Passive Solar at Home

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By Shawna Henderson, Bluehouse Energy

When you are designing a solar house in a cold climate, you need to address solar gain in three ways:

  • How are you going to collect it?
  • How are you going to store it?
  • How are you going to distribute it?

In old school passive solar design, this meant many square feet of south-facing glass for collection, many cubic feet of thermal mass (concrete, water, or masonry) for storage, and some crazy design that often sacrificed privacy and indoor air quality for distribution via natural convection. As an acre of single pane glass has terrible performance at night, old school passive solar houses might have also included a serious daily commitment on the part of the occupants to insert rigid insulation in the window frames at night – and to remove them in the morning.

That’s a fairly extreme version of a passive solar house, but it illustrates a big problem, regardless of whether the house is “sun-tempered” (less than 25 percent of heating requirements met by solar) or relies heavily on direct, indirect, or isolated gain (up to 75 percent of the heating requirements are met by solar): the diurnal temperature swing is a real comfort killer. Storing enough of the solar gain collected during the day to keep the house comfortable through the night is the holy grail of passive solar design.

Passive Solar Costs Less

A well-designed passive solar home is bright and sunny, with few fluctuations in temperature due to the thermal mass, which gives temperature stability and thermal comfort. The great thing about passive solar design is that there is little need for specialty items – perhaps selective glazing to optimize solar gain and minimize heat loss via re-radiation, perhaps a little more concrete or masonry work. Perhaps only a rearrangement of where windows and concrete are in the design. But all in all, the cost of passive solar design is minimal compared to active systems, and can minimize the operating costs along with heating and cooling loads.

While the three elements (collect, store, distribute) will always be the keys to good solar design, the way that houses are put together now, with better-performing windows and higher thermal envelopes, the amount of collector area can be lower. This means a passive solar house doesn’t have to have that acre of glass towering two stories high in the living area any more. In fact, a modest increase in south-facing glass and judicious window placement to the other three orientations can boost a house from “sun-tempered” to “direct gain,” even when passive solar is not the driving force behind the design.

There are more ways to collect solar energy than through windows, of course. Active solar systems that heat liquid or air that can be distributed throughout the house are the most familiar. Liquid systems (i.e., solar hot water systems) have the added bonus of ready storage.

Some Interesting Ideas

I did a little exploration a few years ago on design ideas and proprietary systems that integrate solar collection, storage and distribution into the building envelope. There are some interesting ideas out there – some of which work, some of which could work, some of which are serious pipe dreams.

One design practice used historically in passive solar houses is a thermal storage wall, often called a “Trombe wall.” It is useful for designs that had site-based restrictions that don’t allow for windows on the south side. This technique was patented in 1881 but developed in the 1960s into a usable construction element for “indirect gain.” Typically, it is a dark-colored masonry wall built on the inside of the building, between south-facing windows and the living space. After the masonry soaks up heat from windows during the day, it releases it to the living space through some form of natural or forced convection, with or without vents. For example, a wall constructed of hollow-core concrete blocks creates a series of “chimneys” that rely on the stack effect to move the air into the cooler living space.

Thermal storage walls can work well to heat garages and workshops that don’t typically have a lot of glazing or are not likely to require much heat at night, or on sites where the south-face of a house is up against a busy street for example (the thermal mass having the additional benefit of blocking street noise). A standard, unvented Trombe wall with a large, uninsulated thermal mass right behind the glazing could contribute around 30 Btu per square foot per day to the living space behind the wall.

Sunspaces and Double Envelopes

Another common design practice in passive solar homes is an attached sunspace, where solar gain is collected in an area that can be closed off from the rest of the building. Doors and windows or vents are opened between the sunspace and the house during the day to distribute the collected heat, and then closed at night, before the temperature in the sunspace drops to the point where it reverses the heat flow.

Sunspaces can be brilliant small additions to houses as mudroom/entry areas. In Nova Scotia, we calculated that an attached, airtight, insulated 8×10 sunspace with 4 2×4 windows can contribute about 12.1mil Btu (3500 kWh) to space heating needs over a heating season. In addition, the sunspace has a buffering effect on the area of the house that is no longer exposed to the temperature extremes throughout the heating season.

Using the 8×10 example, the buffering credit to the house-heating load is 315, 400 Btu (about 93kWh). The more the south face of the exterior wall is covered, the higher the buffering credit. There is also a higher credit for more thermal mass in the sunspace. These figures become more significant the lower the overall heating load is for the house.

An old school solar technique that combines collecting, storing and distributing the heat in the building envelope is the double envelope. This design methods features a sunspace that feeds air into a the cavity between two sets of exterior walls. The double envelope acts as a solar generator during the day and a crawlspace full of rock that acts as the thermal battery at night. The cost of materials has pushed this technique far out of the realm of possibility for most homeowners except perhaps the hard-core idealist and his or her custom builder.

Exciting New Materials

There are, however, amazingly cool materials like Vacuum Insulated Panels (VIPs) and other thin-skinned walls with outlandish R-values per inch on the horizon. When these are viable options, we can slap on super-insulated sheathing and insulated drywall, leaving the single stud cavity empty as a plenum. Which, of course, raises crucial and important questions about how indoor air quality is maintained when the distribution of your forced air system is through the building envelope and a crawlspace full of rocks.

For a while, there were some interesting-looking building-integrated products (for example, a patented solar hot water piping system integrated into a roof deck), but they have gone the way of the dinosaurs. Integrating solar collection into the building envelope today is focused on PV (solar electricity), and solar air heaters.

There are two common groups of solar air collectors. Perforated building cladding (typically used to pre-heat ventilation air) and wall- or roof-mounted collectors with through-the-wall fan systems. In Nova Scotia, we have specified a 4×8 wall-mounted unit that comes with a PV cell. This requires no purchased energy to distribute the heat, as the fan only runs when there is enough sunlight to activate the PV. This type of unit can produce over 20,000 Btu per clear, sunny day. The only downside is there is no energy stored for when the sun goes down and the thermometer drops dramatically in the winter.

Storage Is the Challenge

The challenge, as always with solar energy, gets back to storage – whether it’s a battery for solar-generated electricity or a thermal store for heat. So, besides creating an airtight and well-insulated envelope, what can be done to the building envelope to store thermal energy? Not a lot, as it turns out. However, there is a lot of interest in seasonal storage, using different mediums, all of which are related to liquid-based solar thermal systems – and somebody has to come up with a viable building-integrated solar thermal collector soon.

Mid- to long-term storage systems are not too far advanced on a single-house basis. Hydronic distribution systems in sand beds sandwiched between basement slabs and rigid insulation have been promoted as heat sink, but the jury is out on whether or not the cost of a more complex system, extra materials, and the cost of running the pumping equipment is worth the effort. There are several documented systems, but little or no published, third party monitoring of the performance.  Large water tanks have been used, and those have been shown to be an effective way of keeping low-grade heat for several days or weeks, however, they take up a lot of real estate in a house that could (should?) be living space.

Storage is more cost effective at a larger scale. Drakes Landing, in Okotoks, Alberta, is a community-level system where off-the-shelf solar thermal collectors mounted on the garage roofs of 52 houses feed a series of wells. The collectors feed hot water to the wells over the summer, and the wells feed the hot water to the houses where it gets distributed by high-efficiency air handlers. After several years of operation, the wells are apparently now fully charged and the houses are operating with minimal or no purchased energy.

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