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]

So this is cooling?

By efficiency do you mean CoP?

Only a vendor will get you that. But sounds to me like you need 2 x 50 % sized systems.

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

 

[Jon_Doe]

Hi there.

It's actually heating as it turns out after a year of use, there's almost no need for cooling.
Therefore, we're talking about COP.

By 2 x 50% systems, you mean run one from min to max, and when reaching max starting the second one also going from min to max ?
That would mean even smaller systems, that may not even exist.

 

[LittleInch]

If this is heat then I would find the smallest machine I could find and just use resistance heating elements for the occasional days over the max heating of your unit. You should find 8000 btu/ 2 KW machines.

Our use resistance heating for the low demand and then swap it out for the best pump when it gets in the 30% range.

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

 

[MintJulep]

Don't size equipment for a 3% design condition.

Select for 85 - 90%.

Add cheap and simple supplemental capacity of the edge cases might cause a real problem.

Take some time to actually figure out your 90% load, because the range you've stated is too large.

 

[nuuvox000]

That's a tiny heating load, I would use electric-resistance heating for this. If you just have to use a heat pump, you can get some 6,000 BTU/hr heat pumps from Mitsubishi and get two of those (wall-mount).
Sorry, I'm not sure about efficiency at lower speeds, usually two-stage compressors are just a little bit more efficient which you see with SEER 19 models and such so it's definitely more efficient but I don't think you'd see much savings with this small of a load.

 

[Snickster]

I would select the equipment based on required functionality and not be concerned about efficiency at low loads. Like 1 ton max who cares? You are trying to split hairs.

 

[Jon_Doe]

THanks for all the feedback.

Prices are wild, 1kwh electric is ~ 4 times the price of 1kwh natural gas.
The thing here is i only have electricity, no natural gas pipeline.
Even with bottled propane lpg i still get half of natural electricity costs.

An 9kbtu to 12kbtu A+++ ac is $600 or up to $1000 if i want a fancy Mitsubishi or Daikin unit.
These are full inverter devices but would still run at lower power most of the time.

For $600 more i can get an excavator and drop me an 150ft long 8ft deep trench for 600ft of 3/4" PE100 HDPE.
I can connect that to my modified ac unit if outside goes below freezing.
All in the name of better COP, less expenses.

The amounts mentioned above may not seem much but they are significant foe me.
However they are not that much if i consider the flexibility since i don't know what will happen with gas / electricity prices in the future.

So, even if it might seem like splitting hairs, for me it's not.
I just have limited resources and try to make the best of them, and that means headaches.
But i prefer to have them while figuring the solution rather than after, when the implemented solution don't work as expected.

 

[nuuvox000]

I don't know, I understand your concern with electrical costs but if it were me, my next step would be looking into a solar panel, battery, inverter, better insulation, and an electric-resistance heater. Not only are heat pumps not nearly as efficient at the coldest temps, they frequently just stop working altogether. I really think you're asking for a headache.

We never, ever specify heat pumps without backup electric-resistance heating for the coldest parts of the year. A city office building did this in our state and it just wasn't heating like it was supposed to and it was very expensive to put in supplemental heat later on.

I guess we don't know what this space is for. If it's not for comfort heating, just get the cheapest heat pump you can find with a small electric-resistance heater when the heat pump stops working in cold temps. It's really hard to make up the first-cost of a more efficient heat pump in my experience. We just did this exercise with an energy modeling company and the payback for switching to heat pumps from electric-resistance heating was 15 years! By the time they would make their money back from the more efficient equipment, it would be time to buy more equipment.

 

[Jon_Doe]

Thanks for your input.

My primary heating source atm is Propane LPG.
It's half the price of electricity and twice of natural gas (not available in my area - no pipeline - maybe in 10 years - maybe).
So, if i need / want, i can stick to Propane for all needs, for the peak heating load, for DHW or for any combination.
I also have 2 x 30 evacuated tube solar collectors that i but but have to install so they'll add some energy when sun's available.
They can even be used as input to the heat pump if there's a little sun but with not enough temperature for the underfloor heating system.
The heat pump would use the Air source only when outside air is above freezing.
A single loop horizontal geothermal can be used for when outside air goes below freezing.

I'm in the feasibility study right now, that's why i have a lot of questions.
I want to know what solutions i have available so i can calculate the upfront costs and the payback - if any.
It's a matter of costs but also of redundancy / flexibility.

Not keeping all the eggs in one basket if there's not a huge upfront implementation cost is a good future strategy in my books.

 

[LittleInch]

So is this "rather well insulated envelope" your house / dwelling?

In terms of your original question - efficiency will almost certainly be lower at the lower heat rates as there are some fixed elements somewhere, but then it's a much lower number to start with.

If you actually give us all the known details then we can add some value here, but your OP wasn't really very clear.



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

 

[Jon_Doe]

Ok, so i guess my original post was unclear.

Yes, it's my residence.
It was designed and built, by me, to get the max out of the available funds (which were rather low).
It's a timber frame with 20" straw bale infill walls and 2ft of fiberglass in the attic (which is empty, non livable space).
All windows are close to passive house standards.

The condensing gas burner i have now operates on and off, even when outside was 17F.
With 17F outside, a 77lbs Propane bottle lasted 15 days (heating + DHW).
That's 450kWh / bottle which means 450 / (15*24) = 1.25kW if working permanently.
If i consider worst case to need double that, it would be 2.5kW 24/7.
For when temperature does not stay permanently below freezing but just cycles around it during day / night, same bottle lasts for 3-4 weeks.
If outside is around 57F-59F, 4-5hrs of burn every 7-10 days is performed, if there's a need for it.

The temperature entering the hydronic under floor radiant heating is 73F - 75F, and the gas burner does not stay on for more than 4-6 hrs / day.
The air temperature is ~ 70F.

So, with the above data, 12000BTu or 3.5kW at constant rate should cover even the worst of the worst.

This is actual data, not design goals.

So, my quest is to figure out how to have the best bang for my buck regarding to initial investment versus usage costs and safeguarding for market fluctuations.

A standard inverter air source heat pump goes from 6kW upwards and will most likely work poorly at very low compressor speed due to all the fixed losses (electro-mechanical) inherent in the design and manufacture.
A 12000BTU ac inverter is in the required power envelope, although, it seems that it too will work at the minimum power output for most of the time.
9000BTU units are available and i think that even 7000BTU but these are not very common and may not be as efficient.
Having 2 x 9000BTU (or 7000BTU) inverter units in parallel, running one or both, as needed, may be better than 1 x 12000BTU unit but it will be more expensive.
Using 2 external units "as is" has higher costs but less onsite manufacturing.
There is an option of using 2 compressors in tandem with a 14000BTU-18000BTU external (Fan + exchanger) unit.
This has slightly better efficiency and may cost less but it requires basically an onsite build of all components and care must be taken with the tandem refrigerant circuitry, especially the oil part.

Lots of things to consider from technical / financial pov.

 

[BrianPetersen]

Why are you trying to re-invent the wheel?

The unit has to be sized for max load, of course. If you can find a variable-speed one that is a bit more efficient at low speed and you can use it at low speed most of the time, that's great. If low speed still puts out more heat than you need, what's wrong with letting it cycle on and off?

Probably the manufacturer has already put some thought into that minimum turndown. Maybe that's the most efficient condition, below which parasitic losses overcome any further theoretical gains. Maybe there's a technical limitation. Maybe it's just not worth the bother.

Why struggle? Buy something off the shelf, use it the way it's supposed to be, enjoy your valid warranty, and sleep at night!

 

[LittleInch]

That was going to be my comment. Running something as on / off is not that bad a thing providing that the on off bit has some decent time durations associated, so you need to set it up with a decent dead band to start / stop and also some level of thermal mass, be it a water system or concrete.

Also heat pumps do work below 0C, but their CoP does go down a bit and they need to reverse cycle or add extra heat to defrost the eternal unit.

I think you need though to look closely at your power data.

Simply dividing by 24 hours is not a great move.

Your other line which says you burn LPG for 4-6 hours in that day actually gives you a better idea of what max power you need I think.

What is the heat power of your gas burner?

How much water is in your underfloor heating system? ASHP do like operating at those temperatures but along with the screed mass can probably operate for decent periods (2-3 hrs, then shut down for another 2-3 hours without an issue.

So I would start low and then see if you get really cold on occasion and then simply add some resistance heating (convector heater, fan heater etc) for the 7 to 10 days it gets super cold.

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

 

[Jon_Doe]

Thanks for your inputs.

I do want to design for max load but not neglect the minimum load.
Power cycling is not an issue, taking into account the minimum duty cycle.
On inverter driven units this is less of an issue due to soft starting and smooth gliding to the operating point of the compressor motor.
There's no startup, inrush, shock.

The only optimized, off the shelf devices in this power range are ac units.

To buffer the on/off cycle, i have thermal mass.
The floors are 6" concrete/screed (insulated).
On average, each room floor would weigh ~ 5tons of concrete.
That gives a pretty good flywheel effect.
It takes time to get up to speed but once running, it's smooth riding.
That's why i would rather run low power 24/7.
Coupled with an outside temperature sensor, i can up the water temperature on the fly to compensate for extra losses.

Power data and my 24hr division makes lots of sense, i'll explain why.
Gas burner is 24kW but can modulate down to 2kW. Less it's just on/off cycle.
It would run lower but can't.
So it runs a few hours at 2kW and then stops for the rest of the day (or days).

A heat pump/ac i can run at lower power almost 24/7.
If even lower power is needed, cycling on/off will do the trick.

The thing with heat pump and current prices, it doesn't make any sense to run them with COP << 4.
At COP 2 it makes more sense to burn gas.
But the future is unpredictable so there goes that.

As such, the air source ac/heat pump would operate only while outside temp is a few degrees above freezing.
Below that, it makes more sense to switch to an evaporator that is heated by the horizontal geothermal loop as it's EWT is a few degrees above freezing.

While using geothermal, expected COP is between 3.5 and 4.
While using air source, expected COP is > 4

 

[LittleInch]

Just don't forget about the inertia effect. Hence I would always keep your propane burner there for when you need to heat the place up if you're turned it all off for a while. That's an issue some friends of mine are finding is that they need to run the heat pump for a lot longer or continuously as it takes such a long time for the system to warm up.

So overall they are not better off or even costing them more as they don't feel that they can turn it off when they out for the day or overnight.



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

 

[Jon_Doe]

Thanks for the concern.

I've done the calculations and there's no point in having heating turned off while away mainly due to inertia but also the efficiency of the system.
Depending on the exact system and circumstance, it may be cheaper to run permanently at low power than having high power "bursts".
Besides, in my case, there's always at least somebody present.

That's why i designed and built the system this way: maximum inertia, almost permanent heating at low power, no ups and downs, no waiting for the system to respond. Mostly just a slight change in internal temperature, 1 degree tops.

And i don't intend on getting rid of the propane burner just not use it when other sources are cheaper.

Flexibility.

 

[fg37eeehuI]

Are there not problems with oil return from the system when running refrigeration systems at low capacity for long periods?

Your talking about having the heat pump long term, have you thought about the future availablity of the refrigerant it uses?

 

[BrianPetersen]

Hence me questioning why trying to re-invent what's commercially available. It would not be surprising at all for such a device to have a minimum operating speed below which some sort of issue comes up ... and some other operating condition in which efficiency hits an optimum and going slower is counterproductive. If the commercially available unit wants to cycle on and off ... let it.

 

[HVAC-Novice]

You have to defer to manufacturer and install according to their instructions to allow oil return. A brand name manufacturer will have it figured out for you - don't do your own design of HVAC equipment.