inverter minisplit oversized? efficiency at different speeds?

animatt · June 2014

Question about inverter minisplits, at any fixed speed.

Does it operate more efficiently on low vs med or high. Talking about COP. Here.

Would it make sense to buy a larger ac and run it on slow vs a. Smaller one and running on high. Larger unit would have large heat exchangers, but would the compressor be less efficient etc.

I understand about getting ahead of cooling needs. And running on low.

Just seeing these high seer ratings and wondering how different speeds affect the actual efficiency.

I will be living offgrid in a very mass filled structure. admitted low insulation. But the point being i can use a inverter minisplit and run it at peak efficiency regardless of btu output as the mass of my structure wil help regulate temps.

Any idea of where max efficiency of any unit is. Is there a rule of thumb like a generator. Ie inverter generators at full speed are not really super efficient.

Just trying to take into account the mass of my house into design considerations, just unsure where max efficiency lies..

Thanks. Animatt
Sorry for typos. Typed from smart phone.

jonr · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

In every case I've seen, best efficiency is at the lowest speed. Note that if you size the AC for worse case loads (ie, the hottest day), you will typically be running at somewhat reduced loads anyway. There is an argument for over-cooling slightly while the sun is shining (the batteries don't get used) and at night (when the outdoor air is cooler so COP is higher). Try to coast through the entire evening period.

When using AC (vs passive cooling), insulation is much better than thermal mass. Consider adding some on the inside of the wall. And always use ceiling fans.

BB. · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

I agree with Jonr, insulation (and shading with trees/vegetation/roof-attic insulation/etc.) is going to be a much better solution with A/C...

Running an A/C with poor insulation and/or lots of thermal mass is going to take a long time to cool the home--Or simply require you to run the A/C most of the time (during hot weather) to keep the home cool.

-Bill

waynefromnscanada · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

Agree with both Jonr and BB. Mass will smooth out any temperature changes, but only insulation will reduce your cooling load. Assuming the same sized area to be cooled.

Dave Angelini · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

I would say your point about best efficiency at low speed may be correct but not significantly better than the other speeds. The main point here is you would not want to buy an over sized unit for off grid just to try and get this slight gain. You have to size the unit at high speed and the average hourly power your battery/charging system can supply. As a hot day develops, the unit will throttle up and you need to be able to deal with this with multiple or tracked arrays. There is always a generator, but it is probably very wise to start with a 9,000 to 12,000 btu unit, if you want to avoid issues and generators.

jonr wrote: »

In every case I've seen, best efficiency is at the lowest speed. Note that if you size the AC for worse case loads (ie, the hottest day), you will typically be running at somewhat reduced loads anyway. There is an argument for over-cooling slightly while the sun is shining (the batteries don't get used) and at night (when the outdoor air is cooler so COP is higher). Try to coast through the entire evening period.

When using AC (vs passive cooling), insulation is much better than thermal mass. Consider adding some on the inside of the wall. And always use ceiling fans.

animatt · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

I definately agree especially with my climate. It is a humid climate and as such dos not have temp extremes. Local weather is roughly 95f daytime and 72f day time.

If i were in a dry location daily temp swings could be move than 50f. In that case averaging temps get rid of most cooling needs. 50f lows and 100f highs average out nicely. I see thermal mass as averging temps. More mass longer time frame average. Depending on circumstances this maybe good or bad. In my case i see it as being somewhat neutral. But i am trying to design to its benifits. If i were gridtied this would mean more cooling at night to coast through the day. night time being lower electric rates anlower outside temps for bet cop of compressor. But offgrid would be the opposite due to limited batts.. Max cooling that pv would allow in the day and coast through cooler nights. Even with cooler nights i probably will NOT vent at night due to humidity. I am planning in a passive night time radiation. Will be in expirement that will hopefully work.

If day time load is say 5000 btu I would want to supply 9000 btu cooling if PV would allow. Would buying 12000 btu unit be better than running a 9000 at full speed.
Not sure yet how I would automate the process. Just trying to understand where the best cop would be.

jonr · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

Agreed - the efficiency gains are small. And an oversized AC unit is usually less effective at lowering humidity. And costs more and has higher peak energy use.

If you really want efficiency, you can look at water to water geothermal + an insulated chilled water storage tank + fan coils and then run the compressor only when the sun shines. But not if you want simplicity.

animatt · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

Looking to have a thermal tank which is cooled via passive thermal radiators. These same passive radiators would be passive thermosiphon solar waterr heater during the day. 2 seperate tanks. One for cool storage and one for hot water storage. I am trying to figure out how to efficiently connect up mini split to dump heat to this cool store so I get benefit from running compressor against cool temps comparable to night time lows but during the day basically directly from PV. There would batteries to buffer cloudy conditions. But the compressor would be opportunity load. If poor weather several days the would be enough mass to consider firing up generator and running air con full speed.

animatt · June 2014

Re: inverter minisplit oversized? efficiency at different speeds?

I get the argument of poor humidity control in fixed speed compressor but variable speed compressor, unless majorly oversized I see less of an issue, and especially with more thermal massive to that will reduce short cycling without over cooling the house. Walls are 2 ft thick stone walls for other reasons. If being insulated that would happen on the exterior of the wall, but would require some more stone facing. Would have to run numbers to see if cheaper to buy more PV and run ac more or insulate.
I am in a climate that does not get below 50f and no hotter than 100f. Averaging with thermal mass would allow system to work average daily temps that are much closer to desired temps. So insulation is not as important as some one in BC.
-20f outside is alot further from desired temps than a daily average of 60f or 80f.

inverter minisplit oversized? efficiency at different speeds?

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