residential heat pump theory hypothetical

[masraum]

We've got a heat pump with electric aux heat. It was probably a really nice one when it was installed in 1991. It's still doing a pretty good job of heating and cooling our sieve of an old wooden house with practically no insulation. It does a good job of heating the house down to the mid 40s.

My hypothetical is this, would running water over or spraying the outside coils allow the heat pump to work. The thought just occurred to me when I was outside with the dog. WHen the temp is 36º like it is now, the heat pump has a hard time pulling heat out of the air to pump into the house. If I ran water over the coils, and since the water is well water that's probably ~55º, that water should have enough heat for the coils to grab some.

The other thing that's sticking in my head is that usually water cools things off due to evaporation, and with the fan blowing air over the coils, it seems that could be a detriment.

I'm not actually suggesting that I'd do this, it's a theory question.

[oldE]

I can't imagine your well water being much over 40°F. Maybe the first few gallons out of the pressure tank.
On the other hand, ground source heat pumps utilize those temperatures quite well.

Best
Les

[asphaltgambler]

I'm not sure I understand your theory. With HVAC in heat pump mode; the system essentially works in reverse. The evaporator is now the heat sink, the compressor coils / unit become the evaporator; so spraying the coils with water which are cold already does not make any sense to me in theory.

[masraum]

Originally Posted by oldE

I can't imagine your well water being much over 40°F. Maybe the first few gallons out of the pressure tank.
On the other hand, ground source heat pumps utilize those temperatures quite well.

Best
Les

This is Texas, I can't imagine the water ever being that cold.

[masraum]

Originally Posted by asphaltgambler

I'm not sure I understand your theory. With HVAC in heat pump mode; the system essentially works in reverse. The evaporator is now the heat sink, the compressor coils / unit become the evaporator; so spraying the coils with water which are cold already does not make any sense to me in theory.

The theory is that the outside air is currently 36°, but the water temp should be closer to 55°. When the outdoor air temp is 50-55° the heat pump works pretty well. 🎃

[cabmandone]

Originally Posted by masraum

This is Texas, I can't imagine the water ever being that cold.

At a given depth, the water remains the same temperature. Not saying it's 40. There are charts for geothermal that tell you what you should expect for temps at a given depth. And ultimately no I don't believe running water over the coils would do much for increasing efficiency. I could be wrong, it happens!

[nota]

can't be super hard to stick a thermometer in a bit of water fresh out of the well

deeper well should be warmer
is 55 deg a normal number for ground water ?
I remember 55 as a common cave air temp year round

[masraum]

Apparently there's more variation than I've always been lead to believe.

https://assets.supply.com/ul_pdfs/284997_groundwatertemperaturemap.pdf

[asphaltgambler]

While ambient air does affect the efficiency, there is a thermal net difference between the high pressure and low pressure side that really does not change much as ambient air changes. In mobile air conditioning for example; the maximum ability to remove heat is @55 degrees difference from outside ambient air depending on the level of humidity. So spraying cool water over the condenser (while it's running) would lower the high side pressure by directly making the condenser more efficient, which also lowers in proportion the outlet vent temps

[masraum]

Originally Posted by nota

can't be super hard to stick a thermometer in a bit of water fresh out of the well

deeper well should be warmer
is 55 deg a normal number for ground water ?
I remember 55 as a common cave air temp year round

I ran our sink for a while with a thermometer in it and the temp stabilized around 54-55°, but I'm not sure if I ran through all of the water in the pipes between the house and the well or not.

I've had several baths outside using the garden hose recently because of work that we're doing in the main bathroom. I'm fairly sure that if the water was only 35-40°, I'd know.

[zakthor]

I saw the thread title and was prepared to dump all over you. Thermodynamics and all.

But... actually not impossible that it will work. Just depends on how much water you want to waste. I really like your thinking and interested to see how the calcs work out! Doing it quick, sorry if I made mistakes.

Heat capacity of water: 4,200 Joules per kilogram per degree Celsius

Assume well water is 50f == 10c.
Air is 36f == 2.2c.

10-2.2c = 7.7c temp difference.

**Pretend the heat pump is perfectly efficient.**

Liter of water is 1 kg.

7.7c * 4200j per liter of water = 32760j which is 31btu.

W = btu per minute*17.584264210333

So upper bound of opportunity: a liter of 50f water per minute reduced to 36f yields heat at a rate of 545watts.

If you "watered your coils" at rate of 1 gallon/minute, 3.78L on your coils you'd get less than 2060watts.

You've built an open loop geothermal heat pump. Maybe you should cycle the water into the ground to warm it, then reclaim?

https://www.energy.gov/eere/geothermal/geothermal-heat-pumps#:~:text=The%20geothermal%20heat%20pump%2C%20 also%20known%20as%20the,heating%20and%20cooling%2C %20as%20well%20as%20water%20heating.

[oldE]

Thanks for posting the temperature chart. I am just off the NE corner

Best
Les

[BK911]

If it would work trane would have a system already.
Plus like stated above you want to add heat in heat mode, not extract it.
Doubt any evap would happen with large water droplets and cold temps.
Also well pump probably pulls around 30 amps at 240V.
How big are your heat strips? 20 amps at 110?

[masraum]

Originally Posted by zakthor

I saw the thread title and was prepared to dump all over you. Thermodynamics and all.

But... actually not impossible that it will work. Just depends on how much water you want to waste. I really like your thinking and interested to see how the calcs work out! Doing it quick, sorry if I made mistakes.

Heat capacity of water: 4,200 Joules per kilogram per degree Celsius

Assume well water is 50f == 10c.
Air is 36f == 2.2c.

10-2.2c = 7.7c temp difference.

**Pretend the heat pump is perfectly efficient.**

Liter of water is 1 kg.

7.7c * 4200j per liter of water = 32760j which is 31btu.

W = btu per minute*17.584264210333

So upper bound of opportunity: a liter of 50f water per minute reduced to 36f yields heat at a rate of 545watts.

If you "watered your coils" at rate of 1 gallon/minute, 3.78L on your coils you'd get less than 2060watts.

You've built an open loop geothermal heat pump. Maybe you should cycle the water into the ground to warm it, then reclaim?

https://www.energy.gov/eere/geothermal/geothermal-heat-pumps#:~:text=The%20geothermal%20heat%20pump%2C%20 also%20known%20as%20the,heating%20and%20cooling%2C %20as%20well%20as%20water%20heating.

nice, thanks.

I took thermodynamics in college. It was a 7 am class. I understand it and could do/figure out the formulas, but it would have taken me a while.

[908/930]

It should be more efficient as long as you get enough water on the coil, making it a liquid exchanger. But as BK said you will have the cost of running your water pump, the pump itself likely will heat up the water some.

[masraum]

Originally Posted by BK911

If it would work trane would have a system already.
Plus like stated above you want to add heat in heat mode, not extract it.
Doubt any evap would happen with large water droplets and cold temps.
Also well pump probably pulls around 30 amps at 240V.
How big are your heat strips? 20 amps at 110?

That was my theory, running water, not spraying. Also if 50° air works well for the heat pump, then I assume 55° water should work at least as well, but probably better. I would think that adding (relatively) warm water would be adding heat.

I'm not sure how much power the aux heat takes, but it seems to be a bunch.

[billybek]

If you are pumping well water to do that job, you might be better off with your backup heat (gas/electric).
Adding a ground source loop might be feasible and a small brazed plate heat exchanger.

[zakthor]

Here's heat pump that pulls heat from the ocean:

https://nmdgreen.com/nmd-showcase/projects-for-non-profits/ocean-front-geothermal-a-complete-design-concept-for-marthas-vineyard/

Same thing would work if you put the coils directly into an underground tank at ground temp. The energy difference is there but building and maintaining might costs lots of money.

[Superman]

I believe my heat pump does not try to warm my house when the outside temperature is below 45 degrees F. Below a certain temperature, they just work way too hard. Bear in mind, they are trying to take heat from the outside and put it into your house. Like the OP said, when the outside temp is 36 degrees, the system struggles. It probably should move to electric heat at that temperature.

Yes, many systems are built these days to use geothermal. The "coolant" runs through hoses in the ground. The contractor drills holes, line drilling a well, then puts a hose 'loop' in the hole and fills it up with grout. The ground temperature at the bottom of that hole might be 50 degrees, even when the outside air temperature is 20. These systems work well.

So does gas or propane "backup" heat, for when it is really cold outside.

[Superman]

So does insulation, for that matter.