Location, Location and Location

When I became interested in sustainable energy systems, I wanted to understand the nature of wind as an energy source.  An early step was to create wind speed distribution diagrams for several hundred locations around the world taking data from a wide variety of sources such as aerodromes, weather ships, offshore buoys, radiosondes and small weather stations located in backyards.  The principal conclusion from this exercise was the wide variation in both average wind speed and its distribution.

This post is based on my own research and has not been reviewed  by anyone else, thus it should be treated with caution as there may be other and more appropriate interpretations of the same data.  Another caveat is that much wind data is collected at a standard height of 10m in open terrain to assist ground operations.  Thus the bulk of readily available data describes air flow at the base of the boundary layer but the ideal location for wind turbines is as far above this zone as possible.  There may also be gross generalizations.

The variation of wind speed with location is apparent in daily life.  In the coastal town where I live, the wind maybe blowing smoothly at 10 m/s on the seafront whilst in the town it will eddy around buildings appearing as long period gusts struggling to make 8 m/s even on the roof of a multi story car park and in residential areas away from the sea, there may even be still or light air.  For a brief period in my youth, I was a merchant seaman and had ample opportunity to study wind and waves during lookout duty, still air was rare, usually there was a steady breeze with accompanying waves gave the the ship varying amounts of motion.  In winter, gales with speeds of more than 20 m/s had an energy seldom seen onshore.  The energy of wind is proportional to the cube of its speed, thus, a steady stream of air with a velocity of 10 m/s has eight times more energy than one with a speed of 5 m/s, hence the destructive nature of storms where the wind speed exceeds 25 m/s.

The average wind speed over Western Europe at a height of approximately 800m is typically in the range 10 - 12 m/s, this increases to the north and decreases to the east.  At this altitude the influence of terrain and surface cover is small and whilst there are variations, it is reasonable to assume that this is close to the top of the planetary boundary layer over low lying ground.  Wind speeds at this altitude are measured by weather balloons.  The variation at the surface is much greater.  On mountain tops, the average speed can be similar to that observed by weather balloons, at the other extreme are sheltered urban locations where the average speed maybe less than 2.5 m/s which is far too low for the economic operation of wind turbines.  In open inland locations the average speed might be 5 m/s, whilst on a downland ridge it might rise to 6 m/s.  A few miles offshore, the range could be 6 - 9 m/s.  From the work done to date, there is an inference that average onshore wind speed decreases with increasing distance from the coast, but no obvious correlation has been obtained.

This work suggests that, from an energy harvesting perspective, coastal and offshore locations offer significant advantages resulting from higher and smoother wind flow.  Against, these must be set the higher costs of offshore operation and construction.  Most offshore wind farms are located in shallow water with the turbines mounted on a single pile, but there is growing interest in floating support structures such as the semi-submersible platform.

The attraction of semi-submersible platforms is that they minimize the motion caused by waves which is the reason they are extensively used by the offshore oil and gas industry drilling operations.  They are also economically attractive for two reasons.  Firstly the installation cost is largely independent of water depth and secondly because the construction  can be completed at the building yard before the completed unit is towed to the installation site.

Don't talk about.........

I do not know how accurate this graph is, it was literally done on the back of an envelope with a few photocopies of some government statistics around 2007 when I had a period of enforced idleness.  For all practical purposes, it is a statement of the obvious i.e. the higher your level of income, the greater your level of CO2 emissions.

The graph leads to two observations, the first is that the lower the household income, the greater the proportion spent on energy, which has resulted in policies such as the pensioner's winter fuel supplement.  The second is a little more subtle, that as household income increases, the weaker the link between energy consumption and energy price.

Energy sustainability is based on three elements, emission free generation and energy management, sometimes called conservation.  Despite strong opinions for and against, wind farms, rooftop PV and similar concepts are openly discussed in both the media and pubs.  However, conservation is a difficult subject.

A few years back, legislation was introduced restricting the sale and use of incandescent light bulbs.  Back in 2005, most houses were lit with a mix of 60 and 100 watt incandescent light bulbs, Today, most homes that let me through the front door appear to have adopted 10 - 20 watt CFLs.  When the legislation was introduced, I remember some shops selling incandescent light bulbs presenting themselves as champions of civil liberty.  In our case, had we not made the switch from incandescent to CFL, our electricity bill would be more than £100/month.  The bottom line of this paragraph is that if an energy management technology makes some economic sense, it will be adopted.

It is when the discussion moves to cars, that passions are really aroused.  This is illustrated by a couple of song titles "Little Red Corvette" by Prince and the Beach Boys "Fun, fun, fun" which has the line "Til her daddy takes the t-bird away", don't we all want to be part of the story.  There may be a generational difference, mine tended to lust after things like Frog-eyed Sprites whilst the current one favours large SUVs and Pick-up trucks, I appreciate that this observation may not stand up to scrutiny.  Due to dodgy eyesight, I rarely drive and use a bicycle for short trips and public transport for longer ones, prior to making this move, I was as attached to my car as the next guy, but it has been a liberating experience, around town the bike is quicker and cheaper and I can read the news on my smartphone when travelling on buses and trains.  It is clear that cycling is increasing in popularity, but the perceived independence, status and level of control bestowed by being behind the wheel of a car maintain a strong grip on society as a whole.  Making the case that the car brings mixed blessings, except to fellow cyclists, is not a good opening line at parties.

It is hard to make conservation attractive and even harder for high income groups, the link economics (e.g. CFL's versus incandescent light bulbs)  and greater functionality (e.g. bike versus car in urban environments) is often perceived as a reduction in status and self-denial neither of which are much fun.

The Sun does not shine at night

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..

Whilst there are a variety of storage technologies which can buffer short term variations, seasonality is a challenge for renewables and for an off-grid project it may be necessary to have excess capacity  to make make the most of calm or dull months.

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.