Function to calculate energy for wind turbines
First, get rid of the double-spacing. I don't know if that's an artifact of posting on this site, or if you really coded it that way, but fix it!

def WindEnergy_calc(density, surfaceArea, windspeed, powerCoefficient):
    """
    This function returns the value of wind energy given density of the air,
    surface area of blades, and windspeed.

    Args:
    * density (float or int) - the value represents the density of the air
    * surface area (float or int) - the value represents the total surface area of turbine
    * windspeed (float or int) - the value represents the windspeed in the area
    *Power Coeffecient of turbine (float) - value cannot exceed

    Returns:
    * total possible kinetic energy created by the wind on the turbine
    * P = 1/2*row*A*V^3*C
    """
    try:
        if powerCoefficient &gt; 0.59:
            powerCoefficient = "Not Possible" #to break code
    except:
        print "Power Coefficient must be less than 0.59 due to Betz limit"
        return()

    half = 0.5
    windEnergy = float(half*density*surfaceArea*windspeed**3)
    return(windEnergy)

Next, fix the name of your function. This is a function not a procedure. You aren't performing some "helper" operation and leaving your results in global variables or class members. You are performing a computation and returning the result! Pick an appropriate name:

def wind_energy():
def calculate_wind_energy():
def wind_energy_of():


In general, function names should include a verb and some supporting words - adverbs or nouns - to flesh out what is being done. For a "pure" function, it's possible that the verb is implicit: the word "compute" or "calculate". For example sqrt() and cos() have no verb, because the verb form would implicitly be compute_sqrt() or calculate_cosine(), which doesn't add much value. (On the other hand, we fopen() and fread() because the verb isn't quite so obvious.)
For a function taking lots of inputs, you want to have a really obvious ordering of the inputs. That is, open('foo.txt', 'r') should make it obvious that the filename is first, and the mode is second. (It doesn't, btw. But it should!)

In your case, you have four incoming parameters. None of them seem obvious to me. I'd suggest that you take advantage of python's named-only parameters to put a name on each one of them:

def wind_energy(*, density, area, velocity, power):


Above, the * marks the end of positional parameters - meaning there are none. Every parameter after the * is required to be named. So you call this like:

we = wind_energy(density=1.0, area=4.6, velocity=20, power=0.1)

Cite your sources! I found a website that I think contains your source equations:

http://formulas.tutorvista.com/physics/wind-energy-formula.html

You need to include this, and the name(s) of your equations, in your doc comment. Don't write this:

# No!
"""...returns the value of wind energy ... """


Instead, write this:

# Yes!
"""... return the wind energy computed with Farnsworth's Formula
(http://www.farnsworth.com/wind-energy-formula.html) ... """ 

Command your minions. This is purely a "social convention." But the standard convention for commit comments and function documentation is to use the Imperative Mood rather than passive voice. It's almost as if your program is telling the computer what to do. (Wait, what?)

Change this:

This function returns the value of wind energy...


Into this:

Return the value of wind energy ...


Likewise, when you commit your changes to Mercurial, don't write this:

I updated the code based on suggestions from codereview...


Instead, write this:

Make changes from codereview ...

Validate and rename!

You have two constraints, one explicit (the Betz limit) and the other implicit (all the parameters are numeric). Validate them!

I'll suggest you have a look at Python's numbers module, which provides abstract base classes for numeric types, and applies a "hierarchy" to them. The most appropriate type, since you use "float or int" a lot, seems to be Real. The Real type adds "conversion to float" which is right up our alley:

rho = float(density)
V = float(velocity)


(Also worth noting: rho is the name of a Greek letter. row is something you do in a boat.)

You might even consider the fact that Python supports unicode identifiers, like so:

 = float(density)


(Sadly, there's no good way to spell V³ except for V ** 3.)

For your explicit validation, you have totally bungled your code:

    try:
        if powerCoefficient &gt; 0.59:
            powerCoefficient = "Not Possible" #to break code
    except:
        print "Power Coefficient must be less than 0.59 due to Betz limit"
        return()


First, try: / except: is used when you believe that the code inside the try: block will raise an exception. You have an if-statement inside the try block that will not raise. 

Second, this is not a function that should ever call print. I understand you might insert print statements for debugging, but printing anything is outside the scope of this function. If your caller has passed you an invalid value, the correct behavior is to raise an exception - typically a ValueError - and let the caller deal with their own stupidity. (But make sure you document the limit in your doc comment!)

Try this, instead:

# Somewhere global, like outside the function or top of module:
BETZS_COEFFICIENT = 0.593
"""Betz's coefficient- see https://en.wikipedia.org/wiki/Betz's_law"""

# Inside your function:
if power &gt; BETZS_COEFFICIENT:
    raise ValueError("Power coefficient must be &lt;= Betz's coeff. It's the law!")


Also worth noting: when you invoke return() with empty parens, it doesn't mean "return nothing". It means "return a tuple with zero items". The () expression (note: expression) is an empty-tuple constructor. This is consistent with calling a function with no args, since positional args are a tuple:

&gt;&gt;&gt; def f():
&gt;&gt;&gt;     return()
...
...
&gt;&gt;&gt; x = f()
&gt;&gt;&gt; print(x)
()
&gt;&gt;&gt; type(x)
&lt;class 'tuple'&gt;


What you meant to do was just a bare return. Except that was wrong, and what you really meant to do was raise, as above.