Excess solar power to run compressor
 

Hi All,

Anyone know of a method of controlling an AC compressor to utilise excess solar power? I essentially want to control the compressor using a VFD or other form of speed controller but im not sure what types of compressors are available that can be controlled in this fashion.
What I would ideally want to do is purchase an external unit from a heat pump / split air conditioner (common in NZ) and convert it. Many of these units are "inverters" and control the compressor speed and output based on the temperature of a room. Does anyone know if the singling to the controls of such a unit can be intercepted and controlled say by a 0-10v or frequency input.
I would also like to replace the inside unit of such a setup with a refrigerant / water exchanger to use output to heat our under floor heating system.
Im new to refrigeration so am not sure on the specifics of this kind of system but I do have a grasp of PLC and Arduino control from previous projects.

I would have thought that systems on the market should have this function but im unable to find any examples of this. It would seem as tho a heat pump domestic Hot Water heater would also be a good application for this type of control.

Thanks for reading. Any input is greatly appreciated.

MikeC

Lennox makes some AC/heat pumps that are specifically designed to connect to solar. I think it's a great idea but not so useful for those of us who already have an AC unit. It's nice because you don't have to go through the grid connection bureaucracy.

https://www.lennox.com/products/heat...ng/solar-ready

I guess I'm confused about wanting to run the AC while heating the floor.

Hi Andy,
I dont want to run an AC on cooling while heating the floor. Just want to use a heatpump powered by excess solar. Basically I want to control the compressor to match the excess solar power available. Cheers.

Keywords DC brushless inverter compressor

You have to elaborate on the existing solar pv system to get a worthwhile answer. Not knowing what you have or how much extra capacity you have available makes it hard to recommend anything specific. Grid tied system? Battery bank? Solar thermal? None of the above? Hmmm....

Thanks for the reply guys.

Currently we are planning the build but the solar system will be very similar to what we currently have being a 3kw grid tied SMA system. I am using EmonCMS and openenergymonitor CT hardware to monitor power produced and our grid import / export. Im currently using a diverter that measures the grid connection and runs an immersion heater to heat our water in a tank. (some kind of leading or trailing phase power control to "throttle" the resistive load)

Im pretty happy modifying / hacking things to a point. Ideally I could find a common compressor that is 3 phase an can be run with a 1 phase to 3 phase VFD or a "mini split" outdoor heat pump that I could add a heat exchanger to heat water and use the existing inbuilt inverter in the unit and talk to it via the existing controls that would have been attached to the indoor unit?

I dont understand why this is a new problem? Surely in hot environments people have solar installed and want to match the output of AC to the power produced?

Thanks for taking your time and replying to my thread.

I imagine there are very few people who want to match their AC output to solar output. 99.999% of the market wants their AC output to match the thermal load.

I could see it in a utility scale application for load shedding/demand control, but there they would simply switch batches of units on/off rather than scale up/down the individual unit outputs.

take a look at ice energy's ICE BEAR units in California. the utility can turn them on/off of compressor operation to shift demand to suit the grid. the end user still gets thermostat controlled AC.

now if you were doing energy storage, i could see using a a VFD 0-10V input based on solar output to avoid needing large batteries. but as for on-demand operation, instantaneous solar conditions are not typically the demand metric.

someone on here has a thread for a VFD driven 3 phase 3 or 5 ton water to water unit. i believe it ramps down to match the load/motor ratings. you could take that logic and add on a demand variable, secondary to the needs of the mechanical/electrical requirements.

You don't need a super complicated rig to do what you desire. Sure you can use a complicated speed control driving an industrial compressor, but why? It doesn't do more, it just costs more. Just modify an existing outdoor unit. Having a grid tie just makes it easier.

The main concern or three in your unique system would be:

Domestic hot water or space heater duty
Amount of excess electrical power
Direct DC operation from pv panels or tied to ac mains power / inverter

Thanks Jeff,

I would love to find an existing outdoor unit to modify however im still unsure on how I can control the compressor speed and therefore the power consumption. Are the units that specify they are an "inverter" DC or three phase to achieve the speed control? What kind of signalling or control does the outdoor unit receive to control the speed?

Yes, the units that have variable capacity are "premuim" machines in the products offered, and the manufacturers of these units are very proud of them. The boasting is inflated and claimed ratings are highlighted by every sales pitch. These things have been around for over a decade now, so the various manufacturers have developed lines of machines that have fancy names to designate the added functionality. There are also a lot of these types of systems on the second hand market and in the salvage industry available for a lot less than retail price.

Not knowing your operating needs, I will be very general in describing what is available. Also, due to your backwards system goal (matching the source instead of the load), you're probably not going to find a manufactured unit that will suit you. The source matching interface is going to be non-existent. It will need to be built by you or someone who serves you.

Ok, so choosing a suitable outdoor unit, you have 2 major criteria to consider: capacity and method of modulation. There are a bazillion different units on the market and they are all classified to serve specific purposes. In the variable capacity domain, there are 4 main types of systems. Take a look at this table drawn up by Emerson:
https://ecorenovator.org/forum/attac...4&d=1533126451
Using this chart should help you decide which method of modulation to employ to meet your specific purposes.

As you can see, the capacity range begins around 2 or 3 tons. This is because all of the makers of HVAC equipment consider anything smaller a "novelty" item. Minisplit systems are a prime example of this. They don't sell a huge number of these systems compared to the larger capacity machines, and even more so with variable capacity setups.

By capacity, they classify anything 2 to 5 ton as residential and anything over 5 ton as commercial. These 2 classes account for 90 percent or better of the corporate revenue stream. As a result, the small, modulating capacity units are pretty much sold as "what you see is what you get" packages with almost non-existent support except for installation instructions. Even if you buy new, don't expect any help from the store or manufacturers, especially if you aren't an authorized dealer. Even they don't get much help.

On the opposite side of your question, the actual variable speed compressor in nearly all of the "novelty" outdoor unit is going to operate like a super size computer fan. Permanent magnet rotor and multiple phase stator. Depending on how the manufacturer designed the compressor, it may or may not have a 3 phase rotor and stator. This is very important, because a generic 3 phase motor drive might not work. More phases equates to more precise control and maybe higher efficiency, and these smallish capacity variable speed units are uncommon by nature. As such, the designs are all over the place.

Above the 2 ton rating, most of the units are built with 3 phase drives, and as capacity increases the permanent magnet design becomes much less implemented. At a certain point the novelty goes away.

The smaller DC units are pretty much all designed for use with a battery bank in mind, so the drives use multiples of 12 volts. Controls are built for mobile ac or refrigerating duties. The PV residential twist exists, but in general all of these smallish DC units are super uncommon (residential heat pump or mobile commercial refrigeration plus DC power equals single digits parts per million products offered). Not converting source power from DC to AC back to DC has a definite efficiency advantage, but the 120/220 VAC controls are way more common in general.

As to the nature of the units you're looking into, read this article:
https://coolautomation.com/wiki/vrv-or-vrf/
There are more topics in the blog section that will give you lots of quick tidbits to wrap your head around.

Recently the manufacturers have been making 2 stage compressors for the top efficiency outdoor units with variable capacity. They're designed for Sub-Zero temps outside and btu maintenance inside and/or heavenly performance ratings during less demanding conditions. Nearly all of the units are able to run or bypass the second stage of compression on the fly. They can be reduced to 10 percent of the rated output and increased to around 150 percent, so they're very versatile. Naturally these units are the opposite of cheap.

I'm not sure I can agree with the full load efficiency listed for variable speed compressors.

..sure, a 30+ SEER 9k or 12k unit might be at 18 EER(or a little lower depending on this unit) under high load, but this really isn't bad at all because you are still kicking the crud out of 13 SEER 11 EER units.

In some cases the lower efficiency sub-20 SEER variable speed units, especially larger units do seem to be as bad at high load at cheaper 13 SEER units but that's only at peak loads which isn't an all the time scenario and isn't worse than a common old fashioned single stage split unit.

Moral of the story but a high EER rated inverter mini-split if you are planning to run at high load and that's not an issue and you get the best efficiency in any condition. It seems that the outdoor condensor coils on the smallest units often have the same coil on them between the 9k and 12k sizes and sometimes even have the same coil as up to an 18k size, in which case getting the 18k size might be a compromise but the efficiency becomes clear looking at the EER number. ..of course aiming for a high SEER will give you the best information for cooler conditions and lower load but I wouldn't oversize a unit based on the chart above if buying a good top efficiency unit.

I'm still not sure how you'd arrange getting the compressor to ramp load with the excess solar generation though, that's the trickiest part.

Jeff, i've never seen anything but VS (inverter) or 2 stage/step in a <5 ton standard split system. the only application i'm readily familiar with digital scrolls in is CRACs. I don't see those as a good fit for purely running on solar - they modulate the scroll on/off full load, so if it's not actually grid-tied, it might be a problem. otherwise average draw should work for % modulation.

I'm just giving the op a survey of what's out there as far as variable capacity pots. I agree with you guys, most of the sub-heavenly priced stuff in the mini and whole home residential range is going to be brushless permanent magnet stuff.

I'm still kinda intrigued about the whole supply matching strategy here. As of now, the proposal seems like a leveraged heat dump to me. The thing that baffles me is how much extra heat can be dumped into the floor? At a certain point, enough should be enough. Then what?

what MIGHT work and be simple (haha):
get one of those Bosch BOVA 3T or 5T ODUs, then put a coax/brazed plate exchanger indoors. they modulate from 25-110% based on load calculated by the ODU. all you'd have to do is control the water flow rate through the indoor hx to get the ODU to modulate up/down.

i've seen the 3T (0.75-3.6 ton range) available online for under $2k USD.

Hi All, Been reading through this over the years and figured I should join. I have seen recently a few products offered which may be of interest. There is a solar powered minisplit made by hot spot energy and there is a solar powered hydronic water to water heat pump system available as well. Scott Hunt has several videos of these systems in operation from his practical preppers youtube channel and of course on his website he sells the systems.

Really help out
 

It must be so helpful to follow up. These techniques would have worked with the heat pump.