Читать книгу Introduction to Energy, Renewable Energy and Electrical Engineering - Ewald F. Fuchs - Страница 14

Figures and Table P.1 require that the continued accumulation of CO₂ and other greenhouse gases in the atmosphere must be avoided by relying on renewable energy based on solar, wind, hydro, and biomass power plants in Germany as indicated in Figure P.5. While during summertime these renewable energy sources generate to date more than 30% of electric energy usage, this is not so during winter when clouds/fog and calm weather conditions prevail with wind speeds too low to operate wind turbines either onshore or offshore as indicated in Figure P.5, where 26 000 wind turbines and 1.2 million solar plants within the German grid failed for almost two weeks to generate expected sufficient energy. This was due to regional atmospheric high pressure resulting in fog and still air conditions as frequently occur in Central Europe during cold weather. Such a “foggy calm [6, 7]” – called in German “Dunkelflaute” – brought the grid near its limits, where 80 GW must be available to serve peak‐load conditions as indicated in Figure P.5.

Figure P.5 Electric power (generation and consumption) requirements in Germany during 16–27 January 2017. The lightly‐shaded area, which exceeds the full line indicating consumption, represents generation, while the full line indicates consumption: generation = (consumption +losses), that is, generation > consumption.

During the period as indicated in Figure P.5, about 90% of the electric energy had to be provided by natural gas, coal, and nuclear power plants, the latter two of which are scheduled to be discontinued or decommissioned during the coming decade. Such a shutdown appears to be not prudent because short‐term (e.g. battery) and long‐term (e.g. pumped hydro) storage plants [8, 9] are unable to have an output power of 70 GW even only during a few days. Note that 70 GW is the power provided, for example, by either 35 nuclear and 70 coal or natural gas‐fired plants each having a rated output of 1 GW and 500 MW, respectively, or 300 hydro storage plants with each 233 MW output power rating. Clearly such a switch from conventional (e.g. coal, natural gas, nuclear) plants to storage plants appears to be not feasible. The transition from combustion engines (e.g. gasoline, diesel) to electric drives in automobiles, trucks, and trains will lead to significantly increased additional power consumption that cannot be provided by renewable energy during the times as indicated in Figure P.5. As a result, the strategy is to keep natural gas power plants each having an output of 200 MW operating together with some of the nuclear power plants that serve as a frequency leader. The supply of electric energy of Guangdong in southern China via high‐voltage DC (HVDC) lines led to a breakup [10] of the alternating current (AC) grid due to the lack of a frequency‐leading AC power plant and through the lack of sufficient reactive power to control voltage and frequency (Figure P.6). It is well known that inverters converting direct current (DC) to AC at the point of common coupling (PCC) can supply reactive power only if the DC voltage at the DC input voltage of the inverters is significantly increased, as discussed in Chapters 10 and 12. The renewable energy approach, where 100% of the energy is generated and transmitted via DC transmission lines, appears to be clearly too optimistic: there is indeed a climate dilemma.

Figure P.6 The breakup of China's southern power grid due to the concentration of high‐voltage DC transmission lines in Guangdong, making the AC grid unstable.