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Micro Renewable Energy System Performance Data |
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When I built the wind turbine I had done a good bit of research on using a PM motor for a DC wind turbine and while there was a lot of general information on how to hook it up, there was not much data on the actual performance. Several people on the web mentioned that around 6 amps was considered a normal output but I had not had the opportunity to really take any measurements that would confirm what my specific turbine was able to output.
In addition, I knew that the blades were a bit short, or at least that was what I thought from reading a lot of article on blade design, but I again had no real data to evaluate the design. The tail seemed to work just fine but again, I thought the size was marginal.
Therefore, when I moved the turbine to the farm and incorporated the Arduino as the brain for the system I was looking forward to taking some real measurements that would give me an idea of the performance that could be expected from a small, home built system.
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Using the Arduino I can quickly plug into the USB port and monitor the wind turbine output, solar panel output and battery voltage. As I expand the system I want to incorporate a wireless link so that I do not need to have a laptop on site to monitor the system.
On a warm and modestly windy day I arrived at the farm to plug in and ended up downloading about three hours of data. The sky was overcast with no real breaks in the clouds so I had modest expectations for the solar panels and Marie had recently run the pump so I expected to see the batteries taking a charge. However, it was really the wind turbine that most interested me.
While I took about three hours of data, I am only presenting the first 30 minutes here. When I looked at the data, it provided me with some interesting information and a very clear picture of what will be necessary to improve the system.
The turbine needs to be running at about 375 RPM’s to achieve a sufficient output to charge the battery and while I had taken some measurements, using a DVM, I did not have more than these to work with. I knew from that modest effort what the sound of the turbine was when it was charging and could estimate how much it was putting out when it was less than that level. What I was not prepared for was the amount of time that it never really did anything.
To clarify that last statement—I could see that much of the time I was recording data that the turbine output was below the battery voltage and consequently the turbine was not charging the battery. This was a surprise for me as I had expected that more of the time the turbine was running would be used for battery charging than actually happened.
A data point was recorded every 2.5 seconds so I would have a very clear picture of the performance. This may sound like a high sample rate but as I had learned in a previous commercial system, it was the only way to get a true picture of the turbine’s operational characteristics. If it is longer then about 2 seconds, the data points tend to smooth and you loose many of the transitions. The chart above is for a 30 minute snapshot in time.
When the wind turbine is turning at a level that is not sufficient to charge the batteries, the output is essentially wasted. Only when the output is high enough to be greater than the battery voltage does charging happen. As can be noted on the graph, most of the time the turbine is not turning fast enough to generate a charging current.
Solar Panel output:
The output from the solar panel was graphed over the same 30 minute period. It too showed some interesting data.
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The sky was quite overcast so the output variations came about when either the sun would briefly shine through or when a less dense layer of clouds would pass over. While I did not measure the actual current output from the panels, I can see that I need to set this up. However, what is useful to understand is that this type of data is necessary to determine the size of a solar system for a given application.
On the farm we do not run the pump every day and consequently our energy demands are quite low. I have noted in the past that we can really survive with just the solar panels but when the area under irrigation is expanded, we will have additional demands placed on the system and this information is very useful for planning. |
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Battery Voltage:
The data from the battery system provided some very interesting information about the way the batteries charge and what is happening to them as they charge. To clarify this comment I need to once again stipulate that the batteries that are being used came from a radio data system in a gas field south of Dallas. These batteries had a very hard life and were considered junk when I got them. They were overcharged and over-discharged. To get them to work at all required a good bit of effort to de sulfate them to the point where they would hold a charge.
What I always knew, but had never documented, was that no two of them were going to behave the same and this is shown in the graph below. In addition I always knew that they never completely recovered but I reasoned that they were free and they worked well enough in my home system. This is really the first time I have documented their performance. |
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Tentative Conclusions Regarding System Performance:
Since the average output of the wind turbine, over the first hour I took data, was about 9.67 volts, it is clear that to improve performance I need to incorporate a “boost converter” that will kick in around 8 volts. This will allow the output of the turbine that is between 8 volts and the battery voltage to be “boosted” to a level that can be used to charge the batteries. As noted above, most of the energy from the wind turbine is lost because the output is to low.
While I had a plan to build a boost converter, I found one at Oatley Electronics that did 90% of what I wanted. It cost me $10.00 plus shipping and was an easy to assemble kit. I have one working on the bench and will install it when the weather clears. It provides 13.85 VDC when the turbine output is between 8 and 12.7 vdc. While the output is only about 2.5 amps this is 2.5 amps I do not get at the moment.
By using the Arduino to sense the output of the wind turbine, I can switch the boost converter out of the circuit when the output of the turbine is high enough to charge the batteries directly. When the output from the turbine falls below the battery voltage, I switch the boost converter back in the circuit. This configuration will allow me to use more of the energy that the turbine produces, not have to build an MPPT controller and can solve a problem with a very small investment.
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Herb & Barbara our interests and family |
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The chart to the right shows in very good detail what is happening with the system. The time period for this graph is less then a minute and this particular time period was selected to illustrate a point regarding the actual charge cycle from both the wind and solar inputs.
Two points are clear—during a ten second period the batteries received a charge from the wind turbine but the remainder of the time, the energy created by the turbine was lost. This would be one of the “blips” in the chart above |
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Charging conclusions:
From the battery interaction over time and the way they are accepting a charge (battery voltage variations), I can deductively concluded that incorporating a PWM charging cycle should improve the way the batteries accept a charge current. Right now I simply route the output of the solar and wind systems directly to the batteries. I monitor for a specific battery voltage to determine when they are fully charged and once it is reached, I divert the charge current to the dump load.
While this is OK, it is not the best way to charge a battery bank. Using a PWM waveform will improve the way the batteries accept a charge and most important, disulfide them in the process. While the batteries were free, I nonetheless want to get the maximum life from them and keep them in the best health I can.
Since the Arduino has a PWM output that is not used at the moment, I will add a pass element for the charging current from the solar and wind system to the batteries and switch it, using the PWM output. The wave form for the PWM output is controlled in software so it can be easily modified as I collect additional data on the battery system. While the first PWM charger will be a simple one with a 50% duty cycle, I am planning to make it smarter by changing the duty cycle in response to the state of charge in the batteries. This will be a simplified PWM charger that I can modify with software.
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The fluctuations noted in the graph came about because of changes in battery chemistry as they charged and from the interaction between the batteries as the charge current is presented to them. While the graph shows a general increase in stored energy, the “blips” are the result of two influences. The first is the actual charge received from the wind turbine when it was operating above the battery voltage and the second is the interaction between batteries. |
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In this 45 minute view the wind turbine output, plotted over the battery voltage, shows that the actual charging time for the wind turbine comes in short bursts. While the average output from the turbine does charge the batteries, the amount of energy that is unused is significant.
From this graph two conclusions are possible: 1.) a lower cut in speed for the turbine would improve the average output and
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2.) using a boost amplifier, as discussed below, would improve the average charge current availability. |
