Some Active Solar Heating
Design Considerations, Part I
In Western Colorado, solar is an abundant free energy source. Active solar heating systems have recently become more affordable for more home and business owners as a result of the 30% federal tax credit and other installation and energy efficiency rebates. Consult www.dsire.org and www.coseia.org for specific incentives.
Solar Domestic Hot Water (DHW) system kits can be bought pre-packaged from plumbing or solar suppliers to replace the existing water heater. Space can be an issue, as the components take up more room than an ordinary water heater. With more panels, solar can also be used as the primary heat source for the entire house (including DHW). Notice I say “primary”. Even a well designed solar heating system will need some sort of backup heat source to get through cold, cloudy periods; usually a boiler is used for this purpose. Unlike with DHW, solar whole-house heating systems must be designed to match the target building. Additionally, the solar plumbing and the control systems need to integrate seamlessly with the boiler subsystem, which is why an experienced solar heating professional should be involved in their design.
One can place whole house solar heating systems into two categories: new construction, and retrofits to existing buildings. Adding solar heating to existing buildings is possible, but the building must already have in place, or be modified, to address the following considerations. Performance of an active solar heating system is greatly affected by:
Site layout, including building orientation, surrounding terrain (hillside, cliff, etc,), trees and shading.
Architectural design and features, especially roof area/slope.
Passive solar inputs, nighttime heat losses, glazing type/area/orientation.
Insulation design, type, and installation quality.
The type of heat delivery system already installed (radiant floor is best, as explained later).
These considerations are best dealt with in the initial building design. Consult with a solar designer very early in the architectural planning stages (before drawings are started!).
Solar heat storage is perhaps the biggest issue for solar heating. The challenge is to store the heat when the sun is shining (when heat is least needed), and later delivering it into the building envelope when it is coldest (and the sun isn’t shining). The two most common storage media for active solar heating systems are large hot water storage tanks and floor mass.
Hot water tanks have these advantages: high heat capacity (water has by far the best heat capacity per unit volume), and the ability to pipe that heat to the house where and when it is needed. Hot water storage tanks allow variation of day and night time temperatures to match your preferences. Tank disadvantages: they’re expensive, and they take up space.
Floor mass storage can eliminate the need for a water tank. Though non-water (masonry materials/sand/dirt) don’t have as good heat storage capacity per unit volume, they are comparatively affordable. A challenge with “floor mass storage” is that the floor may take a long time to heat up, and fine temperature control, for instance, cooling down the living space at night for comfort, is harder (but still possible!).