Immersion Heater - PV electricity

[markthorpe999]

Hi millerman A lot of systems, solarpv, wet and ground source are designed not to heat the water to full temperature, but to pre-heat the water. This enables a great cost saving as your normal boiler or immersion only has to heat the water from the preheated temperature to your required temperature for washing (note usually above 65'c). I fitted a solar wet system recently in a specially designed cylinders dual coils. The solar coil was at the top of the cylinder. As heat rises it heats from the top of the cylinder down, so I think for best efficiency the immersion coil should be at the top. Good luck with your project

 

[pmcalli]

Hi millerman A lot of systems, solarpv, wet and ground source are designed not to heat the water to full temperature, but to pre-heat the water. This enables a great cost saving as your normal boiler or immersion only has to heat the water from the preheated temperature to your required temperature for washing (note usually above 65'c). I fitted a solar wet system recently in a specially designed cylinders dual coils. The solar coil was at the top of the cylinder. As heat rises it heats from the top of the cylinder down, so I think for best efficiency the immersion coil should be at the top. Good luck with your project

I think we need to introduce you to the laws of physics. Hot water from the top of a tank will only heat the water at the bottom by conduction as water is a terrible conductor of heat it happens only slowly. the hot water rises cold water falls under gravity as it is more dense. The whole point of an immersion at the top of a tank to to provide a boost to a small amount of water. Ie it heats the water at the top only. Best type of tank is an economy 7 tank with two immersions so that top of tank is heated first and bottom if top cuts out and solar still available. I have had to add a pump on my tank to get all of it hot with a top immersion.

 

[thushar]

immersion heaters - 1 kW , 1.5 kW and automatic immersion control switches now available on ebay - Business Building Kits items in Fair Trade Hub Shop store on eBay!

 

[methley]

I do hope the people buying and installing these kits realise there is no way the system can know what the house is consuming. Hence they will likely end up importing 'leccy from the grid to heat their water. However, this has been pointed out many times before..... :sad3:

 

[thushar]

I have been using this switch ( Business Building Kits items in Fair Trade Hub Shop store on eBay!) with the 1 kW immersion heater for a while - Works perfectly with no elec import from the grid. The switch was calibrated to turn on the immersion at 1600W, so 600 W set aside for inhouse use. Alternately, when i want to turn on the high loads like washing machine during the day time, the inside neon switch to the immersion is turned off. Will get a full tank of ho****er with 3 or 4 hours of sun shine and I'm saving on gas bill as well.

Forgot to mention, I'm with a 3 kW pv system.

 

[methley]

If you turn your argument on it's head, you could just as easily run upstair and turn the immersion on every time the sun shines!

 

[inie meanie]

There are several simple controllers, like the SolarHeat controller I am marketing, which operate on PV output and a modulating one is being supplied with a monitoring system and PV panels by Engensa. I expect that others will be available later this year as Paul's colleagues on OpenEnergyMonitor seem to have cracked it. They all seem to use CT's or similar but the tricky bit is getting the switching or modulation right as reducing output by half reduces the heat by a quarter (IxIxR). Not much point in stopping the output to the grid if it doesn't actually do anything useful.

Err are you suggestig that if we reduce the power to the immersion heater by half that we will only get a quarter of the heat? If you're not then I apologise for the rest of this comment. You use I x I X R for heat disapation but we are not controling on current we are controling power. If you only put 1500 watts into the water it will take directly twice as long to heat up as 3000 watts, not 4 times. If it did take 4 times as long where did the extra energy go? You can not create or destroy energy only convert it.

 

[echase]

I have been using this switch ( Business Building Kits items in Fair Trade Hub Shop store on eBay!) Alternately, when i want to turn on the high loads like washing machine during the day time, the inside neon switch to the immersion is turned off. Will get a full tank with 3 or 4 hours of sun shine and I'm saving on gas bill as well.

These people are claiming far more than they can deliver. As I said before, even if you track the export properly by using proportional control you only get about £50 - £60 off your annual gas bill, assuming a 3-4kW PV system and an efficient gas water heating system (no pilot light, short pipes and condensing boiler). Let’s say it is £100 for all systems averaged, including the inefficient gas systems like mine and oil boilers. Any of these sort of switching units can’t come close to saving £100. If you use a 1kW element there will be many times you are underusing the available export and some times you import. You have to pay £60 or more for the element on top of controller cost. If you use a 3kW element you will be importing so much that the benefit will be negligible or even negative.

Plus who wants to bother with turning off the immersion every time they boil a kettle, etc?

Let’s have the data to prove otherwise if you really can prove that these switching units can recover their cost in less than 10 years. I have 9 months of data measured every 5 minutes for 2 samples of mine which shows a much shorter payback but I don’t make any specific payback claim as people’s houses are so different.

 

[TechyTim]

Inie,
If you are controlling power directly, eg with a Triac, then you are correct. If you are controlling output by reducing the voltage (as people have done with transformers) the the output is a square law as the heater resistance is fixed.
I am told that The biggest issue with Traics in commercial kit is the EMC regulations and harmonics generated which can be solved at a cost but with annual savings of £50 -£100 it may not be cost effective.

 

[TechyTim]

echase:
"Let’s have the data to prove otherwise if you really can prove that these switching units can recover their cost in less than 10 years. I have 9 months of data measured every 5 minutes for 2 samples of mine which shows a much shorter payback but I don’t make any specific payback claim as people’s houses are so different. "

I think, and this wil only be proved when I have some data in a couple of months, that you have mised the interaction between the PV output and the size of panel. What you say would be true if the output was fixed at 1kW but if the controller is on a 2.9kW system, the output in the summer will continue to increase once it has passed the threshold, reducing the import. I am suggesting that these switches are used in the summer only.

I wonder how the cost of your proportional system compares with the potential annual saving of £50 - £100?

 

[robassais]

so the 4p/kWh is 6.7 p/kWh.

for a system on the south coast which have much higher output figures than the MCS values based on the UK average,

QUOTE]

Let’s use your 6.7p then. With the 10 panel system you quote the MCS approved annual generation is about 2000kWhr. If all that went into heating water you have displaced 2000kWhr of gas at about 6.7p per unit = £134. Due to the house load only about half is available to the immersion so it’s down to £67. Your unit does not track the export and is turned off in winter so halve it again to £33, e.g. for significant periods of time the PV output will be above the house base load of a few hundred watts but below 1.5kW so that energy is exported.

So about 2000/2/2= 500kWhr has gone into the heater which has run for 500/1.5 hours = 333 hours at 1.5kW. Then you are importing electricity to make up the remaining 1.5kw. So every time you have the immersion on you are importing full price electricity up to 1.5kw at a value = 12pence, which would otherwise have been heated by gas at 6.7p. So it’s an extra charge of 333 x (12-6.7) x 1.5 = £26. So the net benefit is £33-£26=£7. That is a bit pessimistic as if the spare electricity is >>1.5Kw less is imported, but does a 10 panel system often exceeds 1.5kW by much?

I live near the south coast and my system does not exceed the MCS by a huge amount. Maybe 10-20%.

ANSWER = YES A 3.3 Kw system should achieve 1.6Kw average in December the worst month and 13Kw average in June the best month assuming system losses of 25%

 

[echase]

ANSWER = YES A 3.3 Kw system should achieve 1.6Kw average in December the worst month and 13Kw average in June the best month assuming system losses of 25%

Answer to what question please? You have your units wrong. Did you mean kWhr average per day? What system losses (inverter, immersion, panels, wiring?)?

Crudely calculated the average of 1.6 kWhr and 13 kWhr is 5.7 kWhr; times 365 = 2080 kWhr per year. Seems about right according to government. A 1kW switching unit is very unlikely to deliver more than a quarter of that to the immersion with the rest exported or used in house, so it’s maybe saved gas worth 2080 x 0.067/5 = £34pa. A 3kW switch will loose all that benefit in the many periods it is importing to part feed the immersion, unless you have huge panel area. A proportional controller is required.

A 3.3kW system only delivers more than 1.5kw for maybe 2-4 hours in middle of each sunny summer day so immersion is not going to be on for much time. But I stress I am guessing here. Need to check my recorded data. Definitely not 13kW.

 

[echase]

echase:

I wonder how the cost of your proportional system compares with the potential annual saving of £50 - £100?

No more expensive than ones like yours, once cost of adding a 1kW element is added in. If there is any small extra cost it would easily pay back in a year due to increased power into the immersion. But all other proportional designs that I know of are significantly more expensive because they use different design concepts to mine, which needs the use of more expensive components and/or a more complex installation. But except for the expensive EMMA I am sure their payback period is shorter than any switched design. I ignore here any one offs made using salvaged/eBay components as clearly that is a way to potentially cut costs.

Having said all that I am pulling out of delivering kits as the design has been handed over for commercial production by a firm to be announced. This was always my intention as this firm encouraged me to do this development on the back of an interest in taking it on if I succeeded. It does not incorporate any circuit design ideas picked up here, apart from using current transformers mentioned here. So very sorry to those who contacted me very recently but I can not do new kit deliveries beyond the ones I have already promised. (Those of you here promoting other designs may now rejoice!)

I will continue to support my existing units out there. These have been useful Beta test units to prove the design works well and is reliable. No one has ever said they regret their choice. I will be contacting early users soon to offer the latest design update.

Thanks for the encouragement/answers from you all. Particular thanks to Paul for starting this very successful thread.

 

[Whip1971]

Good luck. Hope you do well. I'm in the process of making pmcalli circuit.

 

[echase]

A 3.3kW system only delivers more than 1.5kw for maybe 2-4 hours in middle of each sunny summer day so immersion is not going to be on for much time. But I stress I am guessing here. Need to check my recorded data. Definitely not 13kW.

My 2-4 hours guess was not right for mostly sunny days but is for mostly cloudy days where often there are quite short spikes in middle of day (see last 2 graphs). Only about 10% of spring/summer days are nearly fully sunny.

Here are 2 plots of PV o/p for above average spring/summer UK southern weather. First one is a few days ago and had it been fully sunny 1.5kW would have been exceeded 07.30 to 14.00 (6.5 hours). The clouds though have limited this to about 5 hours. The total power delivered is 14.7kWhr according to the software. Turning a 1kW element on for 5 hours uses 5kWhr. A good proportional controller will use everything above the 500W line if base load plus controller threshold is assumed to be 500W on average. The graph area below 500W is about 5 kWhr which goes to house or export and 14.7 – 5 = 9.7 goes to immersion. That is twice that of stepped controller.


View attachment 11377
Second one is for June (2011), the best month, and notice that whilst the hours above 1.5kW are slightly wider it’s not that dramatic, perhaps one more hour. Leave it to you to work out how much different controllers divert to an immersion.

Don’t take too much notice of the absolute time and kW here as the monitoring system was not set for BST properly and the inverter was uprated between the 2 graphs, so top one is with more efficient/powerful inverter. My 3.7kW faces ESE so peaks at 11am GMT and probably is nearly equivalent to a 3.3kW south facing system.


View attachment 11378

Here is my pick of an average day throughout the year. 1.5kW is exceeded rarely and erratically meaning any mechanical relay has to wear itself out by coming on and off regularly, or stay off to save wear and so miss the short peaks. A solid state proportional relay can track reliably and quickly.