We're In Hot Water!
What happens when an MIT engineer designs his own domestic hot water system for use off the grid? Read on...
In our doublewide, hot water is simple... turn the knob and hot water pours out of the cheesy plastic shower head, thanks to grid electricity and a typical electric hot water tank. There are just three major problems as I see them: (1) after 15 minutes in the shower, I start thinking about all the coal that had to be dug up and burned in the power plant to provide me with that cozy hot water (2) after 20 minutes the tank runs out of hot water and (3) a bill shows up every month from the electric company with taxes, surcharges, and gratuities added on. I have succeeded in solving all of these problems in our new house... to some degree.
The neatest part of our new hot water system is the desuperheater on our heat pump. This gizmo taps into the FREE heat that is been sucked out of our house when the air conditioner is running. Few things in life are truly free, but this my friends is as close as it gets. If it didn't go into our hot water tank, the excess heat from our air conditioner would otherwise be dumped into the ground. As it is, the heat pump can only efficiently reject about 10% of its heat to the hot water tank, so 90% of it still goes into the ground... but still... that 10% is free, and it is subtantial. I have already taken a shower with the hot water from our air conditioner (heat pump) and I can tell you it feels great!
What happens if the air conditioner is not running? Well, if the geothermal heat pump is in heating mode (as in the winter time), it can devote about 10% of its heating capacity to the water heater. This heat is not free, since it diminishes the room heating capacity of the heat pump, but 75% of the heat going into the hot water tank is sucked out of the ground, so it's almost free. For every watt of power we put into our ground source heat pump, about 4 watts of heat are generated thanks to the earth's contribution. That's 4 times as good as a typical resistive element hot water heater! Furthermore, all of our power at the new house comes from solar panels, so there's never a bill for hot water generated in this fashion.
OK, but what if the sun is not shining? Well, in the winter on cloudy days, our house will be heated with a wood gasification boiler (not shown in this picture). The boiler's primary mission is to generate hot water for the radiant heat floors (that's the bulk of the plumbing in this picture), but I also plumbed it to our domestic hot water storage tank (through a heat exchanger, so that boiler water and potable water don't mix). Whenever we're heating the house with wood, there are plenty of BTU's to spare for heating hot water. In fact, I can heat the water in the tank to 160 degrees and then use a tempering valve to mix with cold water so that 60 gallons of really hot water is like 120 gallons of regular hot water. This would last several days and could be handy in the fall when the boiler is fired infrequently. (I haven't implemented the tempering valve yet though, so I'll keep the temps below 130.)
So what if the sun is not shining and/or the temperature in the house is already a comfortable 72 degrees, so the heat pump doesn't need to be running? (a common situation in the spring and fall?) Or what if 4 kids and the wife have beaten me to the shower and taken all of the free hot water? Ah, not to worry... in that case I cheat. Downstream of the sustainable hot water systems is a 200,000 BTU propane-fired on-demand hot water heater that can endlessly supply all the hot water I want until my wallet cries uncle. I ease my conscience a bit by calculating that at $1.39 per 80,000 btus, propane is half as costly as grid electricity ($.10 per 3,000 btus) for making hot water. My independence isn't compromised because I can always burn wood if I get fed up with the propane company. And... some day... I hope to put my cows to work producing methane, which I might be able to substitute for the propane in this or a similar hot water appliance.
This is a chart of the various ways of making hot water in our house, and in typical households...
In our doublewide, hot water is simple... turn the knob and hot water pours out of the cheesy plastic shower head, thanks to grid electricity and a typical electric hot water tank. There are just three major problems as I see them: (1) after 15 minutes in the shower, I start thinking about all the coal that had to be dug up and burned in the power plant to provide me with that cozy hot water (2) after 20 minutes the tank runs out of hot water and (3) a bill shows up every month from the electric company with taxes, surcharges, and gratuities added on. I have succeeded in solving all of these problems in our new house... to some degree.
The neatest part of our new hot water system is the desuperheater on our heat pump. This gizmo taps into the FREE heat that is been sucked out of our house when the air conditioner is running. Few things in life are truly free, but this my friends is as close as it gets. If it didn't go into our hot water tank, the excess heat from our air conditioner would otherwise be dumped into the ground. As it is, the heat pump can only efficiently reject about 10% of its heat to the hot water tank, so 90% of it still goes into the ground... but still... that 10% is free, and it is subtantial. I have already taken a shower with the hot water from our air conditioner (heat pump) and I can tell you it feels great!What happens if the air conditioner is not running? Well, if the geothermal heat pump is in heating mode (as in the winter time), it can devote about 10% of its heating capacity to the water heater. This heat is not free, since it diminishes the room heating capacity of the heat pump, but 75% of the heat going into the hot water tank is sucked out of the ground, so it's almost free. For every watt of power we put into our ground source heat pump, about 4 watts of heat are generated thanks to the earth's contribution. That's 4 times as good as a typical resistive element hot water heater! Furthermore, all of our power at the new house comes from solar panels, so there's never a bill for hot water generated in this fashion.
OK, but what if the sun is not shining? Well, in the winter on cloudy days, our house will be heated with a wood gasification boiler (not shown in this picture). The boiler's primary mission is to generate hot water for the radiant heat floors (that's the bulk of the plumbing in this picture), but I also plumbed it to our domestic hot water storage tank (through a heat exchanger, so that boiler water and potable water don't mix). Whenever we're heating the house with wood, there are plenty of BTU's to spare for heating hot water. In fact, I can heat the water in the tank to 160 degrees and then use a tempering valve to mix with cold water so that 60 gallons of really hot water is like 120 gallons of regular hot water. This would last several days and could be handy in the fall when the boiler is fired infrequently. (I haven't implemented the tempering valve yet though, so I'll keep the temps below 130.)So what if the sun is not shining and/or the temperature in the house is already a comfortable 72 degrees, so the heat pump doesn't need to be running? (a common situation in the spring and fall?) Or what if 4 kids and the wife have beaten me to the shower and taken all of the free hot water? Ah, not to worry... in that case I cheat. Downstream of the sustainable hot water systems is a 200,000 BTU propane-fired on-demand hot water heater that can endlessly supply all the hot water I want until my wallet cries uncle. I ease my conscience a bit by calculating that at $1.39 per 80,000 btus, propane is half as costly as grid electricity ($.10 per 3,000 btus) for making hot water. My independence isn't compromised because I can always burn wood if I get fed up with the propane company. And... some day... I hope to put my cows to work producing methane, which I might be able to substitute for the propane in this or a similar hot water appliance.
This is a chart of the various ways of making hot water in our house, and in typical households...
posted by Thomas Massie at Thursday, August 20, 2009
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