It seems almost simple – all you need to do is connect a number of hydrogen atoms to one helium atom, and there you have cheap and clean energy. The sun has been doing it for five billion years, so how hard can it be? The problem is doing it without having the mass of a star. Whoever will find the answer will win the Nobel Prize.

It is tough to imagine life without it. It allows plants to grow and gives us food, provides us with heat and vaporizes water that precipitates into fresh drinking water. But what enables the sun to emit a tremendous amount of energy for billions of years? Why is it so hard to "emulate" the physical process that occurs in the sun to produce energy?

The stuff the sun is made of

The sun’s mass is seventy four percent hydrogen - a flammable gas that is the first chemical element in the periodic table (and therefore the lightest), and twenty four percent helium, an noble gas that is the next element after hydrogen in the periodic table - it is also used for filling helium blimps and balloons. Its name, by the way, comes from the Greek word for sun – helios. The remaining two percent includes oxygen, nitrogen, carbon, neon, iron and other elements found in only small quantities. It was in 1925 that Harvard scientist Cecilia Payne-Gaposchkin studied the sun’s spectrum and discovered its composition.

The discovery that the sun is made mostly of hydrogen and helium was the basis for what is now called "the standard solar model", proposed by Jewish scientist and Nobel Laureate Hans Bethe. According to the model, four hydrogen atoms become a pair of helium atoms, and in that process the mass that is lost is converted into heat energy. According to Bethe’s calculations, the radiation from this process at the current rate will allow the sun to burn for about ten billion years, and since it has already existed for five billion years, there remains another five billion years for it to spread its light and heat.

Nuclear Fusion

The transformation of hydrogen atoms into helium atoms with the emission of energy is called nuclear fusion, which also occurs in many other stars in addition to the sun.

Nuclear fusion is a process that requires energy to initiate and does not occur spontaneously, however the emitted energy exceeds the energy it requires. In stars, this energy source comes from gravitational compression, which is the contraction of the star under the influence of its own gravity. Energy is required to initiate the process since it begins from fusing two hydrogen nuclei that repel each other due to their identical electric charge (positive).

Two hydrogen nuclei can be deuterium (H2), an isotope of hydrogen containing a proton and a neutron, or tritium (H3), an isotope containing a proton and two neutrons. The merge of these two nuclei creates a nucleus of helium-4 (two protons and two neutrons) and the neutron "surplus" transforms into tremendous energy.


Nuclear fusion on the sun. Two hydrogen nuclei (top) become a helium nucleus (bottom right) while releasing energy (bottom left) | Illustration: Wikipedia

Mankind was able to harness the nuclear fusion process for destructive purposes; one of which was the hydrogen bomb – a bomb based on nuclear fusion in contrast to the nuclear fission process is used in atomic bombs like the ones dropped on Japan during World War II. So far the hydrogen bomb has not been used in any war, although the United States did test it in 1954 in the Bikini Atoll in the Pacific, whose inhabitants were evacuated at the time.

The strength of the hydrogen bomb is 100 times greater than the nuclear fission bomb, because the energy emitted by nuclear fusion is larger than that emitted by nuclear fission. The initial energy to jump-start the process of nuclear fusion is obtained from a nuclear fission bomb, which is found within the hydrogen bomb.

Cold nuclear fusion?

Beyond the enormous amounts of energy it emits, nuclear fusion has two distinct advantages over nuclear fission as a source of energy production: one is the fuel for the fusion process, which is a hydrogen isotope that is easy to distil from seawater, unlike the fission process that requires uranium, which is a very rare element on Earth. The second is the product of fusion, helium, which is a non-radioactive atom; while the fission reaction process leaves many radioactive products that require special disposal.

Alongside the many advantages of the process, there is one major limitation that prevents it from becoming a feasible way to generate energy - the initial reaction requires a huge amount of energy. Although there are now many reactors performing nuclear fusion at temperatures as high as the sun’s, they are still not effective enough. For example, the Weizmann Institute has such a device, which produces the temperature of the sun for a short time when fed hydrogen atoms. Clearly for the process to come into current play, we need to strive to perform the process in the laboratory at room temperature, so-called "cold fusion".

In 1989, two chemists Stanley Pons and Martin Fleischmann reported on a process based on the electrolysis of metals such as palladium and platinum in heavy water – water in which the “regular” hydrogen, with no neutrons, is replaced with deuterium. They claimed they discovered that the electrolysis cells produced more heat than the electrical power put into them and this supplementary energy source came from “cold fusion”. Although the announcement caused great excitement, it was soon realized that it is impossible to repeat these results, so it was assumed that the experiment of Pons and Fleischmann unfortunately did not create cold fusion.

Several years later, researchers in the United States returned to the experiment of Pons and Fleischmann and found a mistake in the formulas used to calculate the energy balance. They found that Pons and Fleischmann assumed the efficiency of the trial was one hundred percent, and this false assumption led them to conclude they have achieved cold fusion. This argument has been tested and validated by other research groups. Since then, no one has claimed they managed to create cold fusion under laboratory conditions.

It can be said that the attempt to replicate the sun’s operation on earth has not gone well. If it had been achieved, it could be an alternative to solar energy - a process that is currently highly dependent on the weather and its effectiveness is limited – only forty percent of the solar energy becomes electrical energy, even in the most advanced technology used today. Even the initial energy of this process is significantly lower than that created by the sun itself, because solar radiation is dissipated on its way to us by passing through the atmosphere. So it seems that at least in the near future we will have to count on sources of energy that are much less efficient (solar energy) or much less cheap (fuel) than cold fusion.


In space, it is possible to use solar energy more efficiently, due to the lack of atmosphere | Photograph: NASA

 


 

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2 comments

  • Patricia Watterworth

    Sun's nuclear fusion

    read that the sun's nuclear fusion initiates from its gravitational compression. Please explain, in simple terms please, how gravity compresses mass. The earth has gravity but it isn't "fusing".

  • אנונימי

    saved ny