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

celerity

grahamc2003 wrote: »

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.

I think you're right, that's why I wouldn't spend too much.
By the way, I wouldn't be getting into the water while a cheap gadget I bought was in it I value my life .

Having said all that, given that we will probably be using the hot tub for a few months - everything you've said applies to the internal 3kW pump / heater as well - so a 1kW backup should mean that it works less.

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.

I'm open to other suggestions for how to use my excess power.
I don't like the pump suggestion for afore-mentioned safety reasons though .

I'm hoping someone will start a new thread discussing cost-effective ways of storing power in batteries for pond-pumps, outside lighting etc.

/\dam

thenudeone

Another possibility for £58 - 1.5kw

http://cgi.ebay.co.uk/DXD-T-SHAPED-1-5KW-STAINLESS-STEEL-HOT-TUB-HEATER-/370412417563?pt=UK_Swimming_Pools_Hot_Tubs&hash=item563e4cf21b#ht_1174wt_905

akarobin

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

Hi,
I see your dilemma - a very simple way to use your PV power production is to set the hot tub onto a timer to allow the heater/pump only to operate between the hours of 10 and 2 which is PV peak production. I acknowledge that the demand is probably more than the PV production but you will ensure you use probably all of the peak PV at this time and offset that power you would otherwise import from the grid. Furthermore this is a very cheap solution - good luck !

squinty

Bit off topic, but be aware of the risk of legionella if the tub is kept heated.

See

http://www.hse.gov.uk/pubns/spalegion.pdf

Although this is aimed at commercial tubs, some advice will also apply to household tubs.

gyromain

I have developed an addional hot tub control system. The existing hot tub control system is unmodified and a radio controlled switch switches the hot tub on and off. I use it for making use of Economy 7 but the same princples could be used to apply it to solar PV.
In my control system the hot tub is switched off a calculated time before the E7 period begins, the hot tub cools so that when it is switched on at the start of E7 it is heating for the whole of the seven hours. It is saving me between £1.00 and £1.50 a day.

In a solar PV, an estimate of how much spare energy would be available for the coming daylight hours The hot tub would be switched off problably sometime in the middle of the night. After breakfast time & sunrise, when spare energy is available the hot tub would be switched on.

I use an Oregon Scientific temperature sensors - one air sensor to measure the ambient temperature and floating water sensor to measure temperature of the hot tub water. I use a radio controlled mains switch - bought from Aldi for £7.99. These are monitored by a bespoke piece of hardware & software consisting of microcontroller, 433MHz Rx and Tx. It also has a webserver running on it for monitoring and control anywhere in the world.

EricMears

celerity wrote: »

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 sort of device you'd need is commonly used in laboratories to maintain 'odd shaped' water baths at constant temperature. It's more usual to buy a complete water bath - i.e tank with heater & stirrer unit built in but the heater stirrer units can be bought as a 'clip on' unit for your own bath

e.g.
http://www.grantinstruments.com/products/heating-circulators/
or look at the
UNISTAT HEATER/CIRCULATORS range at :-
http://www.omegascientific.com.au/thermoline_water_page.htmER

Understand your reticence to sit in the bath whilst such a device in use but I would hope that the existing bath let alone any 'add ons' would be protected by a RCD circuit.

ukjoel

gyromain wrote: »

I have developed an addional hot tub control system. . I use a radio controlled mains switch - bought from Aldi for £7.99. These are monitored by a bespoke piece of hardware & software consisting of microcontroller, 433MHz Rx and Tx. It also has a webserver running on it for monitoring and control anywhere in the world.

I loved this post - This is the kind of thing Dr Evil would invent. A Hot tub that could be controlled from all over the world.

I salute you.