End of 2015 reckoning

At the end of a year, it's nice to look back on how one's hydro has done and compare the performance of the past year with previous years.

Here's a type of graph which does that by presenting the data in a pleasingly simple way:

I've chosen to plot 'water years' of output for a reason: had it been calendar years, each year on the first of January the turbine would be certain to be producing maximally in each and every year.  The result on the graph would be that all the plot lines would overlie each other.

By using water years, for which the start date is 1st October, the plot lines become separated because on that date and for some time after it, there is quite a difference in the output depending on how wet the autumn here is proving to be.  Thus in the graph above, water year 2 didn't see any generation until November 11th, whilst year 1 started on October 17th and year 3 carried on from the end of year 2 without a break.  Once year 2 got going, generation came in strongly, as strongly as year 1, as is indicated by the same slope for the two years up to about March 18th when the line for year 2 begins to fall away from that for year 1.

The plot also shows other interesting features:  

• in water year 2, generation continued throughout the 365 days of that year, unlike in water year 1 when generation had to stop on July 7th.  Yet the extra number of kWh's added to the year's total by continuing to generate through the summer months was minimal: precisely 175 kWh over about 80 days.  And overall the year failed to make the total of the previous year despite generation continuing through the summer.
• in water year 3, which has only just begun (green line), maximum generation has been tweaked to be slightly more than the previous two years: peak power now is 782 W as against 750 W. The result is that the slope of the green line is steeper than for the other lines and it has started to cross them as generation in year 3 accumulates kWh's ahead of the rate in the other two years. I wait with keen anticipation to see how it progresses as the year unfolds.

So I end 2015 with clear evidence that water year 2 was not as good a year as the previous one, but with the hope that the new water year just beginning is showing great promise.

The target I'm hoping to hit at some time in the future is a year with 4000 kWh generated !

The system at full stretch

Minimal script in this post to counterbalance the wordiness of the last two,-  just pictures taken this morning of the system working at full capacity:

Gathering the flow

Removing the trash

2.84 lps bottom jet, 0.3 top jet; 1200 rpm

Spent water returning to the stream

3 amps;  305 v dc

Relative humidity 33%, temp 26.8 ℃ (ambient 95%, 14℃)

Working in turbine mode

302 v ± 3  - very steady cf MPPT mode

Wattson: my in-house visibility of output

Sunny Beam: my in-house visibility of cumulative energy

Wattson Anywhere: my on-line visibility of power and energy output, captured the following day

The really accurate figure for energy output for this one day was 18.78 kWh, giving a mean power output of 782.6 watts.  The Wattson and SMA data seen above are not so accurate. The whole system efficiency for the day can therefore be calculated to be 47~ 48% (782.6 / [53.6 x 3.14 x 9.81] ), the inverter efficiency to be 85 ~ 86%, and the pelton/alternator conversion efficiency to be 55 ~ 56% ( [3 x 302] / [53.0 x 3.14 x 9.81] ).

With the whole system efficiency coming in at 47~48%, there doesn't look to be a significant improvement over last year (see here) when I was running in MPPT mode and two equal sized jets.  So maybe turbine mode and having most water coming through one big bottom jet doesn't confer any advantage.

Thoughts about inverters: Part 2

This post is best read after having read Part 1.

When an inverter starts accepting power from a Powerspout and feeding to the grid, an electrical circuit is completed.  In this circuit, the source of power is the SmartDrive alternator and the inverter is the load (resistance)*.  

In common with all circuits, the resistance in the circuit will determine the current. But in this circuit, unlike in other circuits where the source of power is a source which gives constant voltage, eg: a battery, a change in resistance here will not only determine the current but also the voltage.

The reason for this is to be found in the behaviour of permanent magnet alternators (PMA's).  With a SmartDrive PMA, the voltage it puts out is affected by two factors: rotational speed and load. Thus:

• the voltage output is directly proportional to the speed of revolution (rpm).
• the voltage output per revolution (v/rpm) is inversely proportional to the load in the circuit.

So here we have a circuit where the load (the inverter) is variable and can set the voltage by changing the resistance it places on the circuit.  As an aside, we should note that since one determinant of voltage is rpm, the inverter also has some control over the speed of the turbine.

The question to be answered now is: by what process of logic does the inverter decide what load it places on the circuit ?  And the answer is there are two control techniques which are possible, variously named and described as follows:

• MPPT mode (maximum power point tracking) aka: Iterative / adaptive / intelligent load control, -  primarily designed for optimising output from PV.  To quote : Solar cells have a complex relationship between temperature and total resistance which produces a non-linear output efficiency. It is the purpose of the MPPT system to sample the output of the PV cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions. Different methods are used to find the optimum combination of voltage and current which will provide maximum power. In the "perturb and observe" method, the controller adjusts the voltage from the array by a small amount and measures the resulting power; if the power increases, further adjustments in that direction are tried until power no longer increases. This method can result in oscillations of power output. From Wikipedia (abridged)

• Turbine mode aka table mode: primarily designed for optimising output from a rotating generator. In this method: The inverter regulates the input current by reference to generator voltage by using a 'look up' table.  This table, which can also be represented as a curve, defines the relationship which gives best ac power output for any prevailing DC input voltage.  The table, and also the curve, can be programmed by the user to best suit it to the particular turbine and alternator being used. From SMA WindyBoy literature (abridged)

As mentioned in Part 1, both of these control algorithms are to be found in SMA WindyBoy and SunnyBoy 1200 inverters.  Although I'm not familiar with other inverters, eg the EnaSolar range, I believe you can choose either mode in these inverters too.

In the 2½ years I've been operating my Powerspout, I've been keeping a record of the operating dc voltage, (sometimes called the MPPv or Vmpp, ie the voltage at the maximum power point).  For most of that time, I have had a SunnyBoy operating in MPPT mode as the grid interface, but for the past month I've been using a WindyBoy in turbine mode.  The turbine curve programmed into it is the default, factory one without any optimisation by me.

The difference in the way the two modes function is very clearly seen in the plot below of MPPv against ac power out to the grid.  I should add that all data points were taken using the same 42 pole stator: 60-7s-2p-star (which has a v/rpm of 0.509 v when tested in open circuit conditions).

It can be seen that:

• MPPv trends down as ac power rises for the SunnyBoy, whilst the opposite is true for the WindyBoy
• the scatter of MPPv for the SunnyBoy is wide, narrow for the WindyBoy

From these observations, it can be deduced that a WindyBoy holds the dc voltage much more constant and at a lower level than a SunnyBoy.  The plot also shows clearly why I had the problem I had last year when using a SunnyBoy, - the problem of MPPv rising to such a high level at low ac power output levels that it began to knock against the V Clamp's dumping threshold set at 378 v.  This was what prevented continuing generation at low water flows: - so much power got dumped, it wasn't worthwhile continuing. See earlier blog post here.

This year, I used a reduced core stator to get around the problem.  But it would appear that if I use a WindyBoy in turbine mode at low flow times of year, the issue will not arise. 

I never thought I'd hear myself say this: I can't wait for next summer's low flows to check this out !

*(addendum written 27/1/2016) Like all over simplifications, this statement, that the inverter is the load, compromises truth. All grid connected generators run in parallel with each other so that properly speaking the load is provided by the sum total of consumer load on the grid.  It follows that the inverter needs to behave as an 'open window' to the grid, transforming (from dc to ac) as much power to the grid as possible with minimal power being lost within the inverter. Nevertheless, the characteristics of the inverter (its impedance, its capacitance and its resistance) at any point in time have an effect on the SmartDrive output, and so this simplistic statement stands, but purely as a means of gaining understanding of how inverter and PMA interact.

Thoughts about inverters: Part 1

Those Powerspouts which are connected to a national electricity supply are unusual amongst small water turbines in that they interface with the grid through an inverter. I don't know of any other make of water turbine which connects in this way. Small wind turbines more often do.

In the UK, as in other countries, there are strict regulations about connecting a privately owned generating plant to the national electricity network.  In the UK, these regulations are written down in the document: Engineering Recommendation G83 Issue 2 (August 2012).  It is commonly referred to as just G83/2.

In the original version of this document which was called G83/1 and was issued in September 2003, a useful distinction was made (which has been dropped in G83/2) between micro hydros connecting via an inverter and those connecting directly to the grid.  The former were designated Type A, the latter Type B:

There is a clever thing about Type A connection and it is hinted at in the diagram above. It is that a lot of clever electronics have been squeezed into one box. These electronics, both hardware and software, perform two main functions: 

• converting power from dc to ac 
• managing the grid connection according to the requirements of G83/2.  

The development work for these conditioning and controlling functions has been perfected by companies competing in the huge global market for inverters for the photovoltaic industry.  An inverter is, therefore, a sophisticated bit of kit whose price has been forced down by fierce market competition.  For what it is, it's a bargain.

The market for Type B connections is, by comparison with the solar market, tiny. Within this small market it is difficult to develop a grid connection package cheaply: economies of scale are absent and also there is such a variety of rotating generators available for type B installations (induction motors-as-generators, synchronous alternators, 3 phase, single phase) that standardisation is impossible.  Each has to be specially made for its location.  The price is high.

So all in all, Powerspout's use of a standard PV inverter is an elegant and economic solution to satisfying the complicated regulations of grid connection. It is surprising that other small hydro manufacturers have not followed the same route.  

There is, however, a not-so-clever thing about using an inverter: the electricity generated has to be changed first from ac to dc, and then back again to ac, - and at each conversion power is lost, making Type A installations intrinsically less efficient.  Lower efficiency means lower productivity, - quite significant lower productivity over the entire life span of an installation, and that in turn means a return on investment which is not as good. Perhaps this is the reason why others have not followed the same route.

The G83/1 document of 2003 foresaw that inverters used in Type A hydro systems would "normally be an adaptation of a PV inverter".  Today in 2015, it is evident that as a prediction this phrase wasn't precisely correct: the inverters recommended for use with Powerspouts are not adapted PV inverters but standard ones. They operate in the same maximum power point tracking (MPPT) mode that was designed for solar inverters.  EcoInnovation provide on their website 'compatibility tests' for several different inverters and all of them, they say, should be operated in MPPT mode as if they were handling power coming from an array of solar cells.

This adherence to MPPT mode for a Powerspout is something I have wondered about.  There is no doubt that it works and there is every reason to expect, theoretically, that it should control the speed of the pelton to the point where maximum power will be extracted.  But in the two years of running my turbine in MPPT mode, I have noticed that the way the inverter controls the turbine is not always all that it could be. In particular, the control of dc voltage at different levels of power output has given me problems.

Just recently, I have obtained a Windy Boy inverter.  As the name suggests, this was intended for interfacing a wind turbine to the grid.   Its electronic architecture is absolutely identical to the Sunny Boy but the way the inverter is programmed suits it better to a rotating generator rather than a photo-diode. The mode it operates in is 'turbine mode'.

The two modes, MPPT and turbine, are both programmed into all Sunny and Windy Boy 1200's. If you have the right computer connection cable it is possible to re-configure which mode your inverter will perform in. Not having this specialist cable, nor the expertise for the job of re-configuring my existing SunnyBoy, I was happy to find a second hand Windy Boy on Ebay which was already programmed in "turbine mode".

In the second part of this post, I want to try to explain as simply as possible my understanding of how an inverter controls the voltage output from a Powerspout, and illustrate how the two modes end up causing the package of inverter plus turbine to behave quite differently.