This is an ongoing project I am working on so feel free to offer suggestions in the comments!
The story so far:
I live off grid in a small cabin using solar and wind power and I wanted to have a greenhouse for starting plants in spring and a place for a small hot tub and to store my riding mower and tools in winter.
Step 1 was to get a greenhouse and I found one that is 7x15 for $115. It would have cost me more to build one that size so I chose to go the easy route and it looks and works well. You can also build your own and cattle panel and hoop house greenhouse plans are on Instructable and youtube.
The project will be using mostly renewable energy from the sun but will include thermal mass storage, solar water heating and either wood burning or a small propane heater in the project.
I am using a recycled cattle tank made by Rubbermaid for the hot tub because it is just the right size at 70 gallons for one person and it has a drain plug and will hold up for a lifetime of use.
I recycled my small pool filter pump to circulate the water in the tub and I am using a 12 volt Shurflo 3.5 GPM pump and an Eccotemp L5 On Demand propane water heater to bring the tub up to 100-104 degrees. The water pump and pool filter pump are run off a 100 watt solar panel and small inverter.
The greenhouse gets to about 100 degrees when the outside temperature is 70 degrees and that heats the tub up to between 75-80 degrees. So it is being passively heated which means I only need to heat it a little more to reach hot tub temperature.
I can heat the tub using the Eccotemp for just a few pennies in propane in 20 minutes. Yes I know propane is not considered renewable however, propane is a waste gas and bi-product of natural gas and oil drilling and if not used it would have to be flared off. The Eccotemp is very efficient at about 99%.
I also have a clear plastic cover for the hot tub which holds the heat in and reduces evaporation and a thermal blanket to cover the tub at night which reduces heat loss.
I will also be installing a solar water heater that uses black plastic tubing as I progress on this project and my hope is I can heat the tub with only passive solar energy on sunny days and just use the OD water heater on cold sunless days.
The tub acts as thermal mass and also increases humidity in the greenhouse so it helps to balance the temperature in the greenhouse and it does not get outrageously hot in summer. The tub releases heat at night and it helps keep the greenhouse warmer and more humid so plants thrive.
Enjoy the video and please VOTE on the orange tab and leave feedback and suggestions in the comments!
1) stock tank 70 gallon or similar tank that suits you
1) 12 volt shurflo pump 3.5 GPM or similar
1) Eccotemp L5 OD water heater or other heating device- wood, solar or 12 volt element?
1) 80 or 100 watt solar panel
1) 12 volt battery and deep cycle preferred
Small pool filter pump