Solar Hot Water

• Basically, there is the collector - used to capture the heat in sunlight, and...
• the water storage tank which is part of both the heat collection loop, storage, and distribution when hot water is needed. A tank is not, nor may no, be necessary for hot water applications as in pool heating and some radiant floor heating installations.
• Additionally there are elements of a solar water heating system that are applicable. These include an auxiliary heating system used in periods of additional hot water demand; and...
• a control system for monitoring and coordinating the operation of all a solar system’s components in more sophisticated systems. This controller optimizes heat collection, minimizes heat loss, and provides freeze and over-heating protection to the system.

Solar collector - What is it?  Simply - a container with a glass cover, which allows sunlight to impact the interior surface.

Protections
Super hot water - Solar water heating systems can generate water much hotter than conventional water heaters so a mixing valve is usually incorporated at the tank area. This is a protective measure of temperature adjustment, by adding cool water to the hot water from the storage tank when necessary during hot water use.

A variety of solar hot water approaches are used in Arizona. All have the means of capturing the sun and heating water for use - they vary in the details of solar capture, transport of captured heat, and approach to storage and storage placement. Basically there are 2 fundamental approaches - Direct and Indirect  Direct heat exchange is when the water to be used is heated directly by the solar collector. Indirect heat exchange involves heating an efficient heat transfer medium other than the water, then transferring the gathered heat from the collection medium to the water to be used. Direct heat transfer is highly effective, and even more so when attention is given to water quality. Calcium carbonate residue (scale) can form on the inside of heated containers like tea kettles and in a solar collector. Scale reduces collector performance and may shorten a collector’s life.

Water treatment, a common installation due to Arizona's hard water, often mitigates the condition, and regular care and maintenance is always a good practice. Indirect heat transfer solar systems, beside providing higher heating and lower heat loss, are another approach to dealing with scaling and freezing. Fluid heated in the collector is typically, propylene glycol, a non-toxic antifreeze compound. This heated liquid is in a separate line and loop from the water to be used or stored. The glycol flows through the collector and heats up from the sun's impact, then flows to a heat exchanger where it goes up its captured heat, then goes back to the collector for another round of heat gathering. The heat exchanger transfers the heat collected in the glycol  to the water to be used, which circulates, in a separate piping system loop to the storage tank where it gives up its heat and returns to the heat exchanger for another transfer. As a result, there are two separate fluid loops, one that gathers the sun's heat, and the other which contains water to be used.

Hot water  rises and cold water settles. This is because hot water is less dense than cold water due to its molecular "excitement" in being heated. In a standard water heater, colder water is at the bottom of a tank. When heated it becomes less dense and rises to the top of the tank, while being replaced by cooler, upper water, which is, in turn heated, rises, etc.. This cycle is called a convective action.  A thermosyphon solar hot water heating system incorporates natural convection to move fluid heated by the collector to a storage tank. To do this naturally, the storage tank is located at a point higher than the collector. Cool water from the bottom of the tank flows to the bottom of the collector where it is heated making it rise to the top of the storage tank. This process is continuous and occurs whenever there is enough sunlight to warm the liquid in the collector. As a result, thermosyphon systems do not need pumps and for that reason they are considered a passive system .Thermosyphon systems can be used to directly heat and store water, as well as indirectly heat water with the use of an an antifreeze/heat exchanger loop.

- Active Systems/Forced Circulation Systems

These applications, called active systems because a pump is used to move fluid through the solar collector, allow hot water storage to be placed at any convenient location within the building. Forced circulation systems transfer heat either directly by water circulating from the collector to the tank, or indirectly by use of a heat transfer fluid at the collector and transferring that collected heat via a heat exchanger to water in the storage tank. Variations of a forced circulation include Open Loop and Closed Loop systems. Open loop forced circulation systems transfer heat directly to water to be used. A sensor monitors the storage tank temperature and in the collector loop. When water in the collector loop is hotter than the water in the storage tank, the pump is activated and water from the tank is circulated through the collector. State requirements stipulate provision of equipment to prevent freeze damage, and open loop systems come with recirculation and/or drain down configurations, as well as with freeze plugs or a “dribble” valve.

• A recirculation system controller activates the pump when collector temperature nears freezing, and storage tank hot water circulates through the collector loop to raise its temperature.
• A drain down system has a valve located at the bottom of the collector loop which opens when the temperature drops near freezing, and all water in the collector is automatically drained from the collector and piping, into the tank.
• A freeze plug is simply a valve that opens when the pressure in the collector rises above a certain point. As water changes from liquid to ice, it expands which forces the freeze plug to open and relieve that pressure, thereby avoiding freeze damage to the solar collector and piping.
• A “ dribble” valve is much like a freeze plug. Composed of a material that shrinks when it gets cold, it opens, allowing water to drain from the collector. Open loop, and closed loop, systems also are installed with a check valve, which allows fluid in the collector loop pipe to move in only one direction in the collector to prevent undesired reverse siphoning and loss of heat when the sun is not available.

- Closed Loop

Closed loop forced circulation systems transfer heat to water to be used in a 2 loop method. In one loop fluid not susceptible to freezing, is heated at the collector and circulated to a heat exchanger which removes the gathered heat and transfers it to a loop containing the water to be used and/or stored, and the collection fluid is circulated back to the collector. There are two separate fluid loops, one for the heat collecting liquid, and the other for the water to be used.  Separately, each moves through the heat exchanger which implements the heat transfer process. A system controller turns the circulating pump on when the collector fluid is hotter than the storage tank water. There are two primary types of closed loop systems: the drain back and the non-freeze. Drainback forced-circulation systems have an additional tank (drain back tank) for ensuring protection against freezing. When the pump is off, collector fluid drains into the drain back tank. Non-freeze forced circulation systems use an antifreeze-water mixture in the collector loop. The antifreeze mixture provides protection against very high and low collector operating temperatures. An expansion tank is usually included on these systems to allow the collector loop fluid to expand and contract without damaging the pipes.

Collectors are best located in an unshaded area where there is unobstructed access to the sun throughout the year. The ideal location, of course, is the roof. A solar hot water system should be located to minimize piping runs between collector and storage, and, as in all good hot water design, between storage and end use. This reduces materials, and cost, as well as heat loss in the pipes. Collector placement considerations include:

• A collector facing true south gains equal amounts of sunlight in the morning and the afternoon. If more hot water is desired in the morning, the collector should face somewhat east of true south, and if hot water is more desirable later in the day or early evening, the collector should face west of true south.
• Collector performance is maximized when placed perpendicular to the sun. Typically, a collector is placed to operate at its optimum during the winter, with its short days of sunlight exposure, lower sun angles, and colder temperatures. For this reason the upright angle of the collector is important in maximizing solar heating of water during wintertime conditions.
• Optimum collector angle and angle of a roof may not be compatible. This condition may require a support system like a rack or integration into the building form. It is important to note some subdivisions have restrictions regarding equipment on rooftops, and considerations regarding aesthetic integration and maintenance of style. In these cases, rack mounted collectors meet resistance, and resolution may be in the form of collectors placed flush with the roof. While this may reduce optimum performance of a hot water system, it will still provide an extensive amount of solar heated water. It is said that the difference between an ideal angle and a flush roof angle is about $30 per year in savings.

• Energy bills will be lower due to less demand of electricity and/or gas. Savings are directly proportional to efficiency of the system, cost of local gas and electricity, and amount of hot water used.
• Solar energy replacement for heating water means reduction of gas and/or electricity to be provided by the supplier and avoided new and costly generation and transmission systems.
• Solar water heating replacement of electricity and gas systems results in avoiding additional pollution created by generating electricity and burning gas - a solar water heater avoids the equivalent pollution of .3 cars per year.
• Utility providers have various incentive and buy-down programs for solar incorporation and utilization.
• Local, county, and state government provide incentives for incorporation of solar energy equipment. The State has a tax credit of up to $1000 for the purchase and installation of approved solar water heating systems, and there is no sales tax. The community of Marana waives building permit fees for solar photovoltaic and hot water installations.
• A solar water heating system is a good investment. Return on this investment will be reduced energy bills and a cleaner environment over the lifetime of the system.

• Flush all tanks once a year.
• Annual or bi-annual maintenance check-up by a certified service technician.
• Keeping the glazing clean and unobstructed.
• Check for leakage at pipes leading to and from the collector.
• Check the insulation on the pipes and at all joints.
• For active systems, allow plenty of space around the pump and other equipment in order to extend its lifetime.
• Check antifreeze in closed loop systems.
• If performance drops off, check for consumption pattern changes. If the consumption pattern has not changed, check the system's maintenance manual and/or contact a service representative.

A solar water heater can deliver your hot water all year round - In much of Arizona we are fortunate that we have fewer cloudy and cool days than almost anywhere else in the country, so solar energy can carry much of the load.

• All system component and system must meet State requirements. Contact the Az. Dept. of Commerce Energy Office for information
• Az. Registrar of Contractors, the Better Business Bureau and the Az. Solar Energy Industries Association are information sources about solar companies and certified installers.
• Az. Dept of Revenue and Az. Dept. of Commerce Energy Office are information sources re: approved solar systems and tax benefits
• A properly installed, approved system must have warranties for the equipment and installation
• Parts and labor for the entire system for a minimum period of two years from the date of installation.
• Warranty against freezing for a minimum of five years.
• Solar Contractors License (State of Az. Registrar of Contractors)
• Certified Solar Technicians to do the work (Az. Solar Energy Industries Association)
• Product Meets Az. Dept. of Commerce Energy Office Certifications and Requirements Installation Requirements
• Meets or exceeds all applicable Codes
• Conforms to Arizona adopted guidelines and standards that require residential solar water heating systems installed in Arizona to be certified under the Solar Rating and Certification OG-300 rating system.  This certification ensures compatibility of components and provides comparative information for the consumer.  Information can be attained at the State Department of Commerce Energy Office, from solar companies, and at http://solar-rating.org.

Arizona Revised statues 33-439 prohibits Homeowner Associations from imposing restrictions that effectively prohibit the installation and use of solar energy.  Recent Arizona court rulings have upheld consumer rights to install solar energy devices, while noting that Homeowner Associations can identify reasonable restrictions as long as they do not significantly increase a solar system's cost or diminish its' efficiency. For further information go to www.azsolarindustry.org, or contact the Az. Dept. of Commerce Energy Office, or contact the Arizona Solar Energy Industries Association at 888-253-8180.

Conventional water heating uses electric energy or gas. Gas is burned directly in the water heater, but the electric energy released into the water in the form of heat is usually generated by burning a fuel at a central power plant. Burning hydrocarbon-based fuels (such as coal, oil, or natural gas) emits oxides of carbon (Cox), nitrogen (Nox) and sulfur (Sox). Solar water heaters significantly reduce pollutants and contribute to a more clean and healthy environment.

This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Az. Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources, including in big part the Arizona Public Service Consumer’s Guide to Solar Water Heating.