Climate and Sustainability

The the oil and gas industry has, within very broad limits an idea of the resources available to it.  Whilst it is possible to drown in a sea of numbers, it can be summarised as there always being  enough to fuel the next generation, say 30 - 50 years of supply, albeit with an uneven geographic distribution.  When I first became interested in wind and solar energy I simply wanted to know something about the energy resources of my back yard in the south of England.  This can be summarised as no potential for wind technology because my house is located in an suburban valley and sheltered from the prevailing wind, solar could make a contribution during the summer months, but not much during the winter.  Storage would help deal with the short term uncertainty of the weather, but not with seasonal variation.  As a result, I tend to favour buying energy from large scale sustainable sources rather than attempting to generate it myself.  This started me wondering about a framework for evaluating wind and solar energy.  Treat this post with caution, it evolved over a few cups of coffee and time spent looking at weather reports at  randomly selected locations around the world.

One starting point was climate and terrain (defined in such a way to include offshore areas).  I like to see the world in numbers like average wind speed, solar irradiance, the attenuating effect of clouds and likewise measures, but the potential for wind and solar devices in a given location can be felt on the face.  This might be summarised as "If you can wear a hat without fear of loss, it might not be a good place to put a wind turbine" and "If you don't need sunscreen, you might not need solar panels".

Economics has to be part of the scheme.  The volume of oil and gas reserves is related to price, if the price is low reserves which are in geologically complex areas or in harsh environments will not be economically recoverable, when the price rises, such reserves can be included in the resources available.  The same logic applies to sustainable resources, for example average wind speeds are higher offshore due in part to lower surface friction, but the cost of working offshore is significantly higher than onshore.  I am intrigued by the concept of airborne wind turbines which operate in the smooth air above the planetary boundary layer, but I guess the technology and economics are a challenge.  A similar logic can be applied to solar devices, the effects of seasonality can be offset by installing more panels, however, the system cost will increase.

Expectations affect how wind and solar systems are perceived.  I guess these can be summarised with three scenarios.  The first is grid-tied systems where wind an solar power is fed into the grid when it is available causing fossil fuel sources to be run down, when the wind stops blowing and clouds cover the sky, these are bought back on stream.  Off-grid systems rely sustainable sources, probably with storage and some form of fossil fuel backup.  It might seem a pointless distinction, but I would add "starting over" solutions as separate category.  I suggest that starting an energy economy from scratch might evolve some interesting solutions, possibly related to conservation, storage and management.  Along with expectation, goes realism, few people want wind powered railways and schools, hospitals and similar infrastructure need a lot or reliable power, but that still leaves a lot which could be configured not to.

Climate is largely determined by latitude and recorded in weather reports.  The Koppen schema has five top level categories and more than 20 sub categories, however, the relationship between sustainable energy sources and climate can be illustrated by just two diverse classifications.

Hot deserts (Koppen group B), e.g. parts of Arizona, which are within 30 degrees of the equator have relatively minor seasonal variation in clear sky solar irradiance, when clouds appear they are often high in the atmosphere and where they cause less attenuation than water laden low cloud.  Average non-storm wind speeds are relatively low.  This type of climate makes it possible, in conjunction with some storage capacity (if only because the sun does not shine at night) to maintain a more or less constant load from solar sources, onshore wind is less attractive.

Poleward of the hot deserts are the temperate maritime areas (Koppen group C).  Beyond 40 degrees of latitude, sun-earth geometry ensures that solar irradiance will be season, for example, in the south of England, the clear sky solar irradiance is something like 1 - 2 kwh/m2/day in winter and around 6 - 8 kwh/m2/day.  The clear sky irradiance is attenuated by clouds, in summer these are often intermittent layers of cumulus, in winter they can be dense stratus. which can reduce the solar irradiance to less than 1 kwh/m2/day.  Except for small loads, e.g. some traffic signs, off-grid solar systems are not viable in this climate.  In part, due to the proximity to the coast, average wind speeds on exposed locations such as ridges and hilltops can exceed 5 m/s.  Wind too is subject to seasonality it is stronger in winter when the dominant weather is fronts from the Atlantic, but even then, there can be intervals when the prevailing weather is high pressure over Europe resulting in still, clear air.  In general, wind becomes more reliable as an energy source with increasing latitude.