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Saturday, July 2, 2011

Solar Energy Quesions

The cost of solar panels is decreasing.  I have seen advertisements which talk about $1.75US and even $1.50US per nominal watt* (July 2011).  We are getting close to the magic, oft-quoted figure of $1.00US per watt which is said to be the figure at which solar becomes competitive with conventional energy generations.

*Solar panels are rated by the number of watts of power# they will generate when the rays of the sun are shining directly at a right angle to the panel from a clear blue sky.  This is their "nominal watt rating"

#A Watt is a unit of power.  It is the rate at which energy is produced.  A 10 watt panel if it produced energy for an hour will produce 10 Watt hours.  A watt hour (or a kilowatt hour) is a unit of energy. Some people call a kWh a "unit".

I'm trying to get myself educated as to the inns and outs of solar power in preparation for the magic day when panels sell for a dollar per watt.  I'll put down the questions and then put in answers as I get them, either from my own reading or from responses from people who are knowledgeable in the subject.  If you want to put in an answer (or a question), put them in comments and I will put them in the body of the text.  I'm only considering a grid connected system.  The cost of batteries is just too expensive at present. 

In no particular order:

Panel Shading
### With early solar panels, if a few cells were shaded, they acted as resistors and were often burnt out by the operating cells in the panel.  Diodes inserted in the panels solved this problem but the diodes themselves reduces the power output of the panel.  In addition, with present panels, if you have 10% of the cells shaded, you loose much more than 10% of the power.  This whole subject has further implications.  

Suppose panels become sufficiently inexpensive that it is worthwhile to clad your East and West facing roof as well as your North Facing Roof (I live in New Zealand) with panels.  You do this to have power more evenly generated throughout the day.  However the three sets of panels are now no longer co-linear.  Throughout the day, each set is producing energy at a different rate and a different voltage.  What technological solutions are already in place so that you get the full amount of energy which is being produced by all the cells in your various arrays of panels.  There is a further implication for electric cars.  

At present, you can retrofit solar panels to the roof of your Prius.  It is said to produce enough energy for about 10km of driving for each day in the sun.  A nice little bonus.  Before long, it may be possible to clad all the external surfaces of a car with solar cells.  However, on a car, with it's curved surfaces, no two cells will be colinear with each other.  Does the technology exist to  ensure that you are getting all the power that each cell produces into your batteries.

Generation Without Direct Sun
### What percent of the nominal power of a solar panel do you produce when the panel can "see" a clear blue sky but has no direct Sun shining on it

### Do I still generate power on a bright cloudy day (I know this is like asking how long is a piece of string but I am trying to get a feel for how much energy is produced under different conditions).

Optimum Panel Angle
### How much is my power reduced if my panels are not tilted at the seasonal angle*
*The angle of your Solar panels array can be tilted as a unit throughout the year so that at noon, the panels are at right angles to the rays of the sun.  If the panels are fixed at the best yearly average angle for your latitude, they will be producing less power than they otherwise could be if you adjust the seasonal angle throughout the year.

###  I'm a little puzzled by the optimum angle at which your panels should be set.  Clearly, the very best situation in terms of the amount of energy generated would be to have the panels follow the sun both in the East West direction and in their angle above the horizon.  This would ensure that the panels were always normal (at right angles) to the rays of the sun and hence were always generating the maximum possible amount of energy. Incase you are contemplating such a system there are some severe dissadvantages which I won't go into here.  Lets have a look at mid summer's day (Dec 23 in the Southern Hemisphere where I live).

My house is oriented toward true North.  That is to say, I have a roof surface pointing exactly toward each of the cardinal directions (N,S,E,W).  Lets draw an immaginary East West line through my house.  In the middle of summer, the sun rises over the horizon about 20 degrees South of that line.  It only begins to illuminate my North facing roof at around 9:00AM.  Up to that time, I am not getting any direct sun light on the roof.  Before 9:00AM, a panel would only be generating electricity from a clear blue sky.  The same situation occurs in the afternoon.  Somewhere around 4:00PM, the direct sun no longer shines on my panels and the sun sets about 20 degrees south of my East West line.

I wonder how much electricity I would generate, in comparison with panels which are fixed to my roof, if I had my panels fixed horizontally.  I wonder how much I would generate if the whole array changed its angle to the horizon during the day.  It is much easier to change the angle of the panels in one direction rather than in two directions.  The theoretical answer should be obtainable with a little spherical Trig and some slightly more complicated Calculus. 

Panel Cooling
### How important is it to allow a free flow of air below my panels to help cool them.  What is the effect on power production.  What is the effect on the longevity of the panels.

Single or Double Metering
### Is it lawful in my country to simply turn my meter backwards when I am producing more power than I am using or must I have two meters. (I'd like to find what the regulations are in different countries)

Are Meters Reversable
### Are there meters that will not turn backwards if I produce more power than I use or are all meters reverseable.

Protection of Grid Workers
### What sort of device do I need to ensure that if the grid goes off, I am not sending power into the grid (this is necessary so that workers, fixing a fault on the line will not be electrocuted when they think they have turned off the power to the area of the fault but micro generators are still electrifying the grid)

A Few Batteries??
###  If I am grid connected, would it  be economically worthwhile to have a small array of batteries as a buffer.  (note that for a stand alone system, you need enough batteries to store energy to carry you over periods of no sun.  For a really reliable system, this involves a lot of expense).

Peak Hours Geographically
###  Where can I find information on how many peak hours I have in my area


Note that all these sites measure something different.  Read the fine print carefully.  it is my understanding that peak solar hours are measured with a horizontal detector.  Clearly, if that is so, if you orient your panels at the average latitudinal angle of your location, you will generate more power than the peak hour figure would indicate.  If you go one step further and adjust the angle of your panels, say, each month to keep them normal (at a right angle) to the sun at that season, you will get more power still and if you go all the way and track the sun throughout the day, more power still.  ps.  In case you are going all the way with daily tracking, there are some definite dissadvantages of doing so so tread carefully.

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