All the above sound good, but why can we not for go the 3p export tariff and have a meter that goes backwards when we are exporting and we use this free pre generated energy when importing 

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Hello Chris, I have a question about heating water by solar power to reduce my gas consumption. I want to connect the output of 2 x 250W solar panels connected in series to give 96v DC. Then connect 96v DC to my 3kW immersion heater which has 18.4 Ohms resistance. I know I need to bypass the thermostat to avoid contact arcing. Do you know if feeding DC to the heating element will have any adverse effects. Would this be cost efficient based on purchasing panels at £1.10 per Watt? Thanks, Dave

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Am i right in thinking that to make a saving using an immersion heater you only need to be cheaper than the alternative eg. gas.

So if gas is half the cost of electricity then you only have to halve your net electric power to be comparable with gas. By net power I mean grid minus solar.

That is, for a 3kw immersion you need 1.5kw from the grid and 1.5kw from solar. So the break even point is 1.5kw, and I should switch my immersion on when the inverter is outputting 1.5kw.

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HI again JohnnyK

I've just checked my emails, and cannot find anything from you. So not sure what has happened with your previous email!

Mosauer.. Looking at your reference again , and looking into the background of Abeltronics power halver units. I see their unit is simply a half wave rectifier, that cuts off the negative going cycle of an AC sinewave, thus halving the power.

OK, I use the term 'simply' in the best possible way, as the idea is really good for mains based devices. However, if these would be used on renewable power systems, long term issues could arise.

On Wind Turbines: An imbalance of the bearings over a long period, which will cause premature wear. If you can imagine that only half the time of generation (50% on, 50% off) the turbine will be 'juddering' on and off of load.

On Solar PV systems: Similar to the above, but as no moving parts, will put the grid inverter through the same on-off routine 25 times second, which will definitely put the components through some stress.

As mentioned above, using these devices just for the grid, as was designed, they will work just fine as there will be many other power users absorbing power irregularities.

Chris Rudge 

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Hi JohnnyK

Apologies I missed the earlier post somehow, as I could have saved you all the hassle of coming to the conclusion that a standard 110V site transformer  for power reduction.

We have had a system using a site transformer on test all this year (2011) and have been pleased with the results. So much so, that if you have a 3.5 - 4kWp PV system and a hot water tank & immersion heater, you really do not need a solar thermal system.* Obviously there are some qualifications to this statement, but in brief, if you have a low daylight use of power (fridge, occasional washing machine, etc), then this method will work well. 

As you have also found, a bright day with broken clouds will cause some fluctuation, but this generally will not be a huge issue, and the benefits outweigh the downsides.

We have now started selling the auto relay unit that will be used in conjunction with the 110v transformer on a 3kW immersion heater, which if mains voltage is 230v will run the immersion at around 800 - 850W. From later Spring to early Autumn sunny days will provide a tank of hot water.

For more on this, see http://www.chrisrudge.co.uk/immersion.htm 

* This system WOULD NOT attract the upcoming RHI payments, and indeed is not really energy efficient, but does make effective use of your generated electricity rather than simply exporting at 3p a unit.

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Hi again guys,

Thought I'd better acknowledge the comment of John Twibell, about cutting up resistance wire & the effective changes in power ratings.  Thanks John.  I always check the good old Ohm's Law anyway to carefully calculate the resistance, voltage, current & power output of any potential redesign or test.

I've overcome the problem that I outlined a couple of months ago (of utilising surplus power, storing it, whilst not exceeding the PV power generated) & I thought I'd share this with you.  My solution is simple; if you lower the voltage, the power of an electrical heat load will drop.  So I purchased a 110v site transformer via Ebay (3.3kv size).  This transforms the 230v mains & PV output voltage down to 110v.  As the resistance of a heater element remains unchanged & constant, then when the heater is plugged into the 110v side of the transformer, it now draws only about 23% of its originally rated power (400w instead of 1750w).  Thus I can utilise the low amounts of power my PV system generates during the winter more effectively.

Unfortunately, during days of intermittent cloud (most days in the UK - lol), I still have to manually keep rather a close eye on power generated & power consumed.  At present I run around like a madman switching things on & off to keep within the low power generated during our winter months.  I would very much like to automate this, but without spending a couple of grand ££s on an EMMA SP15 power management unit.  I had emailed Chris Rudge, but still haven't received any reply, about his more simple & much cheaper replay switching unit.  Chris, if you read this, would you contact me or email me please - thank you.

Regards to all & wishing you all a happy new year.

JohnnyK

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Further on the power halver unit. I have spoken with the guys at Ableltronics. It seems they are planning a development of this  enabling existing 3 kW immersion to be attenuated variably from 0-3 kw (early 2012 availability). This seems to me to be a preferred option if you can wait, especialy since there are remote controllability features to be offered as option.  Since it's winter there's no hurry I guess.

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A quick warning to JohnnyK 

Be careful about what you do with the heating element. It sound like you are going to cut it into three. Cutting a 3kW element into 3 will not give you 3 x 1kW elements, you will get 3 x 9kW elements or a total of 27kW if you put mains across the cut up pieces. If you cut it into three each piece will have a third the resistance. If you put 240V across that it will pass three times the current of the original element. To reduce the heating power you need to add resistance by putting elements in series.  I use low power heating in my greenhouse by wiring heaters in series. 

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Hi Maurice (Mo),

The link (you requested) for the power halver from Abletronics is:-

http://www.abeltronics.co.uk/products/2kw-immersion-heater-power-halver-and-power-reducer/imm01

I hope that it's alright to put it up on this blog thingy, (not sure what the rules are as yet).

I've just spent the afternoon wiring one of these power halvers to power two standard sockets & making them switchable, so that they will either allow full power OR half power.  Going to try making a portable multi-socket power halver thingy soon.  I think I'm a goin' nuts - lol.

Hope the link works for you Maurice.

Regards,

JohnnyK.

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Hi all. Thanks for the stream of info. I guess how to requests  for leads on low cost means of soaking up excess energy by heating water will grow  and grow. Chris' system seems a nifty low cost little number, plus there are additional thoughts posted on his blog. I will give this system a try but will install low power immersion heater instead of transformer since the solution is tidier and the immersion is bottom fitted. I've been checking a number of british manufacturers to see who will supply, and found that the cost is, I think modest, for a non-mass produced 1kW incaloy unit (say £35 +). One manufacturer does bespoke for not much more with the posibility of dual elements (say 1kW for winter and 2kw for summer) - prices little more than the cost of the transformer unit used by Chris. If details of manufacturers required I'm happy to supply.(maurice.sauer@ntlworld.com).

Mo

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Hi Malcolm

Sorry the email may not have got here. Maybe try http://tinyurl.com/6ymvhnr for the full details.

We are currently awaiting a whole load of the current sense units in from the USA, and will have the units available soon after for £180.

Johnny K

It would be more elegant to use a 1kW immersion heater, and I would guess a non-mainstream part would cost far more than standard stock items.

It has been mentioned to me that by changing the immersion from 3kW to 1kW, would remove the possibility of heating water fast if need be via an override.

Chris Rudge 

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Hi guys 'n' gals,

Just joined this forum as I've found your subjects of conversation very interesting.  I've just installed a 4kw solar PV system & am seeking methods of maximising its effectiveness, saving money & storing surplus energy generated during the day for later use during the evenings.

Could anyone tell me where I can purchase a 1kw 11" long. 2.25" BSP immersion heater for a "reasonable price" please.  The best I've found so far is on Ebay for around £35; but that is still way more expensive than the local price of buying a std 3kw 27" immersion £17.

Also I'm very interested in purchasing one of these devices mentioned above, that 'shunt' excess day power generated into a low powered immersion heater or storage radiator.

I can recommend the Abletronics Power Halver.  Wired one up yesterday & it works a treat.  Going to dismantle an old storage radiator soon & 'cut-up' the heating elements into three separate heating circuits, all with individual fused switches.  Also connect another power reducer to it.  Purpose to warm the house in winter without exceeding the level of self generated power from the PV system.

If anyone has any info upon my two requests above, would you be so kind as to email me please via JohnnyK_A1@Hotmail.com . Many thanks.

Regards to all,

JohnnyK

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Hi Chris,

Could you give me details of the unit please, I e-mailed you on the link provided 16.6.11 but it doesn't appear to be working.

Thanks Malcolm

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Chris, We have a 3.96KWp PV system. In the last year it generated 4200 units of which 3200 were exported, much of which in summer months. How much of a contribution would 3200KWh make to hot water heating?  Is installing your device an alternative to having solar thermal panels fitted? As I understand it, typical domestic solar thermal panels say for a 2 adult household contribute a lot less than 3200KWh per annum, so it appears to me that we could do without the thermal panels altogether if we manage our PV output carefully.

Charles

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Hi Ian

The unit can be fairly easily installed, depending on the location of exisiting inverter and consumer unit. Probably best to email me chris@chrisrudge.co.uk  rather than getting into semi commercial discussion on the blog.

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I have a 3.84kwp pv installation. I also have an oil fired heating instalation with a 3kw standby immersion heater. At the moment I am using the pv power to top up the hot water and switch the heater on and off manually; so its a bit hit and miss. Thus the auto system mentioned by Chris is of interest. What exactly does it comprise off ? What is the cost ? Is it easily retrofitted ?

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The cost is quite high, I suppose, but when installed when heating would be replacing oil or electric it's going to be quickest payback. There are loads of applications of the device too - I installed one today into a couple of panel heaters for some background heat in a community garden office, for example!

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Ah yes, they used to email us quite a bit with this product. I'll email you to discuss (rather than here), as they used to charge quite a lot for the unit, which somewhat outweighed the potential power savings.

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Thought it would be good to check. The product is called EMMA (Energy and Microgenerator MAnager) made by Irish company Coolpower Products (www.coolpowerproducts.com). We are Scotland distributor/installers for these when attached to PV systems.

As an engineer, they are a great piece of kit and work amazingly well. Have a look at their website, and I'm sure it will explain all and point you in the right direction to get hold of one! My email is fox.marty@gmail.com if you want any further info/experience of the product from me - happy to help. 

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Solar Clubs. I've not found these available at all in the last couple of years, though there is a plethora of companies selling fork lift batteries. These units will be far more managable, not requiring the use of lifting gear and can be picked up for almost scrap value. You will need a decent van to collect, as a standard 48v counterbalance forklift set weighs around a ton. Search online to see what is available in your area.

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I've heard that you can buy second-hand submarine batteries from the Royal Navy which are great for this purpose but haven't been able to find any for sale on the internet - is this the case, what do you think?

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Hi Fox Mart. I think it would be OK to mention suitable product sources here if your not blatantly plugging, so fire away!

The units we have put together use easily available CE marked parts, and is a step further from simply fitting timers to the immersion heater. Its really not rocket science. Under £100 in parts.

1kW immersion heater are available for old style 2 1/4 bsp thread, but new style tank threads in the UK seem to be limited to 3kW (unless someone knows otherwise).. 

We have been getting reports from one local customer who has not had his gas boiler running for about 2 months using this system, but of course depends on your lifestyle. Hot water useage varies wildly from house to house.  

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Hi Fox.mart.  We are an installation company based in Hertfordshire and have been seeking effective ways to help customers maximise their returns and I was very interested in your comment re the product you install which controls the immersion.  We have installed low power immersions for customers but they have to be manually controlled and if it is a top entry immersion only heats the top part of the tank.  A customer passed me some details about an extended 1 KW immersion designed for this purpose but we haven't bought one yet.  I tried to e.mail you for details as suggested but the email link didnt seem to be working.  Could you email me with details - rob@solinvictus.co.uk.  Thanks in anticipation  Rob

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Hi. Yes, a great way to do this is by storing the energy in hot water. Chris' method is great, however there is a product out there which we (solar and wind applications) have been installing on several sites which very accurately controls the power sent to an immersion heater to use all of the "excess" generation without importing power. Not sure if it's allowed to post product names on here, but feel free to message me for more information. The kit works really well - best used to displace oil or electricity which would otherwise be used to heat water. 

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