Sizing Active Solar Space Heating Systems

In an active solar space heating system, fans or pumps move heated air or liquids directly from a solar collector directly to the house for heating or to a storage medium, such as a rock bin or water tank. The amount of collector area needed depends on many variables: available solar gain, collector efficiency, local climate, the total house heating load, insulation levels, degree of airtightness of the house, and the lifestyle of the residents. Generally, the solar collector area is equal to 12 to 30 percent of a house's floor area.

Many tools have been developed to help size active solar space heating systems. These include manual calculations, methods for pocket calculators, and computer software. See the Computer Software for Solar Energy Analysis and System Design: Resource List for more information.

General Rules for Sizing Active Air Systems

1. The volume of a rock bed-heat storage unit should be between 50 to 100 percent of the collector area (ex: 0.5 to 1 cubic foot of storage volume per square foot of collector area).
   
   
2. The air flow rate through standard collectors should be 1 to 3 cubic feet (0.03 to 0.76 cubic meters) per minute for each square foot (0.09 square meters) of collector. The velocity should be 5 to 10 feet (1.5 to 3.1 meters ) per second.
   
   
3. Air entering a collector at 70 degrees F (21.1 deg. C) is typically warmed an additional 70 – 90 degrees F (39 – 50 deg. C.)

General Rules for Sizing Active Liquid Systems

1. The storage capacity of the solar tank should be between 1.5 and 2 gallons per square foot (61 and 81 liters per square meter) of net collector area. This allows a small amount of hot water storage for one or two cloudy days.
   
   
2. The flow rate should be between 0.02 and 0.03 gallons per minute per square foot of collector when water is the heat transfer fluid (0.82 to 1.22 liters per minute per square meter of collector). Other flow rates apply for different heat transfer fluids.
   
   
3. Total flow rate, used to size the collector pump, is the product of the above flow rate times the total collector area.
   
   
4. Under normal operating conditions, the solar collector should warm the heat transfer fluid by 10 to 20 degrees F (5.6 – 11 deg. C) near noon on a sunny day.

This fact sheet was reviewed for accuracy in January 2003.

NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

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