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Solar air heat

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Solar air heat

The front façade of the Fire and Emergency Services Training Institute at Toronto Pearson International Airport designed by Kleinfeldt Mychajlowycz Architects Inc. of Toronto, Canada is actually a black SolarWall transpired solar air heating system that heats the incoming ventilation air for the facility.
Solar air heating is a solar thermal technology in which the energy from the sun, solar insolation, is captured by an absorbing medium and used to heat air.[1] Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. It is typically the most cost-effective out of all the solar technologies, especially in commercial and industrial applications, and it addresses the largest usage of building energy in heating climates, which is space heating and industrial process heating.

Solar air collectors can be commonly divided into two categories:[2]

  • Unglazed Air Collectors or Transpired Solar Collector (used primarily to heat ambient air in commercial, industrial, agriculture and process applications)
  • Glazed Solar Collectors (recirculating types that are usually used for space heating)
Close up of the MatrixAir transpired solar air heating absorber panel. As sunlight strikes this panel the negative air pressure produced by the building's HVAC system pulls 5 - 7 CFM of preheated air for every square foot of collector area through the perforations spread across the panel.

Unglazed air collectors and transpired solar collectors

Background

The term "unglazed air collector" refers to a solar air heating system that consists of a metal absorber without any glass or glazing over top. The first system of this type was probably built by architect Arthur T. Brown for Rose Elementary School in Tucson, Arizona in 1948.[3]

The most common type of unglazed collector on the market is the transpired solar collector.[4] This technology was invented and patented as SolarWall by Conserval Engineering Inc. in the 1990s,[5] who worked with the U.S. Department of Energy (NREL) and Natural Resources Canada on the commercialization of the technology around the world.[6] The technology has been extensively monitored by these government agencies, and Natural Resources Canada developed the feasibility tool RETScreen[7] to model the energy savings from transpired solar collectors.

Since that time, several thousand transpired solar collector systems have been installed in a variety of commercial, industrial, institutional, agricultural, and process applications in over 30 countries around the world.[8][9] The technology was originally used primarily in industrial applications such as manufacturing and assembly plants where there were high ventilation requirements, stratified ceiling heat, and often negative pressure in the building. The first unglazed transpired collector in the world was installed by Ford Motor Company on their assembly plant in Oakville, Canada.[10]

With the increasing drive to install renewable energy systems on buildings, transpired solar collectors are now used across the entire building stock because of high energy production (up to 500-600 peak thermal Watts/square metre), high solar conversion (up to 90%) and lower capital costs when compared against solar photovoltaic and solar water heating.[11]

Method of operation

Schematic showing how the SolarWall air heating system works

Unglazed air collectors heat ambient (outside) air instead of recirculated building air. Transpired solar collectors are usually wall-mounted to capture the lower sun angle in the winter heating months as well as sun reflection off the snow and achieve their optimum performance and best return on investment when operating at flow rates of between 5 and 7 CFM per square foot of collector area.

This cutaway view shows the MatrixAir transpired solar collector components and air flow. The lower air inlet mitigates the intake of heated air to the HVAC system during summer operation.

The exterior surface of a transpired solar collector consists of thousands of tiny micro-perforations that allow the boundary layer of heat to be captured and uniformly drawn into an air cavity behind the exterior panels. This heated ventilation air is drawn under negative pressure into the building’s ventilation system where it is then distributed via conventional means or using a solar ducting system. The extensive monitoring by Natural Resources Canada and NREL has shown that transpired solar collector systems reduce between 10-50% of the conventional heating load and that RETScreen is an accurate predictor of system performance.[12]

While many facade mounted unglazed solar air collectors have air outlets near the top of the collector, Matrix Energy has patented a transpired collector using a lower outlet design that greatly reduces heated air entering the HVAC system during summer operation when heated ventilation air is not required, even when bypass dampers are installed.

Transpired solar collectors act as a rainscreen and they also capture heat loss escaping from the building envelope which is collected in the collector air cavity and drawn back into the ventilation system. There is no maintenance required with solar air heating systems and the expected lifespan is over 30 years.[13]

Unglazed transpired solar air heating systems provide low cost, fresh air ventilation yet require no maintenance. This is one of many examples of solar air heating systems being used in military bases across North America.

Variations of transpired solar collectors

Unglazed transpired collectors can also be roof-mounted for applications in which there is not a suitable south facing wall or for other architectural considerations. In these cases the heat from the system is ducted along the roof to the nearest air handling unit.

Matrix Energy Inc. has developed a unique two stage, modular roof mounted transpired collector operating a nearly 90% efficiency each module delivering over 118 l/s of preheated air per two square meter collector. Up to seven collectors may be connected in series in one row, with no limit to the number of rows connected in parallel along one central duct typically yielding 4 CFM of preheated air per square foot of available roof area.

Close up view of Matrix Energy's two stage, roof mounted modular solar air heating system.

Transpired solar collectors can also be combined with photovoltaic (PV) panels to create a hybrid solar system. The heat from the back of the PV modules (which is often 4 times more than the electrical energy produced by the PV module) is removed by the solar air system (which doubles as the PV racking system) and is used for building heating purposes. In cases where there is a heating requirement, incorporating a solar air component into the PV system produces two technical advantages; it removes the PV heat and allows the PV system to operate at its rated efficiency (which is 25C); and it decreases the total energy payback period associated with the combined system because the heat energy is captured and used to offset conventional heating.

Research is also being done by Conserval Engineering in terms of using the transpired solar collector for nighttime cooling.[10][14][15]

Glazed air systems

Functioning in a similar manner as a conventional forced air furnace, systems provide heat by recirculating conditioned building air through solar collectors - Solar thermal collectors. Through the use of an energy collecting surface to absorb the sun’s thermal energy, and ducting air to come in contact with it, a simple and effective collector can be made for a variety of air conditioning and process applications.

Solar Air Heat Collector, Flat Plate Air Collector, Solar Powered Furnace, installed on a vertical wall on the house for optimum winter performance and minimize summer performance to prevent over-heating.
SPF Solar Air Heat Collector

A simple solar air collector consists of an absorber material, sometimes having a selective surface, to capture radiation from the sun and transfers this thermal energy to air via conduction heat transfer. This heated air is then ducted to the building space or to the process area where the heated air is used for space heating or process heating needs.

The pioneering figure for this type of system was [3][16]

Collector types

Due to varying air-ducting methods, collectors are commonly classified as one of three types:

  • a) through-pass collectors,
  • b) front-pass,
  • c) back pass,
  • d) combination front and back pass collectors.

Through-pass air collector

In the through-pass configuration, air ducted onto one side of the absorber passes through a perforated or fibrous type material and is heated from the conductive properties of the material and the convective properties of the moving air. Through-pass absorbers have the most surface area which enables relatively high conductive heat transfer rates, but significant pressure drop can require greater fan power, and deterioration of certain absorber material after many years of solar radiation exposure can additionally create problems with air quality and performance.

Back, front, and combination passage air collector

In back-pass, front-pass, and combination type configurations the air is directed on either the back, the front, or on both sides of the absorber to be heated from the return to the supply ducting headers. Although passing the air on both sides of the absorber will provide a greater surface area for conductive heat transfer, issues with dust (fouling) can arise from passing air on the front side of the absorber which reduces absorber efficiency by limiting the amount of sunlight received. In cold climates, air passing next to the glazing will additionally cause greater heat loss, resulting in lower overall performance of the collector.

Air heat applications

A variety of applications can utilize solar air heat technologies to reduce the carbon footprint from use of conventional heat sources, such as fossil fuels, to create a sustainable means to produce thermal energy. Applications such as space heating, greenhouse season extension, pre-heating ventilation makeup air, or process heat can be addressed by solar air heat devices.[17] In the field of ‘solar co-generation’ solar thermal technologies are paired with photovoltaics (PV) to increase the efficiency of the system by cooling the PV panels to improve their electrical performance while simultaneously warming air for space heating.

Space heating applications

Space heating for residential and commercial applications can be done through the use of solar air heating panels. This configuration operates by drawing air from the building envelope or from the outdoor environment and passing it through the collector where the air warms via conduction from the absorber and is then supplied to the living or working space by either passive means or with the assistance of a fan.

Ventilation, fresh air or makeup air is required in most commercial, industrial and institutional buildings to meet code requirements. By drawing air through a properly designed unglazed transpired air collector or an air heater the solar heated fresh air can reduce the heating load during daytime operation. Many applications are now being installed where the transpired collector preheats the fresh air entering a heat recovery ventilator to reduce the defrost time of HRV's. The higher your ventilation and temperature the better your payback time will be.

Process heat applications

Solar air heat can also be used in process applications such as drying laundry, crops (i.e. tea, corn, coffee) and other drying applications. Air heated through a solar collector and then passed over a medium to be dried can provide an efficient means by which to reduce the moisture content of the material.

See also

References

  1. ^ "Solar Thermal Collectors - Energy Explained, Your Guide To Understanding Energy - Energy Information Administration". Tonto.eia.doe.gov. 2013-05-29. Retrieved 2014-05-04. 
  2. ^ "Advanced Manufacturing Office: Industrial Distributed Energy". Eere.energy.gov. Retrieved 2014-05-04. 
  3. ^ a b Denzer, Anthony (2013). The Solar House: Pioneering Sustainable Design. Rizzoli.  
  4. ^ "Survey of Active Solar Thermal Collectors, Industry and Markets in Canada". August 2010. Retrieved 3 August 2011. 
  5. ^ "SolarWall® Company Profile". Conserval Engineering Inc. Retrieved May 3, 2014. 
  6. ^ US patent 4899728, HOLLICK JOHN C; PETER ROLF W, "Method and apparatus for preheating ventilation air for a building", published 1998-07-17 
  7. ^ "RETScreen International Home". Natural Resources Canada. Retrieved May 3, 2014. 
  8. ^ "Solar Energy Use in U.S. Agriculture Overview and Policy Issues". United States Department of Agriculture. Retrieved 4 August 2011. 
  9. ^ Siegele, Lindsey. "SolarWall Solar Air Heating Technology". Mother Earth News. Retrieved 4 August 2011. 
  10. ^ a b "Transpired Collectors (Solar Preheaters for Outdoor Ventilation Air)". Federal Technology Alert (Federal Energy Management Program). National Renewable Energy Laboratory. April 1998. DOE/GO-10098-528. Retrieved July 25, 2010. 
  11. ^ Brown, David. "An Evaluation of Solar Air Heating at United States Air Force Installations". Air Force Institute Of Technology. Retrieved 4 August 2011. 
  12. ^ "Solar-Heated Fresh Air Cuts Heating Costs". NREL. 1994. Retrieved 4 August 2011. 
  13. ^ "Solar Preheated Ventilation". Naval Facilities Engineering Service Center. Naval Facilities Engineering Service Center. Retrieved 3 August 2011. 
  14. ^ "Solar Savings: An inside look at solar metal walls". Metal Architecture Magazine. Retrieved 1 September 2011. 
  15. ^ Lombardi, Candace. "Roof-mounted solar assists in cooling too". CNET. Retrieved 1 September 2011. 
  16. ^ Denzer, Anthony, "George Löf: Denver's Solar Pioneer", 2012 ASES National Solar Conference 
  17. ^ Rural Renewable Energy Alliance. "Solar Air Heat Basics". Retrieved 07/05/2011. 
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