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

Introduction

Water heating accounts for a substantial portion of energy use at many residential, commercial, institutional, and federal facilities. Nationwide, approximately 18% of energy use in residential buildings and 4% in commercial buildings is for water heating. Solar water heating systems, which uses the sun's energy rather than electricity or gas to heat water, can efficiently provide up to 80% of the hot water needs—without fuel cost or pollution and with minimal operation and maintenance expense. Solar currently represents 1% of the water heating market (about 3% of buildings have solar and it provides about 1/3 of the energy for each). In 2003, 11.4 million sq. ft. of collector area were delivered by 27 manufacturers. Most of these were unglazed collectors for swimming pools, a very cost-effective application.

Solar water heating systems can be used effectively throughout the United States at residences and facilities that have an appropriate near-south-facing roof or nearby unshaded grounds for installation of a collector. They are most cost-effective for facilities with the following characteristics:

• Water heating load that is constant throughout the year (not vacant in summer);
• Water heating load is constant throughout the week (to utilize solar heat every day);
• Cost of fuel used to heat water is high (more than $10/Mmbtu or 40.034/kWh), such as electricity, which represents 46% of the water heating market, or propane which represents 2% of the market in remote locations; and/or
• A sunny climate helps but is not required. In 2003, the three largest markets were Florida, California, and New Jersey.

Examples include swimming pools, residences, hotels, laundries, prisons, and kitchens.

Description

Solar water heating is a reliable and renewable energy technology used to heat water. Sunlight strikes and heats an "absorber" surface within a "solar collector" or an actual storage tank. Either a heat-transfer fluid or the actual potable water to be used flows through tubes attached to the absorber and picks up the heat from it. (Systems with a separate heat-transfer-fluid loop include a heat exchanger that then heats the potable water.) The heated water is stored in a separate preheat tank or a conventional water heater tank until needed. If additional heat is needed, it is provided by electricity or fossil-fuel energy by the conventional water-heating system.

Although solar water heating systems all use the same basic method for capturing and transferring solar energy, they do so with three specific technologies that distinguish different collectors and systems. The distinctions are important because different water heating needs in various locations are best served by certain types of collectors and systems.

Materials and components used in solar water heating systems vary depending on the expected operating temperature range.

Low-temperature unglazed systems operate at up to 18°F (10°C) above ambient temperature, and are most often used for heating swimming pools. Often, the pool water is colder than the air, and insulating the collector would be counter-productive. Low-temperature collectors are extruded from polypropylene or other polymers with UV stabilizers. Flow passages for the pool water are molded directly into the absorber plate, and pool water is circulated through the collectors with the pool filter circulation pump. Swimming pool heaters cost from $10 to $40/sq. ft. [2004].

Fig. 1. Small sample of unglazed low temperature solar collector showing flow passages and header pipe.