Cascading Refrigeration System
 

Has anyone done this diy? I'm looking at this for a homemade heat pump with 0 degree farenheit outdoor temperatures to make 160-180 degree hot water.

You don't need a cascade setup to make domestic hot water. Easy to get there with either R134 or R22 single stage. Check around in here for heat pump water heater build threads.

The only thing that you'll have to deal with is defrost on the outdoor heat exchanger.

R22 has been out of production for many years and is really expensive now. There are a number of different (near) "drop in" replacements such as R433b although I'm not sure what's most popular now. In most applications that used R22, the replacement for new equipment is either R410a or R32 which excels at low temperature but doesn't work well at high temperature.

What's the application of the 160-180F hot water? It's difficult to build a heat pump that goes that high, although it's easier if you only need a little at that temperature (which can be obtained with a desuperheater heat exchanger) and a much larger amount of heat at a somewhat lower temperature of 90-130F.

Mike,

Not that difficult to get up into that temperature range, just depends on what pressures you and your condenser are comfortable with. Having been designed to operate at automotive radiator temperature, r134a is easy to get there, and readily available. Oil compatibility is a thing though, as it's not compatible with mineral oil.

I mentioned r22 as a reference refrigerant. I myself use BBQ propane in this pressure range of system. If purity is a concern, r290 can be purchased with relative ease. Not as cheap as BBQ gas, but the peace of mind factor is a thing. However, it's a flammable gas, so beware of the risk.

With stuff in the r12/r134a range, high side pressures are going to be close to or exceeding 400 PSIG at that high of a condensing point. For r22/r290, saturation pressure at 150 degF is just above 400 PSIG, so at 180 destination temperature with a few degrees of delta T plus some subcooling, one would want to run a condensing heat exchanger rated at r410 pressure levels.

Actually, I'm not sure that last few degrees of lift would be highly economical with a "normal" heat pump. Getting up into the 140-150 range might not hurt COP too much, but above there, the pressure skyrockets pretty fast.