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By Charles Rhodes, P. Eng., Ph.D.

This web page identifies practical energy sources that have the capacity to meet the needs of mankind in an environmentally sustainable manner.

As shown elsewhere on this website, all energy in the local universe that can do work originates from the gradual aggregation of free electrons and free protons. This particle aggregation in the sun and the stars causes overlaps of particle energy fields which convert static field energy into kinetic energy. This kinetic energy then converts to photons which are emitted, leaving behind particles that are mutually bound together in potential energy wells. There are various means of harvesting the emitted photon energy flux to do work.

Over a period of billions of years the solar photon flux contributed to the Earth's chemical energy and the Earth's residual thermal energy.

During this same period aggregation of light elements on Earth, in part from the solar wind, formed the Earth's reserve of potential nuclear fusion fuel.

During the early universe stellar nuclear reactions that occurred during super nova events produced the heavy elements that form the Earth's potential nuclear fission fuel.

The sources of energy on Earth that mankind can utilize to do useful work are: stored chemical energy, present solar energy, nuclear fission energy, nuclear fusion energy, the Earth's residual kinetic energy and the Earth's residual thermal energy. All energy forms available to mankind that can do work are derived from these energy sources.

1. Stored chemical energy in the form of coal, oil and natural gas that resulted from net absorption of a small fraction of the past solar energy incident on the Earth.

2. Present solar energy incident on the Earth that can produce biofuels, hydroelectric power, solar electric power and wind power.

3. Nuclear energy that can be obtained via fission of high atomic weight elements.

4. Nuclear energy that can be obtained via fusion of low atomic weight elements.

5. Residual kinetic energy from the Earth's rotation and from the Earth-Moon orbit that can be harvested via tidal power.

6. Residual thermal energy, primarily resulting from nuclear decay in the Earth's core, that can be harvested via deep wells drilled in active volcanic areas.

During the last century mankind consumed solid (coal), liquid (petroleum) and gaseous (natural gas) fossil fuel stocks that took many millions of years to accumulate. At present extraction rates fossil fuels will become progressively more uneconomic due to depletion of accessible reserves, increasing costs of extraction and the consequences of increasing the concentration of carbon dioxide in the Earth's atmosphere and oceans.

As of 2023 the thermal power provided by world wide consumption of fossil fuels was about 21,000 GWt.

In practical application fossil fuels are used to heat water to 540 degrees C. The resulting steam expands through a two stage turbo generator and is condensed at 49 degrees C. This temperature range produces good thermal efficiency while staying within the performance range permitted by readily available materials and a natural draft cooling tower heat sink.

In the future fossil fuels will have to be replaced by synthetic fuels that are produced from water and biomass with the aid of electricity and nuclear heat.

Most of the solar energy that is incident on the Earth consists of electromagnetic photons with wavelengths in the near ultra-violet (.1 um to .4 um), visible (.4 um to .7 um) and near infrared (.7um to 2.8 um) ranges. About 30% of the incident solar photons are reflected back into space. The remaining solar photons are absorbed by various materials such as the ocean surface. A solar photon can cause a change in electron energy state, a chemical reaction, a change in physical phase or a change in temperature. Eventually all the energy carried by the absorbed solar photons is converted into atmospheric temperature heat. A small portion of this heat is directly radiated. However, most of this heat evaporates water that emits infrared radiation into space when it condenses in the cool upper atmosphere.

There are various ways of obtaining work from the processes that convert solar photons into atmospheric temperature heat. Six examples are:

1) Solid Biofuels: Solar photons incident on green plants cause photosynthesis whereby carbon dioxide and water combine to form carbohydrates known as sugars, starch and cellulose plus oxygen gas (O2). The biomatter can be harvested, dried, compacted and directly combusted as a dry fuel. The main difficulty with dried biomass as fuel is transportation and handling problems due to low material density, low energy density and high ash yield. Solid biomatter is generally unsuitable as a transportation fuel.

2) Liquid Fuels: Solar photons incident on green plants cause photosynthesis whereby carbon dioxide and water combine to form carbohydrates known as sugars, starch and cellulose plus oxygen (O2). Various processes involving supply of additional energy and hydrogen can be used to convert carbohydrates into alcohols and oils. Dehydration reactions can remove H2O from alcohols to form oils. Due to density differences and immiscibility oils naturally separate from water. Liquid fuels (alcohols or oils) can be burned in an engine or a combustion turbine to do work. The engine or turbine releases heat into the atmosphere. Note that the dehydration and hydrogenation reactions require externally supplied energy beyond that contained in the biomass feedstock. This extra energy can be obtained either by sacrificing additional biomatter feedstock or by input from another prime energy source such as nuclear reactor or a renewable electricity generator.

3) Hydropower: Solar photons absorbed by lakes and the ocean cause evaporation of water. The resulting water vapor rises and cools at higher altitudes where it condenses emitting part of its energy as infrared photons at the upper atmospheric temperature. The water falls as rain onto high elevation land. This water becomes part of a river that flows downhill to the ocean. Along the way one or more hydro-electric generators can be used to change part of the kinetic energy of the flowing water into electricity. This electricity is transmitted along electricity transmission lines and is converted into heat, chemical, mechanical or radiation energy at the electrical load. The chemical, mechanical and most of the radiation energy ultimately will become atmospheric temperature heat.

4) Wind: During the daytime absorption of incident solar photons by dry ground causes thermal expansion of the adjacent air. Similarly at night the air over dry ground cools. This daily temperature swing of air over dry ground causes back and forth air movement known as wind. Wind is particularly strong near the coast of an ocean or a large lake. The energy contained in this air movement can be harnessed with a wind turbine. The major difficulty is that the wind turbine power output drops to almost zero twice per day. A second difficulty, particularly in Ontario, is that average wind turbine power output is also affected by seasonal issues. Other wind power related difficulties are long distance energy transmission, poor transmission line utilization and expensive balancing energy generation/storage.

5) Solar Thermal Power: During the daytime solar photons are reflected onto a black vacuum insulated tube that contains a pumped heat transport fluid. The hot heat transport fluid is used to form a vapor such as steam which generates electricity via a turbine, After passing through the turbine the vapor is condensed, liberating its heat to the atmosphere. The condensate is pumped back into the vapor generator.

6) Solar Voltaic Power: During the daytime solar photons are absorbed by large area semi-conductor p-n junctions. The solar photons excite electrons over the semiconductor's bandgap causing a voltage between the p and n sections. This voltage causes an electric current in an external circuit that does work.

An advantage shared by all forms of solar/renewable energy is that there is no increase in thermal dissipation in the Earth's atmosphere and hence solar/renewable energy does not contribute to global warming.

Many solar power systems are connected behind customer electricity meters. Such solar power systems reduce electricity energy charges, electricity transmission loss charges and electricity demand charges, which maximizes the financial benefit of the solar power system to the electricity customer. Solar power output also goes through a daily maximum coincident with the daily air conditioning load. Solar energy is radiation resulting from nuclear fusion reactions in the sun. At low Earth latitudes, where solar radiation is intense and usually available, solar energy may assist with daytime electricity production. However, at high Earth latitudes, where solar radiation is weak and is frequently unavailable for long periods, solar energy is muchless useful.

Nuclear fission power reactors have been deployed for over half a century. The origin of nuclear fission energy is stellar end-of-life processes that form heavy atoms such as uranium and thorium. A nuclear fission reaction occurs when a suitable heavy atom absorbs either a neutron or a gamma photon and then breaks into two lighter atoms plus some energetic neutrons. The lighter atoms, known as fission products, are generally radio active and release more energy via a sequence of natural decays. The natural decay products are known as fission daughters and are often radio active.

In a nuclear reactor the energy released by fission reactions becomes heat. Most of this heat is used to make steam in a closed system. Steam turbines convert about one third of the heat into electricity. The remainder of the heat is directly or indirectly removed by evaporation of water at close to atmospheric temperature. High altitude condensation and freezing of the resulting water vapor emits infrared photons which carry the heat energy into outer space.

Typically about 10% of the generated electricity is used to power parasitic loads within the nuclear power plant, including: superheated water pumps, condensate injection pumps, cooling water pumps, moderator pumps and cooling tower fans.

The major advantage of nuclear fission based electricity generation is that it is a well understood method of generating bulk electricity without combustion of fossil fuels. The parasitic power requirements are relatively low. Light water cooled and moderated nuclear fission power reactors for electricity generation have been successfully deployed for half a century and the related issues are well understood by governments, engineers, equipment suppliers, contractors, major electricity utilities, labor unions and regulatory bodies. Light water cooled and moderated nuclear fission technology is well known and is taught at the undergraduate level at numerous educational institutions. However, major disadvantages of Light Water Reactors (LWRs) are poor natural uranium fuel efficiency and poor TRU production efficiency. These disadvantages will likely soon lead to replacement of Light Water Reactors (LWRs) with CANDU heavy water reactors for more efficient use of the uranium isotope U-235 and Fast Neutron Reactors (FNRs) that can sustainably operate without U-235.

A special category of nuclear fission reactors is CANDU reactors. CANDU heavy water cooled and moderated nuclear reactors have extraordinary fuel flexibility, fuel utilization efficeincy, reliability and safety features. CANDU reactors operate with natural uranium fuel or spent fuel from light water reactors and require proximity of heavy water to the fuel for the nuclear reaction to run. If the heavy water is either lost or is replaced by light water the main nuclear reaction stops. In an emergency a CANDU reactor can be flooded with light water which, in addition to providing cooling, forces a nuclear reaction shutdown. That safety advantage has not received adequate public attention.

CANDU reactors, although more expensive to construct than light water reactors, are inherently much safer. Light water reactors use enriched uranium fuel. In a nuclear reactor with enriched fuel core melting caused by fission product decay heat can potentially trigger an uncontrolled nuclear reaction whereas in a CANDU reactor with natural uranium fuel core melting cannot trigger an uncontrolled nuclear reaction.

A minor disadvantage of CANDU reactors is that the energy captured by the moderater is discarded, which makes these reactors about 4% less thermally efficient than a light water cooled and moderated reactor having the same electric power output.

An extremely important issue from the Canadian perspective is that CANDU technology provides Canada significant protection from the whims of US politicians, who have a history of not honoring Canadian-US trade agreements. With CANDU technology Canada is not dependent on the USA for nuclear fuel enrichment or for reactor pressure vessel fabrication.

An important innovation by Atomic Energy of Canada Limited (AECL) was the CanFlex Fuel Bundle. The CanFlex Fuel Bundle provides improved cooling water flow and allows various different types of fuel to be loaded into a standard CANDU nuclear reactor. A CanFlex fuel bundle can be configured to use natural uranium, slightly enriched uranium, thorium, plutonium, etc. An obvious application of CanFlex fuel bundles is to use spent fuel from US light water reactors in CANDU reactors. The Canflex fuel bundle potentially allows major improvements in fuel utilization efficiency and waste reduction while avoiding the greater complexity inherent with liquid metal cooled fast neutron reactors.

CANFLEX reactor fuel bundles can be configured to provide some of the fuel breeding and fuel burn up advantages of fast neutron liquid metal cooled reactors without the difficulties related to liquid metal cooling systems.

Many of the benefits of CANFLEX fuel bundle technology are currently not being exploited because the government of Canada lacks the moral fiber to stand up to a narrow segment of the population that blindly opposes transport and storage of spent nuclear fuel.

Going forward the only sustainable source of dependable power that can displace fossil fuels is Liquid Metal Cooled Fast Neutron Reactors (FNRs). These reactors are fuelled by U-238 which is 140X more abundant than U-235 and can extract 100X as much energy from one kg of natural uranium as can a CANDU reactor.

A liquid metal cooled FNR has the immense advantage that, with suitable fuel processing and recycling, the reactor is fissile fuel self sustainable for millennia and the radio toxicity decay time of its spent fuel can be reduced to about 300 years as compared to 400,000 years for a water cooled and moderated reactor. The Experimental Breeder Reactor II (EBR II) successfully operated for 30 years with liquid sodium coolant at a maximum temperature of 473 degrees C. When a liquid metal cooled FNR is operating properly there is no doubt that it is more thermally efficient, much more fuel efficient and produces much less high level radioactive waste than a water cooled and moderated nuclear reactor of similar power. However, liquid metal cooled reactors and related technology are more complex and more expensive than water cooled reactors and related technology.

1) If liquid metal leaks it makes a solid metal blob that may be extremely flammable, potentially toxic and difficult to safely remove. Liquid metal cooled reactors minimize this problem by use of a secondary and tertiary coolant loops that typically contain non-radioactive NaK, nitrate salts and synthetic heat transfer fluid. However, this more complex heat transport system increases the nuclear power plant capital and maintenance costs.

2) At 460 to 470 degrees C, the planned liquid sodium discharge temperature of a liquid sodium cooled reactor, realizing reliable pipe mechanical joints is difficult;

3) A small superheated water leak becomes a noisy steam jet. A small liquid NaK leak is quiet but potentially lethal stream of hot liquid metal;

4) The NaK pipes all need flex sections to accommodate differential thermal expansion.

5) During a prolonged reactor shutdown the parasitic pumping load of a liquid metal cooled power reactor may be greater than for a water cooled reactor. This issue becomes important in the event of an earthquake, tornado or other event that damages the nearby electricity grid;

6) Realizing the full benefits of liquid metal reactors requires fuel reprocessing which introduces process complexity;

7) Liquid metal cooled reactors frequently have coolant drain down and thermal stress relief issues that are more complex than the corresponding issues in a water cooled reactor;

8) In an emergency a water cooled reactor can be quickly flooded with water from any source. Not so with a liquid sodium cooled reactor. A liquid sodium cooled reactor must have sufficient redundant cooling loops to ensure ongoing safe removal of fission product decay heat. If there is a problem in one heat transport loop it is essential to be able to isolate that loop so that the reactor can continue operating with the remaining heat transport loops. This feature is required to achieve a high reactor capacity factor with sodium cooling. Every time a sodium cooled reactor is taken out of service for maintenance there is about a one week delay to allow Na-24 decay.

9) Liquid metal cooled fast neutron reactors should initially have core fuel containing about 20% TRU. Light water cooled and moderated reactors require only about 5% U-235 enrichment. CANDU heavy water cooled and moderated reactors can operate with unenriched natural uranium or spent fuel from light water reactors;

10) The fuel bundle design and construction of liquid metal cooled fast neutron reactors must be more rigid than for water cooled and moderated reactors because the nuclear reaction in a fast neutron reactor is controlled by thermal expansion of the fuel assembly instead of by delayed slow neutrons.

11) Liquid sodium is a serious fire hazard in air and reacts violently with water.

For practical construction, operating and maintenance reasons in the past electricity utilities have often chosen light water cooled and moderated nuclear reactors instead of more fuel and TRU production efficient heavy water cooled and moderated CANDU nuclear reactors. The issues that will eventually force abandonment of light water cooled reactors in favor of CANDU and liquid sodium cooled reactors are requirements for improved natural uranium fuel utilization and for safe disposal of high level nuclear waste.

Molten salt reactors (MSRs) are reactors that use a molten salt as the primary coolant. Molten salt reactors can theoretically operate at core temperatures in the range 700 degrees C to 1500 degrees C and hence can potentially permit direct use of nuclear energy for high temperature chemical processes such as ammonia production and hydrocarbon fuel reformation. However, MSRs face a legion of practical material problems. In a MSR the fuel is dissolved in the fuel salt and the blanket material is dissolved in blanket salt. The power level of a MSR self regulates via thermal expansion of the molten fuel salt. However, extracting heat from a MSR causes fission products dissolved in the fuel salt to plate onto the relatively cool heat exchange surfaces. There are major problems with moderator materials, reactor power stability, fuel tube materials, selective chemical fuel extraction and isotope separations.

A MSR that uses either lithium or a chloride salt in its fuel requires at least one expensive isotope separation. At this time due to material costs the practical economics of MSR technology are questionable.

Continuing Energy Emission:
The primary problem with nuclear fission energy is that the fission products are radioactive and continue emitting ionizing radiation and heat long after the fission process stops. The thermal output from fission product decay and fission daughter decay shortly after a fission reactor is shutdown is typically about 8% of the reactor's full power thermal output. The parasitic electrical load of the reactor cooling system may be as high as 15% of the reactor's maximum rated electrical output. Inability to adequately remove the fission product decay heat and fission daughter decay heat under adverse circumstances, such as a reactor shutdown in combination with a prolonged loss of grid power, has led to major nuclear reactor meltdown accidents. At Chernobyl and at Fukushima substantial amounts of radioactive dust were released into the surrounding environment. The enormous financial losses related to reactor core meltdown and radioactive contamination of the environment have made the cost of credible third party liability insurance for nuclear fission power plants extremely high.

The issues surrounding long term storage and reprocessing of used nuclear fuel bundles are yet another aspect of long term fission product and fission daughter decay.

Protection From Terrorist Attack:
The second major problem with nuclear fission power plants is potential attack by terrorists. Protecting nuclear fission power plants and their spent fuel storage bays from potential terrorist attack substantially increases both the plant capital and operating costs. Furthermore, there is never complete certainty that the anti-terrorist measures will work as anticipated.

Low Ramp Rate:
A third problem specific to water cooled and moderated nuclear fission power plants is known in the electricity industry as low ramp rate. Some of the unstable nuclear fission products and fission daughters such as Xenon-135 have high slow neutron absorption cross sections and are known as reactor poisons. If the operating power level of a water cooled fission reactor is suddenly reduced in response to a rapid fall in electricity grid load these reactor poisons may force a complete reactor shutdown lasting about two days while the reactor poisons naturally decay. In the mean time an increase in electricity grid load must be met with other forms of electricity generation. This problem can be circumvented via an engineering trick known as steam turbine bypass, but provision of steam turbine bypass capability increases the plant capital cost and operating with steam turbine bypass functioning wastes both cooling water and nuclear fuel.

Poor Thermal Efficiency:
A fourth problem specific to water cooled and moderated nuclear fission power reactors is poor thermal efficiency. These reactors typically operate at a maximum coolant temperature of 320 degrees C and are not thermally efficient, are not fuel efficient and are not waste efficient. It is easy to make an academic argument that to substantially improve thermal efficiency, fuel efficiency and waste efficiency water cooled and moderated nuclear power reactors should be replaced by liquid metal cooled fast neutron reactors. However, utility personnel who have to work with major power reactors under real life conditions sometimes have a different perspective.

U-235 Dependence:
The overriding problem specific to water cooled and moderated reactors is U-235 dependence. Reasonable uranium source projections show that after about 2050 new water cooled nuclear reactors will no longer be economically viable due to their dependence on the relatively rare uranium isotope U-235, the price of which is expected to rise substantially.

Nuclear fusion is the energy source that powers the stars. Nuclear fusion theoretically offers major public safety benefits as compared to nuclear fission. Nuclear fusion power reactors are presently at the prototype construction stage.

During the 1950s the world witnessed the development of "hydrogen bombs". The amount of energy released by a "hydrogen bomb" is truly awesome. One typical "hydrogen bomb" releases energy equivalent to about 1000 fission bombs. Single fission bombs destroyed the cities of Hiroshima and Nagasaki, Japan, in August 1945.

The primary source of controlled nuclear fusion energy is conversion of deuterium (H-2), lithium-6 (Li-6), and lithium-7 (Li-7) into helium-4 (ordinary helium) via multi-step nuclear processes that involves intermediate production of neutrons, tritium (H-3) and helium-3 (He-3). A fusion reaction releases heat that can potentially be used to produce steam, which can then be used to produce electricity.

The fusion fuels deuterium and lithium are products of solar fusion reactions. Deuterium and lithium are sufficiently common on Earth that for the purpose of fueling nuclear fusion power stations their supply is virtually unlimited. In this respect nuclear fusion energy is "renewable energy" and the use of fusion energy on Earth is limited only by the thermal radiation constraint.

A "hydrogen bomb" uses energy from a fission bomb to trigger a larger scale fusion reaction. A practical nuclear fusion based electricity generation system involves many sequential small scale fusion reactions that use only electicity as the initial energy trigger. Development of this trigger apparatus has been a long, complex and expensive undertaking which is only now approaching fruition. Recent progress has been enabled by advances in materials, electronics and understanding of semi-stable plasma configurations.

Unlike a nuclear fission reactor, when a nuclear fusion reactor is shut down its heat output goes to almost zero.

Unlike fission reactors, fusion reactors do not produce significant long lived radio active waste products.

If a terrorist attacks a nuclear fusion power plant there is relatively little opportunity for causing significant damage outside the plant perimeter. From a public safety perspective the security of nuclear materials inside a nuclear fusion power plant is almost a non-issue. These safety features potentially allow future deployment of unstaffed nuclear fusion power plants near smaller communities with potential major cost savings in both skilled labor and long distance electricity transmission.

Fusion power plants are inherently modular. The power output from each module can be rapidly changed to track changes in electricity load. If one module is shut down for service other modules at the same site can continue operating. Hence the overall system reliability is high, the output ramp rate is high and the transmission costs are low making electrical kWhs output from distributed nuclear fusion reactors almost twice as valuable as electrical kWhs output from central nuclear fission reactors.

The major technical challenges with nuclear fusion as compared to nuclear fission are magnetic materials, first wall materials, on-site light isotope separation, fire safety, vacuum systems, plasma confinement, operating temperature, heat transfer, vibration, economics and education. Full understanding of the technical issues related to nuclear fusion is confined to only a tiny segment of the physics and engineering community.

A nuclear fusion power plant inherently needs a liquid lead-lithium alloy cooling system. The lithium will liberate hydrogen gas if it comes into contact with water. In addition the liquid metal cooling circuit will also contain radioactive tritium (H-3), helium-3 (He-3), helium-4 (He-4) and gamma emitting beryllium-7 (Be-7). For safety He-4 isolated heat exchangers should be used to completely isolate the liquid metal cooling circuit from the steam/water circuit, so that a single heat exchanger fault can not cause a fire, tritium release or Be-7 release.

The additional technical complexity of a fusion power plant as compared to a fission power plant makes generation of bulk electricity from fusion appear to be more expensive than generation of electicity from fission. However, when the full costs of spent fission fuel management, public liability insurance, operating labor and electricity transmission are taken into consideration, the long term economics might favor fusion power.

Residual kinetic energy of the Earth's rotation in combination with lunar and solar gravity causes ocean tides. At locations with favorable geography tidal energy can be converted into electricity, which at the load becomes atmospheric temperature heat. The major problems with tidal energy generation are: high capital and maintenance costs, intermittant power output requiring energy storage, long transmission lines, limited geographic applicability and interference with ocean navigation.

Residual thermal energy in the Earth's core resulting from decay of radioactive isotopes can be accessed via deep drilled wells in active volcanic areas. To realize useful energy a pumped flow of water is heated by the hot magma. The resulting superheated water or steam is used to vaporize a working fluid. The working fluid vapor expands through a turbine to generate electricity. Then the working fluid vapor is condensed, releasing heat at atmospheric temperature. The condensed liquid working fluid is recycled. At the electrical load conversion of motive power into heat causes further heat to be dissipated in the atmosphere. Disadvantages of residual thermal energy are: limited geographic applicability, high cost, high thermal dissipation per electrical kWh generated and uncertain working life.

This web page last updated January 18, 2024.

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