COAL POST
COAL POST
COAL POST
Okay so I hadn't mentioned coal until now for one really good reason: If you spend $10 on coal you get about 400kWh worth of coal. If you spend $10 on solar panels, you get about 100 watts of panels which will make 100-200kWh per year, for decades... And since electricity is more valuable than heat, you're probably getting more value in one year with the solar than you do (ever) from the coal.
However, last week I wrote about firewood and woodgas electricity and someone mentioned coal. The reason I brought up firewood is because many climates have an awkward period between around October and March when there is very little solar exposure and solar panels are pretty damned useless. Storing up the tens to hundreds of megawatt-hours of energy needed to get through those months is a real challenge - and most of my energy posts are on one way or another do to just that.
to be clear, if you have any reasonable amount of land, it's quite easy to get enough kWh per year to cover your yearly energy consumption - it's just that most of it comes in April-August and you need to store it up for October-March. The known ways to store it are mostly either:
1. Ridiculously expensive (e.g. 1/2 million dollars of batteries), or
2. Obscenely space demanding (e.g. 1200 cubic meters / 264,000 gallons of iron flow battery juice), or
3. Requiring of special geography / geology (e.g. pumped hydro ponds or underground salt caverns that can store compressed air or hydrogen)
Solutions like coal and firewood are where we give up on storage and accept buying in energy to bridge the winter gap. And the cheapest form of thermal energy seems to be coal.
Using US prices, coal is about 2.5 cents per kWh while firewood is about 5 cents. What you can do with them is largely the same, but there are a few unique twists.
A very promising application for coal is use in a wood gas generator such as the ones sold by https://www.thriveoffgrid.net/
According to their YT video, the ThriveOffGrid generator runs for about 2 hours on a full hopper of charcoal, generating about 5kW of power. Well coal has about 5x the volumetric energy density of charcoal - which anyone who ever picked up a bag of coal knows pretty well, there's a lot there.
If the ThriveOffGrid generator can make 5kW of power for 8 hours on a load of coal, this is pretty interesting, it's 40kWh of electrical power, which is about 2 days worth from my usage.
Now whether using coal, or charcoal, or wood, there is the question of how the heat is extracted and made available for home heating. In my previous post, I just casually proposed a "heat exchanger" that sends heat from the hot gas to the house. Of course heat exchangers are fairly costly and if you want an efficient one, they're even more costly. Moreover, they don't have any real heat *storage*, they just move heat from flow A to flow B.
Here I think we can do much better: Imagine a barrel, perhaps 2 meters (6 feet) in diameter and 3 meters (10 feet) tall. This barrel has a liquid drain at the bottom, a sprayer at the top, an exhaust vent at the top, and an intake vent at the bottom. Then it is filled top to bottom with crushed stone, and insulated from the heat and cold of the outside air. The exhaust gas flows in the bottom, and works its way up, depositing most of its heat on the stone. Cool water is rained down from the top, and warm water is drained out from the bottom.
This construct would be pretty simple and cheap to build, and it would be a HIGHLY effective heat scavenger, leaving the exhaust quite cold. Furthermore, it would also be able to *store* heat for some period of time since the stone can warm up and cool down.
Lastly, if the water that is sprayed on the stone has mixed in CaCO₃ (i.e. pulverized limestone), then this becomes equivalent to a commercial Scrubber - which removes sulfur from the coal exhaust.
Whatever your opinions about the environment may be, I don't think anyone wants to have the sulfur smell of burning coal around their home. So being able to scrub sulfur from the exhaust is a Nice Thing To Have, especially when it can be done using a component that is already present for other reasons.
I think in this configuration it makes sense to have one scrubber between the gas producer and the engine, and then a second one after the exhaust. The first one will get tar out of the gas and cool it down, while the second one will extract heat from the exhaust. Both will remove sulphur, helping improve the longevity of the internal combustion engine and improve the smell of the exhaust.
Warm water from the scrubbers should probably be heat-exchanged because it will be full of crap that's collected from the gas, but that just requires a fairly simple liquid/liquid heat exchanger. After that, the heat can go to in-floor heating or a ground-sourced heat pump, or some other heat user.
There are a couple of other applications for coal than simply to burn it. One interesting one is to upgrade it to methane gas or methanol. The methanol reaction is something I described in a past post, and it involves contributing additional hydrogen to the carbon of the coal and ending up with a liquid fuel. While this is interesting as it allows the coal to store additional summertime energy, there are other ways to store energy and I'm not sure it's THAT interesting. Also sulfur is quite destructive to methanol catalysts so it needs to be pretty much perfectly removed. Generally speaking, I don't think any of the other applications of coal are really worth it.
Sunday Energy Post #30 (one day early):
Number 29 (in quote tweet)
Numbers 1-28: https://pkteerium.xyz/@cjd/posts/BBYLP8nKnlOZvnK8Bc
Today, the product that doesn't exist, but should.
So, you're living off-grid, maybe you're too far from the grid, maybe electricity is too expensive, maybe the power hookup is too far from where you want your house, or maybe you just do not want to associate with the electric company.
30 years ago if you wanted this - and you didn't happen to have a water source for hydroelectric generation - you needed to be okay with burning candles, bucketing your water, and getting your entertainment from a portable battery powered radio. But thanks to advances in solar and battery technology, living a normal life off-grid has become surprisingly realistic.
If you have deployed solar panels, you can generate a lot of power all through the summer months. Large batteries (the likes of which are used in EVs) are available at fairly reasonable prices, and will carry you though one or two days without sun. Solar panels are SO CHEAP that you can massively over-provision such that even with a string of cloudy days, you still generate enough power to keep your batteries charged.
However, the winter months present a unique challenge. Depending on your local climate, you generally expect short days, cold, and lots of cloudy, snowy weather where solar panels generate ~nothing.
Your solar does (massively) over-produce in the sunny days of summer, and storing up that energy for winter is the topic of NUMEROUS posts (see above). But the reality is that long term energy storage remains a very difficult problem.
But if we step away from storage for a moment, there is another technology that is mature, reliable, and fits the resources available to the average homesteader. That technology is wood gas.
But while the technology is mature, the product that should exist still doesn't, and that product is an outdoor firewood boiler which can also make power with a gas engine generator.
There are three technologies for wood burning: they are firewood, wood pellets, and wood chips. Small wood burning applications tend to use wood pellets because they are standardized and easily screw-fed, allowing for small, machine-managed fires. Very large applications tend to use wood chips which can be stored in silos, fed with screw and conveyer, and achieve continuous operation with minimal human intervention. Homestead scale applications tend toward stacked firewood. Stacked firewood is kind of crap, the manual labor involved in production, transport, and burning is huge, but it is a standard. The machinery to produce cut, split firewood is mature and widely deployed, and at the scale there most homesteaders buy wood, firewood is simply the most cost-effective option.
Less than 1 tonne favors wood pellets, more than 100 tonnes warrants semi-truck delivery and favors chips, and the awkward middle is where locally sourced firewood wins.
Firewood based gasification boilers already exist, they produce wood gas from firewood and then burn the gas immediately afterwords to produce an efficient smokeless fire. These are currently the standard in outdoor wood boilers.
Wood chip gasification units also exist, alibaba is teaming with examples like [attached video]. But these machines require wood chips, they produce more energy (and consume more chips) than the average homesteader would want, and it's unlikely that they are capable of standby mode which makes outdoor wood boilers reasonably low maintenance.
But the gap between a modern gasifying wood boiler and a wood boiler with woodgas capability is surprisingly small. The main differentiator is that a gasifying wood boiler introduces enough secondary air to burn away all of the wood gas immediately, while a woodgas generator introduces enough air to keep the temperature hot so that tars are cracked, but not burn away all of the energy from the gas. Furthermore, modern woodgas generators introduces some steam in the secondary air stream because the hot carbonous gas will steal oxygen from the water, leaving hydrogen which is a more ideal fuel for an engine.
The product that should exist is essentially a wood boiler, except with an integrated electric generator. Instead of the classical firebox -> secondary burner -> water heat exchanger -> exhaust, the design is: firebox -> secondary burner -> air heat exchanger (so air into the firebox is hot) -> water heat exchanger (so gas is cool) -> tertiary burner -> water heat exchanger (extract heat) -> exhaust.
At just before the tertiary burner, there's an automatic bypass valve that can take the gas to an internal combustion engine (generator). When heat is required (or if the generator is not connected or out of service), the system makes wood gas as though it was going to run the generator, but it burns it off in the tertiary burner, making heat. When electricity is required, the system starts up as though it were making heat, but once the quality of the gas at the tertiary burner is good enough (some sensor is reading this), the tertiary burner is shut down and the valve to the engine is opened up, then it starts the engine.
There are a few adjustments that need to be made continuously, and a number of failure modes, but this is what computers are good at. Modern outdoor wood boilers are already computerized to a surprising extent, so adding the ability to make electricity is well worth the few extra sensors, servos, and potential error codes. And in case of a failure which makes the generator non-operational, a well designed system can fall back to making heat only.
https://pkteerium.xyz/media/687a09889d28711aef43093716d2c6039f2d8a878c19e5f5becd6be0ca3616f8.mkv
note1alk…e9m8