Mark Jacobson and Mark Delucchi published a claim that all the world's energy needs in 2030, allowing for projected economic growth, can be met with wind, water and solar power. They assume that energy efficiency can reduce demand for energy by 5-15% by 2030.--Editors
Advocates of renewable energy technologies frequently refer to the many available and potential ways of reducing the effect of variability of renew able energy. However they usually do not show that these could be combined to enable constant energy delivery to the grid despite the magnitude of the shortfalls that typically occur in supply from renewable sources. Jacobson and Delucchi (2011a, 2011b) list possible strategies but do not show that these can provide the necessary quantities of energy to plug gaps in supply.
The magnitude of the variability problem
There are periods when there is close to no wind blowing anywhere in a large region, and these times can last for many days. Weather tends to comes from the west in large "synoptic patterns" and these can leave the entire continent of Europe under conditions of intense calm, cloud and cold for a week at a time.
Lenzen's graphs from Oswald et al, (2008) make the magnitude of the problem clear. They show wind energy availability over the whole of Ireland, UK and Germany for the first 300 hours of 2006, in midwinter, the best time of the year for wind energy. For half this time there was almost no wind input in any of these countries, with capacity factors averaging around 6%. For about 120 continuous hours UK capacity averaged about 3%. During this period UK electricity demand reached its peak high for the year, at a point in time when wind input was zero.
Clearly these periods of calm are not rare or minor. For several days in a winter month in good wind regions there would have to be almost total reliance on some other source. The capital cost of having a backup system capable of substituting for just about all wind capacity is rarely focused on.
Davey and Coppin (2003) make the point for Australia, for instance indicating that for 20% of the time a wind system integrated across 1500 km from Adelaide to Brisbane would be operating at under 8% of peak capacity. Mackay (2008, p. 189) reports data from Ireland between Oct. 2006 and Feb. 2007, showing a 15-day lull over the whole country. For five days output from wind turbines was 5% of capacity and fell to 2% on one day. At times the Danish wind system contributes almost no electricity.
A similar problem associated with higher penetrations of wind and solar is to do with periods of over-supply and dumping. Lenzen (2009, p, 94) reports Hoogwijck et, al. 2007 as finding that "... the amount of electricity that has to be discarded grows strongly for penetrations in excess of 25-30%." If wind and PV were to contribute 25% and 30% of electricity, then on sunny and windy days they would be generating more than twice average demand. Some degree of system "over-sizing" will probably make sense but the capital cost implications are easily overlooked. System capital costs should be divided by electricity delivered, not generated, to arrive at a realistic system capital cost per kilowatt [kW].
Solar energy availability exhibits similar variability. Most obviously, even on a sunny day PV panels can provide no energy for about 16 hours of that day. Renewable energy enthusiasts tend to discuss in terms of average supply and demand, whereas it is the times of unusually high or low supply and demand which set the limits
Maxima or peaks in demand are also crucial. Energy supply infrastructures typically have to contain 30-50% more generating plant than would meet average demand, in order to cope with peak demand. In addition, the two events can coincide. That is demand can peak in periods of low renewable energy source availability. Such events are not unusual in winter. For instance Victoria, Australia winter energy demand peaks during periods of calm accompanied by low temperatures. When demand peaks the generating capacity required can be c. 1.5 times that which could meet annual average demand, and again it might not be possible to meet more than a negligible fraction of that demand from wind or solar. These are periods when almost all demand would have to be met by other than solar and wind sources, setting significant implications for the amounts of redundant plant required and total system capital costs, unless this can be done via very large scale energy storage.
Jacobson and Delucchi's solutions
Jacobson and Delucchi recognize the general variability problem but state that it can be overcome by the technologies they list. Their discussion of these options is superficial and far from convincing, and these technologies are not capable of solving the general variability problem. The crucial issue here is quantitative; i.e., the extent to which particular technologies can deal with variability and whether or not all combined can add to a sufficient capacity to get around the difficulties set by variability.
First it is important to again make the general point that Jacobson and Delucchi assume 50% of energy needed will come from wind. However Lenzen's review (2000) concludes that only 20+% of electricity, as distinct from total energy, can be supplied by wind due to integration difficulties created by its variability. Jacobson and Delucchi do not deal. with this contradiction.
"Interconnect dispersed generators."
The half-page explanation of this strategy begins by stating that connecting renewable energy sources "smooths out electricity supply--and demand--significantly." A paragraph then refers to a study in which variation in reduces variability.
These brief statements make the well-known observation that interconnections do reduce variability in supply from individual solar and wind devices, but they fall far short of a satisfactory case for the claim that connecting sources can make a sufficient contribution to overcoming the variability of renewable sources. When most of Europe is experiencing calm and cloudy conditions over large regions for days at a time the crucial question is not whether input from wind and...