Variable speed refrigerant compressor at very low speed - efficiency

[Jon_Doe]

Hi there everyone.

I know variable speed compressors can be overdriven and also run very low.
Some sources indicate down to 10% but most it's no less than 30%.
What i could not really find out is data about efficiency of the system at very low speeds.

The reason i'm pondering this is that i have a situation with a rather well insulated envelope, where peak required power in the worst possible time of year would be ~3.5kW - 12kBTU - 1ton.
That time may be 3% of the year and only on some odd years.
However, most of the time, the required power would be way below this value, like 10%-30%.

Commercial, off the shelf, inverter air to water heat pumps are at least twice this much and would need to run at the lowest setting, and even cycle on/off like a fixed speed unit, negating some of the advantages of variable speed.

So, how would a large compressor run at minimum speed compare, from efficiency point of view, to another one, half it's size, at a medium speed ?

There may be a remote possibility of repurposing a standard mini-split, very efficient, 12KBTU inverter AC to do this but it requires some surgery.
That may not be such a problem but warranty voiding may.

Any thoughts ?

 

[LittleInch]

The other big issue is controls. Using vendor ones could easily lead to short cycling as they won't let you set the dead band between on and off. some do so do you research on the controller first.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HVAC-Novice]

the ones I know (Mitsubishi) offer BACNet integration and digital contacts for your own control. Those are options you need to specify. Standard would be a standalone Mitsubishi thermostat (or different options). If you want BACnet integration ro something else, I recommend contacting them, at least the first time you specify it that way. they have all the information on their website, but it can be overwhelming at first.