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Old 03-02-16, 04:25 PM   #1869
Mobile Master Tech
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I'm posting here because this thread is more relevant concerning getting higher temps from a GSHP efficiently and Mike's comments HERE:

I answered most comments already in the hydronic floor thread but wanted to elaborate concerning limits of the refrigerant and equipment.

Not that we are going to, but you actually can use a refrigerant in its supercritical region as described HERE for CO2. Many new systems are doing this, including most upcoming cars.

The refrigerant doesn't liquefy, so the condenser is thus a "gas cooler" and the expansion valve is now a controlled High Pressure Expansion Device. Despite the fact CO2 is not as efficient as a refrigerant and has ludicrous operating pressures, it is still being used effectively. Where there is a will, there is a way. This tidbit and my deleted post aren't meant to dilute the intent of the Manifesto but rather to encourage out of the box thinking as we hack common equipment and think of new ways to store and use energy from it. We don't recognize that we don't know something if we haven't been exposed to it. AC, you said correctly that hackers and experimenters are often more on the cutting edge of what's possible than the industry as a whole is.

I don't plan on using a refrigerant near it's critical temp, such as R410a, in my GSHP. Most likely I will use R290 with a brazed plate condenser and short piping to minimize the refrigerant amount, since about 60% of the refrigerant in a system is in the condenser when running.

HERE is a plain English guide helping with refrigerant and equipment selection. Most important: compressor inlet temps up to 90F and discharge temps up to 250F are ok as long as you consider the compressor envelope, especially compressor cooling and pressure ratio. FYI, the units we are likely to hack from are the high temp/pressure variety, not medium or low.

You don't want to maintain operation outside the compressor's envelope with the refrigerant in question as explained HERE. "Pull down" temporary operation can vary outside the envelope a bit but has to be kept in check if excursions are extreme.

The pressure ratio and thus efficiency is mainly affected by the temperature spread, not the actual temperature. COP falls off at a pressure ratio greater than 3.5 and most compressors max out around 7.5. Newer Copeland scroll compressors optimized for heating and DHW production can go up to 9.5, and even up to 11.5 with vapor injection.

Pressure ratios are graphed in a straight line on envelopes. I might have to limit flow if the PV panel collectors got especially warm, but my geo/solar/PV/GSHP proposed system temps of approx. 73F source and 140F output are within the envelopes of several compressors. Best of all, they are at a very efficient pressure ratio of 2.8, the same pressure ratio and therefore approximate efficiency as 104F output from a 41F source.
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