How the hot sun can make your house cool
Solar powered air conditioning sounds simple – put a solar electric panel on your roof, capture the sun’s energy, convert it to electricity, and use that electricity to run your existing air conditioning system.
Not only will a solar powered air conditioner provide free air conditioning for your house, it will also (if the panels are on your roof) keep your house cooler, by shading the roof shingles and keeping your attic from overheating!
The catch is in how much energy can be captured from a set of solar panels, versus how much electricity a central air conditioner needs. It turns out that it takes a lot of solar photovoltaic panels, and a ton of cash, to buy enough solar generating capacity to power a typically efficient air conditioner capable of cooling a typically insulated home.
There are companies out there that market themselves as providing solar powered air conditioning, when in fact all they really do is attach a huge whack of solar electric panels to your house in order to generate enough power for your existing air conditioner. That’s a bit like putting a sail on the Queen Mary and saying you’re in the sailboat-building business.
Photovoltaic solar powered air conditioning
The way around the cost dilemma of cooling your house with solar electricity costs is to (A) do home energy efficiency upgrades on your home in order to reduce the amount of heat leaking into your house in hot weather, and (B) look for the most energy efficient air conditioners around, and buy as energy efficient an air conditioning unit as possible, to cut the electricity consumption of the unit. In other words, spend as much as possible to reduce electricity demand, so you can spend as little as possible on the solar electric system.
Let’s look at a hypothetical example, to illustrate how a typical solar powered air conditioning system might be made affordable. Suppose you lived in Phoenix, Arizona – a relatively hot place, and therefore expensive to cool, but thankfully with plenty of sun to power your solar electric system. Let’s assume your modestly sized, modestly insulated house is sized to require 4 tons of AC cooling capacity. Assume that with $6,000 worth of investment, the house could be reduced to using a 2.5 ton AC system. Assume a price of $2,000 for an SEER-16, 4-ton AC system, and $1,500 for an SEER-16, 2.5 ton system. The total costs for the original home and the upgraded home, for both on-grid and solar powered air conditioning choices, work out as follows:
|Savings for 2.5 ton
|Plus upgrades for 2-ton system
|Grid – lifetime costs @ $0.10/kwh
|Solar – lifetime costs @ $0.38/kwh
As the table shows, spending $6,000 on energy efficiency upgrades in order to use a smaller system is not cost-effective for an assumed grid-connected rate of $0.10 per kwh. The total lifetime cost is roughly $3,400 more for the upgraded house. But for a solar powered air conditioning system, doing the home energy efficiency upgrades will save roughly $2,600.
(The $0.38/kwh for solar was the price in effect in 2010 when I wrote the first draft of this article – the price has since dropped substantially, while the price of electricity has risen considerbably. So the benefit of doing the efficiency improvement first has improved for the grid connected case (bringing it closer to financially viable), while the benefit of doing the efficiency upgrade first is less important if you are planning to install solar powered air conditioning.)
But there’s a problem with this calculation: look at the cost for the 4-ton system, grid connected, compared to the 2.5 ton solar powered air conditioning system. The solar powered air conditioning system is still much too expensive – $21,035 versus $7,669. So how do you make the solar solution affordable?
The answer is to go far beyond SEER-16 for the air conditioning system. For example, the Millenia 1.5-ton direct-current air conditioner from SolCool had an EER of 20. Although there is no simple and exact formula to compute EER to SEER, a good approximation is that this unit has an SEER of 24. (See Air conditioner ratings for more on this conversion.) If we budget a total of $7,200 for the air conditioner (based on a rough estimate for US installation received from the manufacturer, SolCool), and another $3,000 for further energy efficiency upgrades so that a 1.5-ton solar powered air conditioning system would be adequate, we have the following cost comparison:
|Savings for 1.5 ton over 4-ton
|Plus upgrades for 1.5-ton system
|Grid – lifetime costs @ $0.10/kwh
|Solar – lifetime costs @ $0.38/kwh
Note: The SolCool product appears to no longer be in production; their website now redirects to the Securusair website, which offers a DC system with an SEER of only 20, which is 20% lower than the SolCool product. However, all is not lost – you can find a number of energy efficient air conditioners in the ENERGY STAR air conditioner product finder, with SEER ratings of 25 or higher.
This scenario shows that the most cost-effective solar powered air conditioning solution combines maximizing your home’s ability to keep hot air out (through energy efficiency upgrades) and maximizing the efficiency (EER or SEER rating) of the air conditioning system you buy.
So do your research on energy efficient air conditioners and find the most efficient unit around, if you’re determined to use a conventional air conditioner and solar electric panels. But there are probably better solar powered air conditioning solutions available – at least three different options that should be cheaper, and in some cases more effective, than running a conventional air conditioner using solar cells.
Solar powered evaporative coolers
An evaporative cooler, as described in How air conditioning works, uses the evaporation of water to achieve a cooling effect. An evaporative cooler is only suitable in a very dry climate, but uses only about 1/4 the electricity of a conventional refrigerant-based air conditioner to achieve the same level of cooling. Therefore, a combined evaporative cooler and photovoltaic electrical generation system could be a much more affordable solar powered air conditioning system than solar cells connected to a refrigerant-based air conditioner, because the biggest cost in a solar electric air conditioning system is the solar array, and you’d need only a quarter as much electric generation capacity in this type of system as in a conventional refrigerant-based system.
How about a simple attic vent?
While solar powered air conditioning sounds like a great idea, let’s face it – it is often too expensive for most of us, or at the least, not the best way to maximize energy savings from a given investment.
One of the most common reasons older homes overheat in the summer is that their attics are not properly ventilated. As a result, the attic air spaces can get as hot as 160F during the day, and all that energy slowly leaks through your attic ceiling insulation as well as back out through the roof, so that your upstairs rooms (often including the bedrooms) stay hot even at night.
Anything you can do to ventilate the unfinished attic space will help reduce the cooling load on your house in summer, and a solar powered attic or gable vent fan can do just that, creating a simple form of solar powered air conditioning.
Install one of these fans on your roof or in the wall of one of your attic gables, mount the solar panel in an area facing the path of the sun (angled slightly towards the west, as the afternoon is the hottest part of the day and you’ll want the most ventilation then), and the fan will help keep your attic cool.
Make sure your attic has air intake vents at the bottom (e.g. under the eaves) as well as near the top, so that the natural upward flow of warm air helps draw cooler air in from below. These vents should improve the airflow considerably over what you would get with natural flow, but if there are no vents lower down, they won’t help much.
Remember, a properly ventilated roof is not only better in terms of keeping your house cool in summer, but will do a better job of preserving the attic rafters and the quality of attic ceiling insulation.
Other types of solar powered air conditioning
There are other technologies available to run solar powered air conditioning. Some technologies, such as passive solar cooling are as old as the Roman Empire, while others, such as absorption cooled solar powered air conditioning, are more recent. Let’s discuss the underlying mechanisms for several possible solar air conditioning systems.
Passive solar cooling
The idea behind passive solar cooling is not so much to capture the sun’s energy and use it to cool, but to avoid heat from the sun entering your house.
One example of passive solar cooling is a green roof, where a garden is planted on a suitably reinforced roof. The soil and plants of the garden provide a high level of insulation year-round; evaporation of water applied to the garden also produces a cooling effect. I’ve had a green roof above my kitchen addition for the last 12 years and I can definitely tell how much the green roof helps keep the area cool.
Another passive solar cooling method is the solar roof pond. Solar roof ponds use a large water-filled bladder placed on the roof of the house (the roof must be suitably reinforced to bear the added weight of the bladder). The bladder is left uncovered during the night, to release heat captured during the day, and to cool the bladder down so that by dawn its temperature should be close to that of the outside night air. During the day the bladder is covered, and because it is colder than the outdoor air, it absorbs a large amount of heat rising from the house below.
Dessicant based systems
A dessicant is something that absorbs moisture (such as those little sacs of crystals that are found in vitamin bottles and packaged electronics). Hot, humid air feels much hotter than hot, dry air, so removing humidity from the air is a good way to reduce the perceived temperature in an enclosed space.
A dessicant-based solar powered air conditioning system would work by placing a dessicant in an enclosed space such as a room, where the dessicant absorbs moisture, thereby lowering the humidity and making the room feel cooler. The dessicant is then moved into a solar-powered heater, which uses heat from the sun to dry out the dessicant and restore its ability to absorb moisture.
Obviously a dessicant-based solar powered air conditioning system is not practical for home use.
Absorption based refrigeration has been around for over a century. This process uses a heat source in order to cool. If you’ve ever been to an off-grid home that has a propane-powered refrigerator, you’ve seen absorption cooling in action.
Absorption coolers work on the following principle. A liquid is injected into a chamber containing a gas, where the liquid is made to evaporate. The evaporation cools the chamber, and this cooling can be used to extract heat from the space being refrigerated. The evaporated refrigerant is then dissolved into a second liquid that has a higher boiling point, and heat is used to boil the dissolved first liquid out of the second liquid. The evaporated liquid is then re-condensed and returned to the start of the process.
Ammonia is one common refrigerant used in absorption-based cooling; lithium bromide is another. Let’s look at the ammonia process for absorption-based cooling to understand how this process works in more detail.
The ammonia is injected into a chamber containing hydrogen gas. This lowers the boiling point of the ammonia, which evaporates, causing the cooling effect. The mixture of hydrogen and ammonia gases is next sent down tubes where water is dripped on the mixed hydrogen and ammonia gases. This water captures the ammonia molecules but does not affect the hydrogen molecules, which can flow freely back to the hydrogen chamber. The water with condensed ammonia then moves to a separate chamber where it is heated from the heat source, causing the ammonia to boil out of the water. The ammonia gas then passes through a heat absorber which causes it to revert back to a liquid, from where it can be returned to the first stage of the process. The water, meanwhile, is returned to the second stage of the process to extract the evaporated ammonia from the hydrogen.
Solar energy can be converted far more efficiently and cheaply into heat than directly into electricity. For a while, absorption-based solar powered air conditioning systems were therefore a much more cost effective way to cool a home than hooking solar panels up and using a conventional air conditioner. But most of the manufacturers of solar absorption chillers I surveyed in the original version of this article a decade ago, companies like ClimateWell, Solair, Adroit Solar and EcoSolarCool, no longer seem to be in business, and I was unable to find any credible new manufacturers selling residential absorption based air conditioning systems. Absorption coolers are common in larger, industrial and commercial insulations, because of their energy efficiency, but are rare in residential installations because of system complexity. I expect the companies that did enter this space around 2005-2015 found it increasingly difficult to compete as the cost of solar electric modules dropped while the efficiency of air conditioning sytsems improved.