Wind and solar energy systems are often described as "renewables" in contrast to fossil fuels which are not renewable unless you live in geological time. An alternative description would be climate and location dependent energy systems. Location defines the terrain over which the wind for a turbine flows and it defines the Sun-Earth geometry for a solar device. Climate describes the seasonal variation in wind and the clouds which pass between the Sun and the Earth's surface. One of the distinctions between wind and solar systems and fossil/nuclear ones is that the weather determines the output, not a person sitting at a control panel. Both wind and solar are subject to significant diurnal and seasonal variation and the sun does not shine at night.
The graph below is a simple simulation of solar irradiance that might be experienced by my back yard in southern England. The extremes are high air mass at solar noon and the the prevalence of stratus during the winter months. In summer, the air mass gets close to one at noon and whilst clear skies are not unknown, cumulus is a frequent sight from my workroom window. It is not uncommon at any time of the year to be unable to get a usable amount of energy out of a small solar panel for a period of several days.
Wind has similar seasonal variations, often peaking around the equinoxes (graph did not get finished in time for this post!).
There are also significant fluctuations during the day and even within the space of an hour as shown by the graphs below..
Storage is the missing link in the evolution of a sustainable energy economy. There has been a significant change in emphasis over the past 30 years. I recently acquired a copy of "Small Scale Wind Power" by Dermot McGuigan which was published in 1978. This work contains a discussion and descriptions of battery based storage for off-grid systems. Fast forward to the 2013 and wind and solar devices have proliferated but as grid-tied systems, diurnal and seasonal variations are absorbed by the the fossil/nuclear grid. The popularity of grid-tied system has been enhanced by incentives such as feed-in tariffs.
My own view is that the base load of Western European and North American energy economics will have to be met from fossil/nuclear sources, few people want transport, hospitals, schools to function at the whim of the weather. However, in some situations, off-grid systems are attractive, not least of which is that they don't increase the demand for fossil/nuclear energy, but more subtly because they provide a challenge to work within an energy budget. Historically our energy economies have evolved on the basis of readily available cheap energy, starting a project with the constraint of having only wind, solar or other renewable energy technology, leads to some different solutions. Whilst I have an open mind on feed-in tariffs, I would like to see some similar incentives for achieving true energy sustainability. This does not exclude using the existing grid to move energy around, for example from an offshore wind farm to an office complex.
The batteries which are emerging for use in automotive applications such as hybrid and electric vehicles may have something to offer homes and offices. I have not studies these in much detail, but a brief look suggests that 10 kwh of storage has a similar or lower cost than a rooftop PV system. Integrating this amount of storage into a home or office significantly changes its energy economy, if the building is fitted with solar panels, then energy harvested during the day can be used for lighting at night. It also allows a closer integration with wind farms and might also improve the efficiency of coal and gas power stations by smoothing out demand. In the UK the demand for electricity peaks in the early evening and is at minimum overnight.
Readers with an interest in maths might enjoy this:
http://www.brighton-webs.co.uk/montecarlo/simulation.htm
It describes a simple simulation of a small energy system.
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