Читать книгу Renewable Energy - David Elliott - Страница 19

Natural energy flows can be tapped and converted into mechanical power and then electrical energy, as in hydro projects, and by wind-, wave- and tide-driven devices. In addition, there are systems which use natural sources of heat, either directly for heating or indirectly to generate electrical energy, and finally there are devices which convert solar energy directly into electricity, using photovoltaic cells (PV solar).

I have explored these options in some detail in a recent book (Elliott 2019a), so here I will simply present a brief summary of the state of play. The most developed option so far is hydroelectric power, with around 1.2 terawatts (TW) of capacity installed globally, supplying about 16% of the world’s electricity, producing it relatively cheaply. Indeed, although they are expensive to build, once in place hydro plants often offer some of the lowest-cost electricity on the grid in many countries.

However, hydro has struggled with negative environmental and social impact assessments, and with occasional dam failures and loss of life. For example, the failure of the Banqiao Dam in China in 1975 led to 26,000 deaths from flooding. In some cases, hydro has also been beset by unreliable rainwater supply due to climate change (Moran et al. 2018). As I will be considering later, smaller projects, including run-of-the-river schemes without reservoirs, are often favoured by environmentalists since they may have lower local impacts, but hydro projects with large reservoirs can play an important ‘pumped storage’ role in grid balancing. In such schemes, surplus green power from wind or PV solar projects is used to pump water uphill into the reservoir, ready for generation when needed to meet lulls in the availability of power from the other renewable sources.

The same thing may be done with large cross-estuary tidal barrages. Tidal barrages capture a head of water behind a dam at high tide and this (along with any extra head created by pumped storage) can be used to generate electricity, as with hydro plants. Smaller artificial tidal lagoons, impounding areas of open water, are another option, and they too can be run in pumped storage mode. Although both these tidal-power generation systems are technically viable, and two medium-scale (250 MW) barrages have been built, large tidal barrages are expensive and are usually opposed by environmentalists as being too invasive (they block off entire estuaries), while as yet no tidal lagoons are in operation.

These so-called ‘tidal range’ systems operate on the vertical rise and fall of the tides. By contrast, tidal current turbines (in effect, underwater wind turbines) operate on horizontal ebbs and flows and have proved to be a more popular and successful tidal option. They have also been easier to develop than the other potentially large sea-power option, wave energy. It is harder to extract energy from waves, and from the often chaotic interface between water and wind on the surface, than from the smooth tidal flows beneath it. Nevertheless, many wave projects, as well as tidal current projects, are under development (JRC 2016) and, although costs for tidal turbines and wave devices are still quite high, we might expect them to fall, as has happened with offshore wind. So there may be GWs deployed soon, possibly 1 TW or more eventually, with low environmental impact.

While these new options are still mostly at the development stage, wind power, on- and offshore, has been the big new technology success, with costs falling dramatically and global capacity heading for 600 GW and 1 TW soon (IRENA 2019a). The rate of expansion in many parts of the world is staggering, in China especially but also in parts of the United States and much of Europe. Longer term, 5–10 TW or more may be possible, including increasing amounts offshore.

It may be a wild card but the wind resource could be dramatically expanded if airborne devices prove to work well and safely. Mounting turbines on autogyro-like flying devices or giant drones, delivering electricity to the ground by cable or using the pull from tethered kites to drive ground-mounted generators is some way off, and there are many issues. However, systems like this, perhaps used in remote offshore areas away from aviation corridors, would give access to the much larger wind resource higher up in the atmosphere (Bown 2019).

That is all very speculative. For the present, large floating offshore wind devices are the main breakthrough technology, able to operate in deep water far out to sea, where direct seabed-mounted supports would be prohibitively expensive or impossible. Systems are under test in the EU and Asia. Like offshore wind in general, they avoid the land-use and visual intrusion issues that have sometimes constrained onshore wind projects (Equinor 2019).

Biomass, in its modern usage for electricity production, has faced even tougher land-use and eco-impact constraints, particularly in relation to the impact of the use of forest-derived biomass on carbon balances and carbon sinks: replacement growth can take time. There have also been major concerns over the land-use and eco-impacts of vehicle biofuel production. As I will be describing, views clearly differ on whether biomass can be relied on as a major source of heat, electricity and transport fuel, but, as noted in chapter 1, some look to the use of bio-wastes to avoid the eco-problems and extra land use.

The International Energy Agency (IEA) says that global biomass electricity-generation capacity (including that using bio-wastes) is anticipated to reach 158 GW by 2023, and it also looks to expansion of biomass use for heating and vehicle fuel production (IEA 2018a). Given the environmental issues, that may be optimistic or even undesirable, but Fatih Birol, the IEA’s executive director, has said that ‘Modern bioenergy is the overlooked giant of the renewable energy field. Its share in the world’s total renewables consumption is about 50% today, in other words as much as hydro, wind, solar and all other renewables combined’ (Chestney 2018).

Direct solar heat use, for example via solar heat collectors on rooftops, or large ground-mounted arrays, has had far fewer problems and is heading for 500 GW (thermal) globally, with, in some cases, large heat stores offering a way to use summer solar heat in the winter, although at a price. Concentrated solar power (CSP) conversion of solar heat to power also has a large potential. It uses sun-tracking mirrors or parabolic focusing dishes or troughs to raise steam or other working vapours to drive a turbine generator. However, CSP has been less successful than PV solar so far, with only around 5.5 GW of CSP in use globally. That is despite the fact that CSP has a heat storage option (using tanks of molten salt), so that the power generators can run at night, enabling the plant to operate 24/7, unlike PV solar. This is a significant advantage, enabling CSP to deliver firm, continuous power, but it comes at a cost and with limits. Unlike non-focused PV, CSP needs direct, as opposed to diffuse, sunlight, so it is mostly used in desert areas and, although arid, deserts do have fragile ecosystems which CSP can disturb.

There is also the solar chimney option. Solar heat is collected in greenhouses in hot desert areas, with the hot air fed to an updraft chimney, driving an internal wind turbine as it rises. Small- to medium-scale prototypes have been tested in Spain, China and elsewhere but to get efficient power output the towers would have to be very tall, several hundred metres. Ocean thermal (OTEC) solar devices, which work on local temperature differentials between the surface and the depth of the seas, in effect using the sea as a vast solar heat collector, also hold some promise, but it is a very location-specific option, most relevant to warm sea areas in the Pacific. Geothermal energy from deep under ground is also location-specific but is making progress (13 GW power, 28 GW heat, so far), and the long-term global power potential is large, 2–3 TWs or more with, once the wells have been established, the local impacts being low (IRENA 2017b).

The big success story, though, is solar PV, now heading for 500 GW globally, with costs falling very rapidly, so much so that some look to mass, multi-TW deployment in the years ahead, maybe as much as 20 TW or, as explored later, even more by 2050. Some of this is due to new technology, including new, more efficient high-tech multi-junction cells with new materials, some of them, with light focusing, getting to 40% or more energy-conversion efficiency.

Even with lower efficiencies than that, more conventional PV arrays have spread around the world, including some very large projects (some over 1 GW) in desert areas but also many smaller arrays (typically of up to 20 MW) in rural solar farms, as well as many millions of individual units (of a few kW) on domestic and other rooftops.

While efficiency is important, so is cost, and there are now cheap, easier to mass-produce thin-film or dye-based cells, increasingly using non-toxic materials. They have low efficiencies so far, but they can be used for many new applications, including solar windows, with flexible, spray-on PV materials opening up many new deployment options. PV of various types is already being used for solar roads, solar car-port canopies and, increasingly, in floating arrays, for example on reservoirs (Chandran 2019). That helps deal with one of the big drawbacks of PV: once you have used all the rooftop space available, it takes up land space.

An alternative is to put PV arrays in space, in geostationary orbits, beaming energy to earth by focused microwave links (Snowden 2019). Although very costly, and technically difficult, that would deal with the other big drawback of using solar: it gets dark on one side of the planet at night. While location in permanent sunshine in space is tantalizing, possibly offering solar supply to sites on earth at night, there may be cheaper ways to achieve round-the-clock supply than by launching PV into space and microwaving power back. New storage options are now emerging, along with new long-distance supergrid transmission possibilities. It may, in time, even be possible to shunt power from the sunny side to the night side of the planet, using terrestrial interlinks between grid systems, the ‘global grid’ idea, which I will be looking at later (see chapter 4).