This is a brief review of last week’s intro. More energy is used in buildings than is consumed by vehicles. I don’t mean to say avoid the transportation issue because it’s a close 2nd to buildings. The easiest solution to both these things is less. Fewer miles driven and smaller homes. Then small, efficient, clean systems and cars can supply our energy and transportation needs. If we make the shell of a building like a thermos we can greatly reduce the energy used for heating and cooling. In our northern NM climate, cooling becomes unnecessary in a well insulated house or even an un-insulated adobe. Just opening and closing a few windows, night and day can provide fresh air and a cool comfortable space.
Wouldn’t it be cool if most of the homes in Dixon had solar hot water? I’ve seen a development of almost 1,000 small houses on Maui, all with solar hot water systems. It’s the most cost effective solar system and heating the house with the same system, sized to do both, makes it even more cost effective. Conservation is the first thing to look at, like turning out the lights when you leave the room. Insulate the hot water pipes and tanks, use energy efficient appliances and lighting. Don’t let the water run down the drain when brushing your teeth. Take a shower sometimes instead of a bath. The left knob is hot water and if you’re not going to wait for it to get hot use the cold water. The pipe coming from the water heater is filled with water that was heated but has since cooled off. Depending on the length of the pipe it could be a gallon of wasted water plus the energy it took to heat it. Once the waste is minimized, then the solar system can be smaller and cost less.
About 50 sq ft of collector can provide most of the hot water for an average family. I have a friend who has 1 – 4 foot x 8 foot collector and in the past has gotten almost all of his hot water from the sun. Somehow solar heated water feels better, even if it’s psychological.
The storage tank is the other half of the system. Size matters, up to a point the more the better. I replaced our 80 gallon tank with a 120 gallon tank about 7 years ago. The purpose of the increased volume was to provide more, cloudy day storage. Un-like a standard 50 gallon water heater which can be heated whenever the tank cools, a solar water heater gets the suns energy about 6 hours a day. This 6 hour period of solar heat needs to provide at least 24 hours worth of hot water, more if you want some cloudy day storage. I designed a house that’s being built in Taos that has 160 gallons of storage, which also provides some extra hot water for visitors and longer cloudy spells. Since this system is meant to keep the solar heated water hot longer, more insulation is important. This can be added with a water heater insulation blanket. These are worth installing on every hot water tank. Even better a tank could be built with R-50 or more insulation.
Typical gas hot water heaters are very inefficient. There is a 3-4 inch diameter pipe through the middle of the tank. It has steel fins in it to transfer heat faster. When the burner is on (which is a small % of the time) it heats the pipe which heats the water. When the burner is off, the heat goes the other way. The water heats the pipe and creates a draft that goes out the exhaust duct. The more frugal a household is with hot water the higher the percentage of heat is lost up the pipe. This standby loss could be more than half the energy consumption by the hot water heater.
Adding a solar collector to the system can provide most of the energy needed for the domestic hot water. When the temperature in the tank is not hot enough, some type of back up heat is needed.
If a gas water heater is used for back up and solar storage, it loses a lot of heat up the chimney 24 hours a day. It’s much better to use an electric water heater that has no vent for combustion gasses. If the system is sized correctly only a small percentage of the time is electricity being used.
One popular type of solar hot water system is called a direct PV system. It uses antifreeze that is circulated through the collector and the water tank. The circulation is powered by a small photovoltaic panel. Whenever the sun is shining the pump turns on. There is a sensor on the collector to insure that it is hot enough to heat the water in the tank. As the tank gets hotter the pump needs to slow down for the collector to get hotter. Unfortunately these 12 volt pumps are not available in variable speed. This system compensates for this by shutting the pump off to allow the collector to get hot enough and then turning it back on. This works and is simple but it’s not as efficient as a variable speed pump that automatically slows to provide a consistent high temperature for domestic hot water heating.
Instead of a direct PV system a thermostatically controlled variable speed AC pump can be used. They consume a bit more electricity and produce a bit more hot water. The amount of electricity used by this pump is comparable to a lap top computer or about 4 compact florescent light bulbs.
Another way to supply back up heat to the solar hot water system is with a gas boiler connected to an insulated water tank. Heated antifreeze from the boiler is circulated through a heat exchanger in or around the water tank. This requires an extra pump and controls but becomes more cost effective if the boiler is also used to back up the space heating.
There are many choices available when choosing a boiler. The two basic things to look for are size and efficiency. Efficiency is the percentage of heat produced, compared to the amount of fuel consumed. High performance condensing boilers are available that are more than 90% efficient. They have the added benefit of eliminating the combustion air requirement of a standard boiler or water heater. The mechanical room is required to provide fresh air for combustion, on rare occasions enough cold air enters to freeze a pipe. There is a lot of energy lost through these ducts. High efficiency boilers have sealed combustion chambers that are fed directly with 2” pvc pipes eliminating the need for venting the mechanical room. Just the fact that they use pvc pipe shows that exhaust gasses are fairly cool and most of the heat is retained.
Using a properly sized boiler is important. One that is too large will put out so much heat that it doesn’t run very long before the water is fully heated. This short cycling uses more fuel and is more polluting. It would be hard to find a boiler that is too small because they are designed for the larger heating needs of the house.
Solar hot water panels are not mounted on a movable frame like many solar electric panels, so they need to be as close to perpendicular to the sun as possible to maximize their collection year round. If the system is just for hot water that means its production needs to be only slightly more in winter. Since the noon day sun at Christmas is at about 30 degrees above the horizon and on June 21st is about 75 degrees, an average angle is needed. Half way between these two angles would give about an equal amount of heat winter and summer. Tipping the panel somewhat toward the winter sun at about 60 degrees is a good idea. If the system is designed for heating the house and the hot water, a lot more energy is needed in winter. Tipping the panel up to 70 degrees will shift the seasonal collection toward the winter and protect from overheating in summer. As I mentioned before about 50 sq ft solar collector can supply enough hot water for an energy conscious family. A rule of thumb for a combined system is 10% of the floor area, so a 1,500 sq ft home would use about 150 sq ft of collector. If you know you’ll use more hot water or like the house at 75 degrees in winter add more.
Next week I’ll focus on combined solar systems that heat the house and the hot water. Before going to the quote of the week I’d like to recommend a U Tube that I think is worth watching
Andrew Maguire exposes fraud by JPMorgan
http://www.youtube.com/watch?v=yLxoeLqQMlw
Quote of the Week
The Howard Zinn quote from last week was so good I’d like to read it again
"To be hopeful in bad times is not just foolishly romantic. It is based on the fact that human history is a history not only of cruelty, but also of compassion, sacrifice, courage, and kindness. What we choose to emphasize in this complex history will determine our lives. If we see only the worst, it destroys our capacity to do something. If we remember those times and places (and there are so many) where people have behaved magnificently, this gives us the energy to act, and at least the possibility of sending this spinning top of a world in a different direction. And if we do act, in however small a way, we don't have to wait for some grand utopian future. The future is an infinite succession of presents, and to live now as we think human beings should live, in defiance of all that is bad around us, is itself a marvelous victory."