Solar Hot Air

Solar hot air systems use the sun’s thermal energy to heat up air to supplement your space heating requirements. They can directly heat individual rooms or pre-heat supply air for the whole house. These systems are the cheapest among PV and solar water systems. Their energy conversion efficiencies are lower than solar hot water systems, but higher than solar PV systems.

Basic components

Solar hot air systems typically consist of three basic components: a solar thermal collector, a distribution system, and an optional heat storage unit. Other peripheral elements include fans, controls, filters, and/or dampers.

SOLAR COLLECTOR

collects and transfers heat from the sun to air for distribution within the home.

DISTRIBUTION SYSTEM

circulates hot air to a heat exchanger, heat storage element, or directly to the indoor environment using ducts and fans.

HEAT STORAGE (OPTIONAL)

thermally conductive or phase changing materials that can be used to store heat from the solar collector for distribution at a later time.

 

Solar Collector

Solar Hot Air Collector - greenbuildingadivor.com

Solar hot air collectors are typically mounted on a home’s wall or roof, and consist of an insulated box with one face having a glass to allow for sunlight penetration, while the inside has a solar absorbing material to collect the sun’s heat. The solar absorbing material heats the air that passes through the box, which can then be distributed into the home.

Transpired collectors are dark metal walls with holes in them that are placed on the south facing wall to preheat ventilation air for buildings. They’re more commonly associated with commercial and industrial applications, where ventilation requirements are large, though smaller transpired air collectors could be used in residential applications.4

 

Distribution Systems

Solar Distribution Example

Distribution of the heated air into the home can be accomplished using either active or passive systems. In active systems, air is passed through the collector and into the home using an electric fan or blower. In passive systems, hot air rises and leaves through the top of the collector and cooler air flows naturally into the bottom of the collector. Passive systems are inherently less efficient than active systems, but they are cheaper to install since they do not require mechanical elements. 

Heat Storage

PCM Example

Heat storage allows your solar collector to store heat for later distribution after the sun goes down. Historically people have used rocks as a thermal mass source in thermal hot air systems, but because condensation on the rocks can lead to mold growth, it is not a recommended thermal storage method any more. More recently systems use phase changing materials (PCM) to store heat. PCMs change phase and store heat at a certain temperature.

Cost and Payback

The cost of a solar hot air system is dependent on its size. For example, a 3.6 x 7.7 ft system can collect 3450 BTU per hour thermal energy and cost approximately $1,500. It’s difficult to determine their payback without knowing weather, local energy cost, and other factors.

Getting started

If you wish to install a solar hot air collector for your home, you should find a trusted contractor in your area.10

 

Resources

1 ASHRAE Solar Variables Presentation 

PowerPoint for understanding solar variables (complements of ASHRAE)

2  "The Zen of Passive Heating Panel Design" by Morris R. Dovey

A passive design strategy presented by a home owner that implemented an active solar collector approach with natural convective air movement. This website presents a step-by-step assembly of the system.

3 "A Consumer's Guide, Heat Your Water with the Sun" by National Renewable Energy Laboratory

A guide to implementing solar water heating technologies including background, investment inventives, how to select a solar heating contractor, and other information related to solar water heating. 

"Active Solar Heating" by The Department of Energy

Introduction to solar heating. 

5 http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm “Solar Collector Efficiency Calculator” by BuildItSolar.com

Calculate the average efficiency and heat output of a solar thermal hot water system using ambient air temperature, collector temperature, and solar intensity.

“Using Phase Change Materials (PCMs) For Space Heating and Cooling in Buildings” by University of South Australia

Research conducted by Dr. Frank Bruno at the Sustainable Energy Centre concerning PCMs and their use in various applications.

7 “DIY Solar Air Heating Collectors: Pop Can vs Screen Absorbers” by Build It Solar

Introduction into solar hot air heating followed by simple solar hot air collector instructions, costs, and conclusions.

8 “Renewable Energy Installers” by Green Energy Ohio

A list of solar installers in Ohio. 

"Solar Hot Air Collectors" by Green Building Adivsor

Thorough review of solar hot air systems and the effectiveness of the technology.