New Home DIY alt-energy ideas

[Brian Church]

Hi.

I work in tech, and I recently wrote some simple software to control relays and monitor dry contacts. I'm building a home and was thinking of applying these skills to a some sort of alt-energy project. The purpose of this post is to submit an idea I had, and also see if I can crowd source some ideas that are better than mine.

I'm in the very preliminary phases, but since everyone like fireplaces, and the property will have a decent amount of land with trees, the low hanging fruit seems like fireplace energy recapture/distribution. Obviously, a blower can help distribute the air around the fireplace, and I've learned that fireplace inserts add efficiency and can include a boiler or a water jacket.

I imagine that a modestly sized home with a couple of fireplaces combined with a traditional wood boiler (to supplement the gas furnace) set up in this fashion can produce more than enough energy for heat and water. The purpose of the custom software would be to not only monitor temps and estimate energy savings, but also to route excess energy to some sort of thermal battery. I've been doing some preliminary research into glycol loops, and I have thoughts of using one to heat a garage, or perhaps even melt snow on a walkway or patio.

This would be mostly a labor of love, and so it doesn't need to be easy or particularly cost-effective to design. With that said, I'm not married to the idea. There may be some maintenance pitfalls I'm not anticipating, for instance.

I'd appreciate any thoughts on this idea, as well as any unrelated ideas for a new home that can use wood as an input fuel and output heat. Bonus points if you can integrate a Stirling Engine and create power too :laugh:

Thanks.

[Joe Blake]

Hi Brian, and welcome to the forum.

I live in (very) sunny Western Australia, where fireplaces are not entirely ubiquitous, so I don't have much experience, but since you are still in the process of building your house, perhaps you might design in a couple of insulated "closets" and fill them with builders' rubble to store the heat. Have a couple of electric blowers to reverse the flow of air.

Have you considered using Peltier devices to generate power for these? I've got a handy-dandy little camping stove (Biolite)

https://row.bioliteenergy.com/products/campstove-2?variant=13844609499245

which uses the heat from the combustion to drive an electric fan that increases the efficiency of the burning, reducing smoke etc. It's so efficient that I can use it to burn gum nuts from the local trees in my yard, and they normally can last for years before they break down.

Sounds like an interesting project you've set yourself. Good luck and let us know how you go.

Joe

[Rob Beckers]

Welcome Brian!

Actually one of the best heat-storage mediums per unit volume is .... (wait for it) ... water! To do better you'd have to move to phase-change materials, where you store heat by changing from solid to liquid and vice versa. Trouble is keeping that heat in, it takes some serious insulation. I know of a passive house that uses a very large amount of water to store heat in summer and use it for heating in winter. Think of a basement filled with over-a-foot-thick styrofoam walls and a water bladder. Not easy to store a bit of heat!

Fireplaces were used to heat a house not too long ago. In general, an old-fashioned open fireplace (while looking very nice and warm when you're in front of it) is a net-loss in energy. With the flue gasses you also suck out hot air from the house. You would need a direct-vent closed fireplace or stove to actually produce meaningful heat for a house.

None of this has anything to do with software though. What did you have in mind for the software to add to this?

-RoB-

[Brian Church]

Thanks for the response, Joe!

Hi Brian, and welcome to the forum.

I live in (very) sunny Western Australia, where fireplaces are not entirely ubiquitous, so I don't have much experience, but since you are still in the process of building your house, perhaps you might design in a couple of insulated "closets" and fill them with builders' rubble to store the heat. Have a couple of electric blowers to reverse the flow of air.

I hadn't considered solid materials. I prefer radiant heat to forced air, and you mention the closet would be insulated (which is good, as a single source of radiant heat is not viable). That said, I assume you could use a heat exchanger to get the thermal energy into a fluid, and not lose too much efficiency if done right.

I'm surmising that, given the additional density of gravel over liquids, I would need to use less medium, and that would be the main advantage. This appears to be true if the other choice is propylene glycol based. I'm not entirely sure that's correct in all cases. As I've just discovered, water is only just over a third the density of granite (a dense rock that comes in a gravel form), but the thermal capacity is five times higher. This is not math I do often, but if I'm not mistaken it would actually take less water in terms of both volume and weight to hold the same amount of heat.


Have you considered using Peltier devices to generate power for these? I've got a handy-dandy little camping stove (Biolite)

https://row.bioliteenergy.com/products/campstove-2?variant=13844609499245

which uses the heat from the combustion to drive an electric fan that increases the efficiency of the burning, reducing smoke etc. It's so efficient that I can use it to burn gum nuts from the local trees in my yard, and they normally can last for years before they break down.

Sounds like an interesting project you've set yourself. Good luck and let us know how you go.

Joe

That thing is really cool. I had not considered a Peltier, but that's because I did not know they existed. I fireplace insert with a USB jack that only works when the fire is running strikes me as an interesting gimmick. It may actually have a practical application in an emergency situation. This would have been great when the power got knocked out for a week a few years back. I was lugging around a 19" rackmount battery backup to places that had power to charge a phone and a tablet once.

There's a lot I have to learn about heat engines, and anything that generates power would be far down the road and not part of the construction scope. My only concern with a Peltier per se is that there is a relatively low power output vs size, even for a heat engine. The models I'm finding are in the 3 - 5W output range. Plans for some hobbyist (I guess) Stirling Engines promise about 200 - 300x more.

EDIT: I did some napkin math, and a heat engine running off a 52.7 MJ/hr fire (50,000 BTU/hr, or about 15 KW) @ only 6.66% efficiency should get you a kilowatt (as if it were that simple). This would require something like 3kg of dry wood burned per hour. Not surprisingly, similar concepts have been discussed around the web, and I'd be surprised if someone wasn't working on this.

Thanks again for the response, and if anything comes to mind, please feel free to keep them coming.

[Brian Church]

Thanks Rob.

Welcome Brian!

Actually one of the best heat-storage mediums per unit volume is .... (wait for it) ... water! To do better you'd have to move to phase-change materials, where you store heat by changing from solid to liquid and vice versa. Trouble is keeping that heat in, it takes some serious insulation. I know of a passive house that uses a very large amount of water to store heat in summer and use it for heating in winter. Think of a basement filled with over-a-foot-thick styrofoam walls and a water bladder. Not easy to store a bit of heat!

Subsequent to reading Joe's post I did some research into materials for the thermal battery, and water certainly came up as a possibility. I need to read more, but I have some concerns. For instance, how dangerous would it be to accidentally create steam, even in an open system? Could an open system become a closed system as mineral deposits build up in the pipes over time and create a steam explosion? This is what has given me pause about designing a water jacket for masonry fireplaces in general. What about bacterial growth? How much and what sort of bacteriostatic agent should I use? Obviously, water is more apt to freeze than many liquids, although that can have some advantages for a system designed to cool (Think Stirling cycle in reverse). Etc...

Just to be clear, I'm definitely not saying it's a bad idea. I just want to convey that in fact I have recently started looking into it, and that's where I am with my thoughts at the moment. (There's a lot to think about, so progress tends to be incremental.)

What about an outdoor pool that you covered in the winter with some sort of insulated top rather than just a tarp (or w/e is currently used), sort of like a big thermos? An added benefit is that it can double as a pool heater in the summer, provided you can input to your loop/battery from a source that won't also heat the house. I could use it as a heat sink too, in the event something goes wrong, I guess.


Fireplaces were used to heat a house not too long ago. In general, an old-fashioned open fireplace (while looking very nice and warm when you're in front of it) is a net-loss in energy. With the flue gasses you also suck out hot air from the house. You would need a direct-vent closed fireplace or stove to actually produce meaningful heat for a house.


As I do more research, I see that at the very least a fireplace insert is necessary to make this work.


None of this has anything to do with software though. What did you have in mind for the software to add to this?

-RoB-

At a minimum, the software could be passive, and just monitor temperatures of inputs and output, and perhaps crunch some energy savings numbers. (I'm imagining a graphical UI with a lightly animated schematic of the system.)

Beyond that, the it could control relays to make some common sense decisions. For example, to prevent smoke from going in the wrong direction, the software could prevent the exhaust of a rocket mass heater from being rerouted to the mass (Joe's post sent me down a thermal mass rabbit hole, and I discovered these yesterday). Similarly, with a traditional fireplace insert, it could delay activation of an insert's water jacket, until the flue is at a minimum temperature, and thus reduce the creosote build up that these are known for.

For a more 'smart' functionality, I can make it think like a newfangled programmable thermostat. One thing that came to mind is integration with the sensors of a home alarm system would make it not difficult to determine reliably that no one is home, and act accordingly. Simpler may be to interface it with one of the home smart assistants so you could use voice commands to operate it.

The final category of functionality that comes to mind is for fail safe operation. To be clear, I want the system to be safe without this. I don't want to kill myself or someone else or burn down the house because on accident I tried to divide by 0. That said, I could use this as an extra level of protection.

To that end, I've seen systems with PEX embedded under a driveway that use electricity to heat gycol, which melts snow. This was intriguing at first, but I began to think that the amount of difficult to service infrastructure required made it not worth the potential for failure. Heating the outside is also not a great use of energy, but like a pool, I imagine that a driveway would make a pretty good heat-sink in an emergency situation, so there may be some merit to the idea after all. E.g., what happens if a mass-based heating system gets uncomfortably warm? If you don't have any way to cool it, I guess you'd just have to wait a day or two, and perhaps open some windows to help. You could then use the open windows to escape my wife, until the domicile is habitable again.

Thanks again for the response, and let me know if you have any thoughts.

[Joe Blake]

Hi Brian,

I did a bit of digging around and came up with this Wiki article. Might be something of assistance in here.

https://en.wikipedia.org/wiki/Thermal_energy_storage

[Brian Church]

Thanks. I hadn't stumbled upon that one yet. This part is pertinent:

Water has one of the highest thermal capacities Heat capacity - 4.2 J/(cm3·K) whereas concrete has about one third of that. On the other hand, concrete can be heated to much higher temperatures – 1200 °C by e.g. electrical heating and therefore has a much higher overall volumetric capacity. Thus in the example below, an insulated cube of about 2.8 m would appear to provide sufficient storage for a single house to meet 50% of heating demand.

I like the idea of rocket mass heaters, and I like the idea of getting radiant heat from stone indoors, but I think that's better for a garage or greenhouse. Even if you could regulate the temperature and relatively rapidly cool it if you slip up, it seems like you'd get better overall efficiency in an averaged sized traditional house by somehow moving that heat around. I can envision using less wood because the living room is too hot while my wife turns up the thermostat because the bedroom is too cold.

I wonder how feasible it is to route the stove's exhaust, for instance, down under the basement before going back up through an existing chimney. (once the flue is hot, of course). One elegant solution may be to have a small firebox in the back of an existing firebox. The exhaust would then go out behind the chimney, where the guts of the flue gas combustion chamber can be hidden by stone matching the existing chimney. I could imagine a brick outdoor grill or something similar, but I'd be worried if both were being used the stone might overheat and crack or worse.

Overall, however, heating concrete or similar, and using that to heat water may be easier and safer in the end. Just thinking out loud.

[Brian Church]

After doing some more research I think I'm at a point where I can draw something on a napkin.

https://pasteboard.co/I6xtWUe.png

I've attached it as a pdf and as an MS Publisher file inside a zip, since the hosted image won't last long.

The goal is to have three chambers which store heat at three temperatures (hot, medium, and mild), and then have a safe way to input heat from two different sources, at two different temperatures (hot and mild), and be able to pull it out at two different temperatures as well (medium and mild).

The purpose of the glycol chamber is to create a buffer between the hot concrete and water, and provide a way to cool the system relatively quickly if needed, and perhaps melt snow.

I left a lot of things out, so as not to make it to noisy. E.g. of course there will be public water going directly to the water heater as well, and I forgot to put an exhaust to atmosphere on the generator over there. It's not intended to be anything like exact or final.

I'm not sure what a safe and effective way would be to exchange heat between these chambers. I was thinking it might be feasible to convince an HVAC tech to install an open boiler of some sort, but rather than flame, it would be hot concrete creating steam.

Also, I'm not proposing to drink water from a bladder under my home. I did not show a liquid-to-liquid heat exchanger there, again, so that it would not be too noisy.

I look forward to any thoughts or criticisms. I'm sure I made some dumb assumptions there. This is above my pay grade.

[Joe Blake]

If you want a cheap heat exchanger, have you considered scrounging some cooling elements from behind refrigerators? (Making sure of course to de-gas the old fridge in an environmentally friendly manner.) The piping is usually not a big diameter, but perhaps you could use taps to cut elements in or out of circuit as required?

A thought.

[Brian Church]

If you want a cheap heat exchanger, have you considered scrounging some cooling elements from behind refrigerators? (Making sure of course to de-gas the old fridge in an environmentally friendly manner.) The piping is usually not a big diameter, but perhaps you could use taps to cut elements in or out of circuit as required?

A thought.

Not a bad idea. I had thought about it for the compressors, but then decided that simplicity and safety should probably be foremost for a first attempt.

Thanks for the tip.

[Brian McGowan]

Hi Brian,
Just thought I'd mention that I have made a controller for my PV power system.
I used an arduino. I made a board that has a resistor divider to cut the battery boltage down to a level that the arduino can measure on it's analog inputs and a 7805 regulator to get the battery voltage down to the operating voltage of the arduino and a ribbon cable connector to go out to a relay board.
Then I made the relay board with 8 relays on it. I use 3 for low voltage disconnect and the program turns them off when they get below their associated low voltage setpoints for 5 minutes so things like me making coffee or the fridge kicking on don't cause them to instantly turn off and they come back on when higher voltage setpoint is reached instantly. These points are set to voltages in sequence so they turn off from least important to most important and vice versa for turning back on from most important to least important.
The second function is for dump load control. Primary dump load is my inverter which I use to send power into the house. The controller closes a relay to turn the inverter on when the battery voltage exceeds 16.5V for 5 minutes and 20 seconds later closes a second relay to connect the inverter to my auxiliary house circuit. The inverter is turned off and the contacting relay opened when the battery voltage get below 15V for 5 minutes. This makes sure the battery is charged and that things like the fridge kicking on and so forth doesn't shut the system off the nanosecond the voltage drops below 15V. I made a 3rd thing that will close a relay to divert power to an additional load if the first load is on and the voltage still gets above 16.5V again for 5 minutes and shuts off if the voltage gets to 15V with no delay so as not to disrupt the first arrangement.
I have 2 additional relays I have not thought of a use for yet.
I have had this run my inverter for 6 hours at a crack with the voltage varying between the high and low set points depending on loads placing demand and clouds and so forth.
I am sure you could do something similar for your heat system.
I am working on a solar liquid based heating system and may use an arduino to control that also having separate relays to control the pump that will pump hot liquid through the 30 feet of tube and fin I have scavenged and will mount under the floor in the basement from the hot tank to the cold tank based on living space temperature and additional relays to control pumps to what I hope will be several collectors outside and will pump from the cold tank, through the collectors and back to the hot tank. I envision having several tanks with water able to flow between them so there would be storage.