On Solar Air Conditioning
Why’s solar-powered air conditioning all the rage now?
Solar-powered air conditioning has been theoretically possible for years. But, in the past, it was neither cost justified nor simple. Solar panels were expensive, and air conditioners were inefficient by today’s standards, meaning even more panels would be required. Plus, attaching the panels to an air conditioner required a power conversion which not only further lessened the efficiency of such a configuration, but it also was expensive. In short, it was too big of a hurdle even for an ardent tree hugger.
So, what’s changed? Just about everything. Panels are around ¼ of the cost they were 10 years ago, approximately following a price cascading called the ‘Swanson Effect,’ which states that solar panel costs decrease around 20% for every doubling of solar panels installed (see Figure 1, Courtesy of OurWorldInData.org). Without even taking greenhouse gas savings or environmental issues into account, they now produce energy at a rate less than burning coal or natural gasi. And, some small air conditioners (especially minisplits) now have electronic controls allowing variance of the motor speed and refrigerant flow, something previously only available on very large commercial air conditioning system. Some air conditioners run on direct current, which is the same power the solar panels produce, so no power conversion is necessary. Finally, some units now allow solar panels to be plugged in directly to the outside unit of the air conditioner, and a printed circuit board inside the unit creates stable power for the air conditioner, potentially with the aid of grid alternating current as a backup to the solar. In short, technology to the rescue.
On the demand side, the global market for air conditioning is projected to grow at a 9.9% compound annual growth rate, dwarfing the $102b in revenue the industry amassed in 2018ii. This increase needs to be tempered with energy-efficient and environmentally friendly solutions. And stateside, many utility users have seen electric bills increase, sometimes inexplicably, given that utilities have also been adopting solar panels to their arsenal to bring down their own costs. Solar units can be installed anywhere, and run for free, or for very little cost, provided it’s sunny, to help offset these utility charges. They can be used in areas like garages, attics, or patios which traditionally would have been difficult or expensive. Where would you want to cool if doing so were free, leveraging the same solar rays that were making it warm? Welcome to the promise of solar-powered air conditioning.
How do the benefits and implementation of solar air conditioners compare to whole-house solar?
For many, whole-house solar is the way to go. The same decrease in the cost of solar panels and implementation is driving over 6x the annual installed watts now vs. 2012. Whole-house solar, in effect, makes your home a node on the grid. If your panels are making more power than you currently need, that energy is fed up to the grid, and you’re given whole or partial credit with your utility for supplying this to others on the grid who will use it. If it's nighttime or your panels are not creating enough power (due to weather, large temporary demand (maybe caused by air conditioning?), you pull that extra required energy down from the grid. To do this, you need an inverter that synchronizes your system to the voltage and frequency of the electricity flowing through the grid, and a net meter supplied by your utility to keep the ‘score’ indicating whether you supplied more energy to your utility or they supplied more to you. Such a system may help reduce your grid consumption to almost nil, and, taking into account their cost amortized over time, reduce what you spend on electricity by 30% or more. The upside is such systems cover the reduction all of your electrical consumption in one fell swoop with these potentially massive savings for you and for the environment. The downsides are i) they’re more of a major commitment financially and aesthetically ii) they require some faith and trust that utility net metering rules for energy credits will stay intact and stable, and this trust hasn't always been warrantediii) this type of system doesn’t work, for safety reasons, when the grid is down, so there’s no power that you can utilize in the event of an outage.
Solar air conditioners or heat pumps (which supply not only cool air in the summer but warm air in the winter) have more of a targeted approach, and go after the ‘low-hanging fruit’ of energy consumption, which is the air conditioning (and, to a lesser extent) the heating energy consumed. For many users, this one area can be 70%+ of their summer utility bill. These units usually are only 12,000 to 24,000 BTU (1 to 2 ton), so they don’t supplant your main air conditioners or heat pumps. But what they do, they do well. They can keep running all day long for free, or nearly free, depending on the weather, continuously extracting heat. They can be placed in an area that is usually too hot (like an attic or upstairs office or a new addition without ductwork), even when the main unit(s) for the house are blasting away. This is the simplest and most affordable way to add the benefits of solar savings to that spot. Also, some units have batteries and some don’t (more on that momentarily), but the units always work when it’s sunny, regardless if the grid power is down. You can imagine the benefits of having a couple of these units on hand when there’s life-threatening heat like there was in Hollywood FL after Hurricane Irmaiv.
When and where does solar-powered air conditioning work? Who can benefit?
Solar energy is really light energy that arrives to Mother Earth via photons from the sun. Some areas are blessed with more solar energy than others. Figure 2 from the World Bank shows solar power potential based on past sampling.
The Southwest U.S. is more of a natural fit (or a no brainer?) for solar than the Pacific Northwest, for instance. As solar panels become more efficient, more powerful and less expensive, the ability to justify solar air conditioning becomes more viable in the yellow areas, and there’s even hope for the green areas, though, because although these areas have less sunlight, they also have less radiant heating, and both solar panels and air conditioning compressors run happier (more efficiently) in these areas. And, some of these areas have high utility rates, too, helping with the justification.
Yes, most solar-powered air conditioners are actually heat pumps, meaning that they have a reversing valve to allow them to function such that, in the winter, the inside unit produces heat. In the winter, a good percentage of the solar photons never reach solar panels, since the sun’s trajectory is lower on the horizon, and so many of the photons are diffused, scattered, or absorbed since they need to travel through more of the atmosphere. Couple that with shorter days in the winter, depending on the heating requirements of your conditioned space (certainly great in more northern areas), you may need to supplement the solar with a grid backup connection or with a battery pack (more on these options momentarily). But that’s okay; remember, these units use the newest technology such that they still function at a high SEER value, so they still cost less to run than traditional heat pumps in the winter, and potentially less than other heating options. A great segue for discussing the two options for solar-powered air conditioning…
What are the two types of solar-powered air conditioning? How are they similar? How are they different?
There are two types of dedicated solar-powered air conditioners (where ‘dedicated’ = a photovoltaic solar array solely focused on running the air conditioner): Off-grid 48v and hybrid 310v. Both have advantages and disadvantages.
In off-grid units, solar panels feed a bank of batteries via an external charge controller which optimizes the solar conversion and ensures that the batteries aren’t overcharged. If it’s sunny, the feed into the batteries from solar will meet or exceed the flow out of the batteries to run the air conditioner. Want the system to run even in cloudy or rainy conditions and/or at night? With deep pockets, it’s no problem to do so, statistically speaking. Just add more battery power and more solar panels. The extra panels will fill the extra batteries to create a reserve for these low-solar times. The challenge hits when it's cloudy or rainy for some days in a row. However, these days may also be cooler, so potentially the draw down of the batteries for cooling may be lower. In summary, the advantage of these units is they are totally independent of the grid. You can literally set them up in the middle of nowhere, and they’ll keep plugging away. For that reason, they’re used heavily in off-grid applications like remote telecom relay stations or remote cabins. And, since they use a 48v DC compressor and DC fans, which is the same voltage as the battery array, they’re simple, since there’s no voltage transformation or rectification of alternating current or blending/combining of the direct current with the alternating current to optimize. The disadvantage is there’s a price for this independence: the cost of these units is higher, since their voltage bus is optimized to accommodate the batteries (48v), and the 48v DC fan and compressor motors are a bit rarer and a bit more expensive. Additionally, the extra solar panels and batteries can easily double the price of the system for 24-hour design vs. daytime design. Finally, the battery setup and maintenance is a component not necessary for hybrid systems.
The newest hybrid solar units are off-grid capable…but only during the day. At nighttime or when it’s super cloudy or rainy, they simple can’t work unless they’re plugged in to the grid. But when plugged in, their advanced electronics prioritize solar first, and blend in any required grid AC current, and step up this blended voltage to 310v DC to run the compressor and fan motors. Unlike off-grid units, all of this magic happens within the printed circuit boards of the unit. So, the solar panels are connected directly to the outside unit of the air conditioner without the requirement of an outside control box. And, these units are based on the latest-technology-but-becoming-standard high-efficiency non-solar minisplit inverter compressors and fan motors, so the pricing is favorable for them, even with the additional high-tech electronics they require. These units offer an affordable way, with the help of the grid, to keep going, regardless of conditions.
In common is that both types of units use their efficient variable-speed motors which are inherently direct current (same as the solar panels) to allow them to run on less energy, which is essential to make solar-powered air conditioning financially attractive.
Solar air conditioning for commercial users
Commercial energy rates (kWh) tend to be around half the rate of residential (depending on your jurisdiction), so where’s the savings? Well, commercial utilities also bill based on facility and demand charges derived from power (kW) requirements. So, if, say you can shave off 1kW of power using a solar-powered air conditioner from your peak 15-minute period of the month, or, in some jurisdictions, your peak 15-minute period of the whole year, the savings may be substantial and cumulative on top of your energy dollar savings.
Niche uses for commercial solar powered air conditioners include dehumidification of grow houses or locations where there have previously only been swam coolers, supplemental cooling to high-BTU areas like server room, or larger solar-powered variable refrigerant flow units (which operate similar to other hybrid units) to save on cooling the entire facility based on both the kWh and kW savings outlined above.