Solar Thermal, Water, and Cold Climates: Can't We All Get Along?
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36,597 square feet of Ritter XL solar collectors were installed on an exhibition hall in Wels, Austria, providing almost 7 million Btu/hr of supplemental hot water for district heating. Photo: Ritter Group Most solar thermal sy... . Focused energy: Compound parabolic concentrating collectors The heart of the Ritter XL system is the company's evacuated tubes. Ritter makes evacuated tubes for companies such as Bosch, Buderus, and others, but the ones used in the XL system have a larger diameter and are made specifically for large-scale use. The tubes are mounted on a CPC mirror that focuses any light that misses the tubes back onto them at a perpendicular angle to maximize efficiency. The tubes are easy to switch out if they get damaged, but the collectors come preassembled. This makes the collectors a bit heavier (about 140 lbs.) and trickier to install than normal evacuated tube systems where the tubes and frames are installed separately, but they are rated with an impressive Solar Rating and Certification Corporation (SRCC)-rated output of 55,000 Btus per day. New York City subway maintenance plant uses 48 Ritter XL compound parabolic concentrating (CPC) collectors to supply almost 2000 gallons of hot water a day for washing the city's subway cars. Photo: Ritter Group Water instead of glycol? Ritter XL is the only large-scale system that uses water as a heat-transfer fluid. Most of these solar thermal systems use propylene glycol (antifreeze), which protects them against freezing, but, according to Michael DiPaolo, president of Regasol USA, water is more efficient at storing and transferring heat; is less viscous so it saves on pump energy; and it doesn't break down at high temperatures like glycol does. (Glycol breaks down and becomes acidic at high temperatures sometimes generated in solar thermal systems, damaging collectors and pipes.) In addition, water can be plumbed directly into boilers in some instances, completely bypassing heat exchangers. Can it overheat? Nope. The Ritter system doesn't require any protection from overheating. The controller just shuts down the pump, and the steam pushes the water out of the collectors and pipes into an expansion tank. When the system cools off again, it restarts. Freezing is the real concern. Yes, freezing is the big concern with solar thermal systems. On a commercial-scale system a broken pipe on a roof midwinter can be a nightmare. Ritter XL manages the risk via a digital controller, precise engineering, and a small amount of stored hot water. According to DiPaolo, "Our controller monitors pressure, temperature, time of operation, flow direction, and about ten other parameters." The controller is constantly analyzing water temperature along the pipes and when it gets cold enough the system sends the minimum amount of water necessary to keep the system clear. The engineering to pull off this balance while not flushing hot water through the pipes constantly is impressive. The company claims this freeze protection uses about 2%–4% of the annual solar heat gain, and that these losses are more than compensated by the efficiencies of using water. And as insurance against, freezing, the system requires an uninterruptible power supply in case the power goes out. Does it work and what will it cost me? So far the systems appear to be working as advertised, with more than 50,000 residential and commercial systems installed in Europe, including a 36,597-square-foot district heating installation in Wels, Austria. They also installed 48 collectors New York City subway maintenance plant on Coney Island that supplies almost 2000 gallons of hot water a day for washing subway cars. DiPaolo said the systems cost about $200 per square meter installed (about $19 per square foot) installed, and Ritter incorporates all of the engineering into the price of each system. Vanir Construction Management installed Ritter XL on six YMCA buildings in Winston-Salem, North Carolina--five have 50 panels and around 600 gallon of storage; the sixth has 210 panels. Donald Haase, senior project manager at Vanir, is impressed by the system, adding that his buildings are unique. "They are challenging because they are very dynamic systems, and the demand for hot water varies depending on site, day, and clientele," he said. "On a typical day we see 25 therms a day coming out of these systems, and we are still fine-tuning them." According to Haase, initial results indicate approximately a 51% savings in natural gas consumption over the previous year. Regasol is going to be bringing this technology to the residential market, possibly by the end of 2011, but is awaiting SRCC-OG-300 certification. The hold-up? Water is frowned upon by SRCC in cold climates, understandably, and it will be up to Ritter to convince them--and a conservative solar thermal industry--that the technology works. Brent Ehrlich is the products editor at BuildingGreen, Inc.