At what price does LFP storage make it possible to go off-grid?

[minerva]

From his inputs that 247 kWh is two day's autonomy.
View attachment 82110
From the utility bills the biggest 1-day usage is 99.8 and the maximum two consecutive days is 181.8 (see post#220 for more on that, it's a bit handwavium to catch outliers when it's okay for the generator to run).

The formula for kWh needed is =Max(E3:E14)/2*B19/B20/0.8, which in englishish is the maximum two-day usage divided by two, multiplied by the days reserve, divided by the inverter efficiency, divided by 0.8 (at the end of life the batteries are assumed to be at 80% capacity, so you need more battery starting out).

If 247kwh is 48h consumption, the calculated cost should be half of that. The calculated battery cost is based on 247/day. That’s 10kw/h average consumption.

 

[eXodus]

I upgraded some insulation recently and have reduced average daily consumption by 20kWh/day over the past six years through a range of efficiency measures.

I have more to do on the demand side though - it's an ongoing process.

One of the first things I learned about Solar:

Reducing consumption is the cheapest way to get solar suited for your needs.
Knowledge is power (and money) Make an Energy audit first

Reduce consumption as much as practical.

Switch to renewables

BTW Thermal storage is cheap. LFP batteries are expensive, I'm currently working on idea just storing heat and cold in water for the night. Since most of night time usage is HVAC (heat or cold).

 

[upnorthandpersonal]

I'm currently working on idea just storing heat and cold in water for the night.

That's what I do. I do radiant heating and cooling, coupled with a 3000L insulated water tank.

 

[Ozark Tinkering]

That's what I do. I do radiant heating and cooling, coupled with a 3000L insulated water tank.

How do you heat/cool the liquid and what liquid are you using?

 

[upnorthandpersonal]

How do you heat/cool the liquid and what liquid are you using?

Water. The underfloor pipes are filled with a mixture of water and glycol. Heating is done with a wood burner (and excess solar if available), cooling with excess solar and a small heat pump. The water tank is essentially a battery with heat exchangers, and also provides domestic hot water for showering etc.

Schematically, it looks something like this (heating part only):

For cooling, there is another coil at the bottom of the tank (which can also be used as additional heating coil for e.g. solar hot water setups) where cooled water is pumped through to extract heat from the tank, lowering it's internal temperature. Then the same heat exchanger that is used for underfloor heating is used to get the cold glycol/water mixture under the floor. The pump and intermix system that controls the heating also controls the cooling, and keeps an eye on humidity levels and temperatures to prevent condensation.

 

[svetz]

If 247kwh is 48h consumption, the calculated cost should be half of that.

Sorry, not getting it.

My thinking is if 247 kWh is two days maximum consumption, and you want two days reserve then you need 247 kWh of battery, then you need to buy 247 kWh of battery. An actual day could consume more as that number is an average, but if the load draws more than the maximum (e.g., an exceptionally hot day) then the generator needs to kick in. Although I think @wattmatters' use case is like mine, all the hot days consume pretty much the same amount of power and there are no outliers (at least the seasonally hot days have the best insolation).

If you only want 1-day reserve, then it would be half of that, but he had two days marked in his spreadsheet. That seems a good number as LFP only has 2000 cycles (5.5 years) at 100% DoD (ss chart in OP).

There are other things you can do to minimize battery kWh that have been mentioned in the thread (e.g., oversize the array as even on gloomy days you'll get some power). The spreadsheet has an "oversize" for the solar costs, but to get it to reflect back to the battery calculation you'd need to move down the two-day maximum correspondingly (no idea how to calculate that).

 

[Ozark Tinkering]

Water. The underfloor pipes are filled with a mixture of water and glycol. Heating is done with a wood burner (and excess solar if available), cooling with excess solar and a small heat pump. The water tank is essentially a battery with heat exchangers, and also provides domestic hot water for showering etc.

Schematically, it looks something like this (heating part only):

How is the heat pump integrated? We've had wood stoves here in the US South for a long time that will provide heat for radiant heating and hot water for washing but I don't think they ever incorporated a heat pump into the circuit?

Hardy Heater

 

[upnorthandpersonal]

How is the heat pump integrated? We've had wood stoves herein the US South for a long time that will provide heat for radiant heating and hot water for washing but I don't think they ever incorporated a heat pump into the circuit?

I made an edit in my post for the cooling aspect. The tank has another copper coil at the bottom which is normally used for additional heating such as solar hot water. I've installed a heat pump on there which extracts heat from the tank, instead of adding it.

I'll get to a full write up of all this on my blog, but I am working on the solar electric system entry first (which might become two entries actually since it's getting so long...)

 

[Ozark Tinkering]

For cooling, there is another coil at the bottom of the tank (which can also be used as additional heating coil for e.g. solar hot water setups) where cooled water is pumped through to extract heat from the tank, lowering it's internal temperature.

Interesting. I'm a little confused though...what do you do with the heat you scarfed from the tank to use when in the cooling cycle? Is there another heat ex-changer on the other end? Is it filled with the same glycol mix?

 

[upnorthandpersonal]

Interesting. I'm a little confused though...what do you do with the heat you scarfed from the tank to use when in the cooling cycle? Is there another heat ex-changer on the other end? Is it filled with the same glycol mix?

It's just radiated away (think refrigirator).

 

[Ozark Tinkering]

It's just radiated away (think refrigirator).

Okay when you have cooled the tank with the bottom coil, what do you do with the cooled liquid inside the tank?

 

[upnorthandpersonal]

Okay when you have cooled the tank with the bottom coil, what do you do with the cooled liquid inside the tank?

The heat exchanger for the floor heating is used to extract the cold (instead of heat as it would if the tank were hot), cool the liquid that gets pumped in the underfloor heating that way (the water+glycol mixture; glycol only being there to prevent potential freezing of these pipes in winter) and thus creating an underfloor cooling system. The cold is pumped into to floor.

To be precise, you would have to say that you extract heat from the floor though the underfloor cooling, bring this heat back to the tank, where you extract the heat again with the heat pump.

Edit:

 

[Ozark Tinkering]

The heat exchanger for the floor heating is used to extract the cold (instead of heat as it would if the tank were hot), cool the liquid that gets pumped in the underfloor heating that way (the water+glycol mixture; glycol only being there to prevent potential freezing of these pipes in winter) and thus creating an underfloor cooling system. The cold is pumped into to floor.

To be precise, you would have to say that you extract heat from the floor though the underfloor cooling, bring this heat back to the tank, where you extract the heat again with the heat pump.

Edit:

View attachment 82126

Oh. That doesn't really seem like an value added really? Seems like once the floor and tank equalize and reach ambient the pump is just a parasitic load. On top of that, floors tend to "sweat" condensate from the air when the surface is cooler than ambient air. Dew point. My shop with concrete floor is a real sweating machine depending on weather conditions and if I happen to leave the roll-up open.

 

[upnorthandpersonal]

Oh. That doesn't really seem like an value added really? Seems like once the floor and tank equalize and reach ambient the pump is just a parasitic load. On top of that, floors tend to "sweat" condensate from the air when the surface is cooler than ambient air. Dew point. My shop with concrete floor is a real sweating machine depending on weather conditions and if I happen to leave the roll-up open.

The heat pump extracts the heat from the tank again, so it stays cold. Just like with heating: the tank is just a heat/cold battery and is well above/below ambient respectively. The floor is kept at a constant temperature (just like when heating), and extracts heat from the building.
One of the key points to make this work is that you need a controller that knows temperature and humidity, so you don't have this condensation you speak of.

 

[Ozark Tinkering]

The heat pump extracts the heat from the tank again, so it stays cold. Just like with heating: the tank is just a heat/cold battery and is well above/below ambient respectively. The floor is kept at a constant temperature (just like when heating), and extracts heat from the building.
One of the key points to make this work is that you need a controller that knows temperature and humidity, so you don't have this condensation you speak of.

So is this a setup that's on the market commercially or did you design/build as a one-off?

 

[minerva]

Sorry, not getting it.

My thinking is if 247 kWh is two days maximum consumption, and you want two days reserve then you need 247 kWh of battery, then you need to buy 247 kWh of battery. An actual day could consume more as that number is an average, but if the load draws more than the maximum (e.g., an exceptionally hot day) then the generator needs to kick in. Although I think @wattmatters' use case is like mine, all the hot days consume pretty much the same amount of power and there are no outliers (at least the seasonally hot days have the best insolation).

If you only want 1-day reserve, then it would be half of that, but he had two days marked in his spreadsheet. That seems a good number as LFP only has 2000 cycles (5.5 years) at 100% DoD (ss chart in OP).

There are other things you can do to minimize battery kWh that have been mentioned in the thread (e.g., oversize the array as even on gloomy days you'll get some power). The spreadsheet has an "oversize" for the solar costs, but to get it to reflect back to the battery calculation you'd need to move down the two-day maximum correspondingly (no idea how to calculate that).

247 kWh is simply not $148,000 at $300/kW but half that.

 

[upnorthandpersonal]

So is this a setup that's on the market commercially or did you design/build as a one-off?

It's commercially available. This is the tank manufacturer:

Front page - Akvaterm

High quality Akvaterm accumulators, tanks and hot water heaters - also custom-made according to measures. Made in Finland.

akvaterm.fi

This is the wood burner:

Jäspi Econature 40 puukattila

Jäspi Econature 40 on moderni puukattila käänteispalotekniikalla.

jaspi.fi

The control unit:

Fluvia-pumppuryhmät | Uponor

Fluvia-pumppuryhmät | Uponor + Uponor Fluvia -pumppuryhmät vesikiertoiseen lattialämmitykseen ja lattiaviilennykseen.

www.uponor.com

The heat pump in this case is one I hooked up myself, but you can find off-the-shelf solutions such as this one:

Radiant Floor Cooling | Articles

Looking for radiant floor cooling? We offer the easiest and cost-effective way to maximize your home comfort to provide both cooling and heating. Contact us at (866) 800-8123.

www.arcticheatpumps.com

Mine is just a simplified version of this since energy efficiency for cooling is not important (too much solar power anyway). I have a smaller hot water tank for summer use (just for showering basically).

 

[12VoltInstalls]

Seems like once the floor and tank equalize and reach ambient the pump is just a parasitic load.

As is a heat pump split air conditioning unit. To cool it takes energy; he’s just doing it with his heating tubes versus a separate air conditioning unit.

On top of that, floors tend to "sweat" condensate from the air when the surface is cooler than ambient air. Dew point

Region will effect this. His region it appears to be a non issue. Here in Vermont it would or could be an issue often enough to not merit floor mass cooling- a liquid to air coil with condensate drain would probably be best

 

[Ozark Tinkering]

Region will effect this. His region it appears to be a non issue. Here in Vermont it would or could be an issue often enough to not merit floor mass cooling- a liquid to air coil with condensate drain would probably be best

Heating from the floor is efficient. Cooling from the floor, not so much.
Heating a water tank from the bottom is efficient. Heating a water tank from the top isn't.
Thermal dynamics.

 

[upnorthandpersonal]

Heating from the floor is efficient. Cooling from the floor, not so much.

Of course, but it depends on how much cooling you need. This is Finland - if it gets 30C for a day or two a year it's a lot.

Heating a water tank from the bottom is efficient. Heating a water tank from the top isn't.

You can actually use this to create two zones in the tank, with the colder water at the bottom for floor heating (which operates at low temperatures anyway), and the hotter water at the top for domestic.