Solar DC cable direct to hot water tank element.

[epsilonGreedy]

I have not given PV much thought during the prior stages of my build and now I am exploring the fringes of the subject with questions like this.

 

Given a 1.1kW 3 panel PV installation oriented 8 degrees east of due south on a 30 degree pitch roof at a latitude of 52 degrees I assume the most I could hope for is that it would provide the majority of DHW for two people 6 months of the year.

 

With such modest expectations I am wondering if the PV installation could be simplified by connecting it direct to a 2kW DC heating element in the hot water tank thereby avoiding the cost of an inverter and the hassle of registration?

 

I don't know if such a DC heating element is actually available and I am mindful of a recent post from @Nickfromwaleswho warned that the DC cable output from a solar array is one of the most dangerous cables in a home. As I said at the start I am just exploring the problem space to comprehend what is possible.

[JohnMo]

You can get immersion designed to plug straight into PV, they have to be DC and not that cheap, was thinking if this myself at the start of my build.  Just an isolator and DC cables required

 

https://www.zerohomebills.com/product/mypv-dc-elwa-solar-pv-water-heater/

Edited November 28, 2021 by JohnMo
Missed word

[ProDave]

7 minutes ago, JohnMo said:

You can get immersion designed to plug straight into PV, they have to be DC and not that cheap, was thinking if this myself at the start of my build.  Just an isolator and DC cables required

 

https://www.zerohomebills.com/product/mypv-dc-elwa-solar-pv-water-heater/

At that price, I would take my chance with a standard immersion heater.

 

With 1.1kW of PV you will not overload a standard 3kW heater.  No doubt a direct connection would not run the panels at full eficciency but it has to be worth a try first.

 

And that would be classed as off grid so no need to notify the DNO.

 

If you could set up a data logger to measure voltage across the heater you could calculate the power going into the heater.  A normal electricity meter won't work on DC so they are no good.

[epsilonGreedy]

12 minutes ago, ProDave said:

With 1.1kW of PV you will not overload a standard 3kW heater.

 

 

When writing my question I did wonder if an AC heating element might work.

 

14 minutes ago, ProDave said:

No doubt a direct connection would not run the panels at full eficciency but it has to be worth a try first.

 

 

Panel efficiency, I overlooked that. Boating folk endlessly debate PV panel configs for optimum efficiency.

 

I suppose such a direct DC setup starts with an advantage of no inverter efficiency loss which will be 5% to 10% based on my boating experiences. Hmm I wonder if the marine industry can offer a suitable DC water heating element that can soak up 1.1kW?

 

19 minutes ago, ProDave said:

If you could set up a data logger to measure voltage across the heater you could calculate the power going into the heater.  A normal electricity meter won't work on DC so they are no good.

 

 

Quite a technical counting challenge I guess if the voltage is yo-yoing up and from a directly coupled PV output. 

[SteamyTea]

35 minutes ago, ProDave said:

At that price, I would take my chance with a standard immersion heater.

The element may be OK, but how about the thermal cutout, and a bog standard thermostat, can they cope with the extra arcing? 

[SteamyTea]

15 minutes ago, epsilonGreedy said:

Quite a technical counting challenge I guess if the voltage is yo-yoing up and from a directly coupled PV output. 

https://www.ebay.co.uk/itm/254103933935

Yes, a 13 and a half quid one if you stick to 48V modules.

[ProDave]

9 minutes ago, SteamyTea said:

The element may be OK, but how about the thermal cutout, and a bog standard thermostat, can they cope with the extra arcing? 

Good point, but with such a low power input, I doubt it would ever need to turn off.

[JohnMo]

A cheaper alternative

 

http://www.cpssolar.net/epages/62063063.mobile/en_GB/?ObjectPath=/Shops/62063063/Products/dcie48v1000u1

[SteamyTea]

1 minute ago, ProDave said:

Good point, but with such a low power input, I doubt it would ever need to turn off.

There was someone, in the dim and distant past, who set his ST up so that it never overheated, then all he needed was a simple controller.

 

There is a problem that if, and it is not very likely, that the DC side electrolysed the water, it would put off the hydrogen economy pretty quickly.

[ProDave]

2 minutes ago, SteamyTea said:

There was someone, in the dim and distant past, who set his ST up so that it never overheated, then all he needed was a simple controller.

 

There is a problem that if, and it is not very likely, that the DC side electrolysed the water, it would put off the hydrogen economy pretty quickly.

But that would assume the heater element had corroded and water was getting in to the actual element.

[SteamyTea]

1 minute ago, ProDave said:

But that would assume the heater element had corroded and water was getting in to the actual element.

Hence

4 minutes ago, SteamyTea said:

not very likely

But there are a lot of nobs that tinker under the assumption that relatively low voltage is pretty 'safe'.

[epsilonGreedy]

5 minutes ago, SteamyTea said:

The element may be OK, but how about the thermal cutout, and a bog standard thermostat, can they cope with the extra arcing? 

 

 

The dreaded DC arcing issue and the ghost of Jeremy returns to haunt another good idea! Ho-hmm.

 

From the age of 5 to 20 my family home was cursed with an immersion tank heater that would literally raise the hot water temperature to boiling and the tank could be heard bubbling. We coped without requiring hospital treatment.

 

In the world of unvented cylinders what is a safe maximum temp and how many kW does it take to raise 300l from 60 degrees to 70. Not looking for the answer, just thinking aloud to establish how often this thermal cutout activate across this direct PV DC feed.  

[SteamyTea]

Just now, epsilonGreedy said:

We coped without requiring hospital treatment.

This woman only needed 9 days treatment.

http://news.bbc.co.uk/1/hi/england/cornwall/3035606.stm

[epsilonGreedy]

7 minutes ago, ProDave said:

But that would assume the heater element had corroded and water was getting in to the actual element.

 

 

Before midnight @SteamyTeawill have convinced me I am likely to create the first nuclear fusion plant in the world.

[SteamyTea]

1 minute ago, epsilonGreedy said:

 

Before midnight @SteamyTeawill have convinced me I am likely to create the first nuclear fusion plant in the world.

I thought you were working on cold fusion, shame it is not taking up more of your time.

[ProDave]

Okay a safer way might be a stand alone, i.e. not grid tied inverter to feed the immersion heater.

[SteamyTea]

32 minutes ago, ProDave said:

Okay a safer way might be a stand alone, i.e. not grid tied inverter to feed the immersion heater.

Or just a cheap, non sine wave inverter do it. The kind you see for 40 odd quid.

 

Do the cheap inverts match an elements impedance better?

Edited November 28, 2021 by SteamyTea

[epsilonGreedy]

1 hour ago, SteamyTea said:

I thought you were working on cold fusion

 

 

I am. Every time I type kw or KW there is a reciprocal pulse of energy in Cornwall greater than the experimental energy input.

[SteamyTea]

9 minutes ago, epsilonGreedy said:

 

I am. Every time I type kw or KW there is a reciprocal pulse of energy in Cornwall greater than the experimental energy input.

(expletive deleted)ing good job I am upcountry then.

[markocosic]

You would like to connect PV panels - without an inverter - to an immersion heater?

 

Bad plan.

 

1) The inverter isn't just making AC. It's also doing MPPT tracking (max power point tracking) so ensure that the "load" seen by the PV panels results in the highest possible generation from the panels. Direct connection won't have this.

 

2) Value for money. I also have a rubbish PV array - 2 strings of secondhand panels in bad orientations connected to a secondhand 2 string inverter. It saves far more money running the fridge / internets / mhvr / computer (and sometimes air conditioner) during the daytime than you'll save by doing diversion to an immersion heater.

 

Buy a little inverter and hook up to the mains to divert to either fridge/internets (most of the time) in my view. 

https://midsummerwholesale.co.uk/buy/sofar-inverters/sofar-1100-tl-g3

 

Registration...? ?

 

Buying an "island mode" inverter just for an immersion feels like a waste of perfectly good PV that could be running your office and A/C during the summer. (or charging a car etc)

 

 

[epsilonGreedy]

12 hours ago, markocosic said:

Bad plan.

 

1) The inverter isn't just making AC. It's also doing MPPT tracking (max power point tracking) so ensure that the "load" seen by the PV panels results in the highest possible generation from the panels. Direct connection won't have this.

 

 

This is a persuasive point. There are DC output  MPPT controllers sold to boat owners but anything with a marine label is overpriced so I may as well generate AC and feed it into the overall house demand as you suggest.

 

12 hours ago, markocosic said:

Buying an "island mode" inverter just for an immersion feels like a waste of perfectly good PV that could be running your office and A/C during the summer. (or charging a car etc)

 

 

My thinking was that two daily showers would consume about 100 litres and it takes about 3.5kW hours to raise 100 litres of water from 7 degrees to 37 degrees. Hot water heating demand would roughly match daily output from a 1.1kW array on average over 200 days of the year. There would however be considerable wastage of PV generation capacity mid summer when people have cooler showers and the array's daily capacity would be above 4kW hours.

 

The prompt for my question was the thought that if I stretch myself now to get PV on the roof could I obtain some benefit from the output as  an interim solution before the house is fully complete and signed off. We are planning to move into the upper floor of the house as soon as it is habitable.

 

I need to find a better solar calculator that provides a detailed breakdown of predicted daily generation across the whole year i.e. a calculator that reports "in May the daily PV generation from a 1.1kW array will be 5kW on average though on the best 10% of days 6.5kW can be expected and on the worst 20% of days it will be 2.8kW".

[SteamyTea]

24 minutes ago, epsilonGreedy said:

need to find a better solar calculator that provides a detailed breakdown of predicted daily generation across the whole year i.e. a calculator that reports "in May the daily PV generation from a 1.1kW array will be 5kW on average though on the best 10% of days 6.5kW can be expected and on the worst 20% of days it will be 2.8kW"

Yes you do as that is power, not energy.

[epsilonGreedy]

12 minutes ago, SteamyTea said:

Yes you do as that is power, not energy.

 

Where is the Yawn emoji?

 

I assume you cannot offer a constructive response to the main question about a PV calculator that can offer a daily breakdown of capacity through the year? 

 

[Nickfromwales]

19 hours ago, epsilonGreedy said:

I am mindful of a recent post from @Nickfromwaleswho warned that the DC cable output from a solar array is one of the most dangerous cables in a home

That’s due to there being no earth or overload / short-circuit protection associated with a PV installation. 
If taking DC to an immersion you would install DC circuit breakers at the very least, therefore making it a lot safer.