Optimizing Collector-Loop Design Temperatures

To meet the increased demand for commercial-scale solar water heating systems, designers and installers familiar with the residential solar thermal market require an expanded knowledge base. This includes awareness of new equipment and a firm understanding of the design subtleties associated with larger commercial systems. One notable area of design that can have a significant impact on system energy production is collector-loop temperature and flow rates.

An installer recently asked a question that serves as a good example of this impact. With input from the collector manufacturer, he had installed a 14-collector system designed to heat an indoor swimming pool located near St. Louis, Missouri. The glazed, flatplate 4-by-8-foot collectors were plumbed in a single row in parallel. The system included a collector-loop pump, a pool pump and a stainless steel plate heat exchanger. After the system was commissioned, the temperature differential between the pool water going to the heat exchanger and the outlet temperature of the collectors was as high as 65°–70°F. As designed, this system is not optimized for maximum efficiency.

Collector-Loop Temperature

Excessive collector-loop temperatures indicate lost energy production. The higher the collector temperature, the more heat is lost to the outside air and the lower the system’s efficiency. This is particularly true of flat-plate collector systems. Evacuated-tube systems are negatively impacted by high collector temperatures, but to a lesser degree than flat-plate units because of the superior insulation of the evacuated collectors’ vacuum tube construction.

In the system under consideration, a more ideal temperature differential between the cold side of the heat exchanger and the collector outlet would be approximately 30°F when the system is exposed to full sun. This lower temperature differential would indicate that the system is operating efficiently, without excessive heat loss to the environment. A 30°F differential also indicates that the system’s flow rate is sufficient, but not high enough to cause short cycling of the control during cloudy weather. In addition— and most importantly—a system that is operating with a 30°F differential produces thousands more Btu per day than a similar system running at a 65°–70°F differential.

The corresponding heat gain associated with lower collector temperatures is illustrated and quantified by the Btu production output matrix of the Solar Rating and Certification Corporation (SRCC)-certified collector shown in Figure 1. Although the SRCC constructs its matrixes using the difference in the inlet temperature and the ambient temperature (not the inlet and outlet temperatures), the concept is the same. For reference, a 4-by-10-foot flat-plate collector with a differential of 36°F produces approximately 13,000 more Btu per day on a mildly cloudy day (1,500 Btu/sq. ft. per day insolation) than if it was operating with a differential of 90°F.

Low Collector-Loop Flow Rate

Excessive collector-loop temperatures are usually caused by lower-thanoptimal flow rates through the loop’s piping. Specified flow meters should be installed in all solar water heating systems according to collector manufacturer’s instructions. Without flow meters, suboptimal flow rates can be detected only by monitoring the system’s temperature differentials. Several factors may contribute to non-optimal flow rates and each illustrates a design best practice.

Pressure drop. As additional collectors are plumbed in parallel in a single row, pressure drop within and between collectors increases. I suspect the number of collectors plumbed in a single row is the likely cause of the excessive temperature differential in the example system. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ashrae.org) Active Solar Heating Design Manual recommends a maximum of eight collectors in a single row. Although this standard varies based on the collector and the collector header sizes, I would never recommend installing more than ten 4-by-10-foot collectors with 1-inch headers in a single row.

Some manufacturers may approve a design with more than 10 collectors in a row, but they do so at the peril of system production. The flow in any set of collectors is always greater with fewer of them plumbed in parallel. The example system would have a better flow rate if it was configured as two rows of seven collectors. The resulting increased flow rate would bring down the collector-loop operating temperature and significantly increase annual production.

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