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Renewable Energy. David ElliottЧитать онлайн книгу.

Renewable Energy - David  Elliott


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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.

      Renewable balancing

      That brings us to grid balancing and system integration, an area that is expanding in importance as the use of variable-output renewables increases. There will be times when there are local or regional lulls in energy available from the wind and the sun and, although some of the other renewable sources are not variable, there will be a need at times to provide balancing grid power (Elliott 2016).

      One view sometime expressed by opponents of renewables is that each renewable energy project will have to be backed up 100% by conventional capacity. A moment’s thought should indicate that this is not the case. All power plants, of whatever type, can have downtimes, and we do not insist that each has its own individual backup. Instead, it is the power system as a whole, including all the other power plants, that provides backup, and it is the same for renewables. They may however need backup inputs more often than conventional plants.

      Initially, with relatively low levels of renewable input, much of this extra balancing input can be provided by the remaining conventional plants, for example gas turbines, so we will not need new capacity – it already exists. It is the main way in which supply and demand variations are balanced on the grid. With variable renewables on the grid, the gas plants have to ramp up and down to full power a few times more. Some of that capacity can be gradually changed over to using non-fossil green gas so as to avoid emissions but, as renewables expand, extra balancing capacity may also be needed.

      Options for longer-term bulk storage include compressed air and hydrogen gas, stored in vast underground salt caverns. The hydrogen can be made by the electrolysis of water, using surplus output from wind and PV plants, converted back to electricity when needed in a fuel cell or gas turbine. Similarly, spare renewable power can be used to compress air for later use in a turbine.

      An entirely different balancing approach is to convert excess renewable output into heat and store that. It is much easier to store heat than to store electricity. However, it is hard to convert low-grade heat back to electricity. That is where combined heat and power (CHP) becomes useful. CHP plants can use either fossil gas or biomass/biogas as a fuel to generate electric power, and they recycle some of the waste heat produced in that process, so they have high overall energy-conversion efficiency. Crucially, the ratio of the heat to power outputs can vary depending on demand, so as to help with balancing. If there is too much green power on the grid, the CHP plant can produce more heat, less power. If heat demand is also low, the heat can be stored. If there is not enough green power on the grid, the CHP plant can increase its electric power output and reduce its heat output, and if demand for heat is also high, the stored heat can be used. Solar heat can also be fed into the stores and so can heat produced from surplus wind and PV electricity and from geothermal sources, all of this heat eventually feeding into local district heating networks.

      This new supply and storage system would be complemented with a new demand-side management system, able to shift demand peaks to times when more power was available, for example by variable pricing so that power costs more at peak times. Optimizing it all will be hard, but as balancing technology develops it should be possible to move towards a balanced sustainable energy system at reasonable cost (see Box 2.1).

       Box 2.1 Balancing costs

      The technical costs of balancing variable renewables have been extensively studied, and one widely accepted estimate in the UK context is that they might add 10–15% to power generation costs at medium levels of renewable capacity, depending on what balancing technology is used, although it would rise at higher levels (Heptonstall, Gross and Steiner 2017).

      At present, most balancing is achieved by using gas-fired plants, their output being ramped up and down to compensate for the variations in supply and demand, sometimes coupled with pumped hydro storage, if available, and batteries for short-term storage. That may be fine, with some extensions, up to maybe a 40–50% renewable power contribution: by 2019 the United Kingdom had reached 38%, Germany was nearing 50% and Denmark had reached 55%.


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