Solar Hot Water
Solar hot water systems circulate a fluid past a dark heat collecting surface in order to gather heat from the sun. That heat is then passed on to water in a tank to be used as the domestic hot water in a home, or occasionally for industrial processes that require heat. The most common application is to heat domestic water, which is the water used to wash and bath in. For that reason solar hot water systems are often called solar domestic hot water, or SDHW.
Many people worry that they might be showering in cold water if we have several cloudy days in a row. The way our systems work is to put the solar hot water tank in front of a conventional hot water tank. The water from the solar tank feeds the standard tank. If that water comes in hot the conventional tank doesnt have to turn on to heat it as it would if the water came in from pipes underground at 55 degrees. If the solar water enters the conventional tank warm the tank only adds a little heat, and if it comes in cold the conventional tank will heat the water to a comfortable temperature as it always has.
We tend to steer clear of single-tank systems, which try to add solar heat to the hot water in a conventional tank. We avoid this because you get less energy out of the solar collectors (panels). The gas or electricity will heat the water quickly back up to the temperature it is set at, say 120 degrees. The solar is then stuck with the challenge of increasing that temperature. If the temperature of the water you are trying to heat is much lower, say 55 degrees as it comes in from underground pipes, it is much easier for the solar to add heat. The collectors therefore work more efficiently. Even super-insulated vacuum tube collectors will loose some heat to the atmosphere and work less efficiently if the difference in the ambient temperature around the collectors is much lower than the temperature of the liquid being heated.
How much energy will the solar panels collect?
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A rule of thumb used for typical flat-plate collectors is that they can harvest about 1000 BTUs per square foot per day on a sunny day in Michigan. A BTU is enough energy to raise one pound of water one degree farenheit. Roughly eight BTUs would raise a gallon of water one degree.
Solar hot water collectors are extremely efficient. When comparing the suns energy that hits the collector/panel to energy out, they typically approach or exceed 70% on a warm summer day. This is EXCELLENT efficiency. Solar electric that achieves 15% is considered good, and wind turbines that get 30% would be tops in their class. Solar hot water has the advantage of not tying to convert the energy. All it does is collect heat and use it in the form of heat. The other technologies collect energy in one form and then convert it to another. This fact, and their simplicity, are the reasons solar hot water tends to be the low hanging fruit for most consumers renewable energy budget.
Learn about different types of Solar Hot Water Systems
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Northern climates like Michigan are serviced by two types of solar hot water systems: drainback and anti-freeze closed loop. Drainback systems have a reservoir for the heat transfer fluid to drain into whenever the pump is not running, so the collectors and the pipes in the freeze zone (outside or in the attic) are empty and will not burst in the winter. Anti-freeze closed loop systems use propylene glycol as antifreeze which stays in the collectors and outdoor pipes at all times. It does not freeze and burst because the anti-freeze wont expand until the temperatures drop below typical winter temperatures in Michigan.
At SUR we provide both types of systems depending on the specific needs of our customers. Our default system is a drainback. The primary advantage of the drainback system is that the collectors are empty at BOTH ends of the temperature spectrum. Draining of the collectors not only protects them from freezing in the cold, it also protects the glycol from overheating in the dead of summer, if in fact glycol is added to the system. When glycol gets over about 300 degrees it starts to turn acidic and can cause undue wear and tear on the copper and pumps. Why might a system overheat? If there were a power outage in the summer most systems have AC pumps that will stop circulating the glycol, so it would turn acidic. Other circumstances that can result in excess heat are those in which the client would like a system that is oversized. This may be the case if they try to use the system for space heat or just want a little more solar hot water. The point is it takes the concern of acidic glycol and critical system sizing out of the picture for a more robust system.
Traditionally drainbacks were developed so that water could be used as a heat transfer fluid rather than glycol. Glycol is 30% less efficient at transferring heat than water, so if you have a 50/50 mix of glycol and water you are 15% less efficient than with straight water. While we have done drainbacks using just water as a heat transfer fluid we always reserve the right to add glycol to the system as an added insurance policy. Some have argued that this defeats the purpose of doing a drainback. We would argue that there are many purposes behind this decision. Most of the large commercial systems out there are drainback, or have some other way to spill excess summer heat, which is the reason and circumstances in which installers that do will use a drainback. Most of the large contractors across the country that have mentored us over the years have the drainback system available as an option to be used wherever appropriate.
The primary disadvantage of the drainback system is that it uses larger pumps to circulate the heat transfer fluid through the system. Also, because the pipes have to drain, you have to be careful about the way the piping is installed.
Antifreeze closed loop systems have a glycol/water mix as the heat transfer fluid and are referred to as closed loop because the transfer loop is a pressurized system where all of the air is removed, and the domestic water doesnt mix with the heat transfer fluid. The transfer loop is closed to atmosphere and all other inputs and never mixes with anything as it circulates.
As mentioned before, over time the antifreeze in closed loop systems tends to become acidic. Traditionally installers using these types of systems plan for a service trip every five years or so to change the fluid. We have heard others claim you can wait 10 years but this goes against convention wisdom.
Advantages of this system are that smaller circulating pumps can be used so they dont need as much electricity. Also, in countries where solar is a more mature industry (our technology is mature but the business is small by comparison) they have simplified the installation of these systems to cut back on labor cost. Systems in Germany and other countries are much more modularized.
Most solar thermal collectors you see are of the flat-plate variety. Typical U.S. made flat-plate collectors are 4x8, plus or minus a couple of feet in the length. Collector area is adjusted according to the loads in the household or business. This is one challenge of solar hot water systems- load-matching. When sizing a system we typically try to match the summer loads to the energy collection at the same time and supplement for the rest of the year. This is one area where grid-tied solar or wind has an advantage over solar hot water. With these electric systems you can be sure any energy you generate is used and never wasted because the grid supplies us with infinite load.
Flat-plate collectors consist of a large thin box with five sides insulated and the sixth covered with a sheet of high strength low iron glass. Inside the insulated aluminum box is a dark surface that has been shown to absorb heat and not let it go (high absorptivity with low emissivity). This sort of testing is done by third-party labs. For more information go to the web site for the Solar Rating and Certification Corporation
. In order to collect the federal tax credit the collectors used are supposed to be rated by SRCC.
The other collectors used a little less often are called evacuated tube. These have a tube of glass with the atmosphere sucked out of them so there is no matter to pass the heat from within the collector out to the atmosphere. If done well, the vacuum is the ultimate in insulation. Traditionally evacuated tube collectors cost more per area of aperture, or the dark heat collecting surface. While SRCC testing shows they do better when there is a large difference in ambient temperature and the temperature of the heated liquid, they tend not to be as efficient as flat plate collectors in the summer. Also, if the vacuum is done well these might not collect as much heat when it is snowy, or they are covered with frost or ice. Our location had high end vacuum tube collectors that had been covered with ice for 3 days while customers with flat plate collectors had solar hot water because the lesser insulated sheet glass spilled enough heat to melt off the snow. On the same day we did see another evacuated tube system that didnt have ice or snow buildup. All the same, for 90% of the work we do, flat-plate collectors are our first choice.
We don't really quote our own prices on the web, and haven't seen any good national data on system costs like that available for PV. From what we understand most installers are charging around $9,000 for a basic hot water system for a family of four. Smaller and larger systems are available.