Rock power: Geothermal power simulations for schools

Elizabeth Devon (Retired Earth science teacher elizabeth@earthlearningidea.com), Chris King (Emeritus Professor of Earth Science Education, Education Department, Keele University, Keele, ST5 5BG, UK. chris@earthlearningidea.com), Peter Kennett (Retired Earth science teacher , peter@earthlearningidea.com)


Abstract

Earth Learning Idea is a web-based, global teaching resource. It provides freely-downloadable activities for teachers of science, geography and related subjects. All are written in the same format, with the activity described first, followed by full teacher back-up, including context, pupil learning outcomes, underlying principles and thinking skills. The activity described here models geothermal power sources and challenges pupils to say whether they are renewable or not. The apparatus, involving a density can and gravel, is simple to assemble and can be used to simulate ‘hot dry rocks’, ‘hot wet rocks’ and hydrothermal power. Ground source heat pumps are also discussed. The statement, found in many science textbooks, that ‘geothermal energy is renewable’ is debated.


Earth Learning Idea, accessible at http://www.earthlearningidea.com/, was set up in May 2007, for the International Year of Planet Earth, with the intention of reaching as many children throughout the world as possible, particularly those who suffer from lack of resources and from lack of thought-provoking teaching. The aim is to foster a better knowledge of the natural world and how it works, encouraging the joy of knowledge about the Earth in those who may not otherwise have the opportunity to receive it.

Earth Learning Idea publishes a new Earth science teaching activity every two weeks. By the end of 2016, 251 activities had been published in English and another 20 activities were in preparation. All are written in the same format, with the activity described first, followed by comprehensive teaching notes, including context, pupil learning outcomes, underlying principles and thinking skills. All activities are made available as free-to-download pdf files with a download rate of more than 40,000 per month and the total number of downloads approaching 3 million (King, 2017; King et al, 2007, 2010, 2013, 2014). The website carries, or links to, more than 820 translations of activities, facilitated through a voluntary collaborative global network. The translations are chiefly in Spanish, Catalan, Portuguese, Norwegian, Italian, Japanese and German, but also in Polish, Slovakian and South Korean. We are hugely grateful for the efforts of our colleagues around the world in translating all these Earth Learning Ideas.

The activity below describes an edited version of just one of the 250+ Earth Learning Idea activities – focussed, in this case, on geothermal power.

Rock power: geothermal power simulations

Modelling geothermal power sources – renewable or not?

Add water to a heated gravel-filled density can, to model three types of geothermal power source, like this:

Fill the density can with coarse permeable gravel and carefully insert a vertical PyrexTM tube until it nearly reaches the bottom, as shown in the diagram, Figure 1.


 

Figure 1: The apparatus (photo courtesy of Chris King).


Then heat the apparatus up in an oven or on a hot plate (e.g. to 100 oC); be aware of the safety risk from the hot can. Once hot, add a thermometer or temperature probe to the top of the gravel, and get a container ready with a thermometer to catch overflowing water and measure its temperature.

Then use this apparatus to model the following forms of geothermal power source, Figure 2:


Figure 2: The geothermal power model ‘in action’ (photo courtesy of Chris King).


‘Hot dry rocks’

Model this by adding water steadily to the PyrexTM tube and catching the overflow, whilst monitoring the temperature of the gravel and overflow water. ‘Hot dry rocks’ are rocks like granite that have become warm over millions of years of decay of the radioactive minerals they contain. The heat can be extracted by drilling two boreholes into the granite, ensuring these are connected by cracks, then pumping water around the system.

‘Hot wet rocks’

Simulate this as described above, but first, fill the hot can with water until it just overflows, then leave it for some time (e.g. 5 minutes). This models how water trapped in deep permeable rocks (aquifers) that are insulated by thick sequences of overlying rocks, can accumulate geothermal heat. ‘Extract’ the heat by adding water to the PyrexTM tube, as above, monitoring the gravel and overflow water temperatures.

‘Hydrothermal power’

To model this, stand the density can on a hot plate and repeat the activity. Hydrothermal power is extracted where there is a source of geothermal heat near the Earth’s surface, as found in places like Iceland, Italy, Japan, New Zealand and the Yellowstone area of the USA. ‘Extract’ the heat from the model by adding water down the PyrexTM tube, as above.

Alternatively, run just one of these models, and use the findings to discuss how the other two might work.

Now ask the pupils to use what has been found from these simulations to discuss which, if any of these geothermal power sources is renewable.

Finally, discuss the proposition, found in many science textbooks, that ‘geothermal energy is renewable’.


Teaching notes

Title: Rock power: geothermal power simulation.

Subtitle: Modelling geothermal power sources – renewable or not?

Topic: Using a density can filled with gravel to model different forms of geothermal power source.

Age range of pupils: 14-19 years.

Time needed to complete activity: 15 minutes per run.

Pupil learning outcomes: Pupils can:

  • describe the different situations in which geothermal power can be extracted from rocks;
  • explain how a density can of hot gravel can be used to model these forms of geothermal power;
  • discuss whether or not these forms of geothermal power can be regarded as renewable.

Context:

These simulations clearly show that:

  • ‘hot dry rocks’ geothermal power is not renewable, since the temperature of the gravel steadily falls as heat is extracted by the overflowing water, so that the temperature of the overflowing water also falls, over time. This is because the heat is extracted much more quickly than it is being generated by radioactive decay in the rock.
  • ‘hot wet rocks’ geothermal power is not renewable because it taps into ‘fossil heat’ accumulated over recent geological time, as a rate much faster than it is being accumulated.
  • ‘hydrothermal power’ can be extracted renewably, if heat is removed at a slower rate than it is accumulating from the heat source below. However, most hydrothermal power stations extract heat more quickly than it is accumulated, so they only have a finite life and will eventually close. In these cases, heat is being extracted at non-renewable rates.

Note: You can do the first demonstration just by pretending the density can has been heated, by touching it and pretending to burn your fingers – whole classes have been convinced by this!

A fourth source of power, which is sometimes described as ‘geothermal’, is ‘ground source heat pumps’ where water from an underground or surface source has its heat extracted for warming buildings and is recycled. However, since some 98% of the power in these systems comes from solar heating of the Earth’s surface, and only around 2% is true geothermal power from the Earth, this is not geothermal power in the sense described above. Air-source heat pumps are also available, where heat is extracted from the air, rather than from the ground.

Following up the activity:

Ask pupils to research how ‘ground source heat pumps’ work, and whether or not this power source can be described as ‘renewable’. It can, since heat can only be extracted at the same rate as it accumulates.

Underlying principles:

  • The Earth generates heat, called geothermal heat.
  • The Earth’s heat is generated by radioactive decay in the rocks of the Earth (with a component of original heat from the formation of the Earth).
  • Earth’s heat flows to the surface and can be tapped through the three forms of geothermal power, described above.
  • Such power is usually not renewable, because the heat is normally extracted at a faster rate than it accumulates.

Thinking skill development:

If one run of the activity is carried out, and pupils are asked to use this to discuss how the set-up might behave in the two other circumstances, they will use the mental ‘construction’ of one model and apply it to the two other models. This will provoke pupils into considering and re-working their pre-conceived ideas of how geothermal power is generated. Discussions around the models, and their links with reality, involve more powerful knowledge of the principles and processes. Linking each model with its ‘real world’ operation develops a greater awareness of how geothermal power works in different geological contexts.

Resource list:

  • a large density or displacement can (density cans are also called ‘Eureka cans’ because they are designed to use the Archimedes method of measuring density)
  • permeable coarse gravel to fill the can to above spout level
  • a PerspexTM tube long enough to penetrate nearly to the bottom of the gravel, and stick out of the top
  • an oven or hot plate (a hot plate is needed for the ‘hydrothermal power’ simulation)
  • heat-proof mitts or gloves to be used in moving the hot container
  • a thermometer or temperature probe to go in the gravel (reading to higher than 100 oC in case the can is heated to over 100 oC)
  • containers to catch the overflow (e.g.. several measuring cylinders)
  • a thermometer to go in the overflow containers
  • a sink or basin
  • a source of flowing water

Useful Links:

Earthlearningidea: ‘Power through the window:  which power source might be built in the view you can see from your window?’

Source:

The model was described by Adrian Cook in the Earth Science Teachers’ Association’s ‘Science of the Earth’, ‘Rock power! – geothermal energy resources’ unit (1991), published by Geo Supplies, Sheffield. It was based on an activity originally described in ‘Introducing Earth Science’ by James Bradbury (1986) published by Blackwell, who gave permission for its use.


References

King, C. (2017) Fostering deep understanding through the use of geoscience investigations, models and thought experiments – the Earth Science Education Unit and Earthlearningidea experiences. In Vasconcelos, C. (Ed), Geoscience Education: Indoor and Outdoor, pp. 3-23. Switzerland: Springer.

King, C., Kennett, P. & Devon, E. (2014) Earthlearningidea – 500 translations and more than a million downloads. Teaching Earth Sciences, 39(1), 56-57.

King, C., Kennett, P., & Devon, E. (2013) Earthlearningidea: a worldwide web-based resource of simple but effective teaching activities. Journal of Geoscience Education, 61, 37-52.

King, C., Kennett, P. & Devon, E. (2010) Earthlearningidea 1 – taller presentado en el XVI Symposio sobre Ensenañza de de la Geologia (Tereul 2010) basado en activdades de Earthlearningidea. Ensenañza de las Ciencias de la Tierra, 18(2), 160-165.

King, C., Kennett, P. & Devon, E. (2007) Earthlearningidea for the International Year of Planet Earth. Science Education International, 18, 209-216.


This article has been published in European Geologist Journal 43 – Geothermal – The Energy of the Future

Read here the full issue: