Active Solar describes energy systems that 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, or photovoltaics, 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 (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 that are interconnected into 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.
The average California household uses 19 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. A 250 SF rooftop- or ground-mounted system (2.5 kW) could cover this energy use.
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.
Many companies offer lease programs, where they will install and maintain your panels. The resident benefits for the power produced, and reduced energy bills, while the company gathers any rebates. For more information about rebates in California, see gosolarcalifornia.ca.gov.
Solar Thermal Systems
Collecting heat from the sun and storing it in the form of water is the most typical active thermal system. Stored heat can be used for domestic hot water and/or space heating. 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 to pre-heat water in a tank 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.
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 through tubing that is buried in a 2-3' 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.
