Considering water

[LouiseSJPP]

I've been kicking around ideas for a pv array, driven largely by the fact I need to roof a log store and pv panels are about as cheap as any other roofing material. But the onward equipment is costly and the complexities considerable. Realising that in the end, the main use I had for the power generated, being an island system, would be to heat the water in the spa pool, I'm now wondering if I should be looking at direct solar water heating instead.

The spa, 4 to 6 person, is wood-fired, but given the forest setting (northern Spain) I think that the convenience of the spa being at temperature through the summer and the lack of worry about sparks created by the log burner would be good.

I have a roof facing south, angle 22°, 7m x 2.5m. What are my options for solar water? I guess I start from the black hosepipe panels covered with a sheet of plastic and go through to some kind of blacker-than-black solar panel. Evacuated tube I can rule out due to cost and poor load match: I couldn't add enough heat over winter to maintain the spa and I have lots of logs!

 

[glandpuck]

Well, the first thing is to figure out can it be done and how so?
You will need to know the volume of water contained in the spa, the ambient water temperature that will be present in different months and the temperature you want to heat up the spa when using the spa.

Since you are using an electric heater for the spa, you can probably simplify the entire design by looking into spa heaters. Common heaters in USA are Raypak, Pentair, Jacuzzi and a few others. If you know the volume of the spa, you can size up the correct model heater to install along with the pump and spa equipment. Then you will know the electric current draw of the heater and spa pumps. Now you will know the energy required to operate the spa (amps drawn per hour) and can calculate how much PV and battery you will need to enjoy your spa. Just remember, early morning spa and later in afternoon and evening will be relying mostly on battery power and not direct PV.

 

[LouiseSJPP]

Well, the first thing is to figure out can it be done and how so?
You will need to know the volume of water contained in the spa, the ambient water temperature that will be present in different months and the temperature you want to heat up the spa when using the spa.

Since you are using an electric heater for the spa, you can probably simplify the entire design by looking into spa heaters. Common heaters in USA are Raypak, Pentair, Jacuzzi and a few others. If you know the volume of the spa, you can size up the correct model heater to install along with the pump and spa equipment. Then you will know the electric current draw of the heater and spa pumps. Now you will know the energy required to operate the spa (amps drawn per hour) and can calculate how much PV and battery you will need to enjoy your spa. Just remember, early morning spa and later in afternoon and evening will be relying mostly on battery power and not direct PV.

Erm, the post says the spa is wood-fired.

Heat loss is the critical factor to maintaining spa temperature, not water volume. Figures are not easy to come by for heat loss, and it varies according to installation.

 

[glandpuck]

Erm, the post says the spa is wood-fired.

Heat loss is the critical factor to maintaining spa temperature, not water volume. Figures are not easy to come by for heat loss, and it varies according to installation.

The OP says he is not happy heating with wood and worried about embers and sparks causing a fire in the area, so he wants an alternative method to heat it. And you don't have to worry about heat loss if you cannot heat the water in the first place. Any heater that can heat the volume of water to the desired temperature can also maintain that temperature via a thermostatic control. Same goes for a furnace. it heats the air, the air cools down and it heats it back up again. The amount of heat loss just determines the initial size of the unit to be installed, along with the volume of water to be heated and the flow rate of the pump through the heater.

 

[Cmiller]

Solar direct water heating is more efficient than solar panels> electric heater> hot water. The reason being that solar panel efficiency is only ~20-25%. (This is the ratio of available power in XX area exposed to sunlight vs. the actual power that gets output from the solar panel.) However, I don't know if a solar direct water heating option will get you the water temps that you will need...

I helped install an evacuated tube type solar direct water heater probably ~12 years ago, but I really don't know much about them. That setup seemed to work great to heat up water in the homeowner's water heater buffer tank, but I don't even remember what temps it took the water up to.

 

[DIYrich]

The spa, 4 to 6 person, is wood-fired, but given the forest setting (northern Spain) I think that the convenience of the spa being at temperature through the summer and the lack of worry about sparks created by the log burner would be good.

Since you are in Spain: I visited Greece last year, and everyone had roof top solar water heaters. Mature technology for them. Look there for a kit.

 

[OffGridForGood]

Solar and an electric spa water heater seems like a better way to go, - no pipework required, easy to control, simple. Other loads could be added to the system when you see that 2.5m x 7m (17.5m^2) will collect a lot of power in a day in Spain. Save the firewood for winter.