Would you plan a trip without checking out where your going first? Well the same applies if you're going to Mars or going to the beach. So in this blog I’m going to look at the weather on Earth, more importantly the sun and the power we can get from it!
NASA have sent quite a lot of hardware into space and all for a good reason. Its very important to carry out experiments and to find out where you're going and what to expect. When traveling to a far planet mankind has sent satellites and probes to test the environment first. Only once they know what they are letting themselves in for, do we start sending larger robots like the Mars rover, and then one day people too.
These experiments don't only make repeat trips more reliable but give vital data that can be built upon. For example knowing how much solar power you actually get can allow you to calculate the size of your solar panels, the angle to fit them at and even know what size battery you will need. So before sending EarthRover off on its mission, we also need to find out the same.
It took me less than 5 seconds to find hundreds of websites telling me how to fit a solar panel and how much money I could make. For the UK I read that a roof between 30’ and 50’ facing south is great. But that can't be the opium results can it? We know the sun is high in the summer and much lower in the winter. We also know just from stepping outside that the sun generates more power the higher in the sky it is. So my quest was to find the best results.
To do this I’m building a probe that I’ll be talking about over the coming months. Named ‘Ra’ from the egyptian Sun God, Ra will use solar experiments to measure the amount of solar energy that can be generated. However the probe can’t move so will need a first best guess when collecting data - no point facing north after all!
So it all comes down to calculation at first, to find the first best guess for Ra. Searching the internet did not take long till I found a really nice little Excel sheet that calculates the solar power for you. This generates a table of results for any time period for any number of days.
The tool first needs some data about where you are. I found my Latitude and Longitude from a website called EarthTools which also helped me find my height above the sea too. This data I entered into the tool and hit go. To get an idea of what we are dealing with I asked for the midday results for each day of the year. Assuming this would give me the highest power reading for each day.
The tool gave me three very important data values; the Azimuth or the angle of the sun from north, the elevation in degrees from the horizon and lastly the estimated power output per square meter. The power output however assumes that the panel is facing directly at the sun, the opium power output. So I then plotted these to see what the results looked like. No surprise to find the power output was highest during the summer! However I was surprised to find that the sun, due to the earth rotation and orbit, is not exactly in the same place at noon every day. In fact it seems to swing back and forward around south. We already know that the height changes so I wondered what this movement looks like. What I found was at noon the sun over a year forms a figure eight shape in the sky. Most movement was during the winter and less during the summer. This was going to make working out which way to point the panel harder-!? For example you need to work out where on the figure eight you get the best power from the sun.
So to calculate the angle our panel needs to be at we now need to consider two factors; direction from north and elevation. Both can be worked out using a little trigonometry. Using the COS rule you can subtract the angle of the panel (lets assume 180’ or due south) from the suns location (say 179’). This then gives you a value that you can use as a scale factor - The closer to 1 the value is the more aligned the panel and the sun is. If you then repeat this for each day of the year you will get a range of values. What we want however is the best direction for the panel that gives us the highest average over the year. You can do this in Excel using the ‘solver’ which searches a range of values to give you the best results. From this I found for my location that a direction of 180.59’ was the best - well as close to due south as you can get. Was a lot of work but atleast we confirmed our results!
This was then repeated for the angle of the panel to the sun with respect to the elevation. This time using the SIN rule we could work out what angle gives the highest average output for the year. Again for my location I got a angle of 51.3’. This fits with the information I read online about being between 30-50’ (at one end of the scale anyway) and with another article that suggested you fit the panel at the same angle as your latitude.
As you can see from the plots I generated the power dips a little mid summer but give the all round best results for the year. If you’re fitting solar panels on your house you could also do this same calculation to get the best results.
However.... And here is the interesting bit. Will our solar panels really work best this way? Will the high temperatures in the sun lower its efficiency? Will the panel just peek out at a set light level meaning we could lower the angle? These can only be answered with real results and thats why building and get test results from Ra is so important.
Another key issue is the winter results. We can see that we will get lower than optimal results at this time. We need to be careful that we don't sacrifice power in the winter to keep the EarthRover running for more than enough power in the summer. This is what I will look at in part 2 and try and look at the effects of tuning the angle for the best power usage when we need it most - winter.
To see more about the Earth Rover project check out the first blog here - paul-clarke-needs-you-help-me-with-my-massive-earth-rover-project, or you can follow the project on facebook and twitter.
Many Thanks
Paul
(aka @monpjc)