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Solar Thermal at CCHRC
The CCHRC Research and Test Facility currently has two solar thermal systems installed and operational. They consist of the two basic market leading technologies for the production of hot water in residential applications. One collector is an evacuated tube (heat pipe) type consisting of a 20-tube arrangement with a pump-station and a differential pump controller. The other collector is a 4’x8’ flat plate type with an integral photovoltaic panel (PV) that powers a DC pump.
The flat plate collector is the oldest and most predominant type of solar thermal system. CCHRC has only the one 4’x8’ unit, whereas a typical house would have two units in series. The system is antifreeze filled and uses the solar powered DC pump to move the fluid from the collector to the storage tank heat exchanger. No controls are used to operate the system. The photovoltaic panel is sized proportionately such that if there is enough sunshine to make a small amount of hot water, then the panel will provide enough power to pump the system by an equally small amount. As a result, the system is self-regulating and will operate in a power outage situation. This system heats only domestic water and has achieved temperatures in excess of 140° F.
A far north anomaly occurs in fall and spring when clear, sunny days combine with very cold outside air temperatures. The PV panel efficiency increases with cold temperatures, while the solar thermal collector efficiency decreases. As a result, the PV pump outperforms the system design thereby circulating hot glycol up to the cold collector where the heat is then radiated back to the atmosphere. This phenomenon is seasonal and short in duration, lasting from 2 to 20 minutes.
The evacuated tube system is more highly controlled and monitored. A sensor in the header at the collector determines a minimum turn on temperature. Sensors in the supply line, return line, and the storage tank, feed information to a differential temperature controller that operates a line voltage pump which transfers the heat to the storage tank. The concept of the evacuated tube design revolves around the ability to operate in lower ambient outside temperatures, as well as having the means to produce higher absolute water temperatures. As it is essentially operating inside an environment similar to that found within an insulated thermos bottle, the absorber heat pipe within the tube retains more of the incoming energy by not re-radiating back to the atmosphere as readily.
Should a power outage occur, the system will reach very high (250-300° F) "stagnation" temperatures. This has happened on one occasion. As previously mentioned, the collector can gather more heat from the sun in cold outside air conditions, provided the glass encasing the collector tubes remains clear of frost and snow.