S6C2: Renewable Energy

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a) Sustainability Vs renewability

The term of renewable energy ought to be clear enough and yet, it is easy to read more than there is into it. Renewable energy refers to sources of energy that are virtually inexhaustible because they replenish at a rate that is superior or equal to that at which they are consumed. In other words, an energy source that is not being depleted at our current consumption rate for that particular source. This rules out fossil fuels, which by their very nature are finite since they come from the decomposition of dead biological organisms, as we have seen in the previous chapter (S6 Section 1.a), and the rate at which they accumulate runs in the millions of years, a geological rather than a human time scale.

What renewable does not imply however, is the absence of pollutant emission, including but not limited to greenhouse gases, those driving climate change and global warming – a topic we covered in S3 Section 3.e. Most of the renewable energy sources happen to fare better, or even much better than fossil fuels in this respect, but every source of energy will have a carbon footprint. If not at the time of energy production, then at the stage before that when the raw materials required in the production of the equipment are being extracted, transported and when the equipment is being manufactured. In that sense, renewable energy is not clean energy, and the very concept of clean energy should be thought of as a target, one that can never be achieved but towards which we should aim by minimizing the alterations to our environment, whether land, aquatic or atmospheric. This is essentially what the concept of sustainability means in the context of energy: it should be renewable and managed responsibly so as to minimize impacts over society and our environment, the former including present and future generations, consisting of all human and non-human animals.

Let us assume now we did not know the answer to the question of which are the main renewable energy sources and think them through. In Part B of Series 1 we looked at energy under all its forms, that is under our human terminology because Nature certainly has not adopted such classification, and saw that energy is never truly created, it is merely converted. So what we need to look for are chemical, nuclear, electromagnetic, thermal, potential, gravitational or kinetic sources of energy. This opens up a lot of options indeed.

In the realm of chemical energy, we could think about food, and non-animal food such as vegetables would fit the bill although they would power our organism rather than manufactured hardware equipment. Nuclear energy could come from fission and fusion reactions and, regardless of what so called environmentalists who have not done their homework say, even accounting for the catastrophic risk and the issue of nuclear waste disposal (refer to S5 Section 9.f), statistically it is cleaner than a lot of other renewable energy sources. Clean no, cleaner yes. Now, there is a limited volume of mineable uranium on Earth but still at least several thousands of years’ worth of consumption so this is indeed an edge case according to the definition of renewable. Kinetic energy could be harvested from anything that moves and converted into mechanical energy. The obvious candidates are the motion of the air (wind), water (tides, waves and oceanic currents) and in theory tectonic plates though this is unlikely to be practical. Electromagnetic is of course the radiation originating from our star and for thermal we need look no further than the ground below our feet and the temperature present in and being radiated away from it, as described in S3 Section 1.c. For gravitational energy, we would need to rely on materials coming sown from the sky, and we do: water is used to drive turbines as it loses height. Finally, potential energy is involved in chemical and nuclear energy and it can be a means to store gravitational energy, as in a hydroelectric dam.

We are going to be looking at several of these in the following sections, in no specific order, keeping nuclear for Chapter 3.

b) Wind of change

Humans have been harvesting wind to power mills and crush grains or draw water up from wells for at least two millennia so what we have achieved with modern technologies is twofold: improving the energy efficiency of the windmills and transform them into wind turbines so they now generate electricity which can then be transported across long distances and fed into a distribution grid.

In the previous chapter, in the section on thermal power plants (S6 Section 1.f), we have explained the physical principles of the electric generator so what we need to elucidate now is how we go from the motion of atmospheric particles to the high rate of revolution of a shaft connected to an electric generator.

Here I would strongly suggest taking a look back at S5 Section 3.b titled “dancing with the wind” and the visual illustration enclosed in Figure 2A showing the breakdown of the wind force onto a sail because this is exactly what happens with the blades of a wind turbine, or the wings of an aircraft (S5 Section 5.a). The blades, typically three of them mounted on a rotor, are facing the wind and their pitch or angle of attack is computed so as to maximize lift without generating excessive drag which pushes the blades hard, thus creating stress and fatigue at the junction with the rotor and in the superstructure, a tall tower. Let’s assume we are looking at the blades from upstream (where the wind is coming from); in the majority of cases, the equipment is designed to turn in a clockwise direction. So when wind passes through a blade on the left side, it will generate an upward lift of the blade that is perpendicular to the blade and if the blade was at the top, in a vertical position, the lift would be towards the right, etc. This creates a clockwise motion of all the blades and means that at every point in time the lift force experienced by the blades is tangential to their velocity. In effect, torque is being applied! And it is this torque which drives the rotation of a horizontal shaft.

The first thing to note is that there are designs with horizontal shafts as well and those can work with winds coming from different directions, though not with the same efficiency. The second is that the blades rotate much more slowly than the revolution rate required for the electric generator. As usual, nothing a little engineering can solve and the most common solution is the adding of a gearbox to achieve the desire ratio.

All of this sounds pretty middle-tech and yet, this is not so. We are talking stupendously large structures with blade length in excess of 100m and rates of energy transfer of up to 15MW per offshore turbine. It takes a lot of research and development to build and design materials that are light yet strong enough such as carbon and glass fibres as well as rotors and gearboxes that can withstand the mechanical abuse exerted by the weight of the blades and the wind drag upon those. Startingly, and as importantly from an economic standpoint, modern equipment achieves up to 75% of the theoretical limit of how much energy can be extracted from wind power – this is called the Betz’s coefficient and it is equal to 16/27 (~59.3%) of the kinetic energy flowing through the surface “catchment area” of the blades.

This strongly suggests the most important factor in electing the location of wind farms, the name for a set of wind turbines, is wind velocity and the consistency of this velocity. Consequently, favoured spots would be those exhibiting significant and recurrent temperature differential since this is what causes pressure gradients and therefore wind. The usual suspects would include mountain ridges and bands near oceanic shores due to the difference in thermal mass between sea and land.

Besides the renewable aspect and the absence of any emission whatsoever onsite, the main advantages of wind power is that the electricity generation cost has fallen to below US$0.10/kWh and the wind farms do not prevent the usage of land when situated onshore. Unfortunately, the same cannot be said about the impact on birds and, less studied and advertised, the underwater vibrations (a.k.a. noise) created by the vibrations of the structures that affect marine life. Also, once the novelty wears off, they quickly become unsightly.

c) Hydroelectricity

The motion of water, termed hydropower, has also long been harvested by human civilizations to drive relatively simple mechanical equipment such as flour mills. However, after a brief interlude where power was transferred through liquid pressure at short distances in the midst of the First Industrial Revolution, hydropower as we know it developed in tandem with electrification, starting in the latter decades of the 19^th century.

Now that we understand how the rotation of the blades in a turbine can be made to generate electricity, it is easy to see how the flow of a river or the kinetic energy embedded in a waterfall can be tapped to move the blades of said turbines through simple, direct physical pressure. Where no reservoir is present, a hydroelectric facility would be called run-of-the-river, and when a reservoir has been created we refer to the construction as a dam. The point of a dam is not to create energy, it is to store it in the form of potential energy: it is kinetic energy in-waiting. We’ll get back to this point in a short while.

If you have seen a hydroelectric dam before, you will have noted there is little water going past the dam, or so it seems because in fact the opening is near the bottom of the reservoir, on the inner side of the dam. From there, water is channelled towards the turbines which drive the electric generators and the electricity produced is passed into the grid via wires while the water continues its odyssey in the riverbed.

Depending on the country in which you live, you may or may not be surprised to know that hydropower supplies about 15% of the entire electricity globally, and it accounts for about a full half of the entire renewable energy supply. Some of these facilities are indeed gigantic and the largest of them all is currently the Three Gorges Dam on the Yangtze River in China’s Hubei Province. It has a nameplate installed capacity of 22.5GW (32 turbines capable of producing 700MW plus 2 smaller ones of 50MW each to power the plant itself), and both it and the Itaipu Dam on the Paraná River marking the border between Paraguay and Brazil have produced in excess of 100 TWh of electricity in a single calendar year.

Hydroelectricity is often billed as a model sustainable source of energy, and yet, this is not always the case. While there is no emission of pollutants during production, the construction of a dam requires extremely large quantities of concrete and earth-moving, and this is energy-and-material-intensive to produce, transport and operate. Furthermore, like all technologies, its impact can also be felt if not wielded responsibly. The creation of large dams and reservoirs may subtract arable land for the local population but, more importantly, it can wipe out or disturb entire ecosystems, both upstream, in the flood zone, and downstream where the traditional rate of flow of water is being altered. Plus, there is always the risk of catastrophic dam failure though, from a pure statistical perspective, this is not a meaningful issue provided the construction is carried out competently.

On the other hand, what makes hydroelectricity very popular is its versatility. Granted, there needs to be a river and therefore upstream precipitations, but this is not a one-size-fits-all and the plants can be small, or even very small (termed micro and pico hydro respectively) and thus developed in relatively remote parts of the world with challenging access to serve small, local communities.

The other major draw of dams, as mentioned earlier in this section, is that they can act as energy storage. This comes in handy to supplement a grid when it faces peak demand and it avoids the need to build other types of power plants that can meet this peak level on an ongoing basis. This means other sources of energy such as thermal and nuclear power plants can ensure the base-load provisioning and hydropower plants can release water as and when required to meet fluctuations above the base-load level. In fact, when there is spare base-load capacity available, at times of low demand, some hydropower plants will pump some water back up into the reservoir, thereby re-storing energy.

d) It comes from the Earth

If wind and hydropower are dependent on atmospheric conditions, i.e. on the weather, geothermal energy is conducted continuously all the way to the surface after which it is radiated into the atmosphere and ultimately space, at all hours of the day and in all seasons. Part of the energy contained below the crust, inside the mantle and the core, comes from the original planetary accretion but the majority is the result of the ongoing radioactive decay of materials. The numbers are not trivial, the thermal energy flow is computed to be 44TW out of a total heat content of 3×10¹⁵ TWh. This meets the renewability criteria, no problem there.

The reason we don’t harvest more of this heat is twofold. Firstly, it has to do with the low average density of the flux: only 0.1MW per km² so the technologies become economical in areas of higher concentration, at the boundaries of tectonic plates that tends to be geologically active, and by tapping into heated fluid media such as hot springs or magma conduits. Secondly: heat is not sought after everywhere and in every season so it is no surprise geothermal energy is more popular in regions located at high latitudes.

The bulk of the utilization of geothermal energy made by humans is for the heating of water or spaces. For this, ground source heat pumps (also known as geothermal heat pumps is some regions) are installed, generally in a shallow layer around 10 to 20 metres underground – certainly below the frost line and at a depth where temperature doesn’t vary much or at all depending on the time of the year – and water is made to circulate in coiled pipes where it gets heated by conduction. The heated water is then pumped back up, so this does require some electrical power to operate, and the heated water can then be stored in tanks for later use or be made to run under the floor or through radiators to heat spaces. Onsens and variant bathing facilities are also a major application of this heat source. Interestingly, the process can be reversed to cool down spaces and if you want to read more about this I include a link to the Wikipedia entry on ground source heat pump at the end of this chapter.

Where heat is superior to 150 °C, this becomes interesting for the industrial production of electricity. These geothermal power plants tend to dig wells much deeper underground and run water through pipes to capture heat from the ground or some liquid at depth before pumping it back up and using the water, now in pressured steam form, to drive steam turbines and generate electricity in a manner similar to the thermal power plants seen in S6 Section 1.f. The spent water vapour is then condensed into a liquid state and injected back into the ground. Cleverly, even when temperatures are not sufficient to vaporize water into steam, it is possible to dissociate the roles of conducting and working fluids. Water is used to conduct heat back to the surface and a heat exchanger transfers some of this heat to a working fluid, other than pure water, with a lower vaporization temperature threshold. It is the pressured vapour of this working fluid which drives the turbines. This method is called binary cycle.

The main advantage of geothermal energy is the high efficiency per volume of steam but the economic drawback is the low density of the flux so that the economics only work in certain locations with high utilization rate and propitious underground heat sources.

e) Solar power

We use sunlight in two different manners to produce electricity. The indirect one, which I will only mention in passing now, consists in tapping its heating properties. Essentially, light beams are concentrated into a specific area via a system of mirrors and this will heat up a heat engine such as a steam turbine, in a manner analogous to a steam engine or the fuel in a thermal power plant. This technology is called concentrated solar power and I am including a link to the relevant Wikipedia entry in the last section for those interested in learning more about this.

The method we are more familiar with, and that has the largest installed capacity, relies on the photovoltaic (PV) effect to generate electric current. If you have read S4 Section 7.d on digital sensors, you would already be acquainted with the photoelectric effect whereby the absorption of electromagnetic radiation above certain frequencies trigger the emission of electrons. The trick to derive electric current from PV is thus to manage the movement of those electrons. This is done using materials with a built-in electric field such as semiconductor p-n junctions (S4 Section 1.c) – and note that electric field suggests an imbalance of electric charges, not the existence of an actual flow of electrons. The freed-up electrons, those that have been “photo-emitted”, move toward one side of the junction leaving electron holes on the other side and this difference in charge concentration gives rise to a voltage. Therefore, all that is left to do is close the circuit between those two regions so electrons can go and fill up the electron holes (again), producing direct electric current in the process. It so happens that the heating up of the materials created by the electromagnetic radiation also has an impact in terms of generating voltage but I will not go into this here, however if you wish to know more about the thermoelectric effect, I include a link to the relevant Wikipedia entry at the end of the chapter.

There are many factors influencing the overall efficiency of solar panels, defined as isa reference to how much of the energy contained in sunlight they can convert into electricity. These include reflectance of the surface, the ability of the materials to separate charge carriers and keep them apart, and the thermodynamic efficiency limit – this has to do with frequencies below the minimum threshold required for photoemission and the coefficient is equal to about 86% for the light originating from our star. In the real world, the best recorded efficiency of solar panels is now approaching 40%.

Unsurprisingly, the main material used in these PV panels is the same as in semi-conductors, it is silicon in crystal form. However, thin-film technology instead relies on the layering, as shallow as a few nanometres, of other semiconductor compounds. The most prevalent of these are cadmium telluride, copper indium gallium diselenide, and a non-crystalline version of silicon. Either way, the cells are clustered into large panels and those are either mounted on rooftops on commercial or residential building, or laid in arrays to create solar power farms.

By the end of 2022, the total installed capacity was about 1.2TW and more than 400MW were added in each of 2023 and 2024, suggesting solar will soon become the main renewable energy source and it is now a better financial alternative to install solar panel farms than to build new coal-fired power plants.

The case for solar power is very strong in place with high irradiance, which is the measure of electromagnetic radiation coming from the Sun measured in watts per square meter. This depends on latitude and climate and the tropical and equatorial regions are those recording the maximum levels on Earth.

There is no free lunch however, and solar power comes with its own drawbacks. The first is the mining of the materials and the manufacturing process which is energy intensive though the return on investment from a greenhouse gas emission standpoint has a rapid payback of less than one year. More worryingly, the upgrading of old panels for new ones, and the future discarding of defective ones, is problematic from a waste disposal perspective, especially for the thin-film versions containing heavy metals such as cadmium. You can refer to S5 Section 9.e on solid waste treatment to understand what it takes to prevent or contain leaching in a landfill for example. So the strategy here should be to reuse where possible and to develop adequate recycling logistic chains and technologies.

f) Trivia – Water in motion

If we can tap the motion of air, that is wind power, surely we can harness the predictable and recurrent movement of water we know as tides and currents. Indeed we can, and there are a few tidal power plants located within estuaries or along seawalls used to prevent flooding. We are not talking small installations; the two largest, in South Korea and France, have capacities of 254MW and 240MW respectively. The principle is very much the same as for wind turbines but the spots with the necessary combination of flow velocity and tidal range are scarce and installation costs are quite high. Furthermore, there is the issue of the impact on marine life: it obstructs a natural passageway, the turbines can be lethal and they create underwater noise.

There is much broader array of potential sites when it comes to marine current power and yet the technology has not really been deployed despite an estimated combined power of 5,000 GW. Unfortunately, other than the blocking of the way, these hydro-turbines will come with the same flaws as the tidal equipment in terms of noise and hazardousness. Moreover, at scale, such installations would reduce the flow of currents, which would undoubtedly cause disruptions to existing marine ecosystems.