Solar/battery calculations

Tim Howard · Sep 26, 2024

Please be gentle - I don't normally venture into this area!

A good and long standing customer has a conference centre with a swimming pool.
The swimming pool has two 1700w filtration pumps that run 24/365.
(the same pump system also handles chemical dosing and water heating via a heat exchanger from bio-mass)

The question has been posed to me whether a nearby roof use could accommodate some solar panels with the specific goal of reducing the operating costs of these pumps.

I will share my attempt at mathematical reasoning and thinking, please feel free to fall about laughing, and maybe once vertical again give some advice....

2 x 1700w pumps = 3400w
In a 24 hour period, consumption is 81.6 kwh

I believe in the UK you can reckon on between 2 and 4 hours direct sunlight on PV panels. Advice seems to be work with a figure of 2.5 hours average for estimation purposes over a year?

I think we could fit (size wise) a 20kw system on a suitable nearby roof and this might be expected to generate 20 x 2.5 = 50kwh.
However during the direct sunlight hours, consumption from the pumps would be between 8.5kwh and 13.6kw depending on time of year.

Sticking the surplus in battery storage seems sensible, but this is where my ability to do calculations fail.

What I'm struggling to work out is how far the 'spare' 36kwh in the sunlight hours would go towards charging batteries, and therefore for how many hours a day the batteries could be the source of power for the pumps before it reverts to grid. I'm also wondering if battery systems are designed to be cycled to this degree.

The ultimate aim is working out if a battery system is feasible at all and eventually what the payback time would be.
Thanks a lot for any hints!

mainline · Sep 26, 2024

Tim Howard said:
Please be gentle - I don't normally venture into this area!

A good and long standing customer has a conference centre with a swimming pool.
The swimming pool has two 1700w filtration pumps that run 24/365.
(the same pump system also handles chemical dosing and water heating via a heat exchanger from bio-mass)

The question has been posed to me whether a nearby roof use could accommodate some solar panels with the specific goal of reducing the operating costs of these pumps.

I will share my attempt at mathematical reasoning and thinking, please feel free to fall about laughing, and maybe once vertical again give some advice....

2 x 1700w pumps = 3400w
In a 24 hour period, consumption is 81.6 kwh

I believe in the UK you can reckon on between 2 and 4 hours direct sunlight on PV panels. Advice seems to be work with a figure of 2.5 hours average for estimation purposes over a year?

I think we could fit (size wise) a 20kw system on a suitable nearby roof and this might be expected to generate 20 x 2.5 = 50kwh.
However during the direct sunlight hours, consumption from the pumps would be between 8.5kwh and 13.6kw depending on time of year.

Sticking the surplus in battery storage seems sensible, but this is where my ability to do calculations fail.

What I'm struggling to work out is how far the 'spare' 36kwh in the sunlight hours would go towards charging batteries, and therefore for how many hours a day the batteries could be the source of power for the pumps before it reverts to grid. I'm also wondering if battery systems are designed to be cycled to this degree.

The ultimate aim is working out if a battery system is feasible at all and eventually what the payback time would be.
Thanks a lot for any hints!

The way they went with the holiday complex pool heating was to use solar tube instead of electric.

Tim Howard · Sep 26, 2024

mainline said:
The way they went with the holiday complex pool heating was to use solar tube instead of electric.

The heating of the water is sorted already, they have a huge biomass pellet system on site.
It's more the running costs of the filter pumps they are trying to reduce (long term)

mainline · Sep 26, 2024

Tim Howard said:
The heating of the water is sorted already, they have a huge biomass pellet system on site.
It's more the running costs of the filter pumps they are trying to reduce (long term)

Is the pool open 24/7 then ? As this one, the pump is shut off overnight.

binky · Sep 27, 2024

Shutting down pumps for even just a few hours overnight will make a big difference.

With solar, it will spend more time working at say 60% of capacity than at full tilt. Less in winter, so I would install the pv array first, with hybrid inverters, monitor the system, then try to work out if batteries are worth the money. I suspect they will be, but try to avoid the mindset that the PV / batteries will do everything - that tends to get v expensive.

It may also be worth looking PV Gis, a free solar calculator.

Tim Howard · Sep 27, 2024

Thank you for the responses. My first question at the time was "Are you obliged to run the pumps 24/7" and was told "We are not obliged to, and there are certainly probably more times we could run only one, but they are filtering the water, heating and dosing the chemicals all at the same time, so it’s preferable one is always running. When the pool is used a lot it takes both filters to keep up."

I think they need to rethink the "always on" idea.
I'm not an expert on the dosing of chemicals and what is required.
I would imagine that stopping circulating hot water for between say 10pm and 4 am isn't going to make a material difference to the pool temperature at 9am.
I'd also imagine that by 10pm the pool with be as filtered as it's going to be if the water contents are unchanged since 5pm.

I like the idea of doing PV array first. Plenty to think about.
Thanks again

mainline · Sep 27, 2024

Tim Howard said:
Thank you for the responses. My first question at the time was "Are you obliged to run the pumps 24/7" and was told "We are not obliged to, and there are certainly probably more times we could run only one, but they are filtering the water, heating and dosing the chemicals all at the same time, so it’s preferable one is always running. When the pool is used a lot it takes both filters to keep up."

I think they need to rethink the "always on" idea.
I'm not an expert on the dosing of chemicals and what is required.
I would imagine that stopping circulating hot water for between say 10pm and 4 am isn't going to make a material difference to the pool temperature at 9am.
I'd also imagine that by 10pm the pool with be as filtered as it's going to be if the water contents are unchanged since 5pm.

I like the idea of doing PV array first. Plenty to think about.
Thanks again

The chemicals are checked in the morning and afternoon for CL, PH, ALK, and Hardness, but the pool remains fairly constant with a few chlorine tabs in the skimmers.

Pool is open air and covered at night, temp doesn't drop by that much.
Heated by Bio mass heat exchanger burning Self grown Miscanthus, oil boiler backup.

Andy™ · Sep 29, 2024

sounds like a big enough place that no matter what you generate, it will be used. so rather than focusing only on the pumps & trying to charge batteries to run them when there is no sun, just fit the most PV you can that'll supply everything. if it gets to the point you are generating more solar and you can use and exporting, then look at batteries

definitely look at getting one of them switch off when its not needed too

Tim Howard · Oct 2, 2024

Ok so next solar-newbie question -
Can you use a hyrbid inverter without batteries connected?

The underlying question is whether to get a hybrid inverter ready for the presumed addition of battery storage, or save a little money initially.

mainline · Oct 2, 2024

Tim Howard said:
Ok so next solar-newbie question -
Can you use a hyrbid inverter without batteries connected?

The underlying question is whether to get a hybrid inverter ready for the presumed addition of battery storage, or save a little money initially.

Apparently from what I read is that batteries can be added to any grid connect inverter using AC Coupling, meaning you don't have to worry about buying a 'battery ready' inverter.
Some may try to convince you to get a 'hybrid inverter'. These are inverters especially designed to have batteries attached using DC Coupling.
Apparently, Hybrid inverters are only compatible with a limited number of batteries (which may not still be on sale when you want to buy batteries in a few years) and they are more expensive than regular inverters.
With AC coupling and a regular inverter, you can add any AC coupled battery you like down the track, and you are not adding any extra cost to the solar system you are buying today.

Tim Howard · Oct 2, 2024

mainline said:
Apparently from what I read is that batteries can be added to any grid connect inverter using AC Coupling, meaning you don't have to worry about buying a 'battery ready' inverter.

I read the same but my knowledge of AC coupled battery setups stops at that Tesla thing….
Thanks for the reply.

UNG · Oct 2, 2024

Tim Howard said:
Ok so next solar-newbie question -
Can you use a hyrbid inverter without batteries connected?

The underlying question is whether to get a hybrid inverter ready for the presumed addition of battery storage, or save a little money initially.

If you can afford a day out to Stoke you could attend this manufacturer training here https://givenergy.co.uk/become-a-givenergy-installer/ ( check out the bottom of the page for dates) I haven't done it myself yet but I have been told by a few people I know that it is very informative and the lunch is very good, hoping to do it myself in the next few weeks

Edit Just booked for the 15th

UNG · Oct 2, 2024

mainline said:
Apparently from what I read is that batteries can be added to any grid connect inverter using AC Coupling, meaning you don't have to worry about buying a 'battery ready' inverter.
Some may try to convince you to get a 'hybrid inverter'. These are inverters especially designed to have batteries attached using DC Coupling.
Apparently, Hybrid inverters are only compatible with a limited number of batteries (which may not still be on sale when you want to buy batteries in a few years) and they are more expensive than regular inverters.
With AC coupling and a regular inverter, you can add any AC coupled battery you like down the track, and you are not adding any extra cost to the solar system you are buying today.

I did read somewhere that using AC coupled inverters with PV does create a lot of system losses due to the number changes of DC to AC and AC to DC

binky · Oct 2, 2024

Tim Howard said:
Ok so next solar-newbie question -
Can you use a hyrbid inverter without batteries connected?

The underlying question is whether to get a hybrid inverter ready for the presumed addition of battery storage, or save a little money initially.

You can use a hybrid without batteries, and many so called AC battery inverters are hybrids, without solar panels connected. Hybrids are dearer, but it will cost less than having two separate AC battery and solar inverters. Also it helps with DNO application, if you retrofit an AC battery system, they regard it as another load on the grid supply, so you may have say 16A for the solar, and another 16A for the battery, the DNO regard that as 32A,whereas a hybrid doing both would be 16A.

arwooldridge · Oct 3, 2024

Yes, what Binky and UNG said. 100% go hybrid., plus you can add batteries in stages as funds allow, even a small battery is hugely beneficial as the Sun is not very consistent frequently hiding behind clouds! a small battery will cover those events, a bigger one could cover overnight too.

binky · Oct 3, 2024

UNG said:
I did read somewhere that using AC coupled inverters with PV does create a lot of system losses due to the number changes of DC to AC and AC to DC

There's akways system losses, but the percentages are quite small. I think the hybrids freed DC direct to the battery, do cuts out one of the losses. The other source of losses from multiple inverters is down to how the amp clamps behave, and sometimes the inverters are slower to respond to load demands. But again any losses are fairly small, and compared to the energy savings more of an irritation than significant

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