Geothermal - what if?

[quicksandfarmer]

A passive system is going to have to be far larger than a heat pump to get the same result.

Let's say you want to keep your space at 40F and your groundwater is at 45F. The heat released by a radiator (which is what your slab will be) is determined by two things: the different between the surface temperature and the air, and the are of the radiator. Let's say a typical heat pump puts out water at 110F. The most efficient case for a radiator is when the water leaves at the air temperature, that means all of the heat in the radiator is lost to the air. So in both cases the water is leaving at 40F. In the passive system it's entering at 45F and leaving at 40F, the average temperature is 42.5F. The difference between the radiator temperature and the air is 2.5F. In the heat pump case the water enters at 110F and leaves at 40F, the average temperature is 70F, the difference is 30F -- twelve times what it would be for the passive case. To get the same result, the area of the passive radiator has to be twelve times as big!

Let's look at circulator pump and piping sizing. That's determined by the drop in temperature of the water as it circulates. Passive case, it comes in at 45F and leaves at 40F, a five degree drop. Heat pump, it comes in at 110F and leaves at 40F, a seventy degree drop. The passive case needs to move 14 times as much water to get the same result, which means much bigger pumps, bigger pipes, and electricity used pumping water.

It's the same on the underground side. With the passive system you're taking water out of the ground at 45F and returning it at 40F. With the heat pump, it depends whether you have an open or closed loop. With an open loop your fluid is water and you can't chill it below freezing, so you'd be returning it around 33F. With a closed loop you can use antifreeze and return it at perhaps 15F. So your temperature delta with a heat pump is 2.5 to six times as big as with a passive system. The bigger your delta the smaller your pumps and pipes.

 

[SPYDERLK]

It's hard to figure anything without hard data. Loop size, actual ground temps, just how much insulation, etc.

As I said, my hunch is that you willninput MORE energy into the system pumping water than you will return in BTUs.

If you put x amount of wattage in moving water, and that same x amount of wattage can yield more heat otherways, its not a smart or efficient design. And I think that is going to be the case.

And already with the need for supplement for when you are down there, I think the $5k is well spen for a refrigerant based pump.

I would love to be proved wrong and for this to work out for you, so let us know how it turns out if you proceed

Its going to depend on how many degrees below ground water Temp is acceptable to him.

,,, With 2KW pumping he can move a lot of water at low pressure. ... Say pipe friction limits it to 40gpm and the design of the system brings 50* water based fluid to the slab and exits the slab at 45*. -- Thats 320lb/min x 5* = 1600BTU/m. ... A BTU/m is 17.6 Watts. -- So 17.6 x 1600 = 28,160W. > So in that case he gets 28kW for 2kW invested.

As the slab warmed the efficiency would go down of course. At 50* slab the efficiency would be pretty low.
,,,larry

 

[buckeyefarmer]

Read up on what that freon loop is doing for you. One of the most amazing inventions.

 

[Jim Timber]

I'm well aware of the physics involved in the conventional heat pump process. There's still an energy input needed to compress the Freon/ammonia/what have you.

I also happen to have a 3 cylinder ammonia pump. I should see if I still have the photos of it and maybe some of you guys could ID the maker for me. I bought it years ago off a guy who'd been using it as an air compressor (talk about inefficient) for 50 bucks.

 

[Jim Timber]

As the slab warmed the efficiency would go down of course. At 50* slab the efficiency would be pretty low.
,,,larry

My shop floor right now is at 45F and the inner face of the block sill around it is 39F (it's filled with pea gravel and capped with cement - thermal mass and poor conduction), the sheetrock above it is 48F. It was 12F outside when the measurements were taken, and the I-beam of my bridge crane measures 52F with the thermostat set to 55F. Granted, this is from being heated via a hanging forced air furnace.

If the slab was able to radiate enough heat into the air, it's possible it could keep it above freezing with fluid movement alone. I'm thinking you guys are right about it not being enough to keep the air above warm enough though. Kinda like how lumens dissipate the farther they are from the source. The not-hot slab wouldn't have the energy to raise the air to the desired temps. At least not at our subsurface temperatures.

 

[Redneck in training]

I'm well aware of the physics involved in the conventional heat pump process. There's still an energy input needed to compress the Freon/ammonia/what have you.

I also happen to have a 3 cylinder ammonia pump. I should see if I still have the photos of it and maybe some of you guys could ID the maker for me. I bought it years ago off a guy who'd been using it as an air compressor (talk about inefficient) for 50 bucks.

Ammonia refrigeration is more efficient than R22 or R410. It is not used in residential applications because of danger it presents if released. You can build a HP out of the pump but I would place it outside of the house, equip it with ammonia sensor that would trigger water shower if leak is detected. So it won't be easy to do in MN due to freezing temp.

 

[Jim Timber]

I'll have no less than 4 buildings on this property, and I'm aware of the dangers of ammonia. It'd probably live in it's own structure if I end up using it. I'm not sure how big it is, other than it's three cylinders.

 

[Redneck in training]

I'll have no less than 4 buildings on this property, and I'm aware of the dangers of ammonia. It'd probably live in it's own structure if I end up using it. I'm not sure how big it is, other than it's three cylinders.

Are the cylinders the same volume (stroke and diameter)? If they are different displacement it will makes significant difference in power consumption. Three stage with stages in series with three heat expanders will use about half of the power for the same performance. The problem is complexity.

 

[Jim Timber]

Honestly, I don't know. I haven't looked at it in around 10 years other than passing it in my parents garage. It was still there last week. :laughing:

 

[pat32rf]

When you pour the slab, make sure that you put foam frost walls around the perimeter to keep the ground below the slab warmer. This can make a large difference in your slab temp no matter how you heat it. A few simple things that often are forgotten in the shuffle...
Ground temp below the frost line stays fairly constant year round.
Heat rises, BUT it conducts and radiates in ALL directions.

We have two adjacent houses. One is geothermal with a lake loop, the other just got an old outdoor wood boiler. It will have a cool water feed run from the lake next summer (its about 55 degrees year round) for AC and we will be running a heating loop from the OWB to the geo house as well to save hydro...