I'm not really sure what quantity you intend by 250 watts per hour per square meter. Is that just an extraneous per hour? My insolation numbers come from ASHRAE; the article you cite seems consistent.
I believe that biomass is best reserved for high-temperature applications. In my one-earth scenario, that's sufficient reason not to use it to reach a mere 50C. However, perfect may be the enemy of good. The best solutions I see for space heat aren't easy retrofits, and most US biomass residue probably isn't doing anything useful. In the near term, fuel substitution looks like a good idea.
Unfortunately, I don't believe it makes sense to commit pikans for the next 20 years to collecting and splitting wood, between classes. I suppose a dual fuel system is feasable.
How do you get the 1.2M BTU figure? It seems to me that 700 gallons at 100 C only carries 850K BTU above room temperature, and I assume you don't run it that hot.
The static heat capacity of 700 gallons of water in an unpressurized tank is indeed around 850k BTU. The water in our tank will get up to 195 F. However, the dynamic capacity over the course of the day is around 1.2M BTU. That is because we are constantly drawing heat from the system to heat the house and periodically adding new wood to the boiler to replenish the heat that we used. If I add a phase change material to the storage tank then I could store even more heat. By replacing half the water in the tank with say paraffin (melting at 150 F) I could store 5 to 20 times what I store now. I have not needed to do this but it is interesting.
The 250 watts per hour per square meter of solar collector comes from the fact that the total energy collected is averaged over a 24 hour day.
You could add a wood burning boiler to your existing heating system and then use the original system as a backup. I still have my original natural gas fired heater with forced hot air although it never comes on.
Am I correct in understanding that 1.2M BTU is the daily energy use of your heating system? You weren't kidding about getting a good workout splitting wood. I suppose that would make 700 gallons a reasonable tank capacity, for carrying the building through 8 hours without reloading the furnace.
You're still adding an extraneous clause in the units. You want W/m^2 avg, where 1 W/m^2 avg = 86.4 kJ/day/m^2. Your 250 W/m^2 is substantially higher than my 15 MJ/day, but I'm trying to be conservative.
With regard to low flow shower heads - they don't work. If you calculate the extra time needed to rinse, the amount of hot water used is the same. You spend more time in the shower or you don't get as clean. Ask anyone with long hair.
A better way would be to look at how the hot water is generated. Having a gas or electric fired 80 gallon hot water heater is wasteful. Using a tankless instant hot water heater saves about 50%.
I recall from my undergrad dorm long-haired people specifically using certain showers for that one could actually shower (rinse one's hair) in a reasonable number of minutes.
Although the conventional wisdom (and intuition) is that an on-demand heater is more efficient, it's pretty easy to nickel and dime yourself to net losing with one. (Especially since an electric model (220V, 20A = 4.4kW = ~1050kcal/sec) can't get you more than 63°C*Liter/minute, or 20mL/sec = 43oz/min for a 50°C rise; and gas models have switching costs, or, even worse, a pilot light)
As I understand it, the "right" thing to do, since it avoids manufacture costs and gets you most of the efficiency, is to turn a pre-existing tank heater to as cool as you can abide, so that when you shower, you're using just hot water. Then radiative loss from the heater is minimized because it's ~>20°C cooler (and radiative loss for a black body is proportional to T^4)
Yes yes yes. I abhor low-flow shower heads--all they do is make my showers take longer and get me less clean, without alleviating guilt for using too much water since I have to spend longer in the shower.
You can also turn your hot water heater down to 120 or lower if you can stand it. You can wrap an insulation towel around it and around piping. As for showers, there's also taking less of them. Our electric bill is $50-60 for a house in months that we don't use heat (April-November). Most of that is I need to use hot water to wash Naftali's diapers, also the computer. Went down pretty dramatically when we stopped using the dryer and the lights and unplugged everything.
The latest electric bill for my house reports 344 kWh, which works out to 3.7 MJ/year per person, considerably better than pika. We paid $65; I assume your electricity price is about the same here and there . Our hot water (and space heat) is gas, but our dryer is electric, and while I line dry my clothing, two of our four residents still use the electric dryer.
pika's water heater is cool to the touch. I'd like to stick a contact thermometer on it (and the pipes) at some point, and work out what the losses are, but pika actually uses a lot of water relative to the size of their tank, so static losses are a pretty small fraction.
I don't have a good model of pika's basement, but it's uninsulated, so the prevailing theory is that the losses from the hot water tank keep the cold water pipes from freezing. Clearly a bad situation, but we need to insulate the basement walls, first.
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I believe that biomass is best reserved for high-temperature applications. In my one-earth scenario, that's sufficient reason not to use it to reach a mere 50C. However, perfect may be the enemy of good. The best solutions I see for space heat aren't easy retrofits, and most US biomass residue probably isn't doing anything useful. In the near term, fuel substitution looks like a good idea.
Unfortunately, I don't believe it makes sense to commit pikans for the next 20 years to collecting and splitting wood, between classes. I suppose a dual fuel system is feasable.
How do you get the 1.2M BTU figure? It seems to me that 700 gallons at 100 C only carries 850K BTU above room temperature, and I assume you don't run it that hot.
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The 250 watts per hour per square meter of solar collector comes from the fact that the total energy collected is averaged over a 24 hour day.
You could add a wood burning boiler to your existing heating system and then use the original system as a backup. I still have my original natural gas fired heater with forced hot air although it never comes on.
-v-
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You're still adding an extraneous clause in the units. You want W/m^2 avg, where 1 W/m^2 avg = 86.4 kJ/day/m^2. Your 250 W/m^2 is substantially higher than my 15 MJ/day, but I'm trying to be conservative.
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A better way would be to look at how the hot water is generated. Having a gas or electric fired 80 gallon hot water heater is wasteful. Using a tankless instant hot water heater saves about 50%.
-v-
Reply
Although the conventional wisdom (and intuition) is that an on-demand heater is more efficient, it's pretty easy to nickel and dime yourself to net losing with one. (Especially since an electric model (220V, 20A = 4.4kW = ~1050kcal/sec) can't get you more than 63°C*Liter/minute, or 20mL/sec = 43oz/min for a 50°C rise; and gas models have switching costs, or, even worse, a pilot light)
As I understand it, the "right" thing to do, since it avoids manufacture costs and gets you most of the efficiency, is to turn a pre-existing tank heater to as cool as you can abide, so that when you shower, you're using just hot water. Then radiative loss from the heater is minimized because it's ~>20°C cooler (and radiative loss for a black body is proportional to T^4)
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pika's water heater is cool to the touch. I'd like to stick a contact thermometer on it (and the pipes) at some point, and work out what the losses are, but pika actually uses a lot of water relative to the size of their tank, so static losses are a pretty small fraction.
I don't have a good model of pika's basement, but it's uninsulated, so the prevailing theory is that the losses from the hot water tank keep the cold water pipes from freezing. Clearly a bad situation, but we need to insulate the basement walls, first.
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