More on moisture

Eleven weeks ago, I wrote about Managing moisture. It described my attempt to create a warm and dry atmosphere in the electrical side of the turbine by packing in a kilo of silica gel beads and blocking the ventilation ports. The aim was to improve insulation resistance and reduce corrosion. What's happened since?

At the time I sealed the compartment, generation was only 300W, so temperature inside did not immediately rise very much.  But as winter generation picked up (it has been > 780W since 1st December), the temperature rose to 18-20℃ above outside temperature, reaching nearly 40℃ on warm days. There was nothing worrying about this. But at about this time I began to recognise that the warmer the SmartDrive operated, the lower was its output, about 6 watts less per 5℃ temperature rise (see last diary entry and also here).  After thinking about it and deciding that maximal efficiency was my main aim, I re-established ventilation by putting back the louvred ports in two of their three locations. Ever since, the temperature inside has been lower at between 19 and 24℃.

Restoring ventilation predictably made relative humidity inside rise. When sealed, it was as low as 10%, but unsealed it rose to 20 - 38% because more air exchange occurred with humid, outside air.  The inside humidity now fluctuates in the 20-38% range, tracking the humidity of outside air, and the temperature in the compartment. I have been assuming the silica beads are still taking up moisture as the inside RH is well below the outside value. Later on I'll take the bags out and weigh them to see just how much water they've taken up.

Today, I've been measuring the temperature of the water powering the turbine.  It was 8℃.

It's the temperature of the bulkhead that I really want to know and I'm assuming it will be the same as the water. The temperature of the bulkhead determines when moisture will condense on its surface and this measurement was the only factor I'd never got round to measuring in my quest to predict the conditions which will create condensation.

In the diary entry "Managing moisture" I gave a link to a dew point calculator.  This is a handy tool for working out at what point condensation will occur, handy because the computation is complicated: there are three variables to consider: the relative humidity of ambient air, the temperature of the air, and the temperature of the surface on which condensation is to occur. 

Convenient as the calculator is, a graph gives an alternative way, a more visually predictive way, of appreciating how relative humidity and ambient temperature can be manipulated to avoid condensation. These two variables, temperature and humidity, are the only ones which can be manipulated, since the temperature of the bulkhead obviously cannot be changed.

To understand the graph requires an understanding of humidity: humidity is the amount of water carried as invisible water vapour in a mass of air; the amount of water vapour carried will depend on how moist the air happens to be: deserts are dry, rain forests are wet; but the maximum amount of water air can carry in either region depends on the temperature of the air: the warmer the air, the more water; the cooler the air, the less water.  When a body of air at a certain temperature and carrying a certain amount of water (as invisible vapour) comes into contact with a surface which is colder, the layer of air immediately above the surface is cooled; in being cooled it finds it can no longer carry as much water vapour as the warmer air further away from the surface, and if it cools below a certain point, water comes out of being in a vapour phase and condenses on the cold surface as liquid, first as misting, later coalescing to droplets.

What the graph shows is a family of curves. Each curve describes the ambient humidity and ambient temperature at which condensation will occur on a surface having a given temperature. Several curves are needed because each shows the humidity / temperature relationship for a different surface temperature. In my graph, curves are shown for surface temperatures of 0, 4, 8 and 12℃. 

The measurements I took today recorded the temperature of the bulkhead as 8℃, the temperature in the compartment as 23℃, and the humidity in the compartment as 28%. The red arrow in the graph has its point at the place for the two temperature measurements: where today's compartment temperature 23℃ intersects with the 8℃ curve.

At this point it can be read on the Ambient Relative Humidity (RH) axis that dew (condensation) will form if ambient relative humidity is 38%.  It will of course form too if the RH is at any figure higher than that. Since the ambient RH inside the compartment today was only 28%, it follows that condensation would not have been possible under today's conditions.

We can use the graph in an alternative way to see at what ambient temperature condensation will occur when the RH is 28% and the bulkhead temperature is 8℃: it will be 29℃ (as best as can be discerned from this crude graph).  Now 29℃ is a temperature which might easily be reached if ventilation was blocked off so we might conclude that blocking off ventilation would be a bad idea from the condensation point of view (it will certainly be bad from a power generation viewpoint); but if ventilation was blocked off, ambient RH can be expected to be lower, maybe only 20%, perhaps less, perhaps as low as 10%, and then it can be seen by extrapolating the green 8℃ line to the right, that ambient temperature could be allowed to rise to perhaps 45℃ before condensation would occur, but only so long as the bulkhead temperature is at 8℃.  And if the bulkhead was colder, say 4℃, we would have to jump to the 4℃ curve which gives a whole new set of temperature / humidity relationships.

You can begin to appreciate what a complex business condensation is, and yet it is a very precise and predictable subject ... if only one knows the values of the variables. 

What a fun occupation it is measuring things and using the data to better understand the world about us!  If it prevents a V-Clamp board failing from poor insulation resistance caused by dampness, it will be useful as well as fun.  Here's to hoping!

Update on output & more on temperature effect

Just a couple of light weight matters to post in this diary entry.  I call them light weight to mark them as more easy to follow than the heavy weight stuff discussed in the last entry and in the "Thoughts about Inverters" entry further back.  
Much useful discussion resulted from those posts, only some of it in the comments section after them. I'm still working to understand it all and perhaps will post about it if I can get my newly found understanding into an intelligible form. The comment which encouraged me most (which was not published), came from someone who really understands motors and generators when he said, after giving as lucid an account as anyone could give: "I hope this makes some sense, there are few explanations that make any reasonable attempt to explain what is really happening in an alternator as you load it", - my thoughts entirely.

So the light weight matters are:

1. Generation so far this year.
It has been extraordinarily wet here in Wales, (UK) and the Powerspout has had an uninterrupted spell at full power since 1st Dec.  This, together with it generating 30 watts more for the same flow than in previous years, has seen the record of energy generated outstrip previous years; long may it continue!

2. Confirmation of temperature effect on generation.
Early in January we had a brief period of cold weather.  Night time temperature dropped to below freezing for a few nights making the temperature inside the SmartDrive housing 19.4 deg C. Compared to the normal operating temperature of about 26-29 deg C, it was thus some 7 to 10 degrees cooler than usual.  As the records of power output below show, power increased by 15 watts when it was colder, confirming my earlier observation.

Both these records were with a de-finned rotor.  The aim of de-finning was to reduce energy lost to the extra work of revolving a finned rotor. But de-finning necessarily reduces ventilation and thereby raises temperature.  Since raising the temperature clearly reduces power, it has to be wondered whether de-finning gains anything at all.  When I looked at this a while back, the gain from de-finning was only 1 to 2 watts but no consideration was given in that study to holding temperature constant. 

So there's another research project in the making ! 

The relationship between power, speed and voltage.

Note added after this post was published: the comments section at the end corrects some of what I write below.

2016 and the start of a new year, - but the science theory I write about in this entry dates from 1831. How true it is that we stand on the shoulders of the giants of former times, - Faraday and Lenz.

In April 2015 I installed a tachometer, (which you can read about here) and ever since I've been measuring the Powerspout's rpm and dc voltage at different levels of power output. 

Gathering this data has produced some interesting results which have helped me understand better how shaft speed and operating voltage* vary according to the power generated.  In this post I want to write about what I've found, and to explain my understanding of the physics behind why the results are as they are.  

I am not someone who is a little qualified to write about the matter, - I am not qualified to write about it at all ! - and so some may find my explanation incomplete and insufficiently technical. But others like me, trying to work it through for themselves might be helped by an attempt at a simple explanation. For the subject is complicated,  and it gets more complicated the deeper you venture into it.  Reasoning it all through carries a serious headache warning. You have been warned !

When operating voltage and rpm are plotted on the same graph against power output to grid, this is what the two plots look like:

Take note that the scales on the two vertical axes, rpm and dc volts, are numerically the same. Because the scales are the same, we can make an observation, a valid one, about the slopes of the two plots: the slopes are not the same.  To put it another way, operating voltage does not increase proportionately with rpm. 

This seems a strange finding.  The first axiom of a pma (permanent magnet alternator) is that its voltage rises as the speed of rotation rises.  What can be happening to make voltage rise more slowly?

When 'volts per rpm' (v/rpm**) is plotted at different levels of power output, this is what we find:






Note that the scale along the horizontal axis is the same as in the first graph. This allows us to say that over the same range of power as is depicted in the first graph, the v/rpm decreases. 

Putting the findings of these two graphs together, as power increases we can conclude that: rpm increases, v/rpm decreases and the overall effect of the two together is an increase, albeit a modest one, in operating voltage. 

The second axiom of a pma says that voltage is inversely proportional to load, ie voltage goes down as load goes up; having established that voltage (expressed as v/rpm) goes down as power increases, it should follow that load increases as power increases, which is to say: more power is more circuit load. Is this the case or have we violated the second axiom?

The way through to answering this is best reached by thinking about the current flowing in the pma stator coils as power increases.  

When more water is made to strike the pelton (from having changed the nozzles for larger ones), the torque on the pelton is increased.  This increase in torque translates through to the electrical side mostly as increased current flowing through the stator coils. We have to say 'mostly' because it is evident from the first graph that rpm increases too and that must mean some increase in voltage, but the main effect nevertheless is to increase current: more hydro power is more current.

Going back to the second axiom in its re-arranged form, it said "more power is more circuit load". Is this the same as saying "more power is more current ? - and the answer is "yes, it is", because load and current equate to each other.  (load is resistance; more load is less resistance; less resistance (at constant voltage) is more current (Ohms Law): therefore more load equates to more current).  The second axiom is thus seen to remain intact.

Why should v/rpm decrease as power output increases? It's because of an effect of the higher current flowing when hydro power is more, and that effect is of a magnetic field being created by the current as it passes through the coils of the stator.  The field so created opposes the magnetic field which caused the current in the first place, - so it opposes the field created by the spinning magnets of the rotor. The opposing field induces a correspondingly opposing voltage in the coil, called a 'back voltage'***.  And because the back voltage opposes the voltage induced by the rotor field, the 'net' voltage actually leaving the pma gets to be reduced. 

Well, net voltage (= operating voltage) would be reduced if rpm remained constant. But as we saw above, rpm did not remain constant: it rose with increased power generation to make operating voltage go up, albeit modestly. So the unit we have to use to see that the 'net' voltage does indeed go down is the unit of v/rpm. 

The laws of physics that describe these phenomena are Lenz's and Faraday's Laws.  For those interested, they can be looked up under the links given, - although some descriptions I found quite difficult to follow because they usually talk about these effects in motors rather than generators. For me, working it out from observed data and relating it all specifically to a Powerspout, has been the way to greater understanding. But as I said at the start, there is much more to it than this basic run through.

They do say it's therapeutic to keep the older brain thinking !

*in an earlier version of this post I used the terms 'operating voltage' and 'MPPv' (maximum power point voltage) to be the same thing. But as Hugh has pointed out in a comment, if I am using a WindyBoy in turbine mode, the term MPPV is not correct: the mode does not seek a maximum power point.  So I have dropped the term MPPV as of today: 27Jan 2016.

** v/rpm is simply obtained by dividing the value for operating voltage at a given level of power output by the turbine rpm at that power level. The value for v/rpm in open circuit (Voc) is used to designate stators with different coil configurations.  When measured in open circuit no current flows, so the v/rpm at Voc will be the highest v/rpm ever possible for that stator.

*** back voltage is more properly termed back electromotive force, or back emf.