Solar hot water- is it cost effective?

[Tony1]

 

I'm wondering if I can prompt a discussion about the costs vs benefits of solar hot water, via a question:

Imagine two boaters install identical sets of lithium batteries, and they use identical amounts of charge each day for their 'normal' systems, apart from one thing- boater A also uses his batteries and inverter to power the immersion heater (for say 50% of the year).

Lets assume purely for discussion purposes that boater A is handsome and dashing, and boater B is a grimy troglodyte.

And just for discussion, lets say that in heating up his water, boater A cycles 50% more energy through his batteries each day than boater B. 

So over a year, boater A is making his batteries cycle through 25% more energy than boater B. 

But here is the burning question- will the troglodyte boater B's batteries last 25% longer than those of the handsome boater A?

It's one thing for us dashing solar hot water bods to count our savings in terms of engine running and diesel costs etc, but if our batteries only last 7 years instead of 10, are we really making a saving? 

Asking for a (handsome) friend. 

 

 

[Jon57]

3 minutes ago, Tony1 said:

 

I'm wondering if I can prompt a discussion about the costs vs benefits of solar hot water, via a question:

Imagine two boaters install identical sets of lithium batteries, and they use identical amounts of charge each day for their 'normal' systems, apart from one thing- boater A also uses his batteries and inverter to power the immersion heater (for say 50% of the year).

Lets assume purely for discussion purposes that boater A is handsome and dashing, and boater B is a grimy troglodyte.

And just for discussion, lets say that in heating up his water, boater A cycles 50% more energy through his batteries each day than boater B. 

So over a year, boater A is making his batteries cycle through 25% more energy than boater B. 

But here is the burning question- will the troglodyte boater B's batteries last 25% longer than those of the handsome boater A?

It's one thing for us dashing solar hot water bods to count our savings in terms of engine running and diesel costs etc, but if our batteries only last 7 years instead of 10, are we really making a saving? 

Asking for a (handsome) friend. 

 

 

If boater A and boater B share hot water ( shower with a friend) I would predicked (see what I did there) a long and happy battery life. 😜😜

[IanD]

13 minutes ago, Tony1 said:

 

I'm wondering if I can prompt a discussion about the costs vs benefits of solar hot water, via a question:

Imagine two boaters install identical sets of lithium batteries, and they use identical amounts of charge each day for their 'normal' systems, apart from one thing- boater A also uses his batteries and inverter to power the immersion heater (for say 50% of the year).

Lets assume purely for discussion purposes that boater A is handsome and dashing, and boater B is a grimy troglodyte.

And just for discussion, lets say that in heating up his water, boater A cycles 50% more energy through his batteries each day than boater B. 

So over a year, boater A is making his batteries cycle through 25% more energy than boater B. 

But here is the burning question- will the troglodyte boater B's batteries last 25% longer than those of the handsome boater A?

It's one thing for us dashing solar hot water bods to count our savings in terms of engine running and diesel costs etc, but if our batteries only last 7 years instead of 10, are we really making a saving? 

Asking for a (handsome) friend. 

 

 

The simple answer is "yes" -- lifetime of lithium batteries depends on total energy in and out, some manufacturers even specify their batteries like this now instead of "number of cycles".

 

(e.g. BYD LVL15.4 (48V 15.4kWh) is specified as 46MWh over lifetime, which is 3000 cycles at 100% SoC used or 6000 cycles at 50% SoC used)

 

Of course this only applies if your BMS treats the batteries nicely (e.g. no floating at too high voltages) or does anything else to shorten their lifetime... 😉

 

Mind you, unless you do this (cycling a large extra part of capacity) every single day all year for many years it's unlikely to be a problem anyway -- if you normally use 50% SoC range (which is a lot!) every single day 6000 cycles is 16.5 years, this would drop to 4500 cycles (12.3 years) with the extra use. I doubt that any boater actually hammers their LFP batteries this hard...

Edited February 21, 2023 by IanD

[Jen-in-Wellies]

Or go solar thermal hot water and it has next to no effect on the batteries.

 

[Tracy D'arth]

For hot water heating, direct solar is the best way, cutting out the dual energy conversion of electricity. Cost zero.

[Tony1]

4 minutes ago, IanD said:

The simple answer is "yes" -- lifetime of lithium batteries depends on total energy in and out, some manufacturers even specify their batteries like this now instead of "number of cycles".

 

(e.g. BYD LVL15.4 (48V 15.4kWh) is specified as 46MWh over lifetime, which is 3000 cycles at 100% SoC used or 6000 cycles at 50% SoC used)

 

Of course this only applies if your BMS treats the batteries nicely (e.g. no floating at too high voltages) or does anything else to shorten their lifetime... 😉

 

Thanks Ian, the 3,000 thing does make it easier to make a rough calculation.

Hopefully most batteries will fall roughly into that same sort of performance?

 

So in my case, if my batteries started out at about 400Ah capacity, and if I use a third of their capacity (133Ah) per day, in theory I can get 3,000 x 3 = 9,000 days of use from them?

That appears to mean over 24 years of use- which won't happen- but they'll certainly outlast me on the boat.  

And even with solar hot water, they should still last for more than 10 years, by which time we'll be in antigravity hover boats anyway.

 

 

4 minutes ago, Jen-in-Wellies said:

Or go solar thermal hot water and it has next to no effect on the batteries.

 

 

Begone with your evil Morlock technology. 

That makes far too much sense. 

 

 

[IanD]

1 minute ago, Tracy D'arth said:

For hot water heating, direct solar is the best way, cutting out the dual energy conversion of electricity. Cost zero.

 

That's true if you've got enough roof space to install both, or you use the *extremely* expensive (~5x normal cost?) dual heat/power panels (hot water underneath mono solar panels).

 

For most people nowadays bigger solar panels are a better option, they're cheap (and dropping in price) and give power for all uses, not just heating.

[IanD]

42 minutes ago, Tony1 said:

 

Thanks Ian, the 3,000 thing does make it easier to make a rough calculation.

Hopefully most batteries will fall roughly into that same sort of performance?

 

So in my case, if my batteries started out at about 400Ah capacity, and if I use a third of their capacity (133Ah) per day, in theory I can get 3,000 x 3 = 9,000 days of use from them?

That appears to mean over 24 years of use- which won't happen- but they'll certainly outlast me on the boat.  

And even with solar hot water, they should still last for more than 10 years, by which time we'll be in antigravity hover boats anyway.

 

Begone with your evil Morlock technology. 

That makes far too much sense. 

 

 

Most LFP batteries with a properly designed BMS which controls charging/discharging and balancing should have similar lifetime -- again, the key is that the BMS really has to be the master, in charge of everything including alternators/generators/shoreline/solar/inverter. This tends to mean that they need CANbus (or maybe Bluetooth?) interfaces to talk to the other equipment, and somehow take control. Suppliers like Victron officially support various batteries capable of doing this, and unofficially some DIY systems. Most standalone drop-in LFPs don't, especially the cheaper ones.

 

I gave BYD as one example (who guarantee this lifetime since the BMS logs and reports usage) , there are undoubtedly others just as good, even including properly designed DIY solutions. There are also a lot which are not so good -- usually cheaper ones -- and won't give such long lifetime, probably including all the hybrid LA/LFP solutions favoured by bloggers and posters on Facebook... 😉

Edited February 21, 2023 by IanD

[Jen-in-Wellies]

10 minutes ago, IanD said:

 

That's true if you've got enough roof space to install both, or you use the *extremely* expensive (~5x normal cost?) dual heat/power panels (hot water underneath mono solar panels).

 

For most people nowadays bigger solar panels are a better option, they're cheap (and dropping in price) and give power for all uses, not just heating.

In the fifteen years or so since I first built my solar thermal system, photovoltaic panel prices have crashed through the floor. At least an order of magnitude. Solar thermal made sense back then, but not so much now on a boat.