Chapter 7 Summary Part 2: Energy Solutions by Rector and Yu, Climate Change for Astronomers

Ocean power is an approach in which energy is captured from the ocean’s waves and movements. It can involve placing a device at the surface of the ocean, such as a tidal turbine – these are analogous to wind turbines but operate underwater. Alternatively, there are various kinds of wave energy converters. 

Pros:  Ocean tides are generally quite predictable; some wave energy converters that rely on ocean waves are somewhat weather dependent (since wind creates waves), but less so than wind turbines. Furthermore, during operation, no emissions are produced from these devices. Unlike wind turbines, they don’t create much noise or impact views. 

Cons: There is a high cost associated with the maintenance of these devices due to corrosion from water. The presence of these devices may also have impacts on marine environments that need to be considered. Ocean power may be readily available to cities close to the ocean (which most cities are), however they are only suitable when strong tides are present.

Nuclear fusion power captures energy released by nuclei fusing together, in contrast to nuclear fission, in which energy is released by heavy nuclei decaying into daughter particles. Unlike in the Sun, in which the fusion process starts with the proton-proton chain, in reactors it starts with the fusion of deuterium and tritium. The deuterium and tritium react to create an alpha particle (helium without electrons) and neutrons; most of the energy in the reaction is transferred to the neutrons, which are captured by a blanket (a layer of material). The energy of the neutrons heat water, which creates steam, which spins a turbine, resulting in electricity.

Pros: The energy density associated with the fuel in nuclear fusion is even greater than that of fission, meaning that fusion is much more efficient in that more energy is available in the same mass of fuel used for fission than fusion. There is also no risk of radioactivity, nuclear meltdowns, or radioactive waste (at least in ideal operation). 

Cons: Nuclear fusion reactors require tritium to start the reactions, but tritium has a half life of ~12.3 years and is thus not found naturally. Some tritium is produced in the fusion reactors, but generally the output is not as much as the required input, leaving a bottleneck. If this problem cannot be resolved by finding another way to produce tritium, nuclear fusion is not feasible on industrial scales as an energy source. While the blanket that captures the neutrons shields the surroundings from excess neutron and gamma ray flux, some of this radiation still escapes. This damages the steel enclosures for the reactors and damages it. It can also thus create radioactive waste and components of the reactors need to be replaced.  There is a possibility that the energetic neutrons that escape the reactor can be used to build nuclear weapons.

Carbon capture refers to storing CO₂. This includes capturing CO₂ from power plants or industrial plants, before it is released into the environment. Carbon capture and sequestration (CSS) was first used to enhance the recovery of gas and oil from underground; in the process of recovering oil or gas, pressurized CO₂ is pumped underground where it ends up permanently captured and sequestered in rock formations. However, CSS can involve capturing carbon from industrial plants by mixing the CO₂ with chemicals that bind with it, which results in a solution that is heated and leaves the CO₂  as a concentrated liquid. This captured CO₂ can be stored underground. Another approach called mineral carbonation involves a chemical reaction of CO₂  with minerals to form carbonates that are unlikely to return to the atmosphere; the CO₂ is injected into a mineral-rich environment in this process. Alternatively CO₂ can be used to produce synthetic fuels that are thus carbon neutral, although this approach is unlikely to be economically viable. 

Pros: Under the right conditions captured CO₂ will be stored for thousands of years in rock formations. Carbon capture can and has been used to enhance the recovery of oil and gas from gas processing plants successfully, with 70% of proposed carbon capture operations in these plants still in operation (although there is still a net release of carbon dioxide from the recovered oil and gas in these cases). 

Cons: Since the process of CCS uses some of the generated energy from a power plant it can drive up the price of electricity. Furthermore, carbon capture has historically had a high failure rate for coal-fired power plants. This is due to large capital costs, and that this process has to go through a number of testing phases before it can be employed at an industrial scale, leaving little incentive to go through with it. There is a large financial risk associated with the CCS operations, which has led to the failure of many of them. The high failure-rate for coal-fired power plants is attributed to the decline in price of natural gas which has made coal-fired plants more expensive to operate, and retrofitting for CSS increases these costs.

Another approach is to capture the CO₂ that has already been released into the environment. For example, direct air capture of CO₂  involves pulling it directly from the air by binding it with chemicals embedded in porous filters. Alternatively, one can remove CO₂  from the ocean in various ways; one way involves promoting the growth of algae, which absorbs CO₂ and takes it down to the bottom of the ocean when it dies, or promoting the growth of coastal ecosystems like mangroves, which absorb and store CO₂ in the soil along the coastline. The planting of trees and reforestation can also achieve this.

Pros: Removing CO₂ from the ocean is efficient (if taking this approach) since the ocean absorbs so much of it. The storage of CO₂ in coastal ecosystems is also highly efficient; they can store ~100 times more carbon than rainforests. Furthermore, removing CO₂ with these approaches provides an opportunity to better manage various ecosystems, and these approaches are cheaper than storing carbon in rocks and minerals. 

Cons: Removing CO₂ from the air (in the direct air capture approach) can be challenging due to the low density, so a large volume of air needs to be processed. This process also requires electricity and heat to run. CO₂  that is stored in flora has an associated risk; natural disasters like fires can allow for the stored CO₂ to be quickly released. In general, the monitoring and verification of how much carbon has been removed in these methods is poorly standardized at present. These approaches are risky to rely upon as a sole approach to addressing climate change and require further maturation.

Energy storage with batteries can aid in storing excess energy from sources such as wind or solar power. Batteries are currently used already in electric vehicles, phones, laptops, grid-scale energy storage and more. Different kinds of batteries (lithium-ion, lead-acid, nickel-cadmium) have different ideal operating conditions, uses, and various strengths and weaknesses. As such, batteries can be designed in various ways for different purposes.

Pros: Generally, they are useful when energy output or requirements and demands vary on short timescales. Batteries in electric vehicles have bi-directional charge capability, allowing energy to be transferred between the vehicle and grid in both directions. These batteries can also hold enough energy to sustain a typical household for about 2 days. R&D is underway for batteries that recharge faster, are safer, have lower cost and are easier to recycle. 

Cons: Batteries can lose charge over time, making them less ideal for long term storage, as well as losing their capacity over time. However ‘flow batteries’ (or regenerative fuel cells) are more well suited for long term storage. Lithium-ion batteries can be damaged in extreme temperatures (hot or cold), and can be dangerous if they catch fire or explode. Lead-acid batteries have safety risks while charging. Batteries require a lot of materials including nickel, lithium, manganese and cobalt. Only Australia, Chile and China mine lithium which is the most in-demand for the growing market for these batteries. The mining of these materials creates their own environmental concerns.

These allow us to store excess energy (such as from wind or solar) and include: 

• Gravity – storing energy using gravitational potential; mass is raised when excess energy is available, and lowered when energy is required (thus turning an electrical generator). ‘Pumped hydro’ involves storing water pumped uphill in a reservoir, and accounts for 90% of global energy storage.
• Flywheels – these devices store energy in their rotational motion. They can be slowed as required to extract energy, but are more useful for short timescales.
• Hydrogen energy – this was discussed in the previous bite for the first half of the chapter, but essentially this is the storage of energy in hydrogen fuel cells, which is a device that can produce energy from oxygen and hydrogen. 
• Heat – heated water, for example, stores energy that can be used at a later time by converting it to electricity. Solar-thermal power is one such approach, in which a system collects heat during the day that can be used in the evening or when there is cloud cover. 
• Compressed air – air can be compressed and stored in a container and released (spinning a turbine) when energy is required.

There are various ways we could simply reduce our energy requirements. By following green building standards, we can reduce energy required to heat or cool buildings by optimizing their insulation, using heat-resistant materials, and reducing air leakage (see for example Energy Star or LEED). Likewise, optimal choices can be made for heating and cooling systems in buildings; load on these systems can be reduced by using ‘dynamic glass’ that changes in opacity in response to sunlight. Energy-efficient LEDs can reduce electricity usage, and building automation systems can monitor and control the use of lighting, heating and air-conditioning. Newer buildings can be designed to maximise natural ventilation, block or allow natural sunlight, use more sustainable materials, and take the impacts on the surrounding environment into consideration. Homes built with more efficient energy usage can also have the cost of their electricity bill reduced. Wasted food also contributes to energy loss and inefficiency, and is responsible for about 8% of greenhouse gas emissions. Reducing global food waste will thus reduce emissions.