Solar air heat

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The front facade of this building is a transpired solar air heating system that heats the incoming ventilation air for the facility. GTAA - SolarWall.jpg
The front façade of this building is a 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, 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.

Contents

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

Collector types

Solar collectors for air heat may be classified by their air distribution paths or by their materials, such as glazed or unglazed. For example:

Unglazed air collectors and transpired solar collectors

Background

The term "unglazed air collector" refers to a solar air heating system that consists of an absorber without any glass or glazing over top. The most common type of unglazed collector on the market is the transpired solar collector. [3] This technology was invented and patented by Canadian engineer John Hollick of Conserval Engineering Inc. in the 1990s, [4] who worked with the U.S. Department of Energy (NREL) and Natural Resources Canada on the commercialization of the technology around the world. [5] The technology has been extensively monitored by these government agencies, and Natural Resources Canada developed the feasibility tool RETScreen [6] to model the energy savings from transpired solar collectors. John Hollick and the transpired solar collector were honored by the American Society of Mechanical Engineers (ASME) in 2014 as being one of the best inventions of the industrialized age, alongside Thomas Edison, Henry Ford, the steam engine and the Panama Canal – in a New York exhibition recognizing the best inventions, inventors and engineering feats of the past two centuries.

Several thousand transpired solar collector systems have been installed in a variety of commercial, industrial, institutional, agricultural, and process applications in over 35 countries around the world. [7] [8] 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. [9]

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. [10]

Method of operation

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 return on investment when operating at flow rates of between 4 and 8 CFM per square foot (72 to 144 m3/h.m2) of collector area.

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 solar heated ventilation air is drawn into the building’s ventilation system from air outlets positioned along the top of the collector and the air is then distributed in the building 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. [11]

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. [12]

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. A number of companies offer transpired air collectors suitable for roof mounting either mounted directly onto a sloped metal roof or as modules affixed to ducts and connected to nearby fans and HVAC units.

Higher temperatures are also possible with transpired collectors which can be configured to heat the air twice to increase the temperature rise making it suitable for space heating of larger buildings. In a 2-stage system, the first stage is the typical unglazed transpired collector and the second stage has glazing covering the transpired collector. The glazing allows all of that heated air from the first stage to be directed through a second set of transpired collectors for a second stage of solar heating.

Another innovation is to recover heat from the photovoltaic (PV) modules (which is often four times more than the electrical energy produced by the PV module) by mounting the PV modules onto the solar air system. In cases where there is a heating requirement, incorporating a solar air component into the PV system provides two technical advantages; it removes the PV heat and allows the PV system to operate closer to its rated efficiency (which is 25 C); 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.

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. 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.

SPF Solar Air Heat Collector Solar Air Heat Collector.JPG
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 George Löf, who built solar heated air system for a house in Boulder, Colorado, in 1945. He later included a gravel bed for heat storage. [13]

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.

Solar 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. [14] 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.[ citation needed ]

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.

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.

Night cooling applications

Radiation cooling to the night sky is based on the principle of heat loss by long-wave radiation from a warm surface (roof) to another body at a lower temperature (sky). On a clear night, a typical sky-facing surface can cool at a rate of about 75 W/m2 (25 BTU/hr/ft2) This means that a metal roof facing the sky will be colder than the surrounding air temperature. Collectors can take advantage of this cooling phenomena. As warm night air touches the cooler surface of a transpired collector, heat is transferred to the metal, radiated to the sky and the cooled air is then drawn in through the perforated surface. Cool air may then be drawn into HVAC units. See also [9] [15] [16]

Ventilation applications

By drawing air through a properly designed air collector or air heater, solar heated fresh air can reduce the heating load during sunny operation. Applications include transpired collectors preheating fresh air entering a heat recovery ventilator, or suction created by venting heated air out of some other solar chimney.

See also

Related Research Articles

<span class="mw-page-title-main">Solar energy</span> Radiant light and heat from the Sun, harnessed with technology

Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy, and solar architecture. It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.

<span class="mw-page-title-main">Heating, ventilation, and air conditioning</span> Technology of indoor and vehicular environmental comfort

Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.

A Trombe wall is a massive equator-facing wall that is painted a dark color in order to absorb thermal energy from incident sunlight and covered with a glass on the outside with an insulating air-gap between the wall and the glaze. A Trombe wall is a passive solar building design strategy that adopts the concept of indirect-gain, where sunlight first strikes a solar energy collection surface in contact with a thermal mass of air. The sunlight absorbed by the mass is converted to thermal energy (heat) and then transferred into the living space.

<span class="mw-page-title-main">Passive solar building design</span> Architectural engineering that uses the Suns heat without electric or mechanical systems

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

<span class="mw-page-title-main">Solar thermal energy</span> Technology using sunlight for heat

Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors.

<span class="mw-page-title-main">Solar water heating</span> Use of sunlight for water heating with a solar thermal collector

Solar water heating (SWH) is heating water by sunlight, using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.

A solar chimney – often referred to as a thermal chimney – is a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.

<span class="mw-page-title-main">Solar thermal collector</span> Device that collects heat

A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar cooker, solar air heaters.

<span class="mw-page-title-main">Heat recovery ventilation</span> Method of reusing thermal energy in a building

Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) or energy recovery ventilation (ERV), is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is used to reduce the heating and cooling demands of buildings.

Renewable heat is an application of renewable energy referring to the generation of heat from renewable sources; for example, feeding radiators with water warmed by focused solar radiation rather than by a fossil fuel boiler. Renewable heat technologies include renewable biofuels, solar heating, geothermal heating, heat pumps and heat exchangers. Insulation is almost always an important factor in how renewable heating is implemented.

<span class="mw-page-title-main">Ground-coupled heat exchanger</span> Underground heat exchanger loop that can capture or dissipate heat to or from the ground

A ground-coupled heat exchanger is an underground heat exchanger that can capture heat from and/or dissipate heat to the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes.

Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, such as in the opposing season. For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. Waste heat from industrial process can similarly be stored and be used much later or the natural cold of winter air can be stored for summertime air conditioning.

Solar air conditioning, or "solar-powered air conditioning", refers to any air conditioning (cooling) system that uses solar power.

<span class="mw-page-title-main">Passive cooling</span> Building design that reduces inside temperatures without air conditioning

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from entering the interior or by removing heat from the building.

<span class="mw-page-title-main">Photovoltaic thermal hybrid solar collector</span>

Photovoltaic thermal collectors, typically abbreviated as PVT collectors and also known as hybrid solar collectors, photovoltaic thermal solar collectors, PV/T collectors or solar cogeneration systems, are power generation technologies that convert solar radiation into usable thermal and electrical energy. PVT collectors combine photovoltaic solar cells, which convert sunlight into electricity, with a solar thermal collector, which transfers the otherwise unused waste heat from the PV module to a heat transfer fluid. By combining electricity and heat generation within the same component, these technologies can reach a higher overall efficiency than solar photovoltaic (PV) or solar thermal (T) alone.

HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

The following outline is provided as an overview of and topical guide to solar energy:

Renewable thermal energy is the technology of gathering thermal energy from a renewable energy source for immediate use or for storage in a thermal battery for later use.

SolarWall is the brand name of a transpired collector designed and manufactured by Conserval Engineering. Conserval Engineering is based in Toronto, Ontario, Canada.

<span class="mw-page-title-main">Concentrated photovoltaic thermal system</span>

The combination of photovoltaic (PV) technology, solar thermal technology, and reflective or refractive solar concentrators has been a highly appealing option for developers and researchers since the late 1970s and early 1980s. The result is what is known as a concentrated photovoltaic thermal (CPVT) system which is a hybrid combination of concentrated photovoltaic (CPV) and photovoltaic thermal (PVT) systems.

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. ↑ "Survey of Active Solar Thermal Collectors, Industry and Markets in Canada" (PDF). August 2010. Archived from the original (PDF) on 1 October 2011. Retrieved 3 August 2011.
  4. ↑ "SolarWall® Company Profile". Conserval Engineering Inc. Retrieved May 3, 2014.
  5. ↑ USpatent 4899728,HOLLICK JOHN C; PETER ROLF W,"Method and apparatus for preheating ventilation air for a building",published 1998-07-17
  6. ↑ "RETScreen International Home". Natural Resources Canada. 10 March 2010. Retrieved May 3, 2014.
  7. ↑ "Solar Energy Use in U.S. Agriculture Overview and Policy Issues" (PDF). United States Department of Agriculture. Archived from the original (PDF) on 3 September 2019. Retrieved 4 August 2011.
  8. ↑ Siegele, Lindsey. "SolarWall Solar Air Heating Technology". Mother Earth News. Archived from the original on 30 March 2012. Retrieved 4 August 2011.
  9. 1 2 "Transpired Collectors (Solar Preheaters for Outdoor Ventilation Air)" (PDF). Federal Technology Alert. Federal Energy Management Program. National Renewable Energy Laboratory. April 1998. DOE/GO-10098-528. Retrieved July 25, 2010.
  10. ↑ Brown, David. "An Evaluation of Solar Air Heating at United States Air Force Installations" (PDF). Air Force Institute Of Technology. Retrieved 4 August 2011.{{cite journal}}: Cite journal requires |journal= (help)
  11. ↑ "Solar-Heated Fresh Air Cuts Heating Costs" (PDF). NREL. 1994. Retrieved 4 August 2011.
  12. ↑ "Solar Preheated Ventilation" (PDF). Naval Facilities Engineering Service Center. Archived from the original (PDF) on 28 March 2012. Retrieved 3 August 2011.
  13. ↑ Denzer, Anthony, "George Löf: Denver's Solar Pioneer" (PDF), 2012 ASES National Solar Conference
  14. ↑ Rural Renewable Energy Alliance. "Solar Air Heat Basics". Archived from the original on 29 December 2013. Retrieved 5 July 2011.
  15. ↑ "Solar Savings: An inside look at solar metal walls". Metal Architecture Magazine. Archived from the original on 31 March 2012. Retrieved 1 September 2011.
  16. ↑ Lombardi, Candace. "Roof-mounted solar assists in cooling too". CNET . Retrieved 1 September 2011.
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