How Does Solar A/C Work?

A few years ago, John and I and another friend of mine looked into solar a/c .  It wasn’t ready for primetime. I experimented with some schemes, but just went with brute force to run my old-timey Rheem the old fashion way. Here’s how the new-fangled contraptions John is selling work.  BTW, check out his posts at johnssolarblog.com.

First we’ll look at how the old-timey units work.  Outside, there’s a big box with a motor and a compressor.  When your thermostat clicks, it sends a 24 volt  alternating current signal to a relay in the outside box.  That connects 240vac to the motor and the compressor spins.   So does your power meter, or at least the mechanical ones.  Cha ching!   This is an all or nothing thing.  Thousands of watts are flowing for the next 10 minutes until the thermostat clicks off.

The compressor compresses the refrigerant gas into a liquid.  Compressing the gas makes it hot and coils around the sides of the box get rid of the heat.  The now-warmish liquid goes to a closet in the house where the air handler lives.  There the liquid expands back into a gas in another radiator called an A-coil because of its shape.  The evaporating gas gets nearly freezing cold and a blower blows the cold off the A-coil through the ductwork to the various rooms.  Got that?

Evolution.  Some years back, electronic speed controls got added with an inverter circuit.  This is not exactly like the inverter that runs your house.  It is more of a variable frequency device to speed up or slow down a motor.  There are some variants, but basically we speed up or slow down the compressor.  These inverter a/c units run pretty much ALL the time at lower rate instead of full speed ahead for a few minutes and then stopping for a while.  They are very efficient and very quiet.

When I was helping my friend Courtney refurb his Georgia farmhouse, we lived in the carriage house.  Each bedroom had a window unit and the one in my room would put a freight train to shame at making noise.  He bought a dual head mini split system and put it in himself one weekend.  Mini splits work kind of like a regular split system, except they have an air handler for each room instead of the closet with an air handler for the whole house.  Anyway, this was a variable rate inverter system and it was glorious.  You had to listen hard to even know it was running inside and might walk by the outside unit without noticing it was running.  The power bill went down.  Life was good.  But the unit was not solar powered.

Ok, we have established two general types of a/c.  There’s the all or none conventional systems and there are the variable speed inverter systems.  Let’s look at how the inverter system works a little closer.

The compressor cannot use the 240 vac directly because the 60 Hz frequency of the power line will only run a motor at one speed.  The 240 vac goes to a rectifier to make a DC power supply.  Some use a high voltage of several hundred volts and some use a lower voltage, say, about what a solar panel puts out.  The inverter takes the DC power and chops it up at whatever rate is needed for the a/c to keep the temperature right.  If you come in from vacation, the unit was off and the house is hot, it will come up to full speed ahead, just like my antique unit, and then slow down when it hits temp.  If the thermostat detects it is getting cooler, it slows down.  Warmer, faster.

Courtney and I looked into modifying a standard unit a few years back, but decided that R&D mistakes could get expensive and void the warranty.  Never mind product liability issues if we sold one and it caused trouble.  We both went the brute force route at our homes.  John was demoralized because he really wanted to sell solar a/c.  Oh there were some commercial units, but who would pay 5 grand for a window unit the size of a Kia Seoul?

Still, we were on the right track and folks like Gree, big players in the Asian and European markets fine tuned their DC and inverter sections for solar.  Here’s how it works.  The AC is converted to DC, same as always, but only as needed to keep up.  If a solar power source is connected, there is little or no draw from the power company.  Wait a minute, though!  I’ve seen units the capacity of my antique being run with 1000 watts or less of solar.  MY unit draws 3 kw!  Ah, now we’ve hit upon the secret.

MY unit has to have lots of kilowatts of solar, battery and inverter because MY unit starts hard and runs full blast when it is running.  The solar unit, once the set temperature is reached, slows down to the point where all the power is coming from that/those panel(s)!  There is a good chance, unless it is very hot. that the solar is all you need during the day.  Of course, the power company runs it at night and cloudy days.  Can you add a few more panels and a battery to go 24/7?  Sorry, I can’t answer that question unless I take a look at specific units.  Can you add more panels and have it solar powered more of the time?  Looks that way, but get a copy of the manual.  I just talked to John and he says he will send me all the literature.

So what’s the economic outlook here?  Like most things where you save money through efficiency, there is a higher price up front.  The savings come over time.  Should you change out your 5 year old a/c for a new solar model?  A fairly new conventional system may already be pretty efficient and you should have a lot of life left in it.  A solar energy tax credit might help with the numbers, though.  If you have an antique like mine and it is powered by The Power Company, it is probably a no brainer.  The economics over the brute force method are good.  Smaller inverter, smaller battery, fewer panels. 

There’s a lot going for this cool update.  Check with John or Roberto for available models and pricing.    Looks like 4 ton units will be arriving soon.–Neal