Passive Solar

Passive Solar Design takes advantage of site, climate, and the energy of the sun to provide thermal comfort - both heating and cooling. Shaping and orienting buildings and paying careful attention to design, building systems and detailing are key to optimizing the passive performance. Because the strategies are so variable depending on the specifics of the location, passive solar design results in buildings that are finely tuned to their settings and site.

In most climates buildings can achieve passive comfort for a majority of the time, with additional energy inputs for extreme periods of weather being met through supplemental systems, ideally from renewable energy resources (aka 'active solar'), as discussed below.

Factors that influence the siting of a structure include orientation relative to solar access, protection from harsh sun or winds, and relationships to slopes or existing vegetation. Generally buildings should have the majority of their glazing facing within 30° of due south, and we find that in most instances the optimal orientation is roughly 17.5° east of due south, so a building has more morning gain and less later in the day. Microclimate conditions (i.e. fog, diurnal temperatures, etc.) and the specifics of the site require further consideration when determining the optimal orientation.

Passively heated buildings typically feature high insulation levels and tight construction, south-facing glazing, and thermal mass within the building's insulation envelope. Thermal mass is defined as heavy, dense materials, which might include slab-on-grade floors, thick soil or plaster wall finishes, thick or double gypsum board finishes, and masonry elements such as fireplaces, masonry heaters, or planters. Properly sized shading over windows and doors is key to controlling unwanted solar gain. Generally one wants to limit east, west and north facing glazing, though with high quality windows and advanced glazing systems one can enjoy views in these directions without high energy loss or gain penalties.

Passive cooling similarly features proper shading and thermal mass within the building envelope, and in many climates a comfortable environment can be maintained with natural ventilation alone. Here in California where nighttime temperatures are lower, night flushing via fans or natural convection (warm air rising) can be used to remove heat stored in the thermal mass from the building. Windows or fan openings are then closed in the morning and the mass helps keep the building cool and comfortable.

Other passive design features that can reduce the active energy needs of a building include daylighting, air-to-air heat exchange, radiant barriers and/or ventilated roof systems, and movable shading systems such as shutters or awnings. Employing these passive strategies can reduce or eliminate the mechanical systems, saving both direct costs and long-term energy costs.

Active Solar

Active Solar describes energy systems which capture the sun's energy and store it in some manner for later use, through mechanical or electrical means. The two basic types are electrical systems, such as photovoltaics, wind, or hydro, and thermal systems that heat liquid for domestic hot water and/or space heating needs.

Renewable Electricity Systems

Residential scale renewable energy typically means electricity generation via photovoltaic panels, but if the conditions allow, wind or micro-hydro are attractive renewable energy options. The discussion here is focused on photovoltaic (aka PV) systems.

Framed PV panels are the most common form of photovoltaic energy collection. These panels laminate solar cells (thin slices of mono-crystalline or polycrystalline silicone) onto glass surfaces which are interconnected in arrays generating DC (direct current) electrical energy, typically between 12 and 48 volts, though some grid-intertie inverters operate at higher voltages. An inverter converts the DC current into typical household AC (alternating current) 120 volt power.

Regarding residential projects, the average California household uses 20 kWh (kiloWatt-hours) of electricity per day. With some basic energy conservation (efficient lighting and appliances, limited or no air conditioning, and conscious use) a household should be able to get that use down to 10 kWh or less. Assuming an average of 5 hours of sunlight, and some electrical inefficiencies (line loss, inverter, etc.) this would require an array of approximately 2500 Watts (2.5 kW).

Typical PV panels generate 10 Watts per square foot, so a 2.5 kW array will cover an area of roughly 250 sq. ft. For top performance panels are sloped and oriented close to due south. Optimal orientation and slope depend on ones latitude and weather patterns; here in the San Francisco Bay Area the optimal angle for a fixed array facing due south is approximately 30° above horizontal. With seasonally adjusted panels, one can achieve an additional 10% of production. Fully tracking arrays can increase output by about 35% at our latitude, but tracking systems are prone to significant maintenance.

Solar Thermal Systems

Collecting heat from the sun and storing it in the form of water is the most typical active thermal system, with the heat then being used later for domestic hot water and/or space heating needs. There are a few basic variations we employ, but we aren't limited to these.

Batch collectors use domestic water pressure to push fresh water through the hot water collector, for domestic needs. This type of collector can only be used in climates that do not experience hard freezes. When the tap is turned on, heated water is pushed from the panel to the faucet where it is replaced with cool, incoming water. Often this is piped through a hot water heater, either as a pre-heat to a tank type heater, or through a solar-calibrated instant hot water heater, which doesn't turn on if the water is already hot.

Solar hot water systems in freezing climates will typically feature a closed loop system running anti-freeze (glycol) treated water through the collectors to a heat exchanger which in turn heats water in a solar storage tank. Unless the tank is located above the panel, this water must be pumped mechanically, either with a thermostatically controlled pump, or a 12V DC pump powered by a photovoltaic panel, which is elegant system in that it only runs when the sun is shining, heating the water in the collector.

Space heating can be accomplished with a variation on the closed loop option, by increasing the size of the collector array and, following the heat exchange, piping some of the heated fluid to either a large hot water storage tank, or through tubing that is buried in a 2' to 3' deep insulated bed of sand beneath the floor slabs. This combined solar direct hot water and space heating system was pioneered by Shelter Systems in Wisconsin, and made popular by Bob Ramlow. (This system and all things related are discussed in Bob's book 'Solar Water Heating'). We've combined this system with masonry heaters and wood-burning boilers as well as with air-source and geo-exchange heat pumps.