermal

23 ASHP + solar thermal

24 Electric storage heating + solar thermal

25 Direct electric heating + solar thermal

26 Electric boiler + solar thermal

Key barriers to suitability considered in this study include the following constraints.

Heat demand (specific heat loss): Homes with specific peak heat loss rate above 150 W/m² were considered unsuitable for the use of conventional heat pumps, as the heat delivered even by large emitters would unlikely meet the heating demand, due to the low flow temperature delivered.

Heat demand (fuse limit): The installation of electric resistive heating or heat pumps was considered unsuitable in homes with large heat demand, if resulting in peak electrical power consumption exceeding the maximum fuse limit of 100A. A sensitivity analysis was performed for fuse limit values of 60A, 80A and 100A. Homes with use rating smaller than 60A are expected to be uncommon.

Dwelling type: Communal ASHP were assumed to be unsuitable for detached, semi-detached and end-terrace homes. All other types of dwellings were considered suitable, due to the reduced length of pipes needed for the connections and thus higher cost-effectiveness. The dwelling type was also utilised to estimate the overall suitability of homes for the installation of GSHP and high-temperature GSHP, as detached homes are more likely than other dwelling types to have the available outdoor space for the installation of a horizontal ground loop.

Space constraint: In order to identify dwellings in which space is constrained, total dwelling floor area per habitable room was calculated. Homes with total dwelling floor area per habitable room smaller than 18m² were classified as space constrained. These dwellings were assumed to be unsuitable for the installation of conventional, high-temperature and hybrid heat pumps, due to their additional requirement of a large hot water cylinder for the production of hot water.

District heating: The potential availability of district heating for Scottish homes was assessed on the basis of local heating demand, assuming that district heating networks will be put in place if not already existing in areas where demand for heat is sufficiently high and concentrated. District heating potential was attributed to all homes located in areas in which current annual heat demand density is above a threshold of 40kWh/m²/year. Finally, 80% of homes for which a connection to district heating is possible were assumed to be suitable for district heating, based on expected connection rates and prospective deployment of heat networks in areas with high heat density.

Roof orientation: The suitability of a dwelling for solar thermal was based on the orientation of its roof, and suitability was assigned to homes with roofs facing South, South-West or South-East.

Coastal location: The relative distance from the coast was calculated for each home based on its geographical location. Homes located less than 5 km from the coastline were assigned 'Coastal location' value 'yes'. While the coastal location of a home alone was not considered a sufficient barrier to the suitability of any technology, additional costs were assumed for the use of air-source heat pumps, due to the necessary measures required to prevent accelerated corrosion of the heat exchanger.

Gas Network location: Homes located outside of areas supplied by the gas grid were considered unsuitable for the adoption of hydrogen boilers, biomethane grid injection, and any of their combinations with heat pumps in a hybrid heating system, as these technologies rely on the fuel being delivered by the gas grid. On the other hand, homes located in areas supplied by the gas grid were assumed to be incompatible with bioLPG and bioliquid boilers. In fact, given the similarities in technology and operations of both bioLPG and bioliquid boilers with hydrogen boilers, in a scenario with limited bioenergy resources it is expected that hydrogen boilers would be preferred to bioLPG or bioliquid boilers in homes where both grid and bioenergy options are available. Additionally, the adoption of bioLPG and bioliquid boilers is associated with a higher level of disruption, due to the necessity to store the fuel onsite.

Considerations around heritage homes

Listed buildings and homes in conservation areas are respectively buildings and areas categorised to be of architectural or historic interest. Planning permission may be required to make changes to the external appearance of these homes. Additionally, listed building consent may be required to make changes to both external appearance and internal fixtures of listed homes. Additionally, old dwellings may often present similar issues to those of heritage buildings, according to our consultation with Historic Environment Scotland.

The impact of the peculiar characteristics and restrictions of heritage and old homes on the suitability and costs for the implementation of low-carbon heating was not assessed, due to the complexity and case-by-case nature of the barriers to retrofit.

Listed homes account for 3% of Scotland's housing stock and homes in conservation areas but not listed account for an additional 7% of the stock. Older homes that are neither listed nor located in conservation areas amount to 12% of the total housing stock. Given the large portion of homes that may be affected by additional requirements and restriction in the implementation of low-carbon heating and given the uncertainty about the impact of these restrictions on suitability and costs, it is advisable that these parameters are further investigated.

Suitability of Scotland's housing stock for low-carbon heating technologies

The expected suitability of each home for the considered low-carbon heating technologies in 2040 is reported in the figure below, for various combinations of peak specific heat demand and fuse limit.

Figure description:

A horizontal bar chart shows the percentage of homes compatible with each technology under examination in 2040. The sensitivity of suitability was tested against three combinations of peak specific heat demand (measured in W/m2) and fuse rating (measured in A); the first combination is suitable (100W/m2, 60A), the second is some risk unsuitable (120W/m2, 80A) and the third is high risk unsuitable (150W/m2, 100A). The investigated technologies are air source heat pumps, ground source heat pumps, high temperature air and ground source heat pumps, communal air source heat pumps, electric storage heating, direct electric heating, electric boiler, biomass and biofuel boilers, hydrogen boilers (both electrolysis and reforming), biomethane grid injection, hybrid heat pumps (with gas, gas and hot water cylinder, hydrogen, hydrogen and hot water cylinder, resistive heating, resistive and hot water cylinder) district heating, and heating technologies in combination with solar thermal (air source heat pump and solar, electric storage and solar, direct electric and solar and electric boiler and solar).

ASHP and high-temperature ASHP: While thermal comfort is at risk of not being met without appropriate energy efficiency fabric measures being carried out in some of the suitable homes with the installation of an ASHP in 2017, the installation of a high-temperature ASHP would ensure thermal comfort in homes where energy fabric measures or an upgrade to low-temperature wet system may not be feasible. While overall suitability of both technologies does not vary significantly in 2040 (the year by which it is assumed by this work that energy efficiency fabric measures will have been carried out), the advantage of high-temperature ASHP over conventional ASHP is lost, as the reduced space heating demand allows for both technologies to meet thermal comfort equally.

GSHP and high-temperature GSHP: The suitability of GSHP and high-temperature GSHP is much lower than that of ASHP due to the additional requirements for the installation of a ground loop in a sufficiently large, accessible and geologically suitable plot.

Communal ASHP: The suitability for communal ASHP is lower than that of individual ASHP due to additional constraint posed on the suitable dwelling type.

Electric resistive heating: While very few homes are limited by fuse rating in the implementation of electric storage heating, a large portion is restricted for the installation of direct electric heating by the same constraint. This is due to the larger number of domestic appliances that are consuming electricity while direct electric heating is operating, thus reducing the amount of power that the heating devices can draw before exceeding the fuse limit. This also results in a larger share of suitable homes in 2040, due to a reduced space heating demand. Similar considerations are also valid for electric boilers, which are operating during daytime and at an even lower efficiency than direct electric heating.

Bioenergy boilers: Solid biomass boilers, bioLPG boilers and bioliquid boilers are not suitable in all homes, due to the requirement of space for the volume of their storage/equipment. In addition, bioLPG boilers and bioliquid boilers are overall less suitable, as they were assumed to be only considered for homes that are located off the gas grid.

Low-carbon gas boilers: Hydrogen boilers and biomethane grid injection are assumed to be technically suitable in all buildings that are located on the gas grid.

Hybrid heat pumps: The suitability for hybrid ASHP technologies is based on the combined suitability of its two main components: the ASHP and the additional heating source. The suitability of the ASHP component is evaluated applying the same constraints as for conventional ASHP but considering a lower peak heat demand, compatibly with the reduced load delivered by the ASHP component in a hybrid system.

Solar thermal: The suitability for systems including solar thermal collectors is based on the suitability of the main heating system component, with the additional requirement of the availability and appropriate orientation of a roof on which to secure the collectors.

Dwellings with limited suitability

A significant share of Scottish homes is likely to be suitable only for a limited range of heating technologies. A restriction in the choice of heating system may carry some risks for the decarbonisation of domestic heating. Most relevant risks are associated with following low-carbon heating technologies:

• Electric resistive heating: the reliance on electric resistive heating technologies for the decarbonisation of domestic heating might result in high costs. Included technologies are electric storage heating, direct electric heating, electric boilers and their combinations with solar thermal.
• Bioenergy: the implementation of low-carbon heating based on bioenergy relies on the future access to sufficient biomass feedstock and might therefore be subject to fuel availability uncertainty. Considered technologies are solid biomass boilers, bioLPG boilers, bioliquid boilers and hybrid heat pumps with bioliquid boilers as secondary source.
• Decarbonised gas grid: the availability of technologies that are supported by a decarbonised gas grid is tied to the future delivery of low carbon gas through the grid and might therefore be subject to implementation uncertainty. In fact, a strategic decision on the decarbonisation of the gas network though the supply of hydrogen will likely be based on the outcome of further studies on the technical suitability and safety of the use of hydrogen. Affected technologies are hydrogen boilers and hybrid heat pumps with hydrogen boilers as secondary source. Biomethane grid injection is also included in this category, but it is also affected by the fuel availability uncertainty of the bioenergy category.
• District heating: similarly, the availability of district heating will predominantly depend on the scale of the development of heat networks and could therefore be subject to implementation uncertainty.

The portion of the stock resulting unsuitable for heat pump technologies in 2040 is reported in the table below, broken down into 16 categories with various combinations of technology choice restriction. The reported figures on the right refer to the portion of homes that can choose from all groups indicated in the respective row on the left at the same time.

Number of homes with restricted choice of suitable technologies, considering fuse limit of 80A and peak specific heating demand of 120 W/m ² in 2040

	Suitable technologies for homes with restricted choice				Homes	% of stock
1	District heating	Decarb. gas	Electric resistive	Bioenergy	20,000	0.8%
2	District heating	Decarb. gas	Electric resistive	-	112,000	4.5%
3	District heating	Decarb. gas	-	Bioenergy	1	0%
4	District heating	Decarb. gas	-	-	40	0%
5	District heating	-	Electric resistive	Bioenergy	22,000	0.9%
6	District heating	-	Electric resistive	-	0	0%
7	District heating	-	-	Bioenergy	70	0%
8	District heating	-	-	-	0	0%
9	-	Decarb. gas	Electric resistive	Bioenergy	136,000	5.5%
10	-	Decarb. gas	Electric resistive	-	90,000	3.6%
11	-	Decarb. gas	-	Bioenergy	60	0%
12	-	Decarb. gas	-	-	60	0%
13	-	-	Electric resistive	Bioenergy	78,000	3.2%
14	-	-	Electric resistive	-	0	0%
15	-	-	-	Bioenergy	1,200	0.05%
16	-	-	-	-	0	0%
Total					459,000	18.7%

Four combinations in particular are identified as most at risk, due to their restricted selection of suitable technologies and the relatively large number of affected homes:

• Row 9: 136,000 homes that must choose among decarbonised gas, electric resistive and bioenergy technology groups only. These homes are potentially at risk of incurring in high costs (electricity) or the uncertain availability of the selected technology (decarbonised gas and bioenergy).
• Row 10: 90,000 homes that must choose between decarbonised gas and electric resistive technology groups only. These homes are potentially at risk of incurring in high running costs (electricity) or the uncertain availability of the selected technology (decarbonised gas).
• Row 13: 78,000 homes that must choose between electric resistive gas and bioenergy technology groups only. These homes are potentially at risk of incurring in high running costs (electricity) or the uncertain availability of the selected technology (bioenergy).
• Row 15: 1,200 homes that can only choose bioenergy technology group only. These homes are potentially at risk of uncertain availability of the selected technology.

Conclusions

The key findings of this study provide interesting learnings on the composition of Scotland's housing stock and on its potential interaction with a wide range of low-carbon heating technologies that will be essential for the decarbonisation of domestic heating in Scotland.

• The characteristics of the insulation of the existing stock will change over time in order to comply with the requirements set by the Energy Efficient Scotland Route Map. The most effective fabric energy performance upgrade measures investigated in this study are wall and roof insulation. These result in a reduction of the space heating demand of most dwellings enough for it to have an impact on the suitability of some of the considered heating technologies.
• The feasibility of the implementation of low-carbon heating in heritage homes will need to be assessed individually. Listed buildings and homes in conservation areas require planning consent to make changes to the external appearance or to the internal fixtures. Additionally, old dwellings built before 1919 were reported by Historic Environment Scotland to often present similar issues to those of heritage buildings. Heritage and old homes account for ~22% of Scotland's housing stock.
• The fuse limit constraint affects the implementation of electric resistive heating more than heat pump technologies. The number of homes that might be constrained by fuse limit in the installation of ASHP is significantly lower than for electric storage heating and direct electric heating, due to the higher efficiency of heat pumps. Furthermore, the number of homes affected by the fuse limit constraint decreases substantially between 2017 and 2040, due to the lower space heating demand enabled by the implementation of energy performance upgrade measures.
• The installation of heat pumps is not advisable in homes with peak specific heat demand above 150 W/m². The installation of an ASHP results to be advisable for heat loss rates of up to 100 W/m², while homes with heat loss rates comprised between 100 W/m² and 150 W/m² might incur in risk of thermal comfort not being met by the heat pump system.
• In 2040 ASHP and high-temperature ASHP are expected to be equally suitable in Scottish homes. The overall suitability of the stock for ASHP and high-temperature ASHP is similar, as it is almost exclusively determined by the space constraint. However, in 2017 a portion of homes that are suitable for ASHP are still at risk of not meeting thermal comfort. The advantage of high-temperature ASHP over conventional ASHP is lost in 2040, as the reduced space heating demand allows for both technologies to meet thermal comfort equally.
• Electric storage heating is expected to have a larger suitability than direct electric heating. While very few homes are limited by fuse rating in the implementation of electric storage heating, a large portion is restricted for the installation of direct electric heating by the same constraint. This is due to the larger number of domestic appliances that are consuming electricity while direct electric heating is operating.
• Homes with limited choice of suitable low-carbon heating options may be more subject to implementation risk. Four main groups of dwellings with limited suitability were identified, including homes that are suitable for (a) decarbonised gas, bioenergy and electricity only, (b) decarbonised gas and electric resistive heating only, (c) electric resistive heating and bioenergy only, (d) bioenergy only. These homes may be at risk of incurring in high costs (electricity) or not being able to rely on the availability of the technology (decarbonised gas grid and bioenergy). The number of homes with restricted suitability could amount to up to ~20% of the housing stock.
• While there are no concerns around sufficient availability of bioenergy to cover heating demand, bioenergy resources may be directed to use in other sectors. In fact, the Net Zero report by the CCC advises against the use of the available biomass for domestic heating and recommends its use in other sectors, in combination with CCS.