Mostly off-grid shed with AC safety net

[SunEagle]

Have you seen Bob Ramlow's high-mass solar heating system? It's easiest to deploy on a new build, rather than retrofitting. You basically do a 2' deep sand bed under the foundation, run PEX through it, and run the solar heated liquid through the pex in the sand bed. It's not as efficient as using water as the thermal mass, but due to sand's thermal characteristics, the sand will slowly release the heat through the day. I've been at his house in Wisconsin in the winter, and it was toasty warm. http://www.arthaonline.com/Word%20Fi...Today_ND07.pdf

BTW, altE Store got its start in Maine, focusing on the off-grid Mainers. Maine is still a major region for us. A couple of us spend most of our weekends in Maine, although not as remote as organic farmer.

Thanks Amy.

That is the type of thermal storage system I was refering too a few posts back. Although now with J.P.M. providing some data I never realized that water has better thermal transfer then stone or sand.

[Amy@altE]

Have you seen Bob Ramlow's high-mass solar heating system? It's easiest to deploy on a new build, rather than retrofitting. You basically do a 2' deep sand bed under the foundation, run PEX through it, and run the solar heated liquid through the pex in the sand bed. It's not as efficient as using water as the thermal mass, but due to sand's thermal characteristics, the sand will slowly release the heat through the day. I've been at his house in Wisconsin in the winter, and it was toasty warm. http://www.arthaonline.com/Word%20Fi...Today_ND07.pdf

BTW, altE Store got its start in Maine, focusing on the off-grid Mainers. Maine is still a major region for us. A couple of us spend most of our weekends in Maine, although not as remote as organic farmer.

[RedDenver]

You forget to mention that resistance heating you are calling for is maybe 25% as efficient as a heat pump.

Yes, the efficiency for a heat pump is much better. But for an air-source heat pump the efficiency depends strongly on the air temperature, so resistance heat will vary from 25-50% as efficient. And the heat pump may not work at the coldest air temps, whereas the resistance heat will. So depends only the efficiency and complexity the designer wants to achieve.

[J.P.M.]

I have friends that live nearby, in a university project house built in the 1980s. It has Passive-Solar, and it has Active-Solar, and it has a woodstove. It circulates warm air through an underground thermal-mass stone-bed.

The retired professor of thermal-dynamics who directed the project lives next door. He did a presentation once, that I sat through, on the design features of the house and construction.

There were Phd thesis' done on determining the type of stone, their shapes and sizes.

It was all built so they could do direct comparisons of different heat methods, etc.

A group of permaculture students lives in the house today.



A 2,000 gallon tank of water is only a tiny speck as compared to the volume of sorted and sized stones needed for such a thermal-bank.

I may know/know of the person you refer to as well as a lot of the data on pebble/rock/dry storage particulars.

FWIW: Depending on void fraction, a rock bed/bin needs about 2.5 times the volume of water to store the same quantity of heat. The rock density is greater, but the specific heat of rock is less, and H2O has a void fraction of zero.

[organic farmer]

Have you looked into other types of thermal banks like stone or gravel? I know that once stone is heated up it can take a while for it to cool down.

I have friends that live nearby, in a university project house built in the 1980s. It has Passive-Solar, and it has Active-Solar, and it has a woodstove. It circulates warm air through an underground thermal-mass stone-bed.

The retired professor of thermal-dynamics who directed the project lives next door. He did a presentation once, that I sat through, on the design features of the house and construction.

There were Phd thesis' done on determining the type of stone, their shapes and sizes.

It was all built so they could do direct comparisons of different heat methods, etc.

A group of permaculture students lives in the house today.



A 2,000 gallon tank of water is only a tiny speck as compared to the volume of sorted and sized stones needed for such a thermal-bank.

[russ]

a simple electric heating setup to use excess solar power is much simpler than a heat pump.

You forget to mention that resistance heating you are calling for is maybe 25% as efficient as a heat pump.

[J.P.M.]

FWIW, I spent some time studying and experimenting with thermal storage systems in the late '70's/'early '80's.

One thing I learned several times was the concept of a thermal time constant, somewhat similar in concept to charging (discharging) a capacitor through a resistance in an electrical circuit with the voltage change as f(time) analogous to the temp. change, inside to outside.

Cut to the chase: A dwelling with a large thermal mass will change internal temp. slower than one with a smaller thermal mass. Also, a dwelling with a small total heat loss coeff. (i.e.,well insulated) will change internal temp. slower than one with a larger total heat loss coeff. The building with a large thermal mass and a low total heat loss coeff. will change temp. much slower than either of the others somewhat proportional to: (Effective Building Thermal Mass/Total Building Heat Loss).

Using the analogy of the thermal mass to a capacitance and the (1/total heat loss) to a resistance, most stick/tract type dwellings have building time constants of the order of very approx. ~ 12-24 hrs. or less. High mass passive solar designs can have time constants of the order of several days or more. Seasonal storage dwellings have effective time constants on the order of 6 months - Store in the summer- use in winter - possible but usually impractical and not cost effective.

A common misconception is that high mass alone equals low heat loss. Not true - in act, usually the opposite because most brick/stone/masonry is a lousy insulator, pretty much regardless of thickness, and so a high heat loss --->>> high heat loads --->>> high bills.

Water is the most compact storage medium and is (was) the most common. Rock bins/ pebble beds are OK, but may have long term problems with mold and critters after a while. Think access to the middle of a 20 X 20 rock bin that's 8 ft. deep. Heat distribution in pebble beds can be a tricky issue as can flow rates, and as f(partical size/shape/void fraction, etc.). Also, it's difficult/impossible to add and withdraw heat from a bed simultaneously.

On a retrofit, the most practical, common and cost effective methods to reduce a dwellings HVAC demand is to insulate the living crap out of it, and tighten it to the safest level possible from an air quality standpoint. BTW, air/air HX are a nice idea, that doesn't usually produce effective results or trouble free operation (read, lousy performance and real maint. issues, including health problems). Adding thermal mass in an effective way, in the quantity necessary to be effective beyond a feel good placebo sort of way is pretty difficult. One exception to that might be effectively insulating a brick, stone or masonry dwelling on the outside. It's a tough job with lots of details, and may involve what's called environmental trashing, but again, it's possible.

[RedDenver]

Have you looked into other types of thermal banks like stone or gravel? I know that once stone is heated up it can take a while for it to cool down.

Can be used, but water has more heat capacity.

organic farmer, if you're only using the thermal bank for heating, then a simple electric heating setup to use excess solar power is much simpler than a heat pump. Of course wood is a different form of storing solar energy and works great for heating. If you need cooling, then using a heat pump to store cold water might make sense in some cases.

[SunEagle]

We heat our home with wood. Our woodstove sits in the center of our house. It also heats water, which circulates through a Thermal-bank, which then circulates through our radiant flooring. We think that by capturing as much heat as possible, storing it, transferring it to where and when we want the heat, this is a very efficient home heating system. They say that a heated floor is a much more efficient system of home heating, because you need so much less heat when your feet are warm. We can tell a big difference, between when we heat the floor as compared to when we dont.

Our problem is that our thermal-bank is only 200 gallons.

I have attended a few workshops on this topic. As they workout the math, you need a thermal-bank of 1500 to 2000 gallons to really make it work the best.

When it is -20F, we can fire-up our stove at 3pm, and burn it until 10pm, at which time we bank it. By 7am our house is just beginning to cool enough that we need a bit of heat to keep it comfortable, until our passive solar gain really kicks in around 11am.

I think if we had a 2,000 gallon thermal-bank instead, than it would hold the heat longer and keep our home comfortable from sunset to sunrise much better.

We live in Maine, we generally burn between 3 and 4 cords of wood each winter. Our neighbors commonly burn 10+ cords of wood.



So when it comes to thermal-banks you do need them big. That is in the nature of how they work.

I issue with heat-pumps is that using electricity to turn a motor to compress Freon is a massive power load.

Electricity is great for lighting, computers, fans, and even circ pumps. You can do a lot of neat stuff with electricity. But when it comes to generating heat [coffee pots, microwave ovens, electric stoves, space heaters], or compressing freon [refrigerators, freezers, A/C, or heat-pumps] using electricity sucks.

I do not believe it is the appropriate energy form to be using for those categories of activities.

Wood makes heat very nicely, as does coal.

Have you looked into other types of thermal banks like stone or gravel? I know that once stone is heated up it can take a while for it to cool down.

[organic farmer]

I've seen a couple setups where the heatpump uses excess solar power to heat or cool a tank of water for hydronic HVAC, but the tank needs to be quite large (1000+ gallons) to have more than a few hours of heating/cooling. Not sure if that's sensible, but it can be made to work.

We heat our home with wood. Our woodstove sits in the center of our house. It also heats water, which circulates through a Thermal-bank, which then circulates through our radiant flooring. We think that by capturing as much heat as possible, storing it, transferring it to where and when we want the heat, this is a very efficient home heating system. They say that a heated floor is a much more efficient system of home heating, because you need so much less heat when your feet are warm. We can tell a big difference, between when we heat the floor as compared to when we dont.

Our problem is that our thermal-bank is only 200 gallons.

I have attended a few workshops on this topic. As they workout the math, you need a thermal-bank of 1500 to 2000 gallons to really make it work the best.

When it is -20F, we can fire-up our stove at 3pm, and burn it until 10pm, at which time we bank it. By 7am our house is just beginning to cool enough that we need a bit of heat to keep it comfortable, until our passive solar gain really kicks in around 11am.

I think if we had a 2,000 gallon thermal-bank instead, than it would hold the heat longer and keep our home comfortable from sunset to sunrise much better.

We live in Maine, we generally burn between 3 and 4 cords of wood each winter. Our neighbors commonly burn 10+ cords of wood.



So when it comes to thermal-banks you do need them big. That is in the nature of how they work.

I issue with heat-pumps is that using electricity to turn a motor to compress Freon is a massive power load.

Electricity is great for lighting, computers, fans, and even circ pumps. You can do a lot of neat stuff with electricity. But when it comes to generating heat [coffee pots, microwave ovens, electric stoves, space heaters], or compressing freon [refrigerators, freezers, A/C, or heat-pumps] using electricity sucks.

I do not believe it is the appropriate energy form to be using for those categories of activities.

Wood makes heat very nicely, as does coal.

[RedDenver]

What I do not see making any sense is powering A/C or heatpumps.

I've seen a couple setups where the heatpump uses excess solar power to heat or cool a tank of water for hydronic HVAC, but the tank needs to be quite large (1000+ gallons) to have more than a few hours of heating/cooling. Not sure if that's sensible, but it can be made to work.

[organic farmer]

I can see the benefit of combining grid-power with off-grid. In theory the days and occasional week when the grid is down, maybe the sun will be shining. Hopefully when a snow-storm lasts for more than 2 days the grid will be up.

Both have their weaknesses, but they should be able to offset one another.

What I do not see making any sense is powering A/C or heatpumps.

[russ]

Sounds like a great idea.

Pardon - It is a meaningless idea.

[hogan646]

I couldn't decide whether to put this in the on grid or off grid sub forum, so let me know if this isn't in the right place. Figured the off-gridders have had to get a bit more resourceful .

To get to the crux of the question, I would like to have a system where there's a battery charger on AC hooked up to the batteries 24/7, but only keep the batteries at 80%. The solar panels would be responsible for pushing the charge above 80%. Therefore, the AC wouldn't kick in until the batteries dipped down to 80%.

The smart chargers available seem to be configured to keep the batteries topped off to the brim continuously (rather than the desired 80%). Does anyone know of an existing solution/equipment or clever DIY that I can deploy to achieve this? While there is intended to be a computer running 24/7 in this shed, I would like to avoid using computer controlled relays soldered onto existing meters/chargers to program this behavior.

A tiny more background is that I have the luxury of running AC to the shed 24/7, but I want to have solar contribute to the energy source. The shed is not supposed to use much power, but even so, I've calculated that going securely off the grid will be pricey. But going mostly off the grid, where AC can kick in on those stretches of cloudy days etc is significantly cheaper.


So my thought for doing this would be to have the solar panels connected to the battery bank with the charge controller as usual, and to simultaneously have the bank connected to the AC battery charger set to keep the batteries no lower than 80%. The computer electronics/lights would also be continuously drawing from the bank through an inverter.

Any ideas?

Thank you.

Sounds like a great idea.

[organic farmer]

Wow you sure made that sound tempting. At least on some days.
I think I'd love it if I made it past my media addiction withdrawls. I suspect I'd get by with just my 80 gig ipod
How about a small sunnyboy with standby power. A 2-3 kw system grid tied generating power many days while running AC and laptop. When power drops you loose A/C but PC will still have runtime with ups. For longer outages switch the ups to the offline outlet. May be better answers but I think this could work in the situation.

Most homes that use electricity here, have two generators. A large one that consumes a gallon of fuel an hour, and a small one that consumes a gallon of fuel over 8 to 10 hours. The small generator can power lights and PC, show you can watch movies, etc. The large one can power the well pump [so you can flush the toilet], chest freezers [to keep your meat frozen], and refrigerator.

These homes run the large generator for one hour each day, while the toilet gets flushed, laundry is done, etc. The small generator runs all the rest of the time, except for when they are sleeping.



If you know that the grid will be down for a week at a time, and you expect this to happen a couple times every year [separate from the normal 8-12 hour outages that we see every month]; folks all get used to it.