The goals set out in the Paris Agreement almost certainly require the global energy system to move to close to zero emissions, as other emission sources may be harder to abate (Masson-Delmotte et al., 2018). This transition is normally thought of as a shift from carbon-based fuels to zero carbon fuels. This is understandable, but not an adequate approach to understanding a systemic transition. Not only will primary energy sources change, so also will the ways that they are converted and used. In particular, the direct use of fossil fuels in buildings, transport and industry will need to end, and this has significant implications for the types of final energy used and the efficiency of both upstream and downstream conversion processes.

There is an increasing consensus that the lowest cost sources of zero carbon energy will be renewable electricity sources (RES), and that bioenergy, solar photovoltaics (PV), wind energy and hydroelectricity are likely to be particularly important (Grübler et al., 2018; Gielen et al., 2019). With the exception of biomass, these generate electricity directly rather than via combustion and heat engines. Hydropower has been an important source of electricity in favourable geographies for over a century and provides 16% of global electricity (IEA, 2020a). The economics of PV and wind have been transformed in less than a decade, with typical generation costs falling by 82% and 39% respectively in the decade to 2019 (IRENA, 2020a).

There is therefore increasing evidence that global energy will rely on RES. They will initially substitute for other, largely fossil fuel, electricity generation. However, the later stages of the transition are less straightforward. Electricity currently provides less than 20% of global final energy (IEA, 2020a), and therefore a zero carbon energy transition will also involve the decarbonisation of other end uses. Most analysis shows that this will require electricity to substitute for fossil fuels in many other end uses, notably in transportation and heating in buildings and industry (IEA, 2018a). The energy transition can therefore not be achieved without fuel switching in the large number of energy end uses that provide the energy services on which modern life depends.

Most analysis of the transition shows that improvements in energy efficiency will play a role in the energy system change by reducing energy demand below ‘business as usual’ levels (Edenhofer et al., 2014; Masson-Delmotte et al., 2018). Many projections see energy efficiency improvements mitigating but not reversing global energy demand growth. However, detailed analyses show that the global technical potential for efficiency improvement is very large (Cullen & Allwood, 2010; Cullen et al., 2011). Recent analysis shows that global energy demand could be reduced, even whilst delivering goals for energy access, poverty alleviation and decent living standards (Grübler et al., 2018).

In summary, it is broadly accepted that there is a major role for energy efficiency improvement in the transition, but that the scale of energy demand reduction is disputed. The dominant conceptualisation is that demand reductions will alter the scale of the decarbonisation challenge, and therefore the amount and cost of the zero carbon energy supply system needed.

This paper will argue that this is not a satisfactory conceptualisation. Given the systemic nature of transition, and in particular the need for fundamental changes in the type of energy used in transport and heating, the flows of energy in the global economy need to change substantially. Changes in the fuels used, and the efficiency of their use, are not just an enabler of the transition, but rather a defining characteristic.

The phrase energy efficiency is potentially confusing in that it is used in both physical and economic analyses of energy use. In the latter case, there is a very large literature on the scope for improving economic productivity through the use of higher efficiency technologies and techniques (see e.g. Ayres and Warr, 2010; Kümmel, 2011; Kümmel, 2013; Laitner, 2015). In this sense of energy efficiency, the phrase ‘energy productivity’ is often used synonymously. This paper does not seek to add to that literature, but focusses on the physical analysis of energy efficiency.

Even within this restricted use, the term ‘energy efficiency’ is, confusingly, used with respect to individual processes, sectors and whole systems. Figure 1 sets out the framing and terminology used in this paper. The energy efficiency of the whole system may be improved in three broad ways: firstly, by increasing the efficiency of conversion of primary energy into the final energy delivered to energy users (‘upstream conversion efficiency’); secondly, by increasing the efficiency of conversion of that final energy into useful energy at the point of use (‘final energy conversion efficiency’); and thirdly, by techniques that allow more energy services to be delivered by the same useful energy (‘service delivery’). The latter two categories are often combined in the phrase ‘end use efficiency’, but are conceptually distinct.

Fig. 1
figure 1

Conversion steps in the production of energy services from primary energy, highlighting the focus of the paper

Upstream conversion efficiencies are affected by the transition to RES, in particular through the phasing out of thermal power stations using fossil fuels. However, this paper focusses on the processes highlighted in Fig. 1, i.e. the conversion of final energy into useful energy. As the remainder of the paper shows, these efficiencies are also likely to be affected significantly by the energy transition.

The paper does not explicitly address other contributions to end use efficiency improvement (service delivery), for example through material resource efficiency, transport modal shift and building insulation. This is not because these approaches are not important, indeed it is well-established that they, but because their potential is less likely to be affected by the shift to RES.

The paper poses two research questions:

  • What are the broad qualitative changes to energy flows in the global economy that will happen as a result of the energy transition? and

  • What are the likely quantitative effects of these on final energy conversion efficiencies and, through these effects, on energy demand?

The ‘The thermodynamics of the zero carbon energy transition’ section analyses the shift in energy flows that will happen as a result of a shift from fuels that provide heat to RES that provide work, showing that this is a critical issue for the overall energy system. The following two sections undertake a thought experiment on the implications of this change for end use conversion efficiencies and therefore for global final energy demand. The ‘Quantification methodology’ section sets out the methodology for the quantitative assessment, and the ‘Quantification results’ section the results of the analysis. The ‘Discussion’ section discusses some implications for the zero carbon energy transition and the ‘Conclusions’ section provides key conclusions.

The thermodynamics of the zero carbon energy transition

Most energy analysis uses the concept of the ‘energy system’. Whilst precise definitions differ, the critical understanding is that there are complex links between the ways that different energy sources are converted and transported to provide energy services. Although heat and work are conceptually different, they are strongly linked, and therefore a systems approach is helpful. A common tool to illustrate the point is the Sankey diagram (Sankey, 1898), which shows energy flows from energy sources to energy services.

A highly simplified and non-quantitative Sankey diagram of the global energy system is shown in Fig. 2. In this case, rather than showing fuel types or carbon contents, the diagram makes a distinction between heat and work, both for the energy sources and the energy services provided. Fossil fuels, and some other energy sources, tend to provide heat, as does nuclear power and some RES such as biomass and geothermal energy. Other RES tend to provide work. Most notable in this category are hydropower and the ‘new renewables’, solar photovoltaics and wind, which have experienced dramatic recent cost reductions (IRENA, 2020a, 2020b). Some energy services require heat, most obviously those associated with space heating, water heating, cooking and many industrial processes; many other energy services, notably movement, require work.

Fig. 2
figure 2

A stylised Sankey diagram of the energy economy

The ability to inter-convert between heat and work is characteristic of modern energy systems, and the constraints on these conversions described in the discipline of thermodynamics. In principle, any mix of energy services can be met by any mix of any energy sources, but often only by utilising heat/work conversions. A more detailed and nuanced analysis of the relationships of different energy sources and energy services to work is provided in Appendix 1.

In pre-industrial societies, the systems of provision for heating services and work were largely separate. Heat was largely produced from wood and was used for cooking, thermal comfort and services requiring hot water, as well as a few other specialised applications such as metal working. Work was provided, largely for motion, from different systems, mostly from the physical labour of humans (hence the name work) or by domesticated animals, with hydropower and wind power in specialised applications, such as milling grain.

The technological changes of the industrial revolution allowed the two systems to connect. This was principally by allowing heat to be converted into work, initially in the late eighteenth century using steam engines, but then from the late nineteenth century by the two dominant technologies of modern energy supply, the internal combustion engine and the turbo-generator. Both provide work from combustible fuels using a heat engine. The former is principally used for decentralised motive power, mainly in transportation. The latter, driven by steam turbines and more recently gas turbines, has become the main source of electricity generation, with electricity then distributed to provide a multiplicity of energy services, including motive power. The science of these conversions has been understood through the discipline of thermodynamics since the mid-nineteenth century.

The combination of internal combustion engines and thermal electricity production has enabled the delivery of modern energy services anywhere on the planet, at least in principle. Energy use has multiplied many-fold and fossil fuels have become the main source of energy. Figure 3 shows a stylised version of such an industrial, fossil fuelled energy system. Fuels are combusted to produce heat as in pre-industrial societies. What the industrial revolution changed fundamentally was that heat was also converted to work to provide transportation, stationary power and other energy services.

Fig. 3
figure 3

A stylised Sankey diagram of an industrial economy

The change to low-carbon, and ultimately zero carbon, energy systems seems highly likely to be a transition as fundamental as the changes of the industrial revolution. Current evidence indicates that the cheapest forms of low-carbon electricity will be renewable rather than nuclear and/or fossil fuels with carbon capture and storage (CCS). Already fossil fuels are being displaced from electricity generation by renewables (IRENA, 2020a). Whilst very high levels of variable RES create new challenges for system operators (Jones, 2017), these look soluble, allowing a transition to zero carbon electricity (NGESO, 2020).

A zero carbon energy system will need electricity decarbonisation to be followed by the substitution of fossil fuels elsewhere in the energy system, i.e. for heating, transport and industrial processes. The heat-producing renewables—bioenergy and geothermal energy—may make a contribution. However, most evidence points to this being limited. Use of bioenergy is constrained by land availability, competition for other uses of biomass (notably for food) and relative costs (Smith et al., 2013). The global resource of geothermal energy is very large and economic in specific locations, but it is not expected to be competitive in most large, densely populated areas (Goldstein et al., 2011). If this analysis is correct, the main work-producing renewables (solar, wind and hydropower) will need to become the major fuels for transportation and heating, as well as the services currently provided by electricity. The dominant flows of energy in such a ‘post-transition’ energy system will then be as shown in Fig. 4.

Fig. 4
figure 4

A stylised Sankey diagram of 100% renewable economy

This wider role for electricity in low-carbon economies (Edmonds et al., 2006; Sugiyama, 2012) has become better understood as more ambitious carbon mitigation targets have been established and has been summarised by the IPCC (Edenhofer et al., 2014). The idea that the whole world’s energy can be supplied from renewable energy is not new (see e.g. Lazarus, 1993), but only more recent studies of 100% renewable energy systems (e.g. Lund & Mathiesen, 2009; Jacobsen et al., 2015; Hansen et al., 2019a; Hansen et al., 2019b) rely heavily on renewable electricity and therefore electrification. Increasingly, it is understood that there are therefore major challenges around the conversion of the 80% of global final energy use that is not currently electrified. Analyses of sectoral electrification have been undertaken for transport (e.g. McCollum et al., 2014), buildings (e.g. Deason & Borgeson, 2019) and industry (e.g. Lechtenböhmer et al., 2016). These studies show the potential for increased efficiency through electrification at the level of the individual process and sector.

Some non-academic literature has framed the overall process as a move away from combustion (Lovins, 2013; Patterson, 2014). However, the change is arguably even more fundamental. As shown in Figs. 3 and 4, it is a reversal of the changes that occurred during the industrial revolution. Instead of energy services that require work being provided from fuels via combustion and heat, energy services that require heat will be provided from energy sources that provide work. The whole architecture of energy systems will change, away from converting heat into work, towards converting work into heat.

The scale and rate of change are, of course, likely to vary as a function of resources, investment and politics. However, once the assumption that work-producing renewables will become dominant is accepted, this broad conclusion is unavoidable. Perhaps surprisingly, there is no literature that conceptualises the whole energy transition in this way.

Work-producing RES are already replacing heat-producing fuels, predominantly fossil fuels, in electricity generation. This reduces primary energy demand by eliminating heat losses in thermal power generation. However, it has no direct impact on final energy demand.

With the conceptualisation of a shift from heat-producing to work-producing energy sources, the zero carbon energy transition may be thought of in three subsequent steps, each with implications for the efficiency of final energy conversion, and therefore the scale of final energy demand.

First, without fuels to supply heat engines, end uses of energy requiring work will be converted, where possible, to electricity. The early stages of this step of the transition are already beginning, in particular with electric vehicles substituting for internal combustion engines. As electric motors have a much higher efficiency than heat engines, this change decreases final energy use substantially.

Second, without fuels to produce heat, electricity will also be used for heating. At temperatures below 100 °C, this can also be achieved with a significant reduction in final energy use to supply the same useful energy. This is because electric heat pumps can achieve efficiencies well in excess of 100% (see Appendix 2 for a discussion of the counter-intuitive idea of efficiencies that exceed 100%).

Thirdly, it will be necessary to substitute fossil fuels with other options, in cases where electricity use is not a viable alternative. The precise limits to electrification are not agreed, but seem likely to include at least some aviation, shipping, road freight and manufacturing process applications (CCC, 2018a; IEA, 2019). There may also be practical economic limits for the large, seasonally peaked energy service of space heating (Eyre & Baruah, 2015).

There is a variety of options for alternative carbon-free energy vectors. Hydrogen is the most obvious and most extensively studied (Philibert, 2017). The current dominant production route, using steam reforming of methane, is not carbon neutral, and often referred to as ‘grey hydrogen’. It can be converted into a low-carbon vector, ‘blue hydrogen’, by adding carbon capture and storage to the reformation. Hydrogen can also be produced from water by electrolysis (Brandon & Kurban, 2017). This ‘green hydrogen’ is likely to be the major route in an energy system dominated by work-producing RES.

The electrolytic conversion process can approach 100% efficiency in theory, but in practice involves an efficiency penalty. However, conversion of hydrogen chemical energy to work using fuel cells enables it to be used more efficiently than fossil fuel in heat engines. The theoretical maximum electrical efficiency of a hydrogen fuel cell (determined by the ratio of the Gibbs free energy to the enthalpy) is 83% and achievable efficiencies exceed 50%. These are significantly higher than in a heat engine, where the theoretical efficiency is the Carnot limit and materials limit feasible operating temperatures and therefore the efficiency of conversion to work (Lutz et al., 2002). In summary, hydrogen may be used more efficiently than fossil fuels at the point of final energy conversion, because the chemical energy may be converted directly into work rather than heat.

All three stages will tend to increase conversion efficiencies at the point of final energy use, and thereby reduce global final energy demand. The size of the combined effect for the global energy system is investigated in the next two sections. Likely impacts on primary energy use are more complex due to the additional upstream conversion inefficiencies in the third stage and are addressed in the ‘Discussion’ section.

Quantification methodology

The principles of the quantification methodology follow from the analysis of the previous section.

For the purposes of this thought experiment, it is assumed that energy is generated entirely from work-producing RES. This is not intended to be a prediction or even a realistic scenario. Even in a 100% renewables scenario, bioenergy and geothermal may be expected to make a contribution. However, as set out above, it is widely expected that a zero carbon global energy system will be supplied largely by work-producing RES. It is therefore a reasonable first approximation, on which to base an assessment of the impact of the zero carbon transition on the end use conversion technologies needed.

Global energy demand, using a base year of 2020, is estimated from available sources. It is divided into different categories of energy use designed to provide different energy services (e.g. boilers for space heating, high-temperature industrial processes, heavy road freight transport). The extent of disaggregation is determined by the level required to make reasonable allocations into the three categories set out in the previous section, i.e.

  • Energy services delivered by work and that can be electrified

  • Energy services delivered by heat that can be electrified

  • Energy services that cannot practicably be electrified.

Detailed assumptions about current global energy use are set out in Appendix 3. For each category, current global energy use is split into electricity and heat-producing fuels.

The literature on energy efficiency is reviewed to identify existing efficiencies in both electrified and non-electrified cases. Detailed assumptions about conversion efficiencies are set out in Appendix 4. The same process is repeated for the same set of energy services delivered in a zero carbon system, using a mix of electricity and electrolytic hydrogen as the zero carbon vectors to supply final energy demand. The differences in efficiency are applied to current global final energy demand to calculate the size of global demand reduction.

In order to ensure transparency, other efficiency options (e.g. building insulation, vehicle aerodynamics, industrial process control) and energy service demands (e.g. for material resources, thermal comfort or mobility) are held constant. This is not, of course, realistic, as the potential for these to change is very large (Grübler et al., 2018). But this thought experiment approach enables calculation of the effect of supply mix changes on final energy conversion efficiencies without being obscured by these other effects.

Quantification results

The overall impact of the changes from converting the whole energy system, with a constant level of energy services and useful energy demands, to an efficient work-driven system is approximately a 40% reduction in final energy demand (see Fig. 5) from 416 to 247 EJ/year. The major efficiency gains are in buildings and transport where demand reductions exceed 50%. These are primarily due to converting building heating from fossil-fuelled boilers to electric heat pumps (EHPs) and from switching transport propulsion from internal combustion engines (ICEs) to electric vehicles (EVs). Energy demand reductions in industry are smaller (20%), as set out in Appendix 4, reflecting the high efficiencies already achieved in energy intensive industry sectors and the difficulties in switching some demands to electricity, especially where fuels play some other role such as a feedstock or chemical reducing agent.

Fig. 5
figure 5

Changes in global final energy demand due to a shift to using work

Demands that are not electrified are assumed to be provided by hydrogen. The change in the split between electricity and fuel for each major type of process is shown in Table 1. The share of electricity use doubles in industry (38% to 76%); almost triples in buildings (33% to 97%) and rises more than 30-fold in transport (less than 2% to 54%). Overall electricity provides 189 EJ (77%) with hydrogen the remaining 57 EJ (23%). These compare to the current figures of 110 EJ (26%) for electricity and 306 EJ (74%) for other fuels. In other words, fuel use falls to well under 20% of existing levels, whilst electricity use rises by 70%.

Table 1 Final energy by process type and fuel: current and post-transition (EJ/year)

The detailed results are dependent on the precise assumptions set out in the appendices. In particular, the results depend on the assumptions about the energy supply mix and the conversion technologies deployed at the point of energy use.

The major finding that energy services will be delivered much more efficiently in a 100% work-powered global energy system is robust, provided that EHPs largely replace boilers for low-temperature heating services and EVs largely replace ICE for light vehicles.

In essence, the efficiency gains are a direct result of the fact that the second law of thermodynamics allows greater efficiencies in conversion of work to heat than vice versa. An alternative framing is that ‘energy’ is a misleading metric, with which to cover both work and heat (and heat at different temperatures). The analytical option of ‘exergy analysis’ is discussed in Appendix 2. However, the key conclusion of this paper is robust against shifting to exergy analysis. In either framework, the important point is that a unit of work energy will generally provide more useful energy and energy services than a unit of heat energy.

Two major simplifying assumptions made in the thought experiment warrant some consideration: first, that there is no deployment of energy efficiency measures other than the final use conversion efficiency improvements driven by the change to a work-based energy system; and secondly that the demand for energy services is unchanged. Of course, neither of these is a realistic scenario.

Firstly, there remains significant scope for improvement in the energy efficiency with which energy services are produced from useful energy, for example in building insulation and vehicle design. The technical potential for such changes has been estimated to be 73% (Cullen et al., 2011). End use efficiency improvements, in the broadest sense, are therefore not restricted to those shown in Table 1 and Fig. 5. More substantial improvements are possible by combining the conversion efficiency improvements discussed in this paper with other energy efficiency techniques.

Secondly, demand for energy services seems very likely to rise in order to meet rising living standards, especially in the Global South. On the other hand, there are large opportunities for higher-income energy users to reduce consumption of energy services without lower welfare (Ivanova et al., 2020). Combining these effects requires more detailed socio-technical analysis. Doing this for the whole world is complex; the most comprehensive analysis shows that a sustained effort to reduce energy demand whilst enabling all of the UN Sustainable Development Goals to be met can reduce final energy demand by approximately 40% (Grübler et al., 2018). The similarity of this 40% reduction number to the conversion efficiency improvement calculated above is a coincidence. The implication is that the projected rise in global energy service demands can be balanced by improvements in the production of energy services from useful energy. The large rises in conversion efficiencies in moving to a work-based energy system then drive the 40% reduction.

The analysis uses the simplifying assumption that renewable energy is produced as electricity, with direct use of fossil fuels replaced by either electricity or electrolytic hydrogen. To the extent that bioenergy and, perhaps, geothermal heat might contribute to renewable energy supply, this constraint is too limiting. The best uses of limited biomass resources in a low-carbon transition remain disputed (CCC, 2018b) and may be very dependent on local resources. Analysis of the issue is outside the scope of this paper, but some production of heat and liquid fuels from bioenergy is likely, and it would be expected to substitute for some of the electrolytic hydrogen projected in the results above. This will tend to increase final energy demand, as biofuels can generally be used less efficiently than hydrogen at the point of final energy use. On the other hand, upstream conversion losses associated with hydrogen production would be reduced. More research is needed on the overall impact of a realistic share of bioenergy on the analysis.

The results imply a growth in use of hydrogen to 57 EJ/year. This is four times the existing level of annual global hydrogen production from all sources of 14.4 EJ (IRENA, 2020b). Producing this entirely by electrolysis, assuming an electrolyser efficiency of 80%, which is the median of a major review (Parra et al., 2019), would require 70 EJ/year of electricity, i.e. over 60% of total current global electricity production. It would need at least 2500 GW of electrolysis. This vastly exceeds current electrolyser capacity, although it is rising quickly with new investment expected to exceed 1GW/year in 2023 (IEA, 2020b). For the reasons explained above, in practice, use of heat-producing renewables in some applications would be expected to reduce the scale hydrogen demand.


Policy implications

Critical assumptions in the analysis are that EHPs provide most low-temperature heat, EVs predominate in the light vehicle fleet and hydrogen is used primarily in fuel cells in heavy vehicles. These are the high-efficiency options. Based on current analysis, they appear to be the economic options in a zero carbon system. However, it is well-established that a variety of market failures mean that economically optimal technologies are frequently not deployed (Brown & Wang, 2015; Eyre, 1997). Without policy intervention, some actors in equipment supply industries are likely to market electric resistance technology or hydrogen boilers for low-temperature heating and alternative fuels in ICEs for vehicles. Policy decisions, in particular regulatory standards for heating systems, vehicles and appliances, will continue to be important in ensuring high-efficiency options are used.

In the context of the transition to technologies that use work instead of heat, technical standards and product regulation can be very powerful. The analysis above shows that efficiency standards for heating and vehicles can be approximately a factor of 3 stricter than those currently in place for fossil fuel use. These standards could be set for either or both of the final energy conversion efficiency or the wider service delivery efficiency, e.g. for an electric vehicle, either motor efficiency (%) or vehicle efficiency (kWh/km). Setting standards at a level that can only be achieved by well-designed EHPs and EVs effectively will require both the use of final energy that can be used efficiently (electricity or hydrogen) and end use product designs that achieve that efficient use. For this reason, they will be more effective than traditional product policy, which only affects product design. Such standards therefore have the potential to be pivotal in the energy transition.

Interactions between energy service demands and energy efficiency improvements should also be considered. Two may be significant: the impact of embodied energy and economic rebound effects.

Investment in more efficient technologies may increase embodied energy, and therefore industrial energy use. The methodology used in the analysis above treats industrial energy use as independent of the material input for efficiency improvement, and therefore analysing the size of this effect is outside the scope of the paper. It is a significant effect in a few cases, notably for EVs, where manufacturing emissions are currently double those of an ICE vehicle (Wolfram & Wiedmann, 2017). In principle, these lifecycle considerations could be included within product standards and regulations. However, any such adjustment would be a second-order effect compared to differences between operational energy use in ICEs and EVs.

By treating demand for energy services as fixed, the methodology also neglects any rebound effects from improved energy efficiency. Direct rebound effects are typically 10–30% (Sorrell, 2007). And historical economy wide effects have been found to be larger (Brockway et al., 2021), which follows from the importance of exergy efficiency in the economy. However, rebound effects specific to the improved conversion efficiencies addressed here are unknown. Rebound is driven by energy cost reductions, increased incomes and higher levels of economic productivity, not physical metrics. Many of the conversions envisaged will not lead to significant energy cost reductions, as they involve switching to higher-priced fuels, and therefore historical relationships may not be reproduced. In any event, rebound cannot be addressed by technical standards. To the extent it is judged undesirable, it requires economic disincentives.

Implications for primary energy demand

The analysis above relates to final energy demand, not primary energy. However, it is straightforward to make a preliminary assessment of the effects described above of primary energy demand. International Energy Agency data shows that, in the current global energy system, 70% of total primary energy supply (TPES) is used for final energy use (IEA, 2020c). The remaining 30% is lost in various upstream conversion processes. The main loss (20% of TPES) is in thermal power generation, with 8% in other fuel industry processes (largely in coal conversion to coke and oil refining). Losses in electricity transmission and distribution are 1.6% of TPES (approximately 10% of electricity generated). In a fully work-based energy system, these numbers will be very different. Losses in thermal power generation and fuel industry processing will be eliminated. As electricity becomes the dominant energy vector, distribution losses would be expected to rise to approximately 10% of TPES.

The other new source of upstream conversion losses will be in hydrogen production. Assuming an 80% efficiency in electrolysis, with 23% of final energy delivered as hydrogen, these conversion losses will be 5% of TPES. As explained above, these might be reduced if bioenergy replaces some uses of hydrogen. Depending on the resources used to balance electricity systems with high levels of variable resources, there may also be additional losses in upstream electrochemical storage and electricity generation from stored hydrogen. However, total upstream losses seem very likely to be smaller than in the existing global energy system. A 40% reduction in final energy demand will therefore be reflected in a reduction in primary energy demand of at least 40%.

Timescales of change

The analysis compares the current global energy system with that after a complete transition to work-producing energy sources. The time taken to achieve this is not explicit in the analysis. There is no expectation that different aspects of the transition will have similar timescales. Indeed, it seems very likely that, for example, the adoption of EVs in some developed countries will be more rapid that the complete phase-out of traditional cooking fuels in the Global South. The results are simply a comparison of starting conditions and a projected future end state. However, the overall reconceptualization does have implications for the timescales of the transition. The timescales for different technology transitions are very variable, providing some encouragement that system change by mid-century is plausible (Sovacool, 2016). However, it is clearly also true that large-scale transitions of the type seen in industrial revolutions have tended to be slower (Grübler et al., 2016). The revised conceptualisation set out in this paper shows that the zero carbon transition has the character of an industrial revolution in that it is a complex, multi-stage process, involving multiple technologies, infrastructure systems and many actors. This tends to support arguments that a very rapid complete transition is probably unrealistic.


The energy transition is normally conceptualised as a shift from fossil fuels to zero carbon energy sources, with the role of energy efficiency limited to reducing the scale of demand for energy, and therefore the amount of decarbonised energy required. This paper has shown that this is a seriously inadequate representation of the changes in energy systems implied by the transition to zero carbon. Recent evidence shows that the key energy sources are likely to be ‘work producing’ renewables. As these replace ‘heat producing’ fossil fuels, the changes in the energy system are more profound than simply fuel switching. They constitute a systemic change on the scale of the changes observed during the industrial revolution.

Substitution of fossil fuels by renewables is only the first stage of the transition. To achieve a zero carbon energy system, it needs to be followed by changes that enable ‘work producing’ renewables to supply energy services not currently supplied by electricity. The dominant energy conversion processes in the global energy system will therefore be different. In particular, electricity will be converted to heat and other zero carbon fuels. The transition as a whole cannot adequately be conceptualised as independent processes of shifting to zero carbon fuels and improved energy efficiency. The two have strong positive synergies.

The paper presents a thought experiment on the implications for global final energy conversion efficiency in an energy system in which all energy is supplied by work-producing energy sources. It assumes that all final energy is supplied as either electricity or electrolytic hydrogen, but the broad conclusion is robust to inclusion of some other renewable energy sources. Both electricity and hydrogen can be used more efficiently than the other fuels that dominate final energy use, and this is a very significant effect. Making transparent and plausible assumptions about the end use conversion processes used, the paper finds that there is a very large increase in the overall efficiency of final energy conversion. Total useful energy to deliver the same energy services is reduced by approximately 40%, with the main effects in buildings and transport. Holding other drivers of final energy demand constant, this will have a proportional effect on global final energy demand. A similar impact on primary energy demand is likely.

Technical standards and product regulation for end use conversion efficiency and/or service delivery efficiency seem likely to be key policy instruments. They have the potential both to ensure product designs that use energy efficiently and to drive the transition to final energy sources that enable such use.