Solar hot tub possible

pixiefree

New Member
I stumbled on YouTube videos of Will and was hooked watching him all evening. Such a talented young man.
I initially was watching to help for when I start building my camper van.
Have since purchased the E book, to help.
so i apologies in advance if this question has been asked already, I did have a read through some chats, but being a novice I'd rather get clear answers to my particular situation.
But as I currently have solar panels (6×300w grid tied to my flat, they are currently on my shed at bottom of garden, electrician says I can't tap into the existing system as its already grid tied to flat)
I now have a hot tub, (I suffer from severe joint pains) and wondered if I could build a stand alone system for my hot tub just for it to heat up during the night, for use in the morning. (added bonus if it could supply electricity to my shed also, tv/small 2kw heater, floor lamp)
I have space for 3/4 further panels (300w panels)
I live in Scotland so my electricity is 240v, my hot tub holds 245 gallons water, the instruction book says 220-240v, 50hz, total power 2100w, (heater 1500w/bubble 600w/filter pump 12v/35w)
Is it possible to build a stand alone solar system running via an Inverter.
Or another possibility I could explore would be wind turbine.
I get sun morning right through to nite, or when windy it is very windy in my garden, (destroyed to 2 pop up gazebos already....🤭🤭😫)
Extreme thanks in advance
 

snoobler

Solar Honey Badger
Moderator
It is possible, but 1500W of heating is massive.

Just picking numbers - to heat the water from 90°F to 106°F, it will take about 6.5 hours assuming no system losses (well insulated).

That's 10.6kWh of energy after inefficiencies, not counting run-time. Assuming you get great sun (in Scotland????), your 3-4 panel array could produce 4.5 to 6.0kWh of energy daily.

If heating outside of peak solar hours, you'd need a sizable battery.

If you want to throw thousands that this, and you don't care that it will take 8-10 years to break-even financially, it can be done.

Wind is a losing proposition in almost all cases unless you have SUSTAINED winds and very clean flow (no turbulence) on top of a 10m tower.
 

chrisski

Solar Addict
To store the 10.6 kwh of energy mentioned, you‘d need 16 golf cart sized GC2 batteries, which would store 20 KW of stored energy and 10k of this would be usable. I’d want to put 4000 watts of panels on this. Done on the same scale as my RV, I would expect the cost to be around $20k US. With my math, that is what I consider a real $20k where you have a little left over, quality system you built yourself. This is not a $5k system quote where $15k of parts are left off like wiring, mounts, batteries, and all those other things that add up.
 

Hedges

Photon Sorcerer
If you already have a grid-tied PV system wired to the power of your flat, why not power the hot tub from the grid?

Generally the most cost-effective solution is to convert PV to AC and backfeed the grid (net metering), draw power from grid when you want to.
 

pixiefree

New Member
I do currently have it plugged into grid power, but at £130 a month electric bill, compared to approx £20 a month when tub is turned off, I thought poss solar was a option.
Going on the other folks valuable knowledge this doesn't seem possible....😥
 

pixiefree

New Member
Thanks George, certainly food for thought there.
Will look into some of ur ideas, I certainly don't mind using used veg oil 👍
 

Hedges

Photon Sorcerer
I do currently have it plugged into grid power, but at £130 a month electric bill, compared to approx £20 a month when tub is turned off, I thought poss solar was a option.
Going on the other folks valuable knowledge this doesn't seem possible....😥

If I understand correctly, you already have a grid-tie PV system with net metering. You've now added more load (the spa heater) so your bill went up. If you add more grid-tie PV you should be able to offset that.

1) Can the existing PV inverter handle more PV panels? Even if peak production approaches rating of inverter, during much of the day it is lower. Adding another string of PV panels oriented toward sun at a different time of day will deliver more Wh/day. Those could be on a different MPPT input if available, or can be paralleled with the existing PV panels (in which case all PV strings must be similar Voc & Vmp).
2) Or add another PV inverter.

Heating oil ... what does that cost? Looks like $0.60/L (in the US)? Then 25L/week 100L/month is $60/month or 45 GBP? An improvement and less capital investment than PV.

How many hours per day does your heater run? Snoobler estimated your four PV panels would cover about 60% of your estimated power consumption.

How well insulated is the tub and cover?
 

fat_old_sun

Solar Addict
I now have a hot tub, (I suffer from severe joint pains) and wondered if I could build a stand alone system for my hot tub just for it to heat up during the night, for use in the morning. (added bonus if it could supply electricity to my shed also, tv/small 2kw heater, floor lamp)
I have space for 3/4 further panels (300w panels)
I live in Scotland so my electricity is 240v, my hot tub holds 245 gallons water, the instruction book says 220-240v, 50hz, total power 2100w, (heater 1500w/bubble 600w/filter pump 12v/35w)
Is it possible to build a stand alone solar system running via an Inverter.
Or another possibility I could explore would be wind turbine.
I get sun morning right through to nite, or when windy it is very windy in my garden, (destroyed to 2 pop up gazebos already....🤭🤭😫)
Extreme thanks in advance
If you have the rooftop space for four 300 Watt panels, consider installing similar sized solar thermal panels to heat the water. Solar thermal is 3-4 times more efficient than PV panels. Conservatively, each thermal panel would harvest 900W x 5 hrs or 4.5 kWh a day. Install a small PV panel to provide power for a pump to circulate the hot tub's water thru the thermal panels.
 

Hedges

Photon Sorcerer
If you have the rooftop space for four 300 Watt panels, consider installing similar sized solar thermal panels to heat the water. Solar thermal is 3-4 times more efficient than PV panels. Conservatively, each thermal panel would harvest 900W x 5 hrs or 4.5 kWh a day. Install a small PV panel to provide power for a pump to circulate the hot tub's water thru the thermal panels.
Ahh, but PV panels in the summer,
backfeeding the grid with net metering,
providing credits for electric heating in the winter ...

could be a better way to go. At least, may give at least as much power in the winter (more if cloudy), and works with existing thermostat system.

If you decide to try solar thermal as Solar_Gordo suggests, consider evacuated tube solar collectors (thermos tubes), with a propylene glycol solution circulating through their manifold and a copper tube in the hot tub.
 

fat_old_sun

Solar Addict
Ahh, but PV panels in the summer,
backfeeding the grid with net metering,
providing credits for electric heating in the winter ...

could be a better way to go. At least, may give at least as much power in the winter (more if cloudy), and works with existing thermostat system.

If you decide to try solar thermal as Solar_Gordo suggests, consider evacuated tube solar collectors (thermos tubes), with a propylene glycol solution circulating through their manifold and a copper tube in the hot tub.
I doubt if there'll be too much excess power from the PV panels during the summers, since the hot tub would need heating summer or winter.

It doesn't have to be an all or nothing proposition. It could be 2 PV panels & 2 thermal panels.
 

pixiefree

New Member
If I understand correctly, you already have a grid-tie PV system with net metering. You've now added more load (the spa heater) so your bill went up. If you add more grid-tie PV you should be able to offset that.

1) Can the existing PV inverter handle more PV panels? Even if peak production approaches rating of inverter, during much of the day it is lower. Adding another string of PV panels oriented toward sun at a different time of day will deliver more Wh/day. Those could be on a different MPPT input if available, or can be paralleled with the existing PV panels (in which case all PV strings must be similar Voc & Vmp).
2) Or add another PV inverter.

Heating oil ... what does that cost? Looks like $0.60/L (in the US)? Then 25L/week 100L/month is $60/month or 45 GBP? An improvement and less capital investment than PV.

How many hours per day does your heater run? Snoobler estimated your four PV panels would cover about 60% of your estimated power consumption.

How well insulated is the tub and cover?
Hi the tub is within an enclosure but with still be subject to wind be it minimal, sits on its own thermal ground sheet, under that I have a rubber mat, that's all sitting on top of decking, its cover again by its own thermal cover.
I was thinking of getting the silver foil and wrapping it up it that also.
 

Hedges

Photon Sorcerer
Hi the tub is within an enclosure but with still be subject to wind be it minimal, sits on its own thermal ground sheet, under that I have a rubber mat, that's all sitting on top of decking, its cover again by its own thermal cover.
I was thinking of getting the silver foil and wrapping it up it that also.
An extra 110 GBP just to heat the tub is a considerable expense.

I have gas & electric. If just water heater and stove, bill is $8/month. Of course a water heater is better sealed and insulated than the hot tub.
If I heat the house with gas, cost is more like $100 per month, but it's been a while since I've done that.
Electric heat costs way more than gas, but since I have surplus credits I consume several $100 per month for heat (use it or lose it)

The heating oil suggestion is also worth considering, compute what it costs.

I figure your solar (whether PV or thermal) will produce 4x as much in summer as in winter. The hot tub will probably consume 2x as much in winter as in summer. That's why I'm guessing that even if solar thermal produced 4x as much as PV, it would be less effective than storing power as accounting on your utility bill.

Reflective insulation works by reflecting IR back, so I'm not sure that wrapping it around the outside of something insulated helps much. Finding heat and air leaks should help. An IR imager or just IR thermometer might help. Bubblewrap pool cover may help; in contact it should reduce evaporation which would carry heat away. Inside of tub enclosure probably isn't as well heated as it could be, might be a place to look. Could be that fiberglass insulation bats with reflective surface would be good if that area is accessible.
 

fat_old_sun

Solar Addict
I figure your solar (whether PV or thermal) will produce 4x as much in summer as in winter. The hot tub will probably consume 2x as much in winter as in summer. That's why I'm guessing that even if solar thermal produced 4x as much as PV, it would be less effective than storing power as accounting on your utility bill.

Reflective insulation works by reflecting IR back, so I'm not sure that wrapping it around the outside of something insulated helps much. Finding heat and air leaks should help. An IR imager or just IR thermometer might help. Bubblewrap pool cover may help; in contact it should reduce evaporation which would carry heat away. Inside of tub enclosure probably isn't as well heated as it could be, might be a place to look. Could be that fiberglass insulation bats with reflective surface would be good if that area is accessible.
Did you get the summer/winter production & usage figures swapped? I dont see how the panels can produce 4X in summer.

Reflective insulation is also a poor radiator due to its low emissivity. Any heat that makes it thru (conductively) to the exterior, shiny surface won't be easily radiated. This is true only if there's an air gap between its exterior surface & the surroundings.
 

Hedges

Photon Sorcerer
Did you get the summer/winter production & usage figures swapped? I dont see how the panels can produce 4X in summer.
Which location was this again?
Here's a link with insolation maps for UK


Looks like July/January is 14/1 or 21/3.5, considerably more than 4/1
Probably due to that famous weather.

Usage:
Assume tub is 100 degrees F. Ambient is 50 degrees F in summer, 0 degrees F in winter.
Winter temperature is 100 degrees F, which is 2x summer delta of 50 degrees F.
So energy loss in winter is 2x in summer.

But I think evaporation is particularly significant. Screw a lid on that thermos bottle if you can. Otherwise, float bubble wrap to cover almost all exposed surface area.

Reflective insulation is also a poor radiator due to its low emissivity. Any heat that makes it thru (conductively) to the exterior, shiny surface won't be easily radiated. This is true only if there's an air gap between its exterior surface & the surroundings.
True, but radiating matters more when hotter. If you wrapped fiberglass around a tub with reflective surface facing out, the surface is relatively cold so low emissivity matters little.

At low temperatures though, with outside even lower, you'll get plenty of convection. Or a draft carrying heat away from that shiny surface.

So I suggest such insulation oriented to face the surface of the tub (with an air gap) so the greater IR coming off the tub hits the shiny surface and is reflected back towards the tub.


Maybe blowing in styrofoam packing peanuts would be better.
 
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