A detective story solar: explaining how the output power varies hour by hour

Editor's Note: Scientific American George Musser will be on their experiences in the installation of solar panels in the solar home (formerly 60-second solar). Read his introduction here and see all the posts here.

Item favorite owners solar conversation is the performance of your arrays. As part of the sales pitch, the installer estimated how much power you can generate and most of the systems come with a meter (separated by the meter utility) to handle continuous output power.But how can I know if its parent actually meets expectations? this simple question put me in a mathematical hunt could enjoy other solar owners - and that would make a good project document term for a high school science class.

There are many tools to estimate the performance of a solar array. NOAA provides an applet and the Excel spreadsheet to determine the angle of the Sun at any time of the year, based on astronomical, geometry with corrections for atmospheric refraction and the displacement between the clock and solar time. The Web site of PV Watts combines solar geometry with historical climate data for calculating the expected return on a panel at a location given with a certain orientation.PV performance also factors in the properties of the solar cells and inverter that converts the DC output in home power CA.Para specific issues such as shading, you need to do more detailed measurements. The inspector of the State of New Jersey that signed on my system uses a nifty 360-degrees, called the Solar Pathfinder light meter.

Both for prediction tools. To measure the actual output minute by minute, I have two independent energy monitors that are installed on my electrical service panel: TED5000 and locus.Show the total electric array output through a web interface.

Useful though, these devices not separate the multiple factors that affect the output power: solar geometry, time, diffuse vs direct lighting, shading, reflection on the surface of glass panels, the outlet temperature of the solar cell, solar cell and inverter efficiency unit and thus sucesivamente.estos effects operate in concert. For example, if the temperature gets so hot that cuts off the output of solar cells, total matrix output can remove the low enough that the investor drowns.

I in spite of some of these effects quantitatively, put together my worksheet to calculate the expected power output for June 30 - a pleasant, mild day off clouds, when the temperature of air in our site stayed fairly steady reduction of clima.Aquellos confusion who wish to choose an also opportune day can search its records local time Weather Underground.

In the course of a day, the intensity of sunlight varies sinusoidally over time. The amplitude and phase of this change of sine wave with the seasonal cycle, but in hindsight really does not need to worry about by. I have only attracted a cosine and stretches and displaced curve to match the midday peak power. Between 9 a.m. and 3 p.m., the output array followed by a sine wave almost perfectly (see the blue curve in the previous frame).

Early in the morning and mid - a-tarde, however, production fell faster than explained the lighting conditions alone.So I went down the list of other factors.

First of all took a little more careful with my techo.Por geometry generally, is supposed to go in the direction South general of panels in the northern hemisphere tilted upwards. My roof, however, also tracks towards the West, by what my panels are tilted in their long and short axes. Had some linear algebra to take into account this orientation. The effect was to give the sine wave a slight bias.

Secondly, sunlight passes through a greater mass of air in the early in the morning and the afternoon, so attenuating.Bradley Hibberd, director of engineering at Borrego Solar systems, I pointed out towards a model adopted by the American Society of heating, refrigeration and air-conditioning Engineers (ASHRAE).(For those who are concerned about the technical details, the model uses the law of Beer-Lambert, which reduces the solar intensity by the exponential of the secant of the angle between the Sun and the zenith).This helped to explain why the power fell so dramatically during peak hours.

Thirdly, considered losses due to sunlight glinting off of the glass pane.A role at Sandia Labs has a chart showing how reflection becomes a serious issue when the angle of incidence of the rays of the Sun is greater than 55 degrees, which for me meant before 9 a.m. and after 4 p.m.The delivery will be more or less like the cube angle.Incorporate this into my worksheet more had reduced power tip.

Finally, took into account the effectiveness of my investor communications below a certain level of power, the investor is unable to convert DC to AC very effectively, offering the double whammy of less light from the Sun and less energy per unit of sol.Aproxima fall light used as a power law.

Between investor, reflectance and atmospheric attenuation, I was able to take into account the result of the decrease array during the hours of the morning (see the red curve) .Que left abrupt decline in the average afternoon for explicar.Tengo suspicion that would be sombreado.Un tree and the fireplace are just to the West of the matrix, and while they are not very high, even partial shading could cause a strong loss of power. the panels are cabled electrical series by affecting everything that one of them affects all measurements of Solar Pathfinder ellos.Las showed something more than shaded in the West than in the East, to be cut in the power output starting around 3 p.m.Uno these days when I'm at home in mid afternnoon, you want to go the roof to see what is happening.

My analysis was really only the first step and I'm not sure that I really isolated significant effects between all posibles.Me would love to hear how other people have tried to explain the performance of your systems!

Graphic courtesy of George Musser