Solar Hot Tub Questionsby Admin
19
Mar 2013


Message:

 

 

Good Afternoon,

I'd like to augment my hot tub with solar and have a few of questions about how it would work:

1) Can you give more information about how the heat is exchanged to the hot tub.  Do you submerge one of your stainless heat exchangers into the actual water or do you have to use another pump and use a flat-plate heat exhanger?  If those are both options, which would you suggest?  I live in Virginia and we experience hard freezes so would need a heat pipe system.  

2) If the system was installed with one of your PV powered pumps (instead of a thermostatic controller) is there a concern that the collector manifold might overheat?

3) I have a 600 gallon tub and wonder about the correct sizing?  And with that number of tubes what % can I expect to be solar and how much will I need to rely on electricity?

Thanks so much!

James

Dated on : 21-01-2012
 

 

 

 

Replies :

 
James the best way to heat a Hot Tub is as a second heating loop to an existing solar domestic hot water system.  I have attached a diagram.  The solar would be integrated into the hot tub using a special tube style heat exchanger.  As the tub needs heat and if the solar has heat to give, a second pump will turn on and circulate the heating fluid to the hot tub.


To have proper control it is not recommend to use a PV DC pump as the RESol controller is designed to optimize the solar system based on many variables.


Generally we target 60% of your heating capacity via solar.





Dan


Post By : Dan Jung Dated On : 23-01-2012


 

 



Message:

 

Dan,



Thanks for that.  I was hoping that you could give me some more specific numbers and parts that I'll need so I can try to figure out if this is plausible.  If I just wanted to heat a hot tub and not make it a component of a domestic hot water system, what parts would I need to do that?  Think of it as an off-grid application--this is why I would like to use a PV pump and avoid an expensive pump station.  



I think I've seen the heat exchanger that you're talking about on your website (right?).  I'm assuming it is submersible as there is only one inlet and one outlet (presumably for the fluid from the collector)...so how exactly does it interface with the hot tub?



I had another question, I feel like, but I've since forgotten it!  Thanks for your help with this.




James

 

 

Post By : James Dated On : 23-01-2012


 

 

 



Replies: 




James, where are you located?   There are many ways to run a solar system.  You could integrate without the storage tank but you will loose control.  In other words there may be times that the hot tub is too hot as there is no way to shut it off during the day and store the energy.   The pump can also be a PV pump if you want.  The heat exchanger is a stainless steel exchanger, your solar loop is on one side and the hot tub circulates through the other.  It is not a submersible pump.  Attached is a loop using solar direct with a heat exchanger.   A basic one panel system such as this would cost about $2000 plus piping  and Glycol costs.  This would include collectors, PV panel and Laing CD-5 pump, expansion tank, and Heat exchanger




Dan

Post By : Dan Jung Dated On : 23-01-2012

 

 

 

 

Message:



Great, thanks Dan.  The hot tub is in Bath County, Virginia.  It sounds like there would be some issues with that plan, like overheating.  Is it a concern also that the collector might overheat or does the expansion tank protect it from any pressure/overheating issues?  And the expansion tank won't have any problems with freezing weather because of the glycol mix?



It sounds like the heat exchanger would need a second PV powered pump, in this scenario, to pump the water from the hot tub.  



Thanks again for your help...still trying to work out if this'll make sense.



James


Post By : James Dated On : 23-01-2012
 

 

 

 Replies:

 


James,



This Dieter Jung, I am the engineer for the company. I'll answer each of your questions.


1) Can you give more information about how the heat is exchanged to the hot tub. Do you submerge one of your stainless heat exchangers into the actual water or do you have to use another pump and use a flat-plate heat exhanger? If those are both options, which would you suggest? I live in Virginia and we experience hard freezes so would need a heat pipe system.


The answer to this depends on the details of your existing tub.  If it is a standard acrylic hot tub or even a wood tub, but in either case with an existing heating system, then a shell and tube heat exchanger, our SP155 unit for example , is installed just before the supply wall fitting or just before the jets tee or manifold.  The primary side of the heat exchanger installs into the solar loop, and use a glycol/water mixture. So the water side is protected against freezing using the freeze protection controls that came with the hot tub spa pack.


Two complications arise.  One you have already identified - What happens if there is excess solar heat, how to you prevent the solar system from overheating?  The second complication is how do you control both the solar heat and the conventional hot tub heating in a manner that the controls are not fighting one another and that you have one common, seamless control system?  How good of a solution one wishes to solve these two problems, significantly increases the complexity and the price.


The most elementary addition you need in the glycol circuit is the addition of an expansion tank and if not already mentioned previously by Dan a pressure relief valve. When the solar heat is being produced but the control system has shut off the flow of heat to the hot tub(solar circuit pump has shut off), then the glycol in the collector manifold(s) can reach the boiling point and the manifold(s) will boil empty.  This requires the liquid volume that was in the manifold(s) to be displaced into the expansion tank.  If the sun keeps shining, the empty manifolds will keep going up in temperature until the stagnation temperature is reached.  At this temperature the collectors loose as much heat to the environment as they gain from the sun. No one likes the idea of routinely sending the collectors into stagnation, and further sophistication is added to solve the requirement: " I don't want boiling and stagnation to routinely occur, only if there is a failure, as a safety precaution, should it occur".  In that case the additional choice is to divert the excess heat either to a storage tank, in which case it can be used after the sun has stopped shining, or you can divert the excess heat to some other heating zone domestic hot water, a swimming pool, etc.  Our standard solar Controllers handle the flow of heat to the primary and secondary loads.


You want the hot tub control system to be controlling the hot tub temperature.  Thus when the hot tub has reached the setting, not only does it shut off its normal heater (be it electric or gas), but it needs to send a signal to the solar system to stop the flow of heat to the tub from the solar system.  This signal would then lead to the solar heat being diverted to the storage tank or the second heat load.  Further it is usually desirable to add logic so that the normal hot tub heater is turned off when the solar heat can provide all the heating energy.  This is doable, automatically if the the normal heater is gas or if an auxiliary electric heater is used.  Common electric spa packs with their integrated in-line electric heaters do not provide a means to disable the electric heater in response to an external signal.  So what we can do is physically disconnect the electric element and instead use an auxiliary electric heater which can be controlled in such a manner.

2) If the system was installed with one of your PV powered pumps (instead of a thermostatic controller) is there a concern that the collector manifold might overheat?


From this question, I can see you are thinking "keep it simple and inexpensive".  Keeping it simple is not possible if freezing has to be considered. The freezing possibility cause considerable headaches on both sides of the heat exchanger and biggest problem is in the water(hot tub) side of the heat exchanger.  Circulation must be maintained in such a way that there is no possibility of water in this piping freezing.  Yes you can used a pump that runs directly off a PV panel to circulate the glycol mixture when thermal solar energy is being collected. Call this solar circuit pump, P1.  You do need a second pump on the water side of the heat exchanger.  If this is a completely separate heating system, and there is no hot tub spa pack, then this pump needs to run not only when heat is to be moved to the hot tub water but also run when the ambient temperature is low enough to cause freezing. This also means this pump, call it P2,  cannot run directly off a PV Panel. So either it would need to have an assured power supply available such as grid power of from storage batteries with a sufficient PV panel wattage to do the job, even with days of no sunshine.  A plate type heat exchanger would be appropriate.  Capillary type thermostats could be used for the hot tub temperature and freeze protect control.  An expansion tank and pressure relief valve would be needed.


3) I have a 600 gallon tub and wonder about the correct sizing? And with that number of tubes what % can I expect to be solar and how much will I need to rely on electricity?


I consider the peak power that a 30 tube collector will produce to be 2 kW (6800BTU/hr).  How much energy you will collect over the entire day will depend on your location and the season.  When you need it most, in the winter, you will have the least amount of solar energy available, and in the summer you will have to deal with what to do with the excess energy.  If you don't have a storage tank, it should be intuitive that that the solar system can at best only supply energy during the day, and on the days the weather isn't sunny the amount gets derated.  So even if you sized the number of panels so that they can more than adequately cover the heat loss of the tub during the day, conventional power will have to handle the heat loss at other times, and temperatures are cooler at night time.  Thus without storage you would be hard pressed achieving more than 1/3 of the tubs energy requirements.  Dan or Alan can actually do a simulation for you, with and without storage.  This would then provide you with an appreciation of energy contribution over the whole year.  The big variable however is what the heat loss factor is for your specific tub. The 600 gallons only matters when you are heating up cold water.  Once the tub is up to temperature the controls and heating sources simply balance the supply of heat to match what the tub's heat loss is. 



Dieter


Post By : Alan Anderson Dated On : 24-01-2012

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  Comments: 1
Comments
   01:21 PM , 25 Apr 2012
Awesome info! I have a 1200 gallon swim spa that I am trying to figure out how I can integrate a solar heater with. I would like to tie the system into our incoming cold water supply to our water heater to act as a pre-heater. This info helps point me in the right direction. Thanks!
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