Electrical meters and DC

[femail]

Hello,

I have a question.

A local community support agriculture has a small PV system with batteries and an inverter driving LED lighting. The aim was to power a water pump inorder to irrigate crops with rainwater. The original pump (AC) wasnt being driven enough by the inverter. The installation company replaced the pump with a DC pump, instead of upgrading the inverter to handle the pump. Since we claim a subsidy for off-grid power generation, this has raised a problem. The DC isnt being metered.
Is it possible to feed DC through a meter?
Can this be done with the same meter that the AC goes through or does it require its own meter?

Hope you can help!

BoB

 

[Marvo]

I've put this in the Solar PV forum for you Femail.

 

[davesparks]

You'll need a DC meter

 

[Worcester]

I don't believe that there are any approved DC meters ( I may be wrong) your installer should know better.

You are correct in that you are now using the generated energy and not getting paid for it, they'll need to ensure that the inverter is suitable for the purpose intended. - Did you specify the requirements at the start of the project - what does your quotation and contract say.

If you specified the pump and the pump wasn't up to the job which means a new pump and that triggers the requirement for the new inverter then it isn't your installers problem or mistake and you will need to pay for the upgrades.

 

[TedM]

It is impossible for MID to cover DC meters as the definitions used in the testing standards are written to only cover mains AC electricity where the voltage has an actual frequency and a reference frequency - e.g. 50Hz ±2%

MID would have to be re-written to allow DC meters to comply.

 

[CAB Marine]

As I understand, you are charging a battery bank via a (hopefully MPPT) charge controller and then using an inverter to supply your AC electrical requirements. You have no tie to the National Grid for feed in/emergency power. If I am wrong, please clarify.

There are ways of measuring usage, although none that I know of are fool proof. Whether they meet the requirements of the off grid subsidy I have no idea. I was unaware that such a thing exists although I am interested as I am totally off grid myself.

Who makes your inverter and solar regulator? How big is your PV array? How big is your battery bank? Who is in charge of maintenance (assuming flooded lead acid batteries) Victron, Outback, Mastervolt and NASA to name a few make this sort of thing but none is "fit and forget" and I need some more info to offer more than an unhelpful guess!

 

[Worcester]

As I understand, you are charging a battery bank via a (hopefully MPPT) charge controller and then using an inverter to supply your AC electrical requirements. You have no tie to the National Grid for feed in/emergency power. If I am wrong, please clarify.

There are ways of measuring usage, although none that I know of are fool proof. Whether they meet the requirements of the off grid subsidy I have no idea. I was unaware that such a thing exists although I am interested as I am totally off grid myself.

Any solar PV system designed and installed to MCS standards that generates AC could be eligible for Generation Tariffs under the UK feend in Tariff Scheme. See here: https://www.ofgem.gov.uk/publicatio...ariff-scheme-tariff-table-1-july-2014-pv-only and https://www.ofgem.gov.uk/publications-and-updates/feed-tariff-factsheet-about-scheme-and-how-apply Systems have to generate AC however they also need to be at a 'fixed' location so mobile homes (unless static ...) / canal boats etc are not eligible.

They are paid for what is generated as AC power, and it is used for an eligible purpose. Hence although there may be losses in generating AC and charging batteries via an AC charger or else dedicated device such as SMA's Sunny Island, then it makes financial sense to do so.
Hence they enquiry about AC pumps...

 

[CAB Marine]

Absolutely my understanding of the situation - and why I asked for clarification about "off grid" status. I have no knowledge of grid-tie systems, although I would assume that they are installed with a meter close to the grid tie inverter in order that the electricity cost and subsidy worked out. I presume that this would be measured in kWh. For an off grid situation, powered entirely by battery, measurement would more usually be in Ah because the important factor becomes not "how much is my electricity bill?" but "how long until my batteries are flat" and battery capacity is usually stated in Ah.

As I read the OP, they are being subsidised according to how much power they provide for themselves. This seems like a hell of a wheeze. If, in fact, they have a hybrid system that is off grid until they run out of battery power and then the grid kicks in, I can see the potential need to meter the pump, although depending on the set up, the pump may only run off their own produced electricity with no grid back up…..the more I think about it, the more of a minefield it becomes!

 

[Earthstore]

What would be wrong with using a DC pump connected to the batteries via a switch, instead of running it direct from the panels? Easy solution?

 

[Worcester]

What would be wrong with using a DC pump connected to the batteries via a switch, instead of running it direct from the panels? Easy solution?

Nothing so long as it is all configured correctly.

 

[femail]

Thank you all for your responses.

The set up involves about 8 PV panels mounted on a shed roof.
These panels connect to the management electronics that govern the charging of the batteries.
The batteries feed to and inverter and a consumption meter and on to the LED lighting for 2 sheds.
The pump was a domestic circulation pump (for domestic radiator applications) but was not man enough to drive water from the storage butt through to the polytunnels and trickle tape. I got involved post installation. realised the pump wouldnt do the job, and switched out the circulation pump for the pump that was originally purchased for the task that was in store when the irrigation system was originally envisioned. This pump worked fine unloaded, but failed to rotate under load. This is with 2 meters of head provided by the water in the butt. I suspected that the pump wasnt getting enough power and was being throttled by the inverter. Checking the pump stats and th inverter stats proved that this was likely to be the case. I got the management committee to contact Celtic renewable energy to address the problem and change the inverter for one that would allow enough 'juice' through to power the more powerful pump. This company visited, and for their own reasons, eventually swapped out the more powerful pump for a DC pump. The DC pump provided water to the polytunnels with enough power to deliver rainwater to the butt to the top of the mainstorage butt ( about 2 meters high). I judged this as an improvement but highlighted that the proof of 'the pudding would be the eating' and that I wouldnt know if this solution was adequate until I had seen water coming from the trickle tape. I realised a few weeks later that DC power was bypassing the AC consumption meter and that therefore any power derived from the panels and batteries wasnt being metered for the dc pump as it bypasssed the meter.

It is because of this situation that I contacted this forum to find out if a dc pump could be run through the existing meter through which the AC is running through on the basis that electrons are electrons the only problem I had was if the DC might bleed back through the AC circuits to the inverter and damage it from the ac side.

I have had a response suggesting that I require a DC meter ( consumption?) and was just starting to wonder where I could find such a beast when I came back to the forum and re-read the further advice.

Does this 'tome' clarify the situation and change the advice?

Thanks again,

BoB

 

[SibertSolar]

It does help clarify, thanks. Bottom line - you aren't going to find a DC "meter", not one that's Ofgem or MID certified anyway (as has already been advised) so it's a waste of time trying to measure the consumption of the pump from a FiT point of view. It's a valid idea to run a DC pump though, albeit taking an unmeasured/unquantified amount of power from the battery bank - what size pump is it and how long does it run for each day, on average?

 

[femail]

Well I will find out and get back to you on this cause im at home at the mo not on site.

I can tell you that the idea is to run the pump for an hour or so a day to pump water to irrigate.
When the ibc containers (which capture the rainwater) fill up, we open a valve to redirect the pump output from the irrigation pipework to flow into the larger 5500 liter butt, to store the rain water.

The aim is to reduce our financial dependance on mains water which we buy from a golf course at normal cost plus their added fees. If we run out of rainwater we can switch mains supply on and operate that way, we use mains for drinking water anyway.

The irrigation system isnt functional at the moment - I rely on other members to purchase parts to complete the system. Its take 3 months for them to provide the additional valve and 90 degree bend to complete the irrigation piping. They seem incapable of buying what I specify is needed. When they do manage that I will be able to irrigate each polytunnel individuall, initially, until we can prove the system for watering each tunnel before trying irrigating two and finally all three.
I will fit a flask under a portion of trickle tape to measure the throughput per unit time and use that figure to assess the efficacy of watering on the resultant crop yeild etc etc.

I will be onsite on Monday so I will gather the Data then.

Cheers,

BoB

 

[femail]

It does help clarify, thanks. Bottom line - you aren't going to find a DC "meter", not one that's Ofgem or MID certified anyway (as has already been advised) so it's a waste of time trying to measure the consumption of the pump from a FiT point of view. It's a valid idea to run a DC pump though, albeit taking an unmeasured/unquantified amount of power from the battery bank - what size pump is it and how long does it run for each day, on average?

The System ( as best I can tell...) is comprised of the following.

7 PV panels feeding
2 Tristar MPPT -45 feeding
1 battery (based on wiegt and guestimation ( it was in a sealed box and I didna have a screwdriver on me to open it and have a gander)
The battery feeds ...
1 AJ Inverter 350-24 feeding
1 elster a100c consumption meter feeding
4 led light clusters

Battery also feeds
1 Aquatech demand/delivery waterpump (window cleaning warehouse)
delivering 6L/min open consuming 7A at 80psi

Previously the pump was a domestic circulation pump aka radiator pump operating on the AC circuit.

Better?

Thanks again!

BoB

 

[Worcester]

Well the spec says:
"This AJ Inverter is rated at 300 watts for 24V systems, it also includes a solar charge controller for battery management."

The best person to answer all this is Solarfred - he's the DC expert around here

Until he comes along - the inverter SHOULD provide: Continuous output: 300 watts AC

That is way more than enough for a pump even something like this : Clarke Swimming Pool Pump - SPP3 - Machine Mart is only 180 watts.

I would guess that your battery bank is way, way undersized ESPECIALLY if you can pick it up!!

What is the rating of the panels - what make, and model and how many cells does each one have?
At what angles are they mounted and orientated?

I'm also confused why 7 panels? - Is that the maximum you could get on the roof?

Do you know how they have wired them to the Tristar controllers? - series, parallel, how many in each group of panels that are wired together?

 

[femail]

Lol Ok i need more data!

Yes only seven fit on the roof. Sw facing. Estimate of 40 degrees.
I'll look at the wiring and see if I can establish its mode of connection.

I'll ring along my drill driver and open the box to work out what the battery is.

If I can get an AC pump to work thats the ideal option from the metering perspective (even if I need to pop a rectifier in to convert to DC at the pump) But this raises the problem of getting a pump to work given the inverter rating. The DC pump is I suppose 3 phase equivalent.

Anyway I'll be able to get you new data next Friday.

BoB

 

[CAB Marine]

Wot Worcester said!

You need many many more batteries to make anything of that solar array....my boat has 6 12v batteries (675 Ah) supplied by 500 watts of solar (2 panels) and I've room for expansion on a charge controller comparable to the pair you have! To make the most of your system (assuming around 200 w/panel or so) I would think 800-1000 Ah (at 24v) would be about right. That would look like about a pallet full.

Back to your original problem though....I'm confused by your subsidy....and hence your metering. Normally, a shunt would be placed in the negative battery cable, this is then hooked up to a meter to tell you what the battery state of charge is. They often have an amp hour counter, which is at best, an estimate of what has gone in and out. They need zeroing out regularly to remain synchronised and can, in the wrong hands be far more trouble than they are worth! Your Elster sounds like it does something totally different.

 

[Earthstore]

The elster is a generation meter, of which the op recieves FIT payments for generating electricity from the solar panels for a off grid system.

Hope this helps

 

[Worcester]

The elster is a generation meter, of which the op recieves FIT payments for generating electricity from the solar panels for a off grid system.
Hope this helps

As Earthstore said, it is for measuring the generated 230V 50Hz AC (i.e like you would an off grid house) - CAB Marine, as you live on a boat, then unfortunatley even if you did something similar then you would not be eligible for the generation tariff.