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Ironically, there is really nothing passive about passive solar heating. Sure there are no motors, pumps, fans, blowers, or other electrical components involved, but on a broader scale, utilizing solar power in any capacity is actively improving your home. You might say that you are making your home smarter, more efficient, and a lot more sustainable. On top of that you’ll save big bucks on energy bills.

Sounds great. But how does it work? How do we get hot water and comfortable living space without the use of a single furnace, pump, or fan? Below I will attempt to divulge the Whats, Whys and Wherefores of passive solar heating; divided into two distinct categories: water heating and space heating. The following tutorial should give all the basics you’ll need to understand the art and architecture of passive solar heating. Questions beyond what I can say here — such as those specific to your home or region — should be taken up with a qualified, local solar installer.

WATER HEATING

Of the three main ways to collect solar energy to heat water supply, there is one specifically designed for passive water heating. Fortunately for this tutorial, it is also the simplest design (no moving parts has its advantages). This passive solar system is known as an Integral Collector Storage (ICS) system. It is integral because the water, solar collector, and glazing are all essentially the part of the same body.

There are several advantages to an ICS system, such as its energy-free state, simplicity, and low cost, as well as some limitations, which we’ll get to shortly. In the meantime, here is how it works:

• Water is fed into the system from a tank (off-grid) or water supply.
• The system consists of a tank — painted black, holding 30-50 gallons of water — and surrounded by a clear glazing or mounted in an insulated box with glazing on its south face.
• Water is fed through pipes into the bottom of the tank to be heated.
• During the day sunlight shines through the glazing and is absorbed by water held within the black tank.
• Due to convection — a heated fluid’s tendency to rise — the hot water is drawn from the top of the tank.
• Cold water for the house simply continues right on through the bottom of the tank without being heated.

ICS systems are really that simple. Many homeowners, at least those not looking to heat that much water, choose to build the units themselves. More complex systems exist that will run water through the collector and then on to a storage tank inside the home where the water will remain warm for use at night. Otherwise water in an ICS system will cool at night and only be available during the day.

The main limitation for ICS units referred to earlier lies in water’s tendency to freeze. For this reason, ICS systems are typically used only seasonally in temperate and colder climates. They can be used year round in arid or tropical climates. Also, a good amount of sunlight is needed to provide an adequate hot water supply — another reason that ICS units are most popular in regions like the desert southwest.

PASSIVE SPACE HEATING

Passive solar space heating has a much more diverse spectrum of applications. In fact, every home in existence uses passive solar energy to some extent — it’s the reason we turn on the AC in summer. Yet when we design our homes with passive solar heating in mind, we can manage to make good use of solar heat in winter and cooler months while minimizing it in summer. So in a way passive solar heating automatically comes with some cooling elements to its equation.

Passive space heating involves strategic design and placement (in relation to the sun), as well as the material makeup of the structure. That is, the thermal properties of walls and floor are every bit as important as windows to a passive solar home.

Design and Placement

A passive space heated home will be founded on an east-west axis with a good portion of exterior square footage facing south toward the sun. Rooms on the south side will be living rooms, bedrooms, and other places in need of light and heat. An open floor plan is also crucial to allow for optimal collection of solar energy. Concrete floors and walls, for example, are often used because they absorb heat well and slowly release it into the ambient indoor air. The north side will tend to have a much smaller footprint (in area) and have less windows so that precious heat has less chance to escape “out the back door.” The home will also strategically incorporate trees and foliage to bring shade during hot summer months when solar space heat gets a big No Thank You.

There are three main ways to capture solar energy for space heating. These are:

Direct Gain

The direct gain approach is, well, direct. The home itself is the solar collector. It will consist of plenty of south-facing windows, thermal mass floors and walls to collect heat. Skylights are another popular way to collect heat, although they also tend to be routes for heat loss too.

Indirect Gain

Indirect gain adds storage to the equation. That is, such a system will store solar heat between the south-facing windows and the living space. Take, for instance, a Trombe wall, the most common indirect approach. A Trombe wall is a thick masonry wall — concrete, CMU, etc. — that has a glass layer on its south (exterior) side. This glazing collects solar heat, which is then trapped by the masonry wall and slowly released into the home as warm, comfortable air. Other indirect gain options include water walls, roof ponds, and others.

Isolated Gain

In isolated gain space heating systems, the actual solar-collecting structure is separate from the main house. These typically manifest themselves as sunrooms or solar greenhouses. The sunspace uses some type of thermal mass — be it trombe wall, water wall or other — that is lined with glazing on at least its south side. The thermal mass absorbs the solar heat (indeed these rooms will usually get quite hot during the day). That heat is then slowly released into the living space at night when heat is needed.

Water and Space

Passive solar space heating works best and installs easiest with new construction, but incorporating passive design elements into the home is always possible. Remember that space heating and cooling, as well as water heating, account for close to half the energy costs for the average family home. Incorporating passive solar design can and will have a huge impact on your home’s efficiency.

For folks looking to upgrade or remodel rooms, install new windows or skylights, anything to help improve the solar quality of your home, be sure to address your home’s overall efficiency first and foremost. That is, a home with a hundred south-facing windows backed by water walls will not matter much if the rest of the home is uninsulated and drafty. Passive solar space heating revolves around trapping heat for immediate or eventual use. On the other hand, more is not always better. There are sometimes problems with overheating on hot days.

Therefore it is important to work closely with an architect and your contractor to make sure this water wall or that sunspace, or even that water storage tank is the right size for your needs. Every home is different in many intricate ways, but one thing is for sure, they can all benefit from passive solar heating.

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Posted on July 20th in Solar Information by Dan.