Renewable Energy

I see that at one time on 7 October 2015 Denmark was generating 140% of its local electricity demand from wind turbines enabling it to export to neighbouring countries. The country has a population of only 5.6 million, but a long coastline suitable for offshore generation. The rated capacity of its wind turbines was only 4.8 GW at that time. The country aims to eliminate fossil fuel burning for both electricity and transport by 2050. As a small country operating within an international grid there does seem scope for the former. I wonder how this will pan out in the whole region, with renewables also being pushed strongly in Germany and if offtake from renewables is prioritised over that from fossil fuel baseload generators, which seem almost an endangered species even if still very necessary. Advances in storage would greatly assist.

A more recent story from Denmark is the record (for a commercially available offshore wind turbine) of 215,999.1 kWh produced in 24 hours by a single MHI Vestas Offshore Wind turbine. Its 35 ton blades are 80m long enabling it to sweep an area of 21,125 sq m. Given that the turbine rating was just 9MW this is a rather impressive 24 hour output. [NB For those who like to check such numbers I calculate the radius of a circle with an area of 21,125 sq m to be almost exactly 82m. The 2m discrepancy from the above number is presumably accounted for by the hub but the hub is presumably not a part of the 231,125 sq m swept area that can be driven by wind action and thus not a productive part of the area. If this seems pathetic please send me more useful work!)

Meanwhile Pierre Pretorius, Vestas Eastern Africa’s Project Director for the Lake Turkana Wind Project in Kenya, Africa’s largest, tells me that they have now hit the 335 turbine mark with the last turbine due for installation by 8 March. It will then await the Kenyan government’s completion of the line to Nairobi. His pictures below give some idea of these giant towers stretching into the distance in this remote corner of Kenya and the equipment needed to handle the monster parts.

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Renewable energy (except in minor instances such as biomass) is of necessity generated at the source of its power input (sun, wind, water, geothermal etc) unlike fossil fuel which can be transported to the generating site. This then requires more long distance transmission of electricity. In the early days of long distance transmission this was in the form of AC, in particular because of the relative ease of converting to high voltage for transmission and reduction for retail distribution, compared to DC. But AC has some disadvantages. Thus the cable under the English Channel linking UK and French grids is DC because AC lines through water create electromagnetic interactions, dissipating power. Additionally the alternations in AC require larger cables than for DC, increasing costs and requiring larger pylons. A further problem is that AC power tends to fan out through multiple channels given the chance. Thus renewable energy generated in north Germany destined for the south of the country (where nuclear plants are being decommissioned) tends to inconveniently spread into the Polish and Czech grids. Somewhat offsetting the cost savings of DC transmission is the high cost of the thyristor packed converter stations ($1 billion for the stations on the 4,000 MW Plains and Eastern line in the US).

The current (sorry) answer to these issues is ultra high voltage DC. China has been prominent in developing this for, in particular, transmitting the power from hydroelectric plants in the west (also rich in coal for thermal stations) to the populous eastern parts. One line under construction will transmit 12,000 MW at 800,000 volts.

Renewable energy is still encountering problems or resistance (sorry again). Thus 40,000 homes and businesses were cut off in South Australia a week or two ago when people were returning home after a day when temperatures reached 41° and the wind died down. However the South Australian authorities blamed this on the national market regulator making a commercial decision not to run available gas turbine capacity at high cost, rather than on an over reliance on wind power. (On 10 February the temperature in New South Wales and Canberra reached 47° but no power cuts were reported after appeals were made to refrain from cooking or watching TV. I’m reminded of the request in South Africa’s drought stricken Cape Province not to wear surfer shorts in municipal pools as they absorb more water than speedos.)

(MHI Vestas via Digital Trends, The Economist, Reuters, The Guardian, The Saturday Star) 


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