Using the previous information as a guide, future energy mix scenarios can be proposed. The basic challenge is whether or not sufficient new kinds of infrastructure can actually be built at the capacity of various target goals. In Section 1 we discussed a variety of potential choices for an energy mix but each of these choices has its own set of practical problems of installation and deployment at significantly larger scales. Before discussing various policy scenarios that would lead to changes in the global energy mix, it is quite useful to numerically show the scale of build-out of new infrastructure that would be required to meet new targets. An example scenario, among many that are possible, is shown in Figure 11 where a doubling of energy is predicted for the year 2040 relative to the situation in 2011. In this particular scenario there are two important baseline assumptions that we can adopt for future policy choices.

• Crude oil will be an exhausted resource by 2040
• We start with having very little solar and wind

The principle features in this particular scenario are a) the contribution of nuclear electricity will double b) wind +solar will increase from essentially 0 to 20% of the total electricity portfolio; c) the use of coal dramatically decreases. This particular scenario would apply to a policy choice that prioritizes decarbonization of world energy over the next 30 years. Under this scenario as well as others it remains to be seen if new resources can be built and implemented over a 30-year period. Hence, this same scenario might well apply to the 2020-2050 time period. A major uncertainty in any future scenario is the overall role that conservation might play in determining the timescale over which energy demands double from their current value. This particular scenario does not factor conservation in. The main difficulty is that, while the developed world may begin to practice better conservation habits, much of the world remains undeveloped and their per capita use of energy will accelerate while countries in the developed world may decline.



The current UN population projection for the year 2040 is 9.21 billion compared to the 2011 population of 7.04 billion. This an increase of 31%. If energy use is projected to double over this time then this means that energy scaling demand goes as (pop growth)^2.6. Implicitly, this strong scaling argues that per capita increases in energy use in the developing world far outpace any conservation strategies in the developed world. If we assume the same scaling behaviour for electricity demand then 45,000 TWHs of electricity from new generations sources is required. How can this need be best met? Here the role of CF is paramount. For example, at the end of 2017, both solar PV and nuclear, coincidentally, had a global installed capacity of about 390 GW. Electricity generated from nuclear power in 2017 was about 2500 TWHs (11% of global generation) indicating a capacity factor of 73%. Electricity generated from solar PV was about 375 TWHS (1.6% of global generation) indicating a capacity factor of 11%. This low CF for solar PV is simple to understand. Rated (nameplate) solar power for PVs refers to peak power when the sun is directly overhead. However, fixed PV panels illuminated over the course of the day have an average power of approximately 25% of peak power. As the panels are illuminated only 1/2 of any 24-hour period, on average, then there is another factor of 2 loss leading to an overall throughput of 12%. Under perfect conditions (CF = 100%) 1 TW of power produces 8760 TWHs of electrical energy for that year. For PV, nuclear and wind, 1 TW of nameplate power then produces 963, 6394, and 3942 TWHs of electricity respectively (here we assume that new wind is off shore with CF-- 45%). What weighted combination of sources, given their respective CFs, can produce 45,000 TWHs? One example target would be a combination of 50% wind, 30% nuclear and 20% solar PV? This would require about 9.4 TW of new solar PV install, 2.1 TW of new nuclear, and 5.7 TW of new wind.

In the last few years PV growth has been rather extraordinary due to large scale Chinese manufacturing of panels that has brought costs down on a worldwide basis. At the end of 2013 there were 137 GW of global capacity and by the end of 2017 there was ~400 GW, with 81 GW installed in 2017 alone. In 2014 there were 40 GW installed. Thus, production doubled in 3 years. But exponential scaling at high rates can never occur over many doubling times as at some point, the amount of new facilities one needs to build to make twice as many turbine blades, or twice as many PV panels as the prior 3-year period becomes physically impossible. Nonetheless, if we assume that this doubling time is sustained and start in 2014 then by 2034 9000 GW of installed PV will occur. If we relax the doubling time down to 5 years and start at 2018 then 9000 GW will be realized by 2039. This shows that PVs can potentially meet the proposed target.

The current installed capacity of 392 GW is met by 450 nuclear power plants indicating an average capacity of 0.87 GWs. Recently constructed plants are at nameplate capacity of 2 GW. In the United States the average time it takes for a new nuclear plant to come on line, from design through approval, building, and inspection, is about 17 years. The process is quicker in other parts of the world but still occurs over many years timescale. If each new plant is rated at 2 GW, 1000 new nuclear plants will have to be built over the next 30 years. That is equivalent to building 3 new plants and hence nuclear power would have a very difficult time reaching the target of 2 TW.

The situation for Wind is more favourable due to the emergence of off shore wind power in the UK and Germany as a proven viable technology including the world's first floating off shore wind farm off the coast of Scotland. Off shore wind also has higher CF than onshore with values in the range of 45--50%. In addition, the world’s first OFF shore facility operating 8 MW turbines was recently commissioned. At the end of 2017 there was approximately 540 GW of installed wind capacity, with an average of 56 GW added per year over recent years. Recent world data suggests a doubling time for cumulative global wind power of ~ 5 years. These considerations show that 6 TW of new wind generation could be produced in 23 years. Thus, wind power, and particularly off shore wind power should be regarded as a promising choice for increasing the world's share of renewable electricity generation as it can be build out at a much larger rate than nuclear power.

The above calculations suggest that the world's future mix for electricity can become dominated by renewable energy sources. There is no need to discover additional fossil fuel sources for the generation of electricity given the positive and rapid evolution in solar PV and wind. However, the intermittent nature of these sources absolute requires significant world investment in energy storage in order to deliver electricity 24x7. In that regard, Solar Thermal Electricity (STE) is highly relevant since it can have energy storage built into the facility (heated molten salts). However, for US installations the annual CF is only 22% again indicative of real performance problems in these facilities. As of 2017 STE had a global installed capacity of 5.5 Gigawatts (compared to 400 for solar PV) and there are strong fluctuations in the annual installed capacity additions. Even if CF can increase to 50% and 1 GW of annual install can be realized for 30 years, that only represents ~ 150 TWHS of new electricity generation and this is negligible compared to the requirements above. In addition, there will be small scale deployment of other technologies over the next 30 years, most of which will be ocean wave energy-based but some may be OTEC based. From the data to date, however, its really impossible to predict future amounts, just that there will be "some".

Based on all these considerations, Figure 12 shows a plausible electricity mix of our needed 45,000 TWHs of future electricity. This plausibility is based on real world data over the last few years that shows the physical ability to install devices as well as the physical acknowledgment that we cannot bring renewable energy sources online sufficiently fast to replace all uses of fossil fuels. Given the difficulty of timely construction of nuclear plants, we assume that only 300 new 2 GW facilities can be completed over the next 30 years and that might be optimistic While coal-fired electricity can disappear as source of the needed future 45,000 TWHs, natural gas-fired electricity (currently at 22% of world electrical production), will be expanded to still make an important contribution. For the US it is and unfortunate certainty that it will rely on increasing, in the short term, more NG-fired electricity to utilize new yields from various fracking fields.



For the other two sectors of energy, the future is far more difficult to predict. For transportation, it is clear from a variety of studies that battery powered vehicles are unlikely to ever power more than 2-4 % of the global vehicle fleet. This is a reflection of the limited materials in the Earth left to build the required batteries. The most promising source of alternative fuels is in cellulosic ethanol, particularly switch grass. This is a very scalable solution as most of North America and Europe can grows some form of switch grass. Indeed, Canada can, in principle, get much more automotive fuel from the harvesting of switch grass than it will ever get from drilling out the Athabascan Tar Sands. For space heating, it seems likely that our newly discovered resources of NG are best suited for this. The contribution of renewable energy sources is far less than the contribution of renewable electricity sources. The use of renewable electricity for the purposes of electrical heating would be quite inefficient and not likely to occur as long as some fossil fuels remain.

Regardless of the details, the world must start to strongly reduce the overall contribution of fossil fuels as the major part of our current global energy mix. This needs to be done both for reasons of climate change mitigation and the simple fact that fossil fuel resources are being exhausted. In addition, we need to demonstrate more wisdom on how to live better in balance with Earth resources and abandon the current mentality of solving shortages in energy demand by simply drilling more and more into the Earth. Plausible alternatives to this behaviour have been presented here that can be implanted to scale, over the next few decades. On a longer timescale large scale projects that use the Gulf current flow or the stored heat in the world's oceans can power the energy needs of future civilizations for centuries.