Developing solar air conditioning has long been the holy grail of solar energy scientists and engineers the world over. In a sense, they are a perfect match. Air conditioning is needed the most when the sun is the strongest and providing the most potential power. So, if someone could develop a true solar air conditioning system, it would be an ideal solution to the problem. Unfortunately, the power we receive from the sun much more readily converts itself to heat than it does to cold.
A team of researchers from Stanford University has come the closest of anyone to developing an actual solar cooling system. Professor Shanhui Fan has developed the theory for a roof panel system that reflects light and emits heat, drawing the heat out of the building to cool it. The next step in the process is to build a working model to prove his theoretical work. To that end, he has received a $400,000 grant to fund his working model and test its performance.
While this system shows potential, it is still many years away from being commercially available. To date, we are still stuck with the sun creating heat, rather than being able to harness its power to cool.
Nevertheless, the term “solar cooling” refers to more than just an active system of this type that would cool our homes and offices by drawing off the heat. Any true solar home needs to take into account the fact that there are days in which the temperature will be high enough that there is no reason to heat your home; however, there will be a need to cool it. The home needs to be designed to allow for that as well.
Designing a home for natural cooling is as complex an exercise as designing it for solar heating. It is also something that is dependent largely upon the geographic location of the home and the climate in that location. Strategies which work in some areas may not be as effective in others, so it is necessary to find those strategies which will be the most effective where your home is.
Solar hot water systems and other solar collectors are typically painted black on the inside. This is because the color black absorbs the most light, converting it into heat. While this is excellent for solar heating, it is a problem for solar cooling. When trying to keep a home cool, it is necessary to try and reflect as much light as possible, not absorb it. The color white is the best color for reflecting sunlight, rather than absorbing it.
While it might not be practical to paint your home white every summer, it may be practical to ensure that certain parts of your home are as light a color as possible. I live in the southern part of Texas, where we have lots of sun and lots of heat. One thing which has always amazed me down here is the number of homes which have black or dark brown roof shingles. Those shingles are absorbing light from the sun and converting it to heat, increasing the heat in the attic of those homes.
When we had to replace our roof, I went with the lightest shingles I could find that would work with the color of my home’s brick. While it wasn’t as light as I wanted, I found a tan colored shingle that was a whole lot better than the black that had been on there. I can’t say exactly how much money that saved us, but I know that my attic was at least ten degrees cooler after changing the shingles.
Once again, the ideal colors for you will depend largely upon the area in which you live. You have to balance out the need to heat your home and the need to cool your home. The selection of your home’s colors should meet the greater of those needs.
Besides color, material will affect the reflection or absorption of sunlight and the resulting conversion to heat. A shiny aluminum roof will do an excellent job of reflecting light, preventing the creation of heat. While aluminum is one of the best thermal conductors there is, and the metal of the roof itself will get hot, it will reflect away more light than it will absorb.
The second major ingredient in naturally cooling your home is to provide adequate shade. If sunlight doesn’t directly strike your home, then it will not be converted to heat. While construction of a giant beach umbrella over your home may not be practical, if it could be done, it would go a long way towards maintaining your home in a more cooler and comfortable setting in the summertime.
We lived for several years in a motorhome, traveling around the country for my work. The motorhome we owned was an older Winnebago, which had an aluminum skin. It was essentially an aluminum oven. Trying to keep it cool was a major challenge, one which our two air conditioners worked overtime to unsuccessfully try to handle.
Since we spent a lot of time in the South, I was constantly battling the heat. To help, I built a PVC pipe framework that I could put over my motorhome’s roof, and which overhung its sides. This framework was covered with white nylon cloth. When we were parked somewhere for several days, I would erect this shade over my motorhome, reflecting away as much of the light as possible. This helped us to overcome the heat that the sun was conveying. Parking the motorhome in an east/west orientation helped as well.
The normal way of providing shade to a home is with foliage. Trees planted on the south and west sides of the home can do wonders to help prevent the home from overheating from excessive sunlight. While it takes trees years to grow up to their full height, many homes and properties already have trees available. If you are building a home, make sure that you and the architect take a good look at the trees that are on the property.
While trees take a long time to grow, if you never plant them, they never can offer you their shade. Even though it might be years before you can reap any true benefit from the shade provided by trees, don’t hesitate to plant them. You’ll be glad some day.
Another way in which plants can be used to help shade a home is with vines. Vines grow faster than trees, provide natural shade, and eventually cover the entire wall of the home. The problem with vines is that they also damage the exterior of your home. Small roots from the vine work their way into the exterior brick or wood siding, anchoring the vine to the home.
This problem can be eliminated by erecting a giant wire trellis for the vines. This trellis would consist of a number of wires which run from the ground to the soffit, about 12 inches from the side of the home (industry standard is 12 inches of clearance around the house to allow it to breathe). Some cross wires are needed as well, especially above windows and doors. The vines will grow up the trellis, attaching themselves to it, without damaging the home.
The third natural method of cooling a home is with convection. The basic principle of convection is that heat always rises. This means that the second story of a home will always be hotter than the first story, if there is no air conditioning. If that hotter air has a place to escape, then the home will draw in cooler air to replace it.
Placing skylights or other vents high up on the home provides a way for the hot air to escape. If the home has a crawlspace or basement, the naturally cooler air can be drawn from there up into the home, replacing that hotter air and helping to cool the home. This merely requires providing a way for that cooler air to move into the main part of the home. In other words, more vents.
One nice thing about convection heating and cooling is that they are self-regulating. Convection cooling will never draw hotter air into your home. The air in the main living area will be the coolest air available, whether that is the air that is currently in the living area, or air that is drawn up from the basement. As that air becomes hotter, it will move up towards the higher floors and the exit vents, drawing new cooler air into the living air.
While evaporative cooling isn’t really considered solar cooling, I want to make mention of it. There are evaporative coolers which have their efficiency increased by use of solar power, so I guess we could say that they are half-solar.
Evaporative coolers work by evaporating water. As water goes from a liquid state to a gaseous state, it absorbs an enormous amount of heat. It draws the heat from the air around it, cooling that air. In areas where there is a lot of heat, the evaporation happens faster, increasing the cooling effect that the water is able to provide.
The only problem with evaporative cooling is that it requires a dry climate. Temperature isn’t the only thing that affects evaporation—humidity does as well. In humid climates, evaporation can be slowed immensely, even if the temperature is extremely hot.
The last thing I want to mention is the importance of good insulation. Any cooling system is only going to be as good as the insulation of the home. A well-insulated home will retain both heat and cold much better than a poorly insulated one. Adding insulation can make it possible that a home not need cooling, or not need as much cooling as it did before.
The most important place to insulate in a home is the attic. Attic insulation is usually laid on the drywall of the second story ceiling, without anything to hold it in place besides gravity. Over time, that gravity gradually packs down the insulation, reducing its thickness and effectiveness. To counter this, more insulation needs to be added, making it thicker again.
In hot climates, as much as R-60 worth of insulation needs to be installed in the attic. That means that there would need to be 16 inches of fiberglass batt insulation. If you measure it and find less than that, you do not have R-60 insulation in your attic.
Don’t forget about sealing air leaks either. Air leaks, especially those around doors and windows, can allow hot air to blow through your home, adding to the internal heat and removing your cooler air. Proper sealing helps all of these solar cooling methods work that much more efficiently. However, retain proper venting through soffit and attic vents. Sealing your home too tightly can create adverse conditions where moisture is actually retained in the structure, damaging your house. You don’t want to offset one problem only to create another.