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Rapid scoping review of reviews

What is the nature and the extent of the review evidence for decarbonising home heating?

We searched for systematic reviews relevant to decarbonising home heating against the areas identified in the system map. Areas of the systems map where there are systematic review indicate where the research effort has been concentrated, or where the field of study is more mature and can give a more reliable overview of evidence than for than single studies. 

Overall the quality of the reviews and reliability of the findings was medium to low, there were very few reviews that were systematic, where a review question was defined a priori, and methods were explicity described and therefore could be replicated; the risk of selection bias  of included studies were high as methods for selection were often not described, and so may not be a complete picture or generalisable to the UK context.

Few reviews evaluated the quality of the included studies and findings from these were given equal weight. Many of the reviews were reviews of modelling or simulation studies and again, may not be conceptually comparable to synthesise to give an overview of effect or replicable in a real world setting.  There was a greater concentration of research effort in the area of Air Source heat pumps technical performance either as a stand alone, or enhanced with other technologies. There were fewer reviews in the individual domain of behaviour, attitudes and beliefs.

  • Ahmad (2023)
  • Type of review
  • Systematic map / Scoping review
  • Number of studies
  • 15
  • Outcomes measured
  • Take up
  • Socio-ecological framework
  • Individual motivations for purchase
  • Context-setting
  • UK
  • Quality of evidence
  • Medium Low
    Unclear methods, lacks QA or methods of synthesis
  • Findings
  • ● Positive
    Saving money, increasing household energy independence, and reducing greenhouse gas emissions.
  • ● Negative (adverse) effect
    The current imbalance in energy taxes and levies (environmental levies for electricity and gas bills are 23% and 2% respectively) weakens the economic competitiveness of heat pumps in the UK.
  • ● Mixed
    Private dwellings’ larger average size and relatively newly built units with underfloor heating are positively associated with system efficiency, wheras social housing’s mainly retrofit projects with conventional radiators are negatively associated with efficiency.
  • Factors impacting on outcomes
  • ● High costs
    Higher capital and running costs
    Operational issues addressing stakeholders’ concerns such as manufacturers, traders and installers in such a way that end-users positively experience the journey of HPs (pre and post-installations).
  • ● Consumer confidence
    Appropriate knowledge and awareness – choices, technical and financial knowledge – help in the adoption of heat pumps which can be achieved through adopting a bottom-up approach like community-based energy plans.
  • ● Knowledge and awareness
    Appropriate knowledge and awareness – choices, technical and financial knowledge – help in the adoption of heat pumps which can be achieved through adopting a bottom-up approach like community-based energy plans.
  • Recommendations and gaps in knowledge
  • ● Costs of installation
    Addressing stakeholders’ concerns such as manufacturers, traders, and installers in such a way that end-users positively experience the journey of HP’s (pre and post-installations).
    Evidence base.
  • ● Information
    Increase demonstrator projects (provides real-world data on all forms of benefits, including energy saving).
    Issues related to skills development targeting the right segments of households who are potential innovators and early adopters.
    Effectiveness of incentives.
  • ● Incentives
    Enhance financial incentives and make an economic case for HPs on the demandside among consumers by reducing the imbalance in levies on electricity and gas bills.
  • Arif (2014)
  • Type of review
  • State of the art review
  • Number of studies
  • 33
  • Outcomes measured
  • ● Economic
    The biggest disadvantage and the main challenge of this technology is the fouling problem related to the impurity of the waste water (WW). If treated WW is used as the heat source, the WWSHP needs to be located near the WWTP. Therefore the pipeline for heating/cooling may become longer, which results in higher costs.
  • ● Performance
    The temperature of the WW is constant all over the year and its temperature is almost 20°C higher then the ambient air temperature.
  • Socio-ecological framework
  • Socioeconomic technological maturity
  • Population-user
  • Heat network developers
  • Findings
  • ● Positive
    Due to its main characteristics, availability, high amount, small variation of temperature and being warm in winter and cold in summer, WW is a low-grade heat source which can be utilized in HP systems.
  • Factors impacting on outcomes
  • Operational issues
  • Recommendations and gaps in knowledge
  • Issues related to skills development
    Fouling problem related to the impurity of the waste water.
  • BEIS
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • Not reported
  • Outcomes measured
  • ● Economic
  • Studies into heat decarbonisation generally assume, explicitly or implicitly, that new frameworks of law and governance will be required to overcome barriers to change and to manage effectively the risks and complexities involved.  Economic costs and benefits Strategic inferences: economic costs and benefits.
    All approaches to decarbonising heat will require very substantial new capital investment in energy production and infrastructure. At the same time, this will provide opportunities to grow the low carbon heating sector
  • ● Social
  • Numerous studies and commentaries have argued local government and other local organisations can provide important leadership in the effort to decarbonise heat.
  • ● Attitudes and beliefs
  • Key barriers to more rapid consumer take up of low carbon heating, including: low levels of awareness and familiarity of low carbon alternatives; the hassle and disruption involved in switching; and the relatively high costs compared to natural gas heating.
    Any successful framework for the long- term decarbonisation of heat will need to  address each of these factors.” “Low public awareness of the case for change and the options available remains a key strategic challenge.
    Current Government support for low carbon heating is targeted at a small subset of consumers – mainly those in buildings off the gas grid – and many consumers do not think about their heating until they need to replace it.
    92% of the public are unfamiliar with renewable heating technologies, having never heard of them, knowing little about them, or not understanding the technologies involved.
    Current low desirability and low uptake of low carbon heating systems may be due to poor awareness, not an inherent bias against the possibility of change.
  • ● Behaviour
  • The transition to low carbon heating will result in significant change for consumers, but the nature and scale can vary significantly depending on the low carbon technologies used and how the change is managed.
  • ● Performance
  • Energy system impacts
    Given the size of heat as a proportion of UK energy demand, and the far-reaching role of the current UK gas grid, the decarbonisation of heat is likely to have broad impacts across the whole energy system. These need to be understood in order to ensure the necessary changes are practically deliverable and acceptable and that key parameters of security and resilience are maintained throughout and beyond transition.
  • Socio-ecological framework
  • Socioeconomic
    Looking at policy actions (but could involve other levels)
  • Population-user
  • Heat network developers
  • Findings
  • Not reported
    not addressing a defined RQ
  • BEIS (2023)
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • 73 documents: 63 white (peer reviewed academic) and 13 grey (industry) documents
  • Outcomes measured
  • Social
    Attitudes and beliefs
    Performance
  • Socio-ecological framework
  • Community
  • Context-setting
  • Existing heating and cooling installer workforce in England and the extent to which they wish to, and can, be upskilled to install heat pumps.
  • Quality of evidence
  • Medium Low
     This is a non-systematic literature review. It is unknown how studies were selected, and the criteria to include/exclude studies, particularly those that are not related to heat networks. Regarding “white” literature, no consideration is given to type of study.
  • Factors impacting on outcomes
  • Performance
    Policy stability
    Social factors such as gender, diversity, etc.
    Skills
    Scale up learning and best practice
    Knowledge and awareness
    Stability of employment
    Standardisation and consistency in training
  • Recommendations and gaps in knowledge
  • Evidence base
    Issues related to skills development
    Need for more comprehensive metrics
  • BEIS (2020)
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • 76
  • Outcomes measured
  • ● Economic
    The literature describes the policy and support interventions that have been implemented by the UK government and devolved administrations to support creating a sustainable heat network market and industry
  • Socio-ecological framework
  • Socioeconomic
    Heat network sector and its supply chain
  • Quality of evidence
  • Low
    no methods (not replicable, selective) no QA of included studies or processes.
  • Factors impacting on outcomes
  • ● High costs
  • “the low uptake of the heat pumps is linked to low awareness, financing constraints, concerns around disruption and difficulty in finding trusted installers with the right skills.”
  • Policy stability
  • the need for stable and long-term energy policy, which many reports referenced as having been lacking. This is required to create certainty for investors to expand the heat network supply chain and to help facilitate the development and provision of ‘green skills’ “chopping and changing of UK Government policy has inhibited skills development in housing design, construction and in the installation of new measures”.
  • ● Skills
  • “the low uptake of the heat pumps is linked to … difficulty in finding trusted installers with the right skills.”
  • ● Scale up learning and best practice
  • Research from the Energy Technologies Institute (ETI)25 notes that “the skills base in the UK needs to be coordinated and supported to increase capacity rapidly, ensuring that best practice is both achieved and incrementally improved through innovation and learning”
  • ● Standardisation and consistency in training
  • (there is a need for) professional standards and skills across the building, heat and ventilation supply trades to be reviewed, with a nationwide training programme to upskill the existing workforce and expand the supply chain. The Competition and Markets Authority (CMA) calls for better protection of heat network customers through the enforcement of technical standards and an accreditation scheme.
    • The implementation of minimum technical standards as a necessary step to protect customers from poorly designed, built and operated heat networks. (These technical standards would need to be codified clearly and made mandatory for the construction and operation of all new heat networks).
    • An accreditation scheme to develop the skills needed to build and operate new heat networks, ensuring networks deliver intended benefits and an efficient and good service for customers. The CMA suggests that over time, existing networks should also be required to move towards compliance with such standards.
    • The sector continuing activity around a voluntary quality assurance scheme to ensure that heat networks are built to sufficiently high standards and to improve the quality of service received by customers.
  • Recommendations and gaps in knowledge
  • Evidence base
    Heat Pump Manufacturing Supply Chain Research Project. The aim of the project will be to
    • Investigate the manufacturing supply and value chains for different sectors of the heat pump and related markets.
    • Identify where in the heating supply chain the UK has particular expertise that could make it internationally competitive.
    • Understand how the UK can become an attractive proposition for heat pump manufacture.
    • Gain a realistic understanding of the growth in the heat pump supply chain.
    • Determine which policy levers could maximise growth of the UK’s heat pump supply chain.
    • Provide recommendations for government intervention.
    The research, which started in early 2020, is expected to be completed in summer 2020. The project team were unable to engage with this research project
  • Carroll (2020)
  • Type of review
  • Comprehensive review
  • Number of studies
  • 34
  • Outcomes measured
  • Behaviour
    Performance
    Carbon Emissions
  • Socio-ecological framework
  • Households
  • Findings
  • ● Positive
    The best performance is achieved using the ASHP for direct space heating. A thermal storage, and combined heat and storage mode were less efficient. High system efficiency was recorded for users who left heating on all night and when they were not in the house. System efficiency was higher in privately owned homes, than in social housing.
    ● Negative (adverse) effect
    Financial incentives can have undesirable behavioural responses giving the example of older refrigerators being retained as ‘‘beer fridges’’ in the US rather being disposed of. Homes that did not receive incentives for replacing their ASHP reduced electricity usage by nearly 17% while those that received a rebate had lower reductions 0%–15%. The larger the rebate, the less of an effect it had on electricity consumption. The noise associated with heat pumps with users saying that intrusive noise was a problem
    ● Mixed
    ASHPs exhibit more variable performance compared to other types of heat pumps due to frosting during winter months. The defrosting processes need to be carefully managed, particularly in cold climates as noted in [17] and [19]. potential ‘‘comfort taking’’ behaviour where users may use more energy to maintain the whole house at an ideal comfort level, or use more energy because it is perceived as being cheaper or free. In all configurations, the HT was more expensive than any gas boiler system (based on UK prices), but did produce less carbon emissions
  • Factors impacting on outcomes
  • Performance
    Thermal efficiency of buildings
    The economic analysis highlighted that ASHP with or without the improved insulation are the least expensive option of the retrofit measures.
    Availability of variable electricity tariffs
    ASHPs are shown to be economically attractive in Canadian cold climates given Canadian time-of-use electricity prices [44].
  • Recommendations and gaps in knowledge
  • ● Access to data
    Little ASHP field trial data is publicly available, that data is very useful but further studies across a broader range of ASHP systems, building types, and geographic locations is warranted
    ● Future needs in a context of climate change
    Further analysis of future climate scenarios, and consumer energy usage post retrofit must be modelled and understood if the full intended benefits are to be harnessed.
    ● The role of end users
    The role of users in both adopting, and efficiently operating ASHP technology is highlighted in the SLR papers. While cost benefit analysis shows that policies can encourage uptake, careful management will be required to ensure neither fatigue nor comfort taking behaviours negate the beneficial effects
  • Conrado (2022)
  • Type of review
  • Systematic review
  • Number of studies
  • 59 studies included: 25 are experimental, 16 numerical simulations, 1 analytical, while the remaining 17 employ a combination of these approaches.
  • Outcomes measured
  • Economic
    Performance
  • Socio-ecological framework
  • Households
  • Context-setting
  • The use of thermal energy storage systems to help air-source heat pumps perform in cold climates
  • Quality of evidence
  • Medium Low
    The study covers all basic elements of a SR, however, there is no description of studies’ design nor consideration of their quality. There is no mention of validity measures such as double vs individual screening and data extraction.
  • Findings
  • Mixed
    Thermal storage increases the ASHP coefficient of performance by 27% on average, albeit with higher initial cost compared to conventional fossil-fueled heating systems
  • Factors impacting on outcomes
  • High costs
    Operational issues
  • Recommendations and gaps in knowledge
  • • Evidence base
    • Need for more comprehensive metrics
    Present-day applications may require more comprehensive metrics to determine the system performance
    • Effectiveness of incentives
    Government incentives allied with public policies may foster the replacement of traditional fossil-fueled combustion appliances
    • Methodological issues
    There is a lack of methods to estimate the storage efficiency in the community of researchers, companies, and final users and a need for setting common criteria in the evaluation of thermal storage systems.
  • DECC
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Economic
    Carbon Emissions
  • Socio-ecological framework
  • Community
    Heat pumps for district heating
  • Context-setting
  • • Helsinki, Finland: large city centre scheme with central water source (seawater, sewage) heat pumps retrofitted in existing heating and cooling networks
    • Wandsworth, UK: residential development with central heat pumps in heating and cooling networks using an ATES system, with separate DHW provision.
    • Derbyshire, UK: residential development with central ground source heat pumps in a heat network providing space heating and DHW, in an off-gas area.
    • Duindorp, The Netherlands: large city centre scheme with a central water source (seawater) heat pump, a low temperature network and building-integrated heat pumps.
  • Quality of evidence
  • Medium High
    Relatively old review, however as costs and status has remained stable this does not reduce the relevance of the findings. potential selection bias due to small number of case studies and purposive selection. Two UK cases included
  • Findings
  • • Positive
    Alongside a decarbonising grid, integrating heat pumps into district heating offers large CO2 emissions reduction potential. As may be expected from simple thermodynamic arguments, we showed that the CO2 savings are greater where the following scheme characteristics are combined:
      • Heat pumps provide a larger fraction of the heating
      • Heat pumps operate with a lower source-sink temperature difference, leading to increased efficiency
      • Network thermal losses are lower, typical for lower temperature networks
    • Negative (adverse) effect
    The premium for the price of heat for district heating schemes incorporating heat pumps is in the range 35-74%. The main reasons for this include:
      • High capital cost of heat pumps (particularly MW-scale heat pumps)
      • High electricity price compared to gas price, projected to continue over the next few decades
      • Lost revenue from electricity sales when compared with schemes involving gas-CHP
      • Higher capacity of heating plant required (versus gas-based district heating) where building-integrated heat pumps serve the peak demand in individual dwellings
      • Higher network costs (versus gas-based district heating) where low temperature networks require larger diameter pipes (assuming conventional pipe materials are used)
  • Factors impacting on outcomes
  • • High costs
    The premium for the price of heat for district heating schemes incorporating heat pumps is in the range 35-74%. The main reasons for this include:
      • High capital cost of heat pumps (particularly MW-scale heat pumps)
      • High electricity price compared to gas price, projected to continue over the next few decades
      • Lost revenue from electricity sales when compared with schemes involving gas-CHP
      • Higher capacity of heating plant required (versus gas-based district heating) where building-integrated heat pumps serve the peak demand in individual dwellings
      • Higher network costs (versus gas-based district heating) where low temperature networks require larger diameter pipes (assuming conventional pipe materials are used). The cost of large heat pumps carries significant uncertainty, and has a large impact on the price of heat Due to the low number of operational schemes, there is significant uncertainty around the cost of large, bespoke (MW-scale) heat pump systems
    • Performance
    The ability to provide cooling as well as heating has been found to be a key driver of the use of heat pumps in heat networks.
    • Thermal efficiency of buildings
    In schemes in which there is scope to lower the network temperature (i.e. when supplying thermally efficient buildings), doing this is beneficial, with the majority of the benefit being attributed to enhanced heat pump performance as opposed to reduced network losses.
    • Energy/ heat storage
    Using a separate heat pump to meet the heating and hot water demand in each dwelling – in the case of a residential development – is likely to entail significant oversizing outside times of peak demand unless substantial thermal storage is also installed.
    • Site specific issues
    The optimum heat pump in district heating scheme design is strongly dependent on site specific issues
  • Fischer (2017)
  • Type of review
  • State of the Art review
  • Number of studies
  • 240 studies from which “the most interesting and relevant ones (in the author’s opinion) were selected”
  • Socio-ecological framework
  • Community
    Heat pumps in smart grids
  • Context-setting
  • Smart grids
  • Quality of evidence
  • Low
    High likelihood of selection bias, studies selected if they were of interest to the author
  • Findings
  • Positive
    Predictive controls are successfully used in the majority of studies, often assuming idealized conditions. In all fields heat pumps – when controlled in an appropriate manner – can help easing the transition to a decentralized energy system accompanied by a higher share of prosumers and renewable energy sources
  • Factors impacting on outcomes
  • Stable and economic grid operation
    Integration of renewable energy
    Availability of variable electricity tariffs
  • Recommendations and gaps in knowledge
  • Evidence base
    A transfer of control approaches from simulation to the field, a detailed techno-economic analysis of heat pump systems under smart grid operation, and the design of heat pump systems in order to increase flexibility
  • Freier (2022)
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Economic
  • Socio-ecological framework
  • Socioeconomic
  • Population-user
  • Heat network developers
  • Quality of evidence
  • Low
    Given that this is a non-systematic literature review, basic elements to evaluate transparency and rigor are lacking.
  • Context-setting
  • German households
  • Population-user
  • Heat network developers
  • Factors impacting on outcomes
  • Consumer confidence
    Availability of variable electricity tariffs
    Knowledge and awareness
    Lack of economic incentives
  • Recommendations and gaps in knowledge
  • Effectiveness of incentives
  • Hanna (2016)
  • Type of review
  • Rapid evidence review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Uptake
  • Socio-ecological framework
  • Socioeconomic
    Information, regulation and standards Political support
  • Context-setting
  • European countries
  • Population-user
  • Investors
    Heat network developers
  • Findings
  • Positive
    Incentives, taxation and subsidies have proved successful in different markets. Subsidies for replacing oil and electric heating can also be effective in stimulating demand both for heat pumps and heat networks. Investment grants appear to be particularly important for heat networks where energy markets have been liberalised (and where district heating markets are less developed).
  • Factors impacting on outcomes
  • • Operational issues
    • Policy stability
    Promotes industry, consumer and, in the case of district heating, local authority confidence
    • Consumer confidence
    Enhancing the reputation of the industry through standards and regulations has helped tackle low consumer awareness and confidence in countries with high uptake of low-carbon heat
    • Investor confidence
    Planning and regulatory frameworks are helpful for giving heat network developers confidence that they will secure a high enough percentage of the local heat market to justify the initial capital expenditure in liberalised energy markets. low industry confidence in future support schemes meant investment in skills or supply chains was not triggered
    • Availability of gas and cost of supply
    In the UK, high consumer satisfaction with gas central heating systems means that in recent surveys many consumers say they would be unwilling to consider alternatives. Across Europe, heat pumps have been widely deployed where natural gas networks are less extensive, as gas heating is typically cheaper than alternatives. Off-gas consumers in the UK may be more willing to consider alternative heating technologies.
  • Hanna (2021)
  • Type of review
  • Systematic review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Economic
    Job creation
  • Socio-ecological framework
  • Community
    Low carbon job creation
  • Quality of evidence
  • Medium HIgh
  • Findings
  • • Small effect
    We find limited evidence identifying where the benefits of potential net job creation in renewables and energy efficiency may accrue, i.e. internationally, nationally, and regionally. Demand for renewable energy construction/ installation and operation/ servicing jobs is generally created locally or within countries.
    • Positive
    Domestic construction projects such as insulation retrofits or building wind turbines could be particularly favourable and less prone to offshoring services overseas. d replacing fossil fuel boilers with heat pumps can help to alleviate fuel poverty, meet the UK’s longer-term net zero climate target, and support a just transition: creating jobs in a distributed way around the country, including ‘levelling up’ in regions most affected by unemployment and lack of investment (EEIG, 2020; Jung and Murphy, 2020; Webb, Emden and Murphy, 2020) Building interventions such as heat pumps and energy efficiency are likely to create labour activity which is more evenly distributed around the country. This suggests that supporting a comprehensive national building energy efficiency and heat decarbonisation retrofitting programme could be beneficial in terms of creating employment in a distributed way across regions while contributing to reaching net zero.
  • Factors impacting on outcomes
  • • Policy stability
    There is also a need for a new nationwide programme of energy efficiency and heat decarbonisation retrofitting in UK buildings which could help to stimulate ongoing, countrywide demand f
    • Skills
     A key uncertainty (in the evidence) is the extent to which labour and supply chain services may be imported from other countries.
  • Hepstonstall 2023
  • Type of review
  • Systematic review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Economic
    Single year costs, experience rate data, % cost reduction, The total installed costs of domestic heat pumps varies widely, spanning different designs and contexts, from over €30,000 to just over €5000 – a six-fold range. Higher values typically refer to larger GSHP installations, perhaps involving significant building fabric and heating system upgrades. Lower values typically refer to ASHPs for new build housing, or retrofit installs which avoid building fabric or heating system upgrades. Median costs in the year ranges 2015-19 and 2020-29 reduce by 22% in the RoW dataset, compared to only 6% in UK-only.
    Uptake
  • Socio-ecological framework
  • Households
  • Quality of evidence
  • High
  • Findings
  • Negative (adverse) effect
    In the UK, there has been little or no reduction in the average total installed cost of heat pumps over the past decade. Over the same period, some cost reductions have been achieved internationally, and over time, particular countries have successfully aligned market growth with reduced installed costs. almost all cost reduction forecasts are significantly less than UK policy targets, and in the short term, given rising international demand and supply side challenges, heat pump installed costs face inflationary pressures. Achieving cost parity in ASHP installed costs would therefore require a 70% reduction. For this to be achieved by 2030, over approximately four doublings in cumulative capacity, implies an experience rate of around 26%
  • Factors impacting on outcomes
  • High costs
    Heat pumps also offer energy security and efficiency benefits, and jobs and local economy opportunities. Over time, they can also offer households lower and more stable heating costs. This suggests a wider framing of the heat pump proposition in the UK, going beyond possibly unrealistic goals for installed cost reduction.
  • Recommendations and gaps in knowledge
  • • Costs of installation
    Heat pump cost data is relatively scarce and is often inconsistent across different studies. Given the importance of heat pump costs for energy transitions, there is a vital need for improved data, and greater data access, in the UK and internationally.
    • Access to data
  • Mata (2020)
  • Type of review
  • Systematic review
  • Number of studies
  • 27
  • Outcomes measured
  • Behaviour
    Performance
    Carbon Emissions
  • Socio-ecological framework
  • Households
  • Quality of evidence
  • Medium HIgh
    All the basics of a systematic review have been followed including inclusion and exclusion criteria. No consideration of quality of the studies
  • Findings
  • • Positive
    The results show that between 2% and 18% of residential sector electricity in thefour countries could be shifted, resulting in total emission reductions of 10MtCO2 from peak shaving, or 24MtCO2per year if optimizing the deployment of renewables
    • Negative (adverse) effect
    Recognized risks include higher peaks and congestions in low price-hours and difficulties in designing electricity tariffs because of conflicts with CO2 intensity as well as potential instability in the entire electricity system caused by tariffs coupling towholesale electricity pricing
  • Factors impacting on outcomes
  • High costs
    Operational issues
    Performance
    Integration of renewable energy
  • Recommendations and gaps in knowledge
  • Evidence base
    Less on energy saving potentials. A lack of standardized calculation approaches, linkages between modelling exercises with different goals (e.g. simultaneously addressing the economic and environmental value), and upscale estimates that synthesize the learnings from individual case studies into generally applicable knowledge.
  • Meek (2021)
  • Type of review
  • Literature (non systematic) review
  • Number of studies
  • Not reported
  • Outcomes measured
  • Performance
  • Socio-ecological framework
  • Households
  • Quality of evidence
  • Medium Low
    Non systematic review, no QA methods or RQ. Relevant to UK context
  • Findings
  • • Negative (adverse) effect
    Given the mean ASHP efficiency was found to be less than 2 (for ASHPs), the EST Phase 1 results attracted widespread concern about UK heat pump installation
    GSHPs tend to perform with higher efficiency as do those installed in new buildings.
    (the) concern (is) that UK field trial results generally indicate that UK heat pump installations do not perform as well as those in other European countries
    • Mixed
    Two other detailed reports refer to field trials designed to assess the performance of hybrid heat pumps: where a combination of ASHPs with conventional heating is used to provide space and water heating in domestic properties. Neither are directly comparable to the UK field trials noted above or to the analysis carried out for this paper
  • Recommendations and gaps in knowledge
  • Evidence base
    It is surprising that there is not more robust, recent field trial research on the performance of heat pumps in UK dwellings. There is a lack of recent and definitive field trial evidence regarding heat pump performance in the UK and a resulting lack of evidence related to the consumer value case for heat pump installation. As a consequence, a range of relevant models have been developed to explore the potential benefits and challenges of heat pump roll-out. The suppositions that underpin these studies are important because they are often based assumed SPF values.
  • Stabler 2020
  • Type of review
  • Systematic review
  • Number of studies
  • Not reported
  • Socio-ecological framework
  • Socioeconomic
    National energy transitions
  • Quality of evidence
  • Medium Low
    Very little on changing energy use at home specifically, considers energy transition more broadly. Recommendations not linked clearly to outcomes in included studies
  • Factors impacting on outcomes
  • • Operational issues
    Plan for the cross-sectoral, multi-level coordination challenges that arise from the decentralisation of energy governance (e.g. increased need for coordination between municipalities, electricity providers, OFGEM, etc.).
    • Policy stability
    Integrate district heating as a funded, statutory duty into local planning policy (e.g. Heat Network Partnership for Scotland, 2015). This recommendation is informed by best practice and, as well, the observed barrier that time-limited grants and unpredictable changes to UK national policy/funding opportunities pose to LA-led transition
    • Politics and institutional factors
    Reverse LA budgetary cuts to support local experimentation and engagement with energy governance and reduce disruption in institutional memory and political buy-in. For renewable heat technologies that are closely coupled with natural systems (e.g. ground source heat pumps), Government should avoid prohibitive, blanket regulations at the national level (e.g. the number and depth of downhole heat exchangers and the rate of groundwater extraction). Actively manage phase-outs of competing, unsustainable heating systems.
    • Skills
    Upskill LA officers in technical, legal, and commercial expertise rather than relying on industry consultants to improve municipalities’ long-term capacity to steer transition whilst  ressing the national skills gap for conducting technical feasibility studies
    • Scale up learning and best practice
    Government should continue and scale-up grant competitions that support the development of LA energy concepts/masterplans to support experimentation at the local level that accounts for cross-sector interactions.

    Affordable long-term finance or financial guarantees for non-profit or joint public-private ventures should be provided to LAs having established a promising energy concept or masterplan. This could come from UK infrastructure funds or GIB finance structured to underwrite risks for local enterprises. Institutionalise bottom-up and horizontal learning in the Heat Network Delivery Unit (HNDU), to ensure this ‘experimentation’ phase helps develop case studies and Government learning regarding LA capabilities, opp
  • Recommendations and gaps in knowledge
  • Evidence base
    It is highly recommended that Government adopts a systems approach to the analysis of development pathways and the analysis of policy interventions, meaning linear econometric methods are employed within a systems approach. When building the evidence base, Government should give greater weight to social and ecological aspects of the heat transition. Physical interactions with ecological processes should be explicitly modelled and SSH findings should go beyond market research on technology adoption, to include research areas such as multilevel governance and energy justice, and the role of power in mediating transition processes.
  • Wang (2020)
  • Type of review
  • Systematic review
  • Number of studies
  • Not reported
  • Outcomes measured
  • • Economic
    Energy payback time Energy return on energy invested
    • Performance
    • Carbon Emissions
    “equivalent CO2” (CO2e) emitted per unit input energy, which is determined based on the fuel type and efficiency of the energy conversion devices.
  • Socio-ecological framework
  • Households
    Solar assisted air pumps
  • Context-setting
  • Lab and field trials
  • Quality of evidence
  • Low
    Minimal methods reported. Unclear as to what kinds of studies were included or the inclusion criteria, or QA.
  • Findings
  • Mixed
    Among the three systems, the (solar thermal) ST-ASHP system is mostly studied, which has relatively low COP at mean value of 2.90, but requires much smaller investment with 3.5e10 years as payback time. Either (photovoltaic) PV-ASHP or (photovoltaic-thermal) PV/T ASHP has a bit higher COP value than that of ST-ASHP, at 3.75 and 3.03 respectively, which, however, needs larger capital cost (longer payback time) and more complex control strategy.
  • Factors impacting on outcomes
  • Performance
    Energy/ heat storage
  • Recommendations and gaps in knowledge
  • • Performance enhancing technologies
    Systems: (1) (BI)PV/T-ASHP system type needs largely developed; (2) optimization of the systems is required, including structure, control and operation to improve performance of the system; (3) combine two or more system concepts may have a prospective development, e.g. combination of the heat recovery system and solar ASHP system disadvantages in single system and provide more solutions to buildings energy systems.
    • Evidence base
    Methodologies: (1) develop a common simulation tool; (2) measure the system performance under a standard testing conditions, and (3) collect field testing data from pilot projects and validate the simulation/experiment results.
    • The role of end users
    Standard indicators: common standards and official certifications can help users to make better choice and comparison among different solar assisted ASHP systems.
  • Willem (2017)
  • Type of review
  • Systematic review
  • Number of studies
  • 377 published between 1987 and 2015
  • Outcomes measured
  • Performance
    Reliability, energy savings
  • Socio-ecological framework
  • Households
    Water heating
  • Findings
  • Positive
    Coefficient of performance COP range of 1.8–2.5, there are some potential technological updates that could augment the current systems and increase COP to a range of 2.8–5.5.
  • Factors impacting on outcomes
  • High costs
    Operational issues
    Performance
  • Recommendations and gaps in knowledge
  • Performance enhancing technologies
    Incorporating defrost strategies to boost performance under low or extreme ambient conditions, considering suitable refrigerants, their application in HPWHs, and alternatives to vapor-compression technology; applying advanced materials and components to increase capacity of energy (heat) storage and improve operation efficiency; introducing smart control strategies which optimize energy use based on water use schedule; and developing products with multi-function applications.