Heating a hot tub by Solar PV? (in a smart way)

celerity · 27 June 2011 at 6:11PM

OK, I realise my "green" credentials are about to get blown apart, but we have inherited a hot tub, and given how nice the weather is, it seems a shame not to use it...

What I would like to do though, is help the internal 3kW pump / heater by installing a portable water heater, that only pulls around 1kW.

The idea is that on clear days, I can switch this on and it will effectively run for free thanks to the output from my solar panels.

It would also need an in-built thermostat, so it automatically shuts down once the water reached, say 30C.

Unfortunately, this is as far as my thinking has got, as I don't have a DIY background and would have no idea where to look for such a device.

Any help appreciated!

/\dam

Cardew · 27 June 2011 at 7:36PM

http://www.hottubs2buy.co.uk/hot_tub_product_list.php?Category=heaters&gclid=CPS4q7Hc1qkCFcEd4QodZhveMw

Smallest is 1.5kW. With that power more like a 'tepid tub'

celerity · 27 June 2011 at 8:18PM

Cheers, but they all look like integrated units that would replace what I already have?
I'm after something like an aquarium heater, that I can just bung in the water and remove as needed.

I'm also not expecting it to do too much work heating the water, I'm just hoping it can help the main heater enough to make the exercise worthwhile.
I suppose some calculations couldn't hurt - corrections welcomed, it's been a while since my physics GCSE

:

volume of water in the tub = 1375L = 1375000g
specific heat capacity of water = 4.18 (at 15C)

So, to heat up the water by one degree, I reckon it is 1375000 x 4.18 x 1 = 5747500J

1 watt equals 1 joule per second, so a 1000W heater would take 5747.5 seconds or 95.79 minutes to raise it by one degree - let's call it 90 minutes to make the calcs easier.
My 3000W heater should in theory do it three times quicker, so 30 minutes to heat it by one degree. So, for every three hours the solar PV heater would be on, it would save one hour of the main heater being on, and would heat the water by two degrees.

Saving an hour of a 3kW heater would work out at around 36p of electricity saved, and I reckon a maximum of two hours a day could be saved, so that's 72p per day, or £5 per week. We'll probably only run the hot tub for 12 weeks a year, so the saving would be a maximum of £60 per year.

I've just done a search for "pond heaters" hoping I'd find something cheap, but they all seem to start at £160+ which is way too expensive in my opinion. Those koi carp owners have too much money

.

Also, as discussed in another thread - in practise you couldn't possibly just leave even a 1000W heater on all the time and expect the solar panels to run it. A slight bit of cloud and you might end up having to buy in power from the National Grid. Given that, my budget for this exercise is £50 tops - if anybody can source a suitable heater that would be appreciated!

UPDATE: found this: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=360320688560 - could be suitable...?

/\dam

thenudeone · 27 June 2011 at 8:58PM

Well - they certainly exist:
http://www.aliexpress.com/store/803114/200259976-418936772/1kw-spa-heater-for-spa-free-shipping.html
http://www.poolandspacentre.co.uk/hotspot-1kw-heater.html
but whether you can source one for £50 is another question.

zeupater · 28 June 2011 at 2:10PM

celerity wrote: »

Cheers, but they all look like integrated units that would replace what I already have?
I'm after something like an aquarium heater, that I can just bung in the water and remove as needed.

I'm also not expecting it to do too much work heating the water, I'm just hoping it can help the main heater enough to make the exercise worthwhile.
I suppose some calculations couldn't hurt - corrections welcomed, it's been a while since my physics GCSE :

volume of water in the tub = 1375L = 1375000g
specific heat capacity of water = 4.18 (at 15C)

So, to heat up the water by one degree, I reckon it is 1375000 x 4.18 x 1 = 5747500J

1 watt equals 1 joule per second, so a 1000W heater would take 5747.5 seconds or 95.79 minutes to raise it by one degree - let's call it 90 minutes to make the calcs easier.
My 3000W heater should in theory do it three times quicker, so 30 minutes to heat it by one degree. So, for every three hours the solar PV heater would be on, it would save one hour of the main heater being on, and would heat the water by two degrees.

Saving an hour of a 3kW heater would work out at around 36p of electricity saved, and I reckon a maximum of two hours a day could be saved, so that's 72p per day, or £5 per week. We'll probably only run the hot tub for 12 weeks a year, so the saving would be a maximum of £60 per year.

I've just done a search for "pond heaters" hoping I'd find something cheap, but they all seem to start at £160+ which is way too expensive in my opinion. Those koi carp owners have too much money .

Also, as discussed in another thread - in practise you couldn't possibly just leave even a 1000W heater on all the time and expect the solar panels to run it. A slight bit of cloud and you might end up having to buy in power from the National Grid. Given that, my budget for this exercise is £50 tops - if anybody can source a suitable heater that would be appreciated!

UPDATE: found this: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=360320688560 - could be suitable...?

/\dam

Hi

Quick rule of thumb is 1kW raises 1Tonne by 1DegreeC in 1Hour .... not totally accurate, but works as a quick estimate for most materials, domestic thermal mass etc ....

HTH
Z

Ben84 · 29 June 2011 at 8:00PM

Turning electric from solar PV in to heat, which is generally worth less than electric, may not be such a good option. You could use the electric for appliances in your house that must use electric, or sell it back to the grid, so when there's options to realise its full value, reducing its value in this way isn't ideal.

You could instead make a solar water heater? There are many designs that use everyday items you can buy in hardware shops.

celerity · 29 June 2011 at 10:10PM

Thanks Ben, but I'm talking about "spare" electric - I keep a close eye on my generated output and usage. Many of us on here are on a 50% deemed export rate, so ways of using "extra" energy to either help out existing appliances or reduce the use of gas or oil are of interest.

Also, I'd be unlikely to have much success building a solar water heater - my DIY skills pretty much end at Lego

.

/\dam

felixDcat · 30 June 2011 at 3:07PM

Why not use a solar water heater? Easy to make one from a long black pipe laid out on the ground or roof, has water circulating around it powered by a smallish electric pump. Woud be much more efficient than trying to heat water with electricity. Could circulate the actual water from the hot tub or use an indirect system.

Regards

thescouselander · 30 June 2011 at 4:10PM

Why use PV pannels - you are just creating inefficiencies changing the light/heat to electricity and then back to heat again. Surly its best to use the type of pannel that heats water directly.

celerity · 30 June 2011 at 6:14PM

Maybe I'm not being clear?
I have solar panels that generate more than my house can use during clear days.
On those days, I would like to utilise this spare energy to power a "pond heater" type device.
I have neither the time nor the skills to build a dedicated solar thermal water heater.

This is exactly the same principle as people wanting to use an immersion heater for their house powered by solar PV, but I think it is more useful, as you don't want house heating in the summer - but you do want to use the hot tub

.

/\dam

grahamc2003 · 30 June 2011 at 6:20PM

While I can see what you are trying to do, I think any benefits from such a small heater in a hot tub would be very small. The energy you're adding over a few hours won't have much of an impact on the volume of water in a hot tub, as you have demonstrated. Even a small rise would be met with rapidly increasing heat losses from the tub, especially if you are in it and the cover is off. I think if you also use the proper tub heater to get it to proper tub temperature, then the best you can expect is a slightly lower rate of cooling.

But as others have said - electricity is high quality energy, and using it as heating is always a waste, since you can use it for any other purpose, and still get the heat out as a freebe if you like. I'd bet 1kw would be enough to drive some powerful pumps to create water jets - I'd just turn those on when it's sunny, then you get the jets as well as the heating - no need for a small heater, a 1kw pump would give you 1kw of heating.

Heating a hot tub by Solar PV? (in a smart way)

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