Active vs. Passive Solar

There are two basic types of solar homes Active and Passive.

Passive Solar - The homes we designed and built are passive solar. That means it works passively with no moving parts, no special equipment. A passive solar home is a solar collector by it very design. This limits you in that a passive solar home can not store a large amount of heat to hold for several cloudy days. Additionally uou must design the home so that most windows are on the south side. With some lots this is not very feasible.

Active Solar - An active solar home has solar collectors and a heat storage area. As a general rule an active solar home will cost more to build. The higher cost is due to building a collector area and a heat storage area in addition to the house. Because the collector area is not part of windows the collectors can be mounted on any south facing wall or roof area. So one has more options in the design of the house. An active house normally keeps a more stable temperature as the heat collecting system handles the temperature swings. One can think of an active system as a solar furnace to supplement your standard furnace. But because you have to allow for several cold cloudy days in the winter you can not do away with a standard heating system unless you are willing to don sweaters.

Below are some pictures of an active solar home in the the area.

As you can see this active solar home appears like a standard home from the street.

But on the south side they have a large wall of solar collectors. Over 500 sqft of collector area is on this one wall. This is about double the south facing windows we have in our passive solar house. It would be very difficult to add this much window area to a house without it looking very odd, or have it overheat on a sunny winter day.

In the basement they have a large insulated heat storage box to store the heat for use at nighttime and on cloudy days. The box measures 5' tall, 8' wide and 12 foot long. As you can see this requires significant extra space and additional ductwork is required.
During the day fans draw warm air from the collectors into the box to heat the materials in the box. At night they close off the flow to the collectors and direct the heat to be circulated through the house.

Initially the insulated box was filled with a phase changing salt. While non-toxic it did manage to make a mess as the plastic broke down over the years. The advantage of phase changing material is it stores a large amount of heat. 1000 lbs of phase changing material can store 100,000 BTUS of heat with only a 20 degree temperature rise.

The owners are changing over to water to store the heat it takes 5000 pounds of water or 630 milk jugs, to store the same amount of heat as 1000 lbs of phase change salts. But water is a great low cost heat sink.

So while an active system gives more flexibility in house design, the system adds costs to cover the heat storage and collector system.