U.S. Department of Energy - Energy Efficiency and Renewable Energy

Solar Energy Technologies Program

Current Research on Solar Heating

A researcher examines polymer glazing samples from a testing chamber that subjects polymer glazing samples to UV light carefully matched to the spectrum of natural light.

The National Renewable Energy Laboratory and Sandia National Laboratory support the development and testing of new solar technologies for buildings applications, to help consumers and companies reduce their energy costs, and help the nation reduce it dependence on foreign oil. Current research includes following:

• Innovative concepts to reduce the cost of solar water-heating systems
• New applications for solar water-heating systems
• The use of plastics in residential solar water-heating systems
• Powering air-conditioning systems using solar-energy systems, especially focusing on compound parabolic concentrating collectors
• The use of flat-plate collectors for residential and commercial hot water
• Ventilation air preheating for commercial buildings using transpired air collectors
• Concentrating and evacuated-tube collectors for industrial-grade hot water and thermally activated cooling.

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.

Figure 1.

On the left: Freeze probability map for ¾-in. pipe with 1-in. insulation, with zero-freeze-probability areas in green. On the right: Water-consumption map, with the areas consuming fewer than 1,000 gal/year highlighted in green. Passive systems using insulation should be installed only in the green areas.

Figure 2.

An indirect thermosyphon system with unpressurized storage is shown, with a freeze protection valve mounted at the roof-line immediately before the return pipe enters conditioned space. The valve vent should also proceed downward into the house to a drain to avoid ice damming.

Materials testing

Durability is the key issue in the manufacturing and use of low-cost polymer materials. Continued research his helping identify optimal materials for these applications. To identify and predict the useful lifetime of the materials, NREL tests glazings and absorbers for optical and mechanical degradation. Passive solar water-heating and active cold-climate solar water-heating technologies require glazing and absorbers that operate in harsh environments. The research efforts include analysis of glazings and absorbers.

• Glazings. These glass or plastic covers are subjected to light and heat weathering using three complementary forms of exposure. They are tested outdoors, in accelerated weathering chambers, and at the NREL UV-concentrator facility. NREL has identified many glazing materials and coatings that won't work well and several that will work well and last at least 20 years in a solar domestic water heater. Particular emphasis has been on testing UV coatings on polycarbonate glazings.

• Absorbers. These dark-colored objects soak up heat in solar collectors and are tested for thermal stability in dry (up to melting point) and wet chambers (95°C) at NREL.

Sandia National Laboratory (SNL)

Manufacturing assistance

The SNL Advanced Manufacturing Center works with manufacturers of solar-thermal and other related products on technical issues involving product realization. This includes product concept, design prototyping, testing, manufacturing processes, production, field evaluation, and disposal.

SNL lends its expertise in the following areas:

• Manufacturing engineering
• Machining
• Welding
• Corrosion
• Electronics and materials manufacturing.

Companies benefiting from SNL manufacturing assistance include Sun Systems, Sun Trapper Solar Systems, Thermal Conversion Technology, and Industrial Solar Technology.