The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Saturday, 17 February 2018

Meter deviousness

The 'big six' electricity companies in the UK understandably get concerned when they see from their records that a customer's bills have suddenly become smaller.  If they see that decrease in their income to have coincided with the installation of a 'micro-renewables' at the customer's premises, the conclusion they come to is: the supply meter must be being made to run backwards when energy is exported to the grid.

Such has been the accusation levelled at me in this past week by SSE (Scottish and Southern Electricity).

It is completely reasonable that SSE should want to investigate when they think such a situation has arisen, but the somewhat irritating thing is the way they go about it.  Instead of writing to say what their suspicions are, they say this: "we have come to know that your meter is incompatible with the  micro-renewables system that you have had installed. On a regular basis we extract data from flows that are sent between ourselves and your local network operator. It was highlighted on this report in January that we have come to the knowledge that the meter at your property is incompatible".

Part of the reason leading them to write such utter nonsense is that in the UK, the government has set a target of having 'Smart meters' installed in all domestic premises by the year 2020.  The onus for meeting this target is upon the "big six" electricity companies. So given a situation where they think a meter is running backwards, rather than saying so they just say "your meter needs to be replaced".

From a consumer's point of view however, having an old-style meter replaced by a Smart meter carries implications, - possibly quite major financial implications affecting the income he gets from his "micro-renewables" installation.  This comes about because a Smart meter will measure precisely how much energy is exported, so instead of export payments being based on "deeming", they will come to be based on a metered reading.  For a Powerspout owner where deeming is reckoned at 75% of generated kWh's, this will mean a severe reduction (read here for more about why).  And for people who have any of the devices (such as an Immersun or a SolarCache) that divert surplus energy to alternative loads within the home, - again Smart metering will be a hit on the income they receive from that part of the Feed-in-Tariff which comes from the export payment.

Now SSE is, in my opinion, a good company.  I have held them in high regard ever since they helped with the installation of a 300 kW hydro at a small hospital in Uganda where I used to work (see here).  So when I took up my pen to object to having my meter replaced, and to object to the manifestly fabricated evidence they put forward for replacing it, I did so with a feeling of respect.

The incontrovertible weapon I have for resisting is that my meter does NOT run backwards. So if that argument is their sole reason for insisting on replacement, - their case fails.  If, on the other hand, they were just using that as a Trojan Horse argument in order to get one more old-style meter replaced by a Smart meter to help meet the 2020 target, then they will have to think again.  Although the supply meter at my house is the property of SSE, any change to the installation needs to be made with the agreement of the customer, - and from me that is not likely to be forthcoming.

Post script added 21 Feb 2018: an interesting, if trifling, point of detail came to light after writing the above entry when SSE sent another email explaining why they were so insistent about my meter being able to go backwards. They had the wrong code in their records for the type of meter I had. Apparently the 'H' at the end of the type number of this make of meter is all important: lacking it means the meter that can go backwards; having it means it can't. So now we know !




...to show it had a symbol indicating it could not go back wards.  My correspondent

Saturday, 27 January 2018

Comparative efficiencies

In this diary, I seem to go on rather a lot about efficiency.  For some the subject may hold little interest, but for me knowing the efficiency at which my Powerspout is operating and trying to improve it, are matters which engage me.

To recap as to what efficiency is in relation to a water turbine, - it's a measure of how good the installation is in translating, into useful electricity, the energy which is available from the fall and flow at a site.  But a consideration of efficiency need not apply only to generators which use hydro power.  It can be applied to generators using any of the energy sources commonly used: coal, gas, nuclear, solar etc.

Recently, in searching for information about how efficient other people's hydro sites are, I have come across the web site of the US Energy Information Administration. It's quite informative. The site can be found at this web link and there is much of interest answering the question: "What is the efficiency of different types of power plant?"

Summarised below is the data the link provides, - the efficiencies are given for the different types of power station in the US, in the year 2016, averaged across the efficiency of all power stations using each form of energy. The point to be noted is that these figures are real figures, derived from actually measuring the efficiency of each and every power plant in the US. It's an exercise done every year and the data presented includes how the figures change year-on-year for the last 10 years. That too is interesting because it shows how efficiencies in each category are tweaked to be better as technology improves over time.
Whilst noting that the efficiency for 'conventional hydroelectric'is 90% (conventional meaning that pumped storage hydro is excluded), it should be made clear that the power plants included in the survey are all large scale installations generating MW's of power, - like the Hoover dam which can put out 1,345,000 kW.  It is the size of such power stations which makes possible efficiency benefits at every stage of the energy transformation process, so that an overall efficiency of 90% becomes possible. The same would never be possible for a Powerspout turbine generating barely 1 kW. Small size is inherently less efficient.

So my 'best' efficiency of 55% comes a long way below the Hoover dam efficiency ! But what I would really like to know is the efficiency seen by other operators of really small sized turbines. How does my 55% compare with them. The web seems to be silent on the matter. Anyone with any reliable info in this area please comment.


Sunday, 31 December 2017

System efficiency at top-end flows

December has been a better month, - we've had 118 mm of rain, which is double what fell in November and three times what fell in October, - and it's now working its way through for me to see a pleasing uptick in generation.  Units of energy generated in December were 312 kWh (a figure which is poor for December) and the capacity factor for the month was 56%.  At the moment the turbine is putting out 892 W from a flow of 3.26 l/s and these are heights of power and flow which I have never before aspired to.

The explanation behind such an unprecedented level of generation is that I've been waiting for a time when there was copious flow so I could run some tests to check what the system efficiency is at the top end of the range of flows I see at my site. To do this I have had to run the installation a little beyond the limits imposed by the various authorities that licence small hydros: strictly the output should not exceed 750 W and the maximum flow 3 l/s.

Previously in this blog, I presented a plot like the one below to illustrate how operating with one jet in the bottom position was more efficient than operating with both jets:



With the data I had at that time, the plot seemed to show there was a clear advantage to using one jet on the bottom - it gave an improvement in system efficiency of about 2 %, and this advantage seemed to apply throughout the flow range, - although it should be noted that data points for single jet operation are sparse for higher flows, there being only one.

Having copious water available at the moment, it has been possible to get data readings for single jet operation very easily. This has meant I've been able to fill in the gaps in the earlier data series and better define what happens at the top end.  The latest plot, which is based on a completely new and more refined data set, now looks slightly different:





... it will be seen that when a very large single jet is employed, one that delivers 3.12 l/s, the efficiency starts to drop off and begins to fall below the trend line for two jet operation.

I'm not sure why this should be. There shouldn't be any question of water from this bigger nozzle missing the cups on the runner: the cups are 70 mm wide and the diameter of the jet 12.2 mm. Maybe it's just that there's a lot of splash from the force of this big jet and that impairs the efficiency.

Some of the reason for the fall-off in efficiency beyond a flow of 2.5 l/s in both single and two jet operation is that shaft speed begins to rise away from the optimum speed for the runner. I have found that with flows greater than 2.5 l/s I have needed to change from the standard Type 2 rotor to the more strongly magnetised Type 2+ rotor. This keeps the shaft speed down to nearer the optimum speed, which for my site is 1000 +/- 100 rpm.  At the moment, generating as I am 892 W from 3.26 l/s, the shaft speed is 1165 rpm.  Were I to push the experimenting and deliver even more water to the runner, the downward trajectory of the efficiency curves for both single and double jet operation would fall off steeply as shaft speed began to rise despite the 'braking effect' of the Type 2+ rotor.

As it happens, I don't want to do such an experiment: the inverter I have is a WindyBoy 1200w and for continuous operation (as is the case when the inverter is coupled to a Powerspout) the output rating of 1200 W has to be down-rated to 900W.  Not wanting to roast the inverter, I think I'll just stay at 892 W.