So I'm building a house and planning to go off-grid. I briefly explained some of my reasons here
My gut is telling me to be practical and get something off the shelf that is easy to put together; however, this idea keeps nagging at me.
The idea is this:
Heat engine + very large insulated storage tank + very large array of homemade flat-plate collectors = Long term energy independence
I know this is an old idea with a high upfront cost.
Could someone check my math? I compared lithium-ion batteries because that seems to be where the technology is going.
The energy density of water at a practical 50 degrees C temperature difference is 0.2093 MJ/L.
However, taking the highest possible conversion efficiency into account, it is 0.0314 MJ/L. I bet in reality it will be half this.
Lithium-ion batteries are 0.9 MJ/L.
This makes lithium-ion at least 29 times more dense than hot water.
Lithium-ion batteries are definately more compact than hot water, but cost, fires, and longevity of any chemical battery are concerns.
At some scale heat storage makes more sense than battery storage.
Heat engines at these temperatures can be designed to last lifetimes. This is one of my favorite aspects, set it and mostly forget it.
The average home in the US uses ~900 kWh per month, this is ~3.24GJ.
This means you would need 103144 liters of hot water to power a home for a month.
This is 27248 gallons, or 3643 cubic feet.
This is a square pond 30.18' long and 4' deep.
More efficiently a cube tank with a length of 15.39', but hydrostatic pressure makes shorter tanks more practical.
In reality I would build a much larger tank(s). I've built a couple 100k gallon ones before and I have plenty of space.
The thing is that the vast majority of my energy usage will be for heating, not electrical appliances. Cooling is another big user, but I want to build an icehouse too.
I am already going with hydronic heating and I won't have to deal with conversion inefficiencies.
With a large enough heat reservoir I could even capture excess summer sun for the winter.
Still toying with flat-plate designs, but think I will try something like this
. We have very similar property and I will learn from his mistakes and keep them vertical.
My plan was to find a couple student mechanical engineers at a local college and pay them to design and help build the heat engine. I'm talking something big and slow and steady, like 60 RPMs.
So what do you think? No nasty PV manufacturing and degradation and few if any batteries to go bad. Terrible efficiencies, but that's the price you pay I guess.