48v battery solar air conditioners vs. 310v hybrid solar air conditioners

Battery vs. Hybrid Solar Air Conditioners

How are battery solar air conditioners and hybrid solar air conditioners similar and how are they different?

The last few years have allowed something splendid to be possible.  Air conditioners, one of a home’s biggest power eaters, can now be run directly using solar!  This splendor results from a technological culmination leveraging much more efficient and lower-cost solar panels, and much more efficient and lower-cost air conditioners, both brought by advanced electronics made easy for us.  And, there are two main types of home (or garage, or dog house?) units we’ll discuss here:  the battery solar unit and the hybrid solar unit. 

What they have in common

Minisplit heat pumps…Both are mini splits, meaning that they have an inside unit with a heat-exchanging coil, an outside unit with a heat-exchanging coil, and refrigerant and communication lines running between the inside unit and outside unit.  And, both, it’s fair to say, are mostly used for their air conditioning, but most all also have a reversing valve to allow them to act as heaters.

BTUs…Both are generally availably in mini split-type BTU output, so from around 9,000 BTU to 24,000 BTU, or from around .75 tons to 3 tons.  So, as of now, these units aren’t going to run a whole house, but they work great for keeping your house relatively cool for pets and plants when you’re away, or being used for garages or barns, or for spot cooling in upstairs bedrooms where you spend a lot of time, thus allowing you to raise the setpoint temperature on your main unit.

DC motors…The compressor, condenser fan motor, and inside fan motor run on DC power, thus allowing them to run on solar’s inherent DC power.  So, there’s no need to invert the solar power to alternating current.  These babies are both built with solar in mind, so no power is lost in translation.

Efficiency/power requirements…Both use very little power compared to a traditional air conditioner or a traditional mini split heat pump.  How?  They both run on something called VRF, or variable refrigerant flow.  Whereas air conditioners in the past were either on or off, VRF units ramp up to their max speed slowly, and run at a speed hovering around the setpoint you’ve designated with your thermostat.  This does three things:  i) it avoids the surge, or inrush power normally required to start the compressor on non VRF units, essential for the units to easily run using solar ii) it makes you more comfortable, since the unit can maintain a tighter temperature dead band, since it’s not constantly passing your setpoint temperature on the cold side and then starting up again when the room feels too warm ii) it make it so the units can run more slowly if there’s not enough solar available.  It’s a win/win/win.

Ability to run ‘gridless’ when it’s sunny…Neither requires a grid connection to run when the solar irradiance is strong.  But, they achieve this differently.  More on that momentarily.  As such, neither needs to be net metered, so both are much simpler than selling extra power back to the grid for an eventual credit (or these days, an eventual partial credit).  And, since they are not net metered, they can run when there’s no grid power, such as after a bad storm.

What’s different?

Input power source for solar…battery vs. directly off of solar.  Battery solar air conditioners (as you might surmise) need batteries to run, in this case 48v of batteries.  And, so, in addition to the batteries, a voltage controller is needed to make sure that the batteries are not over or under charged.  Meanwhile, the hybrid units allow their users to plug in solar panels (usually around 80v to 380v) directly into the outside unit using standard solar MC4 connectors.

Control system to maintain running voltage…48v vs. 310v DC.  You may have guessed that the battery solar air conditioner that needs 48v of batteries as input is the one that runs using the 48v.  So, it’s simple.  The voltage control and power management are both done with the MPPT voltage controller outside the unit near the batteries.  Think of the air conditioner (and, well, its motors) as just a node on that 48v network.  As the system needs more cooling, the motors speed up and more current flows from the batteries to run the unit.  Solar then fills the batteries.  The advantage is the simplicity.  The disadvantage is that the 48v motors that run this unit are rarer, and thus more expensive, than the DC motors in the hybrid. The hybrid solar air conditioner, meanwhile, takes the input voltage of the solar panels and regulates it inside the outside unit to be 310v, still DC, for the hybrid unit’s power bus.  80v to 310v solar input usually means 2 or more solar panels.  Obviously, users of this types of unit work to optimize the system by getting enough solar panels to run the unit off of solar, but not much more, since these panels only can be used to run this one unit.  Luckily, 3 or 4 panels totaling around 1,500 watts of solar is plenty.  And, all (let me now if you’ve seen an exception) standard inverter compressors in mini splits run using 310v, so these units have components that are off the shelf.  They just need enhanced electronics to have the ability to run directly off of solar

Power attainment when there’s no sun…batteries vs. the grid.  If you need to run an air conditioner at nighttime, and there’s no grid (or inverted solar battery power to create ‘grid’ power) available, you need to run the 48v battery solar air conditioner.  It’s just a matter of adding more batteries and more solar panels.  After all, even in the summer, the best of solar locations only have around 8 hours of peak sun to run in the summer. So, without backup power, you’re good to run around 1/3 of the day under the best of conditions (perfect sun, perfect south-facing solar panels tilted for summer). So, triple up on panels and batteries, and you’re good to take on the hot hot heat of mid-summer.  Off-grid air conditioning at all costs is a necessity for applications like remote telecom shacks.  Hybrid units, meanwhile, have a cheap work-around if the user has grid access:  they plug in to any standard 110v outlet (or, in some cases, 200v breaker, in if the unit requires 220v alternating current).  The advantage of this is that there’s substantial up-front cost savings.  And, usually, the power attained from the grid will be at a cheaper hour for those who are enduring a variable rate plan with their utility.  That is, solar, overall and in most markets, is still beaming brightest concurrently as grid power rates are the most expensive.  Hybrid air conditioning users thus escape these costly periods and pull power mostly when it’s less expensive.

Fill-in power when there’s partial sun…batteries vs. the grid.  Similarly, if it’s partly cloudy, then the battery bank continues to fill in the lack of solar for the battery unit, and the grid fills to help out the hybrid one.  Solar is always optimized for both, in effect.  The 48v battery unit always runs off of the batteries, whether it’s sunny, cloudy, or nighttime.  But if it’s sunny, you can think of that topping off solar power just passing through the batteries to run the unit.  If it’s a bit cloudy, then the batteries are becoming depleted even during the day.  Not a problem, most days, in Palm Springs in the summer, but Wilmington, NC?  Lots of cloudy summer air conditioning days.

Ability to always run…battery units can’t, hybrid units can.  What happens if it’s hot and rainy for 3 days in a row?  The battery unit is vulnerable to being fully depleted.  The hybrid unit keeps going.  It’s not running for free, but the baseline efficiency of most of these units is around 20+ SEER (Seasonal Energy Efficiency Ratio) on grid power, which clocks in around 45% more efficient than most air conditioners running today.  So, there’s still savings to be had.

The bottom line

For most users, including those who live off-grid, the newer hybrid solar air conditioners are likely the way to go, since they…

  1. Are less expensive to get running (since the units are more affordable, and since no batteries or MPPT are needed)
  2. Are easier to set up (since no batteries or MPPT needed).
  3. Are less expensive to maintain (since batteries go bad faster than air conditioners, and since replacement parts are likely less expensive)
  4. Are easier to maintain (since, more people are knowledgeable on how to fix the kissing cousin to these hybrid solar units…the high efficient units which have most of the same components, electronics, and wiring)
  5. Are more likely to stay running, as long as grid backup power is available, in times when it’s less sunny.