Extension Bulletin 0948 Solar Flat Plate Collectors Steve Denner and Stan Price-Washington State University

The flat plate collector performs an important func­ temperatures can exceed 300 oF. Special care should tion in a solar space or water heating system. The flat be taken to prevent stagnation of collectors built with plate collector absorbs solar energy and converts it to wood. Repeated exposure of wood to high tempera­ heat. This heat can be transported from the collector tures can lower wood's combustion temperature, for immediate use or stored for later use. thus creating a potential fire hazard.

There are many different types of solar flat plate col­ Glazing lectors available commercially. These solar collectors The metallic flat plate collector is covered by a can be generally grouped into two categories; metallic transparent or translucent glazing material. The glaz­ flat plate collectors and those made from non-metallic ing material transmits the solar radiation onto the col­ materials. Both metallic and non-metallic collectors lector plate and helps trap heat within the collector. have proven to be successful in both space heating In Western Washington, the use of a single layer of and domestic water heating applications. In either glazing is usually cost effective for solar water heating case, consumers should investigate manufacturers' systems. The added insulating value of a second layer warranties and expectations of product life. The well­ makes double glazed collectors more appropriate for constructed flat plate collector can offer effective per­ solar space heating systems. formance over a lifetime of 20 to 25 years. There is a relatively wide choice of glazing materials Metallic Flat Plate Collector Design for use with flat plate collectors. Glass, acrylics, poly­ The basic design of the metallic flat plate collector carbonates, fiberglass reinforced plastics, and thin consists of an absorber plate housed in a well­ clear films have all been used as collector glazing. insulated box. The absorber plate is most often made Four criteria should be considered in selecting a col­ of copper, aluminum, or steel, which have good heat lector glazing: 1) it must transmit most of the solar absorbing and conducting properties. The metal ab­ radiation; 2) it must retard the outflow of heat from sorbing plate is typically painted flat black to heighten inside the collector; 3) it must withstand environmen­ solar absorption. Instead of using flat black paint, tal conditions such as wind, hail, ultraviolet light ex­ some flat plate collectors coat the absorber plate with posure, and stagnant collector temperatures; and 4) a selective surface coating such as black chrome. cost. While selective surface coatings do increase collector efficiency in most operating conditions, selective sur­ Aid Versus Liquid Flat Plate Collectors face coatings are not essential for good collector per­ Some metallic flat plate collectors use liquid to collect formance. There have been instances in the past the heat from the absorber plate. Liquid-type collec­ where selective surface coatings have deteriorated tors commonly use water, or a water/anti-freeze mix­ and flaked off the absorber plate. It would be wise to ture, or a silicone fluid as the heat transport medium. check the warranty of the collector before buying a Copper tubes transport the liquid over the surface of selective surface coated collector. the collector plate. Heat from the absorber plate is conducted to the liquid in the tubes. In other flat plate The absorber plate is housed in a collector box most collectors, air is blown over the entire surface of the commonly made of wood, aluminum, or plastic. The absorber plate to collect heat. Often, the surface of box must be constructed to be weathertight to avoid the absorber plate is made irregular by corrugation or air or moisture infiltration which could lower collec­ piercing the plate. This sets up a turbulent air flow tor efficiency. The collector box is insulated on the which increases air to surface contact and heightens back and sides to minimize heat loss. Commerical flat efficiency. plate collectors typically use polyurethane, iso­ cyanurate, urethane board, or fiberglass insulation. Both the liquid type and air type flat plate collectors The insulation used in the collector should be able to have advantages. The air type flat plate collector is withstand high collector temperatures. In a stagnant free of worry from the problems of freezing and cor­ collector in which no heat is being drawn off, rosion that can damage liquid collector systems.

College of Agriculture Washington State University Pullman CooperativeExtension In cooperation with the United States Department of Agriculture. @ Issued in furtherance of the Acts of May 8 and June 30, 1914, by Washington State Cooperative Extension, J. 0. Young, Director