Journey from the Centre of the Sun

Click image for a larger version. Some of the stages in the energy conversions and transfers that allows me to have hot water in the mornings without any carbon dioxide emissions. Simple heh?

Friends, just the other day I wrote about how my heat pump produced hot water each day.

The way in which the heat pump extracts heat from the air is ingenious in the extreme.

But as I reflected on it, I realised that this ingenuity occurred in the middle of a long series of energy transformations taking – very roughly – 170,000 years.

Please allow me to explain.

#1 Where does the energy come from?

The source of nearly* all the energy humans exploit on Earth is sunlight.

Using sunlight and carbon dioxide plants produce oxygen (thank you) and carbohydrates. This so-called ‘photosynthesis‘ captures energy from the sunlight in the form of re-arranged chemical bonds within carbohydrate molecules.

When we use animals for work – horsepower and ox-power – the energy the animals use is derived from the carbohydrate molecules in their food.

When we burn plants – primarily wood – for heat, the energy released is from the reverse of the reaction that created the carbohydrate molecules. And so, the carbon dioxide which was captured when the plant grew, is released. But since burned wood is generally only a few decades old – burning plants can be (almost) neutral in the production of carbon dioxide.

Fossil fuels are all derived from plants, and the energy of the captured sunlight has been ‘distilled’ over thousands of years by a variety of physical processes into coal, oil and gas. Unfortunately, burning fossil fuels also releases carbon dioxide, but not carbon dioxide that was recently captured. It releases carbon dioxide that was captured eons ago.

Burning fossil fuels is still the main way in which we make electricity – so electrical energy is in some sense the energy of ancient sunlight. How charming.

#2 Where does the energy of sunlight come from?

That the Sun is hot has been obvious to all humans since the dawn of time.

But the source of its immense heat was a mystery until just about 100 years ago when it was suggested that hydrogen nuclei (a.k.a. protons) might ‘fuse’ together to make helium nuclei, and release energy.

Segueing past a few decades of research and speculation, we now know for sure that nuclear fusion deep within the Sun is indeed the source of the energy that makes the Sun hot.

The environment deep within the Sun is extraordinary, with a temperature of roughly 15 million degrees Celsius.

The hot dense gas emits electromagnetic radiation – γ-rays, X-rays, ultra violet and visible light – in all directions. The nuclei and electrons in the Sun are a plasma – which is opaque to radiation. So the radiation is constantly absorbed, causing local heating, and then being re-emitted by the nuclei and electrons in the various layers of the Sun.

Because the Sun is so vast and so opaque – it takes roughly 170,000 years for the energy to travel from the core of the Sun to the surface of the Sun. Just so you know it was not a typo: I did indeed say 170,000 years.

Eventually the energy reaches the outer layers of the Sun which are at a tepid 5,500 °C (ish). The glow of this hot plasma sends visible light out in all directions and after roughly 8 minutes, a tiny fraction of it reaches Earth.

#3 My hot water: a summary

Click image for a larger version. Some of the stages in the energy conversions and transfers that allows me to have hot water in the mornings without any carbon dioxide emissions. Simple heh?

So where does the energy that heats my hot water come from? The letters in the bullet points below refer to the diagram above.

  • Nuclear fusion (A) around 170,000 years ago created energy from the fusion of hydrogen nuclei that were themselves created in the primordial ‘big bang’.
  • This energy travelled through the Sun’s layers as a variety of forms of electromagnetic radiation – γ-rays, X-rays, ultra violet and visible light – until it reached the outer layers when the radiation could travel uninterrupted into space (A, B).
  • A tiny tiny fraction of this radiation was intercepted by solar panels on my roof, which converted some of the visible light into an electrical current (B, C, D).
  • This electrical energy was stored in a battery by electrically forcing ions of lithium metal (shown in the figure as red dots) to cluster together against their desire to diffuse away from one another (E).
  • This energy was then released at night by allowing ions of lithium metal to diffuse away from one another (F), forcing electrons around an external inverter circuit that created AC currents to power a motor in a compressor (G) and electronics which ran the heat pump.
  • The heat pump then chilled a refrigerant (G) that extracted heat from molecules in the air that had also been heated by sunlight over the preceding few days.
  • This energy was transferred as heat to water flowing in a circuit through the heat pump (G).
  • And then this energy was further transferred as heat to fresh water in a hot water tank (H).

And then finally (I) a few hours later, this energy was transferred to the outer layers in my face and hands where transient receptor proteins in thermoreceptors in my skin sent signals to my brain that caused me to realise the water coming from the tap into the sink was ‘just right’.

Simple heh?

* Nearly?

Humans do exploit one source of energy which did not originate in the fusion of nuclei within the Sun: nuclear power.

Nuclear power exploits energy released by splitting the nuclei of heavy atoms that were created – as I understand it – during the last few destructive moments of a previous generation of stars.

These elements – uranium primarily – were then deposited on the primordial Earth as it formed at the same time as the Sun was ‘born’.


5 Responses to “Journey from the Centre of the Sun”

  1. Sam Gibbs Says:

    I always tell my students we are solar powered…….it takes the “stepping stones” of the process you have beautifully outlined here to make that clear. We also discussed heat pumps the other day and a student asked how this worked in winter……The critical element is that energy can still be extracted from anything that is above absolute zero. Am i correct in saying this?

    • protonsforbreakfast Says:


      Yes, heat pumps can operate at all temperatures. A 3-He dilution refrigerator can run from around 0.100 K down to about 0.001 K!

      The key trick for normal heat pumps is to understand that they working fluid – a substance that circulates within the heat pump which is neither degraded or lost – but constantly recycled. In my heat pump (Vaillant Arotherm 7 kW), this working fluid is propane.

      You can see for yourself how the pump using a relative of propane, butane – this is the working fluid in my fridge. YOu can buy butane as ‘lighter fluid’ in a pressurised can.

      At room temperature in the can butane is pressurised to about 2 bar, and is a liquid. When it expands through the nozzle, it expands and cools – so much that it re-condenses as a liquid at around – 2 °C (if I remember correctly). If you squeeze it into a balloon you can capture a few cubic centimetres. Now seal the balloon. Because the liquid is colder than room temperature, heat flows from the air into the liquid. So the liquid captures heat from its surroundings and becomes a gas – you can see the balloon inflate.

      In a heat pump, this gas is then re-compressed and gets hot. You can’t do this compression thing in a balloon but the heat pump is configured to do this. So heat has been captured from the room temperature air and moved to somewhere hotter!

      Best wishes

  2. Dan Grey Says:

    I believe there are actually four sources of energy on earth; fusion/solar & fission, but also geothermal heat (part fission/part leftover heat from formation of the earth) and tidal, the kinetic energy of the spinning earth made accessible by the drag of the moon, exchanging PE to KE as it spirals in towards us.

    All are products of gravitational potential energy, I guess. What set that up? I suppose at that point you’re looking at process like cosmic inflation and whatever the Big Bang was…

    • protonsforbreakfast Says:

      Mmmm. I think you are right. Thank you.

      In my mind I had lumped tidal energy in with other renewable energy sources, but its regularity should have alerted me to its different origin.

      I think I may need to write another article!

      Best wishes


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