# Solar energy design question

sngtdt
I am an amateur science/engineering enthusiast (AA electrical engineering), I would like to design/build a solar energy system consisting of a small tracking parabolic mirror heating a steam boiler to drive a small turbine-generator. The mirror I am thinking 3 foot diameter and I have some idea how to go about constructing it. The questions come in on the boiler design, pressure, safety, how to pump in the cold water feed, check valving, turbine construction, cooling the steam, and many phases of constructing same. Any info on any of the above would be appreciated.

Gold Member
I am also an amateur, so I can't answer much of that. What I would seriously recommend, however, is that you have your pressure vessel built by a professional shop, with an appropriate relief valve provided by them. Safety always has to come first.

Gold Member
Well, a useful number that you will want to know is this- solar energy incident on the Earth at sea level is about 125 W/m^2. With a 3 ft diameter mirror, this means you will only have a maximum power of about 80 W to work with; that doesn't sound like a whole lot when you're wanting to drive a steam turbine and an automatic tracking system...

Staff Emeritus
Homework Helper
Well, a useful number that you will want to know is this- solar energy incident on the Earth at sea level is about 125 W/m^2.

Actually it's closer to 1000 W/m^2. You may be thinking of a photovoltaic cell, where the low efficiency results in an output power close to what you are saying.

See the paragraph under "Solar constant" here:

sngtdt
Thanks for the info, I am aware the pressure vessel would need special design cosideration, as far as the area of the mirror, this would be more or less a model. If it worked, something larger could be made.

Gold Member
Actually it's closer to 1000 W/m^2. You may be thinking of a photovoltaic cell, where the low efficiency results in an output power close to what you are saying.

See the paragraph under "Solar constant" here:

That would be the amount of power incident at the edge of Earth's atmosphere (still in space). By the time it gets through Earth's atmosphere to the ground, I think it is anywhere from 100-375 W/m^2 average. That's at sea level in North America. The number will rise as you go higher in the atmosphere or closer to the equator, and peak power will incease or decrease depending on time of day and weather conditions.

Wikipedia Solar Energy Article said:
...in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).
http://simple.wikipedia.org/wiki/Solar_energy

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Staff Emeritus
Homework Helper
That would be the amount of power incident at the edge of Earth's atmosphere (still in space).

Well no, that number is 1300 to 1400 W/m^2. According to your link.

By the time it gets through Earth's atmosphere to the ground, I think it is anywhere from 100-375 W/m^2 average. That's at sea level in North America. The number will rise as you go higher in the atmosphere or closer to the equator, and peak power will incease or decrease depending on time of day and weather conditions.

http://simple.wikipedia.org/wiki/Solar_energy

Checked out your link. They are averaging in the time the surface is in total darkness, and cloudy weather, to get those numbers. They also factor in that a horizontal surface is not oriented at a perfect 90 degrees to the sun.

http://simple.wikipedia.org/wiki/Insolation

The 1000 W/m^2 figure is more applicable to the purpose at hand: boiling water during daylight hours, where one can aim the setup directly at the sun.

sngtdt
... that doesn't sound like a whole lot when you're wanting to drive a steam turbine and an automatic tracking system...

The part I am afraid will use the most energy is the pump to introduce cold water into the boiler

Con
The pump you may use you can find it by apply the formoula of the 1st low of thermodynamics.

Q= m . C .Dt

m= mass flow rate , c= water heat coefficient 4200, dt= temperature difference between the boile imput and output.

Q= heat gain you may have.

Note: the above formula is only valid for closed systems.

CFDFEAGURU
Before you get to far along on a design, you should get a copy of the ASME Sections 1 and 8 for boiler and pressure vessel code requirements. Also, a copy of B31.1 Power piping would be needed. These are the American Society of Mechanical Engineers code books that govern the minimum design requirements. Also, make sure they are the 2007 editions and for Section 8 you should use Division 1 not Division 2.

rocky14159
sngtd, I realize that what you're building is just a model, but you probably won't get much power at all with a 3-foot mirror, so the turbine generator isn't going to be able to be big.