Sunday, 19 January 2014

Understanding the Weather and making Use of the Knowledge

Understanding the Weather and Making use of the Knowledge

Back in April 2013 I bought a weather station, only a cheap one, but it allows samples of wind, rain and temperature to be taken automatically and stored on a computer.

As of now, I have around 400,00 samples of the weather that have been taken every 10 minutes for the last 9 months.

The software that came with it only works on Windows and allows you to download the weather and look at the data and also plot graphs, how ever the software was so awful that the graphs are worthless. The software was little better than rubbish.

Being a computer programmer and all round nerd I decided to export the data to a Linux computer and write a program so that I could analyse it. Although the program isn't finished it does work and gives most of the information I want.

I was interested in knowing a few things such as is it worth having a wind turbine, how much water falls on our little field and what weather and when provides good growing conditions and whether these conditions would allow Soya and other plants to grow well. Also how much wood for my heating do I need to heat the house each month.

I've now been able to answer some of my questions and have the data and a method that will enable me to gleam far more information in the future.

One of the obvious things that I wanted to know was the temperature of our site.

Monthly Temperature (click for larger image)

And also the rainfall...

Monthly Rain Fall (Click for larger image)
The rainfall was interesting but also raised another question. How much water falls on the field because the field floods and I am trying to make a drainage system and although my efforts have mostly worked the field is now saturated and my drainage channels are full. My drainage system can't take much more water. If I knew how many gallons it takes to saturate the field I then can also know when it will flood and therefore how many gallons on top of this I need to shift.

The amount of water that falls on our plot was quite staggering and caused me to recheck the answer many times. In the end I know I am correct and the amount of water that falls is huge.

(These next lines are taken directly from my program. The overall plot is around 0.7 acres)

159.71 cubic meters  (159705 litres / 35130.3 Gallons)  of rain fell on the plot of land in December

30.54 cubic meters  (30531.9 litres / 6716.08 Gallons)  of rain fell on the house area (House stables yard garden) in December

129.18 cubic meters  (129174 litres / 28414.2 Gallons)  of rain fell on the field in December

35,000 Gallons of water! one month for the plot! 28,000 Gallons for the field. All this falling on a compacted clay soil, no wonder it floods. I haven't gone back through the figures to see how much it takes to saturate the ground before a flood but that is one of the next jobs. I only know roughly when the ground saturated but that will be good enough.

The next question was how windy is the site and where does the wind come from. The met office says that it comes from the South West where we live but the results show differently. What ever the met office say the fact is that they don't do their measurements from my exact location and hills houses and trees have an effect on strength and direction.

The data I had was for each 10 minutes and I had to produce a wind rose which involved grouping the data from 16 directions into 8 and working out what was the predominant direction for each day, then working out how many days of each direction we had.

(Below is a direct output from the program)

Wind Rose for December 2013
 N (average:  0 mph) - 0 days
 NE (average:  0 mph) - 0 days
 E (average:  0 mph) - 0 days
 SE (average:  9.18 mph) - 29 days
 S (average:  0 mph) - 0 days
 SW (average:  4.24 mph) - 2 days
 W (average:  0 mph) - 0 days
 NW (average:  0 mph) - 0 days

 It's important to know that an average of 0 mph doesn't mean that no wind came from that direction only that on no day was the predominant wind in that direction. What this shows is that my location has my house and stables on the North and North West side, a small hill (I mean small) in the West and East and a town (centre 1 mile away) in the South West. These landmarks help to funnel the wind from a mainly South or South West direction to a South East direction. The met office wind is measured 10 metres up where as mine is measured around 3.5 meters up. The lower to the ground you measure the lower the wind strength will be.

The difference of wind from 10 metres up to about 3.5 metres is around 18% which explains why my measured wind is less than what the met reported but also the land marks reduced it a bit. I don't want to measure the wind 10 metres up as my plants aren't up there. I measure at 3.5 metres because that is above my green house but also closer to where the plants are so the measurements are accurate although the plants will be hit with another 10 or 20% less wind.

The highest gusts were interesting. We had a storm where winds of 90 to 100 mph were seen around the South East but I measured:

December 2013 Maximum gust of wind was day: 5 at 42.56 mph (samples taken every 45 seconds)

The met office measures the average wind speed and my highest wind average was:

Windiest day was: 27 with an average speed of: 18.91 mph predominantly from a South Easterly direction.

On that day the met office were predicting an average wind speed of 30+ mph so we can see how sheltered my site is. Even adding 18% to take into account the height difference we only saw 22 mph winds..

All these figures effect, massively, how well or not a wind turbine would operate.

Wind Turbines
How much potential power there is in wind is fairly easy to work out. You need to take into account pressure, air density, size of a propeller and wind speed. The air density and pressure aspect can be simplified which helps a lot.

Most wind turbines have a cut in speed of  around 6 mph and stop producing when the speed reaches 25 mph so I calculated how much wind there was between 6 and 25 mph and fed this into the general formula for calculating wind power. Next I had to take into account that this is only theoretical and the real world designs of turbines only capture and convert 10 to 40% of this power. I used 20% as a realistic figure because the cheaper a turbine is the less efficient it will be, and if I were to buy one it would be on the cheaper side.

Calculating how much power I would have generated with a 2 metre Wind Turbine situated on a tower 10 metres above ground at a rate of 13 pence per Kilo Watt unit I came to:

May: £10.34
June: £4.27
July: £1.60
August: £2.53
September: £2.43
October: £11.49
November: £5.54
December: £39.63

You can see that each month varies a huge amount. My own monthly spend on electricity is £70 per month and therefore I would be well short with a turbine of this size.

I will leave to another post many more details and all thoughts regarding whether a wind turbine is worth having and the costs involved plus the actual calculations but for now what I have achieved is a proper data set, a few months short at present, but one that is capable of answering my questions. 


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