Passive Solar Heating Basics

Categories: Construction Methods

The very root of what makes a home green is how effectively it responds to its surrounding environment. You could say that this has defined the primary material pursuit of mankind for all time – building better shelters to keep us warmer, cooler, and drier. Many of the native building techniques employed centuries ago are still reliable in similar climates today, and used as optimal models for environmentally conscious architects. That said, since global acceptance of the air conditioner in the 1950s, the benefits of responding to a unique climate have been left by the roadside.

The principles of passive solar heating, such as basic types of systems, their description, and the components making up any passive system are presented in this course. Sources of data for heating requirements and available solar radiation throughout the U.S are identified and discussed along with a method for estimating the rate of heat loss from a home. The use of these three inputs in a method for estimating performance of a passive heating system of specified size at a specified location is presented.

Passive Solar Heating Definition

Passive solar heating of a building means using the sun’s radiant energy to provide heat by converting the radiant energy to thermal energy (heat) when it is absorbed by the building. Some of the incoming thermal energy may be used to directly heat the building and some may be stored in components of the building. In a completely passive system, energy flow within the building is by natural means (conduction, natural convection and radiation) only. An active solar heating system, on the other hand, uses devices like blowers, pumps and/or fans to move heated fluid, from the collectors to the heated space, from thermal storage to the heated space, and from the collectors to thermal storage. 

Components of a Passive Solar Heating System

A passive solar system is made up of components that have functions very similar to the components of an active solar heating system, but those components look much different and are arranged much differently. The typical components of either a passive or active solar heating system are: aperture (opening for solar radiation to go through, absorber (to absorb the radiant energy and convert it to thermal energy, thermal mass (for storage of excess thermal energy for later use), distribution system, controls, and a backup heating system. In a passive solar heating system, the aperture and absorber are separated physically, while in an active system they are typically both part of the collectors.

The aperture(s) in a passive solar heating system will be south-facing window(s). It is important that these windows not be shaded by other buildings or trees from 9:00 a.m. to 3:00 p.m. during the heating season. In a passive system, the absorber and the thermal mass for storage are both part of the same unit(s), which are components of the building such as floors and walls in the direct path of sunlight. The floor and/or wall surfaces are typically dark colored so that they will absorbs solar radiation well and convert it to thermal energy, which is stored in the mass of the floors and/or walls. In an active system, the absorber is typically part of the collector and the heat storage system is a separate unit.

A distribution system is used to circulate heat from the collection and storage points to different areas of the house. In a strictly passive system, heat will be circulated solely by one or more of the three natural heat flow methods, conduction, natural convection, and radiation. Fans, and/or blowers are sometimes used to help with the distribution of heat throughout the house in an otherwise passive system. In a passive heating system, Controls include items such as operable vents or dampers, moveable window insulation, and/ or roof overhangs or awnings that shade the aperture during summer months. For active systems and nearly passive systems that use fans and/or blowers, controls typically include electronic sensing devices, such as a differential thermostat that signals a fan to turn on or off or a damper to open or close. The backup heating system for either a passive or active system may be any type of non-solar heating system.

Basic Passive Solar Heating Systems

Types The five basic types of passive solar heating systems are: direct gain, thermal storage wall, attached sunspace, thermal storage roof, and convective loop. Each of the types contains the components described above. Three of these types, thermal storage wall, attached sunspace and thermal storage roof, are sometimes called indirect gain systems. The convective loop is sometimes called an isolated gain system. A brief description of each of the five types follows. 

Direct Gain is a simple, straightforward approach to passive solar heating. In this type of system, sunlight enters the living space through south facing windows during the daytime. The incoming solar radiation strikes the walls, floor, and/or ceiling (thermal storage mass) of the living space directly or is reflected to them and is absorbed and converted to thermal energy in that thermal storage mass. The heated wall, floor, and/or ceiling will keep the living space warm. Also, during the daytime, when the storage surface temperature is high enough, heat will be conducted from the hot surface of the walls, floor, and/or ceiling to their interior, where it is stored. When the surface of the thermal storage mass are no longer being heated by the suns rays, the room and the wall, floor and/or surface temperatures will decrease and heat will be conducted from the heated interior of the thermal storage mass to the cooler surface and will heat the living space by convection and radiation. 

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