5.1 Main results and discussion
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. Main results are the following:
- 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, with the target for all owner-occupied homes to reach EPC band C by 2040. Most effective fabric energy performance upgrade measures investigated in this study are wall and roof insulation. These result to reduce 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. While the installation of low-carbon heating technologies in heritage homes provides relatively similar obstacles as in non-heritage homes, the implementation of energy performance upgrade measures might be more difficult and will require to be assessed on a case-by-case basis. In fact, 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, with ~3% of listed homes, ~7% homes in conservation areas but not listed and ~12% of homes built before 1919 but not listed or in conservation areas.
- The fuse limit constraint affects the implementation of electric resistive heating more than heat pump technologies. While there is uncertainty about the fuse limit of individual Scottish homes, it is assumed that most likely values are 60, 80 and 100 A. The number of homes that might be constrained by fuse limit in the installation of ASHP is significantly lower than for the installation of 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. Peak specific heat demand could constitute a barrier to the implementation of heat pumps, due to the low flow temperature at which space heating is delivered. 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. Finally, the installation of heat pumps in homes with specific heat loss above 150 W/m2 is considered not suitable.
- 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 and does not change over time with the reduction of space heating demand, 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. On the other hand, the installation of a high-temperature heat pump in those homes would guarantee thermal comfort. Finally, 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, thus reducing the amount of power that the heating devices can draw before exceeding the fuse limit.
- 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. Depending on the chosen technology, these homes are 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). Depending on the chosen specific heat demand and fuse rating, 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, the resource may be directed to use in other sectors. The bioenergy resource required if all dwellings in restricted categories were supplied by bioenergy-based heating technologies is many times smaller than the current bioenergy availability in the UK. However, the Net Zero report by the CCC advises against the use of the available biomass for domestic heating: "The level to which sustainable low-carbon biomass production can be increased is finite, given land constraints and competition from other uses (e.g. food production). It is therefore important to pursue ways of using this finite resource that maximise its contribution to emissions reduction. This means combining bioenergy with CCS."
5.2 Recommendations for further work
Given the complex nature of the application of retrofit measures in heritage and old homes, it is advised that this area is investigated in more detail. In particular, the suitability of energy efficiency measures in homes should be assessed separately for each of the different materials available. Such study would greatly benefit from the engagement of local stakeholders involved in retrofit projects (e.g. Scottish Land and Estates), who can provide real data and information on the types of materials applied, their suitability and costs currently in use.
Additionally, more detailed information could be gathered on the values of fuse rating of Scottish homes, in order to assess suitability for the correct values of each home, rather than through a sensitivity analysis.