Is it possible to harness the latent heat of freezing in a heat pump? 2

[microhydro]

Is it possible to harness the latent heat of freezing in a heat pump?

I have a water source heat pump application. The water is 40 F. and limited in flow. Is there a way to freeze some ice crystals at the evaporator heat exchanger which could flow out the water source discharge, without freezing the entire water source line?

 

[Dave41A]

microhydro:

Yes, I think you should pump the spring water straight through the barrel and side-step the anti-freeze. As proposed, the anti-freeze would just cool to some (below freezing) equilibrium temperature with the evaporator coil, and the spring water would then be susceptible to freezing in the tube. At least with the spring water in the barrel, you can clean off any ice that forms more easily.

However, to avoid ice formation, the maximum amount of heat that can be extracted from the water is Q=M*c_p*8, where M is the mass flow rate, and "8" is the temperature drop from 40 down to 32 (Fahrenheit).

Your ice-rink idea is really an air-source heat pump with an intermediate substance (the ice). If you plan to have an ice-rink anyway, this is fine, but an air-source pump will likely give much better performance due to the fact that air is a fluid and ice is a solid. In the ice-rink case, the ice, once it forms, will be the limiting factor to your rate of heat transfer since the ice, once it forms, will be transferring heat entirely by conduction (i.e. it cannot "convect" away) and the heat conduction coefficient of ice is fairly low.

Good luck,
Dave

 

[microhydro]

In January, the average temperature is 24 F. I'm looking for solutions for the coldest parts of the winter.

What if my ice rink were more of a pond, with an upper and lower evaporator coil? The refrigerant would flow into the top coil first, freezing the top and creating an insulating layer. The water below the ice would be availiabe for freezing in cold weather. The refrigerant would flow into the bottom coil next, and warm to 32F. Then it would flow into the spring water barrel and warm further.

My calculations show that I only need to make about 24 gallons of ice a day. A 24 x 12 x 2 foot deep pond would hold six months of ice.

 

[Dave41A]

microhydro:

Start by recalling how water freezes. In general, water gets more dense as it cools, but below about 40F, the water starts becoming less dense. What this means is that typically, as the pond water cools, the coldest water will naturally make its way to the top. As the temperature of the water reaches 32F, ice will start to form on the surface. If this worked right, you could get away with one coil (the lower one).

However, this is all assuming that the water is well-mixed. In actuality, the thermal driven advection (mass transport) of the water is slow, which means that the evaporator coil will quickly become ice-bound, severely limiting the rate of heat transfer to the pond. If the average temperature of the pond is above 32, and your heat pump system cycles off, then heat from the pond will eventually migrate to the ice, melting it--and the situation will resolve itself until the heat pump re-starts and the coil becomes ice-bound again. But if there is ice on the surface (which will eventually happen), then the liquid water below the surface will also be at or very close to 32F. Any ice that forms on the evaporator coils at this point will stay there until spring*. The reason? There is still "heat" in the liquid water of the pond, but it is latent heat, and heat transfer can only take place if there is a temperature difference (a sensible heat difference).

On top of all this, I anticipate that a 2 foot deep pond would freeze solid on its own due to the weather, making the proposed design impractical at best. Do people ice fish or skate in the area? How much ice forms on those ponds?

Given the climate, I would suggest a ground-source heat pump if you are set on the heat pump idea. You're probably trying to avoid drilling an expensive well by using the spring water, which is certainly a noble effort. Are you sure the water is only 40F (even in the winter)? That seems a little cold for ground water--but I don't know the area. Maybe you could drill or dig a shallow well near the spring and get at some warmer ground water. It seems like you only need a few degrees and I suspect the spring water may be losing heat to the surface rocks as it comes to the surface if this temperature really was what you measured in the winter.

Good luck,
Dave

* Some ice will dissipate and possibly re-form elsewhere due to the equilibrium between the water and ice... individual molecules will enter the ice structure and depart. However, this process is very slow and I would not count on it to remove the ice.

 

[microhydro]

Thanks for helping me think through freezing dynamics in a pond. I wonder if a single oversized evaporator at the bottom could have enough surface area to extract latent heat for the entire heating season?

Eight inches of ice is typical. The spring water temperature and flow measurements are for the spring water piped to my site. There is some cooling in transit because the spring line is buried for 1700' next to a 32F water line.

The plan for this heat pump is to directly drive the compressor with a turgo runner (like a pelton wheel). Take a look at my thread "Microhydro-powered Heatpump" at "autoacforum"

http://www.autoacforum.com//messageview.cfm?catid=2&threadid=21548&forumid=1

 

[Dave41A]

microhydro:

Thanks for the link. I can see now (and admire) what you are trying to do.

The fundamental problem you will encounter is the freezing of the water on the evaporator coil. Once ice starts forming on the surface of the pond, the water below the ice will be liquid water at 32F. Hence, any heat extracted from it will immediately start to form ice. This ice will form on your evaporator coil and greatly reduce performance.

To avoid ice formation, you can really only extract a little over 13,000 BTU/hr of heat from the 40F water (assuming 3.3 gpm or 27.4 lbm/min). That's probably not enough to heat the building. I think you need to find another source of heat (your heat pump can still supplement whatever you settle on).

Good luck,
Dave