Solar Basics

Here you'll find information about the different types of water heating systems. In the future, we will beef this page up a bit with some more basic terminology and access to more educational resources.

For now, you can ASK THE SOLAR MAN your specific questions via email. Please send your inquiries via our General Inquiry Online Contact Form.

He wrote the article below for a Basic overview of solar water heating systems produced by AET.

Active Systems

Active Systems require a pump to circulate the water from the solar storage tank through the collectors. There are two general categories of active solar water heating system – open loop and closed loop systems.

Open Loop System

In these types of systems the water stored in the solar storage tank is circulated directly through the solar collectors where it is heated and returned to the storage tank. Open loop systems are very basic and were the first to appear on the market many years ago. The concept is very simple. A pump controller (called a differential controller) will turn the pump on when the collectors are hotter than the stored water and shut it off when they reach the same temperature. The benefits to this system are cost and ease of installation, but the drawbacks often outweigh these benefits. First, there is no totally reliable form of freeze protection other than manually draining the collectors. Therefore they should only be considered in areas that do not freeze, ever. A second problem is that there is no high limit protection. By this I mean that without daily consumption of hot water, the water in the storage tank and solar collector can boil. If this occurs, relief valves (when operating properly) will release. This can be messy and a waste of energy. Finally, in open loop systems, the solar collectors are susceptible to clogging from mineral deposits in areas of problem water. Figure 1 depicts a typical open loop system.

Figure 1. The open loop system.

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Closed Loop Systems

Unlike open loop systems, closed loop systems do not directly heat the water in the storage tank. By this I mean the water in the tank does not circulate directly through the solar collectors. There are two types of closed loop systems – the glycol indirect system and the drainback system. In this type of system a separate fluid is circulated through the collectors and a heat exchanger is used to transfer this heat to the water in the storage tank. This heat exchanger can either be built into the storage tank, or separate from it. If separate an additional pump is required to circulate water from the storage tank through the exchanger. The heat exchanger fluid is typically propylene glycol. Additional items such as an expansion tank, pressure gages, and fill valves are also required.

Glycol Indirect System
Like the open loop system, the glycol closed loop system has been around a very long time. It also alleviates two of the three problems encountered in an open loop system. By using glycol as the heat transfer fluid the chance of freeze damage is eliminated. Additionally it eliminates the problem of fouling the collectors in areas that encounter hard water. There still remains the problem of high limit protection and the potential of failure of the additional components added (expansion tank, pressure gages, etc). Figure 2 depicts a typical indirect glycol closed loop system.

Figure 2. Indirect Glycol Closed Loop System.

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Drainback System

The drainback system was first introduced in a large scale back in the 1980’s. They have been proven very reliable and are more efficient than the indirect glycol system. This is a non-pressurized closed loop system using water as the heat transfer fluid. A small drainback reservoir is installed in the collector loop. When this system is filled with water it is only filled to the top of reservoir. Since it is located below the collectors, they remain dry when the pump is not circulating. When the collectors are hotter than the water in storage, the pump circulates the water in the reservoir through the collectors where it is heated. The heat from this water is then transferred to the solar storage tank through a heat exchanger located either in the storage tank or drainback reservoir. When the collectors approach the same temperature as the water in the storage tank or this water has reached a preset temperature, the pump shuts off and all the water drains back the reservoir. The drainback system eliminates all the problems inherent in the other types of systems. First, freeze protection is based on gravity. Second, the pump is shut off when the storage tank reaches its high limit setting and all water is drained from the collectors. And finally, the problems encountered in the collectors in hard water areas are eliminated. The drainback system also uses fewer components than any of the other types of active system, thus the potential of component failure is drastically reduced. Figure 3 depicts a typical drainback system.

Figure 3. Drainback Closed Loop System.

Date written: December 8, 2003

Benefits of Going Solar

Why have over 1.5 million Americans invested in solar? Surveys taken over the last ten years confirm that the vast majority of owners (94% or more) consider that investment a wise decision. Consumer investment and satisfaction have spawned a small but growing solar hot water (SHW) industry here in the U. S. that is providing even broader benefits to our nation and has the potential to contribute much more.
Readily Available Resource The U. S. Department of Energy estimates that Americans consume approximately 2.5 quads of end-use energy annually to produce hot water at a cost of over $20 billion dollars. Solar energy currently provides only a tiny fraction of that demand, but huge portions of our country possess sufficient insolation to produce much greater quantities of energy. Enough sunlight reaches the earth’s surface each yearto produce approximately 1000 times the same amount of energy produced by burning all fossil fuels mined and extracted during the same period. Sunlight does not have to be explored, mined, extracted, transported, combusted, transmitted — or imported. Quality, Reliability, Durability Solar water heating technology, pioneered in the U. S., is the oldest and most developed of all renewable energy systems. Modern solar water heating systems can provide a large portion (40 - 80%) of household hot water demand depending on local climate conditions and the size and type of system. Most systems pay for themselves in four to sevenyears and continue to provide hot water for many years thereafter.
Solar pool heating often provides an even better investment. Payback can be as low as two years and the solar system can extend the swimming season by several weeks without additional cost. Many homeowners have regretted the purchase of a conventional pool heating system after receiving their first utility bill. Another cost-efficient application for solar energy is preheating ventilation air for commercial and industrial facilities. The vast majority of U. S. manufacturers of solar equip­ment voluntarily comply with national consensus standards devel­oped by the Solar Rating and Certification Corporation (SRCC). The SRCC tests and certifies collectors as well as complete systems for performance, reliability and durability. In addition, manufacturers and installers comply with the SRCC’s strict requirements for proper installation, labeling and homeowner information regarding operation and maintenance. Assurances of performance and quality are backed by warranties that in many cases exceed the guarantees of other household appliances. Made in America In 1960, jobs within the energy industry (including coal mining, oil and gas extraction, petroleum refining, electric and gas utilities) represented about 1.8 percent of total U. S. employment. By 1990 that share fell to 1.2 percent. “This ratio likely will decline further over the next decade,” the U. S. Center for Global Climate Change reported in 1993. Employment patterns resulting from conventional energy technologies are dominated by the capital-intensive nature of the industry. When measured in jobs per million dollars of annual expenditure, coal, oil, gas and nuclear technologies support among the fewest jobs of any economic activity.” The solar water heating industry is a good example of the type of manufacturing needed to create both new skilled and unskilled jobs. Realizing the Potential Several utilities all across the country—from the Sun Belt to the Midwest and Northeast— offer consumers a variety of programs to reduce the initial cost of solar systems. In turn, the utility avoids the cost of installing additional generating capacity, especially power to meet peak energy demand, and using solar energy helps the utility comply with every-increasing restrictions on pollution emissions.
In 1992, the U. S. Department of Energy along with the Edison Electric Institute, American Public Power Association, and their utility and industry members established “Utility Solar Water” or USH2O. USH2O will serve as a forum for interested parties to develop and expand utility programs for residential and commercial solar water heating. These systems are examples of installations resulting from a demand—side management program at Sacramento Municipal Utility District. Many state and federal government officials have ac­knowledged the importance of solar hot water systems in many of their programs.
The federal government builds, owns and operates more buildings than any other sector of the economy. Managers of these buildings are learning from the Federal Energy Management Program (FEMP) how solar and energy efficiency can be combined to reduce the energy load of their facilities and save millions of taxpayers’ dollars. The federal government also subsidizes the monthly utility bills for millions of low- income families. The State of Florida and the U. S. Department of Energy have established a pilot program to demonstrate that SHW on low-income housing can also save millions of dollars in federal subsidies. The Housing and Urban Development agency has undertaken a project to establish criteria and mechanisms that would allow mortgage lenders to give homeowners an advantage for solar and energy efficiency measures when purchasing a new or used home. Solar water heating systems are already eligible for the energy efficient mortgage programs which allow homeowners to have higher debt-to-income ratios, thus allowing them to qualify more easily for financing. In fact, when SHW is incorporated into a mortgage, the monthly finance charges attributed to the new system fall below the monthly energy costs and give homeowners money in their pocket every month. In 1993, the National Association of Home Builder’s Research Center included SHW, as well as other solar options, in the Resource Conservation House, a single-family resi­dence built to demonstrate to builders the characteristics and advantages of the many new products on the market that are conserving our nation’s energy resources. The United States is striving to reindustrialize with new technologies that create jobs yet are environmentally benign. The U. S. solar water heating industry is comprised of the manufacturers profiled in this brochure plus many distributors, installers and maintenance companies nationwide. The question many people ask is “How can the U. S. save money, help our environment, and create jobs? The answer has been up there all the time - solar energy