Clouds and Irradiance from the sky

This was quick-and-simple study to look at the distribution of solar irradiance under a cloud sky.  The intention was to get some insight into the mounting of solar panels.  Under a clear sky, the greatest output is obtained by pointing them at the Sun.  For fixed mountings the optimum is facing south at an angle to the ground close to the latitude of the equipment.  This may not be the case under a cloud sky where the direct beam irradiance is attenuated and a higher proportion of the total is diffuse from the hemisphere of the sky.

The equipment was constructed from materials to hand which included a length of waste pipe, part of a broken bed and a roll of packing tape.  The main component was a light dependent resistor mounted at the base of a 300 mm length of white waste pipe whose translucence was reduced with a wrapping of parcel tape. The photo shows it in position sitting on top of a dust bin.

Operation consisted of setting the altitude to 15, 30, 45, 60 or 75 degrees, then rotating the instrument from 0 to 330 degrees in 30 degree increments and recording the resistance of the light dependent resistor.

The light dependent resistor has typical resistance of 20k at 100 lux. A simple calibration gave the relationship between resistance an luminescence of:

The results should be treated as relative irradiance which are consistent for the experiment, but are unlikely to represent accurate absolute values.

Overcast Sky - 26-Jul-2010 15:00: The irradiance increased slightly with altitude, but was relatively constant with azimuth.  The cloud was thick and low, there was a small break to the northeast, hence the higher irradiance in that direction:

The conclusion drawn from this observation is that irradiance from an overcast sky is more or less uniformly distributed around the sky, albeit at a level much attenuated from that expected under clear sky conditions.

Broken Cloud - 27-Jul-2010 14:00:  The early afternoon sky consisted of broken cloud, with the sun 

occasionally visible through the cloud.  Compared to the overcast sky the day before, there was significantly greater irradiance in the part of the sky in which the Sun was located.

Typically, in our part of the world, cloud described as broken, is a discrete layer and can sometimes have similar appearance to an overcast sky with only occasional glimpses of the blue sky above. This was the case on the day these readings were taken. There is an increase in irradiance in the direction of the Sun, but a significant amount of irradiance is coming from the sky as a whole.

Few Clouds - 28-Jul-2010 11:30:  As so often happens when there are a few clouds in the sky, there was some fluctuation in the readings. The maximum irradiance was to the south east at an elevation of 45 to 60 degrees.

The irradiance under a sky with a few clouds is similar to that of a clear sky with exception that there may be short periods of attenuation.

Clear Sky - 16-Aug-2010 15:00 - The irradiance is clearly coming from the sun's disk:

Under a clear sky the maximum irradiance received by a device is when it is pointing directly at the Sun. In this case the maximum irradiance was around 230 degrees at an altitude of 45 degrees which is close to the altitude and azimuth predicted by Sun-Earth geometry.

Doris B - Instrumentation

It’s a sad fact that almost everything I make gets built at least twice.  I’m trying to figure out what needs to go into the wind turbine’s “power box” and how it’s going to fit, then I’ll rebuild it so it looks less like a building site and use components with appropriate values.

It monitors the rectified output of the generator, the voltage across the load and the current through it.  This allows the power generated and being delivered to the load to be estimated.  A Fourier transform is used to determine the rotational speed.  The original intention was to work with AC output of the generator, but it proved a lot simpler to work with DC throughout the whole system.

The Arduino Nano has exceeded expectations, a previous attempt used op-amps and many other components, however, this time around, the only components used in the instrumentation are resistors for voltage divides and zenor diodes for protection.  voltages from three measurement points are fed to the Nano’s ADC.  After some processing with a couple of blocks of Python code running on a Raspberry Pi, the output looks like this:

Testing is being done using a 47 ohm resistor as a load, this keeps the current low but at the expense of high voltage on the upside of the regulator.  When a battery pack is used as a load, the voltages look a little more sensible.  Still some work to do, but the basic design is in place.

The next task is to determine if pulse width modulation can be used to control the load, if so, if so, it may be possible to optimise performance.  Ideally, I want the rotor to start to turn at 4 m/s, but the high torque of a large load may prevent this.  Maybe if the duty cycle is 0 when the turbine starts, it can be increased as the rotational speed increases.  Also, it can be adjusted to take account of low wind speeds.  All that comes after I’ve soldered the bits together.

Doris B - Power Curve(2)

Progress (and more to learn and some bugs to find).  The Arduino captured a one minute time series of the voltage across a 22 ohm resistor attached to the generator’s output terminals whilst it was hand cranked.  The load on the crank varies during a rotation so there are constant variations in speed and this can be seen in the time-series, the flywheel effect of the wind turbine’s rotor will probably have a smoothing effect.  The circuity between generator and the ADC needs to include a zenor diode to provide some protection for the chip, once that’s in place, the DC biasing can be setup sensibly.  However, the time series was good enough to passed through a Fourier Transform routine which allowed the rotational speed to be estimated.

The speed and voltage data can then be combined into a power curve, the variations in power are due to uneven cranking.  The 22 ohm resistor was chosen because I had one in a box somewhere.  To get peak performance from the generator will require matching the impedance of the load to the impedance of the generator.  The planned load is a USB power pack.

Whilst messing with electronics and Fourier transforms has been instructive, so too has hand cranking the generator, clearly the torque required varies with the value of the resistor, with a 10 ohm resistor the torque required almost pulls the generator of its mounting.

So on the next trip to the beach, I’ll take a bag of resistors and see how they effect the ability of wind to turn the rotor.

Doris B - Power Curve(1)

In order to match the rotor to the generator, it is necessary to understand the behaviour of both. The first step is to grab data, starting with the generator because this can be done on the workbench rather than the beach. Much the same solution can be used for both. Currently, the instrumentation is an Arduino Nano which provides analogue to digital conversion (ADC) and a Raspberry Pi for processing. Electricity and electronics were part of my OND but that was a long time ago, so setting up the tests involved a little vexation, but eventually a Python programme managed to capture a sample of the output of the generator.

Two things became apparent, first the output of the generator is complex and not a simple sine wave, Secondly the starting torque increased significantly from the open circuit value when a resistor was placed across the terminals.

The next step is to analyse the data, the plan is to use a Fourier transform to determine the rotational speed and the area under the curve will provide the energy generated, putting the two together will provide a power curve (I hope). The Ardunio is a brilliant tool and may offer a solution to the high starting torque. If pulse width modulation is applied to the load, the turbine can start with no load and as speed increases the load on it is increased, this in turn could provide a means of optimising performance.

Doris B - First outing

 On Wednesday evening I took Doris B up to Green Ridge and assembled it.  The wind speed was  roughly 5 - 7 m/s.  Whilst held aloft, the rotor was 2.5m above the ground, it turned smoothly and quietly, which was pleasing, an earlier version had struggled to turn at 10 m/s, so progress is being made.  The next step is understand the relationship between the rotor and the generator,  The speed of this type or rotor is proportional to the wind speed and it is important that the rotor turns fast enough for the generator to produce a useful output.  On returning home I ordered some bridge rectifiers and  an Arduino Nano.  The first use of these will be to obtain a power curve for the generator.

My favourite place for messing with this stuff is on the beach at Aldrington, when the wind is from the SW there is little turbulence.  Most of the other people are beach fishermen and we occasionally swap observations on the state of the sea.  There was a curious incident at Green Ridge, a dog walker scooped up his spaniel, carried it over to my rucksack and let the dog have a good sniff, then walked off carrying the dog, I said "good evening" politely.

Doris B - Trial Assembly

10-April-2019 - Completed the partial assembly of the Doris B5/6 small vertical axis wind turbine this morning before giving it a trial outing on the Downs later today. 

The objectives of the initial trials are to determine if it rotates in a wind of 5 m/s and does not disintegrate at 10 m/s. If it survives, the next step will be add some instrumentation and to try and figure out how it works. The design is based on some maths, guesswork and wishful thinking. There have been four previous versions, mostly made of Meccano and sawn up bits of plastic pipe, this one is mainly made of wood. This is not a finished design but part of a process of learning how to make small wind turbines for rural and urban environments, hopefully the design will evolve. The objective for this series is an output of 2.5 watts (similar to a standard USB port) at 5 m/s.

The early days of electricity - Diverse decsions and technologies

Public electricity supplies started to evolve in the UK during the 1880s.  Today electricity is a utility accessible by a high proportion of the population, but in its early days it was part of the luxury goods industry, a unit might cost between 4d and 1s 3d (2p to 6p) which is around £1 in today's money.  Its attraction was that it was convenient and clean and therefore perceived as being healthier than the gas lamps that it was to displace over the next half century.  Arc lamps improved the lighting of streets and public spaces and this gave local authorities an interest in the industry.  The industry grew using both public and private capital, and some local authorities proved to be adept in managing the evolution of a new technology.  Street lighting was often managed by the gas committee, because that was how the streets were lit, so decisions about borrowing substantial sums against the rates were being made by men who were often involved in decisions about the lighting of urinals

The caption on this cartoon was Electricity for the Ballroom

Electricity works, especially those supplying DC had to be close to the consumer because of the limitation of the early distribution systems.  This resulted in generating plant being located in unlikely places like London's Carnaby Street: its contribution to the swinging sixties is well remembered but its power station is long forgotten. The search for early power stations often takes one to the posher parts of town.

Most of the early dynamos and alternators were turned by reciprocating steam engines.  The early engines were relatively small and built by companies also known for their traction engines and road rollers such as Robey or Fowler.  Until the development of high speed engines such as those of Willans which facilitated direct coupling, the generators were connected to the engine by a belt drive.  Belts would sometime break, in 1882,  the Mansion House was provided with electricity from a generator installed in the basement, where the belt was driven by a gas engine.  During a dinner the belt broke giving the diners the impression they were being attacked by gunfire.  In 1888, the first turbine driven alternator was installed at the Forth Bank power station, close to the centre of Newcastle.

Large generators were steam driven because steam engines could be built to supply hundreds of horsepower and it was a mature technology. At this time steam engines were almost as common as electric motors are today, they powered mills, railways, ships, sawmills, pumps and anything big which needed turning.  Smaller plant in urban areas often used gas engines.  By 1880, most urban and some rural areas had a gas supply and whilst this was mainly used for lighting, it could also used as fuel for engines such as those made by Crossley.  The attraction of gas engines was that there was no need for a boiler and most could be hand cranked into life when needed.  Judging by the number of adverts for fractional horsepower gas engines in pre-1900 magazines, many modest homes may have generated their own electricity from gas.  Such a system is described in a biography of Magnus Volk, the house in which it was installed is comfortable, but not grand.  Many micro systems incorporated a bank of lead acid accumulators making it unnecessary to run the gas engine continuously.

During 1880s and 90s AC and DC systems competed for supremacy.  The AC system would eventually win because it facilitated transmission over long distances allowing big power stations to be sited away from city centres.  However, DC did have the advantage of being able to use lead acid accumulators for storage.  Then as now, the demand for electricity peaked in the early morning and early evening and if only a few hundred homes were being supplied overnight demand could be met from the accumulators allowing the steam plant to be shut down or the boilers banked up.  Accumulators also provided some back-up in the event of plant problems, for this reason, some consumers perceived DC systems as being more reliable.

In the early days, the demand for electricity was measured in kW rather than MW making it possible to supply rural communities using small plant connected to consumers by wires hung from wooden poles. Some of these used water power, Godalming claims the distinction of having the first public electricity supply, this was from a generator turned by a water wheel in a mill. Reeth in Yorkshire had a similar arrangement.

Initially, electricity was an urban industry relying on clusters of high income households, some smaller communities were still not connected to a central generating station until well into the interwar period.  It was during this time that many small electric companies were formed, examples include the Steyning Electric Light Company and the Peacehaven Electric Light and Power company.  I've seen photo's of these companies' plant, both show belt driven generators, in both cases it looks like the motive power is coming from an industrial internal combustion engine, but it is not clear if the fuel was oil or gas.  One of the more interesting of these companies is the High Salvington Electric Light Company, this served a small development of houses on the Downs to the north of Worthing. The generator was turned by a wind mill/turbine similar in design to those of West Texas -the electricity was used to charge up accumulators which in turn supplied the consumers.  There was an oil engine back-up for days when the wind did not blow.  Now that utility scale storage, like Tesla's installation in Adelaide is becoming available, High Salvington can claim to be pioneer in the field of sustainable energy.