Heating, Cooling and Ventilation

In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25 percent (4.75 EJ) of the energy used in commercial buildings and nearly half (10.1 EJ) of the energy used in residential buildings.Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy.

Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, concrete, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption. A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a hollow thermal mass connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. These systems have been in use since Roman times and remain common in the Middle East.

Solar space heating with air solar collectors is more popular in USA and Canada than heating with solar liquid collectors since most buildings already have a ventilation system for heating and cooling. The two main types of solar air panels are glazed and unglazed.

Glazed Solar Collectors are designed primarily for space heating and they recirculate building air through a solar air panel where the air is heated and then directed back into the building. These solar space heating systems require at least two penetrations into the building and only perform when the air in the solar collector is warmer than the building room temperature. Most glazed collectors are used in the residential sector.

Unglazed Solar Collectors are primarily used to heat ventilation air or ambient air and not building air. As these solar panels heat the fresh air for a building, they are ideally suited to commercial, industrial and institutional buildings with a high ventilation load. They only require one penetration into the building, or if existing fan inlets are used, then no additional penetrations are necessary. Heating ambient air allows solar energy to be utilized whenever the temperature in the collector is above ambient, not room temperature. This can provide twice the solar energy gain over space heating designs. The efficiency of a solar collector is highest when the temperature of the air entering the solar panel is equal ambient temperature. This occurs with solar heaters that draw outside air into the solar heater instead of room air.

The transpired solar panel is a low cost and high performance unglazed solar panel and is building integrated. It is currently the most popular type of solar air heating in North America. A painted metal panel, with small holes spaced uniformly across the entire absorber, is the main feature of the transpired collector. Sunlight strikes the dark surface which absorbs the heat. Solar heat conducts from the surface to the thermal boundary layer of air 1 mm thick next to the plate. This boundary layer of air is drawn into a nearby hole before the heat can escape by convection, virtually eliminating heat loss off the surface of the plate.

A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat.

Solar roof ponds are unique solar heating and cooling systems developed by Harold Hay in the 1960s. A basic system consists of a roof-mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building’s interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling.

Active solar cooling can be achieved via absorption refrigeration cycles, desiccant cycles, and solar mechanical processes. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device. Thermal mass, smart windows and shading methods can also be used to provide cooling. The leaves of deciduous trees provide natural shade during the summer while the bare limbs allow light and warmth into a building during the winter. The water content of trees will also help moderate local temperatures.