Distributed Renewable Energy Operating Impacts and Valuation Study

Section 2
• Thermosyphon systems are an economical and reliable choice, especially in new homes.
These systems rely on the natural convection of warm water rising to circulate water
through the collectors and to the tank (located above the collector). As water in the solar
collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the
cooler water flows down the pipes to the bottom of the collector, enhancing the circulation.
Some manufacturers place the storage tank in the house’s attic, concealing it from view.
Indirect thermosyphons (which use a glycol fluid in the collector loop) can be installed in
freeze-prone climates if the piping in the unconditioned space is adequately protected.
Typical Installations
In general, SHW systems are mounted on a south-facing roof, or adjacent to the house at ground
level. In either case, the SHW system is generally remote from the backup and supplementary
storage water heater and its tank. This distance, or the amount of finished space the loop must
traverse in a retrofit installation, impacts the method and cost of installation. The most
fundamental distinction is between systems that must resist freezing (closed-loop systems), and
those located in climates where freezing is very rarely severe enough to threaten the integrity of
the system (open-loop systems). Because closed-loop systems require either drain-back
provisions or a separate freeze-protected loop to indirectly heat water in the storage tank, they
generally have active components (pumps) and are more complex.
Current Market
Currently, the U.S. market for SHW systems, excluding pool heating, is in the range of 6,000
units per year, with more than half of these sales in Hawaii. 2 This number compares with annual
sales of almost 10 million conventional gas and electric storage water heaters. 3 In general, SHW
systems have not been a priority for many organizations seeking to promote energy conservation.
Indeed, the principal solar trade association, the Solar Energy Industries Association (SEIA),
gives this set of technologies only passing reference. Groups that have been more active in
promoting, testing, and/or certifying solar hot water technologies include the Florida Solar
Energy Center and the SRCC.
SHW technology is relatively simple and the materials and manufacturing involved have been
well understood for decades. Historically, market penetration and promotional activity have
depended primarily on financial incentives that lower the up-front cost burden to consumers. 4
System Performance
The SRCC currently administers a certification, rating, and labeling program for complete SHW
systems. Appendix G presents the SRCC certification information. The SRCC also provides
estimates of annual SHW system performance in the Phoenix area. SRCC uses a computer model
to estimate the thermal performance of SHW systems under specified conditions.
A total of 445 different systems, produced by 23 different manufacturers, have been rated by the
SRCC for Phoenix. Table 2-4 shows the spread of energy savings for these units. According to
the SRCC, over two-thirds of the installed units save their owners more than 2,700 kWh per
year.
2 U.S. Department of Energy, Solar and Efficient Water Heating, a Technology Roadmap, Washington, D.C., 2005.
3 American Council for an Energy-Efficient Economy (ACEEE), Emerging Technologies Report: Solar Water
Heaters, April 2007.
4 ACEEE Emerging Technologies Report: Solar Water Heaters (2007) and discussions with manufacturers..
2-18 R. W. Beck, Inc. Arizona Public Service