Cheapest Solar Energy Storage Off-Grid
I love solar energy, the idea that you can just put this panel out in the Sun and it creates electricity, for you is phenomenal, but you need a way to store that energy. Now I built this lithium battery bank behind me out of a used Chevy Volt car, but even this used battery pack was very expensive at $ 2,000.
So today we’re, going to talk about a much cheaper, simpler and much more old-fashioned way of storing energy, that’s just with hot water. So here we go Applause. So what I’m? Trying to do on my site with my family is generate my own power store my own power and use my own power and be independent from the power grid.
In several of my past, videos, where I’ve, been talking about an ongoing experiment between the solar photovoltaic and the solar thermal. I’ve received a lot of comments from you guys, suggesting that I just use a heat pump water heater.
Now heat pump, water heater is kind of like a air conditioner bolted on top of a large tank of water, save 50 or 80 gallons. While this makes a lot of sense for people who are on grid, I don’t think it makes a lot of sense.
For me, where I want to go off-grid, you need to think about your days of autonomy or how many, basically, how many cloudy days are a row. Do you expect reasonably before you get another sunny day and these badboys recharge your batteries, where I live in Massachusetts? State I’m, usually looking at 3 or 4 cloudy days in a row that I’m planning.
My storage around a heat pump water heater is usually a 50 60 or 80 gallon tank of hot water. Now that that volume of hot water is not going to be able to last my family for 3 or 4 days in a row now, knowing that it means that I would have to increase the size of the batteries in order to store the electrical energy.
That’s needed to run the compressor for all those cloudy days in a row, but you know what these lithium batteries these are expensive. Even my used batteries at $ 2,000. These are over $ 140 per kilowatt hour of energy.
So if we can instead just store enough hot water to get us through all those cloudy days, this can potentially be a lot less money. So thermal energy storage in the form of hot water makes a whole lot of sense and that’s.
One of the reasons that I’m running this extended experiment right now between the solar, photovoltaic and the solar thermal. This is a fairly simple concept. You heat up a lot of water, you insulate it really well, and then you’ll.
Still have a hot water a few days later. That makes sense. I love it now exactly what is the math and – and I do like math – we need a starting point and what is that? Well to me, that would be a minimum temperature water of 120 degrees.
Fahrenheit and that’s, actually the limit of what my state allows hot water tanks to be set at anything higher than that and you need a thermostatic mixing valve now. I actually have this downstairs in my crawlspace right now where, because my thermal storage tank can be hotter than 120 degrees.
Well, my thermal storage tank down in my crawlspace is what we call atmospherically vented or you could also call it just non-pressurized. There is no pressure that can be stored in that tank coming out of the shower head.
You want pressurized water. So how do you have a pressure water system get heat from a non pressure system? Well, the answer is that you use a heat exchanger down at my crawlspace, that is a 300 foot coil of PEX tubing.
Now this PEX tubing actually makes for a pretty decent heat exchanger, but full disclosure. If I was to do this again, I would actually switch it over to a copper coil inside the heat. Exchanger is the pressurized potable water, which just means it’s, the drinking water that comes out the taps outside of that just in the regular vessel that water does not get replaced or replenished that water just sits there for the life of the tank And, and is a thermal battery just like the chemicals that are inside this lithium battery, they don’t, get replaced just the electrons move in and out right, just like in a thermal battery.
The heat energy moves in and out now the last piece of the puzzle to remember when your work with a heat exchanger is that the thermal battery, the large storage tank of hot water, it needs to be hotter than the water that you want coming out of Your tap, so if you want a hundred and twenty degree Fahrenheit water coming out your tap, the thermal battery needs to be hotter than that.
Now this has to do with the heat transfer rate and how big is the the heat exchanger? What the flow rate of the waters, but just to keep it simple for the sake of the video, let’s, say you need a 10 degree Delta or 10 degree difference in temperature between the thermal storage tank and the water that you want coming Out so in this example, we’re, going to set the bare minimum temperature of that thermal water 130 degrees Fahrenheit.
If we can always stay above that, we’re good, then we can talk about what’s. The high limit, so we don’t want the water to boil and turn to steam. So we want to keep it under 212, but is that really the working limit? Well, the working limit of a thermal storage tank is probably lower.
In my case. The working limit of my tank is 180 degrees Fahrenheit, because I have an e PDM rubber liner and I have PEX tubing as my heat exchanger, alright cool. So we know that 180 degrees is our high point and 130 is our low point.
The difference between them is 50 degrees, Fahrenheit, so 50 degrees is our Delta awesome. Now let’s, move on to the next phase. Now I’m, going to start talking about BTUs or British thermal units, because I’m in the United States and that’s, just what we use here.
The heat capacity of water is 8.3, 3 BTUs per gallon per degree Fahrenheit. Now we’ve already established that our thermal battery, our big water tank, has a 50-degree delta of temperature of degrees Fahrenheit.
Now that means that, for every gallon of water we can store 400 16.5 BTUs. Then we can ask the question: how much energy do you actually need? Well from my own family? Oh, I recorded our energy use just on hot water over the course of the year.
We’re at almost 24,000 BTUs per day, all right cool, in addition to knowing how many BTUs are going to be pulled out of that tank because of our heat exchanger in our use. We also need to know what is our heat loss if our tank is made out of this styrofoam.
Well, there’s. Only a certain thickness to this, and this styrofoam is only so good. It’s, not perfect, so this is gonna lose some of its heat energy through the walls to the surrounding environment and it’s gonna.
Do that all the time? So how do we calculate for that heat loss? Now? The rate of heat loss is going to vary depending on the temperature of the water inside the tank and the temperature of the surrounding environment.
So we’re gonna have to make a few assumptions to work this out, but you know we’re working with really big numbers here. So just go with me on this one. So let’s say that the average air temperature around the tank is 65 degrees and the average temperature inside the tank is 155.
Well now we have a 90 degree Delta difference in temperature. Now, if we take the case of my own personal tank that I’ve built down in the crawlspace week, we have a area of 90 square feet around the all six sides and it’s insulated with two layers of poly iso.
Foam board the lid has a third, but let’s, just keep it to to make it nice and easy right now, so that’s, a total of our 26 all the way around with the 90 square feet. Well, if we actually take the inverse of that, our value will come up with the you factor, which is 0.
03 yo. What is it it’s? A long digit here that you factor is zero point zero. Three, eight four! Six! So if we multiply that by the area and by the difference in temperature – and then you also multiply that by 24 hours in a day – and we come up with a heat loss out of that tank and we come up with a heat loss out of that Tank of 7400 BTUs per day.
So if we add the heat loss out of the tank to the heat loss that we’ll be drawing from the tank for heating our water, then we need a total of thirty one thousand three hundred and sixty BTUs per day.
If we multiply that by, however many days of autonomy, you want or cloudy weather days in a row, I’m, going to use four for this example. Then we need one hundred and twenty five thousand BTUs stored inside the tank.
Now, if we go back to our heat capacity of the tank and look at our gallons and we’re, going to come up with three hundred and one gallons of water that need to be stored in that tank now, my particular tank is 250 Gallons, so I’m, not there.
I’m, probably good up to three days of autonomy, but on the fourth day, not so much now when I built my storage tank down in the crawl space, I just used a lot of stuff that I had on hand. But if you’re gonna go and buy all the material new, it would probably cost you about $ 300 in supplies, so my storage tank being 250 gallons.
I can store just over a hundred thousand BTUs now, if you were to create that enter that heat energy using just an electric resistance element, that is the equivalent of thirty point five kilowatt hours now remember this battery bank is fourteen point three kilowatt hours.
So if you go and you build a 250-gallon storage tank for three hundred dollars and you can store just over a hundred thousand BTUs – that’s, the equivalent of having more than two of these remember, I paid two grand for one.
So that’s, equivalent of $ 4,000 worth of batteries that you’d, have to buy. You can build that tank for $ 300, so that actually means that if you transfer lick translate that all into kilowatt hours, then instead of spending a hundred and forty three dollars per kilowatt hour for these lithium’s in a thermal storage bank, you’re, actually paying just $ 10 per kilowatt hour or seven percent of the cost.
Now that is so cheap that it’s just ridiculous. So I I just love the idea that you can store several days worth of hot water in heat energy in a large, well insulated thermal battery bank yeah. Now, if you oversize your thermal tank, that could potentially be bad.
If you can’t heat the water to the temperature that you want, but assuming that you can get it up to the temperature, you want an oversized tank can be a benefit to you, because you could also then use it for things like space Heating, which I need a lot of being in Massachusetts, so I hope that helps you understand why I love the idea that when the sun is shining go ahead and make make the electrical energy stored in batteries also make the thermal energy and store it in hot Water and do both and that’s, that’s, part of the reason for my big experiment here and why I care so much about how I can most efficiently and cost-effectively take solar energy and produce 180 degree Fahrenheit water.
You know I mean because if I can take my big giant tank of water and heat it up to 180 degrees, that’ll. That’ll swing me through for the you know for the next four days of cloudy weather and that’s. Awesome.
Thank you guys so much for watching. I hope you enjoyed the video, so please like subscribe and leave your questions and comments below thanks a lot for watching