The National Renewable Energy Laboratory's Guide to Solar Water Heating

Mid-temperature systems produce water 18 to 129°F (10 to 50°C) above outside temperature, and are most often used for heating domestic hot water (DHW). However, it is also possible to use mid-temperature solar hot water collectors for space heating in conjunction with fan-forced convection or radiant floors.

Mid-temperature collectors are usually flat plates insulated by a low-iron cover glass and fiberglass or polyisocyanurate insulation. Reflection and absorption of sunlight in the cover glass reduce the efficiency at low temperature differences, but the glass is required to retain heat at higher temperatures. A copper absorber plate with copper tubes welded to the fins is used. In order to reduce radiant losses from the collector, the absorber plate is often treated with a black nickel selective surface, which has a high absorptivity in the short-wave solar spectrum, but a low-emissivity in the long-wave thermal spectrum. Mid-temperature systems range in cost from $90 to $120/sq. ft. [2004] of collector area.

Fig. 2. Small sample of mid-temperature flat plate collector showing cover glass, insulation, and copper absorber plate and flow passages.

High-temperature systems utilize evacuated tubes around the receiver tube to provide high levels of insulation and often use focusing curved mirrors to concentrate sunlight. High temperature systems are required for absorption cooling or electricity generation, but are used for mid-temperature applications such as commercial or institutional water heating as well. Due to the tracking mechanism required to keep the focusing mirrors facing the sun, high-temperature systems are usually very large and mounted on the ground adjacent to a facility. Evacuated tube collectors themselves cost about $75/sq. ft., but use of curved mirrors and economies of scale get this cost down for large system sizes to a relatively low cost of $40-70/sq. ft. [2004].

Fig. 3. Close-up view of an evacuated glass tube with black copper absorber plate inside.

A. Components of a Solar Water Heating System

Solar Collectors Solar collector efficiency is plotted as a straight line against the parameter (Tc-Ta)/I, where Tc is the collector inlet temperature (in °C), Ta is the ambient air temperature (in °C), and I is the intensity of the solar radiation (W/sq. m.). Notice that inexpensive, unglazed collectors are very efficient at low ambient temperatures, but efficiency drops off very quickly as temperature increases. They offer the best performance for low temperature applications, but glazed collectors are required to efficiently achieve higher temperatures.

In addition to solar collectors, all solar hot water systems have thermal storage, system controls, and a conventional back-up system.

Thermal storage Storage is generally required to couple the timing of the intermittent solar resource with the timing of the hot water load. In general, 1 to 2 gallons of storage water per square foot of collector area is adequate. Storage can either be potable water or non-potable water if a load side heat exchanger is used. For small systems, storage is most often in the form of glass-lined steel tanks.

Controls Active systems have a "delta T" (temperature differential) controller to start and stop the pumps. If the temperature in the solar collector outlet exceeds the temperature in the bottom of the storage tank by a set amount (say, 6°C), the controller starts the pump. When this temperature difference falls below another set value, say 2°C, the controller stops the pumps. The controller will also have a high-limit function to turn off the pumps if the temperature in the storage tank exceeds a third setting, say, 90°C. Due to the simplicity and low cost of a delta-T controller, it is wise to keep controls independent of any whole-plant energy management system, although it is desirable to include some indication of system performance, such as output from a BTU meter or preheat tank temperature in the building control system.

Conventional Back-Up Heater Solar water heaters save energy by preheating water to the conventional heater. Solar DHW systems are usually designed to meet 40% to 70% of the water-heating load. A back-up, conventional heater is still needed to meet 100% of the peak hot water demand for cloudy days or for when the solar system is down for service.