Solar Heating Options

Solar Air Heating Options Compared, and Recommendations

Space heating for commercial, industrial and agricultural buildings can account for over half of a building’s total energy requirements and solar energy can displace a large percentage of this heating requirement.

This site is dedicated to explaining the various types of solar air heating systems (not water heating) and to assist buyers in selecting the most appropriate solar air technology for their buildings.  The initial selection of products to be found on this site are building integrated systems typically found on the larger commercial industrial buildings. Future reviews may look at the residential sector and the solar panels being offered to homeowners.

Let’s first look at the various types of solar air heaters on the market and then evaluate them according to performance, cost, appearance and ease of integration into a building.

The two main types of solar air panels are unglazed and glazed:

• Unglazed Solar Collectors are primarily used to heat ambient air and not building air. These 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.
• 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 and thus will be dealt with at a later time.

There are two types of unglazed panels, perforated (transpired collector) and non perforated (back pass).

Perforated Solar Collector

USA Department of Energy has rated the SolarWall transpired solar panel invention in the top 2% of all energy innovations. This low cost and high performance unglazed solar panel is building integrated, elegantly simple in operation and 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.

Perforated collector panels are installed several inches from an appropriate wall, creating an air cavity. As sunlight heats the solar collector surface, ventilation fans create a negative pressure in the air cavity, drawing in solar heated air through the perforations in the panel. A connection to an HVAC intake allows air to be preheated before entering the air handler, reducing the load on the conventional heater. Heated air is then distributed into the building through the existing HVAC system or with separate air makeup fans and perforated ducting.

The SolarWall System by Conserval, inventors of the Transpired Solar Collector
(photo courtesy of Conserval Engineering, Inc)

Transpired SolarWall Solar Collector

Note: The transpired solar collector was invented by Conserval Engineering Inc, who market the technology as SolarWall. Conserval owns numerous patents on the transpired solar heating system.

Back Pass non-perforated unglazed collector

An old design that was used in the 1980’s and has recently appeared is the back pass unglazed collector. The difference between a back pass non perforated collector and the transpired collector is the lack of holes. Unfortunately, the lack of holes prevents the capture of the external thermal boundary layer which represents 50% of the solar heating. In other words an unglazed back pass solar collector os much less efficient than the transpired solar collector. The lack of holes means that the incoming air must pass as close to the back side of the metal panel if it is to remove the heat from the back side (referred to as back pass collector).

A small back pass test panel with a short distance for air to travel, less than 3 m (10 ft) may produce solar efficiencies about half that of a perforated panel, but it is not scalable for large walls and roofs. Field experience has shown that the longer the distance the air must travel, the less efficient is the solar panel and if more air is to be heated, then the air cavity depth must be increased to accommodate the larger air volumes. Increasing the depth reduces the amount of air coming in contact with the back side of the collector and further reduces the heat transfer rate which lowers the solar efficiency.

Attempts by some designers to increase air flows and reduce the cavity depth will increase pressure drop and fan power which negates the potential solar heat gain. A back pass panel for a typical industrial wall where air must travel a long distance to reach the fan inlet will likely have solar efficiencies in the 20% to 30% range. By comparison, a similar sized transpired collector will have a solar efficiency in the 50% to 80% range or up to 400% more solar energy gain.

Typical back pass collector draws air from bottom and misses the surface heat on the panels

PV Thermal Solar Cogeneration

The rise of hybrid systems

One of the most exciting trends in solar technology is the hybrid solar photovoltaic/thermal (PV/T) technology, which combines PV with a solar thermal component to generate up to four times the energy from the same surface area. This hybridization also helps reduce the ROI timeframe on a photovoltaic system from decades to less than one decade, making PV electric systems more financially accessible to building owners.

A diagram showing the modular rooftop solar air heating unit with PV panels on top

With a modular PV/T system, the all-metal solar air-heating system panels double as the racking system, while removing heat from the back of the photovoltaic modules and utilizing it to offset the building’s heating load.  There are significant synergies between photovoltaics and solar air-heating technologies based on the fact PV modules generate electricity, but the electrical output is only one component of the total energy produced by a photovoltaic array. A typical PV module has an ideal electric conversion efficiency of about 15 per cent. The remaining energy produced is heat, which is neither captured nor used. This heat increases the operating temperature of the photovoltaic modules, which actually decreases their overall performance.

Scientific testing done three years ago in conjunction with the International Energy Agency Solar Heating and Cooling (IEA SHC) Task 35, “PV/Thermal Solar Systems,” at Canada’s National Solar Test Facility (NSTF) has shown it is possible to capture almost two to three times more thermal energy than electricity from a PV array. Photovoltaic panels from various manufacturers were tested under normal operating cell temperature (NOCT) conditions. The results indicated when photovoltaic modules were fixed atop wall-mounted solar air collector panels, the total solar energy conversion increased to more than 50 per cent, compared to the typical 10 to 15 per cent for PV modules alone.

Removing the excess heat generated by these modules increases electrical output. Modules can operate at temperatures of 50 C (90 F) above ambient, or at 80 C on hot summer days, reducing electrical performance by 25 per cent. By removing heat from the module and lowering its operating temperature, significant gains can be made in system performance; the dissipated heat can also be used for practical heating purposes.
Photovoltaic manufacturers rate the electrical output of modules at 25 C. For every 1 C above 25 C, the electrical output drops by 0.4 to 0.5 per cent. A typical rooftop PV array may measure 55 to 75 C, which means their electrical output would fall by 12 to 25 per cent below the name plate rating.

For example, a 10 kW array only generates 7.5 to 8.8 kW under these temperature conditions. A PV/T system lowers the photovoltaic temperature by 10 to 20 C, which increases the electrical output by five to 10 per cent, or an extra 0.5 to 1 kW for a 10 kW array. These values are extremely important for individuals taking advantage of Ontario’s new Feed-in-Tariff rate that pays a certain amount of money for 20 years based on the actual (i.e. not projected) solar electric energy delivered.

The modular units are easy to install and are angled at an ideal orientation for maximum solar gain.

A diagram showing SolarDuct PV/T

A roof mounted SolarDuct PV/T system

Two Stage – Unglazed & Glazed Perforated Panels

High Temperature, Space Heating or Windy Locations

The unglazed perforated panel system offers the lowest cost and highest efficiency building integrated solar system on the market today. It is designed to increase fresh air temperatures as much as 30 degrees C (54 F) over ambient. If your application calls for higher temperature air or roof mount panels such as with PV thermal systems, or if the building is in a cold windy climate, consider a two stage solar air heater.

For a small premium, a second stage glazed section can be installed over a portion of the unglazed panels to allow air to be heated twice by the perforated absorber panel, improving the solar efficiency. The concept is relatively simple. After the air passes through the standard transpired panel (first stage), the preheated air rises to a narrow cavity between a glazing and a second perforated panel where the air is heated again by the sun as it passes through the second set of perforations. After the second stage of heating, the air travels to the inlet of the fan or HVAC unit in the same manner as with the single stage transpired solar heater.

Two Stage wall mounted solar heater, with the lower half unglazed and the upper half covered with glazing

When to Use a Two Stage System

Roofs receive stronger winds than walls.  If the air to be heated has a flow rate of less than 40 m3/h.m2 (<2.1 cfm/ft2) and the panels are to be roof mounted, the effects of wind may reduce the performance of the unglazed design. By incorporating a two stage system, the air passes through the perforated panels twice which doubles the flow rate, i.e. a 2 cfm/ft2 flow rate becomes 4 cfm/ft2 which will minimize wind effects and increase the heat gain.

When unglazed perforated panels are roof mounted, the panels above the air intake or fan  opening must be covered with non perforated panels to prevent rain from entering the fan air intake. Using a glazed section along the top eliminates the need for a non perforated metal panel as the glazing doubles as the rain protective cover while increasing the heat gain.

Cutaway of two stage solar heater

Two stage solar system installed on a roof (courtesy of SolarWall).