Solar

Solar Energy = Solar Saves Money + Solar Saves the Earth

Solar Heating

Solar heating uses the energy of the sun to heat air or water, which can be used for a variety of applications. Active solar-heating uses pumps that move air or liquid from the solar collector directly to a load (such as the building space heating or hot water system) or to storage for use later. Passive solar heating relies on the design and structure of the house to collect and distribute heat throughout the building

Solar Heating Basics

Solar water heaters and solar space heaters are constructed of solar collectors, and all systems have some kind of storage, except solar pool heaters and some industrial systems that use energy "immediately." The systems collect the sun's energy to heat air or a fluid. The air or fluid then transfers solar heat directly to a building, water, or pool. Learn more about:

Solar Thermal

Solar thermal devices use direct heat from the sun, concentrating it to produce heat at useful temperatures. Solar thermal devices do everything from heating hot water, heating swimming pools to creating steam for electricity generation. Solar heating system saves energy, reduces utility costs, and produces clean energy.

The efficiency and reliability of solar heating systems have increased dramatically, making them attractive options in the home or business. But there is still room for improvement. The U.S. Department of Energy (DOE) and its partners are working to design even more cost-effective solar heating systems and to improve the durability of materials used in those systems. This research is helping make these systems more accessible to the average consumer and helping individuals reduce their utility bills and the nation reduce its consumption of fossil fuels.

Check with your accountant before purchasing. To help more Americans benefit from these systems, the U.S. Energy Policy Act implemented a 30% tax credit for consumers who install solar water heating systems. To be eligible for this tax credit, the systems must be certified by the Department of Energy's non-profit partner, the Solar Rating & Certification Corporation (SRCC). Alternatively, residents of Florida and Hawaii can use their state certification programs.

Solar Photovoltaic

Photovoltaic devices use semiconducting materials to convert sunlight directly into electricity. Solar radiation, which is nearly constant outside the Earth's atmosphere, varies with changing atmospheric conditions (clouds and dust) and the changing position of the Earth relative to the sun. Nevertheless, almost all U.S. regions have useful solar resources that can be accessed.

National Renewable Energy Laboratory (NREL)

NREL is working with the solar industry to decrease the cost of solar water-heating systems and to develop systems that work in mild and cold climates helping bring these technologies to vast regions of the country where they previously were not viable. Researchers assist with the prototype development of new polymer systems through modeling and optimization, characterizing system performance, and testing the durability of the materials.

In collaboration with FAFCO Inc., Davis Energy Group/SunEarth Inc., the University of Minnesota, and others, NREL is helping to develop the next generation of low-cost polymer-based solar water-heating systems for mild climates. The work to date has focused on material and thermal-performance issues related to passive systems. This includes unpressurized polymer integral collector storage (ICS) systems that use a load-side immersed heat exchanger and direct thermosyphon systems.

Thermal modeling

NREL is building and testing different simulation models for the polymer-based systems in different climates all in an effort to expand the market for these technologies by making them viable in more regions of the country. Recent work includes the following:

• Developing new simulation models to investigate three cold-climate solar water-heating system types: glycol, drainback, and indirect thermosyphon

• Developing new simulations for ICS with immersed load-side heat exchangers

• Developing new simulations for unglazed collectors and systems

• Developing new simulation models for combined PV-thermal collectors, using liquids and air as the heat-transfer medium

• Working with the University of Minnesota to test, characterize, and optimize heat exchanger performance, predict the lifetime of tubing used in polymeric heat exchangers, and protect pipes from scaling (the accumulation of calcium carbonate around the heat exchangers and tanks)

• Working to develop and protect solar-heating systems from overheating. Overheat protection through venting the front and/or back of the absorber has been characterized and with appropriate design can limit maximum temperatures and allow the use of commodity plastics for fabrication of the absorber.

Pipe freeze protection

Passive systems have the advantage of lower initial cost and higher reliability as a result of the absence of pumps, controllers, or sensors. However, they have the disadvantage of possible pipe freeze.

The market for lower-cost passive-solar domestic water heaters (PSDWH) has been limited by the problem of freezing and bursting of insulated supply and return lines that connect to the thermal storage on or under the roof. Using insulation to protect the pressurized piping—as is current industry practice with PSDWH—severely restricts the market for PSDWH, (Figure 1).

Freeze-protection valves (FPV)—also called "dribble valves"—protect water pipes by inducing a small flow of warm water through the piping when the temperature drops below the FPV setpoint (typically ~35°F for piping protection), thus preventing freeze. The valves are commonly used in many industries to protect piping from freezing in exposed locations as far north as Alaska. It is possible to use FPV to protect the supply/return piping from freezing in passive systems, when the valve is mounted as shown in Figure 2.

NREL conducted an experiment to better understand the flow rate through the valves as a function of ambient temperature and warm water temperature. The data was the basis for predicting the long-term annual water consumption of using FPV for pipe-freeze protection. If 1,000 gallons/year is considered acceptable water consumption, the potential market for passive systems (considering only the pipe-freeze aspect) is shown on the right side of Figure 1.

NREL's research showed that markets could be extended by more than 2 orders of magnitude with the 1,000-gallon limit and approximately 1 order of magnitude for a very small 100-gal/yr limit (less than 1 day of average household use). Continued research in these technologies will help further penetrate existing markets and expand to new geographic markets.

The FPV must be used in conjunction with piping that can withstand freezing, because the valve (or any other freeze-protection mechanism) may fail. One way to assure against catastrophic failure is to use only freeze-tolerant piping (e.g., piping that can be frozen solid hundreds of times without chance of bursting). Three brands of PEX pipe (cross-linked polyethylene) are now under freeze-thaw testing at NREL and are at about 400 freeze-thaw cycles without breaks in the longer lines with fittings as of December 2005. To date, two 5-in. samples of one pipe/connector combination have broken. One brand of pipe has proven freeze-intolerant with all samples busting in under 10 cycles of freeze-thaw. However, PEX piping has an upper temperature of 210°F and must be used with a system that cannot exceed that temperature.