PV to immersion heater

[siwhi]

Hi All,

 

We have a 3.3KW PV system on our south facing roof. We are moderate - low electricity users and the system works well for us. However, we use most of the electricity and have little left over for other uses such as water heating which is be a big draw. I am considering getting another 2.2KW of panels and installing them against the south facing back garden fence using a basic frame, with the sole intention of using it to heat water. We have a 300L immersion tank with 3KW element to it which feeds water at up to 65C into our gas combi boiler which tops up the domestic water temperature if required, and separately also heats the radiators. I assume this is a resistive dump load compatible hot water tank element (meaning it takes any level of power input), I think they all are, but if not I can swap it for one.  My question is, what is the best way to connect the PV panels to the 300L hot water tank? Can I do something simple myself. The options seem to me to be:

 

1) Buy from eBay a used DC/AC inverter and connect it to the main fusebox as normal. Then connect (plug in) the immersion tank element to the house (as it is currently set up). But I would then need to de- prioritize the boiler element somehow, getting it to cut out when the power output is low, otherwise it will draw from the grid. Currently I have to manually switch it on and off, which is a hassle and I could easily forget about it causing a big electricity bill. Using a My Eddi or similar would be very expensive. Are there any other products or options to identify when we have excess solar and divert part of it to the water tank?

2) Buy a more basic resistive load inverter which I wire straight into the tank element, (bypassing the main house fuse box altogether). Questions then are - is this DC>AC or is it all DC? If all DC would the element work OK? Would the element which has a high / low knob on it, cut out power automatically when the water got sufficiently hot, or would I need another temperature sensor switch? I would of course have fuses in the wiring as well. 

3) Any other suggestions for a cheap-ish solution to hook up PV to an immersion heater? 

 

Overall option 1 is more flexible, meaning when the water was hot enough we could use the power for other things, whereas option 2 would be cheaper to set up. 

 

Thanks for your thoughts!

[ProDave]

If you connect the new system wit a normal grid tie inverter, you will need prior permission of your DNO as you can only automatically install up to 3.68kW.  There may or may not be a network upgrade charge to allow that.

 

The off grid idea might be simpler and put it all to the immersion heater, no need to tell the DNO anything,. 

 

As to how to use it?  Well don't heat the hot water tank in the morning.  Leave it for the solar PV to heat during the day and heat it as necessary wit the boiler in the evening. But that is tricky if your family want to shower in the morning.

[Marvin]

You may wish to find out how  much PV energy (therefore hot water you will get in the winter).

 

I use this to calculate:

 

https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html 

 

First choose your exact location on the map.

then choose set up (i.e. off grid) tab to the right of the map.

Then fill in the figures: panel wattage etc. 

Then hit the button at the bottom which says visualise results.

 

Depending on your situation you may want to look at a small ASHP. I am not sure. The £ and the reliability need looking at. I always hate wasting the PV power and batteries are expensive...  I am working on spliting my mains power and having the MVHR running on PV with battery back up because it only uses about 22watts per hour and the excess power produced in the summer (about 4 times the winter PV generated) to supply my lights, with mains back up.

 

Best of luck

 

 

Marvin

[Marvin]

A 2.2kW system on my roof (23 degree slope) where I live would give me about an average 71kW of power in the month of January and a peak of about 324kW in the month of June.

 

Although there is loss of heat through the tank related to the temperature of the water, the insulation and air /room temperature, if we take a simplistic approach and just look at the water usage and temperature of water refilling the tank, we can have a rough idea of how much energy you would use to heat up the replacement water used each  day

 

Say the use is 2 good showers a day. Assume each 10 minute long (I do about 4 mins) at about 43 centigrade (hot temp my wife likes not me). Shower flows at the shower head say 10 litres a minute (I love simple maths) That would be 200 litres of 43 degree water used. (at this point there is lots of minute detail about cold water in the tank being heated to 63 (I love simple maths) and then blended down to 43ish degrees therefore using less of the hot stuff but 200 litres at 43 degrees or 136.5 litres at 63 degrees blended with cold needs about the same energy to produce (ish) if its from the same source, so lets stick with it.)

 

 Ok the question is how much energy is required to raise 200 litres of (say winter cold) 5 degree C water up to 43C

 

The raise in temperature is 43 - 5 = 38

I believe the Specific heat of water is about  4.19     kJ/kg K (That's every litre of water you want to raise 1 degree C means you use 4.19 killajoules)

So the maths is 200 litres raised in temperature by 38 degrees is 7600 units. Times these by the the kJ required for each unit (4.19) gives you 31,844 kJ

 

So we have 31,844 kJ which converts to kilowatt/hours (if I'm right 1kW hour equally 3600 kJ) So divide by 3600 gives you 8.85 kw of energy needed per day or approximately 264kW hours per (winter) month

 

I am hoping that this will be corrected by those who know better and then used by those who want to check their own hot water use.

 

Marvin

Edited April 30, 2021 by Marvin
cant count number changed

[siwhi]

ProDave and Marvin, thanks so much for that input, it's useful and interesting. 

 

Yes I can see the advantages of missing out the DNO in the issue, but I'm wondering whether I would miss the flexibility. I need to think about whether I could justify the expense of a MyEddi. 

 

Marvin, my back of the envelope calculations were: 

2400 W rated system, so 2000w good case scenario output for 8 hours max per day = 16KW. Just to get an attainable upper benchmark. 

 

A 300 Litre tank, with ideally temperature increasing from 5C to 65C (an increase of 60C), would require this much power: 

Pt = (4.2 × L × T ) ÷ 3600

 

L = litres = 300

T = temperature = 60

 

Pt = (4.2 X 300 liters x 60) ÷ 3600

= 21KWH

 

If we only wanted to heat it to say 45C, and only needed 200L, then it's:

Pt = (4.2 x 200 x 40) ÷ 3600

Pt = 9 KWH

 

So, basically the same number you got, which is reassuring! 

 

So this system would, on a good day produce enough to sort us out for hot water. Obviously, as with any scenario with PV, when there's less sun, less output, but even so, it would be a helpful addition for us at what might be quite low cost. Hopefully I can get an old inverter (of either type) for say £300 and 6x 400w panels for say £115 each, so about a grand. I have a roll of 6mm cable. No idea what the ROI would be, but probably around 10 years (10% ROI), plus reduced CO2 emissions. 

 

But still not sure what the best set up would be and how to automatically prioritize (or not) where power gets used if it's via the domestic AC system, or else how to set up a direct system. 

[ProDave]

As regards ROI.  It cost me a total of £1500 (by some very careful buying) to put together my 4Kw grid tie system and I am typically self using £250 worth of electricity each year giving a 6 year payback.

 

Most of the self use is the big appliances in the daytime and other house loads, with about 1/4 of the generation going into the immersion heater.  Rarely enough to heat all the water for the day but a big contribution on a sunny day.